Background: Based on the theory of"five Zang-organs corresponding to the seasons"in traditional Chinese medicine (TCM), physiological functions including emotions vary with the seasons. We aimed to investigate the seasonal photoperiod effects of melatonin (MT) released from the pineal gland on the MT receptor (MTR)-Gs/Gi-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMP response element-binding protein (CREB) signaling pathway in the hippocampus.Methods: Rats were divided into three groups: control, operation (surgery with pineal gland removal), and pseudo-operation groups (same surgery as operation group but without removing pineal gland), and fed at specific time across the four seasons. The levels of MTR, adenylate cyclase (AC), cAMP, PKA, and CREB in the hippocampus were analyzed using ELISA. The concentrations of Gs and Gi were analyzed using Western blot. The expression of CREB mRNA was detected by PCR. Results: For intragroup comparisons, compared with spring, the levels of Gs/Gi in the control group were higher in summer, autumn, and winter (P=.009 in summer;P<.001 in autumn and winter);the levels of MTR, cAMP, PKA, and CREB in the control group were significantly higher in autumn and winter than in spring (all P<.001). The levels of MTR, cAMP, PKA, and CREB in each season were significantly lower in the operation group than in the control group (all P < .05). Significant differences were noted in Gs/Gi levels between the operation group and control group in spring, autumn, and winter (all P<.05). Conclusion: Our findings suggest that MTR-Gs/Gi-cAMP-PKA-CREB signaling pathway is involved in the seasonal photoperiod effects of the pineal gland on the hippocampus and may underpin seasonal changes in emotions. It can support the prevention and treatment of the seasonal onset of mental dis-orders, and enrich the theory of"five Zang-organs corresponding to the seasons".