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In cats anaesthetized with pentobarbital sodium, the posterior hypothalamus was superfused and electrically stimulated with a push-pull cannula. The pressor response to stimulation of this hypothalamic area was inhibited when the hypothalamus was superfused with drugs blocking either alpha-adrenoreceptors (piperoxan, tolazoline), or beta-adrenoreceptors--(+/-)-propranolol, (-)-propranolol, practolol, sotalol, metoprolol. (+)-Propranolol and a concentration of procaine equianaesthetic to propranolol were ineffective. During superfusion with tolazoline in the presence of practolol the inhibition was twice as that when the hypothalamus was superfused with either tolazoline or practolol. In another series of experiments the push-pull cannula was inserted into the anterior hypothalamus. The depressor response to stimulation of this area was inhibited by the hypothalamic superfusion with the alpha-adrenoreceptor blocking drugs phentolamine, tolazoline, piperoxan or yohimbine. Hypothalamic superfusion with phenylephrine abolished the inhibitory effect of phentolamine on the depressor response. The results indicate that adrenoreceptors are present in the hypothalamus and that they are involved in blood pressure changes elicited by hypothalamic stimulation.

In vivo microdialysis coupled with HPLC and electrochemical detection was used to monitor extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) of the locus ceruleus (LC) in halothane-anesthetized rats. The identity of DOPAC was confirmed by experiments showing that the chromatographic peak was totally suppressed after inhibition of monoamine oxidase by pargyline. Histological examinations allowed to relate the quantity of DOPAC measured in the dialysates with the localization of the probe implantation site. We found that the DOPAC concentration was inversely proportional to the distance between the probe and the lateral border of the LC. Regardless of the caudorostral level of the nucleus, concentrations were maximal when the axis of the probe was 100 microns from the lateral border of the LC and decreased by 53% when this distance reached 300 microns. Activation of LC noradrenergic neurons by systemic administration of the alpha 2-antagonist piperoxane increased by 100% DOPAC concentrations in LC dialysates. These results suggest that the DOPAC measured by microdialysis could be considered an indicator of the functional state of LC noradrenergic neurons.

The process describes the concise synthesis of (R/S)-enantiomers of doxazosin, an antidepressant drug and α-adrenergic receptor antagonists like prosympal, piperoxan, and dibozane in practical yields from easily available (R)-2,3-O-cyclohexylidene- d -glyceraldehyde and (S)-3-(benzyloxy)propane-1,2-diol.

Starting from commercially available inexpensive optically active diol (I) the synthesis of optically active 2-hydroxymethyl-1,4-benzodioxane (V) is developed and its use as common precursor in the synthesis of the title compounds doxazosin (XI), prosympal (XIIIa), piperoxan (XIIIb), and dibozane (XV) is demonstrated.

Effects of clonidine and tizanidine, which have antinociceptive and alpha 2-agonistic actions, were studied on the release of substance P from slices of spinal cord from the rat. Veratridine-evoked depolarization induced a 2-3-fold increase in the release of substance P from the slices of spinal cord. Exposure of the cord tissue to 10 microM clonidine and tizanidine significantly reduced the release of substance P. The inhibitory effects of clonidine and tizanidine were attenuated by pre-exposure of the tissue to 10 microM piperoxane, which has alpha 2-antagonistic activity and the inhibitory effect of clonidine was attenuated by 10 microM yohimbine. Moreover, the inhibitory effects of clonidine and tizanidine were also blocked by a small dose of prazosin, an antagonist for alpha 1- and alpha 2B-receptors. None of the antagonists had any effect on release of substance P, when given alone. These results suggest that alpha 2B-adrenoceptors are involved in the inhibitory effects of clonidine and tizanidine on the release of substance P.

In connection with the structure of piperoxan 1 and the recently described pharmacological properties of N -(diethylamino-2 ethyl) 2,3-dihydro 1,4-benzodioxin 2-carboximidamide 2a (R = C 2 H 5 , R′ = H) and of idazoxan 3 , different structural analogues have been prepared. These derivatives showed marked antiarrhythmic properties leading to the disappearance of extrasystoles and to a persistent regularization of the cardiac rhythm of rabbits perfused with aconitine. In addition, sedative effects have been detected using classical tests.

The role of descending noradrenergic fibers in the spinal motor systems was investigated using spinal reflexes in acutely spinalized rats. In rats pretreated with the MAO inhibitor clorgyline-HCl (1 mg/kg, i.v.), L-3,4-dihydroxyphenylalanine (L-dopa) (5 mg/kg, i.v.), a precursor of dopamine and noradrenaline, markedly potentiated the mono- (MSR) and polysynaptic reflexes (PSR). Selective blockade of alpha 1-adrenoceptors by pretreatment with prazosin-HCl abolished these facilitatory effects on the MSR and the PSR and revealed the inhibitory effect of L-dopa on the PSR. The depression of PSR was antagonized by the alpha 2-antagonist piperoxan. Clonidine-HCl (0.05 mg/kg, i.v.), a so-called alpha 2-agonist, and tizanidine-HCl (0.1 mg/kg, i.v.) decreased the MSR and the PSR in rats pretreated with prazosin. These inhibitions were antagonized by piperoxan. These results suggest that alpha 1- and alpha 2-adrenoceptors mediate facilitation and attenuation of motor transmission in the rat spinal cord, respectively.

Continuous free flow electrophoresis was investigated as a tool for the preparative chiral separation of piperoxan enantiomers using sulfated beta-cyclodextrin (sbeta-CD) as the chiral additive. Bulk migration of sbeta-CD was confirmed using LC-MS analysis of the individual fractions collected and visualized with the addition of crystal violet to the separation buffer. In the absence of sbeta-CD, the crystal violet-containing buffer was reddish/purple and the crystal violet was deflected cathodically in the chamber. In the presence of sbeta-CD, the crystal violet-containing buffer was blue and was deflected anodically. However, formation of accumulation and depletion zones was apparent in both cases. The addition of sbeta-CD to the cathodic wash solution allowed for almost complete resolution of the piperoxan enantiomers with a processing rate of 0.45 mg/ h.

Continuous free flow electrophoresis was investigated as a tool for the preparative chiral separation of piperoxan using a sulfated cyclodextrin chiral additive. In the absence of chiral additive, the sample stream was deflected cathodically. However, the presence of sulfated cyclodextrin in the run buffer caused anodic deflection and splitting of the sample stream into two streams, each enriched in one enantiomer. Although the sulfated cyclodextrin used was comprised of a mixture of homologues and isomers, this polydispersity did not seem to significantly impact band dispersion. Sample introduction rates ranged from approximately 0.9-7.2 mg h-1. Maximum resolution was 0.53, using an applied voltage of 220 V, buffer composition of 0.075% sulfated cyclodextrin, 7.6 mM citrate (pH 3), 4.5 degrees C.

