Differential tolerance to repeated daily injections of N-allylnormetazocine and its enantiomers in the rat

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 120 min, 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.     

Differential neuropharmacological effects of mu,kappa and sigma opioid agonists on cortical EEG power spectra in the rat: Stereospecificity and naloxone antagonism

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.     

Sigma receptors mediated the psychotomimetic effects of N-allylnormetazocine (SKF-10,047), but not its opioid agonistic-antagonistic properties

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.     

(+)-SKF-10,047 and dextromethorphan ameliorate conditioned fear stress via dopaminergic systems linked to phenytoin-regulated σ1 sites

Mice exhibited a marked suppression of motility when they were re-placed in the same environment in which they had previously received an electric footshock. (+)-SKF-10,047 ([2S-(2α,6α,11R1)]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol hydrochloride; (+)-N-allylnormetazocine hydrochloride) and dextromethorphan, putative σ receptor agonists, have been reported to reverse this psychological stress-induced motor suppression, defined as conditioned fear stress, through phenytoin-regulated type σ₁ receptors. In the present study, we investigated the involvement of dopaminergic neurons in the ameliorating effects of (+)-SKF-10,047 and dextromethorphan on conditioned fear stress. (+)-SKF-10,047 and dextromethorphan attenuated conditioned fear stress at low doses (4 and 20 mg/kg, respectively) when they were co-administered with phenytoin (10 mg/kg), an anticonvulsant drug. The effects were antagonized by the σ receptor antagonists, NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride) and BMY-14802 (a-(4-fluoro-phenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-butanol hydrochloride). Furthermore, the effects of (+)-SKF-10,047 or dextromethorphan in combination with phenytoin were blocked by the dopamine D₁ receptor antagonist, SCH 23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine), and the dopamine D₂ receptor antagonist, (−)-sulpiride, and they were also attenuated by 6-hydroxydopamine-induced lesions of dopaminergic neurons. The ameliorating effects of (+)-SKF-10,047 and dextromethorphan on conditioned fear stress at high doses (5 and 30 mg/kg, respectively) were also blocked by both the dopamine receptor antagonists. These results suggest that the stress-induced motor suppression is restored by the activation of dopaminergic neuronal systems as a result of the stimulation of phenytoin-regulated type σ₁ receptors.     

Characterization of opioid, sigma, and phencyclidine receptors in the neuroblastoma-brain hybrid cell line NCB-20

Opioid, sigma, and phencyclidine (PCP) receptors were characterized in the mouse neuroblastoma--Chinese hamster brain hybrid cell line NCB-20. Quantitative receptor assays under equilibrium binding conditions with highly specific radioligands demonstrated the presence of delta, but not mu or kappa, opioid receptors on NCB-20 cell membranes. NCB-20 cells were shown to possess two distinct sites specific for sigma opioids and PCP derivatives. One site was labeled by (+)-[3H]N-allylnormetazocine [(+)-[3H]SKF-10,047] (Kd = 69 nM; Bmax = 4100 fmol/mg of protein). The rank order of potency of drugs at this site was (+)-3-(3-hydroxy-phenyl)-N-(1-propyl)piperidine [(+)-3-PPP] greater than haloperidol greater than (+)-SKF-10,047 greater than (+/-)-ethylketocyclazocine greater than (+/-)-bremazocine greater than N-[1-(2-thienyl) cyclohexyl]piperidine (TCP) greater than dexoxadrol. This site is similar in its ligand selectivity to the haloperidol-sensitive sigma receptor of rat brain. The other site was labeled by the potent phencyclidine derivative [3H]TCP (Kd = 335 nM; Bmax = 9300 fmol/mg of protein). This density is equivalent to approximately 60,000 sites/cell. The rank order of potency of drugs at this site was TCP greater than (+)-3-PPP greater than PCP greater than dexoxadrol greater than haloperidol greater than cyclazocine greater than levoxadrol greater than (+)-SKF-10,047; mu and delta ligands were inactive. This site is similar to the rat brain PCP receptor. The NCB-20 cell line is the only cultured cell line that has been demonstrated to have PCP receptors.     

