THE EFFECT OF α‐ADRENOCEPTOR ANTAGONISTS AND METIAMIDE ON CLONIDINE‐INDUCED LOCOMOTOR STIMULATION IN THE INFANT RAT

Abstract

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 pA 2 ‐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 H 2 ‐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 H 2 ‐receptors in the central nervous system of the infant rat.