Clonidine antagonizes pressor effect of N-methyl-D-aspartate in the rostral ventrolateral medulla of rats

The purpose of the present study was to investigate the modulatory actions of adrenoreceptor agonists on N-methyl-D-aspartate (NMDA)-induced pressor effect in rostral ventrolateral medulla (RVLM). These drugs were locally applied into RVLM of urethane-anesthetized Sprague-Dawley rats through multibarrel pipettes. Microinjection of NMDA increased the arterial pressure, an effect which was abolished by pretreatment with clonidine, whereas neither the β-adrenergic agonist isoproterenol nor the α1-adrenergic agonist phenylephrine did alter this pressor response. Previous experiments demonstrated that clonidine binds to noradrenergic α2 and imidazoline receptors in the RVLM. Norepinephrine, which has high affinity for the α2 receptor and low affinity to the imidazoline receptor, partially antagonized NMDA-induced hypertension. On the other hand, administration of selective imidazoline receptor antagonist idazoxan partially reversed clonidine-mediated antagonism of NMDA. Taken together, these results suggest th...