Evidence for multiple opiate receptor involvement in different phencyclidine-induced unconditioned behaviors in rats

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

Comparison between intraperitoneal and subcutaneous phencyclidine administration in Sprague-Dawley rats: a locomotor activity and gene induction study

In a putative model of acute phencyclidine (PCP)-induced psychosis we evaluated effects of the drug on locomotor activity (LMA) and immediate early gene (IEG) induction in the rat using two routes of drug administration, intraperitoneal (i.p.) and subcutaneous (s.c.). Adult male rats received saline or PCP (1.0-5.0 mg/kg) either i.p or s.c. and were assessed for LMA for 60 min. At the end of the LMA testing animals were culled and blood and brain samples were collected for PCP concentration analysis. Separate cohorts of animals received 5.0 mg/kg PCP (i.p. or s.c.) and were used to investigate (1) the pharmacokinetics of PCP or (2) induction of IEG (Arc, c-fos, BDNF, junB, Krox-20, sgk-1, NURR1, fra-2, Krox-24, and egr-3) mRNA expression in the prefrontal cortex (PFC). Administration of PCP resulted in locomotor hyperactivity which was more robust and longer-lasting in animals dosed s.c. compared to i.p.-treated-animals. Differences in hyperlocomotion were paralleled by higher concentrations of PCP in the blood and in the brain of s.c.-treated animals compared to i.p.-treated animals. The differences in the concentration of PCP between the two routes of administration were detected 30 min after dosing and persisted for up to 4 h. Administration of PCP via the s.c. route resulted in induction of more IEGs and consistently larger magnitudes of induction than that via the i.p. route. Therefore, we have outlined the dosing conditions to induce rapid and robust effect of acute PCP on behaviour, gene induction, and pharmacokinetic profile, to allow investigation of this as a potential animal model of acute psychosis.

A simple procedure for assessing ataxia in rats: effects of phencyclidine

The present study describes an objective, cost- and time-efficient procedure for characterizing the ataxic effects of psychoactive drugs. Male Sprague-Dawley rats were administered an intraperitoneal injection of either saline or one of three doses (1, 5 or 10 mg/kg) of phencyclidine (PCP) 15 min prior to being placed into an empty standard operant conditioning chamber (all manipulanda were removed). The floor of the test apparatus consisted of parallel rows of metal rods spaced approximately 1.5 cm apart. During a 5-min test, a single observer counted the frequency with which each animal's paws (front or back) slipped between the rows of bars that constituted the cage floor. The data demonstrated that while saline animals exhibited no instability in their ambulation, PCP-treated animals demonstrated a highly reliable dose-dependent increase in the number of "paw slips" in a single trial. Since animals are known to develop tolerance to the ataxic response to PCP, the validity of the test as a measure of drug-induced ataxia was examined in a separate group of animals treated with the middle (5 mg/kg) dose every other day over the course of a 9-day period (i.e., resulting in five injection trials). In this experiment, each subsequent test produced a reliable reduction in the magnitude of the ataxic response, and by the fifth drug challenge, the PCP animals were performing at near-control levels. These results suggest that the "paw slip test" can serve as a simple, reliable, objective and valid measure of drug-induced ataxia. The relevance of the ataxia data for interpreting the locomotor response of animals treated with PCP is also discussed.

A further study on asymmetric cross-sensitization between MK-801 and phencyclidine-induced ambulatory activity

Our previous study found that MK-801-sensitized rats showed cross-sensitization to the locomotor stimulant effects of phencyclidine, but phencyclidine sensitized rats did not show cross-sensitizaton to MK-801. This study was designed to determine whether the asymmetric cross-sensitization was due to injection-environment conditioning or possibly reduced phencyclidine-like effects following further repeated injections of phencyclidine. Adult female Sprague-Dawley rats were used in this study, and their activity was assessed with an automated photoelectric system. Results confirmed the early finding that four daily injections of phencyclidine (3.2 mg/kg) or MK-801 (0.32 mg/kg) produced locomotor sensitization, and that the two drugs showed asymmetric cross-sensitization. Moreover, injection-environment conditioning was ruled out as a possible cause for cross-sensitization from MK-801 to phencyclidine, and possibly reduced phencyclidine-like effects following further repeated injections was also ruled out as a cause for the failure of cross-sensitization from phencyclidine to MK-801. These additional results further confirm our previous finding, and indicate that there are significant differences in the neural mechanisms underlying phencyclidine- and MK-801-induced sensitization.