We have previously shown that the release of acetylcholine (ACh) in the medial prefrontal cortex of the conscious rat, as measured by microdialysis, is increased following intraperitoneal injection of the selective alpha 2-adrenoceptor antagonist (+)-efaroxan. To characterize further the receptor pharmacology of this response, the effects of other selective alpha 2-adrenoceptor ligands were examined. The alpha 2-adrenoceptor antagonists idazoxan (2.5 and 20 mg/kg), atipamezole (2.5 mg/kg), and fluparoxan (10 mg/kg) increased ACh outflow by up to 250-325% of basal levels over a 3-h period following intraperitoneal injection. The alpha 2-adrenoceptor agonists UK-14304 (2.5 mg/kg) and guanabenz (2.5 mg/kg) reduced ACh outflow by 80 and 60%, respectively. Clonidine (0.00063-0.16 mg/kg) had no significant depressant effect and at 2.5 mg/kg increased ACh outflow to 233% of basal levels. These results indicate a modulatory role for alpha 2-adrenoceptors on the release of ACh in the rat prefrontal cortex in vivo. Based on the facilitatory effects produced by the antagonists alone, this alpha 2-adrenoceptor modulation appears to be tonic and inhibitory. The ability of alpha 2-adrenoceptor antagonists to enhance ACh outflow suggests a therapeutic usefulness in disorders where cortical ACh release deficits have been implicated.

Although amphetamine has profound cardiovascular actions, the role of the sympathetic nervous system in these responses is largely unknown. The purpose of this study was to characterize the sympathetic nerve responses to amphetamine and to determine whether these neural responses involve an action of amphetamine in the rostral ventrolateral medulla (RVLM). In sinoaortically denervated (SAD) and sham-SAD rats, amphetamine dose-dependently increased mean arterial pressure (MAP) and heart rate (HR), while decreasing (-87 +/- 5%, max) renal sympathetic nerve discharge (SND) for 57 +/- 5 min. Comparison of the SND responses in SAD and sham-SAD rats revealed a small but significant contribution of the baroreceptor reflex to the sympathoinhibitory response. In separate studies, the bilateral microinjection of amphetamine into RVLM decreased HR, MAP, and SND. The magnitude and duration of the decrease in SND elicited by amphetamine were significantly attenuated by the prior intravenous (i.v.) administration of idazoxan (alpha 2-adrenergic antagonist). The prior bilateral microinjection of idazoxan or piperoxan into RVLM significantly attenuated the duration of the sympathoinhibitory responses elicited by i.v. amphetamine. Idazoxan and piperoxan also tended to decrease the magnitude of the SND response; however, this reduction was significant at only the highest doses. The MAP and HR responses were unaffected by idazoxan treatment. The microinjection of terazosin (alpha 1-adrenergic antagonist) or propranolol (beta-adrenergic antagonist) into RVLM did not alter the HR, MAP, or SND responses to i.v. amphetamine. We conclude that i.v. amphetamine decreases SND in anesthetized rats, in large part, by a mechanism involving the activation of alpha 2-adrenergic receptors in RVLM.

Piperoxane is an α2-noradrenergic antagonist with demonstrated excitatory effects on neurons in the locus coeruleus, causing a corresponding increase in norepinephrine in many forebrain areas. 16 male Fischer-344 rats approximately 16 months of age were injected with 3 mg/kg of piperoxane or .09% saline. The piperoxane-treated rats lived an average of 127.1 days longer than the saline-treated rats. The results are discussed in terms of the effects of strategies designed to enhance brain levels of catecholamine and their effect on the aging process. A discussion of further research is also presented.

1. The effects of alpha 2-adrenoceptor agonists and antagonists on rat tail skin temperature (tts), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail. 2. In conscious rats, at an ambient temperature of 18.5-20 degrees C, tts was 21.0 +/- 0.2 degrees C and core (rectal) temperature (tc) was 38.2 +/- 0.04 degrees C (n = 126). The alpha 2-adrenoceptor antagonist, delequamine (RS-15385-197; 1 mg kg-1, s.c., n = 6), produced a rapid elevation in tts to 29.1 +/- 0.7 degrees C (P0.001 vs. saline-treated control group), attained 10 min after injection. tc fell slightly, by 1.0 +/- 0.1 degrees C. The tts response was dose-related over the dose-range tested (0.01-1 mg kg-1, s.c.), with an ED50 of 17 micrograms kg-1, s.c. (n = 6 per dose). 3. The maximum increases in tts in response to a dose of 1 mg kg-1, s.c. of alpha 2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+9.6 +/- 0.8 degrees C)yohimbine (+9.0 +/- 1.0 degrees C)WY-26703 (+7.9 +/- 1.3 degrees C)piperoxan (+5.6 +/- 1.7 degrees C)idazoxan (+4.6 +/- 1.3 degrees C)imiloxan (+4.1 +/- 1.3 degrees C)SKF 104078 (+2.0 +/- 1.9 degrees C)BDF-6143 (+1.3 +/- 0.8 degrees C). 4. Prazosin (0.3 mg kg-1, s.c.), hydralazine (10 mg kg-1, s.c.) and nifedipine (3 mg kg-1, s.c.) did not increase tts, whereas propranolol (10 mg kg-1, s.c.) evoked a small increase in tts (+2.9 +/- 1.0 degrees C). Pentolinium (2-10 mg kg-1, s.c.) elicited a dose-related increase in tts, which was elevated by 4.4 +/- 1.3 degrees C after a dose of 10 mg kg-1; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg-1, s.c.) had no effect on tts or tc. 5. In anaesthetized rats, idazoxan (300 microg, i.v.) produced a rapid increase in tts which was detectable 2 min after beginning the injection, reaching a peak after 7 min. When the same dose was administered i.c.v., tts also rose, but more slowly. The peak response (+ 3.6 +/- 0.70C, n = 5) was significantly smaller than when idazoxan was administered intravenously (+ 6.3 +/- 1.2 C, n = 5), which suggests that the increase in tts following systemic administration of M2-adrenoceptor antagonists is not due to a central effect. The change in tts was not secondary to changes in blood pressure.6. In the isolated, Krebs perfused, tail vascular bed of the rat, at an ambient temperature of 20-21C,under constant flow conditions (3.5-4.0 ml min-1; n = 4), baseline perfusion pressure was 57 +/- 4 mmHg.5-Hydroxytryptamine (5-HT; 70-150 nM) increased perfusion pressure by 56+/- 9 mmHg. The alpha2-adrenoceptor agonist, UK-14,304 (10 nmol), elicited a further increase in perfusion pressure by27.5 +/- 15 mmHg but had no effect in the absence of 5-HT; this response to UK-14,304 was abolished by rauwolscine (1 microM).7. Under constant pressure conditions (-100 mmHg; n = 9), baseline mean perfusion flow was 2.1 +/- 0.2 ml min-1, and mean tail skin temperature was 31.6 +/- 0.6C. 5-HT (119 +/- 28 nM) decreased tts.by 3.3 +/- 2.0 C and reduced flow by 1.2 +/- 0.3 ml min-1. UK-14,304 (10 nmol) further reduced tts by 3.0 +/- 0.3 C without significant effect on flow; this effect was also abolished by 1 microM rauwolscine.8. We conclude that post-junctional M2-adrenoceptors in the vasculature of the rat tail have a major vasoconstrictor role, controlling both the flow and distribution of blood within the tail and thereby thermoregulatory heat loss from its surface.

The direct effect of alpha 2-autoreceptors was studied by measuring the effects of piperoxan, an alpha 2-autoreceptor antagonist, and clonidine, an agonist on catecholamine exocytosis, from single bovine chromaffin cells in culture. Catecholamine release was elicited by stimulation with 100 microM nicotine and was monitored electrochemically with a carbon-fiber microelectrode placed adjacent to the cell. These electrodes allowed the number of exocytotic release events to be monitored and reported as total charge for release following a specific stimulus. Repeated stimulation with 100 microM nicotine showed that total release caused by the second exposure to nicotine was 32% of the first, and release caused by the third exposure to nicotine was 80% of the second. Total release of catecholamine increased significantly after application of 20 microM piperoxan relative to a control application of balanced salt solution. Application of 20 microM piperoxan alone did not cause release. After the cells were incubated in culture medium containing 20 microM clonidine, a significant decrease in nicotine-stimulated catecholamine release was observed. These results confirm that there are autoreceptors on chromaffin cells and, when relatively high levels of catecholamine are released, the catecholamine stimulates the alpha 2- autoreceptors, which inhibits subsequent release through a negative feedback mechanism. In addition to piperoxan, the sympathomimetic drug amphetamine also increases quantal release after application of nicotine. Amphetamine increases the extracellular concentration of catecholamine, and these data appear to indicate that at least part of the pharmacology of amphetamine might involve blocking catecholamine autoreceptors.

