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Mitochondria are essential for neuronal survival and function, and mitochondrial dysfunction plays a critical role in the pathological development of Parkinson's disease (PD). Mitochondrial quality control is known to contribute to the survival of dopaminergic (DA) neurons, with mitophagy being a key regulator of the quality control system. In this study, we show that mitophagy is impaired in the substantia nigra pars compacta (SNc) of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. Treatment with the sigma-1 receptor (Sig 1R) agonist 2-morpholin-4-ylethyl 1-phenylcyclohexane-1-carboxylate (PRE-084) reduced loss of DA neurons, restored motor ability and MPTP-induced damage to mitophagy activity in the SNc of PD-like mice. Additionally, knockdown of Sig 1R in SH-SY5Y DA cells inhibited mitophagy and enhanced 1-methyl-4-phenylpyridinium ion (MPP+) neurotoxicity, whereas application of the Sig 1R selective agonist SKF10047 promoted clearance of damaged mitochondria. Moreover, knockdown of Sig 1R in SH-SY5Y cells resulted in decreased levels of p-ULK1 (Unc-51 Like Autophagy Activating Kinase 1) (Ser555), p-TBK1 (TANK Binding Kinase 1) (Ser172), p-ubiquitin (Ub) (Ser65), Parkin recruitment, and stabilization of PTEN-induced putative kinase 1 (PINK1) in mitochondria. The present data provide the first evidence for potential roles of PINK1/Parkin in Sig 1R-modulated mitophagy in DA neurons.

Serotonin (5-HT)1A and σ1 receptors have been implicated in psychiatric disorders. We previously found that combined 5-HT reuptake inhibition and σ1 receptor activation has a synergistic effect on prefrontal dopaminergic transmission in adrenalectomized/castrated mice lacking circulating steroid hormones. In the present study, we examined the mechanisms underlying this neurochemical synergism. Systemic administration of fluvoxamine, a selective 5-HT reuptake inhibitor with agonistic activity towards the σ1 receptor, increased prefrontal dopamine (DA) levels, and adrenalectomy/castration potentiated this fluvoxamine-induced increase in DA. This enhancement of DA release was blocked by WAY100635 (a 5-HT1A receptor antagonist), but not by ritanserin (a 5-HT2 receptor antagonist), azasetron (a 5-HT3 receptor antagonist) or SB269970 (a 5-HT7 receptor antagonist). Individually, osemozotan (a 5-HT1A receptor agonist) and (+)-SKF-10,047 (a σ1 receptor agonist) did not alter prefrontal monoamine levels in adrenalectomized/castrated and sham-operated mice differentially. In contrast, co-administration of these drugs increased prefrontal DA levels to a greater extent in adrenalectomized/castrated mice than in sham-operated animals. Furthermore, co-administration of osemozotan and (+)-SKF-10,047 increased expression of the neuronal activity marker c-Fos in the ventral tegmental area of adrenalectomized/castrated mice, but not in sham-operated animals. These findings suggest that combined activation of 5-HT1A and σ1 receptors has a synergistic effect on prefrontal dopaminergic transmission under circulating steroid deficiency, and that this interaction may play an important role in the regulation of the prefrontal DA system.

Using whole-cell voltage-clamp recordings, hyperpolarization-activated cation currents (Ih) were elicited with hyperpolarizing voltage jumps in CA1 pyramidal neurons of rat hippocampal slices, and the effects of phencyclidine (PCP) and some sigma ligands on Ih were studied. PCP concentration-dependently (0.1-100 microM) suppressed Ih and shifted the activation curve of Ih to the negative direction. D-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM) and MK-801 (30 microM), competitive and non-competitive NMDA blockers, respectively, failed to mimic the inhibitory effect of PCP on Ih, and suppression of Ih by PCP was unaffected in the presence of these blockers. To explore the involvement of sigma1 receptors in the reduction of Ih, the effects of representative sigma1 ligands were studied. SKF10047 (100 microM), a sigma1 agonist, attenuated the maximal Ih and shifted the half-activation potential of Ih to the hyperpolarized direction. In the presence of the sigma1 antagonist NE-100 (1 microM), which alone did not affect Ih, the effect of SKF10047 on Ih was unaltered. By contrast, a higher concentration of NE-100 (10 microM) mimicked the effect of SKF10047. Again, no antagonism of Ih suppression by SKF10047 was obtained with rimcazole (100 microM), a sigma1 receptor antagonist that is structurally distinct from NE-100. This concentration of rimcazole alone resulted in a slight but significant reduction of Ih. Thus these major sigma1 ligands appear to suppress Ih independently of their agonistic or antagonistic properties. The results of this study suggest that PCP and some sigma ligands could modulate cell excitability partly through their action on Ih.

