Phencyclidine-induced rotational behavior in rats with nigrostriatal lesions and its modulation by dopaminergic and cholinergic agents

In Vitro and In Vivo Pharmaco-Toxicological Characterization of 1-Cyclohexyl-x-methoxybenzene Derivatives in Mice: Comparison with Tramadol and PCP

1-cyclohexyl-x-methoxybenzene is a novel psychoactive substance (NPS), first discovered in Europe in 2012 as unknown racemic mixture of its three stereoisomers: ortho, meta and para. Each of these has structural similarities with the analgesic tramadol and the dissociative anesthetic phencyclidine. In light of these structural analogies, and based on the fact that both tramadol and phencyclidine are substances that cause toxic effects in humans, the aim of this study was to investigate the in vitro and in vivo pharmacodynamic profile of these molecules, and to compare them with those caused by tramadol and phencyclidine. In vitro studies demonstrated that tramadol, ortho, meta and para were inactive at mu, kappa and delta opioid receptors. Systemic administration of the three stereoisomers impairs sensorimotor responses, modulates spontaneous motor activity, induces modest analgesia, and alters thermoregulation and cardiorespiratory responses in the mouse in some cases, with a similar profile to that of tramadol and phencyclidine. Naloxone partially prevents only the visual sensorimotor impairments caused by three stereoisomers, without preventing other effects. The present data show that 1-cyclohexyl-x-methoxybenzene derivatives cause pharmaco-toxicological effects by activating both opioid and non-opioid mechanisms and suggest that their use could potentially lead to abuse and bodily harm.

Dose-response characteristics of intravenous ketamine on dissociative stereotypy, locomotion, sensorimotor gating, and nociception in male Sprague-Dawley rats

Clinicians administer subanesthetic intravenous (IV) ketamine infusions for treatment of refractory depression, chronic pain, and post-traumatic stress disorder in humans. However, ketamine is administered via the subcutaneous (SC) or intraperitoneal (IP) routes to rodents in most pre-clinical research, which may limit translational application. The present study characterized the dose-response of a subanesthetic IV ketamine bolus (2 and 5mg/kg) and 1-h infusion (5, 10, and 20mg/kg/h) on dissociative stereotypy, locomotion, sensorimotor gating, and thermal nociception in male Sprague-Dawley rats. The secondary aim was to measure ketamine and norketamine plasma concentrations following IV ketamine bolus at 1, 20, and 50min and at the conclusion of the 1-h infusion using liquid chromatography/mass spectrometry. The results showed that ketamine bolus and infusions produced dose-dependent dissociative stereotypy. Bolus (2 and 5mg/kg) and 20mg/kg/h infusion increased locomotor activity while 5mg/kg/h infusion decreased locomotor activity. Both 10 and 20mg/kg/h infusions reduced the acoustic startle reflex, while 5mg/kg bolus and 20mg/kg/h infusion impaired pre-pulse inhibition. Ketamine 5mg/kg bolus and the 10 and 20mg/kg/h infusions induced significant and prolonged antinociception to the hotplate test. Plasma concentrations of ketamine decreased quickly after bolus while norketamine levels increased from 1 to 20min and plateaued from 20 to 50min. The peak ketamine plasma concentrations [ng/ml] were similar between 5mg/kg bolus [4100] vs. 20mg/kg/h infusion [3900], and 2mg/kg bolus [1700] vs. 10mg/kg/h infusion [1500]. These results support the findings from previous ketamine injection studies and further validate the feasibility of administering subanesthetic doses of IV ketamine infusion to rats for neuropharmacological studies.

Effects of a metabotropic glutamate receptor group II agonist LY354740 in animal models of positive schizophrenia symptoms and cognition

It has been proposed that activation of metabotropic glutamate receptor subtype 2/3 (mGluR2/3) may induce both antipsychotic and anxiolytic effects. The aim of this study was to evaluate further the effect of the mGluR2/3 agonist, LY354740 [(+)-2-aminobicyclo(3.1.0)hexane-2,6-dicarboxylate monohydrate] in animal models relevant to both psychotic and cognitive impairment in schizophrenia. The elevated plus maze was used to select the doses for further experiments, LY354740 induced anxiolytic-like effects at doses of 3 and 10 mg/kg but not 1 mg/kg. At a dose of 10 mg/kg. LY354740 attenuated phencyclidine (PCP)-induced locomotor activity. Administered alone, it had no effect on horizontal activity, but at doses of 3 and 10 mg/kg, slightly decreased vertical activity (rearings). LY354740 (1-10 mg/kg intraperitoneally) affected neither prepulse inhibition in normal rats nor reversed the disruption of prepulse inhibition produced by PCP (2 mg/kg subcutaneously). Moreover, LY354740 (3-10 mg/kg) did not modify PCP-induced working memory deficits assessed in a spontaneous alternation task and had no effect on PCP-evoked amnesia in the passive avoidance test. LY354740 alone (3 and 10 mg/kg) induced working memory deficits, but had no effect on acquisition of passive avoidance. In conclusion, LY354740 was effective in models for anxiety and positive symptoms of schizophrenia but not in models for sensorimotor gating and cognitive impairment.

