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

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.

Cranial Visceral Afferent Pathways through the Nucleus of the Solitary Tract to Caudal Ventrolateral Medulla or Paraventricular Hypothalamus: Target-Specific Synaptic Reliability and Convergence Patterns

“…Although the simplest of these reflexes can consist of as few as two central neurons, a broad complement of CNS nuclei also process afferent signals that are relayed from NTS as part of an orchestrated integration with complex behaviors (Spyer, 1990). Two prominent nuclei that play very different roles in autonomic regulation are contained within the caudal ventrolateral medulla (CVLM) (Jeske et al, 1993;Aicher et al, 1996) and the paraventricular nucleus of the hypothalamus (PVN) (Stern, 2001;Benarroch, 2005). CVLM contains the critical intermediary neurons in the baroreflex pathway that mediate baroreceptorinitiated sympathoinhibition of sympathetic neurons within the rostral ventrolateral medulla (Aicher et al, 1995).…”

Interaction of medullary P2 and glutamate receptors mediates the vasodilation in the hindlimb of rat

“…This discrepancy can be explained partly by excitatory projections from the NTS to inhibitory interneurons in the RVLM. Direct excitatory projections from NTS neurons to the RVLM exist [24,46], and many of these NTS neurons are positive for immunoreactions to VGluT1-3 [47,48]. Inhibitory neurons projecting to RVLM cells are located mostly in the caudal ventrolateral medulla [49] but are also found in rostral segments that include the RVLM [50].…”

“…Thus, we speculate that interneurons within the RVLM may inhibit the sympathoexcitatory neurons. Nevertheless, direct [24] or indirect [51] inhibitory projections from the NTS to the RVLM also exist. We believe that the remaining vasodilation after kynurenate /kg), and after following microinjections of suramin (10 mM) into the nucleus tractus solitarii (NTS).…”

“…Additionally, due to existence of direct projections from the NTS to the RVLM [24], we speculated that neurons in this area mediate responses to activation of the NTS. Therefore, first we aimed to determine whether or not activation of P2 receptors in the NTS mediate the increase in HVC evoked by stimulation of the HDA.…”

Mecanismos purinérgicos no bulbo ventrolateral rostral modulam respostas cardiovasculares e respiratórias promovidas pela ativação dos quimiorreceptores centrais e periféricos.

“…O NTScom é o primeiro núcleo no SNC que recebe informações provenientes dos quimiorreceptores periféricos (FINLEY; KATZ, 1992), e envia projeções excitatórias (glutamatérgicas) para o bulbo ventrolateral rostral (AICHER et al, 1996;. Nossos achados sugerem que a sinalização purinérgica poderia ser co-liberada com glutamato no bulbo ventrolateral rostral durante a ativação de quimiorreceptores periféricos, como já sugerido em outras áreas (LARSSON et al, 2012).…”