Evaluation of cardiovascular control by neurons in the dorsal medulla of rats

The contribution of sympathoexcitatory neurons in the dorsal medulla to the regulation of arterial pressure and the involvement of such neurons in integration of physiological responses or in the genesis of basal vasomotor tone are not well defined. In the present study discharge of neurons in the dorsal medulla of anesthetized rats was increased or decreased by microinjections of amino acids to examine effects on systemic arterial pressure, heart rate and blood flow and conductance of the renal and femoral vascular beds. Microinjections of excitatory D, L-homocysteic acid caused increases in arterial pressure of 18 +/- 2 mmHg, increases in heart rate ranging from 5-40 beats/min and renal vasoconstriction; the femoral bed constricted after some injections and dilated in response to others. Injections of the inhibitory amino acid glycine caused no consistent decreases in arterial pressure and heart rate and injections of the gamma-aminobutyric acid analog, muscimol were ineffective. These data demonstrate that neurons in the dorsal region of the rat medulla can contribute to regulation of arterial pressure and can integrate generalized differential changes in regional vascular resistance, but do not appear to be essential for the genesis of basal vasomotor tone.