Electrophysiological effects of phencyclidine on rat hippocampal pyramidal neurons

Electrophysiological effects of phencyclidine and the sigma agonist ditolylguanidine in the cerebellum of the rat

The electrophysiological actions of phencyclidine (PCP) and the sigma agonist 1,3-di(2tolyl)guanidine (DTG) were examined in the cerebellum of urethane-anesthetized rats. The object of the study was to determine if PCP and sigma agonists shared a common mechanism of action. The cerebellar Purkinje neuron was chosen because it has sigma receptors but not N-methyl-D-aspartate receptors, where PCP has additional effects. Both DTG and PCP decreased the spontaneous discharge rate of cerebellar Purkinje neurons after parenteral administration. When the drugs were applied locally to single Purkinje neurons, using pressure ejection through multibarrel micropipettes, both compounds decreased the spontaneous activity of the neurons with equal potency. Previous studies have shown that the actions of PCP in the cerebellum are dependent upon an interaction with noradrenergic terminals from the nucleus locus coeruleus. A similar finding was made in this study for DTG. Elimination of the noradrenergic input by lesion with the neurotoxin, 6-hydroxydopamine, diminished equally the effects of PCP and DTG. Treatment of the animals with haloperidol had similar effects. It is concluded that PCP and the sigma agonist DTG both act as indirect noradrenergic agonists in the cerebellum.

Facilitation of gamma-aminobutyric acid-induced depression by (+)PCMP and dexoxadrol in the cerebellar Purkinje neurons of the rat

The purpose of this experiment was to investigate the interaction of GABA (gamma aminobutyric acid) with PCP (phencyclidine) and sigma receptor agonists in the cerebellum. Drugs were applied directly to a single cerebellar Purkinje neuron of urethane-anesthetized rats, through a multibarrel pipette. The PCP receptor agonist, (+)PCMP [1-(-1-phenylcyclohexyl)-3-methyl piperidine], significantly enhanced GABA-induced inhibition. On the other hand, its stereoisomer, (-)PCMP, had no such modulatory effect. Dexoxadrol, a sigma receptor agonist, similar to (+)PCMP, potentiated GABA-induced depression. Its stereoisomer, levoxadrol, although inhibiting the spontaneous firings of Purkinje neurons, did not alter the effect of GABA. In conclusion, the findings indicate that the electrophysiological mechanisms of PCP-induced facilitation of GABA-induced reactions are similar to those triggered by sigma agonists in the cerebellum.

Differential effects of phencyclidine (PCP) and ketamine on mesocortical and mesostriatal dopamine release in vivo

The interactions of phencyclidine (PCP) with the mesocortical dopaminergic system were of interest because of the putative role of this pathway in the etiology of schizophrenia. In the present investigation we examined the effects of PCP, and PCP-receptor agonist, ketamine, on dopamine (DA) release by measuring the levels of 3-methoxytyramine (3-MT), the only DA metabolite which is a reliable indicator of DA release in vivo. PCP increased DA release in the amygdala, pyriform and prefrontal cortices, while ketamine was less potent than PCP in this respect. In contrast to the changes in DA release in the cortical regions, ketamine decreased DA release in striatum, while PCP did not change DA release.

Alterations in local cerebral glucose utilization induced by phencyclidine

The effects of phencyclidine (PCP; 0.5, 1,5, 10 mg/kg, i.v.) on local cerebral glucose utilization (LCGU) in the rat were studied with the 2-deoxy-D-[1-14C]glucose method. Significant findings were obtained in 41 of 87 brain regions of PCP-treated rats (25-270% of control). Rates of LCGU increased throughout the limbic system, except the habenula. Although LCGU increased in most sensory structures, it decreased in specific layers of the somatosensory and auditory cortices and the inferior colliculus. Evidence was seen for dissociation between LCGU responses of specific thalamic relay areas and their terminal fields in the cortex. Increases in LCGU occurred throughout the motor system, manifesting a striking pattern of columnar activity in the motor cortex. However, LCGU was reduced in the frontal cortical pole. Elevated LCGU was observed in the pontine nuclei and the nuclei and the nucleus solitarius. Effects of 5 mg/kg PCP diminished with time although 8 regions maintained a metabolic alteration at 180 min. PCP induced several behaviors, including stereotypies, which varied with the dose and time after drug administration. The results demonstrate a PCP-induced activation of various functional circuits in the brain, especially the limbic system, and may provide a physiological basis for PCP's psychotomimetic properties.

