Blockade of long-term potentiation by phencyclidine and sigma opiates in the hippocampus in vivo and in vitro.

Long-term potentiation (LTP) of synaptic transmission in the rat hippocampus in vivo and in vitro, was studied using field potentials. Pretreatment with phencyclidine (PCP) or 'sigma' opiates blocked LTP in vivo while mu and kappa opiates and the antagonist naloxone were ineffective. Scopolamine (20 mg/kg i.p.) neither prevented LTP nor antagonized the LTP-blocking effect of PCP. In vitro, PCP up to 100 microM did not alter synaptic activation of CA1 pyramidal cells by stratum radiatum stimulation but blocked LTP in a dose-dependent manner (ED50: 3 microM). The sigma opiate, cyclazocine, also prevented the induction of LTP in vitro while morphine and procaine were ineffective.