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Opioid receptors comprise a heterogeneous group that can be divided into at least four biochemically and topographically distinct subtypes, designated (μ, κ, σ, and δ. Martin et al. (1976) proposed the existence of μ-, κ-, and α-opioid receptors based on observed differences in pharmacological profiles of drugs seen with variably selective opioid agonists and antagonists. Other groups have provided evidence for the 8 subtype (Hughes et al., 1975).

Phencyclidine (1-(1-cyclohexylphenyl)piperidine) was synthesized in 1957 by the Parke Davis company. Phencyclidine (PCP) is an excellent general anesthetic because it produces anesthesia with analgesia, while producing minimal cardiorespiratory depression. However, in clinical trials PCP was found to produce postanesthetic hallucinations, which could last more than 12 hours, in about 30% of patients [1]. Clinical use of PCP in humans was discontinued. Later, PCP was approved for use as a veterinary anesthetic and an analog, ketamine, was approved for use in humans. Although ketamine can produce psychotomimetic side effects, these are not as prolonged as with PCP.