Phencyclidine. Physiological actions, interactions with excitatory amino acids and endogenous ligands

Phenycyclidine (PCP) produces many profound effects in the central nervous system. PCP has numerous behavioral and neurochemical effects such as inhibiting the uptake and facilitating the release of dopamine, serotonin, and norepinephrine. PCP also interacts with sigma, mu opioid, muscarinic, and nicotinic receptors. However, the psychotomimetic effects induced by PCP are believed to be mediated by specific PCP receptors, where PCP binds with greater potency than sigma compounds. Electrophysiological, behavioral, and neuro-chemical evidence strongly suggests that at least some of the many PCP actions result from antagonism of excitatory amino acid-induced responses via PCP receptors. The recent isolation and partial characterization of the alpha and beta endopsychosins and the identification of other endogenous ligands for the PCP and sigma receptors, is another promising area of research in the elucidation of the physiological role of an endogenous PCP and sigma system.

Anatomical and electrophysiological evidence for a direct projection from Ammon's horn to the medial prefrontal cortex in the rat

Following microinjection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the medial prefrontal cortex (defined as the neocortical area innervated by the thalamic mediodorsal nucleus) labelled cells were observed in the pyramidal layer of the CA1 field of Ammon's horn. Observations made using antidromic stimulation confirmed these results, and revealed that slow conduction velocity of the fibers of the hippocampal cells innervating the prefrontal cortex. Taken together, these data provide evidence for a direct projection of CA1 cells to the medial prefrontal cortex.

Phencyclidine (PCP) receptors: autoradiographic localization in brain with the selective ligand, [3H]TCP

Receptor binding sites for the phencyclidine (PCP) analogue, [3H]TCP, have been localized in the rat and guinea pig central nervous systems by in vitro autoradiography. Quantitation of [3H]TCP binding site densities in rat brain reveals highest levels in the forebrain, in particular the strata oriens and radiatum of the hippocampus, the molecular layer of the dentate gyrus and superficial layers of the cerebral cortex. Moderate levels of binding occur in the amygdala, thalamus, anterior olfactory nucleus, external plexiform layer of the olfactory bulb, olfactory tubercle, geniculate nuclei and deep layers of the cortex. Low levels of binding occur throughout most of the septum, diagonal band, hypothalamus, pons-medulla and cerebellum. Spinal cord grey matter also has low levels of binding. Excitotoxin lesions of the hippocampal formation, which destroy the pyramidal and granule cells, reduce the binding of [3H]TCP to strata radiatum and oriens and the molecular layer of the dentate gyrus by 60% suggesting that [3H]TCP labels intrinsic neurons in these regions. Residual binding is probably on afferent terminals. Ibotenic acid lesions of the caudate-putamen reduce [3H]TCP binding by 70%, indicating that binding sites are localized on intrinsic striatal neurons. 6-Hydroxydopamine lesions do not alter [3H]TCP binding levels in the caudate, suggesting the absence of binding sites on dopaminergic terminals in the caudate.

Interactions of phencyclidine with hippocampal circuitry: evidence for neuronal heterogeneity

The discovery of phencyclidine (PCP) receptors has stimulated the search for specific PCP antagonists. A direct product of this research is metaphit, an irreversible PCP ligand, which has recently been synthesized. In this study we examined the effects of metaphit on the responses of hippocampal neurons to PCP. On the basis of unfiltered action potential durations, hippocampal cells were divided into two groups, complex-spike cells and theta neurons. Local application of PCP caused inhibitions of the spontaneous firing rates of complex-spike cells. Metaphit, locally applied, antagonized approximately 50% of these responses, while the remaining responses were unaffected. In contrast, PCP caused increases in the spontaneous firing rates of theta cells and in almost all cases, these responses to PCP were attenuated by metaphit administration. These effects of metaphit were specific for PCP as the responses to locally applied norepinephrine were not altered by metaphit. The data suggest two mechanisms of action of PCP in the hippocampus. In addition, these mechanisms may be localized in part to different cell types.

