Pharmacological characterization of central alpha-adrenoceptors which mediate clonidine-induced locomotor hypoactivity in the developing rat.

The present experiment was designed to pharmacologically characterize receptors which mediate the clonidine-induced locomotor change in the developing rat. A subcutaneous injection of clonidine (0.78 mumol/kg) produced locomotor hyperactivity in 7-day-old rats but hypoactivity in 20-day-old rats. Phenoxybenzamine (1.5 mumol/kg, 5.9 mumol/kg and 15 mumol/kg) decreased spontaneous activity in a dose-dependent manner but did not antagonize clonidine-induced hypoactivity in 20-day-old rats. By contrast, the significant reversal of the clonidine-induced hypoactivity by pretreatment with phentolamine (1.6 mumol/kg and 6.3 mumol/kg), yohimbine (1.3 mumol/kg and 5.1 mumol/kg) and piperoxan (7.4 mumol/kg) was observed at such doses when the blockers did not cause any hypoactivity by themselves. It is suggested that clonidine could induce locomotor hypoactivity by activating presynaptic (alpha 2-type) alpha-adrenoceptors in the CNS of 20-day-old rat.