The 'presynaptic' blocking potency of sulpiride and haloperidol in the rat is age-dependent.

A low dose of pergolide (0.01 mg/kg) caused a marked inhibition of locomotor activity in male rats. Sulpiride significantly blocked the locomotor inhibition in young (6 weeks of age) rats already in low (2.5 mumol/kg) doses, while in adult (15 weeks of age) rats a blockade was achieved first with high 'postsynaptic' doses (50 mumol/kg). Haloperidol slightly blocked the effect of pergolide in both young and adult rats. However, the blocking action of haloperidol was only found in a narrow pharmacological 'window' (0.05 mumol/kg). This study illustrates the importance of considering age-related variations when using behavioural models of 'presynaptic' dopaminergic agonistic activity. It is suggested that 'presynaptic' dopaminergic events are modulated by hormonal changes during growth.