Enhancement of haloperidol-induced increase in rat striatal or mesolimbic 3,4-dihydroxyphenylacetic acid and homovanillic acid by pretreatment with chronic methamphetamine.

After a drug-free period of 1 week following 2 weeks of haloperidol treatment, the increased response of striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to a challenge dose of haloperidol was significantly reduced. Tolerance to this effect was not, however, seen in the mesolimbic system. Pretreatment of the rats with methamphetamine (MAP) for 8 days prior to chronic haloperidol significantly enhanced the DOPAC and HVA increase produced by the challenge with haloperidol in both brain areas. The reduced response of striatal DOPAC or HVA after chronic haloperidol was prevented by pretreatment with MAP. The data suggest that the long-term dopamine receptor stimulation induced by MAP may antagonize the tolerance produced by chronic haloperidol treatment.