Effects of phencyclidine on schedule-controlled responding following neurotoxic lesions of the striatum

The effects of phencyclidine on an operant task were evaluated prior to and after neurotoxic lesions of the striatum in rats. Subjects were trained to respond on a fixed-interval 90-second schedule for water presentation. The degree to which phencyclidine disrupted responding was first evaluated (dose range 1.0-4.0 mg/kg). The subjects were then divided into three matched groups and received bilateral intraventricular injections of 6-hydroxydopamine (6-OHDA) (100 microg), kainic acid (0.25 microg), or vehicle delivered stereotaxically. 6-OHDA was used to destroy the presynaptic neurons of the nigro-striatal pathway and kainic acid was employed to destroy the postsynaptic neurons whose cell bodies are located in the striatum. Following recovery, the phencyclidine dose-response curve was repeated in the fixed-interval paradigm. It was observed that 6-OHDA-induced damage resulted in a rightward shift of the dose-response curve indicating tolerance to phencyclidine and caused a significant depletion of striatal dopamine and gamma-aminobutyric acid (GABA). Kainic acid-induced damage resulted in a leftward shift in the dose-response curve indicating sensitivity to the schedule-disruptive effects of phencyclidine and produced a significant GABA depletion. The vehicle-treated rats exhibited no shift in their sensitivity to phencyclidine. These observations indicate that the effects of phencyclidine are mediated, at least in part, by striatal dopaminergic neurons.

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.

Alterations in striatal dopamine overflow during rotational behavior induced by amphetamine, phencyclidine, and MK-801

Rats lesioned unilaterally in the medial forebrain bundle with 6-OHDA rotated ipsilateral to the lesion following injections of amphetamine, phencyclidine (PCP), and MK-801. Concurrent measurement of striatal dopamine (DA) in the intact striatum with in vivo microdialysis revealed a dissociation between rotational behavior and alterations in DA overflow induced by the three drugs. Amphetamine produced robust ipsilateral rotational behavior and a substantial elevation in striatal DA (approximately 130% increase at asymptote). PCP produced comparable increases in rotational behavior, but only approximately 30% increase in striatal DA. MK-801 also had a comparable behavioral effect but failed to alter DA overflow in the intact striatum. Since MK-801, a noncompetitive NMDA antagonist which does not enhance extracellular dopamine in the striatum, is able to produce ipsilateral rotational behavior in rats with unilateral nigrostriatal lesions, it is likely that the effects of PCP may also be determined predominantly through NMDA blockade in this model.

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.

The role of nucleus accumbens dopamine in the neurochemical and behavioral effects of phencyclidine: a microdialysis and behavioral study

A series of experiments were conducted in order to characterize the role of nucleus accumbens dopamine (DA) in the neurochemical and behavioral effects of phencyclidine (PCP). In the first study, microdialysis probes were implanted in nucleus accumbens to determine the effects of 4.0 and 8.0 mg/kg PCP on extracellular levels of DA and its metabolites, dihydroxyphenyl-acetic acid (DOPAC) and homovanillic acid (HVA) in behaving rats. PCP increased extracellular DA, DOPAC and HVA in the same dose range that produced increases in locomotor activity, stereotypy and ataxia. The increases in extracellular DA that were induced by 4.0 mg/kg PCP were significantly correlated with the increases in locomotor activity. In the second study, rats received bilateral injections of 6-hydroxydopamine in order to deplete DA in nucleus accumbens. DA-depleted and control rats received injections of saline and 4.0 mg/kg PCP and were tested in an 'intruder' paradigm. In this procedure, saline-and PCP-treated rats were placed in a stable colony of three other rats and social behavior was observed for 30 min. PCP reduced the frequencies of various social behaviors, but accumbens DA depletion did not reverse the effects of PCP on social behavior. Subsequently, all rats received 8.0 mg/kg PCP and were assessed for locomotor activity, stereotypy and ataxia. Depletion of DA in nucleus accumbens attenuated PCP-induced locomotion, but did not alter the effects of the drug on stereotypy or ataxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phencyclidine on spontaneous and N-methyl-D-aspartate (NMDA)-induced efflux of dopamine from superfused slices of rat striatum

Phencyclidine (PCP) acts as an indirect dopamine (DA) agonist by inhibiting the neuronal reuptake of DA, while it also works as a N-methyl-D-aspartate (NMDA) antagonist. Aiming to investigate characteristics of these two properties of PCP in the same experimental system, the effects of PCP on spontaneous and NMDA-induced efflux of DA from superfused slices of the striatum of the rat were examined. Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the samples of superfusate were extracted by alumina extraction and measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC). Phencyclidine at concentrations greater than 1 microM, produced a concentration-dependent increase of the spontaneous efflux of DA. The efflux of DOPAC was also concentration-dependently increased by PCP. However, PCP inhibited the efflux of DA induced by NMDA, even at a small concentration (0.1 microM), which did not alter the spontaneous efflux of the transmitter. The mode of the inhibition by PCP was shown to be noncompetitive, with an estimated IC50 value of 280 nM. These results suggest that PCP, at small concentrations, reduces the synaptic levels of DA by blocking the facilitating effect of endogenous glutamate on the release of DA and, at slightly greater concentrations, the drug also works as an indirect DA agonist, to increase the levels of the transmitter in the synaptic clefts. The clinical significance of the dual effects of PCP is discussed in relation with the unique schizophrenomimetic property of PCP.