The alpha 2 agonist clonidine has been shown to be anxiolytic in a number of preclinical anxiety models. Interestingly, intravenous infusion of the alpha 2 antagonists idazoxan at 10 mg/kg and rauwolscine at 2.24 mg/kg significantly disinhibited lick-shock conflict responding in rats similar to the alpha 2 agonist clonidine (0.022 mg/kg) and the benzodiazepine diazepam (0.5 mg/kg). However, the alpha 2 antagonists yohimbine and piperoxan, the alpha 2 agonists medetomidine, guanfacine, and guanabenz, the non-specific alpha antagonist phentolamine, and the alpha 1 antagonist prazosin did not disinhibit conflict responding in the Vogel lick-shock paradigm. In fact, yohimbine has been shown to be anxiogenic in both animals and man. This may be due to yohimbine's lack of specificity and its ability to inhibit GABAergic release. In addition, all of these agents, except idazoxan, did not increase water consumption in water deprived rats. Idazoxan (10 mg/kg) significantly decreased water consumption by 45%. Therefore, idazoxan increased conflict responding for water reward at a dose (10 mg/kg) which also decreased water consumption in a non-conflict paradigm. These data suggest that agents with selective antagonism at the alpha 2 receptor site may be anxiolytic while agents with less specificity at this site such as yohimbine, piperoxan, and phentolamine are not anxiolytic.

Bulbospinal barosensitive neurons of the rostral ventrolateral medulla (RVLM cells; presumed sympathetic vasomotor premotor neurons) were recorded with iontophoretic electrodes in urethan-anesthetized rats. The majority of these cells were insensitive to intravenous clonidine (Clo; up to 20 micrograms/kg) and insensitive to iontophoretically applied Clo or alpha-methylnorepinephrine (alpha-MNE). These cells (n = 47 of 76) had a spinal conduction velocity of 4.1 +/- 0.2 m/s and a mean firing rate of 20 +/- 1 spikes/s. A second population (n = 29) was powerfully inhibited by intravenous Clo (5-10 micrograms/kg, activity decreased by 83 +/- 11%), iontophoretically applied Clo (decreased by 51 +/- 7%), and iontophoresis of alpha-MNE (decreased by 69 +/- 3%). These cells had a slower conduction velocity (2.0 +/- 0.3 m/s) and a much slower discharge rate (6 +/- 1 spikes/s). Both populations were pulse synchronous at resting arterial pressure. The inhibitory effects produced by iontophoresis of alpha-MNE or Clo were reduced to the same degree (86-98%) by iontophoresis of idazoxan (an alpha 2-adrenergic antagonist with imidazoline structure) and by iontophoresis of piperoxan (65-77%, a nonimidazoline alpha 2-antagonist). The inhibition of RVLM cells by intravenous Clo was reversed by iontophoresis of idazoxan and by intravenous injection of yohimbine (nonimidazoline alpha 2-antagonists). These data suggest that 1) intravenous Clo only inhibits a subpopulation of RVLM sympathetic premotoneurons, possibly the C1 adrenergic cells, 2) this effect of Clo is due to activation of alpha 2-adrenergic receptors rather than nonadrenergic imidazoline binding sites, and 3) these alpha 2-receptors are located on or close to the Clo-sensitive cells and may be continuously activated by endogenously released catecholamines.

The pharmacological characteristics of hindlimb scratching induced by serotonergic compounds were studied. We conclude that hindlimb scratching induced by serotonergic compounds is mediated by a serootonin 1D (5-HT 1D ) or 5-HT 1D -like receptor outside the blood-brain barrier because hindlimb scratching could be induced by s.c. injection of 5-methoxytryptamine (5-MeOT), 5-carboxamidotryptamine (5-CT), bufotenine, 5-hydroxytryptamine (5-HT) and tryptamine. These compounds have high affinity for 5-HT 1A and 5-HT 1D receptors. The 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT 1C receptor agonist MK 212, and the mixed 5-HT 1C /5-HT 2 receptor agonists (dl)-1-(2,5 dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and quipazine did not induce hindlimb scratching. Rather, the latter compounds attenuated 5-MeOT-induced hindlimb scratching. The 5-HT releasing compounds fenfluramine and p-chloroamphetamine (PCA) inhibited whereas the 5-HT re-uptake inhibitors fluvoxamine and indalpine potentiated 5-MeOT-induced hindlimb scatching, 5-MeOT-induced hindlimb scratching could be inhibited dose dependently by the α- 2 -adrenoceptor blockers yohimbine and rauwolsince, which also have high affinity for 5-HT 1D receptors, whereas the α 2 -adrenoceptor blocker piperoxan only weakly counteracted hindlimb scratching. Haloperidol, apomorphine, morphine, clonidine and methiothepin strongly attenuated hindlimb scratching, atropine, naloxone and ICS 205930 attenuated it weakly whereas domperidone, methylatropine and mepyramine were inactive in doses up to 10 mg/kg. Hindlimb scratching induced by 5-MeOT was potentiated by the 5-HT receptor antagonists metergoline, methysergide, mesulergine, mianserin, ritanserin and xylamidine. Hindlimb scratching was not induced by i.c.v. injection of 5-MeOT. This lack of i.c.v. activity of 5-MeOT and the effect of xylamidine suggests that the site of action for induction of hindlimb scratching is outside the blood-brain barrier.

Antagonism of pressor responses to sympathetic outflow stimulation and alpha-adrenoceptor agonists in pithed spontaneously hypertensive rats was used to estimate postsynaptic alpha-adrenoceptor blocking activity of mianserin, phentolamine, phenoxybenzamine, piperoxan and yohimbine. Estimation of presynaptic alpha-adrenoceptor blocking activity of these drugs was obtained by studying their ability to antagonize clonidine-induced suppression of positive chronotropic responses to sympathetic outflow stimulation. In this manner, evidence was obtained that mianserin causes selective presynaptic alpha-adrenoceptor blockade. Mianserin, piperoxan and yohimbine antagonized clonidine-induced avoidance blockade or hypotension in spontaneously hypertensive rats, but methysergide, phenoxybenzamine and phentolamine were ineffective. These results suggest that mianserin may antagonize the central effects of clonidine by blockade of noradrenergic presynaptic or autoreceptors and possibly explain the antidepressant effect of mianserin as due to indirect activation of central noradrenergic neurons.

