Differential changes in dopaminergic receptor sensitivity induced by agonist drugs.

Mice were treated acutely or chronically with apomorphine (APO) (0.1 and 1.0 mg/kg) or L-DOPA (150 mg/kg) plus benserazide (50 mg/kg). The decrease of homovanillic acid (HVA) caused by APO was greater in acutely treated animals. After APO or L-DOPA pretreatment the animals received single doses of haloperidol (HALO) (1.5 mg/kg) or saline and were submitted to behavioral and biochemical analysis in order to determine changes in dopaminergic receptor sensitivity. When the acutely and chronically treated groups were compared, we found no difference in the cataleptogenic effect of HALO nor any difference in the effect of HALO in increasing brain HVA levels. The data suggest that chronic treatment with dopaminergic agonists leads to changes in receptor sensitivity for the agonist but not for the antagonist effect.