Reciprocal connections between nucleus tractus solitarii and rostral ventrolateral medulla

Experiments were done to test the hypothesis that there are reciprocal connections between the nucleus of the tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). Spontaneous activity was recorded from units in the right RVLM or NTS of urethan-anesthetized and artificially ventilated rats. Twenty-four of 42 RVLM and 12 of 21 NTS units were classified as cardiovascular because they were inhibited by baroreceptor activation and displayed a cardiac rhythm. Electrical stimulation of depressor sites in the NTS inhibited 14 and excited 10 RVLM units. Stimulation of pressor sites in the RVLM excited 10 and inhibited 2 NTS units. None of the units in the NTS or in the RVLM could be activated antidromically. These results provide evidence that there are reciprocal excitatory and inhibitory connections between NTS and RVLM and that these connections are not monosynaptic.