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The synthesis and the binding study of new 3-arylesters and 3-heteroarylpyrazolo[5,1-c][1,2,4]benzotriazine 5-oxide 8-substituted are reported. The nature of these substituents (in terms of lipophilic and electronic features) seems to influence the binding affinity. High-affinity ligands were studied in mice in vivo for their pharmacological effects, considering six potential benzodiazepine actions: anxiolytic-like effects, muscle relaxant effects, motor coordination, anticonvulsant action, spontaneous motor activity, and ethanol-potentiating action. Compounds 4d and 6d showed an inverse-agonist profile. These compounds were evaluated also for their binding at benzodiazepine site on GABAA receptor complex (GABAA/BzR complex) subtype to evaluate their subtype selectivity.

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

The supraspinal cellular events involved in H(1)-mediated hyperalgesia were investigated in a condition of acute thermal pain by means of the mouse hot-plate test. I.c.v. administration of the phospholipase C (PLC) inhibitors U-73122 and neomycin antagonized the hyperalgesia induced by the selective H(1) agonist FMPH. By contrast, U-73343, an analogue of U-73122 used as negative control, was unable to modify the reduction of the pain threshold induced by FMPH. In mice undergoing treatment with LiCl, which impairs phosphatidylinositol synthesis, or treatment with heparin, an IP(3)-receptor antagonist, the hyperalgesia induced by the H(1)-receptor agonist remained unchanged. Similarly, pretreatment with D-myo inositol did not alter the H(1)-induced hypernociceptive response. Neither i.c.v. pretreatment with TMB-8, a blocker of Ca(2+) release from intracellular stores, nor pretreatment with thapsigargin, a depletor of Ca(2+) intracellular stores, prevented the decrease of pain threshold induced by FMPH. On the other hand, i.c.v. pretreatment with the selective protein kinase C (PKC) inhibitors calphostin C and chelerytrine resulted in a dose-dependent prevention of the H(1)-receptor agonist-induced hyperalgesia. The administration of PKC activators, such as PMA and PDBu, did not produce any effect on FMPH effect. The pharmacological treatments employed did not produce any behavioral impairment of mice as revealed by the rota-rod and hole-board tests. These results indicate a role for the PLC-PKC pathway in central H(1)-induced hyperalgesia in mice. Furthermore, activation of PLC-IP(3) did not appear to play a major role in the modulation of pain perception by H(1)-receptor agonists.

New 3-aryl-6-(3-thienyl)pyrazolo[1,5-a]pyrimidin-7-ones (2a-j) are synthesized and evaluated in vitro on Bz/GABA(A) receptors and on recombinant benzodiazepine receptors (alpha x beta 2/3 gamma 2; x = 1-3, 5) expressed in HEK293 cells. SAR studies on the new compounds are conducted and molecular modeling is accomplished to better investigate requirements leading to subtype selectivity. Some of the synthesized compounds are tested in vivo to explore their pharmacological effect as a consequence of their high alpha 1 beta 2 gamma 2 subtype selectivity observed in vitro.

Effects of substances that are able to alter the histamine level, a histamine H(1)-receptor agonist and antagonist, and a histamine H(2)-receptor agonist were investigated in an anxiety-like state in mice by means of the light/dark box test. Diazepam was used as positive control. The histamine H(3)-receptor antagonist, thioperamide (2, 5, and 20 mg/kg s.c.), showed an anxiogenic-like effect that reached a maximum with the dosage of 5 mg/kg. The histamine-N-methyltransferase (HMT) inhibitor, metoprine (5 and 20 mg/kg s.c.), also decreased the time in the light at the highest dose used and, likewise, the highly selective histamine H(1)-receptor agonist, 2-(3-trifluoromethylphenyl)histamine (FMPH) (2.65 and 6.5 microg/mouse, i.c.v.). On the contrary, the histamine H(2)-receptor agonist, impromidine (3, 10, 20, and 30 microg/mouse, i.c.v.), dose-dependently showed an anxiolytic-like effect. The selective histamine H(1) antagonist, pyrilamine (20 mg/kg i.p.) was able to prevent the anxiogenic-like effect of FMPH significantly, and that of thioperamide partially, while the effect caused by metoprine remained unvaried. It is suggested that the histaminergic system modulates anxiety-like states via the activation of both postsynaptic receptors in a contrasting manner: activation of the H(1) receptor causes an anxiogenic-like effect, while that of the H(2) receptors reduces anxiousness. However, on the basis of effects observed with the substances capable of releasing endogenous histamine, it seems likely that the anxiogenic-like effect is prevalent.

