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

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