The effect of alpha-adrenoceptor antagonists and metiamide on clonidine-induced locomotor stimulation in the infant rat.

1 Subcutaneous injections of clonidine (3.9 X 10(-8) mol/kg to 3.9 X 10(-6) mol/kg) produced forward locomotion and wall climbing in 7-day-old rats in a dose-dependent manner. 2 The effect was reduced significantly by a preceding intraperitoneal injection of phentolamine (7.9 X 10(-6) mol/kg), phenoxybenzamine (7.4 X 10(-6) mol/kg), yohimbine (1.3 X 10(-6) mol/kg) or piperoxan (7.4 X 10(-6) mol/kg). 3 The pA2-values of the antagonists to the clonidine-induced locomotor hyperactivity were: 5.1 (phenoxybenzamine), 5.2 (phentolamine), 6.4 (yohimbine) and 6.0 (piperoxan). 4 Metiamide (2.5 X 10(-4) mol/kg, 5.0 X 10(-4) mol/kg and 1.0 X 10(-3) mol/kg), a histamine H2-receptor blocker, did not affect the clonidine-induced locomotor stimulation. 5 It is suggested that the receptors which mediate clonidine-induced locomotor stimulation could be alpha-adrenoceptors but not histamine H2-receptors in the central nervous system of the infant rat.