Receptor binding and antinociceptive properties of phencyclidine opiate-like derivatives

Phencyclidine-Based New Psychoactive Substances

The serendipitous discovery of phencyclidine (PCP) in 1956 sets the stage for significant research efforts that resulted in a plethora of analogs and derivatives designed to explore the biological effects of this class. PCP soon became the prototypical dissociative agent that eventually sneaked through the doors of clinical laboratories and became an established street drug. Estimations suggest that around 14 PCP analogs were identified as "street drugs" in the period between the 1960s and 1990s. Fast forward to the 2000s, and largely facilitated by advancements in electronic forms of communication made possible through the Internet, a variety of new PCP analogs began to attract the attention of communities interested in the collaborative exploration of these substances. Traditionally, as was the case with the first-generation analogs identified in previous decades, the substances explored represented compounds already known in the scientific literature. As the decade of the noughties unfolded, a number of new PCP-derived substances appeared on the scene, which included some analogs that have not been previously recorded in the published literature. The aim of this chapter is to present a brief introductory overview of substances that have materialized as PCP-derived new psychoactive substances (NPS) in recent years and their known pharmacology. Since N-methyl-D-aspartate receptor (NMDAR) antagonism is implicated in mediating the subjective and mind-altering effects of many dissociative drugs, additional data are included from other analogs not presently identified as NPS.

Opioid Properties of Some Isomeric Derivatives of Phencyclidine

The phencyclidine analogues (±)-α-, (±)-β-, and (+)-α- and (–)-α-4-hydroxy-3-methyl-4-phenyl-l-(1-phenylcyclohexyl)piperidine, all with known relative and absolute stereochemistry, have been prepared, and their analgesic potencies related to corresponding prodines. In contrast to the prodines, the (±)-α-phencyclidine analogue was a more potent analgesic than its diastereoisomer, while in agreement with observations in the prodine series, the 3R, 4S-α-enantiomer displayed substantially greater potency than its mirror image form.

Naloxone does not produce withdrawal hypothermia in chronically phencyclidine-treated rats

Naloxone-induced hypothermia is a sensitive indicator of mu-type opiate dependence in the rat. The present study investigated whether naloxone produces hypothermia in chronically PCP-treated rats. Rats were given daily injections of saline or PCP (10.0 mg/kg s.c.) for 7 days, and on day 8 challenged with naloxone (2.0 mg/kg s.c.). There were no differences between chronic saline- and PCP-treated subjects, indicating that the repeated administration of 10.0 mg/kg per day of PCP does not produce mu-type opiate dependence as reflected by naloxone-precipitated hypothermia.

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.

Phencyclidine blocks histamine H3-receptors in rat brain

Phencyclidine (PCP) was examined for its ability to modulate histamine release from rat brain slices labeled with L-[3H]histidine. PCP failed to mimic but completely reversed the autoinhibitory effect of histamine at H3-receptors with an apparent Ki value of 13 +/- 3 microM. A direct interaction of PCP with H3-autoreceptors rather than PCP or sigma receptor sites was confirmed by binding studies. PCP inhibited the binding of [3H](R)alpha-methylhistamine to H3-receptor sites in rat cerebral membranes with a Ki value of 25 +/- 2 microM. It is concluded that PCP is a H3-receptor antagonist of moderate potency.

The effects of the acute administration of phencyclidine hydrochloride (PCP) on the release of corticosterone, growth hormone and prolactin in the rat

There is little information on the neuroendocrine effects of PCP. The present study examined the effects of the acute subcutaneous administration of PCP on serum levels of corticosterone, growth hormone and prolactin in the male rat. PCP increased serum levels of corticosterone, decreased serum levels of prolactin and failed to affect growth hormone levels. The results indicate that, like other drugs of abuse, PCP alters neuroendocrine function.

Potentiation of phencyclidine-induced stereotyped behaviors in rats by thiorphan and bestatin

We investigated the effects of thiorphan, a specific inhibitor of enkephalinase A and bestatin, a specific inhibitor of aminopeptidase, on the stereotyped behaviors induced by phencyclidine (PCP) in cannulated rats. The PCP-induced turning was significantly potentiated when thiorphan (50 micrograms) and bestatin (50 micrograms) were injected simultaneously into the rat lateral ventricle. The increase of PCP-induced turning was completely antagonized by the pretreatment with naloxone. Thiorphan (50 micrograms) or bestatin (50 micrograms) alone failed to potentiate PCP-induced turning. Thiorphan and/or bestatin did not affect significantly the PCP-induced head weaving and back-pedalling except that thiorphan (50 micrograms) potentiated the PCP-induced head weaving 15-18 min after the PCP injection. The combination of thiorphan and bestatin alone did not induce any behavioral change. These results suggest that thiorphan and bestatin produce an increase of endogenous enkephalins and that as a result, PCP-induced turning may be enhanced.

A specific acylating agent for the [3H]phencyclidine receptors in rat brain

A derivative of phencyclidine (PCP, 1 in fig. 1) bearing an isothiocyanate moiety on the meta position of the aromatic ring (Metaphit, 3 in fig. 1) has been synthesized and identified as a rapid and specific site-directed acylating agent of the [3H]phencyclidine binding site in rat brain homogenates. The percentage of sites irreversibly inactivated by Metaphit was found to be the same in the hippocampus and striatum and the remaining sites were unaffected by Metaphit treatment under any conditions, suggesting that at least two distinct binding sites are present. An isomeric isothiocyanate derivative did not irreversibly inhibit [3H]phencyclidine receptors, indicating structural specificity for Metaphit in the inhibition of these receptors. The availability of Metaphit should greatly facilitate study of the structure and function of the phencyclidine receptors.

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.

Interaction of two phencyclidine opiate-like derivatives with 3H-opioid binding sites

Small modifications of the basic structure of phencyclidine have produced compounds with potent opioid analgesic actions. Detailed competition studies show that two of these phencyclidine derivatives, the 3'-hydroxy and the 4-phenyl-4-hydroxy analogs, displace 3H-opioid binding in a multiphasic manner. Approximately 25% of the total specific binding of all the radiolabeled opioids is displaced by low concentrations of the derivatives while the remainder of the binding is far less sensitive. The inclusion of these derivatives in saturation studies with [3H]dihydromorphine indicate that both compounds interact with highest affinity with mu1 sites. Furthermore, the prior in vivo administration of naloxonazine 24 h earlier reduces the analgesic potency of the 4-phenyl-4-hydroxy compound by 63% supporting a mu1 mechanism of action.

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.

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].

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.