Effect of Chronic Psychogenic Stress Exposure on Enkephalin Neuronal Activity and Expression in the Rat Hypothalamic Paraventricular Nucleus

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Abstract
This study tested the hypothesis that the activation pattern of enkephalinergic (ENKergic) neurons within the paraventricular nucleus of the hypothalamus (PVH) in response to psychogenic stress is identical whether in response to repeated exposure to the same stress (homotypic; immobilization) or to a novel stress (heterotypic; air jet puff). Rats were assigned to either acute or chronic immobilization stress paradigms (90 min/day for 1 or 10 days, respectively). The chronic group was then subjected to an additional 90‐min session of either heterotypic or homotypic stress. A single 90‐min stress session (immobilization or air jet) increased PVH‐ENK heteronuclear (hn) RNA expression. In chronically stressed rats, exposure to an additional stress session (whether homotypic or heterotypic) continued to stimulate ENK hnRNA expression. Acute immobilization caused a marked increase in the numbers of Fos‐immunoreactive and Fos‐ENK double‐labeled cells in the dorsal and ventral medial parvicellular, and lateral parvicellular subdivisions of the PVH. Chronic immobilization caused an attenuated Fos response (∼ 66%) to subsequent immobilization. In contrast, chronic immobilization did 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 neurons resist habituation, principally within the ventral neuroendocrine portion of the nucleus. This suggests an increase in ENK effect during chronic stress exposure. Homotypic (immobilization) and heterotypic (air jet) psychogenic stressors produce similar responses, including Fos, ENK‐Fos, and ENK hnRNA, within each subdivision of the PVH, suggesting similar processing for painless neurogenic stimuli.