1. Effects of substances which are able to alter brain histamine levels and two histamine H1 receptor agonists were investigated in mice by means of an animal model of depression, the forced swim test. 2. Imipramine (10 and 30 mg kg(-1), i.p.) and amitriptyline (5 and 15 mg kg(-1), i.p.) were used as positive controls. Their effects were not affected by pretreatment with the histamine H3 receptor agonist, (R)-alpha-methylhistamine, at a dose (10 mg kg(-1), i.p.) which did not modify the cumulative time of immobility. 3. The histamine H3 receptor antagonist, thioperamide (2-20 mg kg(-1), s.c.), showed an antidepressant-like effect, with a maximum at the dose of 5 mg kg(-1), which was completely prevented by (R)-alpha-methylhistamine. 4. The histamine-N-methyltransferase inhibitor, metoprine (2-20 mg kg(-1), s.c.), was effective with an ED50 of 4.02 (2.71-5.96) mg kg(-1); its effect was prevented by (R)-alpha-methylhistamine. 5. The histamine precursor, L-histidine (100-1000 mg kg(-1), i.p.), dose-dependently decreased the time of immobility [ED30 587 (499-712) mg kg(-1)]. The effect of 500 mg kg(-1) L-histidine was completely prevented by the selective histidine decarboxylase inhibitor, (S)-alpha-fluoromethylhistidine (50 mg kg(-1), i.p.), administered 15 h before. 6. The highly selective histamine H1 receptor agonist, 2-(3-trifluoromethylphenyl)histamine (0.3-6.5 microg per mouse, i.c.v.), and the better known H1 agonist, 2-thiazolylethylamine (0.1-1 microg per mouse, i.c.v.), were both dose-dependently effective in decreasing the time of immobility [ED50 3.6 (1.53-8.48) and 1.34 (0.084-21.5) microg per mouse, respectively]. 7. None of the substances tested affected mouse performance in the rota rod test at the doses used in the forced swim test. 8. It was concluded that endogenous histamine reduces the time of immobility in this test, suggesting an antidepressant-like effect, via activation of H1 receptors.

The antinociceptive effect of (+/-)-3-alpha-tropanyl 2-(4-Cl-phenoxy)butyrate [corrected] (SM-21) (10-40 mg kg(-1) s.c., 10-30 mg kg(-1) i.p., 20-60 mg kg(-1) p.o., 3-20 mg kg(-1) i.v. and 5-20 microg per mouse i.c.v.) was examined in rodents and guinea pigs by using the hot-plate, abdominal constriction, tail-flick and paw-pressure tests. The antinociception produced by (+/-)-SM-21 was prevented by atropine, pirenzepine and hemicholinium-3 but not by quinpirole, R-(alpha)-methylhistamine, [1-[2(methylsufonyl)amino]ethyl]-4-piperidinyl]methyl-5-floro++ +-2-methoxy-1H-indole-3-carboxylate hydrochloride, N6-cyclopentyladenosine, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide, naloxone, 3-aminopropyl-diethoxy-methyl-phosphinic acid or reserpine. On the basis of the above data, it can be postulated that (+/-)-SM-21 exerted an antinociceptive effect mediated by a central potentiation of cholinergic transmission. Affinity profiles of (+/-)-SM-21 for muscarinic receptor subtypes, determined by functional studies (rabbit vas deferens for M1, guinea pig atrium for M2, guinea pig ileum for M3 and immature guinea pig uterus for putative M4) have shown a selectivity ratio M2/M1 of 4.6 that, although very low, might be responsible for the antinociception induced by (+/-)-SM-21 through an increase in ACh extracellular levels. In the antinociceptive dose range, (+/-)-SM-21 did not impair mouse performance evaluated by the rota-rod and hole-board tests.