Lack of involvement of haloperidol-sensitive sigma binding sites in modulation of dopamine neuronal activity and induction of dystonias by antipsychotic drugs.

The present studies evaluated previous suggestions that haloperidol-sensitive sigma binding sites are involved in the modulation of dopamine (DA) neuronal activity and in the induction of the dystonic effects of antipsychotic drugs. These issues were addressed by evaluating the effects of compounds that have differing affinities for sigma binding sites, on the firing activity of DA neurons in the substantia nigra in chloral hydrate-anesthetized rats and on the ability to induce extrapyramidal motor dysfunction in squirrel monkeys sensitized to the dystonic effects of haloperidol. The agents studied included haloperidol, DTG (1,3-di-o-tolylguanidine), (+)-pentazocine, (+)-SKF 10,047, BMY 14802, 8-OH-DPAT and sulpiride. There was no relationship between affinity for sigma binding sites and the ability to either alter DA neuronal activity or to induce extrapyramidal motor dysfunction.     

Electrophysiological effects of phencyclidine in the medial prefrontal cortex of the rat

The effects of local applications of phencyclidine (PCP) and dopamine (DA) on neurons of the medial prefrontal cortex were investigated using single unit recording techniques. The activity of the majority of cells in the deeper layers of the medial prefrontal cortex was depressed by both phencyclidine and DA, whereas increases, as well as decreases, in the firing rates were observed in cells located in the superficial cortical layers. The stereospecificity of the responses of deeper cells to phencyclidine was demonstrated using the enantiomers of 1-(-1-phenylcyclohexyl)-3-methylpiperidine (PCMP). Phencyclidine was found to be 1.5 times more potent than (+) PCMP and 3 times more potent than (-) PCMP. Finally, the DA receptor antagonist fluphenazine, blocked the phencyclidine-elicited depressions of unit activity in the deep prefrontal cortex. Taken together, the data indicate that the DA-like effects of phencyclidine on neurons of the medial prefrontal cortex are mediated by DA receptors and provide pharmacological support for the idea that psychomotor stimulant drugs have specific actions on targets of the ventral tegmental area (A10) dopamine system.     

Selective sigma receptor agonist and antagonist affect dopamine neuronal activity

Extracellular recording techniques were used to study the effects of the selective sigma receptor agonist (+)-[3H]3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) and selective sigma receptor antagonist BMY 14802 on dopamine (DA) neurons of the substantia nigra. Intravenous administration of (+)-3-PPP produced a dose-dependent inhibition of DA neuron firing rate. Complete inhibition of DA neurons produced by (+)-3-PPP could be completely reversed by administration of BMY 14802. Also, pretreatment with BMY 14802 shifted the (+)-3-PPP dose response curve to the right. These data demonstrate a relationship of the sigma receptor with the dopamine system and further suggest a model system to study agonist/antagonist interactions of sigma ligands.     

Dissociation of haloperidol, clozapine, and olanzapine effects on electrical activity of mesocortical dopamine neurons and dopamine release in the prefrontal cortex.

The aim of the present study was to compare the effects of the typical antipsychotic haloperidol and the atypical antipsychotics clozapine and olanzapine on both extracellular dopamine (DA) levels in the medial prefrontal cortex (mPFC) as well as electrical activity of mesoprefrontal DA (mPFC-DA) neurons. Extracellular single unit recordings and microdialysis experiments were carried out in different groups of chloral hydrate anesthetised rats under identical experimental conditions. Intravenous administration of haloperidol, clozapine, and olanzapine increased the firing rate and burst activity of antidromically-identified mPFC-DA neurons; maximal increase in firing rate of approximately 140, 155, and 70 %, was produced by haloperidol, clozapine, and olanzapine at doses of 0.2, 2.5, and 1 mg/kg, i.v., respectively. Intravenous administration of the same doses increased extracellular DA levels in mPFC by 20%, 190%, and 70%, respectively. Moreover, while haloperidol and olanzapine increased extracellular levels of the deaminated DA metabolite DOPAC, by 60% and 40%, respectively, clozapine was totally ineffective. The D1 receptor antagonist SCH 23390 modified neither DA output nor neuronal firing. To determine whether the effect of the three antipsychotics on DA release might depend on a direct action on the mPFC, rats were perfused locally via inverse dialysis in the mPFC at concentrations ranging from 10(-6) to 10(-4)M. While clozapine and olanzapine increased extracellular DA concentrations by up to 400% of basal level, haloperidol was totally ineffective. The results obtained from this study indicate that the rank potency of the three antipsychotics in stimulating the firing rate of DA neurons projecting to mPFC, correlates with their affinity for D2 receptors and doses used clinically. On the other hand, their stimulating effect on DA release does not correlate with their effect on neuronal firing but depends on a direct action on the mPFC.     

