Arterial baroreceptor and vagal inputs to sympathoexcitatory neurons in rat medulla.

Lumbar sympathetic nerve discharge and unit activity of reticulospinal sympathoexcitatory (SE) neurons located in nucleus paragigantocellularis lateralis (PGCL) were recorded in rats. The sympathoinhibition produced by low-frequency stimulation of vagal afferents was abolished by bilateral microinjections of 20 pmol of bicuculline methiodide (BIC, a GABA-receptor antagonist) into PGCL and was converted into a pressor response by injections of 100 pmol. These BIC injections also inhibited the arterial baroreflex in a dose-dependent manner. In contrast the sympathoexcitation produced by high-frequency stimulation of vagal afferents was selectively blocked by bilateral injections of kynurenic acid (KYN, a Glu-receptor antagonist) into PGCL. Convergence of vagal excitatory, vagal inhibitory, and arterial baroreceptor inputs was detected in all SE neurons recorded. Single-pulse stimulation of vagal afferents produced up to two peaks of excitation of SE neurons, both blocked by iontophoretic applications of KYN and at least one inhibitory period selectively blocked by iontophoresis of BIC. The results emphasize the importance of SE neurons and surrounding area in integrating the brain vasomotor output to spinal preganglionic neurons.