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.
"For instance, the neurons of the iNTS receive mainly afferent information from the arterial baroreceptors and from the slow-adapting pulmonary stretch receptors (SARs) whilst afferent inputs from peripheral chemoreceptors and rapid-adapting pulmonary stretch receptors (RARs) converge to the neurons of the cNTS (Mifflin et al., 1988; Mifflin, 1992; Machado, 2001; Kubin et al., 2006). Therefore, depending on the sensory information, distinct groups of NTS 2nd-order neurons are activated in order to recruit specific efferent pathways (Aicher et al., 1996; Bailey et al., 2006; Alheid et al., 2011; Song et al., 2011). "
[Show abstract][Hide abstract] ABSTRACT: It is well known that breathing introduces rhythmical oscillations in the heart rate and arterial pressure levels. Sympathetic oscillations coupled to the respiratory activity have been suggested as an important homeostatic mechanism optimizing tissue perfusion and blood gas uptake/delivery. This respiratory-sympathetic coupling is strengthened in conditions of blood gas challenges (hypoxia and hypercapnia) as a result of the synchronized activation of brainstem respiratory and sympathetic neurons, culminating with the emergence of entrained cardiovascular and respiratory reflex responses. Studies have proposed that the ventrolateral region of the medulla oblongata is a major site of synaptic interaction between respiratory and sympathetic neurons. However, other brainstem regions also play a relevant role in the patterning of respiratory and sympathetic motor outputs. Recent findings suggest that the neurons of the nucleus of the solitary tract (NTS), in the dorsal medulla, are essential for the processing and coordination of respiratory and sympathetic responses to hypoxia. The NTS is the first synaptic station of the cardiorespiratory afferent inputs, including peripheral chemoreceptors, baroreceptors and pulmonary stretch receptors. The synaptic profile of the NTS neurons receiving the excitatory drive from afferent inputs is complex and involves distinct neurotransmitters, including glutamate, ATP and acetylcholine. In the present review we discuss the role of the NTS circuitry in coordinating sympathetic and respiratory reflex responses. We also analyze the neuroplasticity of NTS neurons and their contribution for the development of cardiorespiratory dysfunctions, as observed in neurogenic hypertension, obstructive sleep apnea and metabolic disorders.
Frontiers in Physiology 06/2014; 5:238. DOI:10.3389/fphys.2014.00238 · 3.53 Impact Factor
"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  . The latter communicate either directly or indirectly through third order neurons with other nuclei including RVLM, hypothalamus or CVLM    . Functionally, activation of cardiovascular afferents (chemo or baroreceptors) enhances the release of excitatory amino-acid L-glutamate within the NTS , which prompts the excitation of NTS-projections to other baroreflex arc nuclei e.g. "
[Show abstract][Hide abstract] ABSTRACT: Cannabinoids elicit complex hemodynamic responses in experimental animals that involve both peripheral and central sites. Centrally administered cannabinoids have been shown to predominantly cause pressor response. However, very little is known about the mechanism of the cannabinoid receptor 1 (CB1R)-centrally evoked pressor response. In this review, we provided an overview of the contemporary knowledge regarding the cannabinoids centrally elicited cardiovascular responses and the possible underlying signaling mechanisms. The current review focuses on the rostral ventrolateral medulla (RVLM) as the primary brainstem nucleus implicated in CB1R-evoked pressor response.
Journal of Advanced Research 03/2014; 5(2):137–145. DOI:10.1016/j.jare.2013.03.008
"Nevertheless, activation of adrenal premotor neurons by an intrinsic drive from brainstem neurons cannot be disregarded. For instance, C1 sympathetic premotor neurons receive excitatory inputs from other brainstem nuclei including the NTS (Aicher et al., 1996), a structure which provides a high proportion of asymmetric synapses onto C1 neurons. By contrast, although sympathetic premotor neurons in the ventral medulla are activated by glucoprivation, evidence supporting the notion that they are intrinsically glucose-sensitive is poor. "
[Show abstract][Hide abstract] ABSTRACT: Glucose is an essential metabolic substrate for all bodily tissues. The brain depends particularly on a constant supply of glucose to satisfy its energy demands. Fortunately, a complex physiological system has evolved to keep blood glucose at a constant level. The consequences of poor glucose homeostasis are well-known: hyperglycemia associated with uncontrolled diabetes can lead to cardiovascular disease, neuropathy and nephropathy, while hypoglycemia can lead to convulsions, loss of consciousness, coma, and even death. The glucose counterregulatory response involves detection of declining plasma glucose levels and secretion of several hormones including glucagon, adrenaline, cortisol, and growth hormone (GH) to orchestrate the recovery from hypoglycemia. Low blood glucose leads to a low brain glucose level that is detected by glucose-sensing neurons located in several brain regions such as the ventromedial hypothalamus, the perifornical region of the lateral hypothalamus, the arcuate nucleus (ARC), and in several hindbrain regions. This review will describe the importance of the glucose counterregulatory system and what is known of the neurocircuitry that underpins it.
Frontiers in Neuroscience 02/2014; 8(8):38. DOI:10.3389/fnins.2014.00038 · 3.66 Impact Factor
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