Interaction of putative neurotransmitters in rostral ventrolateral medullary cardiovascular neurons

Central cholinergic blockade reduces the pressor response to L-glutamate into the rostral ventrolateral medullary pressor area

Injections of the excitatory amino acid l-glutamate (l-glu) into the rostral ventrolateral medulla (RVLM) directly activate the sympathetic nervous system and increase mean arterial pressure (MAP). A previous study showed that lesions of the anteroventral third ventricle region in the forebrain reduced the pressor response to l-glu into the RVLM. In the present study we investigated the effects produced by injections of atropine (cholinergic antagonist) into the lateral ventricle (LV) on the pressor responses produced by l-glu into the RVLM. Male Holtzman rats (280-320 g, n=5 to 12/group) with stainless steel cannulas implanted into the RVLM, LV or 4th ventricle (4th V) were used. MAP and heart rate (HR) were recorded in unanesthetized rats. After saline into the LV, injections of l-glu (5 nmol/100 nl) into the RVLM increased MAP (51+/-4 mm Hg) without changes in HR. Atropine (4 nmol/1 microl) injected into the LV reduced the pressor responses to l-glu into the RVLM (36+/-5 mm Hg). However, atropine at the same dose into the 4th V or directly into the RVLM did not modify the pressor responses to l-glu into the RVLM (45+/-2 and 49+/-4 mm Hg, respectively, vs. control: 50+/-4 mm Hg). Central cholinergic blockade did not affect baro and chemoreflex nor the basal MAP and HR. The results suggest that cholinergic mechanisms probably from forebrain facilitate or modulate the pressor responses to l-glu into the RVLM. The mechanism is activated by acetylcholine in the forebrain, however, the neurotransmitter released in the RVLM to facilitate the effects of glutamate is not acetylcholine.

Catecholaminergic neurons in medullary nuclei are among the post-synaptic targets of descending projections from infralimbic area 25 of the rat medial prefrontal cortex

The infralimbic (IL) 'visceromotor' area of the rat medial prefrontal cortex projects to strategic subcortical nuclei involved in autonomic functions. Central among these targets are the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (rVLM). By combining tract-tracing using the anterograde tracer biotinylated dextran amine (BDA) with immunolabeling for tyrosine hydroxylase (TH; an enzyme marker of catecholaminergic neurons), a limited proportion of BDA-labeled IL axonal boutons in the NTS and rVLM was found to be closely associated with TH immunopositive (+) target structures. Such structural appositions were mainly located proximally over the labeled dendritic arbors of identified TH+ neurons. Quantitative ultrastructural examination revealed that in NTS, TH+ dendritic shafts comprised 7.0% of the overall post-synaptic target population innervated by BDA-labeled IL boutons, whereas TH+ dendritic spines represented 1.25% of targets. In rVLM, TH+ shafts represented 9.0% and TH+ spines 2.5% of IL targets. Labeled IL boutons established exclusively asymmetric Gray Type 1 (presumed excitatory) synaptic junctions. The results indicate that subpopulations of catecholaminergic neurons in the NTS and rVLM are among the spectrum of post-synaptic neurons monosynaptically innervated by descending 'excitatory' input from IL cortex. Such connectivity, albeit restricted, identifies the potential direct influence of IL cortex on the processing and distribution of cardiovascular, respiratory and related autonomic information by catecholaminergic neurons in the NTS and VLM of the rat.

Afferent sources of substance P in the C1 area of the rat rostral ventrolateral medulla

To examine the potential extrinsic sources of substance P (SP)-containing terminals in the C1 area of the RVL, immunoperoxidase localization of SP was combined with retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). In all cases examined, several neurons containing SP and WGA-HRP were found in the nucleus raphe pallidus and a few dual labeled neurons were found in the nucleus of the solitary tract and the paraventricular nucleus of the hypothalamus. In some cases, one or two double labeled neurons were detected in the lateral hypothalamic area and nucleus raphe obscurus. The results demonstrate that although several brain structures contribute SP afferents to the RVL, the nucleus raphe pallidus is the major extrinsic source.

Immunolocalization of putative neurotransmitters innervating autonomic regulating neurons (correction of neurones) of cat ventral medulla

This study investigated possible sites of contact of nerve fibers containing a range of putative neurotransmitter substances onto neurons in the cat ventral medulla oblongata concerned with autonomic, particularly cardiovascular, regulation. The parasympathetic preganglionic neurons of the nucleus ambiguous (correction of ambiguus) were identified by retrograde horseradish peroxidase tracing from the vagus nerve, and the groups of neurons in the A1 and C1 cell areas and the raphe nucleus by catecholamine enzyme or 5-hydroxytryptamine (5-HT) immunohistochemistry, respectively. Immunoreactive (-ir)nerve fibers and terminals in the vicinity if these neurons were visualized by subjecting the sections to a dual-staining technique using a brown peroxidase-diaminobenzidine reaction product and a blue alkaline phosphatase-Fast blue reaction product. By employing monochrome photography with combinations of blue and orange-red filters, it was possible to discriminate neural elements displaying one or the other reaction product, or colocalization of reaction products. The results revealed the presence of calcitonin gene-related peptide (CGRP) and galanin (GAL)-ir in some motoneurons of the nucleus ambiguus, but not in those innervating the heart via the cardiac vagus nerve. The latter group of parasympathetic efferent neurons were found to be densely innervated by fibers immunoreactive for dopamine beta-hydroxylase (DBH, indicating noradrenaline), glycine (GLY), gamma-aminobutyric acid (GABA), 5-HT, enkephalin (ENK), neuropeptide Y (NPY), substance P (SP), and thyrotropin-releasing hormone (TRH), and, to a lesser extent, by other neuropeptide-ir fibers. The catecholamine cells of the rostral C1 and caudal A1 groups showed a broadly similar pattern of innervation, most noticeably by fibers immunoreactive for DBH, GABA, 5-HT, cholecystokinin (CCK), CGRP, ENK, GAL, NPY, and SP. The 5-HT-ir neurons of the raphe nucleus, some also containing SP, TRH, ENK, or corticotropin-releasing factor (CRF)-ir, were most prominently innervated by terminals containing DBH, GABA, CCK, ENK, NPY, TRH, somatostatin (SRIF), and vasoactive intestinal polypeptide (VIP)-ir. Although the proof that these groups of neurons receive functional synaptic contacts from the immunoreactive fibers awaits further ultrastructural studies, the results do suggest that a wide range of putative transmitters may influence the activity of efferent neurons in the cat medulla controlling autonomic functions.