Increases in extracellular dopamine levels and locomotor activity after direct infusion of phencyclidine into the nucleus accumbens

Three studies were conducted to provide a further characterization of the neurochemical and behavioral effects of phencyclidine (PCP). The first experiment utilized in vivo microdialysis to measure extracellular levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens. Administration of PCP (4.0 x 10(-4) M) in the dialysis perfusion medium produced large increases in extracellular DA, and biphasic changes in DOPAC. In the second experiment, chronic indwelling cannulae for drug infusion were implanted bilaterally into the nucleus accumbens. Local infusions of PCP (15.0, 30.0 or 60.0 micrograms per side) produced substantial increases in locomotor activity. Little or no headweaving, stereotypy or ataxia was observed after intra-accumbens injections of PCP. In the third experiment, systemic administration of 0.4 mg/kg haloperidol significantly reduced the locomotor activity induced by intra-accumbens injection of 15.0 micrograms PCP. These results indicate that PCP can increase extracellular levels of DA by direct actions in the DA terminal region, and that these effects are related to some of the behavioral stimulant properties of PCP.

The effects of N-methyl-D-aspartate receptor blockade with MK-801 upon the relationship between cerebral blood flow and glucose utilisation

The local cerebral circulatory and metabolic effects of MK-801, a selective non-competitive N-methyl-D-aspartate receptor antagonist have been examined in conscious rats with quantitative autoradiographic techniques using [14C]iodoantipyrine and [14C]2-deoxyglucose as tracers. Local cerebral blood flow (CBF) and local cerebral glucose utilisation (GU) were measured in 41 discrete neuroanatomical loci using identical criteria for region of interest localisation. Animals received either saline or MK-801 (0.5 mg/kg in saline) intravenously 10 min prior to the start of GU determination, or 15 min before the CBF measurement. MK-801 effects an immediate transient, elevation in mean arterial pressure (elevated by 30% from baseline) which returned rapidly to preinjection levels and a sustained moderate hypercapnia (arterial carbon dioxide tension increased by 16%) which persisted throughout the measurement periods. Statistically significant changes in GU were observed in 13 brain region structures after MK-801 administration. Glucose utilisation was significantly and markedly elevated with MK-801 in some limbic structures (particularly the hippocampus, posterior cingulate and entorhinal cortices), the inferior colliculus and most of the neocortex displayed moderate reductions in GU after MK-801 treatment. In the majority of brain regions (28 or the 41 studied), MK-801 minimally altered GU. There were widespread alterations in local CBF with MK-801 although in the majority of brain regions (24 of the 41 studied) there was no statistically significant alteration in CBF with MK-801. With one exception (the anterior thalamic nucleus), CBF was increased with MK-801 in all regions in which glucose use was elevated with the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Prenatal phencyclidine in rats: effects on apomorphine-induced climbing

Either 5 or 10 mg/kg of phencyclidine (PCP) in saline was administered by subcutaneous injection to gravid dams during the last two weeks of gestation. A pair-fed control group was administered the vehicle alone and allowed to eat and drink only the amount consumed by the 10 mg/kg group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. Among the dams receiving 10 mg/kg of PCP, food and water intake was initially reduced to 33-43% of nontreated controls, but then returned to control levels. Surprisingly, after 3 days of drug administration, water intake of PCP-treated dams exceeded that of the nontreated dams by approximately 15%. Compared with the nontreated dams, both PCP groups and pair-fed control dams gained significantly less body weight from conception to term. PCP had no significant effect on number of implantation sites or number of live births, however, PCP produced an apparent selective embryolethal effect on males and body weight reduction in all groups at birth. Prenatal PCP did not alter the sensitivity to apomorphine-induced climbing behavior during the second postnatal week. These results are discussed with respect to published animal and clinical studies of PCP exposure during pregnancy.

Ketamine as a pharmacological model for tongue dyskinesia

The purpose of this article is to document that ketamine hydrochloride, administered at an anesthetic dosage of about 100 mg/kg, produces tongue contractile activity in the rat. The methods for monitoring and quantitating ketamine-induced tongue contractions (KITCs) are described. We also found that neuroleptic agents consistently and readily abolish KITCs. On the basis of these observations and other pharmacological properties of ketamine, we propose that KITCs may be a useful model for studying neuroleptic-induced oral dyskinesia, e.g., tardive dyskinesia. Additional findings in support of this model are presented.

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.

Development of tolerance and supersensitivity to phencyclidine in rats after repeated administration of phencyclidine

In rats treated with phencyclidine (PCP) repeatedly (PCP 10 mg/kg per day for 14 days), the back-pedalling, head-weaving and turning induced by PCP were attenuated (tolerance), while PCP-induced sniffing, rearing and ambulation were potentiated (supersensitivity). The behavior induced by the direct and indirect serotonin (5-HT) agonists, 5-methoxy-N,N-dimethyltryptamine and p-chloroamphetamine, was attenuated, while the sniffing, rearing or licking induced by the direct and indirect dopamine (DA) agonists, apomorphine and methamphetamine, were potentiated in the chronic PCP-treated rats. The DA and 5-HT contents in the nucleus accumbens and the ratio of HVA to DA in the striatum increased following the repeated PCP administration. Pentobarbital-induced sleep time did not change in the chronic PCP-treated rats as compared with the control rats. In addition, there was no significant difference between the disappearance rate of PCP in the brain of the rats treated with PCP repeatedly and the rate in the control rats. These results suggest that functional changes in the dopaminergic and serotonergic neuronal systems develop on repeated administration of PCP but that such changes do not develop in the hepatic drug-metabolizing system. In addition, tolerance develops in the serotonergic neuronal system while supersensitivity develops in the dopaminergic neuronal system. Biochemical findings suggest that increased mesolimbic dopaminergic neuronal function plays an important role in the development of the supersensitivity.

