MONOSYNAPTIC PROJECTIONS FROM THE NUCLEUS-TRACTUS-SOLITARII TO C1 ADRENERGIC-NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA - COMPARISON WITHINPUT FROM THE CAUDAL VENTROLATERAL MEDULLA
Sa. Aicher et al., MONOSYNAPTIC PROJECTIONS FROM THE NUCLEUS-TRACTUS-SOLITARII TO C1 ADRENERGIC-NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA - COMPARISON WITHINPUT FROM THE CAUDAL VENTROLATERAL MEDULLA, Journal of comparative neurology, 373(1), 1996, pp. 62-75
The rostral ventrolateral medulla (RVL) contains reticulospinal adrene
rgic (C1) neurons that are thought to be sympathoexcitatory and that f
orm the medullary efferent limb of the baroreceptor reflex pathway. Th
e RVL receives direct projections from two important autonomic regions
, the caudal ventrolateral medulla (CVL) and the nucleus tractus solit
arii (NTS). In the present study, we used anterograde tracing from the
CVL or the NTS combined with immunocytochemical identification of C1
adrenergic neurons in the RVL to compare the morphology of afferent in
put from these two autonomic regions into the RVL. NTS (n=203) and CVL
(n=380) efferent terminals had similar morphology and vesicular conte
nt, 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 li
kely to form asymmetric synapses than CVL efferent terminals. CVL effe
rent terminals were more likely to contact adrenergic somata than were
NTS efferents, which usually contacted dendrites. These findings 1) s
upport the hypothesis that a portion of NTS efferents to the RVL may b
e involved in sympathoexcitatory, e.g., chemoreceptor, reflexes (via a
symmetric synapses), whereas those from the CVL mediate sympathoinhibi
tion (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, C
VL inhibitory inputs may be more influential than excitatory NTS input
s in determining the discharge of RVL neurons. (C) 1996 Wiley-Liss, In
c.