Somatostatin receptor activation is involved in the control of daily torpor in a seasonal mammal

Abstract

Scherbarth F, Diedrich V, Dumbell RA, Schmid HA, Steinlechner S, Barrett P. Somatostatin receptor activation is involved in the control of daily torpor in a seasonal mammal. Am J Physiol Regul Integr Comp Physiol 309: R668 -R674, 2015. First published July 8, 2015 doi:10.1152/ajpregu.00191.2015 show spontaneous daily torpor only after ϳ2 mo in winter-like short photoperiods (SP). Although some SP-induced hormonal changes have been demonstrated to be necessary for the occurrence of seasonal torpor, the whole set of preconditions is still unknown. Recent findings provide evidence that the hypothalamic pituitary growth axis is involved in endocrine responses to SP exposure in the photoperiodic hamsters. To examine whether suppression of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) secretion affects the incidence of daily torpor, we used two somatostatin receptor agonists, pasireotide (SOM230) and octreotide, with different affinity profiles for receptor subtypes. Pasireotide strikingly increased the torpor frequency in male hamsters compared with sham-treated controls, and torpor duration was often increased, which in some cases exceeded 12 h. In contrast, administration of octreotide reduced the body weight of SP hamsters but had only a marginal effect on torpor frequency in males and no effect in females. Together with measured concentrations of circulating IGF-1, the present results strongly suggest that reduced activity of the GH/IGF-1 axis is not critical for stimulation of torpor expression but activation of specific somatostatin receptors is critical. This putative role for certain somatostatin receptor subtypes in torpor induction provides a promising new approach to unravel the endocrine mechanisms of torpor regulation.GH/IGF-1 axis; somatostatin receptors; torpor; seasonality; pasireotide (SOM230); octreotide SPONTANEOUS DAILY TORPOR REPRESENTS one trait of the substantial acclimation to winter-like short photoperiods (SP) in Siberian hamsters (Phodopus sungorus; a.k.a. Djungarian hamster; for review, see Ref. 26). Together with a considerably reduced body weight and improved insulative properties of the white winter fur, daily torpor contributes significantly to reduce energy demands (12). Unlike fasting-induced torpor, spontaneous daily torpor is not necessarily linked to food scarcity. Instead, Siberian hamsters show this energy-saving behavior after ϳ10 wk of SP exposure despite food abundance. Interestingly, the frequency of torpor varies largely between individuals, ranging from animals that show torpor almost every day to individuals that never enter torpor (23). Thus these hamsters exhibit an individual propensity to use this state of reduced metabolism and body temperature (T b ). Despite several decades of study, the underlying metabolic and/or hormonal factors that tip the balance in support of, or against, entry into torpor at the beginning of the (diurnal) resting phase are still unknown.SP-mediated loss of body weight appears to be a prerequisite for the occurrence of torpo...

Thyroid hormone status affects expression of daily torpor and gene transcription in Djungarian hamsters (Phodopus sungorus)

“…SDT is a period of hypometabolism and hypothermia (Tb< 32°C) that occurs unpredictably during the light phase for 0.5-11 hours, even without obvious challenges like low Ta or food restriction (Heldmaier and Ruf, 1992;Ruf et al, 1993). Although different hormonal systems (testosterone, leptin, somatostatin) have been shown to be involved in the expression of spontaneous torpor, the exact driving mechanisms for torpor are still unclear (Freeman et al, 2004;Scherbarth et al, 2015;Vitale et al, 1985).…”

Implications of being born late in the active season for growth, fattening, torpor use, winter survival and fecundity

“…Our results suggest that an increased use of torpor seems to conflict with the energy-demanding processes of structural growth. Supporting evidence for this hypothesis is provided by a recent study of Scherbarth et al (2015) showing that a suppression of growth hormone secretion via pasireotide, an analog of the growth hormone release inhibiting hormone somatostatin, increased the incidence of daily torpor in Siberian hamsters ( Phodopus sungorus ) exposed to a (winter-like) short photoperiod. Hence, high levels of growth hormone required for the activation of the growth axis may prevent individuals from entering a torpid state.…”

What can seasonal models teach us about energy balance?

“…Spontaneous daily torpor only occurs when adaptations of fur, body weight and gonads are completed and it has been shown that the accompanying reductions in prolactin, leptin, growth hormone and testosterone are permissive for torpor behavior (Ruby et al 1993, Freeman et al 2004, Scherbarth et al 2015. When these hormones are supplied, they are able to reduce torpor incidence or entirely block torpor.…”