Blockade of the serotonergic system counteracts the dizocilpine-induced changes in dopaminergic function.

The administration of dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist acting at the associated ion channel, increased the grooming time induced in rats by the D1 dopamine receptor agonist SKF 38393 and the stereotyped behaviour elicited by the D1/D2 dopamine receptor agonist apomorphine, and reduced the locomotor response to the D2 dopamine receptor agonist quinpirole. This supports the view that glutamate deficiency plays an important role in the pathogenesis of schizophrenia by altering the balance between glutamatergic and dopaminergic systems. Blockade of serotonin receptors counteracted the effect of dizocilpine on dopaminergic responses. Both the non-selective 5HT1/5HT2 antagonist methysergide, and ketanserin, which more specifically blocks 5HT2 receptors, given at doses inhibiting serotonin-mediated behaviours but which did not affect spontaneous motility and dopaminergic behaviours, hampered the dizocilpine-induced potentiation of responses elicited by the stimulation of D1 or D1/D2 dopamine receptors and counteracted the dizocilpine-induced reduction of hyperactivity observed following quinpirole administration. The results suggest that the functional integrity of the serotonergic system is fundamental for the occurrence of dopaminergic changes resulting from non-competitive NMDA blockade.