In this study, we investigated the involvement of the interaction between sigma receptors and the nitric oxide/cyclic GMP pathway in short term memory in mice, assessed through spontaneous alternation behavior in a Y-maze. N(G)-Nitro-L-arginine methyl ester and 7-nitro indazole, both nitric oxide synthase inhibitors, impaired the spontaneous alternation behavior. These impairments were attenuated by (+) SKF 10,047 and (+) pentazocine, sigma(1) receptor agonists. Further, the sigma(1) receptor antagonist, NE-100, reversed the improvements made by sigma receptor agonists. Cyclic GMP levels and nitric oxide synthase activity in the hippocampus were reduced by treatment with N(G)-nitro-L-arginine methyl ester. The suppressive effects of N(G)-nitro-L-arginine methyl ester on the cyclic GMP levels were reversed by co-treatment with (+) SKF 10,047, but the decline in nitric oxide synthase activity was not. These results suggest that the nitric oxide/cyclic GMP pathway in the hippocampus is responsible for spontaneous alternation behavior in a Y-maze. Further, the ameliorating effects of (+) SKF 10,047 on the impairment of spontaneous alternation behavior may be mediated through activation of guanylate cyclase, but not nitric oxide synthase in the hippocampus of mice.

The central distribution of sigma sites labelled by di-o-tolylguanidine (DTG), a compound which has specific affinity for sigma sites, and its ability to produce postural movements, are consistent with the hypothesis that sigma sites may play a functional role in the regulation of movement. The aim of the present study was to evaluate the specificity of the circling behaviour induced by unilateral intranigral injection of DTG in rats. As previously described, DTG produced dose-dependent unilateral rotations (2.5-20 nmol/rat). A similar dose-dependent circling behaviour was observed with DMTG and (+) NANM (3-40 nmol/rat), compounds which bind to both sigma and PCP sites, and with haloperidol (3-20 nmol/rat) whereas raclopride and D,L-sulpiride did not elicit any circling (10 nmol/rat). DTG-induced circling after intranigral injection (10 nmol/rat) was decreased in a dose-dependent manner by rimcazole (20-40 mg/kg, i.p.), a selective ligand for sigma sites, and by BMY 14802 (3, 10, 30 mg/kg, i.p.), ifenprodil and eliprodil (1, 3, 10 mg/kg, i.p.), non-selective sigma ligands. In contrast, naloxone (1 mg/kg, s.c.) and CGS 19755 (1, 3, 10 mg/kg, i.p.) did not change the DTG-induced circling. Eliprodil failed to inhibit circling produced by compounds devoid of any affinity for sigma sites such as APV, dizocilpine or muscimol, indicating the specificity of the inhibition observed with eliprodil on the DTG-induced circling.(ABSTRACT TRUNCATED AT 250 WORDS)

The present report investigates the modulation of the hypothalamic-pituitary-adrenal (HPA) axis in the rat by sigma receptors, using a selective ligand, (+) pentazocine, and comparing the effects with (+) SKF 10,047. Both compounds stimulate ACTH release potently after central and peripheral administration. These effects are centrally mediated, since they did not release ACTH from anterior pituitary primary cultures. The effects are not blocked by naloxone, but are blocked by the NMDA antagonist, CPP, indicating a centrally mediated functional interaction between NMDA and sigma receptors, in vivo .

The effects of sigma ligands, (+)3PPP 3-[3-hydroxyphenyl-N(1-propyl) piperidine] and (-)butaclamol, were evaluated in vivo on the metabolism of dopamine (DA) and in the striatum release of adrenocorticotrophic hormone (ACTH) and prolactin in the rat and changes in levels of cyclic guanosine monophosphate (cGMP) in the cerebellum of the mouse and compared with the effects of (+)NANM (N-allyl-normetazocine, SKF 10,047) and (+)pentazocine. Both (+)3PPP and (-) butaclamol decreased the release of prolactin and did not affect the metabolism of DA. N-Allyl-normetazocine and (+)pentazocine increased release of prolactin and have been shown previously to increase the metabolism of DA. All four ligands increased release of ACTH; however, only the increases caused by (+)NANM and (+)pentazocine were reversed by pretreatment with CPP, a N-methyl-D-aspartate (NMDA) receptor antagonist. (+)Pentazocine and (+)NANM inhibited the NMDA receptor-mediated changes in levels of cGMP in the cerebellum of the mouse, while (+)3PPP and (-)butaclamol did not attenuate the response to NMDA. In addition to further confirming a functional interaction between sigma receptors and NMDA receptors, these studies divide the observed effects of putative sigma ligands into two groups, characterized by benzomorphan compounds and non-benzomorphan compounds, suggesting the possibility of subtypes at sigma receptor in vivo.

