The orexin-1 receptor antagonist SB-334867 blocks the effects of antipsychotics on the activity of A9 and A10 dopamine neurons: implications for antipsychotic therapy.

Antipsychotic drugs alter the activity of dopamine neurons in the ventral tegmental area (A10) and substantia nigra pars compacta (A9). As there is a dense projection of orexin neurons from the lateral hypothalamus to A10 dopaminergic neurons, and some antipsychotics have been shown to increase the expression of c-fos in orexin-containing cells in the hypothalamus, we hypothesized that stimulation of orexin receptors plays a role in the effects of antipsychotics on the activity of A9 and A10 dopamine cells. Single-unit recordings in anesthetized rats demonstrated the central effects of the selective orexin-1 receptor antagonist SB-334867 (2 mg/kg, intravenous), as it reversed the excitatory effects of orexin-A administration (6 microg, intracerebroventricular) on the activity of locus coeruleus (LC) cells. Recordings from midbrain dopamine neurons showed that acute administration of SB-334867 alone did not alter the number of spontaneously active A9 or A10 cells, but did reverse: (1) the increase in the number of spontaneously active A9 and/or A10 dopamine cells caused by the acute administration of haloperidol (1 mg/kg, subcutaneous) or olanzapine (10 mg/kg, s.c.) and (2) the decrease in the number of spontaneously active A9 and/or A10 dopamine cells caused by the chronic administration of haloperidol (1 mg/kg/day x 21 days, s.c.) or olanzapine (10 mg/kg/day x 21 days, s.c.). However, SB-334867 did not block a different electrophysiological effect of olanzapine, as it did not block the olanzapine-induced activation of LC cells. These results indicate that activation of orexin-1 receptors plays an important role on the effects of antipsychotic drugs on dopamine neuronal activity and may play an important role in the clinical effects of antipsychotic drugs.