1. Effects of substances which are able to alter brain histamine levels on the nociceptive threshold were investigated in mice and rats by means of tests inducing three different kinds of noxious stimuli: mechanical (paw pressure), chemical (abdominal constriction) and thermal (hot plate). 2. A wide range of i.c.v. doses of histamine 2HCl was studied. Relatively high dose were dose-dependently antinociceptive in all three tests: 5-100 micrograms per rat in the paw pressure test, 5-50 micrograms per mouse in the abdominal constriction test and 50-100 micrograms per mouse in the hot plate test. Conversely, very low doses were hyperalgesic: 0.5 microgram per rat in the paw pressure test and 0.1-1 microgram per mouse in the hot plate test. In the abdominal constriction test no hyperalgesic effect was observed. 3. The histamine H3 antagonist, thioperamide maleate, elicited a weak but statistically significant dose-dependent antinociceptive effect by both parenteral (10-40 mg kg-1) and i.c.v. (1.1-10 micrograms per rat and 3.4-10 micrograms per mouse) routes. 4. The histamine H3 agonist, (R)-alpha-methylhistamine dihydrogenomaleate was hyperalgesic, with a rapid effect (15 min after treatment) following i.c.v. administration of 1 microgram per rat and 3 microgram per mouse, or i.p. administration of 100 mg kg-1 in mice. In rats 20 mg kg-1, i.p. elicited hyperalgesia only 4 h after treatment. 5. Thioperamide-induced antinociception was completely prevented by pretreatment with a non-hyperalgesic i.p. dose of (R)-alpha-methylhistamine in the mouse hot plate and abdominal constriction tests. Antagonism was also observed when both substances were administered i.c.v. in rats. 6. L-Histidine HCl dose-dependently induced a slowly occurring antinociception in all three tests. The doses of 250 and 500 mg kg-1, i.p. were effective in the rat paw pressure test, and those of 500 and 1500 mg kg-1, i.p. in the mouse hot plate test. In the mouse abdominal constriction test 500 and 1000 mg kg-1, i.p. showed their maximum effect 2 h after treatment. 7. The histamine N-methyltransferase inhibitor, metoprine, elicited a long-lasting, dose-dependent antinociception in all three tests by both i.p. (10-30 mg kg-1) and i.c.v. (50-100 micrograms per rat) routes. 8. To ascertain the mechanism of action of the antinociceptive effect of L-histidine and metoprine, the two substances were also studied in combination with the histamine synthesis inhibitor (S)-alpha-fluoromethylhistidine and with (R)-alpha-methylhistamine, respectively. L-Histidine antinociception was completely antagonized in all three tests by pretreatment with (S)-alpha-fluoromethylhistidine HCl (50 mg kg-1, i.p.)administered 2 h before L-histidine treatment. Similarly, metoprine antinociception was prevented by(R)-alpha-methylhistamine dihydrogenomaleate 20 mg kg-1, i.p. administered 15 min before metoprine. Both(S)-alpha-fluoromethylhistidine and (R)-alpha-methylhistamine were used at doses which did not modify the nociceptive threshold when given alone.9. The catabolism product, 1-methylhistamine, administered i.c.v. had no effect in either rat paw pressure or mouse abdominal constriction tests.10. These results indicate that the antinociceptive action of histamine may take place on the postsynaptic site, and that its hyperalgesic effect occurs with low doses acting on the presynaptic receptor. This hypothesis is supported by the fact that the H3 antagonist, thioperamide is antinociceptive and the H3 agonist, (R)-alpha-methylhistamine is hyperalgesic, probably modulating endogenous histamine release.L-Histidine and metoprine, which are both able to increase brain histamine levels, are also able to induce antinociception in mice and rats. Involvement of the histaminergic system in the modulation of nociceptive stimuli is thus proposed.