Quantitative ligand binding studies have been used to characterize binding sites for N-allylnormetazocine ((+)SKF10,047) (SKF), 1-(1-phenylcyclohexyl) piperidine (PCP), N-[1-(2-thienyl) cyclohexyl] piperidine (TCP) and haloperidol in membranes from the brain of rat and guinea pig under conditions which permitted simultaneous analysis of the binding of both PCP and SKF. Using four labelled ligands (SKF, TCP, PCP and haloperidol), each displayed by the corresponding four unlabelled ligands, four classes of binding sites were observed in membranes from the brain of the rat, corresponding to sigma (σ), two classes of PCP sites (PCP1, PCP2) and dopamine (D2) sites. The σ site was suppressed by 50nM haloperidol, while the PCP1 and PCP2 sites were not. These results were confirmed by the studies employing a self- and cross-displacement design and dose-response surfaces for SKF and TCP, with and without blockade by haloperidol of the sigma site. Using mathematical modelling, employing the program LIGAND, it was possible to reject simpler models involving a common “PCP/sigma” site or a model involving only two classes of sites (sigma and PCP). Similar methods were used to identify two classes of σ binding sites and two classes of PCP binding sites, in membranes prepared from the brain of guinea pig. The relative potencies of 18 ligands for displacement of (+)[3H]SKF10,047 and [3H]TCP were compared: there were significant qualitative and quantitative differences in the “sigma” binding sites in the brain of rat and guinea pig, while the PCP binding sites were very similar in two the species.

The potential neuroprotective effects of phencyclidine, ketamine and (+) SKF 10,047 were investigated in the trimethyltin (TMT)-treated rat. Of the three drugs used in this study, only phencyclidine (5mg/kg i.p.) reversed the behavioural hyperactivity and deficits in spatial localization of TMT-treated rats. Neurochemically, phencyclidine and (+) SKF 10,047 were without effect on the neurotransmitters (e.g. noradrenaline, dopamine, serotonin and 5-hydroxyindole 3-acetic acid), examined in the amygdaloid cortex and hippocampal regions, while ketamine increased the steady state concentrations of 5-HIAA in the amygdaloid cortex. These results suggest the involvement of the phencyclidine receptor in reversal of the behavioural impairments produced by TMT in rats. The significance of these results with respect to phencyclidine and sigma receptors is discussed. The lack of effect of (+) SKF 10,047 in this model may reflect behavioural differences between phencyclidine and sigma ligands. It may be concluded that the TMT model can be exploited for studying the mechanism of action of molecules liable to have an effect at the phencyclidine receptor site, as opposed to the sigma receptor.

Our present findings suggest that SKF-10,047, the prototype sigma agonist, has its opioid entity residing with its (-) isomer, while both its (+) and (-) isomers possess psychotogenic properties similar to those produced by PCP. We found that (-)-SKF-10,047 blocks EEG and behavioral effects of morphine in the naive rat, precipitates withdrawal in morphine-dependent rats, produces physical dependence as evidenced by naloxone-induced withdrawal, and displaces [3H]dihydromorphine from brain homogenates. (+)-SKF-10,047 did not produce dependence upon chronic treatment, and it did not displace [3H]dihydromorphine from brain homogenates. Such pharmacodynamic dissociation with SKF-10,047 suggests an association of sigma receptors with psychogenic, but not opioid effects. The latter are most likely mediated by mu or kappa receptors.