Antagonism of phencyclidine action by metaphit in rat cerebellar Purkinje neurons: an electrophysiological study

Metaphit (1-[1-(3-isothiocyanatophenyl)-cyclohexyl]piperidine), a derivative of the psychotomimetic drug phencyclidine (PCP), is postulated to bind irreversibly to PCP receptors. We examined here the electrophysiological interactions of metaphit with PCP in rat cerebellar cortex, since a specific effect of PCP on cerebellar neuronal circuitry has been shown. Metaphit, applied locally to Purkinje neurons by micropressure ejection through multibarreled micropipettes, has a reversible depressant action lasting for 5-20 min. Following this, PCP-induced inhibition is blocked with no recovery despite repeated applications of PCP for over an hour. This blockade was not seen unless the dose of metaphit was sufficient to transiently depress Purkinje neuron discharge. Metaphit does not antagonize inhibitory effects of locally applied norepinephrine or gamma-aminobutyric acid. This electrophysiological data suggests that metaphit is an irreversible antagonist of PCP in the cerebellum.

Effects of phencyclidine on ventral tegmental A10 dopamine neurons in the rat

Extracellular single unit recordings were used to determine the effects of systemically and iontophoretically applied phencyclidine on electrophysiologically-identified A10 dopamine neurons within the ventral tegmental area of the rat. Intravenous injections of phencyclidine inhibited, as well as excited A10 cells. Approximately 42% of the inhibitions induced by phencyclidine were completely reversed by an injection of haloperidol. The excitatory effects of phencyclidine most often resulted in a state of depolarized inactivation and were not sensitive to haloperidol. The alteration of the activity of A10 cells by systemically-applied phencyclidine was effectively prevented in all animals pretreated with a combination of reserpine and alpha-methyl-p-tyrosine, which depleted stores of central catecholamines by over 90%. In contrast to the data obtained with intravenous injections, iontophoretic applications of phencyclidine produced only inhibition of the activity of the A10 cells and, in a few of these cases the decreased firing rate was accompanied by an increase in the amplitude of the action potential. A comparison of the response patterns of dopaminergic neurons to systemically- and iontophoretically-applied phencyclidine would suggest that excitations induced by phencyclidine are not mediated at the level of the A10 cell bodies but through a site outside the ventral tegmental area. The results of this study also indicate that some of the effects of phencyclidine on the activity of A10 neurons are clearly dependent upon an interaction with dopamine and thus, would support the hypothesis that phencyclidine can act as an indirect dopamine agonist. However, other effects of phencyclidine which are not apparently linked to dopamine may represent the response of a pharmacologically-distinct subpopulation of A10 neurons to phencyclidine.

Structure-activity relationships of phencyclidine derivatives in rat cerebellum

The depressant effects of phencyclidine [1-(1-phenylcyclohexyl) piperidine, PCP] and three of its analogs (m-amino-PCP, m-nitro-PCP, and PCP-methyliodide) on the spontaneous action potential discharge of cerebellar Purkinje neurons in urethane-anesthetized rats were examined in this study. Both intraperitoneal injection and micro-pressure ejection were employed as routes of drug administration. The relative potency after parenteral administration corresponded closely with previous findings in behavioral test paradigms. PCP and m-amino-PCP were equipotent, m-nitro PCP was less potent than either PCP or m-amino-PCP, and PCP-methyliodide showed almost no activity. After local administration onto neurons, m-amino-PCP was significantly more potent than PCP, while PCP, m-nitro-PCP, and PCP-methyliodide were equipotent. Tritiated PCP, m-nitro PCP, and m-amino PCP have similar distribution and metabolism in cerebellum. PCP-methyliodide, a quaternary ion, does not cross the blood brain barrier. M-nitro PCP is appreciably less ionized at pH 7.4 than PCP or m-amino-PCP and, therefore, may be more easily sequestered into lipids. Differences between PCP and its analogs found in experiments which employ parenteral administration may reflect differences in drug distribution. These differences are minimized when these drugs are administered directly onto neurons via pressure microejection.

