Melatonin and Chlorpromazine: Thermal Selection and Metabolic Rate in the Bullsnake, Pituophis melanoleucus

Abstract

We investigated the effects of melatonin (MEL) and chlorpromazine (CPZ) on the thermal selection and metabolic rate of the bullsnake, Pituophis melanoleucus. Adult snakes were acclimatized for 5 weeks to a constant temperature of 25 ± 1°C (range) and an L12:D12 photoperiod; photophase was centered on 1200 hr CST and began at 0600 hr. Temperatures selected by snakes in response to intraperitoneal (IP) injections of saline (control), MEL (5 mg kg −1 body mass), and CPZ (25 mg kg −1 body mass), a melatonin antimetabolite, were measured in a linear thermal gradient over a 36-hr experimental period. Using a repeated measures design, we showed that mean preferred body temperature (T b) of snakes when receiving either MEL (19.6°C, SE = 1.86, n = 11) or CPZ (15.7°C, SE = 1.12, n = 11) differed significantly from the preferred T b of animals receiving control injections of saline solution (24.1°C, SE = 1.90, n = 11). Changes in metabolic rate were determined with closed system respirometry to measure oxygen consumption before and 3 hr after treatments of: 1) non-injected control, 2) injected-saline control, 3) MEL (5 mg kg −1 body mass), and 4) CPZ (25 mg kg −1 body mass). Static samples of oxygen consumption before and after treatments showed that MEL and CPZ had no significant effect upon the resting metabolic rate (RMR) 3 hr after injection. A multiple comparisons test of the among-treatment differences indicated that there were no statistically significant changes in RMR ( F = 0.975; df = 3,27; P = 0.419). However, the difference between before and after mean RMR for the CPZ experiment was almost significant ( W = 35; df = 9; P = 0.084), and may be biologically meaningful. Both exogenous and endogenous MEL may play a role in behavioral and physiological thermoregulation of vertebrates and also may influence metabolic rate.