Asymmetric cross-sensitization to the locomotor stimulant effects of phencyclidine and MK-801

Chronic administration of a psychomotor stimulant has been shown to produce progressively enhanced effects, a phenomenon called "reverse tolerance" or sensitization. Sensitization which develops to the psychomotor stimulant effect of a drug generalizes to drugs with similar neurochemical mechanisms of action, a phenomenon called cross-sensitization. The present study compared the psychomotor stimulant effects of phencyclidine and MK-801, examined the effects of the daily injection of phencyclidine and MK-801 on locomotor activity and investigated whether reciprocal cross-sensitization occurred between phencyclidine and MK-801. Adult female Sprague-Dawley rats were used. Their locomotor activity was measured automatically for a 2 h period following drug injection. Phencyclidine and MK-801 both increased locomotor activity. Four daily injections of phencyclidine in a dose of 3.2 mg/kg i.p., or MK-801 in a dose of 0.32 mg/kg i.p., produced sensitization to locomotor activity. Moreover, MK-801 sensitized rats showed cross-sensitization to phencyclidine. However, phencyclidine sensitized rats did not show cross-sensitization to MK-801. This finding suggests that there are significant differences in the neurochemical mechanisms underlying phencyclidine-induced and MK-801-induced sensitization. Phencyclidine sensitization may not be mediated by NMDA receptors.

Phencyclidine-induced behavioral sensitization

Chronic administration of a psychomotor stimulant has been shown to produce progressively enhanced effects, a phenomenon called "reverse tolerance" or sensitization. The present study reexamined the effects of the daily injection of phencyclidine on locomotor activity and stereotypy in rats, and investigated whether drug-environment conditioning was necessary for the development of behavioral sensitization and whether (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cyclohepten-5,1 0-imine hydrogen maleate (MK-801, dizocilpine) blocked behavioral sensitization to phencyclidine. Female Sprague-Dawley rats were used. Locomotor activity and stereotypy were measured automatically with the Digiscan system. The results confirmed an earlier finding that four daily injections of phencyclidine induced sensitization to both locomotor activity and stereotypy. The development of behavioral sensitization did not require drug-environment conditioning. Moreover, MK-801 did not block behavioral sensitization to phencyclidine. The results of the present study suggest that the neuronal mechanisms underlying sensitization to phencyclidine are different from those underlying sensitization to amphetamine and cocaine.

Affinity of 1-aminoadamantanes for the sigma binding site in post-mortem human frontal cortex

The 1-aminoadamantanes memantine (1-amino-3,5-dimethyl-adamantane) and amantadine (1-amino-adamantane) are clinically used as anti-parkinsonian, anti-spasticity, anti-dementia and antiviral drugs. In the present investigation we have tested a series of 1-aminoadamantane derivatives including memantine and amantadine for their ability to compete with [3H](+)-pentazocine in homogenates of post-mortem human frontal cortex. The Ki values ranged from 0.237 +/- 0.019 microM for 1-N-dimethyl-amino-3,5-dimethyl-adamantane to 20.25 +/- 16.48 microM for amantadine. The Ki value of memantine was 19.98 +/- 3.08 microM and was thus very similar to that of amantadine. Memantine, at therapeutic concentrations, probably does not interact with the sigma binding site. Amantadine, at therapeutic concentrations, probably binds both to the sigma site and to the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor.

The sigma receptor ligand 1,3-di-(2-tolyl)guanidine in animal models of schizophrenia

The behavioral effects of the selective sigma ligand 1,3-di(2-tolyl)guanidine (DTG) were studied in rats. In the radial 8-arm maze, DTG (2, 4 and 8 mg/kg i.p.) reduced the number of arm entries in the spontaneous alternation task. In animals receiving 4 mg/kg DTG, the percentage of 135 degrees angles between consecutive arm entries decreased. In the open field, equipped with a holeboard, DTG (8 mg/kg) reduced the number of line crossings, rearings and head dips. Sniffing, measured in an experimental chamber, was also reduced. DTG prolonged the time that the animals were inactive. In combination with DL-amphetamine (4 mg/kg) or dizocilpine (0.16 mg/kg), DTG (8 mg/kg) decreased--but did not antagonize--the induced enhancement of locomotion and sniffing. These results demonstrate motor depressant effects of DTG on locomotion, rearing and sniffing. Since antagonists of sigma binding sites are known to produce opposite effects, we conclude that DTG--in behavioral terms--acts like an antagonist at sigma binding sites.