The effect of the GABAA antagonists, bicuculline and picrotoxin, in the hot plate and writhing tests in mice and the paw-pressure test in rats was assessed. Subconvulsant doses of bicuculline (1.3-4 mumol kg-1, s.c.) or picrotoxin (0.8-2.5 mumol kg-1, s.c.) induced a dose-related increase in latency of licking in the hot plate test in mice, whereas subconvulsant doses of strychnine and thiosemicarbazide (0.9 and 6 mg kg-1, s.c. respectively), did not modify the threshold to thermal stimuli in mice. The effects of bicuculline and picrotoxin were not modified by naloxone (3 mg kg-1, i.p., a dose which inhibited the antinociceptive effect of morphine) or by atropine (5 mg kg-1, i.p., a dose which prevented oxotremorine-induced antinociception) but were antagonized by the GABAB antagonist CGP 35348 (2.5 micrograms, i.c.v., a dose which prevented (+/-)baclofen-induced antinociception). Mice, rendered tolerant to baclofen-induced antinociception by twice daily injection of increasing doses of baclofen (5-18 mg kg-1, s.c.), were unresponsive to the antinociceptive effects of bicuculline and picrotoxin but still responded to morphine. Bicuculline and picrotoxin, in the same range of doses which affected the three models of antinociception used, inhibited pentobarbital-induced hypnosis. Large doses of bicuculline and picrotoxin (4 and 2.5 mumol kg-1, s.c. respectively), reduced locomotor activity and impaired rota-rod performance in mice. The changes in response to noxious stimuli, induced by bicuculline and picrotoxin, are interpreted as an antinociceptive effect. It is then suggested that this effect might depend on an indirect activation of GABAB receptors through release of GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

1. The ability to modify the pain threshold by the two M1-muscarinic agonists: McN-A-343 and AF-102B and by the specific M2-agonist arecaidine was examined in mice and rats by using three different noxious stimuli: chemical (writhing test), thermic (hot-plate test) and mechanical (paw pressure test). 2. In the mouse hot-plate test McN-A-343 (20-50 micrograms per mouse i.c.v.) and AF-102B (1-10 mg kg-1 i.p.) produced significant antinociception which was prevented by atropine (1 microgram per mouse i.c.v.) and by the two selective M1 antagonists: pirenzepine (0.01 micrograms per mouse i.c.v.) and dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by the specific M2-antagonist AFDX-116 (0.1 micrograms per mouse i.c.v.), naloxone (1 mg kg-1 i.p.) or by the acetylcholine (ACh) depletor hemicholinium-3 (HC-3) (1 micrograms per mouse i.c.v.). McN-A-343 and AF-102B were able to increase the pain threshold also in the mouse acetic acid writhing test and in rat paw pressure test. These antinociceptive effects were completely prevented by dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by AFDX-116 (0.1 microgram per mouse or rat i.c.v.). 3. In contrast with the M1-agonists, the M2-agonist arecaidine (0.1-2 micrograms per mouse or rat i.c.v.) did not induce antinociception in all three analgesic tests. However, arecaidine, at the same i.c.v. doses, was able to reduce the pain threshold in the hot-plate and paw pressure tests.4. The site of muscarinic control of the pain threshold is localized in the CNS since drugs which do not cross the blood-brain barrier such as McN-A-343, pirenzepine and arecaidine exerted their effects only if injected i.c.v.5. On the basis of the above findings and existing literature we suggest that the postsynaptic muscarinic receptors involved in antinociception belong to the M1 subtype. Nevertheless, presynaptic autoreceptors (M2 subtype) may play a role in pain regulation since they are involved in modulation of endogenous ACh release.

1. CGP 35348, a new GABAB antagonist, was examined on antinociception induced by (+/-)-baclofen by use of the hot plate and writhing tests in mice and the paw pressure test in rats. CGP 35348 was also studied in mice on (+/-)-baclofen-induced impairment of rota-rod performance. 2. CGP 35348, injected either i.p. (60-100 mg kg-1 in mouse) or intracerebroventricularly (i.c.v.) (0.5-2.5 micrograms per mouse; 25 micrograms per rat) prevented (+/-)-baclofen-induced antinociception. 3. CGP 35348 did not modify oxotremorine- and morphine-induced antinociception in mice and rats. 4. CGP 35348 (2.5 micrograms i.c.v. per mouse) also prevented (+/-)-baclofen-induced impairment of the rota-rod test. 5. Two other GABAB antagonists, phaclofen (50 micrograms i.c.v. per mouse) and 2-OH-saclofen (2.5 micrograms-10 micrograms i.c.v. per mouse) did not modify (+/-)-baclofen-induced antinociception. 7. These results suggest that, at present, CGP 35348 is the only compound able to antagonize (+/-)-baclofen-induced antinociception.

