Cardiovascular responses to microinjection of AMPA into the rostral commissural nucleus tractus solitarius of awake rats.

Previous studies have shown that microinjection of NMDA or non-NMDA receptor agonists into the nucleus tractus solitarius of anesthetized rats produces bradycardia and hypotension. In the present study, we evaluated the autonomic components of the cardiovascular responses to the microinjection of alpha-amine-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) into the nucleus tractus solitarius of awake rats. AMPA (0.05 nmol 50 nl(-1)) was microinjected into the nucleus tractus solitarius before and at 2, 10, 30 and 180 min after intravenous injection of methylatropine (2 mg kg(-1)). The control response to the microinjection of AMPA into the lateral aspect of the commissural nucleus tractus solitarius consisted of bradycardia and hypotension, (-190+/-20 bpm and -49+/-7 mm Hg), which were effectively blocked at 2 min (-5+/-2 bpm and -4+/-5 mm Hg), 10 min (-3+/-2 bpm and -6+/-3 mm Hg) and 30 min (-9+/-4 bpm and -5+/-5 mm Hg) after methylatropine. Although the bradycardic and hypotensive responses remained significantly reduced 180 min after injection of methylatropine, the responses tended to normalize. The data show that microinjection of AMPA into the rostral commissural nucleus tractus solitarius of awake rats produced intense bradycardia essentially mediated by parasympathetic excitation, which was the cause of the concomitant hypotensive response.