Comparison of the effects of the acute administration of dexoxadrol, levoxadrol, MK-801 and phencyclidine on body temperature in the rat

Some of the dioxolanes produce pharmacological effects that have much in common with phencyclidine and phencyclidine-like drugs. Dioxadrol can be resolved into two enantiomers, dexoxadrol and levoxadrol. Dexoxadrol has an affinity for phencyclidine receptors that is much greater than that of levoxadrol, but dexoxadrol and levoxadrol have nearly equal affinities for sigma receptors. The systematic analysis of the relative potencies of dexoxadrol and levoxadrol can be used as an approach to define effects mediated by phencyclidine vs sigma receptors. Compounds that act on phencyclidine receptors, as well as affecting behavior, alter body temperature in the rat. The purpose of the present study was to compare and contrast the effects of the acute administration of dexoxadrol, levoxadrol, MK-801 and phencyclidine on body temperature in the rat. Dexoxadrol and levoxadrol (5.0, 10.0, 20.0 or 40.0 mg/kg), MK-801 (0.12, 0.6 or 1.2 mg/kg) or phencyclidine (5.0, 10.0 or 20.0 mg/kg) were administered subcutaneously and body temperature was measured. Both dexoxadrol and MK-801 produced hyperthermia but levoxadrol did not affect body temperature. In contrast to the hyperthermic effects of dexoxadrol and MK-801, phencyclidine produced hypothermia. These findings indicate that hypothermia induced by phencyclidine is not due to interactions with phencyclidine receptors and, while dexoxadrol, MK-801 and phencyclidine may share some similar receptor binding and behavioral characteristics, they can be differentiated on the basis of their effects on body temperature.

Naloxone does not antagonize PCP-induced stimulation of the pituitary-adrenal axis in the rat

Phencyclidine (PCP) has been shown to stimulate the pituitary-adrenal axis in the rat. The purpose of the present study was to determine whether opiate receptors are involved in this effect by testing whether pretreatment with the opiate antagonist naloxone can antagonize PCP-induced ACTH and corticosterone release. PCP (10.0 mg/kg) produced increases in plasma ACTH and corticosterone 60 min after s.c. administration. Pretreatment with naloxone (2.0 mg/kg s.c.) did not reduce the rise in plasma levels of ACTH or corticosterone produced by PCP. These results indicate that naloxone-sensitive opiate receptors are not involved in the PCP-induced stimulation of the pituitary-adrenal axis in rats.

Inhibition of (+)[3H]SKF 10,047 binding to rat brain membranes by FAB fragments from a monoclonal antibody directed against the opioid receptor

Examination of the binding of (+)[3H]SKF 10,047 to rat brain membranes indicated that at a low concentration most of the binding was to the haloperidol-sensitive binding site. Titration curves exhibited a displacement potency order of haloperidol greater than (+)SKF 10,047 = 1,3-diorthotolyl-guanidine much much greater than (-)SKF 10,047 much greater than phencyclidine analogues. The effect of Fab fragments from a monoclonal antibody, OR-689.2.4, directed against the opioid receptor on the binding of (+)[3H]SKF 10,047 to rat brain membranes was examined. The specificity of this antibody for the opioid receptor has been determined by its ability to inhibit the binding of mu and delta opioid peptides to rat brain but not the binding of kappa opioid ligands or nonopioid ligands specific for other receptors. The Fab fragments blocked and displaced specifically bound (+)[3H]SKF 10,047 in a titratable manner. Increasing the incubation time of the membranes with the Fab fragments increased the percent inhibition obtained. The Fab fragments acted as noncompetitive inhibitors of (+)[3H]SKF 10,047 binding. A (+)SKF 10,047 binding site in rat brain appears to share a common structural domain with mu and delta opioid receptors.

Phencyclidine (PCP) injected in the nucleus accumbens increases extracellular dopamine and serotonin as measured by microdialysis

Phencyclidine (PCP; 20 micrograms in 0.5 microliter) was tested by local brain injection for neurochemical effects in the nucleus accumbens and striatum of rats. Changes in dopamine turnover could not be detected in postmortem tissue assays. In contrast, extracellular levels of dopamine significantly increased as measured by microdialysis in freely moving animals. PCP also increased extracellular levels of serotonin and decreased 3,4-dihydroxyphenylacetic acid (DOPAC), but did not change homovanillic acid (HVA) or 5-hydroxyindoleacetic acid (5HIAA). Microdialysis suggests that PCP acts in some dopamine terminal regions to increase extracellular dopamine and serotonin.

