SUPRAMEDULLARY INPUTS TO CARDIOVASCULAR NEURONS OF ROSTRAL VENTROLATERAL MEDULLA IN RATS

Experiments were done to test the hypothesis that selective activation of cell bodies in different nuclei known to be involved in central cardiovascular control could excite or inhibit the discharge of neurons in the rostral ventrolateral medulla (RVLM). It is known that chemical stimulation of the lateral parabrachial nucleus (LPBN), locus ceruleus (LC), and lateral hypothalamic area (LHA) in anesthetized animals elicits increases (LPBN) or decreases (LC and LHA) in arterial pressure. We therefore recorded extracellularly spontaneous activity from RVLM units in urethan-anesthetized rats and monitored the changes in firing frequency of these neurons during chemical stimulation of one of LPBN, LC, and LHA. Thirty-two units were classified as cardiovascular neurons because their activity was inhibited by baroreceptor activation (1-3 mug phenylephrine iv) and displayed a cardiac cycle-related rhythmicity. Chemical stimulation with sodium glutamate of arterial pressor sites in the ipsilateral LPBN increased the firing frequency (40.3 +/- 1.3 %) of 11 cardiovascular neurons. Activation of cell bodies in arterial depressor sites in the ipsilateral LC inhibited the firing rate (59.1 +/- 7.1%) of 10 cardiovascular neurons and excited 1 unit. Activation of cell bodies in arterial depressor sites in the ipsilateral LHA inhibited the discharge rate (25.4 +/- 4.7%) of six cardiovascular neurons, excited one unit, and did not alter the rate of the remaining three units. These results provide direct evidence for the existence of excitatory and inhibitory pathways from neurons located in the LPBN, LC, and LHA to cardiovascular neurons in the RVLM.