Reciprocal connection between nucleus ambiguus and caudal ventrolateral medulla.

To investigate interactions between parasympathetic preganglionic neurons in the nucleus ambiguus (NA) and sympathoinhibitory neurons in the caudal ventrolateral medulla (CVLM) the activity of CVLM neurons was recorded during glutamate stimulation of cardioinhibitory sites in the NA of urethane-anesthetized rats. Neurons in the CVLM were identified as cardiovascular neurons if they increased their activity after i.v. phenylephrine and displayed cardiac cycle-related rhythmicity. Of 23 cardiovascular neurons studied, 10 decreased activity after glutamate (GLU) microinjection in the NA, five neurons were excited and eight did not respond. In another series of experiments, the nature of the influence of the CVLM on unit activity in the NA was investigated. Sites in the CVLM from which decreases in arterial pressure and heart rate were elicited after GLU microinjection were identified and the activity of cardiovascular neurons in the NA was recorded. Of 22 NA cardiovascular neurons studied, eight decreased activity after microinjection of GLU in the CVLM and 14 did not change firing frequency. These results demonstrate the existence of a reciprocal pathway between the NA and CVLM and provide evidence for functional interactions between medullary sites implicated in the control of the parasympathetic and sympathetic nervous systems.