PCP-induced alterations in cerebral glucose utilization in rat brain: blockade by metaphit, a PCP-receptor-acylating agent

The effects of phencyclidine (PCP) on regional cerebral glucose utilization was determined by using quantitative autoradiography with [14C]-2-deoxyglucose. PCP increased brain metabolism in selected areas of cortex, particularly limbic, and in the basal ganglia and thalamus, whereas the drug decreased metabolism in areas related to audition. These results are consistent with the known physiology of central PCP neurons and may help to suggest brain areas involved in PCP-mediated actions. Moreover, based on the behavioral similarities between PCP psychosis and an acute schizophrenic episode, these data may be relevant to the understanding of schizophrenia. The PCP-receptor-acylating agent, metaphit, blocked most of these PCP actions. In addition, metaphit by itself was found to diminish glucose utilization rather uniformly throughout brain. These results indicate an antagonist effect of metaphit on the PCP system and suggest a widespread action of metaphit, putatively at a PCP-related site, possibly in connection with the N-methyl-D-aspartate (NMDA) receptor.

Neuropathology and amphetamine-induced turning resulting from AF64A injections into the striatum of the rat

The putative specific cholinergic neurotoxin AF64A was prepared by a micro-scale procedure which afforded the neurotoxin in greater than 95% purity and was microinjected unilaterally into the striatum of male albino rats. The AF64A-injected animals displayed amphetamine- and apomorphine-induced ipsilateral turning indicating disruption of normal striatal pathways. These turning effects were absent in the control injected animals. Histological examinations of the brains revealed that AF64A in amounts as low as 1 nmole produced regions of necrosis in the striatum in some cases extending along the cannula tract up to and including the cortex. These results indicate that the striatum is highly sensitive to relatively low doses of AF64A, and that consideration should be given to the possibility of gross tissue damage when interpreting behavioral data.

Phencyclidine-induced dopamine-dependent behaviors in chronic haloperidol-treated rats

This study was designed to assess whether phencyclidine (PCP) produces dopamine (DA)-dependent behaviors such as licking, biting and gnawing at low doses after withdrawal from chronic haloperidol (HAL) treatment in rats. Low doses of PCP (2.5 and 5 mg/kg) produced licking, gnawing, biting and self-biting in rats after withdrawal from chronic HAL treatment, which were not observed in the vehicle-pretreated rats given PCP at the same dose range. These behaviors were similar to DA-dependent behaviors produced by methamphetamine and apomorphine in rats after withdrawal from chronic HAL treatment. The PCP-induced behaviors were attenuated by acute pretreatment of DA antagonist, HAL (0.25 mg/kg, IP). Furthermore, at doses of 5 or 7.5 mg/kg, PCP-induced head weaving and backpedalling, which were mediated by both DA and serotonin (5-HT) neurons, significantly increased in rats after withdrawal from chronic HAL-treatment. These results suggest that dopaminergic systems play an important role for PCP-induced behavioral responses.

Effects of phencyclidine (PCP)-like drugs on turning behavior, 3H-dopamine uptake, and 3H-PCP binding

In this study representatives from three chemical classes which are known to produce a phencyclidine (PCP)-like discriminative stimulus cue in rats were tested for their ability to inhibit synaptosomal uptake of 3H-dopamine (3H-DA) and to compete for a binding site labeled with 3H-PCP. Although there was a good correlation between these two in vitro activities within the arylcycloalkylamine class (PCP, N-ethyl-phenylcyclohexylamine (PCE), and ketamine) it did not hold when representatives from the benzomorphans, N-allynormetazocine (NANM), cyclazocine (CYCL), and ethylketocyclazocine (EKC), or a substituted dioxolane (etoxadrol) were included. At some dose each of these drugs with the exception of EKC also produced ipsilateral turning in rats with a unilateral 6-hydroxydopamine lesion of the substantia nigra. This effect was also not well correlated with inhibition of 3H-DA uptake. However, a significant correlation was found to exist between turning behavior and affinity for the putative PCP/sigma receptor. The possibility that a non-dopaminergic mechanism involving the PCP/sigma receptor underlies the ability of these agents to induce turning behavior is discussed.

Quantitative electrocortical changes in the rat induced by phencyclidine and other stimulants

An automated technique for the continuous analysis of different frequency bands of the electrocorticogram (ECoG) of the rat has been used to quantify the actions of phencyclidine (PCP) and various other stimulant drugs. It has been demonstrated that phencyclidine, etoxadrol and LY154045 produced similar changes in the individual frequency bands whereas amphetamine and apomorphine had different profiles of activity. The phencyclidine-like compounds exhibited extremely strong stimulation of the ECoG with very large increases recorded in high frequency (15-50 Hz) activity and reductions in all other frequency bands. Various compounds have been used in an attempt to antagonise the changes in the ECoG. Chlorpromazine caused a slight shift in the dose-response curves as did chlordiazepoxide when used with phencyclidine. The GABA agonists, THIP and muscimol, had no effect on the stimulation of the ECoG. In contrast another presumed GABA agonist, baclofen, proved to be the most effective agent for blocking the stimulation induced by phencyclidine. The role of the GABAB receptor in the action of phencyclidine is discussed.