Opioid agonists were used to investigate the modulation of seizures mediated by mu, kappa and delta opiate receptors in the seizure-sensitive Mongolian gerbil. Morphine (1.0-25 mg/kg, s.c.) were used as prototypic agonists for mu, kappa and delta opiate receptors. Each opioid decreased the incidence and severity of the seizure as compared to control values. The anticonvulsant effects of morphine (10 mg/kg, s.c.) and ketocyclazocine (0.5 mg/kg, s.c.) were reversed by naloxone (1.0 mg/kg, s.c.), while the anticonvulsant effects of N-allylnormetazocine (2 mg/kg, s.c.) were not significantly changed by naloxone. Additionally, abnormal behavior was observed following administration of the opioids. Morphine (10 mg/kg, s.c.) produced excitation and hyperresponsiveness with intermittent cataleptic-like states. Ketocyclazocine (10 mg/kg, s.c.) predominantly produced a stuporous, immobile state, accompanied by some loss of posture. N-allylnormetazocine (10 mg/kg, s.c.) produced ataxia and stereotypic side-to-side head nodding . Naloxone was able to reverse the behavioral effects produced by morphine and ketocyclazocine but not those produced by N-allylnormetazocine. The data presented are consistent with earlier studies which demonstrated the anticonvulsant effects of beta-endorphin in the gerbil. This study further suggests that opioids have a protective role against seizure activity in the gerbil and the opioid anticonvulsant effect is not specific to one type of opioid agonist.

Morphine at 5, 10 and 20 mg/kg intraperitoneally caused a dose-dependent, naloxone-reversible, antagonism of d -amphetamine-induced ipsilateral circling in rats lesioned with 6-hydroxydopamine in one substantia nigra. Morphine (10 mg/kg) weakly antagonized apomorphine-induced contralateral circling. Morphine, levorphanol and phenazocine were potent antagonists of d -amphetamine-induced circling in rats with a 6-hydroxydopamine lesion of one neostriatum. Pentazocine, pethidine and ethyl-ketocyclazocine were weak antagonists. It is proposed that a major action of morphine and some closely related opiate analgesics is to reduce the release of dopamine from nigro-striatal nerve terminals in the rat by acting upon specific opiate receptors.

The study was designed to determine and compare the acute effects of the enantiomers of mu, kappa and sigma opioid agonists on the cortical EEG with the spectral analysis technique. The relative ability of naloxone to antagonize such effects was also assessed. Adult female Sprague-Dawley rats were implanted with chronic cortical EEG and temporalis muscle EMG recording electrodes, and with permanent indwelling external jugular cannulae. (−)-Methadone (mu agonist) produced increases in spectral power over the zero to 10 Hz range, while (−)-ketocyclazocine (kappa agonist) produced increases in the 5–8 Hz band as a predominant peak. The (+)enantiomers of methadone and ketocyclazocine were inactive. The drug (+)-SKF-10,047 (sigma agonist), produced a predominant spectral peak in the 7–9 Hz band that was associated with behavior that suggested psychotomimetic effects. The effects of morphine (mu agonist) on EEG and EEG power spectra were more sensitive to antagonism by naloxone than those produced by ketocyclazocine. The effects of(±)-SKF-10,047 and (+)-SKF-10,047 were not antagonized by 10 mg/kg of naloxone, while the effects of (−)-SKF-10,047 were partially antagonized by lOmg/kg of naloxone. These findings further delineate the specificity of the differential effects of mu, kappa and sigma opioid agonists on the EEG and EEG power spectra in the rat.

Electrophysiological studies using a cortical slice preparation revealed that MK-801 is a potent, non-competitive, antagonist of N-methyl-D-aspartate (NMDA)-induced depolarisations. Also, MK-801 was a highly selective antagonist, exhibiting a high degree of use-dependency. It completely blocked responses to NMDA and quinolinic acid but had no effect on responses produced by kainic acid or quisqualic acid. Using [3H]MK-801, high affinity binding sites for MK-801 were detected in membranes of the rat brain. The pharmacological specificity and regional distribution of binding sites for MK-801 were consistent with an action of the compound at the level of the NMDA receptor-associated ion channel. Administered parenterally, MK-801 had a remarkable neuroprotective role. It caused essentially complete protection against loss of neurones produced by injection of neurotoxic doses of NMDA or quinolinic acid into the striatum or hippocampus. Furthermore, MK-801 was highly effective in preventing loss of hippocampal neurones following bilateral occlusion of the common carotid arteries in the gerbil.

