| Summary: | Background: Based on the effect of seasonal changes on human visceral function, this study investigated the impact of seasonal photoperiod of the pineal body on hypothalamic-pituitary-adrenal axis-hippocampal-receptor in rats, aiming to reveal the mechanism by which pineal gland melatonin regulates the seasonal secretion of hippocampal neurotransmitters. Methods: Vernal equinox, summer solstice, autumn equinox, and winter solstice were selected as four experimental time points, and rats were randomly divided into normal control group, sham operation group, and pinealectomized group. The seasonal changes in corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), corticosterone, hypothalamic melatonin receptor (MTR), and hippocampal corticosterone receptor (CORTR) were examined by enzyme-linked immunosorbent assay. Results: Comparing the same group between different seasons, we showed that in the normal control group, CRH, ACTH, corticosterone, and MTR were higher, while CORTR was lower in autumn and winter than in spring (all P < .05). Compared with the normal control group, the pinealectomized group showed higher levels of corticosterone (P = .01), MTR (P = .01), and CORTR (P = .03) during spring; reduced levels of MTR and CORTR (both P < .001) during summer; higher levels of ACTH (P = .001) and MTR (P < .001), and lower levels of CRH (P = .001), corticosterone (P < .001), and CORTR (P = .003) during autumn; and lower levels of CRH (P < .001) and MTR (P = .004), and higher level of ACTH (P < .001) in winter. Conclusions: Seasonal photoperiod acts on the pineal gland to secrete different levels of melatonin, resulting in seasonal changes in the hypothalamic-pituitary-adrenal axis-hippocampal-receptor, which may be the pathophysiological basis for the onset of seasonal affective disorder.
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