Phencyclidine (PCP): effects of acute and chronic administration on EEG activities in the rhesus monkey

Effects of acute and chronic administration of phencyclidine (PCP) on EEG activities and gross behavior were studied in monkeys with electrodes implanted in the brain. Administration of PCP (2.0 or 4.0 mg/kg i.v.) in monkeys produced a biphasic pattern of inhibition and excitation of behavior during 6-8 h observation period. The inhibitory phase appeared 1-3 min after PCP injection, and was uniquely characterized by high-voltage slow waves with delta waves (0.6-0.8 c/sec) in the parietal lobe and by those with theta waves (4-5 c/sec) in the occipital lobe and hippocampus during behavioral stupor. This inhibitory phase lasted 1-1.5 h, and subsequently, high-voltage fast wave with enhancement of theta waves and nystagmus appeared during behavioral arousal. Upon chronic PCP administration, the intensity and duration of the inhibitory phase progressively decreased, while the effects of the excitatory phase increased. The results suggest that PCP administration in monkeys produces a biphasic pattern of inhibitory and excitatory effects and upon chronic administration a tolerance develops to the inhibitory effect while an augmentation of the excitatory effect develops.

Phencyclidine (PCP): effects on limbic afterdischarges in the rhesus monkey

Effects of phencyclidine (PCP) on eliciting afterdischarges (AD) by electrical stimulation of the hippocampus (HIPP), amygdala (AMY), thalamus (CM) and midbrain reticular formation (MRF) were studied in rhesus monkeys with permanently implanted encephalic electrodes. Prior to drug administration, the minimal amount of electrical stimulation for eliciting AD, i.e., minimal AD's stimulation (MADS), was determined for each stimulation site and employed as a parameter for evaluating the effects of PCP on AD. Following PCP administration (0.5 mg/kg IM), marked increases (130 to 240% above the control level) in MADS of the HIPP, AMY and MRF were demonstrated, while only slight increase (+40%) in MADS of the CM was found. These results suggest that PCP causes a marked decrement in the ability of electrical stimulation to elicit AD in the HIPP, AMY and MRF, along with only little similar stimulation effects in the CM. This selective increase in MADS in structures of the limbic system and the midbrain reticular formation tends to implicate these systems rather than the thalamocortical system in the modulation of PCP's effects on central nervous system activities.

Evidence for an endogenous peptide ligand for the phencyclidine receptor

Porcine brain contained an active factor that competed with [3H]-phencyclidine (PCP) for binding to rat brain membranes. On reverse phase high pressure liquid chromatography, the active material eluted between 38-42% acetonitrile. Gel filtration chromatography of the factor predicted a molecular weight of approximately 3000 daltons. The endogenous substance appeared to be selective for PCP receptors as it did not interact with either benzodiazepine, neurotensin, nor with mu, delta, or kappa opioid receptors. The active material showed a heterogenous distribution in brain, with highest concentrations found in hippocampus and cortex. It is likely to be a small peptide since various proteases eliminated or markedly reduced the potency of the compound in a [3H]-PCP binding assay. The material also possessed PCP-like activity in two bioassays. Like PCP, it induced contralateral rotational behavior after unilateral intranigral injection and depressed spontaneous cell activity after iontophoretic micropressure application in hippocampus and cerebral cortex. Thus, this small peptide is likely to be an endogenous ligand for the PCP receptor.