The effects of the phencyclidine analogs BTCP and TCP on nigrostriatal dopamine neuronal activity

Extracellular single unit recording techniques were used to evaluate the effects of two phencyclidine (PCP) derivatives. N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine (BTCP) and N-[1-(2-thiophenyl)cyclohexyl]piperidine (TCP) on the electrophysiological activity of antidromically identified nigrostriatal dopamine (DA) neurons in chloral hydrate-anesthetized rats. I.v. BTCP produced a dose-dependent decrease in the firing rate of identified nigrostriatal DA neurons whereas TCP elicited a dose-dependent biphasic effect which was characterized by an activation of cell firing at low doses followed by a reversal of the response with larger doses. A hemitransection of the brain anterior to the substantia nigra significantly reduced the inhibitory effect of BTCP while this surgical procedure did not affect the response to TCP. However, iontophoretic application of BTCP induced a current-dependent inhibition of the spontaneous activity of cells while local application of TCP had no effect on the firing rate of these cells. These data indicate that PCP analogs are able to interact with the nigrostriatal DAergic pathway through distinct and opposing mechanisms. The results are discussed in light of recent observations that BTCP is selective for the DA uptake site while TCP is selective for the high affinity PCP binding site.     

Electrophysiological effects of selective sigma-receptor agonists, antagonists, and the selective phencyclidine receptor agonist MK-801 on midbrain dopamine neurons

Extracellular single unit recording techniques were used to study the effects of selective sigma-receptor agonist [(+)-3-PPP, (+)-pentazocine, and DTG] and selective sigma-receptor antagonists (BMY 14802 and Rimcazole) on dopamine neurons of the substantia nigra. Intravenous (IV) administration of sigma agonists decreased, whereas IV administration of the sigma antagonist BMY-14802 increased the firing rate of dopamine neurons. The other sigma antagonist Rimcazole produced inconsistent changes in dopamine unit activity. These data, in conjunction with anatomic data suggesting sigma receptor localization on dopamine neurons in the substantia nigra (Gundlach et al: J Neurosci 6:1757-1770, 1986; Graybiel et al: Soc Neurosci Abstr 13:28, 1987) demonstrate a relationship of the sigma receptor with the dopamine system and further suggest a model system to study agonist-antagonist interactions of sigma ligands. The selective phencyclidine (PCP) agonist MK-801 was equipotent to PCP in regard to stimulatory properties on dopamine neurons. However, the relative potencies do not correspond to their relative binding affinities, suggesting that non-PCP-receptor properties may mediate this effect.     

Phencyclidine-induced inhibition of rat prolactin secretion: increased portal blood dopamine

Intraperitoneal administration of phencyclidine (PCP, 2.5-20 mg/kg) produced a dose-related inhibition of the increase in serum PRL concentrations produced by alpha-methylparatyrosine (AMPT) or reserpine, but not morphine. Phencyclidine was more potent in antagonizing the PRL-releasing effect of reserpine than that of AMPT, suggesting a greater effect of PCP on the cytoplasmic than the storage dopamine (DA) pool. Phencyclidine had no effect on PRL release from rat pituitary glands in vitro. Intravenous administration of PCP (10 mg/kg) to anesthetized male rats produced a two-fold increase in pituitary stalk (DA) concentrations, suggesting that PCP inhibits rat serum PRL by increasing the release of DA from the tuberoinfundibular neurons, and possibly by blocking its reuptake as well.     

