Non-competitive N-methyl-d-aspartate receptor antagonists, including phencyclidine, ketamine, and MK801, produce vacuoles and induce the hsp70 stress gene in layer III pyramidal neurons of the rat cingulate cortex. This study shows that phencyclidine (50 mg/kg) induces hsp70 messenger RNA and HSP70 stress protein primarily in pyramidal neurons in posterior cingulate and retrosplenial cortex, neocortex, insular cortex, piriform cortex, hippocampus, and in the basal nuclei of the amygdala. Several neurotransmitter receptor antagonists inhibited induction of HSP70 produced by phencyclidine (50 mg/kg): haloperidol (ed50 = 0.8 mg/kg), clozapine (ed50 = 1 mg/kg), valium (ed50 = l mg/kg), SCH 23390 (ed50 = 7 mg/kg) and muscimol (ed50 = 3 mg/kg). Baclofen had no effect. Nifedipine blocked the induction of HSP70 produced by phencyclidine in some regions (cingulate, neocortex, insular cortex) but only partially blocked HSP70 induction in other regions (piriform cortex, amygdala). These results suggest that phencyclidine injures pyramidal neurons via dopamine D1, D2, D4, sigma and other receptors. Several factors appear to contribute to this unusual multi-receptor mediated injury. (1) Phencyclidine blocks N-methyl-d-aspartate receptors on GABAergic interneurons resulting in decreased inhibition of pyramidal neurons. This may help to explain why multiple excitatory receptors mediate the injury and why GABAA agonists decrease the injury produced by phencyclidine. (2) Phencyclidine blockade of an amine transporter helps explain why dopamine receptor antagonists ameliorate injury. (3) Phencyclidine depolarizes neurons and produces high, potentially damaging intracellular calcium levels probably by blocking K+ channels that may be linked to sigma receptors. Since nifedipine prevents injury in cingulate, insula, and neocortex, it appears that calcium entry through L-type voltage gated calcium channels plays a role in the pyramidal neuronal injury produced by phencyclidine in these regions. There are similarities between the cingulate neurons injured by phencyclidine and circuits recently hypothesized to explain receptor changes in cingulate gyrus of schizophrenic patients. The present and previous studies also provide approaches for decreasing the clinical side effects of N-methyl-d-aspartate receptor antagonists to facilitate their possible use in the treatment of ischemia and other disorders.
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