DIURNAL RHYTHMS OF 5-HT(1A) AND 5-HT(2) RECEPTOR-BINDING IN EUTHERMICAND TORPOR PRONE DEERMICE, PEROMYSCUS-MANICULATUS

Deermice display both spontaneous and induced daily torpor bouts, atta ining minimum body temperatures of 15-20-degrees-C. There is evidence that brain serotonin may be involved in the initiation and/or maintena nce of torpor. Inhibition of serotonin 5-hydroxytryptamine (5-HT)! sy nthesis markedly reduces the duration and depth of torpor. Because a c ertain percentage of deermice will not enter torpor under any circumst ances, we were able to compare 5-HT receptor subtypes in deermice that readily enter into torpor (TP) and in non-torpor prone (NTP) animals. Deermice were trapped in the wild and subjected to food rationing and low ambient temperature and then sacrificed either in a normothermic or torpid state at 11:00 p.m. or 11:00 a.m. Whole brain was assayed fo r 5-HT1A and 5-HT2 receptor differences using H-3!8-OH-DPAT and H-3! ketanserin, respectively. The B(max) values for 5-HT1A receptors were significantly greater in both TP and NTP animals sacrificed at 11:00 p .m. compared to animals sacrificed at 11:00 a.m. In contrast, the dens ity of 5-HT2 receptors was significantly greater in animals sacrificed at 11:00 a.m. compared to animals sacrificed at 11:00 p.m. This is co nsistent with the opposing functions of these receptors in the regulat ion of temperature and sleep. The affinity (K(d)) of each receptor was unchanged. A comparison of TP and NTP animals sacrificed at the same time of day revealed no significant differences in either B(max) or in K(d) values, indicating that differences in 5-HT1A and 5-HT2 receptor s may not explain the heterogeneity of deermice in their ability to en ter torpor. However, the diurnal fluctuation in 5-HT receptors describ ed here may be involved in the serotonergic regulation of hormone rhyt hmicity and the onset of torpor in deermice.