Seasonal changes in the hypothalamic vasopressinergic system of a wild Sahelian rodent, Taterillus petteri.

Seasonal variations in the immunoreactivity of vasopressinergic perikarya in the paraventricular (PVN), supraoptic (SON) and suprachiasmatic nuclei (SCN), and in the labelling of vasopressinergic fibres in the internal zone of the median eminence were studied in Taterillus petteri, a rodent that is found in the north Burkina Faso (formerly Upper Volta). In this region, there are four seasonal climatic combinations: the humid and hot, humid and cold, dry and cold, and dry and hot seasons. In the dry hot season, the rodents experience phases of torpor (adaptation to dryness). Immunoreactivity of the PVN and SON is highest during the dry cold season. Labelling is intense during the dry hot and humid hot seasons, and is at its lowest during the humid cold season. In the SCN, labelling of the perikarya is only dense during the dry hot season, whereas for the rest of the year, the immunoreactivity is weak or undetectable. The pattern of immunoreactive variations of vasopressin-positive fibres located in the internal zone of the median eminence is similar to those of vasopressinergic perikarya in the PVN and SON. These results suggest that there is an association between: (1) seasonal modifications in the immunoreactivity of PVN and SON vasopressinergic perikarya and vasopressinergic fibres of the internal median eminence, and (2) climatic conditions, water metabolism, behavioural activity and diet. It is not possible to establish a correlation between seasonal variations in water availability and fluctuations in the labelling of vasopressinergic perikarya in the SCN. However, labelling is intense when the animals are in torpor during the dry hot season.

Localization of melatonin binding sites in the pars tuberalis of the mink at three times during the seasonal testicular cycle.

The strict photoperiodic dependence of gonadotropic function observed in mink provides an excellent physiological model for studying the activity of GnRH hypothalamic neurons. In mink, exposure to more than 10 h light inhibits the activity of this neurohormonal system. Melatonin plays an essential role in this type of regulation of gonadotropic function. In mink, contrary to results in many other photosensitive species, melatonin was observed to mediate the gonadostimulating effect of short days. However, the binding sites and action of this substance have not yet been defined. We thus attempted to identify melatonin binding sites in the mink brain. The study was carried out at three times during the seasonal testicular cycle in male minks maintained under natural environmental conditions. Coronal sections were taken from the whole brain and pituitary gland. Using 2-[125I]iodomelatonin, we observed a strong concentration of binding sites in the pars tuberalis and not in the part of the median eminence surrounded by the pars tuberalis. This concentrated localization of melatonin binding sites brings up the problem of defining the action of this substance in photoregulating the activity of the GnRH neurohormonal system.

In situ hybridization of GnRH mRNA in the rat and the mink hypothalamus using biotinylated synthetic oligonucleotide probes.

The cellular localization of GnRH messenger RNA (mRNA) in the rat and the mink hypothalamus has been examined using a newly developed highly sensitive non-radioactive in situ hybridization procedure. Synthetic oligonucleotides labeled by addition of a biotin-21-dUTP tail at their 3' end can be used to detect GnRH mRNA in both species. Streptavidin-alkaline phosphatase revealed with nitroblue tetra-zolium-bromo-chloro-indolyl-phosphate as substrate makes possible detection of the biotinylated oligonucleotides. In the rat, our findings confirm results previously obtained using synthetic radioactive probes, and demonstrate the potency of and interest in using biotinylated oligonucleotides to identify related sequences of bases in tissues. The principle advantages include rapid signal detection, excellent spatial resolution, and low background. In the mink, the in situ hybridization method clearly confirms the characterization of GnRH-producing cells and also allows detection of GnRH cell bodies in conditions in which they are not detected by immunohistochemistry. Adaptation of the in situ hybridization to the detection of GnRH mRNA in species like the mink which shows seasonal reproductive activity is a crucial step. This method offers a new approach to problems as fundamental as changes in gene expression depending on photoperiod or under a variety of experimental conditions.

Short-day stimulation of testicular activity and immunoreactivity of the hypothalamic GnRH system in mink following deafferentation of the pineal body by bilateral superior cervical ganglionectomy and melatonin replacement.

