Diurnal rhythms of 5-HT 1A and 5-HT 2 receptor binding in euthermic and torpor prone deermice, Peromyscus maniculatus

Abstract

Deermice display both spontaneous and induced daily torpor bouts, attaining minimum body temperatures of 15–20°C. There is evidence that brain serotonin may be involved in the initiation and/or maintenance of torpor. Inhibition of serotonin [5-hydroxytryptamine (5-HT)] synthesis markedly reduces the duration and depth of torpor. Because a certain percentage of deermice will not enter torpor under any circumstances, 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 for 5-HT 1A and 5-HT 2 receptor differences using [ 3H]8-OH-DPAT and [ 3H]ketanserin, respectively. The B max values for 5-HT 1A 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 density of 5-HT 2 receptors was significantly greater in animals sacrificed at 11:00 a.m. compared to animals sacrificed at 11:00 p.m. This is consistent with the opposing functions of these receptors in the regulation 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-HT 1A and 5-HT 2 receptors may not explain the heterogeneity of deermice in their ability to enter torpor. However, the diurnal fluctuation in 5-HT receptors described here may be involved in the serotonergic regulation of hormone rhythmicity and the onset of torpor in deermice.