BETA-ADRENERGIC SIGNAL-TRANSDUCTION IN THE HYPOTHALAMUS OF THE EUROPEAN HAMSTER - RELATION WITH THE SEASONAL HIBERNATION CYCLE AND THE DIURNAL ACTIVITY CYCLE

Mammalian hibernation, an adaptation to survive harsh winter condition s, is one of the most prominent seasonal rhythmic processes exactly re gulated on a low metabolic level. Diurnal variations in vegetative phy siology are missing during hibernation; however, a precisely working d iurnal system is mandatory for both the proper initiation and terminat ion of the annual hibernation phase and the periodical arousal reactio ns. Biorhythms and the vegetative physiological processes connected wi th hibernation are, among others, controlled by hypothalamic noradrena line systems. In this study, the density, binding capacity, and relati ve proportions of beta(1)- and beta(2)-adrenergic receptors (AR) withi n the hypothalamus of: 1) motorically inactive summer; 2) motorically active summer; 3) aroused, motorically active winter; and 4) deeply hi bernating winter European hamsters (Cricetus cricetus) were studied. F or further analysis of the beta-adrenergic signal transduction cascade , the activity of adenylyl cyclase (AC) was measured by formation of c AMP in controls, after stimulation of G proteins, or after forskolin s timulation without or in presence of manganese ions. While beta(1)- an d beta(2)-AR subtypes were nearly equally abundant (50% beta(1):50% be ta(2)) in active summer, inactive summer, and hibernating hamsters, a significant redistribution in favor of beta(2)-AR occurred after arous al (40% beta(1):60% beta(2)). The activity of AC was much higher in ac tive summer hamsters than in inactive summer, aroused winter, and hibe rnating winter hamsters. When AC was stimulated by guanylylimidophosph ate [Gpp(NH)p], MnCl2, forskolin, or by forskolin in presence of MnCl2 instead of MgCl2, the potency to stimulate AC was found to show the f ollowing rank order: basal < Gpp(NH)p < MnCl2 less than or equal to fo rskolin + MnCl2 < forskolin. ((C) Elsevier, Paris).