Putative neurotransmitters involved in medullary cardiovascular regulation.

Experiments were carried out in artificially ventilated pentobarbital-anesthetized male Wistar rats. Following microinjection of muscimol (GABA-mimetic) or kynurenic acid (KYN; glutamate antagonist) into the ventrolateral medullary depressor area (VLDA), microinjection of L-glutamate (GLU; 4.5 nmol) into the NTS elicited a pressor response. This pressor response was attenuated in a dose-dependent manner by microinjection of KYN (0.5-5 nmol) into the ventrolateral medullary pressor area (VLPA). A GLU-induced pressor response could also be elicited from the NTS when GABA receptors in the VLPA were blocked with the microinjection of bicuculline (GABA antagonist, 200 pmol) into this site. The same dose of bicuculline in the VLPA also blocked the depressor responses elicited from the VLDA. With the VLDA or VLPA functionally unimpaired, microinjection of GLU (4.5 nmol) into the NTS elicited a fall in blood pressure and heart rate. This depressor response was attenuated in a dose-dependent manner by the microinjections of KYN (2-20 nmol) into the VLDA. These results indicate that: (1) The NTS sends glutamatergic inputs to both the VLDA and the VLPA; the projection from the NTS to the VLPA mediates pressor responses while that from the NTS to the VLDA represents one component of the pathway mediating the depressor responses elicited from the NTS. (2) The pathway from the VLDA to the VLPA is GABA-ergic and represents another component of the pathway mediating depressor responses evoked from the NTS. (3) The bradycardia evoked from the NTS may involve a pathway from the NTS to the VLDA.