Phencyclidine (PCP): effects on auditory evoked potentials in the rhesus monkey

Effects of acute and chronic administration of phencyclidine (PCP) on auditory evoked potentials (AEPs) were studied in rhesus monkeys with encephalic electrodes permanently implanted. AEPs were evoked by single clicks (1.0 pps in frequency) generated by an externally triggered audiomonitor. Single PCP injections (dose, 2.0 or 4.0 mg/kg IV) in monkeys produced a striking distortion in the amplitudes of major AEP components. This AEP distortion was manifested in superior temporal cortex (ST) by a complex pattern of amplitude reduction and enhancement, and was manifested in the medial geniculate body (MGB) by amplitude reduction. These effects persisted for 4 to 5 hours. Upon chronic administration (dose, 4.0 mg/kg IV daily), the effects of PCP on AEP components in ST were enhanced while little change was noted in its effect on AEP components in MGB. These results suggest that PCP administration in rhesus monkeys causes a significant disturbance of AEPs within central nervous system structures associated with auditory processing (i.e., ST and MGB), and that chronic PCP administration enhances its effects on AEPs predominantly in the primary auditory cortex while having little additional effects on AEPs from the subcortical auditory pathways.

Differential effects of phencyclidine upon hippocampal complex-spike and theta neurons

The effects of local application of phencyclidine (PCP) upon hippocampal CA1 neurons were investigated in urethane-anesthetized rats. Hippocampal neurons were classified on the basis of extracellularly recorded action potential duration as either complex-spike or theta cells prior to PCP administration. PCP depressed spontaneous firing of 46 of 48 complex-spike cells, but excited 12 of 13 theta neurons. This result demonstrates that hippocampal complex-spike and theta neurons may be differentiated of theta neurons were greatly attenuated or absent in rats pretreated with DSP4, a neurotoxin which selectively destroys noradrenergic pathways. This latter finding lends additional support to the hypothesis that the effects of locally applied PCP are mediated via noradrenergic mechanisms.

Effects of the selective noradrenergic neurotoxin DSP4 on cerebellar Purkinje neuron electrophysiology

The effect of the adrenergic neurotoxin DSP4 on cerebellar electrophysiology was studied in the rat. DSP4, administered parenterally, depleted cerebellar norepinephrine by 76%. The depressant response of cerebellar Purkinje neurons to phencyclidine, a drug which acts on adrenergic presynaptic terminals to release NE, was markedly reduced after DSP4 pretreatment. In contrast with 6OHDA, which increased firing rates of the Purkinje cells, DSP4 did not change the rate or pattern of Purkinje cell discharge. Taken together these results suggest that DSP4 may be a valuable tool for studying central adrenergic pathways, but that this drug has properties which differ from 6OHDA.

Electrophysiological responses to adenosine analogs in rat hippocampus and cerebellum: evidence for mediation by adenosine receptors of the A1 subtype

Adenosine has profound depressant effects upon the electrophysiological activity of the brain, but the adenosine receptor subtypes which mediate these responses are uncertain. In order to resolve this question, we have characterized the effects of two adenosine analogs which differ in their relative potencies at adenosine A1 and A2 receptors. The effects of these adenosine analogs were examined on spontaneous firing rate of Purkinje neurons in the rat cerebellum in situ, in cerebellar brain slices in vitro, and on synaptic transmission in the rat hippocampus in vitro. Although the A2 agonist appeared to be more potent with local drug application techniques in situ, our in vitro results suggest that the A1 receptor subtype is involved in the electrophysiological actions of these drugs in both rat cerebellum and hippocampus. Furthermore, these data indicate that the physical properties of some adenosine analogs may reduce apparent drug potencies when they are studied with local application techniques.

Effect of phencyclidine on inhibition in the hippocampal slice

The effects of phencyclidine (PCP) on synaptic transmission were studied in the hippocampal slice. Population spikes evoked by orthodromic or antidromic stimulation were recorded from CAl pyramidal cells. Bath applied PCP (10(-4) M) reduced moderately both the orthodromic and antidromic population spikes. Lower concentrations, 5 X 10(-6) to 5 X 10(-5) M of PCP, which did not depress the population spikes, reduced inhibition of the orthodromically evoked spike in a dose dependent reversible manner. Diazepam (10(-6) to 10(-5) M) restored the inhibition despite the continued presence of PCP. It is suggested that PCP-induced seizures and other signs of hyperexcitability could be a result of reduced inhibition.

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.