1. The effect of atropine on the nociceptive system was examined in mice and rats by use of the hot-plate, writhing and tail-flick tests. 2. Atropine dose-dependently produced analgesia, no effect and hyperalgesia. Analgesia was observed in both species with doses ranging from 1 to 100 micrograms kg-1 while hyperalgesia was obtained with 5 mg kg-1. 3. Atropine antinociception was prevented by pirenzepine (0.1 microgram per mouse, i.c.v.), dicyclomine (10 mg kg-1, i.p.), atropine-methylbromide (0.5 microgram per mouse, i.c.v.) and hemicholinium-3 (1 microgram per mouse, i.c.v.). Naloxone (1 mg kg-1, i.p.), alpha-methyl-p-tyrosine (100 mg kg-1, s.c.) and reserpine (2 mg kg-1, i.p.) were ineffective. 4. The site of atropine analgesia is in the CNS since it exerts its antinociceptive effect also when injected i.c.v. (1-10 ng per mouse). Moreover drugs which do not cross the blood-brain barrier, such as hemicholinium-3, pirenzepine and atropine methylbromide, were unable to antagonize atropine analgesia if administered i.p. 5. Atropine also in vitro, showed a biphasic action on electrically-evoked guinea-pig ileum contractions. Concentrations between 10(-14) and 10(-12) M increased electrically and nicotine-evoked contractions but did not affect acetylcholine- and oxotremorine-evoked contractions. Concentrations above 10(-9) M inhibited both electrically- and drug (acetylcholine, nicotine and oxotremorine)-evoked contractions while they were ineffective on unstimulated ileum. 6. On the basis of the above findings, amplification of cholinergic transmission by very low doses of atropine is postulated, through a selective blockade of presynaptic muscarinic autoreceptors, as the likely mechanism of action. 7. Atropine antinociception, unlike oxotremorine antinociception, was obtained without any impairment of mouse rota-rod performance. 8. The antagonism by pirenzepine and dicyclomine of oxotremorine and atropine antinociception suggests that M1 muscarinic receptor subtypes are responsible for cholinergic analgesia.

1. Some new pharmacological activities of bicuculline were found in isolated rat atria, mouse vas deferens and guinea-pig ileum. 2. In isolated rat atria bicuculline (10-300 microM) induced potent positive inotropic and negative chronotropic effects which were not antagonized by propranolol (1 microM), 6-hydroxydopamine pretreatment (50 mg/kg i.v. twice), ranitidine (3 microM) or atropine (1 microM). Bicuculline (10-300 microM) potentiated electrically evoked contractions in mouse vas deferens and inhibited them (30-500 microM) in guinea-pig ileum. It was inactive on unstimulated mouse vas deferens. 3. The above effects were completely reproduced by the bicuculline related-substance, beta-hydrastine, but not by the bicuculline N-methyl derivative, bicuculline methiodide (BMI), on the isolated rat atria. BMI inhibited instead of potentiating the mouse vas deferens twitches and potentiated instead of inhibiting the guinea-pig ileum twitches. 4. Picrotoxin, the other classic non-competitive GABA A antagonist, was completely devoid of the effects reported for bicuculline. 5. We concluded that, on the three preparations studied, bicuculline possesses some effects which are unrelated to its GABA A receptor blocking activity.

