Effect of long-term haloperidol treatment on striatal neuropeptides: relation to stereotyped behavior.

Behavioral and biochemical responses to D1 and D2 dopamine (DA) agonists were used to evaluate the participation of striatal peptidergic mechanisms in the motor function alterations that attend chronic neuroleptic treatment. Rats, given haloperidol (1 mg/kg, i.c.) for 21 consecutive days, were randomly allocated to one of the following treatments: the D1 agonist SKF 38393, the D2 agonist quinpirole, their combination or saline. Stereotyped behavior and neuropeptide levels were evaluated after 5 days treatment and 4 days washout. Haloperidol increased most oral behaviors including licking, chewing and biting as well as striatal enkephalin and somatostatin levels. Subsequent treatment with SKF 38393 diminished the haloperidol-induced increase in licking and chewing; quinpirole reduced chewing behavior. The administration of both agonists together decreased chewing and biting. Neither DA agonist alone, nor their combination, reduced the haloperidol-induced increase in enkephalin levels. Both SKF 38393 and quinpirole, when given alone, tended to decrease the haloperidol-induced increase in somatostatin levels; when both D1 and D2 agonists were administered together, somatostatin levels declined significantly. These results suggest that somatostatin- but not enkephalin-containing striatal neurons contribute to the expression of haloperidol-induced stereotypies.