Effect of the pineal gland on 5-hydroxytryptamine and γ-aminobutyric acid secretion in the hippocampus of male rats during the summer and winter

The present study aimed to investigate the effect of seasonal variation on neurotransmitter release in the hippocampus of normal rats and rats with pineal excision.

Two time points, the summer and winter solstice, which are the longest and shortest days of the year, respectively, were selected. Male Sprague–Dawley rats that underwent a sham operation without pineal excision were included as a control group. The concentrations of 5-hydroxytryptamine (5-HT) and γ-aminobutyric acid (GABA) were determined by radioimmunoassays and enzyme-linked immunosorbent assays, respectively.

In the winter, the 5-HT and GABA levels in normal rats exhibited a significant difference compared with those in the operation group (P < .01). A difference was also noted in GABA levels between the normal group and the sham operation group (P < .05). The concentrations of 5-HT and GABA in the hippocampal tissues of the normal group exhibited a seasonal rhythm consisting of elevation during the summer and reduction during the winter (P < .01), while the GABA levels in the sham operation group exhibited a significant difference, with elevation during the summer and reduction during the winter (P < .01). In the operation group, GABA showed the same trend (P < .01).

The seasonal rhythm of neurotransmitter secretion by the hippocampus (5-HT and GABA) consisted of elevation during the summer and reduction during the winter. During the winter, the pineal gland exhibited a reverse regulatory effect on the secretion of 5-HT and GABA in the hippocampus, and it exhibited seasonal selectivity with regard to the regulation of 5-HT.