The effects of superior cervical ganglionectomy on testicular function (testis volume and plasma testosterone levels) and the immunocytochemical activity of the GnRH hypothalamic system were studied in the mink, a short-day breeder. Animals reared in a natural photoperiod were (i) ganglionectomized at four different times during the period extending from the end of summer to the end of autumn (September 15, October 20, October 28, and December 1), and (ii) reared for 50 days in a short gonadostimulatory photoperiod (4L:20D). Lastly, an attempt was made to overcome the effects of superior cervical ganglion removal by administering melatonin to mink reared in a natural photoperiod. In mink reared in a natural photoperiod, deafferentation of the pineal on September 15 (L = 12.5 h) or October 20 (L = 10.5 h) resulted in consistently low values of testicular volume and plasma testosterone until the end of the experiment (February). When the operation was performed on October 28 (L = 10 h) testicular activity was initiated but only lasted a short time and did not allow maximal gonadal development. When superior cervical ganglionectomy was carried out on December 1 (L = 8.5 h), during the phase of renewed testicular activity, the increases in testicular volume and testosterone levels were not affected by the operation and the subsequent variation of these parameters was identical to that observed in intact animals. Similarly, in mink reared for 50 days in a photoperiod of 4L:20D before superior cervical ganglionectomy, deafferentation of the pineal did not prevent gonadostimulation induced by short days.(ABSTRACT TRUNCATED AT 250 WORDS)

Suprachiasmatic nucleus lesions abolish photoperiod-induced changes in the testis function and GnRH immunoreactivity in the mink, a short-day breeder.

Testicular activity (testis volume and plasma testosterone) and immunoreactive GnRH hypothalamic system were examined after suprachiasmatic nucleus (SCN) lesion in the mink, a short-day breeding mammal, whose sexual activity is inhibited by day lengths exceeding 10 h. In animals maintained under a natural photoperiod, SCN destruction performed during the period of maximum sexual activity (February) was shown to have no effect on onset of the testicular inactive period which begins at the end of winter and continues through spring. On the other hand, while gonadal activity began again at the end of autumn in intact animals, minks that had undergone SCN destruction remained sexually inactive until the end of the experiment period (February). The SCN could thus be crucial to the onset of sexual activity triggered by the reduction of day length, whereas onset of sexual inactivity is a spontaneous phenomenon. This was confirmed in a second experiment demonstrating that a short photoperiod (4 L:20 D), highly gonadostimulatory in intact animals, had no effect on testicular activity after SCN destruction. An immunocytochemical study of the hypothalamic GnRH system (staining intensity and number of labeled perikarya and immunoreactive endings in the external layer of the median eminence) also showed consistent by very low rates of immunoreactivity and number of labeled perikarya and endings in operated animals.

Morphofunctional evidence for the involvement of hypothalamic dopaminergic and GABAergic neurons in the mechanisms of photoperiod-dependent prolactin release in the mink.

This study was designed to examine possible relationships between the photoperiodic regulation of prolactin secretion and the activity of dopaminergic and GABAergic neurons projecting to the external layer of the median eminence. The study was carried out on the mink whose remarkable photosensitivity has been clearly demonstrated. The animals were reared in short (4L:20D) or long (20L:4D) photoperiods. The experiment began in November when day length is short (9.5 h). Dopaminergic and GABAergic neurons were studied using immunocytochemical methods allowing evaluation of the immunoreactivities of tyrosine hydroxylase (TH) and glutamate decarboxylase (GAD), which are respective markers of these neurons. The results were quantified by image analysis. The plasma prolactin level of animals maintained in 4L:20D decreased after 60 days and TH and GAD immunoreactivity were strongly stimulated. After 110 days, the prolactin concentration and TH and GAD immunoreactivity recovered their starting levels. In animals maintained in 20L:4D, the prolactin level was 3 times higher than at the beginning of the photoperiodic treatment but only dopaminergic neurons showed a change, i.e. a decrease in immunoreactivity. At the end of the experiment, prolactin secretion was no longer affected by the stimulatory effect of long-day treatment, and TH immunoreactivity remained low. These results confirm the generally accepted concept that dopaminergic neurons are potent PIF-producing components. GABAergic hypothalamic system appears to be implicated in photoperiodic PRL regulation, but this remains to be clearly demonstrated.

Peptidergic neurohormonal systems in the basal hypothalamus of the ferret and the mink: immunocytochemical study of variations during the annual reproductive cycle.

The hypothalamic systems secreting corticotropin-releasing hormone (CRF), somatostatin, oxytocin, vasopressin and luteinizing hormone-releasing hormone (LHRH) were characterized using immunochemistry, and variations were studied in relation to the recrudescence of testicular activity in the ferret and the mink, two species with opposite photoregulation of their annual reproductive cycles. Under the present conditions of study, the immunoreactivity of the CRF, somatostatin, and oxytocin systems showed no significant variation in either species. In contrast, in these two species, the immunoreactivity of the LHRH system varied considerably depending on the date of observation. The increase in the number and immunoreactivity of the LHRH-secreting neurons that occurred in November in the mink and in January in the ferret, is in agreement with previous results showing that the photoperiod plays an essential role in regulating the annual activity of the testis and that the photoperiodic environmental conditions required for the activation of the LHRH system differ between the species. Similarly, correlations could be found between an increase in immunoreactivity of the vasopressinergic axons projecting to the external median eminence and the recrudescence of testicular activity.