Effect of chronic psychogenic stress exposure on enkephalin neuronal activity and expression in the rat hypothalamic paraventricular nucleus

This study tested the hypothesis that the activation pattern of enkephaline rgic (ENKergic) neurons within the paraventricular nucleus of the hypothala mus (PVH) in response to psychogenic stress is identical whether in respons e to repeated exposure to the same stress (homotypic; immobilization) or to a novel stress (heterotypic; air jet puff). Rats were assigned to either a cute or chronic immobilization stress paradigms (90 min/day for 1 or 10 day s, respectively). The chronic group was then subjected to an additional 90- min session of either heterotypic or homotypic stress. A single 90-min stre ss session (immobilization or air jet) increased PVH-ENK heteronuclear (hn) RNA expression. In chronically stressed rats, exposure to an additional st ress session (whether homotypic or heterotypic) continued to stimulate ENK hnRNA expression. Acute immobilization caused a marked increase in the numb ers of Fos-immunoreactive and Fos-ENK double-labeled cells in the dorsal an d ventral medial parvicellular, and lateral parvicellular subdivisions of t he PVH. Chronic immobilization caused an attenuated Fos response (similar t o 66%) to subsequent immobilization. In contrast, chronic immobilization di d not impair ENKergic neuron activation within the PVH following homotypic or heterotypic stress. These results indicate that within the PVH, chronic psychogenic stress markedly attenuates the Fos response, whereas ENKergic n eurons resist habituation, principally within the ventral neuroendocrine po rtion of the nucleus. This suggests an increase in ENK effect during chroni c stress exposure. Homotypic (immobilization) and heterotypic (air jet) psy chogenic stressors produce similar responses, including Fos, ENK-Fos, and E NK hnRNA, within each subdivision of the PVH, suggesting similar processing for painless neurogenic stimuli.