Long‐Term Effects of Pinealectomy on Testicular Function, Luteinizing Hormone‐Releasing Hormone Hypothalamic System, and Plasma Prolactin Levels in the Mink, a Short‐Day Breeder

Abstract

Experiments in minks, as in a number of other seasonal breeders, clearly demonstrate that the pineal gland is essential for the photoperiodic control of reproduction. While maintenance of pineal-intact minks under natural photoperiods results in a set of seasonally appropriate changes in testicular activity, pinealectomized minks undergo none of these changes but rather remain sexually inactive as under long-day conditions. Thus, the consequences of pinealectomy differ from one photoperiodic species to another, but the unifying feature is the organism's need for the pineal gland to respond appropriately to changes in day length. Although the precise mechanism by which the pineal regulates hypothalamic-pituitary gonadal function remains unknown, the results of the present study indicate that, in the mink, luteinizing hormone-releasing hormone axonal transport is affected by pinealectomy. Furthermore, our results suggest that the pineal does not act exclusively upon the neuroendocrine-gonadal system but also acts on other functions that are influenced by photoperiod. Pinealectomized minks left in natural conditions cannot adjust their prolactin secretion in response to either long or short photoperiods. Operated animals continued to have plasma prolactin variations but at irregular intervals and with no apparent relation to the time of the year. The data strengthen the hypothesis that melatonin may act at some point on the hypothalamic neuroendocrine systems, which regulate the two functions differently, and that melatonin is not an anti- or progonadal substance but rather a seasonal transducer.

Suprachiasmatic Nucleus Lesions Abolish Photoperiod-Induced Changes in the Testis Function and GnRH Immunoreactivity in the Mink, a Short-Day Breeder

“…Brains of intact and SCNx minks were fixed for the immunocytochemical study of the GnRH neurohormonal system as previously de scribed [5,24]. After deep equi-thesin anesthesia, animals were per fused through the heart with PBS followed by a fresh solution o f 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.2) containing 0.2% picric acid.…”

“…Brains were then removed from the skull, carefully dissected and the hypothalamus was immersed in the perfusion fixative at 4°C for the next 24-48 h. They were then rinsed for 18 h in PBS containing 10% sucrose and frozen in liquid nitrogen. Ten-micron-thick frontal sections were cut in a cryostat and serially thaw-mounted on gelatincoated histological slides before being treated for indirect immunof luorescence microscopy as previously described [5,24]. Sections were incubated for 18 h with a primary antibody diluted 1:200, GnRH rabbit antiserum raised in our laboratory [24] and for 3 h with a secondary antibody conjugated with fluorescein isothyocyanate diluted 1:100.…”

“…In this short-day breeder, photoregulation of the annual neuroendocrinological cycle involves a circadian rhythm of photosensitivity with a res ponsive phase that begins 10 h after dawn and the light coinci dence with the photosensitive phase leads to inhibition of the hypothalamic GnRH-secreting system [1, 2], This circadian rhythm of photosensitivity feature causes the seasonal recrudes cence of gonadal activity to occur in November when day length becomes shorter than 10 h [3,4], The neural and neuroendocrine mechanisms involved in the light-induced inhibition of the GnRH hypothalamic system are still unknown. But the pineal gland is known to be a critical element, since animals exposed to a natural photoperiod have Received: July 27, 1990 Accepted after revision: December 10, 1990 been found to remain sexually inactive for more than 3 years after pinealectomy [5]. Moreover, an immunocytochemical study of GnRH-secreting neurons showed that in pinealectomized minks the perikarya are highly immunoreactive, whereas the axonal endings in the external layer of the median eminence remained small and weakly immunoreactive.…”

Morphofunctional Evidence for the Involvement of Hypothalamic Dopaminergic and GABAergic Neurons in the Mechanisms of Photoperiod-Dependent Prolactin Release in the Mink

“…There is, however, strong evidence that the pineal and particularly melatonin may play a major role in this regulatory system [review in 36]. It should be mentioned that in the mink, pinealectomy abolishes the synchronizing ef fect of the annual photoperiodic cycle on plasma PRL levels [17]. Ongoing experiments are now exploring the possible effects of melatonin on hypothalamic dopaminergic and GABAergic neurons.…”

“…The heparinized blood was immediately centrifuged and the plasma was stored frozen at -25 °C until the hormonal levels were determined. Plasma PRL concentrations were measured using a double-antibody radioimmunoassay previously adapted to the mink [16,17]. All samples were treated in the same way.…”