The nucleus tractus solitarius (NTS) is the major integrative center within the brainstem for cardiovascular homeostasis. The NTS receives primary input from a variety of cardiovascular afferents such as arterial and cardiopulmonary baroreceptors, peripheral chemoreceptors, somatic and visceral afferents. This information is integrated and via projections to the rostral and caudal ventro-lateral medulla (controlling efferent sympathetic activity) and to the ambiguous complex (controlling efferent parasympathetic activity), reflex control of autonomic nerve activity occurs. Integration of primary afferent information in the NTS can also be markedly affected by descending projections from higher central structures such as specific nuclei within the hypothalamus which have reciprocal connections with the NTS. The NTS neurons contain a rich vocabulary of neurotransmitters and neuromodulators which may affect NTS integration and therefore control of the cardiovascular system. Recently, we and others have shown that adenosine markedly affects NTS integration and thus can elicit profound changes in cardiovascular control. Adenosine levels in the NTS can rise via at least two distinct mechanisms, 1) during cerebral ischemia as occurs, for example, during marked hypotensive hemorrhage or high gravitational stress, and 2) via the breakdown of ATP released as a neurotransmitter or co-transmitter. Adenosine, acting pre-synaptically via both A1 or A2a receptors can modulate the extent of neurotransmitter release. Our previous studies strongly support the concept that different neurotransmitters/neuromodulators operating in the NTS are distinctly linked to specific patterns of regional cardiovascular and sympathetic responses archetypal for specific physiological and pathological situations. A key observation is the unique role of adenosine in the activation of pre-ganglionic adrenal sympathetic nerve activity which innervates the adrenal medulla. NTS adenosine receptor mediated activation of the adrenal medulla likely aids in the restoration of homeostasis during stress as well as likely contributes to the distinct cardiovascular patterns of the defense response. This competing renewal application is focused on 1) determining the mechanisms mediating the responses to stimulation of NTS adenosine receptor subtypes and 2) the role of adenosine receptors in the NTS in modulating cardiovascular responses from descending pathways and ascending peripheral inputs.
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