Central actions of norepinephrine, phentolamine and 6-hydroxydopamine in spontaneously hypertensive rats.

The effects of norepinephrine HCl, isoproterenol HCl, phentolamine and 6-hydroxydopamine HCl on mean arterial blood pressure (MABP), heart rate (HR), and pulmonary respiration were examined in 67 urethane-anesthetized, spontaneously hypertensive rats (SHR) with carotid arterial and femoral venous catheters and stereotaxically positioned intracerebral cannulae. Norepinephrine produced significant pressor responses when it was instilled into the posterior hypothalamic region and/or the pars magnocellularis region of the ventral medial thalamus. Central pretreatment with either phentolamine mesylate or phentolamine HCl, 30 min before central administration of norepinephrine, blocked the norepinephrine-induced pressor responses. Central administration of 6-hydroxydopamine HCl in the hypothalamic-thalamic region of the diencephalon produced significant depressor responses within 60 min. Neither norepinephrine nor isoprotereol produced their centrally-induced pressor and depressor responses when they were instilled centrally into the reticular formation. The results suggest that receptors that respond to norepinephrine are located in the posterior hypothalamic-ventral medial thalamic region of the diencephalon. The results also suggest that cells located in these areas send projection fibers either directly or indirectly to the vasomotor center located in the brain stem.