To determine whether the hyperactivity of noradrenergic neurons that occurs during opiate withdrawal might be associated with a reciprocal alteration in noradrenergic receptor response, isoproterenol-stimulated adenylate cyclase activity was examined in the cerebella of morphine-addicted monkeys at two time periods: a) prior to withdrawal, and b) during late withdrawal. Compared to controls, the chronically-addicted group showed a significant increase in maximal enzyme velocity, a finding consistent with the observed hypoactivity of noradrenergic neurons which occurs during opiate administration. In contrast, the morphine withdrawal group demonstrated a significant decrease in enzyme activity. Piperoxan, known to mimic the effects of withdrawal in causing noradrenergic hyperactivity, also decreased enzyme activity. In no group was there a change in Ka for isoproterenol. These findings, indicating that opiate withdrawal is associated with a subsensitivity of noradrenergic receptor response, suggest a possible etiology for certain of the physiological changes of noradrenergic hypoactivity seen during the secondary (or protracted) abstinence syndrome.

Measurements were taken for the acetylcholine content of animal and human pelviureteral muscle and for the release of acetylcholine at rest and during field stimulation of the isolated renal pelvis and ureter. Release was frequency-dependent, with the maximum output obtained at 10 Hz. The release of acetylcholine from reserpine-pretreated and piperoxan-treated tissues remained unchanged, but tetrodotoxin (1.10(-6) g/ml) and noradrenaline (2.10(-6) g/ml) significantly reduced the output.

The acute hypotensive and bradycardic effects of the new compound B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin-dihydrochloride) were investigated in anaesthetized cats and rats. In spite of the molecular structure of B-HT 933, which differs considerably from that of clonidine, its cardiovascular reaction pattern is comparable, although the former is much less potent. B-HT 933 showed a central nervous origin for its acute hypotensive action in anaesthetized cats, since its effects on blood pressure after infusion via the left vertebral artery were much greater than after intravenous application of the same amounts. Pretreatment with the alpha-adrenoceptor-blocking drug, piperoxan (100 micrograms/kg), completely abolished the hypotensive response to B-HT 933 (30 micrograms/kg) injected subsequently via the left vertebral artery. It is concluded that B-HT 933 is a centrally acting hypotensive drug. Central alpha-adrenoceptors are involved in a similar manner as the mechanism of clonidine.

Clonidine (150 Μg/kg s. c.) depressed avoidance conditioned reflexes in Long-Evans rats. Some α-adrenoceptor blocking agents piperoxan, tolazoline, phentolamine, dibenamine and phenoxybenzamine also depressed these reflexes. Yohimbine (1–2 mg/kg) was found to have no significant effect. Atropine (10 to 15 mg/kg i. p.) antagonized the depression of conditioned avoidance reflexes induced by α-adrenoceptor blocking agents as well as the depression produced by clonidine. Yohimbine (1–2 mg/kg) reduced the depressing effects of clonidine on avoidance conditioned reflexes. In rats treated with piperoxan (10 mg/kg) the effects of clonidine were reduced. In these rats the depression of conditioned reflexes was less than in rats treated with clonidine or piperoxan alone. These experiments suggest that clonidine depressed avoidance conditioned reflexes in rats by activating central α-adrenoceptors. It is proposed that α-adrenoceptor blocking agents might act as partial agonists.

Dextromoramide (0.01−0.2 mg/kg i.v.) and fentanyl (0.005−0.05 mg/kg) induced a dose dependent decrease in blood pressure and heart rate in dogs. These effects appear to be to a centrally mediated decrease in sympathetic tone, (i) Both drugs are 10−20 times more potent when injected into the vertebral artery or the cisterna magna than when they were injected intravanously, (ii) Both drugs reduced splanchnic nerve activity in intact animals. The effect appears to be localized to the medulla oblongata as suggested by central injections; transection of the brain stem at midpoint levels did not change the effects on blood pressure and heart rate induced by dextromoramide in intact bivagotomized dogs. After transection of the spinal cord at C1, dextromoramide induced a strong decrease in heart rate and a hypertension. Cutting of both vagus nerves abolished the decrease in heart rate and blood pressure. The hypotensive effect was therefore due to the bradycardia reducing cardiac output. In contrast in intact and midpontine dogs, a reduction in sympathetic tone contributes to the hypothensive effect. Nalorphine prevented and reversed the hypotensive, bradycardic and sympatho-inhibitory effects of dextromoramide. Piperoxan and yohimbine, two potent peripheral and central α-adrenoceptor blocking agents did not change the effects of dextromoramide. However oxotremorine or acetylcholine administered into the cisterna magna and vertebral artery administration of physostigmine abolished or reversed the haemodynamic effects of dextromoramide. A tropine abolished the antagonistic effects of cholinomimetic agents on the central cardiovascular effects of dextromoramide. Muscarinic mechanism appear to be involved in the haemodynamic effects of analgesic agents.

In pithed rats prazosin (10μg/kg, i.v.) caused a prolonged antagonism of the hypertensive response to clonidine and (−)-noradrenaline, probably due to inhibition of vascular, postsynaptic α-adrenoceptors. The clonidine-induced reduction of the tachycardia evoked in pithed rats by electrical stimulation of cardiac sympathetic nerve fibres was antagonized by piperoxan and less effectively by prazosin, thus suggesting that prazosin displays a modest degree of cardiac presynaptic α-adrenoceptor blocking activity apart from its predominantly postsynaptic affinity. Prazosin (1 mg/kg, i.p.) significantly affected the hypotensive effect of clinidine (2 and 6 μg/kg, i.v.), but not the bradycardia induced by clonidine in pentobarbitone-anaesthetized, normotensive rats. Prazosin proved to be an effective hypotensive drug in anaesthetized cats. This action was peripheral as no central nervous origin could be demonstrated. Prazosin in low doses significantly reduced the central hypotensive effect of clonidine (1 μg/kg), injected into the left vertebral artery of chloralose-anaesthethized cats. Since the intravenous pretreatment with low doses of prazosin did not alter the central hypotensive response to clonidine, the interaction was likely to have occured within the brain-stem. Presumably, postsynaptic α-adrenoceptors in the brain, similarly to those in the periphery are inhibited by prazpsin, thereby preventing the central hypotensive effect of clonidine. It is submitted that clonidine and prazosin should not be combined in antihypertensive therapy in patients.

Earlier investigations of sympathomimetic drugs overcoming the amnesic action of cycloheximide (CXM) in day-old chickens were extended to biochemical studies in vitro. The effects of amphetamine, norepinephrine, α and β noradrenergic stimulants and receptor blockers on Na + /K + ATP'ase activity in total homogenate of chicken forebrain were investigated. Norepinephrine and the β stimulant, isoprenaline significantly stimulated the activity of this enzyme, while the β blocker, propranolol inhibited activity. Amphetamine, the α stimulant, methoxamine and the α receptor blocker, piperoxane had no effect on Na + /K + ATP'ase activity in total homogenate. In a purified synaptosomal preparation, both amphetamine (5 × 10 −5 M) and norepinephrine (1 × 10 −4 M) produced a slight stimulation of Na + /K + ATP'ase activity. A similar concentration of amphetamine (1.12 × 10 −4 M) did not inhibit 1 4 C-leucine uptake or incorporation into protein in the synaptosomal fraction. Nor was it able to alleviate CXM inhibition of 1 4 C-leucine incorporation into synaptosomal protein. The results are interpreted in terms of amphetamine (via release of norepinephrine) norepinephrine and isoprenaline stimulating and maintaining the labile, sodium pump-dependent, phase of memory formation for a sufficient length of time until protein synthesis inhibition by CXM wears off.