Effects of acute and chronic administrations of phencyclidine on the levels of serotonin and 5-hydroxyindoleacetic acid in discrete brain areas of mouse

The effects of phencyclidine (PCP) on the levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in discrete brain areas of mouse were investigated. Following a single administration, PCP significantly increased at 60 min the level of 5-HT but not 5-HIAA in the cortex. However, acute administration of PCP induced no changes of 5-HT and 5-HIAA levels in other brain areas investigated. On the other hand, chronic treatment of PCP produced a significant increase the striatal 5-HT and 5-HIAA levels by about 30% and 20%, respectively. These increased levels were gradually returned to the control levels, and there was no difference of these levels between the control group and the 48 hr withdrawal group. The changes of 5-HT level in the hypothalamus were similar to those in the striatum. These results suggest that the pharmacological actions of PCP and tolerance development to PCP may be related to the functional changes of serotonergic neuronal activity.

Circling behavior induced by phencyclidine in mice and its inhibition by naloxone

Phencyclidine (PCP), when given to mice, induces general hyperactivity and rapid circling, similar to that caused by morphine. These effects are partially antagonized by naloxone.

The effects of phencyclidine and N-allylnormetazocine on midbrain dopamine neuronal activity

Single unit recording techniques were used to determine the effects of intravenously and microiontophoretically administered phencyclidine (PCP) and the enantiomers of N-allylnormetazocine (SKF-10,047) on the activity of midbrain dopamine (DA) neurons. Intravenous PCP produced a biphasic effect on substantia nigra zona compacta (A9) and ventral tegmental (A10) DA neurons which consisted of excitation followed by inhibition below baseline firing rates as the dose was increased. The high-dose attenuation of firing by PCP, but not the excitatory effects, was antagonized by haloperidol pretreatment. Intravenous (+)- and (-)-SKF-10,047 increased the firing rate of most A9 and A10 DA neurons. In contrast to the intravenous findings, iontophoretic PCP generally exerted only very weak inhibitory actions on neuronal activity, while (+)- and (-)-SKF-10,047 produced no consistent effects. The results suggest that these drugs indirectly influence the activity of DA neurons and that this may be a property shared by drugs classified as sigma receptor agonists.

Serotonergic involvement in phencyclidine-induced behaviors

Administration of 5-10 mg/kg of phencyclidine (PCP) caused stereotyped behaviors including sniffing, backpedalling, head weaving and turning in rats. The PCP-induced stereotyped behaviors (backpedalling, head weaving and turning) were attenuated by serotonin (5-HT) depleters [reserpine, p-chlorophenylalanine, p-chloroamphetamine (PCA)] and 5-HT receptor antagonist (cyproheptadine). PCP-induced head weaving and turning were potentiated by 5-HT precursor (tryptophan) and 5-HT releaser (PCA). PCP-induced head weaving were potentiated also by monoamine oxidase inhibitor (pargyline) and 5-HT reuptake inhibitor (imipramine). PCP 5-10 mg/kg significantly increased the content of 5-HT in the thalamus/hypothalamus at 30 and 60 min after the injection, except PCP 5 mg/kg at 60 min. PCP 7.5 and 10 mg/kg increased the rate of increment of 5-HT by pargyline in the thalamus/hypothalamus at 30 and 60 min after the injection, respectively. PCP 10 mg/kg significantly increased the contents of 5-HIAA in the striatum and thalamus/hypothalamus at 30 min, but decreased that of 5-HIAA in all discrete brain areas except the stratium at 60 min after the injection. PCP also significantly prevented the depletion of 5-HT by PCA in all discrete brain areas except the stratium at 60 min after the injection. From these results, PCP-induced stereotyped behaviors are related to an increased serotonergic neuronal activity due to 5-HT releasing action and/or inhibitory action of 5-HT uptake-by this drug.

Genetic effects on PCP-induced stimulation in recombinant inbred strains of mice

The stimulation of motor activity by phencyclidine was found to differ significantly in BALB/c and C57Bl/6By inbred strains of mice. Phencyclidine-induced stimulation was compared for these strains, their reciprocal F1 hybrids, and their recombinant inbred offspring. There were significant differences in responsivity among the strains, suggesting a genetic influence on the PCP response; however, the strains did not segregate into two distinct groupings, suggesting that this genetic influence was not carried by a single gene. In addition, there was no relationship between the responsivity of these strains of mice to PCP and their previously-reported responses to amphetamine or scopolamine, which suggests that PCP-induced stimulation is not a simple cholinergic or amphetamine-like response.

Effects of mecamylamine, nicotine, atropine and physostigmine on the phencyclidine-induced behavioral toxicity

Phencyclidine (PCP) has multifaceted actions on the cholinergic functions, including interaction with the central and peripheral cholinergic receptors. Therefore, to evaluate the possible involvement of the nicotinic and muscarinic acetylcholine (ACh) receptors during the behavioral toxicity of PCP, influence of various cholinergic modifiers on the PCP-induced behavioral effects in male Swiss mice was studied. PCP-induced (45 mumol/kg, IP) behavioral toxicity (circular movements, side-to-side head movements, and hyperactivity leading to convulsions) was blocked by pretreating the animals with secondary- or tertiaryamino -cholinergic modifiers, mecamylamine (ME; 14.9 and 29.9 mumol/kg), nicotine (NI; 12.3 and 30.8 mumol/kg) and physostigmine (PH; 0.16 and 0.31 mumol/kg). NI at 1.5 mumol/kg significantly potentiated the PCP-induced convulsions. Atropine (AT; 14.4 and 28.8 mumol/kg) pretreatments shortened the onset of circular movements. The locomotor activity of PCP (16.4 mumol/kg) was blocked by ME, NI, and PH. AT at 7.2 mumol/kg significantly potentiated the PCP-locomotion by 62%. These observations indicated that the behavioral actions of PCP, at least in part, are mediated by the central nicotinic and muscarinic ACh receptors. The involvement of cholinergic receptors in conjunction with the dopaminergic actions of PCP during these behaviors also has been discussed.

