The effect of phentolamine on the pressor response to centrally administered clonidine in rats.

In the present study we investigate the effect of the previous injection of phentolamine (a nonspecific alpha-adrenergic antagonist) into the lateral hypothalamus (LH) and lateral ventricle (LV) on the pressor and bradycardic responses produced by the injection of clonidine (an alpha 2-adrenergic agonist) into these same areas of conscious rats. The injection of clonidine into the LV and LH produced pressor (39 +/- 5 and 38 +/- 3 mmHg, respectively) and bradycardic responses (-65 +/- 16 and -94 +/- 13 bpm, respectively). Previous injection of phentolamine into the LH or LV reduced the pressor response to clonidine injected into the same areas (delta MAP = 13 +/- 6 mmHg for LH and 1 +/- 3 mmHg for LV). No reduction was observed when clonidine was injected into the LV after the injection of phentolamine into the LH. No changes in bradycardic responses were observed after treatment with phentolamine. The present results show the participation of alpha-adrenergic receptors in the pressor response to centrally administered clonidine but not in the bradycardic response. The data also suggest that the pressor effect produced by the injection of clonidine into the LH is due to the activation of alpha-adrenergic receptors located specifically in this area. The pressor response after injection of clonidine into the LV and of phentolamine into the LH is due to the action of clonidine on other cerebral areas.