Photic cues induce multiple neuroendocrine adjustments in testicular function.

Neuroendocrine responses were evaluated in an outbred population of deer mice (Peromyscus maniculatus nebrascensis) after exposure to an inhibitory photoperiod (8:16 light-dark) for 10 wk. Deer mice were chosen as an animal model for this study because they are typical of naturally selected species that rely on environmental factors to signal the onset or cessation of annual reproductive effort. Short photoperiods induced multiple neuroendocrine adjustments as judged by three types of spermatogenic responses: normal, intermediate, and azoospermic individuals. Plasma concentrations of luteinizing hormone and testosterone coincided with gradations in spermatogenic activity. In contrast, plasma concentrations of follicle-stimulating hormone were unaffected. Prolactin secretion was lowered in all mice exposed to short day lengths, regardless of sperm count. These results demonstrate that short photoperiods engage at least three types of neuroendocrine adjustments: 1) a suppression in luteinizing hormone and testosterone secretion accompanying spermatogenic arrest, 2) a reduction in prolactin secretion independent of changes in testicular function, and 3) a null response in gonadotrophic hormone secretion in which spermatogenesis is unimpaired by short day lengths. The neuroendocrine subsets identified in this model provide new evidence that photic cues induce three types of adjustments in pituitary-testicular function. These subsets are readily identified, and they can be easily exploited to dissect and manipulate the suite of neural, endocrine, and metabolic adaptations triggered by environmental lighting among mammals with annual reproductive strategies.