Reproductive responses to photoperiod persist in olfactory bulbectomized Siberian hamsters (Phodopus sungorus)

Abstract

In reproductively photoperiodic Syrian hamsters, removal of the olfactory bulbs leads to a marked and sustained increase in gonadotrophin secretion which prevents normal testicular regression in short photoperiods. In contrast, among reproductively non-photoperiodic laboratory strains of rats and mice, bulbectomy unmasks reproductive responses to photoperiod. The role of the olfactory bulbs has been proposed to have opposite effects on responsiveness to photoperiod, depending on the photoperiodicity of the reproductive system; however, Syrian hamsters are the only reproductively photoperiodic rodent species for which the role of the olfactory bulb in reproductive endocrinology has been assessed. This experiment evaluated the role of the olfactory bulbs in the photoperiodic control of reproduction in Siberian hamsters (Phodopus sungorus), an established model species for the study of neural substrates mediating seasonality. Relative to control hamsters housed in long days (15 h light/day), exposure of adult male hamsters to short days (9 h light/day) for 8 weeks led to a temporal expansion of the pattern of nocturnal locomotor activity, testicular regression, decreases in testosterone (T) production, and undetectable levels of plasma folliclestimulating hormone (FSH). Bilateral olfactory bulbectomy failed to affect any of these responses to short days. The patterns of entrainment to long and short days suggests that pre-pineal mechanisms involved in photoperiodic timekeeping are functioning normally in OBx hamsters. The absence of increases in FSH following bulbectomy in long days is incompatible with the hypothesis that the olfactory bulbs provide tonic inhibition of the HPG axis in this species. In marked contrast to Syrian hamsters, the olfactory bulbs of Siberian hamsters play essentially no role in the modulation of tonic gonadotrophin production or gonadotrophin responses to photoperiod.

Influence of the olfactory bulbs on blood leukocytes and behavioral responses to infection in Siberian hamsters

“…Hamsters were weighed (±0.1 g) weekly, and estimated testis volumes (ETVs) were determined at two-week intervals under light isoflurane anesthesia. In this experiment, reproductive neuroendocrine responses to photoperiod (testis sizes, plasma testosterone and FSH concentrations) in OBx hamsters were comparable to those of sham-operated hamsters and have been reported elsewhere (Prendergast et al, 2008b). …”

“…Gonadal responses to photoperiod are capable of substantial immunomodulatory effects in this species (Prendergast et al, 2008a), but a recent report from our laboratory indicates that the Siberian hamster OB is entirely unnecessary for normal photoperiodic regulation of the reproductive system (Prendergast et al, 2008b). Entrainment of the circadian system to long and short days (and presumably, the downstream generation of photoperiod-specific nightly melatonin signals) is likewise comparable among OBx and intact hamsters, eliminating differences in entrainment as a potential mechanistic explanation for the modest effects of OBx observed.…”

The influence of food restriction and photoperiod on reproduction in male spiny mice ( Acomys spinosissimus ): Evidence for terminal investment?

“…Changes in photoperiod are consistent between years and therefore photoperiod is an ideal environmental cue to time reproductive activity in many mammals (Bradshaw and Holzapfel, 2007). In most temperate rodents, short-day photoperiods (SD) induce the regression of the reproductive organs and inhibit reproduction, whereas long-day photoperiods (LD) cause reproductive recrudescence (Hegstrom and Breedlove, 1999;Prendergast et al, 2009;Trainor et al, 2006). In the tropics and sub-tropics where photoperiodic changes are much less pronounced, both reproductively photoperiodic and non-photoperiodic mammals have been identified.…”

Photoperiod Mediated Changes in Olfactory Bulb Neurogenesis and Olfactory Behavior in Male White-Footed Mice (Peromyscus leucopus)

“…Olfactory input is necessary for normal photoperiodic responses in gray mouse lemurs [40] and Syrian hamsters [41]–[42], potentially due to OBX-dependent increases in gonadotropins. However, OBX in Siberian hamsters has no affect on photoperiodic responses [43]. Thus, the exact role of olfaction and the olfactory bulbs in photoperiodic responses is species-specific, and in white-footed mice remains uninvestigated.…”