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PubMed2002

Evidence for tonic disinhibition of RVLM sympathoexcitatory neurons from the caudal pressor area.

Previous studies in the rat have shown that a significant proportion of the tonic activity of presympathetic neurons in the rostral ventrolateral medulla (RVLM) is dependent on the tonic activity of neurons within the caudal pressor area (CPA), located in the most caudal part of the caudal ventrolateral medulla (CVLM). In this study, we determined the extent to which tonically active neurons in the CPA contribute to sympathetic vasomotor tone, and we also investigated the pharmacological mechanisms by which these neurons affect the tonic activity of RVLM presympathetic neurons. In anaesthetised rabbits, bilateral injections of the neuroinhibitory compound muscimol into the CVLM at the level of the most caudal part of the lateral reticular nucleus, which corresponds to the anatomical location of the CPA as mapped in the rat, resulted in an immediate profound hypotension and almost complete abolition of renal sympathetic nerve activity (rSNA). In contrast, microinjections into surrounding regions had little or no effect or else evoked a delayed hypotensive response. The hypotensive and sympathoinhibitory response evoked by inhibition of the CPA was greatly delayed by prior injections of the GABA receptor antagonist bicuculline into the RVLM. In contrast, injections of the glutamate receptor antagonist kynurenic acid into the RVLM did not alter the hypotensive and sympathoinhibitory response. The results indicate that neurons within the CPA tonically inhibit other neurons, which, in turn, inhibit RVLM sympathoexcitatory neurons, via a GABAergic synapse. This disinhibition of RVLM neurons by CPA neurons is essential for maintaining resting sympathetic vasomotor tone.

Animals↗
PubMed1967

Observations on carotid body chemoreceptor activity and cervical sympathetic discharge in the cat.

1. Parallel recordings have been made of the cervical sympathetic and carotid body chemoreceptor activity in the anaesthetized cat.2. The sympathetic activity remains remarkably constant while the chemoreceptor activity is varied by changes in the blood gas tensions of relatively short duration.3. Changes in intrathoracic pressure by obstruction to the airway or thoracic compression were associated with changes in the activity of both nerves.4. It is likely that, under normal conditions, the sympathetic nervous activity provides a stable vasomotor tone within the carotid body.

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