Effects of sigma(1) receptor ligand MS-377 on D(2) antagonists-induced behaviors

(R)-(+)-1-(4-Chlorophenyl)-3-[4-(2-methoxyethyl)piperazin-1-yl]methyl-2-pyrrolidinone L-tartrate (MS-377) is a novel antipsychotic agent with selective and high affinity for sigma(1) receptor. The present study was carried out to clarify the interaction of MS-377 with dopamine D(2) receptor antagonists (D(2) antagonists) in concurrent administration, and then the involvement of sigma receptors in the interaction. The effects of MS-377 on haloperidol- or sultopride-induced inhibition of apomorphine-induced climbing behavior and catalepsy were investigated in mice and rats, respectively. In addition, the effects of (+)-SKF-10,047 and SA4503, both of which are sigma receptor agonists, and WAY-100,635, which is a 5-HT(1A) receptor antagonist, on the interaction due to the concurrent use were also investigated. MS-377 potentiated the inhibitory effects of haloperidol or sultopride on apomorphine-induced climbing behavior in a dose-dependent manner. In contrast, MS-377 did not affect the catalepsy induction by these drugs. The potentiation of the inhibitory effects of haloperidol or sultopride on apomorphine-induced climbing behavior by MS-377 was not inhibited by WAY-100,635, but was inhibited by (+)-SKF-10,047 and SA4503. These findings showed that MS-377 potentiates the efficacy of D(2) antagonists, but it does not deteriorate the adverse effect. Moreover, sigma(1) receptors are involved in this potentiation of the efficacy of D(2) antagonists by MS-377.     

Effects of SKF-10047 in the phencyclidine-dependent rat: evidence for common receptor mechanisms

The repeated administration of phencyclidine (PCP, 72 mg/kg/day) to rats led to physical dependence, as evidenced by a withdrawal syndrome exhibited approx. 24-48 h following suspension of drug. All components of the withdrawal syndrome were suppressed by s.c. injections of PCP (16 mg/kg), (+/-)-N-allylnormetazocine (SKF-10047, 16 mg/kg) and (+)-SKF-10047 (16 mg/kg), but not by injections of saline or (-)-SKF-10047. Moreover, tolerance to the behavioral effects of PCP, as well as cross-tolerance to (+/-)-SKF-10047 and (+)-SKF-10047 were observed. These data indicate that PCP and the sigma opiate SKF-10047 share mechanisms of action, which are mediated by the (+)-isomer of the sigma agonist.     

Neurosteroids ameliorate conditioned fear stress: an association with sigma receptors.

Mice exhibited a marked suppression of motility (conditioned fear stress) when placed in an environment in which they had previously received an electric footshock. This conditioned fear stress response was dose-dependently attenuated by neurosteroids such as dehydroepiandrosterone sulfate (DHEAS; 25 and 50 mg/kg, s.c.) and pregnenolone sulfate (PREGS; 10-50 mg/kg, s.c.), and by a putative sigma(1) receptor agonist, (+)-N-allylnormetazocine ((+)-SKF-10,047; 3 and 6 mg/kg, s.c.). However, progesterone (PROG; 10-50 mg/kg, s.c. ) and allopregnanolone (5 and 20 mg/kg, s.c.) had no effect on this stress response. The attenuating effects of DHEAS (50 mg/kg, s.c.), PREGS (50 mg/kg, s.c.), and (+)-SKF-10,047 (6 mg/kg, s.c.) were reversed by NE-100 (5 mg/kg, i.p.), a sigma(1) receptor antagonist and PROG (5 or 10 mg/kg, i.p.). When DHEAS (25 mg/kg) was co-administered with (+)-SKF-10,047 (3 mg/kg) at doses that do not affect the conditioned fear stress response by themselves, motor suppression was significantly attenuated. In mice showing the conditioned fear stress response, the serum concentration of DHEAS was lower than that in non-shocked mice. These results suggest that the attenuating effects of DHEAS and PREGS on the conditioned fear stress response are mediated via sigma(1) receptors and that PROG has a sigma(1) receptor antagonistic property. Further, the endogenous DHEAS may be involved in the expression of conditioned fear stress response in mice.     

