Anatomical evidence for volume transmission in the dorsal vagal complex of the rat

Injection of either calcitonin gene-related peptide (CGRP) or substance P (SP) into the cerebrospinal fluid or dorsal vagal complex (DVC) of the rat has an effect on gastric function via a vagally-dependent mechanism. These effects are both slow in onset and of prolonged duration, characteristics of non-synaptic events. Although these peptides have been previously detected within the DVC, the anatomical basis for their effects has remained unclear. In order to examine the possibility that these peptides act via a non-synaptic, volume transmission mechanism within the rat DVC, a combination of retrograde tracing, immunocytochemistry, confocal microscopy and 3-D reconstruction techniques were used to investigate the spatial association between nerve fibres immunoreactive for both CGRP and SP, the substance P receptor (NK-lr) and identified gastric efferent neurons within this region. In 3-D reconstructions, it was found that the majority of nerve fibres containing either SP- or CGRP-IR surrounded identified gastric efferent neurons without making contact with the membrane of these cells. While most of the CGRP-IR fibres were preferentially associated with the dendrites of retrogradely-labelled neurons around the level of the obex, SP-IR fibres surrounded all surfaces of these cells at all levels of the DVC. The somatic membrane of a subpopulation of gastric efferent neurons (7%) was labelled with NK-lr-IR. In addition, NK-lr-IR was localized to nerve fibres, mainly in the rostral DVC. NK-lr-IR covered almost the entire membrane of some DMV neurons, with the highest density of receptors localized to the dendrites. The distribution of neutral endopeptidase 24.11 (NEP), the principle cleavage enzyme for SP within the CNS, was compared to that of SP-IR, NK-lr-IR and retrogradely-labelled gastric efferent neurons within this region. The enzyme was not associated with vagal neurons or fibres at any level of the DVC, but rather was localized to components of the blood-brain barrier. In conclusion, the small proportion of neuropeptide-IR nerve fibres that made direct contact with identified neurons, the non-synaptic localization of NK-lr-IR and the absence of NEP-IR associated with identified neurons provide anatomical evidence in support of neural communication via volume transmission within the rat DVC.Medicine, Faculty ofCellular and Physiological Sciences, Department ofGraduat