Cats were anaesthetized with pentobarbital sodium and the anterior hypothalamus was superfused with artificial cerebrospinal fluid through a push-pull cannula. Electrical stimulation of the superfused area with the tip of the cannula elicited a fall of the arterial blood pressure which was dependent on frequency and voltage. Maximal depressor response was obtained at 60 Hz with 2-4V; further increase of the voltage often led to a rise of the arterial blood pressure. Superfusion of the anterior hypothalamus with the alpha-adrenoreceptor blocking drugs tolazoline, piperoxan, yohimbine or phentolamine caused a dose-dependent inhibition of the depressor response to hypothalamic stimulation. Tolazoline was less effective than the other drugs. Superfusion of the anterior hypothalamus with the alpha-sympathomimetic drug phenylephrine prior to and during superfusion with phentolamine abolished the inhibitory action of the latter drug. It is concluded that alphaadrenoreceptors are present in the anterior hypothalamus and involved in the depressor response to electrical stimulation of this hypothalamic area.

Bilateral injections of adrenaline (0.01-10.0 microgram) into the anterior hypothalamic (AH) region, in urethane-anaesthetised spontaneous hypertensive (SH) rats, elicited dose-dependent falls in blood pressure and heart rate. The bradycardia was immediately in onset while the hypotension was preceded by a short-lasting rise in blood pressure. Bilateral injections of adrenaline into the anterior preoptic area (POA) and areas surrounding the AH had little or no effect on blood pressure and heart rate, while injections into the posterior hypothalamus (PH) induced tachycardia and hypertension followed by a smaller fall in blood pressure. Pretreatment with dl-propranolol (25-100 microgram bilat. AH) appeared to potentiate the hypotension induced by adrenaline (1 microgram bilat. AH) in a dose-dependent manner, but did not affect the falls in heart rate. On the other hand, pretreatment with metoprolol (25-100 microgram bilat. AH) effected a dose-dependent antagonism of the adrenaline-induced hypotension and bradycardia. Pretreatment with 1-propranolol (25 microgram bilat. AH) also antagonised the adrenaline-induced cardiovascular depressor effects, while pretreatment with d-propranolol (25 microgram bilat. AH) abolished the initial hypertensive effect. Pretreatment with piperoxan (25-100 microgram bilat. AH) antagonised adrenaline (1 microgram bilat. AH) induced hypotension and bradycardia only at the highest dose used. The results give further support to the concept that hypothalamic adrenaline receptors may be involved in the central regulation of blood pressure and heart rate. Furthermore, while an involvement of hypothalamic alpha-adrenoceptors cannot be ruled out, it is suggested that hypothalamic beta-adrenoceptors may be involved in mediating the cardiovascular depressor effects of adrenaline injected into the AH.

1. Intraventricular clonidine and BAY 1470, administered in small doses to conscious renal hypertensive cats, produced a fall in mean blood pressure lasting for a period of 3 h. This fall in blood pressure was accompanied by a marked bradycardia. 2. Pretreatment with intraventricular phentolamine (0.3–6 μmol), piperoxan (0.18–0.74 μmol) or tolazoline (0.35–1 μmol) abolished the hypotensive effects of intraventricular clonidine (74 nmol), whereas pretreatment with haloperidol (2.6 μmol/kg, intraperitoneally), or desmethylimipramine (3.3 μmol/kg, intraperitoneally, or 1.7 μmol, intraventricularly) did not modify the cardiovascular responses to clonidine. 3. Emesis was observed 1 min after intraventricular administration of clonidine (18–112 nmol) or BAY 1470 (0.07–0.14 μmol), which always preceded the cardiovascular actions and was still observed after pretreatment with haloperidol, desmethylimipramine, phentolamine, piperoxan or tolazoline. 4. In conscious hypertensive rats clonidine (0.6 μmol/kg, intraperitoneally) produced a marked fall in blood pressure that was antagonized by centrally acting α-adrenoreceptor blocking agents but not modified by pretreatment with either 6-hydroxydopamine (three doses of 1 μmol, intraventricularly) or 5,6-dihydroxytryptamine (0.1 μmol). 5. It is concluded that the anti-hypertensive responses to clonidine are mediated via stimulation of central α-adrenoreceptors and are independent of central dopaminergic receptors, intact central serotonergic neurons and intact adrenergic uptake mechanisms.

Piperoxan (50-100 micrograms/kg) injected into the cisterna magna of anaesthetized dogs increased blood pressure, heart rate and resting rate of sympathetic nerve discharge. The baroreflex curve was shifted to the right with an elevation of the upper plateau (when blood pressure was lowered below resting values, renal sympathetic nerve activity rose to an upper plateau) and with no change in baroreflex sensitivity. Catecholamine depletion, produced by reserpine and alpha-methyl-p-tyrosine, did not change the effects of piperoxan on resting sympathetic nerve activity and on the baroreflex curve. Piperoxan (50 micrograms/kg i.c.) enhanced the rate of renal sympathetic nerve discharge in baroreceptor-denervated dogs. Piperoxan (5 micrograms) injected bilaterally into the nucleus tractus solitarii (NTS) increased blood pressure and resting sympathetic nerve activity with a shift of the sympathetic baroreflex curve to the right and no change in baroreflex sensitivity. Piperoxan (5 micrograms) injected bilaterally into the ventrolateral pressor area did not change blood pressure or the rate of sympathetic discharge. However, the baroreflex curve was shifted to the right with an elevation of the upper plateau level and the baroreflex sensitivity was increased. The present findings suggest that tonically active alpha 2-adrenergic mechanisms located within the NTS regulate resting and reflex sympathetic activity. An alpha 2-adrenergic mechanism in the rostral ventrolateral medulla modulates reflex sympathetic activity but has no influence on resting activity.

1. A study was made of the effects of centrally administered alpha-adrenoceptor agonist drugs on the pressure changes elicited by somatic afferent nerve stimulation using chloralose-anaesthetized cats with autoperfused hindlimb circulations. 2. Clonidine, noradrenaline and adrenaline, given intracisternally, differed in their effects on the magnitude and duration of the systemic component of the somatic pressor reflex. None of these drugs significantly affected the hindlimb component of this reflex. 3. The selectivity of these drugs for the different reflex components may be explained by the different haemodynamic characteristics of the systemic and hindlimb circulations. 4. Intracisternal injection of the alpha-adrenoceptor antagonist piperoxan after the agonist drugs resulted in a complex pattern of responses affecting both the systemic and hindlimb reflex components. This suggests that the central pharmacological mechanisms involved may modulate more subtle changes in cardiovascular function than might be indicated by observations made in intact animals.

Postdecapitation convulsions (PDC) were quantified by recording the number of movements of the trunk of decapitated rats with an Animex motility meter. PDC were inhibited by α 1 -adrenoceptor blocking agents, phenoxybenzamine and prazosin, but unaffected by α 2 -adrenoceptor antagonists, yohimbine and piperoxan, or a β-adrenoceptor blocker, propranolol. Chronic administration of antidepressant drugs: amitriptyline, imipramine and zimelidine augmented the PDC. Preliminary experiments with [ 3 H]prazosin binding suggest that the increased PDC response is related to an increase in α 1 -adrenoceptor binding, most likely owing to an increase in receptor density.

The antihypertensive effects of Clonidine (0.15 mg kg−1, i.p.) were studied in conscious DOCA/saline hypertensive rats having chronically implanted arterial cannulae. The response to Clonidine was markedly reduced by simultaneously administered desipramine (3 mg kg−1, i.p.), antagonized dose-dependently by piperoxan (2–10 mg kg−1, i.v.) and prevented by pretreatment with phentolamine (0.2 mg, i.c.v.). Pretreatment with 6-hydroxydopamine (3 × 250 μg, i.c.v.), halo-peridol (1 mg kg−1, i.p.), p-chloro-N-methylamphetamine (3.5 mg kg−1, i.p.) or 5, 6-dihydroxytryptamine (50 μg and 25 μg, i.c.v.) did not significantly modify the antihypertensive response. It is concluded that the antihypertensive response to Clonidine is mediated via stimulation of central α-adrenoceptors and is independent of central dopaminergic receptors and intact central serotoninergic neurons. The necessity for intact central noradrenergic neurons remains uncertain.