The antipyretic, analgesic, antinflammatory and antiulcerogenic properties of a new compound 1,4 dihydro-1-ethyl-7-phenylpyrrol (1,2-a)-pyrimidine-4-one (V33) are described. V33 on a mg/kg basis possesses antipyretic and analgesic properties at doses lower than paracetamol and which do not produce hypothermia or motor impairment. V33 possesses antiinflammatory activity and decreases the production of LTB4 from inflammatory exudates without affecting PGE2 content. V33 is not only devoid of gastric ulcerogenic properties but exerts antiulcer activity toward various ulcerogenic stimuli. Lethal dose 50% (LD50) of V33 is higher that of paracetamol.

The protective action of eseroline--(3aS,8aR)-1,2,3,3a,8,8a-hexahydro-1,3 a, 8-trimethyl-pyrrolo[2,3-b]indol-5-ol--salicylate against (DFP) diisopropyl fluorophosphate and carbamate poisoning of cholinesterases (ChEs) has been examined in-vitro with human erythrocytes and purified preparations of electric eel acetylcholinesterase (AChE) and of horse serum butyrylcholinesterase (BuChE), and in-vivo using mice. Eseroline afforded 50% protection (ED 50) of erythrocyte AChE against inactivation by 1 microM DFP, physostigmine or neostigmine, at concentrations of 4.3, 22 and 23.5 microM, respectively, while for eel AChE protection against 10 and 30 microM DFP, 0.3 and 1 microM physostigmine and 1 microM neostigmine the eseroline ED 50 values were 0.3, 0.4, 0.7, 1.9 and 5.6 microM, respectively. On the other hand, up to 0.3 mM eseroline did not appreciably affect the inhibitory action of the same drugs on horse serum BuChE. Eseroline concentrations in the range 0.1-1 mM were able to reactivate 20-42% of erythrocyte AChE previously inhibited by 100 microM physostigmine, but failed to reactivate the DFP (10 microM)-pretreated enzyme to any extent. Finally, eseroline salicylate injected into mice (10 mg kg-1 s.c.) protected an average of 82 and 26% of the animals against lethal doses of DFP (7 mg kg-1 s.c.) and physostigmine sulphate (1 mg kg-1 i.p.) respectively, which were administered 15 min later. These results indicate that the protective activity of eseroline correlates well with its own anti-ChE profile, and that the effectiveness of the protection depends largely on the rate of AChE inhibition by the agents used to inactivate the enzyme.

1 The antinociceptive effects of systemically-administered procaine, lignocaine and bupivacaine were examined in mice and rats by using the hot-plate, writhing and tail flick tests. 2 In both species all three local anaesthetics produced significant antinociception which was prevented by atropine (5 mg kg-1, i.p.) and by hemicholinium-3 (1 microgram per mouse, i.c.v.), but not by naloxone (3 mg kg-1, i.p.), alpha-methyl-p-tyrosine (100 mg kg-1, s.c.), reserpine (2 mg kg-1, i.p.) or atropine methylbromide (5.5 mg kg-1, i.p.). 3 Atropine (5 mg kg-1, i.p.) which totally antagonized oxotremorine (40 micrograms kg-1, s.c.) antinociception did not modify morphine (5 mg kg-1, s.c.) or baclofen (4 mg kg-1, s.c.) antinociception. On the other hand, hemicholinium, which antagonized local anaesthetic antinociception, did not prevent oxotremorine, morphine or baclofen antinociception. 4 Intracerebroventricular injection in mice of procaine (200 micrograms), lignocaine (150 microgram) and bupivacaine (25 micrograms), doses which were largely ineffective by parenteral routes, induced an antinociception whose intensity equalled that obtainable subcutaneously. Moreover, the i.c.v. injection of antinociceptive doses did not impair performance on the rota-rod test. 5 Concentrations below 10(-10) M of procaine, lignocaine and bupivacaine did not evoke any response on the isolated longitudinal muscle strip of guinea-pig ileum, or modify acetylcholine (ACh)-induced contractions. On the other hand, they always increased electrically-evoked twitches. 6 The same concentrations of local anaesthetics which induced antinociception did not inhibit acetylcholinesterase (AChE) in vitro. 7 On the basis of the above findings and the existing literature, a facilitation of cholinergic transmission by the local anaesthetics is postulated; this could be due to blockade of presynaptic muscarinic receptors.