MODULATION OF SEROTONIN BINDING-SITES IN THE BRAIN OF THE DJUNGARIAN HAMSTER, PHODOPUS-SUNGORUS, DURING ADAPTATION TO A SHORT PHOTOPERIOD

During the physiological adaptation of the Djungarian hamster, Phodopu s sungorus, to a short photoperiod in autumn the modulation of specifi c serotonin (5-HT) binding sites of synaptic membranes was investigate d in two brain regions, i.e. cerebral cortex and basal brain (CNS with out cerebral cortex, cerebellum, pineal gland, and spinal cord). The r adioligands [H-3]5-HT and [H-3]ketanserin were used to characterize to tal 5-HT1 and 5-HT2 binding sites, respectively. An increase of 5-HT1 and 5-HT2 binding sites was observed in both brain regions within 14 d ays after reduction of the photoperiod from a 14:10 h light/dark (l/d) cycle to an 8:16 h l/d cycle. The increase was still present after 56 days of the short photoperiod. Binding kinetics assayed after 4 days of the short photoperiod show that maximal specific binding of [H-3]5- HT and [H-3]ketanserin was increased, while dissociation constants (K( D)) were not changed. The membrane anisotropy of synaptic membranes, m easured by fluorescence polarization, was reduced transiently during t he early part of the adaptation. Neither the phospholipids nor the mol e ratio of cholesterol to phospholipids were significantly affected by adaptation to short photoperiod. The results suggest an important rol e of the central nervous 5-HT system in the physiological adaptation o f the Djungarian hamster to a short photoperiod.