1 Spinal alpha-adrenoreceptors involved in cardiovascular control have been investigated using selective alpha-adrenoreceptor agonists and antagonists in urethane-anaesthetized rats. 2 Intrathecal injections of clonidine, alpha-methylnoradrenaline, guanfacine and M7 at the C7-T1 level reduced blood pressure and heart rate. In contrast, phenylephrine, 5-hydroxytryptamine and procaine had little or no effect. These results suggest the involvement of spinal alpha 2-adrenoreceptors. 3 The fall in blood pressure produced by clonidine appeared to be attributable to a reduction in heart rate and stroke volume. Lower body vascular resistance was unchanged. 4 The clonidine-induced bradycardia was antagonised by prazosin, WB4101, piperoxan or yohimbine. Their relative potencies suggest that alpha 1-rather then alpha 2-adrenoreceptors mediate this response. 5 piperoxan and yohimbine clearly prevented the clonidine-induced fall in blood pressure; prazosin and WB4101 also appeared to antagonise clonidine but these results were complicated by the fact that these antagonists themselves reduced blood pressure. 6 It was difficult to interpret these results simply in terms of alpha 1- or alpha 2-adrenoreceptors. Thus spinal alpha-adrenoreceptors may be different from peripheral alpha 1- or alpha 2-adrenoreceptors.

The adrenergic receptors in the isolated feline middle cerebral artery were characterized pharmacologically using a sensitive system for recording circular contractions in vitro. Epinephrine. norepinephrine, isoproterenol. and phenylephrine contracted the vessel in the mentioned order of potency. Together with the inhibitory patterns obtained with graded doses of piperoxan (reversible competitive inhibition) and dibenamine or phenoxybenzamine (irreversible competitive inhibition), these results showed that the contraction was mediated by alpha receptors. With piperoxan and norepinephrine, the mean value for pA 2 was 7.06 and for K H 1.24 x 10 -7 M. The mean value for K A calculated for norepinephrine before and after partial irreversible blockade of the alpha receptors with phenoxybenzamine was 1.73 x 10 -6 M. The norepinephrine response was not directly proportional to the amount of receptors occupied; ED 50 was reached when only about 11% of the receptors were occupied and the E Am response was obtained when 75% of the receptors were occupied. Dilation was achieved only after an active tonic contraction had been induced (with 5-hydroxytryptamine) in the vessels, and the order of potency was isoproterenol > norepinephrine - epinephrine > terbutaline. Inhibition with INPEA and propranolol was competitive, as confirmed by Arunlakshana-Schild plots, showing that the dilatory response was a beta-receptor effect. The values for pA 2 (8.78 and 9.17) and K H (2.31 x 10 -9 M and 1.77 x 10 -9 M) for propranolol were indistinguishable in tests with terbutaline and isoproterenol, respectively. Comparison of the relative potencies of norepinephrine and epinephrine as well as isoproterenol and terbutaline suggested that the receptors were of the beta 1 type.

Intraperitoneal injection of 20 mg/kg of xylazine causes loss of the righting reflex (LRR) and exophthalmia in the rat. The experiments determined some of the pharmacological characteristics of these responses to xylazine. The putative α2-adrenoceptor antagonists yohimbine, piperoxan, CGS 7525 A, and idazoxan blocked the LRR response; tolazoline, phentolamine, and salbutamol attenuated the response to some extent, whereas prazosin, aceperone, and phenoxybenzamine exerted no apparent effect. The putative α2-agonists clonidine, guanabenz, guanfacine, guanoxabenz, tiamenidine, and naphazoline were identified in an earlier study as acting only as partial agonists in depressing rat behavior and were found here to antagonize partially the LRR response. Other compounds that attenuated but did not block the response either have some affinity for and perhaps act as partial antagonists at α2-receptors (e.g., phentolamine, LSD, lisuride) or exert some arousing or behaviorally stimulating action (e.g., cocaine, nomifensine, tranylcypromine, apomorphine). Arousing sensory stimulation also produced a partial attenuation of the LRR response. The xylazine-induced exophthalmia appeared responsive to the putative α1-antagonists prazosin and aceperone, and other drugs exerting α1-antagonist activity also antagonized the exophthalmia. The doses at which compounds antagonize xylazine-induced LRR and exophthalmia in rat might offer an estimate of the compounds' in vivo potency in acting as antagonists at a2- and a,-adrenoceptors, respectively.

Experiments were performed to determine the effects of yohimbine, piperoxan, and mianserin on the flash-evoked afterdischarge (FEAD) in rat. All three drugs are α-adrenergic antagonists, which block the inhibition of norepinephrine-release mediated by presynaptic (α2) receptors. It was predicted that these drugs would exert dose-dependant biphasic effects of FEAD: at low doses, which act presynaptically, the FEAD would be suppressed; while at higher doses, which also block postsynaptic (α1) receptors, the FEAD would be disinhibited. The dose response curves for yohimbine and piperoxan were, in fact, biphasic with low doses decreasing the amount of FEAD, while higher doses returned the amount of FEAD to baseline. No biphasic effect was seen with mianserin. These observations are consistent with the fact that yohimbine and piperoxan are most potent antagonists at α2- than at α1-receptors, whereas ththe affinities of mianserin for the two receptor types is comparable. The results of these experiments support the hypothesis that norepinephrine modulates FEAD.

3 H-clonidine labeled two binding sites in rat cortex membranes with apparent K D values of about 1.0 and 5.9 nM. These sites appeared analogous to “super-high” (SH) and “high” (H) affinity states of the α 2 -receptor described in human platelets. 10 mM magnesium increased the number of SH receptors by 30% whereas 100 μM GTP reduced SH 3 receptor number by 45% with no significant change in the K D of 3 H-clonidine at α 2 (SH) sites. In drug competition studies using 1.0 nM 3 H-clonidine, 100 μM GTP reduced the affinity of clonidine and increased the affinity of yohimbine, whereas 10 mM magnesium increased the affinity of clonidine and reduced the affinity of yohimbine. The effect of magnesium on the affinity of several antagonists at cortex 3 H-clonidine sites ranged from none (phentolamine) to a 6-fold reduction (piperoxan). These data indicate that different states of the α 2 -receptor exhibit different affinities for some antagonists.

