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Monosynaptic projections from the nucleus tractus solitarii to C1 adrenergic neurons in the rostral ventrolateral medulla: Comparison with input from the caudal ventrolateral medulla

Department of Neurology and Neuroscience, Cornell University Medical College, New York, New York 10021, USA.
The Journal of Comparative Neurology (Impact Factor: 3.51). 09/1996; 373(1):62-75. DOI: 10.1002/(SICI)1096-9861(19960909)373:1<62::AID-CNE6>3.0.CO;2-B
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ABSTRACT The rostral ventrolateral medulla (RVL) contains reticulospinal adrenergic (C1) neurons that are thought to be sympathoexcitatory and that form the medullary efferent limb of the baroreceptor reflex pathway. The RVL receives direct projections from two important autonomic regions, the caudal ventrolateral medulla (CVL) and the nucleus tractus solitarii with immunocytochemical identification of C1 adrenergic neurons in the RVL to compare the morphology of afferent input from these two autonomic regions into the RVL. NTS (n = 203) and CVL (n = 380) efferent terminals had similar morphology and vesicular content, but CVL efferent terminals were slightly larger than NTS efferent terminals. Overall, efferent terminals from either region were equally likely to contact adrenergic neurons in the RVL (21% for NTS, 25% for CVL). Although efferents from both regions formed both symmetric and asymmetric synapses, NTS efferent terminals were statistically more likely to form asymmetric synapses than CVL efferent terminals. CVL efferent terminals were more likely to contact adrenergic somata than were NTS efferents, which usually contacted dendrites. These findings 1) support the hypothesis that a portion of NTS efferents to the RVL may be involved in sympathoexcitatory, e.g., chemoreceptor, reflexes (via asymmetric synapses), whereas those from the CVL mediate sympathoinhibition (via symmetric synapses); and 2) provide an anatomical substrate for differential postsynaptic modulation of C1 neurons by projections from the NTS and CVL. With their more frequent somatic localization, CVL inhibitory inputs may be more influential than excitatory NTS inputs in determining the discharge of RVL neurons.

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    • "The most cardiovascular-relevant part of the NTS is located at the most caudal part of the NTS, which contains synapses from chemo and aortic baroreceptor processes that contact with secondary order neurons within the NTS [55] [56]. The latter communicate either directly or indirectly through third order neurons with other nuclei including RVLM, hypothalamus or CVLM [57] [58] [59] [60]. Functionally, activation of cardiovascular afferents (chemo or baroreceptors) enhances the release of excitatory amino-acid L-glutamate within the NTS [54], which prompts the excitation of NTS-projections to other baroreflex arc nuclei e.g. "
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    • "The peripheral chemoreceptors are essential to detect changes in the oxygen in the arterial blood (PaO 2 ) and its activation by hypoxia produces respiratory, autonomic, and behavioral responses (Haibara et al. 1995, 1999; Machado 2001). Anatomical and functional evidence demonstrate that the first synapse of peripheral chemoreceptor afferents in the central nervous system is located in the nucleus tractus solitarius ([NTS] Finley and Katz 1992; Vardhan et al. 1993; Chitravanshi et al. 1994; Chitravanshi and Sapru 1995; Paton et al. 2001) and several studies demonstrated that the sympathoexcitatory and respiratory components of chemoreflex involve NTS neurons sending projections to the ventral medulla ([NTS-VLM neurons] Ross et al. 1985; Urbanski and Sapru 1988; Seller et al. 1990; Koshiya et al. 1993; Aicher et al. 1996). "
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    • "The distribution of RVLM-projecting neurons within the nTS in the current study was similar to that reported previously (Ross et al., 1985; Aicher et al., 1996; Chan and Sawchenko, 1998b; Kantzides et al., 2005; Kantzides and Badoer, 2006). Most of the retrograde labeling appeared to be localized in the caudal two-thirds of the nTS, and ventral to the area postrema. "
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