Testosterone Suppresses Circadian Responsiveness to Social Cues in the Diurnal Rodent Octodon degus

Abstract

The diurnal, social rodent Octodon degus displays a robust sex difference in the ability to use social cues to facilitate reentrainment following a phase advance of the light cycle. Adult females housed with a female social cue donor reentrained 25% to 40% faster than did females reentraining alone. However, reentrainment rates of males were unaffected by exposure to female social cues during reentrainment. The authors hypothesized that males were less sensitive to the reentrainment-enhancing effects of social cues and that their higher threshold to the stimuli could be overcome if the social cues were either increased in strength or salience. Housing a male with two females significantly shortened the time to reentrain following a 9-h phase advance (p = 0.002). Housing with a sister had no effect on reentrainment. Therefore, male degus are able to respond to social cues but require the stimulus to be stronger than that for females. The effect of testosterone was tested by comparing reentrainment rates of castrated males before and after testosterone replacement both with and without a female social cue donor. Castrated males responded to a single female social cue donor, reentraining 35% faster than when housed alone (p = 0.006), whereas the time to reentrainment of intact males and males with testosterone capsule implants did not differ. Intact females were also implanted with testosterone and phase shifted with and without donors. Testosterone treatment eliminated the increase in reentrainment rates in the presence of social cues. The authors conclude that the rate of recovery from odor-enhanced phase shifts is modulated by activational effects of testosterone in male degus. Testosterone is also effective in suppressing social cue responsiveness in females, suggesting that testosterone's effects on responsiveness are not sexually dimorphic. This hormonal effect likely occurs by altering sensory system functions or CNS response to sensory information.

The regulation of neuroendocrine function: Timing is everything

“…Because hormones can have a widespread influence over physiology and behavior, and provide a means by which circadian information can be communicated systemically, it is important to determine how these rhythms are regulated. Not only are hormones modulated by the circadian system, but hormonal feedback to the SCN also influences circadian function (Dubocovich et al, 1996;Ellis and Turek, 1983;Hastings et al, 1997;Jechura et al, 2003;Labyak and Lee, 1995;Lewy and Sack, 1997;Morin et al, 1977). Together, these mechanisms controlling endocrine timing entail regulatory actions by the circadian system and provide extensive opportunities for empirical investigations of behaviorally relevant systems.…”

Odor-specific effects on reentrainment following phase advances in the diurnal rodent, Octodon degus

“…Intact males (when increased in number), intact females, and females with progesterone and estrogen replacement, but not OVX females, produce odors able to enhance reentrainment rate. It is unclear whether androgens are directly effective in male odor production, or whether testosterone is first converted to estrogen to produce the effective odor (22). We also found that circadian timing of the production of the odor cue by donors is not critical, since continuous exposure to odors from used bedding of intact females effectively accelerated reentrainment.…”

“…There may be no advantage to knowing the donor (sister) or to an increase in odor intensity by using a group of donors because female shifters are experiencing a ceiling effect with a 6-h phase advance with odors from a single, unfamiliar, female donor. In contrast, males were able to use strengthened odor intensity from multiple females to accelerate their reentrainment (22). However, the odors from a familiar conspecific or a group of conspecifics might show a greater benefit under conditions where recovery from a phase shift is more difficult (27).…”

“…In experiment 3, animals were housed in pairs similar to earlier experiments for the first 2 shifts (15,16,22), with the phase-shifting animal and the entrained odor donor separated by a wire mesh divider in a Nalgene cage with dimensions of 48 cm ϫ 38 cm ϫ 19.5 cm. Only shifting females were provided with running wheels when cohoused with another female.…”

Social forces can impact the circadian clocks of cohabiting hamsters

“…One interesting hypothesis is that low levels of testosterone, here due to social subjugation [22], render the circadian system of hamsters more susceptible to environmental influences, which could be social or not. Castration decreases and testosterone replacement restores t precision in male Syrian hamsters [23], and in degus (Octodon degus) testosterone decreases the responsiveness of the circadian system to social cues that facilitate re-entrainment to shifts in the LD cycle [24].…”