Preferential stimulation of ventral tegmental area dopaminergic neurons by nicotine

The effect of intravenous (i.v.) nicotine on the single unit activity of midbrain dopamine (DA) neurons was studied in rats under either local or general anesthesia. Nicotine (50-500 micrograms/kg) produced a dose-related increase in the firing rate of nigral pars compacta DA cells (A9), up to 25% above baseline, irrespective of the preparation. The same range of doses was more than three times as effective on ventral tegmental area DA cells (A10) in rats paralyzed and given a local anesthetic. By contrast, the majority of these cells were temporarily depressed in deeply anesthetized animals. All of the above effects were reversed and prevented by i.v. mecamylamine suggesting the involvement of nicotine cholinergic receptors. Moreover, after nicotine-induced stimulation, low doses of i.v. apomorphine inhibited the firing rate similar to controls indicating that dopamine receptors are not directly involved in the nicotinic action. The results suggest that acute nicotine shares with other drugs of abuse the characteristic of being more effective in stimulating A10 than A9 neurons.     

Effects of acute thioridazine, metoclopramide and SCH 23390 on the basal activity of A9 and A10 dopamine cells

Extracellular single-unit recording techniques were employed to examine the effects of various dopamine (DA) antagonists on the basal activity of spontaneously active DA cells. Metoclopramide and thioridazine were both effective in reversing apomorphine-induced suppression of A9 and A10 DA cells. SCH 23390 produced only a partial reversal of this suppression. When the antagonists were given without any pretreatment, thioridazine preferentially increased the firing rate of A10 DA cells, and was relatively ineffective in altering A9 activity. Metoclopramide, on the other hand, increased the activity of most A9 DA cells, but was less effective in doing so with A10 cells. SCH 23390 did not significantly affect the basal activity of either cell subpopulation. These data support the hypothesis that the so-called 'atypical' antipsychotic drugs act preferentially on the A10 DA system. Taken together with previous results, they also suggest that the acute effects of DA antagonists on DA cell subpopulations coincide with their chronic effects.     

Flunarizine attenuates cocaine-induced inhibition of A9 dopaminergic neurons.

The intravenous administration of cocaine (0.5-8 mg/kg) produced a dose-related inhibition of the firing rate of substantia nigra (A9) dopaminergic (DA) neurons. Pretreatment with the calcium antagonist flunarizine (5 mg/kg i.v.) prevented cocaine-induced effects but failed to antagonize the inhibition of firing induced by a low dose of apomorphine (10 micrograms/kg i.v.). This finding rules out the possibility that flunarizine antagonism of cocaine effect might depend on DA autoreceptors blockade by flunarizine. It is suggested that flunarizine, by blocking calcium influx into DA neurons, prevents DA release from dendrites, thereby counteracting the main mechanism by which cocaine inhibits dopaminergic neuronal activity.     

Phencyclidine/SKF-10,047 binding sites: evaluation of function

Results of correlation analyses comparing rank-order affinities with rank-order potencies of (+)SKF-10,047, phencyclidine (PCP), and several PCP analogs support the involvement of [3H]-1-[1-(2-thienyl)cyclohexyl]piperidine binding sites (TCP sites) in mediating both the discriminative stimulus properties of PCP and production of 180 degrees perseveration in a 4-arm radial maze. For the same group of drugs, no significant relationship was found to exist between affinities at haloperidol-sensitive (+)[3H]SKF-10,047 binding sites (H-S-SKF sites) and potencies. Also, H-S-SKF sites were found to lack pharmacological selectivity and to be localized in the microsomal fraction of cells. It is concluded that TCP sites may represent receptors which mediate effects not only of PCP, but also of (+)SKF-10,047. In addition, the possibility that H-S-SKF sites may represent a type of membrane-bound enzyme is discussed.