The effects of different photoperiods on circadian 5-HT rhythms in regional brain areas and their modulation by pinealectomy, melatonin and oestradiol.

Differential circadian rhythms in 5-HT levels were found in the hypothalamus and pineal (but not in the cortex, hippocampus or midbrain) in ferrets kept in either long (14 h light/10 h dark) or short (8 h light/16 h dark) photoperiods. 5-HT decreased during the first 6 h of illumination in all areas examined from animals kept in short photoperiods. In long photoperiods, 5-HT in the hypothalamus (particularly the anterior region) increased during the first 6 h after onset of light and levels in the pineal became arrhythmic. There were no differential effects of light on 5-HIAA/5-HT ratios. Removal of the pineal or the superior cervical ganglia abolished these differential rhythms, as did subcutaneous implants of oestradiol (releasing about 5 micrograms/day). Melatonin (1 mg/day) injected 8 h after the onset of light into animals kept in long photoperiods resulted in circadian 5-HT rhythms resembling those from animals exposed to short photoperiods, whereas melatonin given at 14 h after onset of light did not have this effect. It is suggested that 5-HT containing neural systems may play a role in the way the pineal transmits information about the duration of the photoperiod to the neural structures controlling the pituitary.