Immunocytochemical Study of GnRH and GnRH-Associated Peptide in Male Syrian Hamsters as a Function of Photoperiod and Gonadal Alterations

Abstract

Hypothalamic luteinizing-hormone-releasing hormone (GnRH) and gonadotropin-releasing-hormone-associated peptide (GAP) biosynthesis and storage were estimated by immunocytochemistry in male golden hamsters maintained in different photopenods. Intact or castrated male hamsters with subcutaneously inserted testosterone implants were exposed to long-day (14:10) or short-day photopenods (10:14) for 4-8 weeks. Exposure to short photoperiod for 4 weeks, an interval characterized by a suppression of gonadotropin secretion but not gonadal regression, was associated with an increase in the number of GnRH- and GAP-immunoreactive cells in the diagonal band of Broca/medial septum. Furthermore, morphometric analysis revealed that these animals displayed significantly more GnRH but not GAP immunoreactivity in the median eminence as opposed to hamsters exposed to long-day photopenods. In additional studies, gonadally regressed hamsters exposed to short day lengths for 8 weeks had equal numbers of GnRH cells as did the long-day controls. These patterns suggest that reproductive quiescence in golden hamsters is not the result of depletions of neuronal GnRH stores available for secretion.

Photoperiod Affects the Gonadotropin- Releasing Hormone Neuronal System of Male Prairie Voles <bolditalic>(Microtus ochrogaster)</bolditalic>

“…Taken together, these data suggest that short photoperiods inhibit GnRH secretion, at least in part, by inhibiting the release of this decapeptide at the level of the hypothalamus. These results are reminiscent of those reported for other rodent species [10, 11, 12], although the effect in previous studies both within and between species has not been consistent [13, 23, 24, 25]. …”

“…GnRH immunoreactive (GnRH-ir) cell numbers are increased in discrete brain regions of Syrian hamsters (Mesocricetus auratus [10, 11], and white-footed mice (Peromyscus leucopus) [12]housed in short days relative to long-day conspecifics. One study failed to find an increase in GnRH-ir cell numbers in Syrian hamsters exposed to short photoperiods, but did report that GnRH neurons were significantly larger in animals exposed to inhibitory photoperiods relative to long-day controls [13].…”

“…One study failed to find an increase in GnRH-ir cell numbers in Syrian hamsters exposed to short photoperiods, but did report that GnRH neurons were significantly larger in animals exposed to inhibitory photoperiods relative to long-day controls [13]. Additionally, the number and density of GnRH-ir fibers extending into the median eminence are increased in animals housed in inhibitory photoperiods [10, 12]. Taken together, these data suggest that inhibitory photoperiods in rodents suppress GnRH release sufficiently to prevent the maintenance of normal gonadal function [10, 12].…”

“…Additionally, the number and density of GnRH-ir fibers extending into the median eminence are increased in animals housed in inhibitory photoperiods [10, 12]. Taken together, these data suggest that inhibitory photoperiods in rodents suppress GnRH release sufficiently to prevent the maintenance of normal gonadal function [10, 12]. Additionally, GnRH synthesis appears to continue in rodents with prolonged exposure to short days; GnRH mRNA levels in the brains of Syrian hamsters are not decreased in short-day hamsters relative to long-day animals [14].…”

The GnRH system in the European sea bass (Dicentrarchus labrax)

“…This strategy was developed because of the divergence of the GAPs within the same species and because the distribution of immunoreactive GAPs was consistently reported to be similar to that of immunoreactive GnRH (Bloch et al 1992, Ronchi et al 1992, Polkowska & Przekop 1993.…”