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In seasonal species, the photoperiod (i.e. day length) tightly regulates reproduction to ensure that birth occurs at the most favourable time of year. In mammals, a distinct photoneuroendocrine circuit controls this process via the pineal hormone melatonin. This hormone is responsible for the seasonal timing of reproduction, but the anatomical substrates and the cellular mechanisms through which melatonin modulates seasonal functions remain imprecise. Recently, several genes have been identified as being regulated by the photoperiod in the brain of seasonal mammals. These genes are thought to play active roles in the regulation of seasonal biology, notably for the adjustment of reproduction and body weight. Here, we briefly review findings associated with the control of seasonal breeding and describe recent data ascribing photoperiodic roles to type 2 and type 3 deiodinases, to the Kiss1/GPR54 system and to the RFamide-related peptides.Interestingly, these systems involve different hypothalamic nuclei, suggesting that several brain loci may be crucial for melatonin to regulate reproduction, and thus represent key starting points to identify the long-sought-after mode and site(s) of action of melatonin. Such findings raise great hopes for the future and could herald a new era of research in the field of seasonal biology.

Kisspeptin (KP) neurones in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (Arc) are important elements in the neuronal circuitry regulating gonadotropin-releasing hormone (GnRH) secretion. KP and co-synthesised neuropeptides/neurotransmitters act directly on GnRH perikarya and processes. GnRH neurones not only form the final output pathway regulating the reproductive functions of the anterior pituitary gland, but also provide neuronal input to sites within the hypothalamus. The current double-label immunohistochemical studies investigated whether GnRH-immunoreactive (IR) projections to the RP3V and/or Arc establish morphological connections with KP-IR neurones at these sites. To optimise visualisation of KP immunoreactivity in, respectively, the RP3V and Arc, ovariectomised (OVX) oestrogen-treated and OVX oil-treated female mice were studied. Confocal laser microscopic analysis of immunofluorescent specimens revealed GnRH-IR axon varicosities in apposition to approximately 25% of the KP-IR neurones in the RP3V and 50% of the KP-IR neurones in the Arc. At the ultrastructural level, GnRH-IR neurones were seen to establish asymmetric synaptic contacts, which usually reflect excitatory neurotransmission, with KP-IR neurones in both the RP3V and Arc. Together with previous data, these findings indicate reciprocal connectivity between both of the KP cell populations and the GnRH neuronal system. The functional significance of the GnRH-IR input to the two separate KP cell populations requires electrophysiological investigation.

We have examined the effects of adrenalectomy (ADX) on neuropeptide Y (NPY) content in the posterior pituitary of the rat. In rats given 2% saline to drink for 12 days, plasma osmolality increased to 343 +/- 18 compared to 291 +/- 5 mosm/kg in control animals on water (mean +/- SEM). Hypertonic saline treatment resulted in increased posterior pituitary NPY content measured by radioimmunoassay (2.42 +/- 0.36 ng) compared to the control group (1.18 +/- 0.16 ng). Total NPY immunoreactivity in tissue extracts coeluted with synthetic NPY(1-36) on reversed-phase high-performance liquid chromatography. In ADX rats given 2% saline to drink, plasma osmolality increased to 355 +/- 35 mosm/kg, but posterior pituitary NPY content was not different from the control group (1.41 +/- 0.11 ng). Immunohistochemical staining for NPY in the posterior pituitary also revealed a saline-induced increase in the number of NPY-immunoreactive nerve fibers, an increase which was not evident in the ADX group. No NPY immunoreactivity was observed in the intermediate zone. These results are the first evidence that the saline-induced increase in NPY in the posterior pituitary is under corticosteroid control.

Glucagon-like peptide-1 (GLP-1) immunoreactivity has been found in autonomic and neuroendocrine brain regions, whereas only limited data are available regarding the characterization and localization of brain GLP-1 receptors. In the present study, using quantitative in vitro autoradiography, a high density of specific binding sites for GLP-1 was characterized on sections of the posterior pituitary lobe of the rat. Low specific binding of radiolabeled GLP-1 was found in the anterior lobe and no specific binding in the intermediate lobe. To examine the specificity of GLP-1 binding sites, sections of the posterior lobe were incubated with radiolabeled GLP-1 in the presence of various peptides. Radiolabeled [Tyr39]exendin-4, a specific GLP-1 agonist, bound to these receptor sites with the same affinity as GLP-1, while glucagon and vasoactive intestinal peptide (VIP) were unable to displace 125I-GLP-1. Both unlabeled exendin-4 and GLP-1 inhibited this binding with equally high affinity. Using 125I-[Tyr39]exendin-4 as radiolabel, the concentration of biding sites was found to be 7.8 +/- 0.4 fmol/mg tissue. Further analysis of the binding data from experiments with tissue slices revealed the presence of high and low affinity binding sites. In experiments with unlabeled [Tyr39]exendin-4, the KdS were 6.2 +/- 1.4 x 10(-12) and 9.3 +/- 1.5 x 10(-10) M, respectively, and in experiments with unlabeled GLP-1, 3.4 +/- 1.8 x 10(-12) and 5.9 +/- 1.5 x 10(-10) M, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution of substance P was demonstrated by immunohistochemical methods, and receptor binding with 125I-8-iodinated 8-tyrosine-substance P as an analog was used to describe possible specific binding sites in the hypothalamo-hypophysial system of the rat. The immunohistochemical experiments showed fibers exhibiting substance P-immunoreactivity in the external and internal layer of the median eminence. In the external layer some of the fibers were located in apposition to the portal capillaries. In the posterior pituitary substance P-immunoreactive fibers were identified. These terminals varied in size and were located near the capillaries as well as in the neuropil. The anterior lobe contained a number of substance P-immunoreactive fibers distributed around the pituitary cells. The kinetic analysis on crude membrane fractions from anterior and posterior pituitaries revealed the presence of a specific binding site for substance P in the anterior pituitary. These data indicate that substance P may be released from the median eminence and posterior pituitary to the vascular system and reach specific receptors in the anterior pituitary. However, a direct innervation of the anterior pituitary is possible. The lack of binding sites in the posterior pituitary indicates that fibers in this organ release substance P into the blood stream.