The effects of superfused epinephrine (E) and norepinephrine (NE) on the membrane potential of primary afferent fibers of the isolated frog spinal cord were studied by sucrose gap recordings from the dorsal root. In all preparations both E and NE, applied in concentrations ranging from 0.1 μM to 1.0 mM, produced a hyperpolarization of afferent terminals. In many instances this was followed by a slow depolarization and, in a small number of cords, a small depolarization preceded the increase in membrane potential. E- and NE-induced hyperpolarizations were blocked by the selective α2-antagonists yohimbine and piperoxan, but not by the selective α1-antagonists prazosin and corynanthine or by the s-blockers propranolol and sotalol. The α2-agonists clonidine, α-methylnorepinephrine and guanabenz also hyperpolarized terminals, causing a change in potential that was reduced by yohimbine and piperoxan. Taken together, these results suggest that α2-receptors mediate the hyperpolarizing effects of E and NE. The s-agonist isoproterenol evoked a slow depolarization similar to that produced by E and NE. The isoproterenol-depolarization was antagonized by propranolol. Sometimes, application of E and NE after superfusion with yohimbine produced only a depolarization of the dorsal root and this depolarization was sensitive to propranolol. It would appear therefore that the late depolarization seen after the application of E and NE is produced by activation of s-receptors. In contrast, the α1-agonist phenylephrine elicited a short latency, short duration depolarization similar to those seen preceding approximately 10% of the E- and NE-hyperpolarizations. Such short-latency depolarizations were blocked by prazosin and corynanthine. The major component of the response to both E and NE is indirectly mediated through a synaptic process: application of Mn2+, Mg2+, procaine or tetrodotoxin in concentrations sufficient to block synaptic transmission substantially reduced, but never eliminated, the actions of the catecholamines. Interneurons are probably involved because mephenesin, which reduces interneuronal transmission, significantly decreased the E and NE effects. Furthermore, interneurons which secrete excitatory amino acids and/or GABA may mediate the indirect effects of the catecholamines on afferent terminals because (−)baclofen and d,l -α-aminoadipate decrease, and picrotoxin and bicuculline increase, the dorsal root (DR) effects of E and NE. These results indicate that catecholamines can alter the membrane potential of afferent fibers indirectly and probably directly and' that such effects are produced by activation of both α- and s-receptors. These results may be important in understanding the pharmacological basis of previous reports which have shown that catecholamines can affect the transmission of sensory information in the spinal cord.

1 Subcutaneous injections of clonidine (3.9 X 10(-8) mol/kg to 3.9 X 10(-6) mol/kg) produced forward locomotion and wall climbing in 7-day-old rats in a dose-dependent manner. 2 The effect was reduced significantly by a preceding intraperitoneal injection of phentolamine (7.9 X 10(-6) mol/kg), phenoxybenzamine (7.4 X 10(-6) mol/kg), yohimbine (1.3 X 10(-6) mol/kg) or piperoxan (7.4 X 10(-6) mol/kg). 3 The pA2-values of the antagonists to the clonidine-induced locomotor hyperactivity were: 5.1 (phenoxybenzamine), 5.2 (phentolamine), 6.4 (yohimbine) and 6.0 (piperoxan). 4 Metiamide (2.5 X 10(-4) mol/kg, 5.0 X 10(-4) mol/kg and 1.0 X 10(-3) mol/kg), a histamine H2-receptor blocker, did not affect the clonidine-induced locomotor stimulation. 5 It is suggested that the receptors which mediate clonidine-induced locomotor stimulation could be alpha-adrenoceptors but not histamine H2-receptors in the central nervous system of the infant rat.

Cortical slices incubated with [3H]noradrenaline (NA) were used to study quantitatively the infelucne of the NA concentration in the synaptic cleft on electrically induced release of [3H]NA from adrenergic nerve terminals. Stimulation-induced [3H]-overflow was regarded to be proportional to the NA concentration in the synaptic cleft. High concentrations of piperoxan or clonidine were used to block, or maximally stimulate, respectively, the presynaptic alpha-receptors and thus to eliminate feedback control. These two extreme conditions were thought to delineate maximal feedback range. With alpha-receptors not artifically manipulated, [3H]overflow increased with stimulus intensity, yet not in proportion to the [3H]-overflow when the presynaptic alpha-receptors were blocked, demonstrating increasing feedback inhibition with increasing NA concentrations in the synaptic cleft. Feedback inhibition of NA release was shown to depend in an exponential fashion on NA concentration in the synaptic cleft. The slope of the regression line indicated that maximal inhibition of NA release occurred with NA concentrations in the synaptic cleft 65,000--700,000 times higher than threshold concentration. Data from experiments with different stimulus rates also supported the notion of feedback control of NA release.

Microinjection of noradrenaline and clonidine into the nucleus ambiguus elicited dose-dependent bradycardia with insignificant alteration of blood pressure. Phenylephrine failed to elicit any cardiovascular effect. The bradycardic effects of noradrenaline and clonidine were antagonized by piperoxan but not by phenoxybenzamine. Adrenergic neurone blockade with local guanethidine pretreatment also abolished the response to clonidine. No significant cardiovascular effect of clonidine microinjection into the nucleus ambiguus was observed in bilaterally vagotomized animals. The baroreflex bradycardia induced by volume loading was abolished by yohimbine and piperoxan by volume loading was abolished by yohimbine and piperoxan but not by phenoxybenzamine, microinjected bilaterally into the nucleus ambiguus. These results demonstrate the presence of cardioinhihibitory, presynaptic α2-adrenoceptors in the nucleus ambiguus and their involvement in baroreflex bradycardia.

The effects of various α-adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic α-receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic α-receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated 3H-noradrenaline-overflow and the concentrations required to shift the EC50 for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA2, ratio 1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic α-receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic α-adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic α-receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with α-adrenergic receptors in the central nervous system.

To assess the influences of central noradrenergic neurons on both serotonergic and dopaminergic systems, the neurochemical effects of clonidine, piperoxane, and 6-hydroxydopamine were examined. Using quantitative fluorescence histochemistry and high performance liquid chromatography, we have demonstrated that clonidine, much like apomorphine, preferentially augmented intracellular serotonin (5-HT) fluorescence in the dorsal raphe without affecting 5-HT cells in the median raphe nucleus. Clonidine also produced a significant decrease of extraperikaryal catecholamine (CA) fluorescence in the same region. Piperoxane, at a dose having no significant effect alone, antagonized the effects of clonidine on 5-HT and CA. 6-Hydroxydopamine lesions of the locus coeruleus produced a similar increase of 5-HT fluorescence in the dorsal raphe and decrease of CA fluorescence in both the dorsal and median raphe. Biochemically, clonidine decreased while piperoxane increased a measure of 5-HT turnover in the corresponding terminal region of the dorsal raphe, the striatum. Similarly, dopamine turnover was also decreased by clonidine and increased by piperoxane in the striatum. These effects may be mediated by noradrenergic projections from the locus coeruleus to both the dorsal raphe and the substantia nigra. These results support the hypothesis that the effects of clonidine on serotonergic and dopaminergic neurons are indirectly mediated through noradrenergic receptor stimulation.

The effects of alpha 2-adrenoceptor agonists on sulfobromophthalein (BSP) disposition in mice were studied. It was found that agents with both central and peripheral activities (clonidine, guanabenz, B-HT 920 and methyldopa) as well as peripherally acting alpha 2-adrenoceptor agonists (para amino-clonidine and St 91) inhibited sulfobromophthalein disposition in the mouse, and also caused substantial hypothermia. The effects of these agonists were inhibited by yohimbine, a specific alpha 2-antagonist, except for those of para amino-clonidine where only partial reversal was achieved. The effects of clonidine on BSP disposition were also reversed by piperoxan but not by the alpha 1-adrenoceptor antagonists, prazosin and phenoxybenzamine, nor by the beta-blocker, propranolol. These results suggest that, in mice, peripheral alpha 2-adrenoceptors are involved in the effects of the alpha-agonists on BSP disposition.

