Early suppression of striatal cyclic GMP may predetermine the induction and severity of chronic haloperidol-induced vacous chewing movements.

Haloperidol persists in brain tissue long after discontinuation while haloperidol-induced tardive dyskinesia often worsens after withdrawal of the drug. The mechanism of haloperidol-associated tardive dyskinesia is unknown, although neurotoxic pathways are suspected. Nitric oxide (NO) synthase (NOS) inhibitors exacerbate haloperidol-induced catalepsy, while haloperidol itself is a potent neuronal NOS inhibitor in vitro. Since NO and cGMP are involved in striatal neural plasticity, this study investigates a possible relation between cGMP and extrapyramidal symptoms as early predictors of haloperidol-associated tardive dyskinesia. Sprague-Dawley rats were administered either water or oral haloperidol (0.25 mg/kg/d p.o.) for 17 weeks, followed by 3 weeks withdrawal. Saline (i.p.) or the nNOS/guanylate cyclase inhibitor, methylene blue (5 mg/kg/d i.p.), were co-administered with haloperidol for the first three weeks of treatment. Vacous chewing movements (VCM's) were continuously monitored, followed by the determination of striatal cGMP and peripheral serum nitrogen oxide (NOx) levels. Chronic haloperidol engendered significant VCM's, with acute withdrawal associated with significantly reduced striatal cGMP levels as well as reduced serum NOx. Furthermore, suppressed cGMP levels were maintained and VCM's were significantly worse after early administration of methylene blue to the chronic haloperidol group. However, serum NOx was unchanged from control. We conclude that the central effects of chronic haloperidol on striatal NO-cGMP function persist for up to 3 weeks post-withdrawal. Moreover, suppression of striatal cGMP constitutes an early neuronal insult that determines the presence and intensity of haloperidol-associated motor dysfunction.