Taste aversions can be conditioned in rats by a variety of psychoactive drugs, including those with reinforcing properties. Previous research, however, has not established clearly whether phencyclidine and related drugs are active in such procedures. The present study was carried out to investigate whether phencyclidine would induce a conditioned taste aversion and whether several other compounds (MK-801, the stereoisomers of NANM and ifenprodil) which, like phencyclidine, are known to antagonise the actions of N- methyl- d -aspartate (NMDA), would produce similar effects. When rats received injections of these compounds, after consuming a novel solution of saccharin, their subsequent consumption of the same solution decreased. The smallest doses of the different drugs which induced clear taste aversions were: phencyclidine 3 mg/kg, MK-801 0.3 mg/kg, (+)-NANM 10 mg/kg, (−)-NANM 3 mg/kg and ifenprodil 10 mg/kg. Thus, all the drugs were active. However, as neither the potencies nor the efficacies of the different compounds in inducing taste aversions correlated with their other behavioural effects or with their relative potencies in antagonising the effects of NMDA or in displacing phencyclidine from its binding sites, the mechanisms involved are unclear.

The present authors gave mu, delta, kappa, epsilon and sigma opiate receptor agonists intracerebroventricularly to rats both singly and in combination while monitoring the electroencephalogram from cortical and depth electrodes. Dose-response curves were plotted with naloxone against the changes produced by each agonist, and the effect of a number of anticonvulsant drugs on agonist-induced seizures was ascertained. Each opiate agonist produced a different seizure pattern with a different naloxone dose-response curve and anticonvulsant profile. The order of convulsive potency was epsilon greater than delta greater than mu greater than sigma much greater than kappa. Petit mal-like seizure activity was unique to the delta agonist, leucine-enkephalin, while only the mu agonist, morphine produced generalized convulsive seizures. These experiments raise the possibility that opiate systems in the brain may be involved in the pathogenesis of a wide spectrum of seizure disorders.

This study was designed to compare the development of tolerance to the effects of N-allylnormetazocine (SKF-10,047) and its enantiomers on the EEG and on behavior. Adult female Sprague-Dawley rats were implanted with chronic cortical electroencephalogram (EEG) and electromyogram (EMG) recording electrodes in the temporalis muscle and with permanent cannulae in the external jugular vein. In non-tolerant rats, 10 mg/kg (i.v.) injections of SKF-10,047 racemate produced primarily aroused wakefulness for about 120min, that was associated with alternation between desynchronized EEG and theta waves in the EEG. After these rats received a series of automatic, intravenous injections of SKF-10,047 racemate, the aroused wakefulness state induced by SKF-10,047 racemate lasted for about 40 min. In non-tolerant rats, 2.5 mg/kg (i.v.) injections of (+)-SKF-10,047 induced a psychotomimetric EEG and behavioral state for about 30 min, which included continuous theta wave activity in the EEG. After chronic treatment with (+)-SKF-10,047, the psychotomimetic state induced by (+)-SKF-10,047 persisted for about 20 min. In non-tolerant rats, 2.5 mg/kg (i.v.) injections of (−)-SKF-10,047 produced an aroused EEG and behavioral wakefulness for about 30 min, which was then followed by slow-wave bursts in the EEG and associated behavioral stupor for about 90 min. After chronic treatment with (−)-SKF-10,047, injection of (−)-SKF-10,047 produced predominantly aroused wakefulness for about 45 min. The data suggest that (+)-SKF-10,047 exerts psychotogenic properties, but not opioid properties. On the other hand, the data suggest that (−)-SKF-10,047 possesses opioid properties.

The depletion of mesolimbic dopamine by 6-hydroxydopamine injections into the A10 region of the ventral tegmental area effectively blocked locomotor hyperactivity induced by SKF 10,047, phencyclidine and amphetamine, whereas the stimulatory effects of caffeine and scopolamine were not altered by the lesion. These results suggest that SKF 10,047, like amphetamine, and phencyclidine acts through presynaptic dopaminergic mechanisms in the mesolimbic nucleus accumbens system to stimulate locomotor behavior.

Dose-effect curves were determined for phenazocine (0.64–2.5 mg/kg), ketazocine (1.25–80 mg/kg) and ethylketazocine (1.25–80 mg/kg) in pigeons responding under a multiple fixed-ratio 30-response, fixed-interval 5-min schedule of grain presentation. All three opioid agonists decreased responding with the larger doses. The effects of phenazocine were completely antagonized by small doses of naloxone (0.01–1 mg/kg), whereas the effects of ethylketazocine required larger doses of naloxone (1–10 mg/kg) to be completely antagonized. The behavioral effects of ketazocine were partially attenuated by naloxone, but were not antagonized completely even by a 10 mg/kg dose of naloxone. These data from the pigeon are consistent with previous interpretations that the effects of phenazocine are mediated by actions at a mu opioid receptor, whereas the effects of ketazocine and ethylketazocine are mediated by actions at a kappa opioid receptor.