Stress Adaptation Upregulates Oxytocin within Hypothalamo-Vagal Neurocircuits

Abstract

Stress plays a pivotal role in the development and/or exacerbation of functional gastrointestinal (GI) disorders. The paraventricular nucleus of the hypothalamus (PVN) contains neurons that are part of the hypothalamic-pituitary-adrenal axis as well as preautonomic neurons innervating, among other areas, gastric-projecting preganglionic neurons of the dorsal vagal complex (DVC). The aim of the present study was to test the hypothesis that stress adaptation upregulates oxytocin (OXT) within PVN-brainstem vagal neurocircuitry. The retrograde tracer cholera toxin B (CTB) was injected into the DVC of rats which, after post-surgical recovery, were pair-housed and exposed to either homo- or heterotypic stress for five consecutive days. Fecal pellets were counted at the end of each stress load. Two hours after the last stressor, the whole brain was excised. Brainstem and hypothalamic nuclei were analyzed immunohistochemically for the presence of both OXT-immunopositive cells in identified preautonomic PVN neurons as well as OXT fibers in the DVC. Rats exposed to chronic homotypic, but not chronic heterotypic stress, had a significant increase in both number of CTB+ OXT co-localized neurons in the PVN as well as density of OXT-positive fibers in the DVC compared to control rats. These data suggest that preautonomic OXT PVN neurons and their projections to the DVC increase following adaptation to stress, and suggest that the possible up-regulation of OXT within PVN-brainstem vagal neurocircuitry may play a role in the adaptation of GI responses to stress.

Keywords: brainstem; gastrointestinal motility; vagus.

Figure 1.. The decreased fecal pellet output in CHo rats correlates with a larger number of oxytocin immunoreactive neurons in the PVN.

A: graphical summary illustrating the number of fecal pellets excreted during the periods of stress loading. B: graphic representation showing the negative correlation (r²=0.563) between the number of oxytocin-immunoreactive neurons in the PVN and the fecal pellets during the stress loading. Although the regression was calculated using the entire data set, for reasons of clarity only a few data points have been included in the graph.

Figure 2.. CHo increases co-localization of CTB- and OXT-IR in preautonomic PVN neurons.

A-C: representative micrographs showing oxytocin-immunoreactive (OXT-IR, blue) and CTBimmunoreactive (CTB-IR, brown) neurons in the PVN of control (A), CHo (B), and CHe (C) stressed rats. Bar, 100μm. D: Enlarged area showing co-localization (arrow) of OXT- and CTB-IR. Bar, 50μm. E: summary graphic showing the number of OXT-IR preautonomic neurons in the PVN. Note the increased OXT-IR in rats that underwent CHo, but not CHe, stress. *P<0.05 vs. control and CHe.

Figure 3.. CHo increases the density of OXT axons in the DVC.

A: representative micrographs showing OXT-IR fibers in the dorsal vagal complex (DVC) from a control rat. Insert at the top right shows a higher magnification of the dotted area. Bar, 100μm. B: representative image showing reconstruction of OXT-IR fiber tracing from the image in A. Bar, 20μm. C: representative micrographs showing OXT-IR fibers in the DVC from a rat that underwent CHo stress. Insert at the top right shows a higher magnification of the dotted area. Bar, 100μm. D: representative image showing fiber reconstruction tracing from the image in C. Bar, 20μm. E-F: summary graphic showing the density of OXT-IR fibers in the caudal (E) and intermediate (F) DVC. Note the increased oxytocin-IR fibers in rats that underwent CHo, but not CHe, stress. *P<0.05 vs. control and CHe. #P<0.05 vs control.