Different effects of ethylketocyclazocine on phencyclidine- and N-allylnormetazocine-induced stereotyped behaviors in rats

The effects of ethylketocyclazocine (EKC) on the stereotyped behaviors induced by intraperitoneal injection of phencyclidine (PCP) or N-allylnormetazocine (SKF 10,047) were examined. EKC markedly antagonized PCP-induced stereotyped behaviors such as sniffing, head-weaving, turning and backpedalling. On the other hand, EKC failed to antagonize SKF 10,047-induced stereotyped behaviors, which are PCP-like stereotyped behaviors, except sniffing and head-weaving at 0-15 min after the SKF 10,047 injection. PCP-induced turning and backpedalling were potentiated by pretreatment with SKF 10,047, while PCP-induced sniffing and head-weaving were not. EKC failed to affect the enhancing effect of SKF 10,047 on PCP-induced turning and backpedalling. These results suggest that part of the PCP- and SKF 10,047-induced stereotypy may be mediated by different neuronal mechanisms.

Rat brain PCP receptors: alterations in binding parameters following chronic administration of opiate agonists and antagonists

Phencyclidine (PCP) is a widely abused drug of the arylcyclohexylamine class which is capable of producing symptoms of acute psychosis in man. PCP interacts with a specific CNS receptor, for which a putative endogenous peptide ligand has been identified. We have investigated whether PCP receptor binding parameters are modulated by activity in central opiate pathways. We have found that chronic administration of both an opiate agonist (etonitazene) and an opiate antagonist (naloxone) are able to decrease the affinity of the PCP receptor for TCP, a thienyl derivative of PCP. Furthermore, naloxone, but not etonitazene, resulted in a significant increase in the Bmax of TCP binding to the PCP receptor. These results suggest that neural activity mediated by CNS opioids systems is capable of affecting the binding parameters of the PCP receptor.

A simple and rapid method for assessing similarities among directly observable behavioral effects of drugs: PCP-like effects of 2-amino-5-phosphonovalerate in rats

Directly observable behavioral effects of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (AP5) (10-1,000 mg/kg IP, 0.18-5.6 mumol/rat ICV) and of phencyclidine (PCP) (3.2-56 mg/kg IP, 0.032-3.2 mg/rat ICV), ketamine (10-100 mg/kg), amphetamine (1-18 mg/kg), apomorphine (0.1-5.6 mg/kg), chlordiazepoxide (1-100 mg/kg), and pentobarbital (3.2-56 mg/kg) were studied in rats. Pharmacologically specific results were obtained rapidly and reliably, using a cumulative dosing procedure. Cluster analysis grouped the drug treatments, on the basis of their similarities in producing different behavioral activities, into three main clusters; characteristically, stimulant drugs (amphetamine, apomorphine) produced sniffing and gnawing; PCP-like drugs (PCP, ketamine) produced locomotion, sniffing, swaying and falling; sedative drugs (pentobarbital, chlordiazepoxide) produced loss of righting. The behavioral effects of ICV administration of AP5 were more similar to the effects of PCP-like drugs than to the effects of either stimulant or sedative drugs, thus supporting the hypothesis that the behavioral effects of PCP-like drugs may result from reduced neurotransmission at excitatory synapses utilizing NMDA preferring receptors. The present procedure is simple, rapid and may provide a useful approach in the classification of behaviorally active drugs.

Sensitization to amphetamine and tolerance to cocaine and phencyclidine stimulation in mice

Amphetamine (1.0-7.0 mg/kg), cocaine (5.0-40.0 mg/kg) and phencyclidine (1.0-7.0 mg/kg) increased acoustic startle responding in mice. These drugs, however, had varying effects on habituation of the startle response after repeated exposure to the auditory stimulus. The primary effect of phencyclidine was to disrupt the habituation process, whereas increased startle responding after cocaine developed without modification of the habituation curve. Amphetamine facilitated acoustic startle at all doses, and after administration of 3.0 mg/kg a significant response sensitization as a function of repeated stimulus presentation was evident. Consistent with previous reports the excitatory effects of cocaine and amphetamine on acoustic startle were blocked by pretreatment with haloperidol. Haloperidol, which decreased startle responding, attenuated the facilitating effects of PCP on acoustic startle as well. Chronic exposure to amphetamine, cocaine and phencyclidine had differential effects on startle responding. The facilitating effects of amphetamine on startle were further enhanced after long-term exposure to the drug and the sensitizing effect of repeated amphetamine exposure was observed only when animals were tested with amphetamine. In contrast, tolerance developed after chronic exposure to both cocaine and phencyclidine, and the response attenuation was evident when animals were tested for acoustic startle after cocaine, amphetamine and phencyclidine.