Effects of phencyclidine (PCP) on the visual evoked potentials in the rhesus monkey

Effects of acute and chronic administration of phencyclidine (PCP) on both neocortical and subcortical visual potentials (VEPs) and on spontaneous EEGs were studied in the rhesus monkeys with permanently implanted brain electrodes. VEPs were evoked by brief single photo-stimulator flashes (0.8 pps. 10 microsec duration). Injection of PCP (0.5 to 4.0 mg/kg doses, IV) in monkeys produced a significant inhibition on the peak-amplitude of major VEP components predominantly in the occipital lobe and hippocampus. The PCP-induced VEP inhibition persisted in the presence of occipital and hippocampal theta-activities. Nystagmus persisted throughout the 6 to 8 hours course of PCP-induced behavior. A biphasic pattern of inhibitory and excitatory effects on EEGs and behavior was also observed during the 6 to 8 hours observation period. Chronic administration of PCP (2.0 and 4.0 mg/kg dose IV daily) produced a significant decrease in its inhibitory effects on VEPs, suggesting tolerance development to the inhibitory effect on VEPs. The results suggest that the hippocampus has important implications in the modulation of PCP effects on CNS activities related to the visual function of the rhesus monkey.

Effect of phencyclidines on hippocampal pyramidal cells

Phencyclidine (PCP) and several behaviorally active or inactive structural analogs were administered i.v. to urethane-anesthetized rats in order to determine their effects on CA1 pyramidal cell discharges elicited by contralateral CA3 (cCA3) stimulation. PCP and the behaviorally active m-amino derivative (m-NH2 PCP) depressed, in a dose-dependent manner, the amplitude of the population spike evoked in CA1 by a cCA3 stimulation (ED 50s: 0.9 mg/kg for PCP, 0.5 mg/kg for m-NH2 PCP). However, the behaviorally inactive derivatives m-nitro (m-NO2 PCP) and PCP methyliodide (PCP CH3I) were ineffective up to 10 mg/kg. PCP (0.1-0.3 mg/kg i.v.) also decreased the duration of inhibition of CA1 discharges in a paired-stimulus paradigm; this was in contrast to the effects of thiopental and diazepam. In midcollicular-transected, urethane-anesthetized rats, the inhibitory effect of PCP on cCA3-CA1 transmission was not observed but the drug was still as effective as in intact rats in the paired-stimulus paradigm. In animals subjected to 6-hydroxydopamine lesions of the hippocampal noradrenergic innervation (average 85%) decrease in NE content), the potency of PCP in inhibiting cCA3-CA1 transmission was the same as in a group of sham-operated controls. These results suggest the following conclusions: (i) PCP exerts at least 2 separate types of effects in CA1, both of which result from a central action of the drug; (ii) PCP decreases the monosynaptic excitation of CA1 pyramidal cells and this action requires the integrity of brainstem afferents; (iii) PCP may decrease recurrent inhibition or afterhyperpolarization in CA1 via a mechanism which is independent of these connections and, therefore, could result from a direct action of the drug at the level of the hippocampus; (iv) finally, no evidence was found to suggest that the noradrenergic innervation of the hippocampus is critically involved in the action of PCP on CA1 discharges.

Interactions of a neuroleptic drug (fluphenazine) with catecholamines in hippocampus

The interactions of fluphenazine with the electrophysiological responses to catecholamines were studied in the rat hippocampus and parietal cortex. In the in vitro hippocampal slice, changes in synaptically evoked responses induced by norepinephrine, isoproterenol and dopamine were not altered by superfusion of fluphenazine. Both alpha- and beta- components of adrenergic responses were unaffected by neuroleptic administration in this preparation. Similarly, alterations in the spontaneous firing of single hippocampal pyramidal neurons in situ to adrenergic agonists or dopamine were not affected by local fluphenazine and administration using pressure ejection through multibarreled micropipettes. In contrast, norepinephrine- or isoproterenol-induced inhibitions of parietal cortical neurons in situ were potently antagonized by fluphenazine. A similar interaction was observed from a hippocampal basket neuron. It is concluded that while fluphenazine can antagonize well-defined noradrenergic effects in some brain regions (e. g., cerebellum, cortex), this property is not generalized to all brain regions receiving noradrenergic input.