Effects of phencyclidine, haloperidol, and naloxone on fixed-interval performance in rats

Phencyclidine (PCP), haloperidol, and naloxone were administered alone and in combination to rats responding under a fixed-interval schedule for water presentation. Lower doses of PCP (0.25-2.0 mg/kg) and naloxone (0.001-0.1 mg/kg) produced increases while higher doses produced dose-dependent decreases in response rate. Haloperidol (0.0625-0.5 mg/kg) produced a monotonic dose-dependent decrease in responding. When a dose of naloxone (8.0 mg/kg) that did not alter responding was administered prior to the PCP, the PCP dose-response curve was shifted to 6.5-fold lower doses of PCP. When a dose of haloperidol (0.0625 mg/kg) that did not alter responding was administered prior to the PCP, the PCP dose-response curve was shifted to 1.5-fold higher dose of PCP. These observations are discussed in relation to current views of the mechanism of PCP action.

Phencyclidine-induced locomotor activity in the rat is blocked by 6-hydroxydopamine lesion of the nucleus accumbens: comparisons to other psychomotor stimulants

The present study was primarily designed to explore the relationship between phencyclidine(PCP)-induced hyperactivity and the mesolimbic dopamine (DA) system. In addition, the motor-activating and behavioral effects of amphetamine (1.5 mg/kg), SKF-10,047 (25.0 mg/kg), scopolamine (1.0 mg/kg), and caffeine (10.0 mg/kg) were also measured and compared to PCP action. While all compounds produced a moderate to large degree of hyperactivity with varying time courses for effect, gross behavioral observations indicated a greater similarity between PCP and SKF-10,047 than between any of the other drugs. Following bilateral 6-hydroxydopamine lesions of the nucleus accumbens the robust locomotor-stimulating action of 5 mg/kg PCP was significantly reduced. Such lesions also successfully prevented amphetamine- and SKF-10,047-induced hyperactivity, but not the behavioral activation produced by scopolamine or caffeine. These results suggest that PCP and SKF-10,047, like amphetamine, elicit locomotor activity through presynaptic DA mechanisms within the mesolimbic system.

In vitro and in vivo evaluation of cis-methyl-phencyclidine (CIS-MPCP) as a potential antagonist of phencyclidine (PCP)

In four preparations/tests (isolated nerve, ventricular strip, rotarod, and mouse acute lethality), cis-N-phenyl-4-methyl-cyclohexyl piperidine (cis-MPCP) was consistently less active than PCP and trans-MPCP. As expected, cis-MPCP, at 10(-4)M, which did not depress the action potential evoked on frog sciatic nerves, reduced by half both the nerve block and prolongation of relative refractory period caused by PCP. However, cis-MPCP at 10(-6)M, which by itself had little effect, failed to reduce the positive inotropic effect of PCP on the field-stimulated rat ventricular strip. Cis-MPCP also failed to decrease the ataxic effect of 6 mg/kg PCP (ED80) in the mouse rotarod test. Finally, at a dose that was neither ataxic nor lethal to mice (20 mg/kg), cis-MPCP failed to reduce the 24-hour LD50 of PCP. These data suggest that the actions of PCP are mediated through a multiple receptor system.

Effect of lesions in the striatum, nucleus accumbens and medial raphe on phencyclidine-induced stereotyped behaviors and hyperactivity in rats

The effect of lesioning the striatum, nucleus accumbens and medial raphe on phencyclidine(PCP)-induced stereotyped behaviors and hyperactivity was investigated to determine the site or sites of actions of PCP in rats. Bilateral lesions of the striatum diminished or abolished all the parameters of PCP-induced stereotyped behaviors, including sniffing, back pedalling, turning and head weaving 7 days after the operation. The medial raphe lesion significantly reduced PCP-induced back pedalling and head weaving. Bilateral lesions of the ventral portion of the nucleus accumbens did not affect the PCP-induced stereotyped behaviors. On the contrary, none of the lesions altered the sensitivity to PCP-induced hyperactivity 7 days after the operation. These results suggest that PCP-induced stereotyped behaviors may be mediated in the striatum and the medial raphe but not the nucleus accumbens. Furthermore, PCP-induced hyperactivity may not result from PCP effects on these discrete brain areas.

The effects of phencyclidine on glutamic acid decarboxylase activity in several regions of the rat brain

Glutamic acid decarboxylase (GAD) activity in several regions of the rat brain were monitored after administration of phencyclidine. Sub-acute (4 injections over 12 h) treatment decreased cerebellar GAD activity 6 and 12 h after the last dose; recovery was noticed by 24 h. This effect occurred with doses of 5 and 10 mg/kg. GAD activity in other brain regions was not affected by this treatment. Acute and chronic treatments with phencyclidine caused no change in GAD activity in any of the brain regions examined.

