Differential Regulation of Luteinizing Hormone and Follicle-Stimulating Hormone in Male Siberian Hamsters by Exposure to Females and Photoperiod

Abstract

Siberian hamsters have decreased gonadotropin levels and testis size after short-day (SD) exposure. Upon transfer from short to long days, FSH and testis weight increase rapidly, whereas LH and T remain low for much longer. We investigated whether an additional environmental stimulus, specifically a female, could trigger an earlier release of LH and whether the response to the female was dependent on photoperiod. An increase in serum LH was induced in long day (LD), but not SD, males within minutes of female exposure. The ability of SD males to secrete LH upon female exposure was regained within 4 d of photostimulation. FSH was not secreted after female exposure, but varied with photoperiod. Thus, FSH and LH are differentially regulated by photoperiod and female exposure. In subsequent studies melatonin injections and a GnRH antagonist were used to show that photoperiod modulates the endocrine responsiveness of a male to a female via melatonin and that female-induced LH release is GnRH dependent. Collectively, these results suggest separation of gonadotropin signaling pathways by environmental stimuli and provide an excellent model to elucidate the effects of photoperiod on the processing of social and chemosensory inputs to the GnRH neurons of the hypothalamus.

The Preoptic Area and the RFamide-Related Peptide Neuronal System Gate Seasonal Changes in Chemosensory Processing

“…Responses within these systems to female chemosignals are gated seasonally, and responsiveness is not rescued by T replacement in SD animals. These results mirror photoperiodic regulation of behavioral and neuroendocrine responses to female chemosignals (Morin and Zucker 1978;Miernicki et al 1990;Anand et al 2002). Thus, changes within the POA and RFRP neuronal system present a likely means by which photoperiod regulates processing of chemosensory cues to restrict reproduction to the appropriate season.…”

“…As mentioned previously, exposure to female chemosignals elicits a rapid increase in LH and T in LD hamsters, presumably to facilitate reproductive behavior and function upon encountering a potential mate (Anand et al 2002;Richardson et al 2004;Nyby 2008). Investigation of the kisspeptin and RFRP systems, both potent upstream regulators of GnRH and LH release in male Syrian hamsters, revealed RFRP to be differentially responsive to female chemosignals across photoperiods.…”

“…In contrast to reproductively active males, hamsters housed under SD photoperiods fail to show either behavioral or neuroendocrine responses to these same chemosensory cues (Morin and Zucker 1978;Anand et al 2002). Differences in responses to female chemosignals are not solely attributable to differences in circulating T. Expression of male sexual behavior is T-dependent (Whalen and Debold 1974;Powers and Bergondy 1983), and hamsters housed under SD photoperiods undergo gonadal regression and have correspondingly low concentrations of circulating T (Sisk and Turek 1983).…”

“…For Syrian hamsters (Mesocricetus auratus), reproduction is limited to long-day (LD) photoperiods of spring and summer and exposure to winter-like, short-day (SD) photoperiods drives reproductive quiescence (Gaston and Menaker 1967). Quiescence can be observed through gonadal regression (Zucker and Morin 1977), but also through alterations in the responsiveness to sexually-relevant chemosensory cues (Morin and Zucker 1978;Anand et al 2002). The neural mechanisms by which photoperiod regulates chemosensory processing to limit reproduction to the appropriate season are currently unknown.…”

Photoperiodic effects on the male gonads of the Namibian gerbil, Gerbilliscus cf. leucogaster from central Namibia

“…In natural ecosystems, populations of wild rodents and some domesticated animals rely on photoperiod as a major proximate cue to control reproduction (Muteka et al 2006;Bronson 2009). However, some studies have shown that photoresponsive animals do not rely on photoperiod alone, but also on other changes in environmental conditions (Nelson et al 1983;Sicard et al 1993;Prendergast et al 2001;Anand et al 2002;Mutze 2007;Meheretu et al 2015;Fabio-Braga and Klein 2018). These may include ultimate factors such as food availability and social factors (Anand et al 2002;Meheretu et al 2015).…”