The neuroendocrine basis for short-day induced testicular regression was studied in Syrian hamsters of the LSH/Ss Lak strain. Adult animals were maintained either under long or short days (14:10D or 6L:18D, respectively) and given single, daily ip injections of iV-methyl-D-aspartate (NMDA) (25 mg/kg BW); control animals received injections of saline. As expected, the testes of the short-day controls had completely regressed to a prepubertal condition within 6 weeks, a change that was associated with significantly reduced mean plasma gonadotropin levels. In contrast, the NMDA-treated hamsters from both the long-day and short-day groups, as well as the long-day controls, all maintained large testes and elevated plasma gonadotropin levels, although plasma luteinizing hormone (LH) was partially suppressed in the short-day group. In a second experiment which lasted 2 weeks, short-day hamsters with completely regressed testes were either transferred to long days, maintained further on short days, or maintained on short days and given a daily ip injection of NMD A (25 mg/kg BW). The short-term exposure to long days caused an expected increase in plasma and pituitary concentrations of follicle-stimulating hormone, pituitary LH, and testicular weight. Similar, but even more marked, changes were observed in the short-day hamsters that were treated with NMDA, including significant increases in plasma LH and serum testosterone concentrations. Moreover, histological examination revealed that the recrudescing testes from this latter group already contained mature spermatocytes and in some individuals even spermatozoa. These results demonstrate that NMDA receptors may play a pivotal role in both the termination and onset of the breeding season in photoperiodic species.
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