Anorexia and hyperphagia produced by five pharmacologic classes of hallucinogens

The acute actions of five prototype hallucinogens administered SC on food consumption in 23 hr food deprived dogs were compared with the anorexic effect of d-amphetamine and the hyperphagic effect of sodium pentobarbital. Comparisons were made on the basis of dose-response relationships. Among the hallucinogens decreasing food intake, both LSD and atropine produced substantial anorexia, but the slopes of their dose-response curves were clearly different from d-amphetamine. Phencyclidine and the opioid SKF 10,047 suppressed food intake also; their individual dose-effect curves were parallel to the amphetamine curve, although both were less potent. Of the hallucinogens tested, only delta-9-tetrahydrocannabinol (delta 9-THC) stimulated food consumption, and though it was less potent, it resembled pentobarbital both qualitatively and by having a parallel dose-response curve. The appetitive responses are discussed in relation to other pharmacologic actions of these hallucinogens in the dog, and consideration is given to the possible modes of action for phencyclidine- and SKF 10,047-induced anorexia.

Effects of phencyclidine on aggressive behavior in mice

The effects of phencyclidine on aggressive behavior in mice and the possible mechanism of action for these effects were examined. PCP at a dose of 10.0 mg/kg significantly decreased the number of attacks by resident mice toward intruders. Significant increases in the number of attacks by non-drugged residents toward the intruders who were given high doses of PCP (6.0 and 10.0 mg/kg) were observed. Only the higher doses of PCP (6.0 and 10.0 mg/kg) significantly increased the duration of locomotion. The increase in locomotion was dependent upon the time after administration of the drug. Hyperactivity was present at 30 minutes for both doses and hypoactivity was present at three hours after administration of 3.0 mg/kg. PCP did not significantly alter the frequency of attacks in an unfamiliar test locale. Pretreatment with haloperidol (1 mg/kg) partially blocked the PCP-induced hyperactivity but pretreatment with methysergide (3 mg/kg) did not. Neither haloperidol nor methysergide blocked the suppressive effects of PCP on aggressive behavior. It is concluded that PCP does not increase aggressive behavior in mice but high doses will decrease aggression. PCP-treated intruder animals provoke more aggression by non-drugged animals. PCP-induced hyperactivity appears to be mediated by dopaminergic systems.

The effects of aggregation on the lethality of phencyclidine in mice

The lethal effects of d-amphetamine and phencyclidine (PCP) were compared when adult male mice were placed into isolated or aggregated (12 mice/cage) conditions. The LD50 of d-amphetamine decreased approx. 30-fold between isolated (87.9 mg/kg) and aggregated (2.8 mg/kg) conditions. In contrast, PCP showed only a 1.3-fold increase in toxicity between isolated (64.5 mg/kg) and aggregated (48.4 mg/kg) conditions. These results suggest different mechanisms for the acute lethal effects of d-amphetamine and PCP in mice.

Phencyclidine-induced release of [3H]dopamine from chopped striatal tissue

Phencyclidine was examined for its ability to release [3H]dopamine ([3H]DA) from prelabelled chopped rat striatal tissue. A dynamic perfusion system was used in order to minimize the effects of drugs on uptake mechanisms. Cocaine and S-(+)-amphetamine were used to distinguish uptake inhibition from a neurotransmitter releasing action. Phencyclidine, starting at 3 microM caused a dose-dependent increase in efflux of [3H]DA from chopped striatal tissue. In this same preparation, cocaine, a known neuronal uptake inhibitor of dopamine, was unable to release [3H]DA except in the largest dose of 100 microM. S-(+)Amphetamine, a known releaser of neuronal dopamine, was found to be about ten times more potent then phencyclidine in causing a dose-dependent release of [3H]DA. The results of the above experiments are discussed in relation to the ability of phencyclidine to decrease the synaptosomal accumulation of [3H]DA. It is concluded that some of the psychoactive effects of phencyclidine may be due to the ability of phencyclidine to elicit a release of dopamine from dopaminergic neurons.

Phencyclidine intoxication: assessment of possible antidotes

Phencyclidine (PCP), a widely abused drug currently, has multiple pharmacological actions, including psychotomimetic [1], anesthetic [2], sympathomimetic [2], anticholinergic [3-7], and dopaminergic [8-10]. Similarly, PCP intoxication in man can present with diverse symptoms: schizophrenia-like delusions and hallucinations; mania; violence, dyskinetic, catatonic, or stereotyped movements; hypertension; and coma [11, 12]. There is general agreement that the treatment of PCP intoxication includes support of vital functions and acidification of the urine [13]. However, there is no known specific antidote for PCP toxicity. Although diazepam [13], haloperidol [14, 15], and chlorpromazine [16] have been reported to improve the agitation and psychotic symptoms caused by PCP, the therapeutic efficacy of these agents has rarely been documented with objective clinical measures. Recently we found that intramuscular physostigmine and haloperidol [17, 18] improved several symptoms of acute PCP intoxication as measured by the Brief Psychiatric Rating Scale (BPRS) [19].

Effect of phencyclidine on [3H]QNB binding

Intraperitoneal injection of phencyclidine before intravenous injection of [3H] Quinuclidinyl benzilate (QNB, 1.6 micrograms/kg) significantly increased the amount of radioactivity found in the brains of female C57BL/6J mice one hour after the 3H-QNB administration. This effect was found in hypothalamus, cortex, hippocampus and striatum and was decreased by pretreatment of the animal with atropine. The magnitude of the enhancement varied as a function of dose but did not change across the time span studied. These data are in contrast to our findings and those of others of inhibition of the specific binding of 3H-QNB to muscarinic cholinergic receptors by PCP in vitro. When atropine or PCP was administered in vivo and the tissue later analyzed in vitro, no effects of the drugs were observed on 3H-QNB binding. The reasons for the differences remain a matter of speculation.

