Decreased sensory responsiveness of noradrenergic neurons in the rat locus coeruleus following phencyclidine or dizocilpine (MK-801): role of NMDA antagonism.

The effects of the schizophrenomimetic compound phencyclidine (PCP) on baseline activity and sensory-evoked responses of noradrenergic locus coeruleus neurons were studied with extracellular single-cell recording techniques in the chloral hydrate-anaesthetized male albino rat. PCP dose-dependently decreased firing rate, induced a more regular firing pattern of the neurons, and decreased neuronal responses to a peripheral sensory stimulus (electrical stimulation of the hindpaw). These effects of PCP were significantly decreased by pretreatment with reserpine or yohimbine, indicating that the effects of PCP were largely indirect and mediated through noradrenaline, i.e. by inhibition of its re-uptake, resulting in stimulation of alpha 2 autoreceptors. The effects of PCP were, however, mimicked by dizocilpine (MK-801), a selective non-competitive antagonist at excitatory amino acid receptors of the N-methyl-D-aspartate (NMDA) sub-type, suggesting a role also for NMDA receptors in the suppression of sensory responsiveness of locus coeruleus neurons by PCP. In view of the purported physiological role of the locus coeruleus, this effect of PCP may well contribute to the psychotomimetic properties of the drug.