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

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Abstract
Summary 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 μmol/kg) produced locomotor hyperactivity in 7-day-old rats but hypoactivity in 20-day-old rats. Phenoxybenzamine (1.5 μmol/kg, 5.9 μmol/kg and 15 μmol/kg) decreased spontaneous activity in a dosedependent manner but did not antagonize clonidineinduced hypoactivity in 20-day-old rats. By contrast, the significant reversal of the clonidine-induced hypoactivity by pretreatment with phentolamine (1.6 μmol/kg and 6.3 μmol/kg), yohimbine (1.3 μmol/kg and 5.1 μmol/kg) and piperoxan (7.4 μmol/kg) was observed at such doses when the blockers did not cause and hypoactivity by themselves. It is suggested that clonidine could induce locomotor hypoactivity by activating presynaptic (α1-type) α-adrenoceptors in the CNS of 20-day-old rat.