Effects of naloxone, metenkephalin, and morphine on phencyclidine-induced behavior in the rat

The effects of naloxone, metenkephalin, and morphine were tested on phencyclidine(PCP)-induced stereotyped behaviors, ataxia, and hyperactivity in the rat. Naloxone (8 mg/kg) significantly decreased stereotypy, ataxia, and hyperactivity across all PCP doses tested (2.0, 4.0, and 6.0 mg/kg). Metenkephalin (40 micrograms/kg) and morphine (5 and 10 mg/kg) increased ataxia at the 4.0 and 6.0 mg/kg PCP doses. Stereotypy was altered by the opiates in a dose-dependent manner; enhanced by metenkephalin (40 micrograms/kg) at 2.0 mg/kg and inhibited by metenkephalin (40 micrograms/kg) and morphine (10 mg/kg) at 4.0 and 6.0 mg/kg PCP. Locomotor activity was increased by morphine (5 mg/kg) at 2 mg/kg PCP. These results suggest an involvement of central opiate receptor mechanisms in the mediation of PCP-induced behaviors in the rat.

A comparison of the effects of neuroleptics on phencyclidine-induced behaviors in the rat

The dose-response effects of neuroleptic pretreatment on phencyclidine (PCP; 3 or 5 mg/kg)-induced locomotor activity, stereotyped behaviors and ataxia were quantified in groups of male rats using rating scales recently developed in this laboratory. Three butyrophenone neuroleptics consistently produced dose-dependent antagonism of the behavioral effects of PCP administration. Fluphenazine antagonized the behavioral effects produced by 3 mg/kg PCP but not those produced by 5 mg/kg PCP. Each of the other neuroleptics examined (chlorpromazine, thioridazine, mesoridazine, triflupromazine, cis-flupenthixol) had no consistent antagonistic effect or actually enhanced one or more of the behavioral effects of PCP. Some neuroleptics slightly reduced PCP locomotion or stereotypies at high doses, but these effects were probably a non-specific consequence of the synergistic ataxia-producing properties of these drugs. In a second set of experiments, atropine sulfate pretreatment increased PCP-induced locomotor activity and stereotyped behaviors but had no effect on ataxia; pretreatment with physostigmine produced opposite effects. Combined pretreatment with haloperidol and atropine sulfate significantly reduced only haloperidol antagonism of PCP-induced ataxia, thus suggesting that non-dopoaminergic effects of neuroleptics may interfere with their ability to antagonize PCP.

Activation of striatal dopamine receptors induces pain inhibition in rats

In the rat, elevating dopamine content in corpus striatum with electrical stimulation of substantia nigra or direct administration of apomorphine (50-200 micrograms) into the lateral cerebral ventricle or apomorphine (2-10 microgram) into the caudate-putamen complex decreased pain sensitivity (as shown by an increase in the latency to hind-paw lick in the hot plate test). Furthermore, the decreased pain sensitivity after the central administration of apomorphine was antagonized by pretreatment with haloperidol (a dopamine antagonist). On the other hand, lowering dopamine content in corpus striatum with electrolytic destruction of substantia nigra and 6-hydroxydopamine lesions to the substantia nigra, as well as direct injection of haloperidol into the lateral cerebral ventricle or caudate-putamen complex increased pain sensitivity. The data indicate that activation of striatal dopamine receptors in rat brain induces pain inhibition.

Acute and chronic phencyclidine effects on locomotor activity, stereotypy and ataxia in rats

Behavioral rating scales, developed to measure phencyclidine (PCP)-induced stereotypy and ataxia in rats, were tested using acute dose-response and chronic paradigms with concomitant assessment of locomotor activity by automated counters. Also, effects of chronic PCP on apomorphine-induced stereotypy were assessed as a test of dopamine supersensitivity. A linear dose-response effect was found for measures of all three behaviors through moderate dose levels (2-6 mg/kg), but only ataxia ratings continued to increase, showing a linear relationship through the higher (8 and 10 mg/kg) doses. Chronic daily PCP administration showed progressive augmentation of stereotypy, tolerance to ataxia at 10 min post-injection, and a biphasic increase followed by decrease to day 1 levels in locomotor activity over 14 days. No significant change was found in apomorphine stereotypy following chronic PCP treatment. The chronic behavioral changes demonstrated in this study may provide a model of PCP-induced psychological and cognitive changes seen following chronic usage in man.

Stimulation of synaptosomal tyrosine hydroxylation by phencyclidine in vitro

Phencyclidine (PCP), a potent psychoactive drug, produces some animal behaviors that are believed to be mediated by dopaminergic and/or cholinergic neurons in the basal ganglia. In this study, we have monitored the effects of PCP in vitro on the synthesis, uptake, and release of dopamine (DA) in rat striatal synaptosomes. Using tyrosine hydroxylation as an index of DA synthesis, we observed a concentration-dependent stimulation of DA synthesis by PCP. The stimulatory effect was antagonized by reserpine (1 micro M) and was observed only when synaptosomes were preincubated under conditions which prevented the spontaneous release of [3H]DA. Two hydroxylated metabolites of PCP were also tested and found to have little effect on tyrosine hydroxylation. Like PCP these metabolites are potent inhibitors of synaptosomal [3H]DA uptake, but they apparently lack PCP's ability to release synaptosomal DA. Taken together, these results support our hypothesis that PCP stimulates synaptosomal DA synthesis by releasing DA from an inhibitory pool.