The hypotensive action of a new compound, B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin) was characterized to be of the “clonidine-type” by the use of 6 pharmacological test procedures. (1) Intravenous injection of the substance elicited a typical cardiovascular reaction pattern of initial hypertension, followed by hypotension with bradycardia and decreased cardiac output. (2) A direct α-adrenoceptor stimulation at peripheral vascular sites was demonstrated by a hypertensive action in spinal rats. (3) A central nervous site of action was shown as 30 μg/kg injected intracisternally (i.ci.) lowered blood pressure and heart rate in cats, but the same dose i.v. was practically ineffective. (4) The decrease in sympathetic activity was revealed by a decrease in preganglionic splanchnic nerve discharges in cats. (5) The drug facilitated the vagally-mediated baroreceptor reflex as shown by the increase in reflex bradycardia following i.v. angiotensin injection in dogs. (6) The involvement of central nervous α-adrenoceptors was suggested because the α-adrenoceptor blocking drug piperoxan antagonized the facilitation of the baroreceptor reflex induced by B-HT 933. Furthermore, the drug facilitated the flexor reflex in rats with spinal section at midthoracic level. These results were unexpected as the chemical structure of B-HT 933 differs considerably from hitherto known substances of the “clonidine-type”.

The interaction between piperoxan and alpha 2-agonists on sympathetic tone was studied in rats. The sympatho-inhibitory effect of alpha 2-agonists (clonidine, guanfacine, B-HT 933) was assessed by recording heart rate in normotensive bilaterally-vagotomized rats. Clonidine (3 micrograms/kg, i.c.v.) and B-HT 933 (100 micrograms/kg, i.c.v.) induced a bradycardia which was fully reversed by piperoxan (30 micrograms/kg, i.c.v.). However, in rats treated with guanfacine, piperoxan induced a partial recovery of the bradycardic effect. The injection of a small dose of the specific alpha 1-adrenoceptor blocking drug, AR-C 239 (10 micrograms/kg, i.c.v.) which, by itself did not modify heart rate, completely inhibited the reversal effect of piperoxan in rats treated with clonidine, B-HT 933 or guanfacine. In rat brainstem membranes, B-HT 933 was found to bind to both alpha 1- and alpha 2-adrenoceptors and was as potent as clonidine in competing for alpha 1-sites bound by [3H]prazosin. On the other hand, in bilaterally vagotomized rats, piperoxan (30 micrograms/kg, i.c.v.) induced an increase in blood pressure and heart rate which was inhibited by previous administration of AR-C 239 (10 micrograms/kg, i.c.v.). These data suggest that, by inhibiting central alpha 2-adrenoceptors, piperoxan unmasks central alpha 1-adrenoceptor stimulation by endogenous catecholamines leading to an increase in the sympathetic tone, but a full recovery in heart rate could be observed only with the mixed alpha 1- and alpha 2-adrenoceptor agonists, clonidine and B-HT 933. In addition, these data further indicate that alpha 1-adrenoceptors are implicated in a tonic control of the sympathetic nerve activity in normotensive rats.

The effect of the α 2 -adrenoceptor antagonists RX 781094, rauwolscine and piperoxan on the metabolism of striatal dopamine and acetylcholine and of cerebral noradrenaline has been investigated in the rat. All compounds increased hypothalamic DOPEG-SO 4 levels, cerebral MOPEG-SO 4 concentrations and noradrenaline utilization. Rauwolscine and piperoxan also enhanced HVA and DOPAC levels and dopamine utilization in the striatum whereas RX 781094 did not. Moreover, rauwolscine and piperoxan, but not RX 781094, reduced striatal acetylcholine concentrations in vivo and antagonized the dopamine-induced inhibition of potassium-evoked release of [ 3 H]acetylcholine from striatal slices. Finally, rauwolscine and piperoxan competitively antagonized the apomorphine-induced decrease in striatal DOPAC levels. These results altogether indicate that rauwolscine and piperoxan, but not RX 781094, possess dopamine receptor blocking properties. The divergent effects of these α-adrenoceptor antagonists on striatal dopamine turnover, together with the inability of clonidine to antagonize the rauwolscine and piperoxan-induced elevation of striatal DOPAC, strongly argue against an involvement of α 2 -adrenoceptors in the regulation of striatal dopaminergic transmission.

The influence of clonidine on α-adrenoreceptors in the central nervous system of rats and mice has been investigated. Both functional events due to postsynaptic receptor stimulation (flexor reflex activity, motor activity) and biochemical changes have been considered. 1. Clonidine was less potent in stimulating the hindlimb flexor reflex activity of spinal rats than in inhibiting the α-methyltyrosine-induced disappearance of noradrenaline in the spinal cord and in the whole brain of rats. 2. The increase in flexor reflex activity due to clonidine (0.4 mg/kg) was virtually completely inhibited by phenoxybenzamine (20 mg/kg) and haloperidol (10 mg/kg), was partially inhibited by yohimbine (10 mg/kg) and piperoxan (60 mg/kg) and was not significantly inhibited by yohimbine (3 mg/kg) and tolazoline (50 mg/kg). 3. The potentiation by clonidine of the apomorphine-induced locomotor stimulation of reserpinetreated mice was almost completely inhibited by phenoxybenzamine (20 mg/kg) but was not significantly affected by yohimbine (10 or 3 mg/kg) and only slightly inhibited by tolazoline (50 mg/kg). 4. Clonidine (0.1 mg/kg) caused a considerable inhibition of the α-methyltyrosine-induced disappearance of noradrenaline in the spinal cord and brain of rats and in the brain of mice. This effect of clonidine was completely antagonized by yohimbine (10 mg/kg). It was markedly antagonized by yohimbine (3 mg/kg), piperoxan (60 mg/kg) or tolazoline (50 mg/kg) but not by phenoxybenzamine (20 mg/kg) or haloperidol (10 mg/kg). 5. Clonidine (0.1 mg/kg) caused an inhibition of the accumulation of Dopa after decarboxylase inhibition in the noradrenaline-rich regions of the rat central nervous system. This effect was counteracted by yohimbine (10 mg/kg), piperoxan (60 mg/kg) or tolazoline (50 mg/kg) but not by phenoxybenzamine (20 mg/kg). 6. The postsynaptic functional effects and the biochemical effects of clonidine may be due to stimulation of different α-adrenoreceptors since the two effects were inhibited differently by various α-adrenoreceptor blocking agents and since the two effects were produced by different doses of clonidine. The α-adrenoreceptors mediating the biochemical changes might be located on the noradrenergic neurones.

1 The effects of several drugs upon the excitatory junction potential (e.j.p.) in the guinea-pig vas deferens have been investigated. 2 Amiodarone, a noradrenergic neurone blocker which also blocks both alpha and beta-adrenoreceptors, did not reduce the e.j.p. 3 The alpha-adrenoreceptor antagonists, azapetine, piperoxan and prazosin, only enhanced the e.j.p. irrespective of their relative potencies at alpha 1 and alpha 2-adrenoreceptors. 4 Sotalol, a beta-adrenoreceptor antagonist, was without effect upon the e.j.p. 5 Clonidine and lysergic acid diethylamide, alpha-adrenoreceptor agonists, produced a dose dependent inhibition of the e.j.p. without apparently affecting the frequency or size of spontaneous junction potentials. 6 The effects of clonidine were antagonized by piperoxan in a competitive reversible manner. 7 It is argued that these results confirm the presence of alpha-adrenoreceptors prejunctionally upon the sympathetic excitatory innervation of the vas deferens but that although an endogenous alpha-adrenoreceptor agonist is released by nerve stimulation either the transmitter that is responsible for the e.j.p. is not noradrenaline or that the postjunctional receptors responsible for the generation of the e.j.p. are not adrenoreceptors.