Immunocytochemical demonstration of GAP-like immunoreactive neuronal elements in the human hypothalamus and pituitary

Afferent neuronal control of type-I gonadotropin releasing hormone neurons in the human

Understanding the regulation of the human menstrual cycle represents an important ultimate challenge of reproductive neuroendocrine research. However, direct translation of information from laboratory animal experiments to the human is often complicated by strikingly different and unique reproductive strategies and central regulatory mechanisms that can be present in even closely related animal species. In all mammals studied so far, type-I gonadotropin releasing hormone (GnRH) synthesizing neurons form the final common output way from the hypothalamus in the neuroendocrine control of the adenohypophysis. Under various physiological and pathological conditions, hormonal and metabolic signals either regulate GnRH neurons directly or act on upstream neuronal circuitries to influence the pattern of pulsatile GnRH secretion into the hypophysial portal circulation. Neuronal afferents to GnRH cells convey important metabolic-, stress-, sex steroid-, lactational-, and circadian signals to the reproductive axis, among other effects. This article gives an overview of the available neuroanatomical literature that described the afferent regulation of human GnRH neurons by peptidergic, monoaminergic, and amino acidergic neuronal systems. Recent studies of human genetics provided evidence that central peptidergic signaling by kisspeptins and neurokinin B (NKB) play particularly important roles in puberty onset and later, in the sex steroid-dependent feedback regulation of GnRH neurons. This review article places special emphasis on the topographic distribution, sexual dimorphism, aging-dependent neuroanatomical changes, and plastic connectivity to GnRH neurons of the critically important human hypothalamic kisspeptin and NKB systems.

Neuropeptides in hypothalamic neuronal disorders

A few examples of hypothalamic, peptidergic disorders leading to clinical signs and symptoms are presented in this review. Increased activity of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) and decreased activity of the vasopressin neurons in the biological clock and of the thyroxine-releasing hormone (TRH) neurons in the PVN contribute to the signs and symptoms of depression. In men, the central nucleus of the bed nucleus of the stria terminalis (BSTc) is about twice as large and contains twice as many somatostatin neurons as in women. In transsexuals this sex difference is reversed, pointing to a role of this structure in gender. Luteinizing hormone-releasing hormone (LHRH) neurons are formed in the fetal olfactory placade and migrate along the terminal nerve fibers into the hypothalamus. In Kallmann's syndrome the migration process of the LHRH (gonadotropin-releasing hormone) neurons is aborted, which explains the joint occurrence of hypogonadotropic hypogonadism and anosmia in this syndrome. In postmenopausal women, the neurons of the infundibular nucleus hypertrophy and become hyperactive because of the disappearance of the estrogen feedback and contain hyperactive peptidergic neurons. Climacteric flushes may be caused by hyperactivity of the neurokinin-B or LHRH neurons in this nucleus. The hypocretin (orexin) neurons in the perifornical area are involved in sleep. In narcolepsy with cataplexy, a loss of these neurons, probably due to an autoimmune process, is found. Obese subjects with a mutation in the gene that encodes for leptin, the preproghrelin gene, or the alpha-melanocyte-stimulating hormone (alpha-MSH) gene have been described. Decreased numbers and activity of the oxytocin neurons in the PVN may be responsible for the absence of satiety in Prader-Willi syndrome. Moreover, a glucocorticoid receptor polymorphism is associated with obesitas and dysregulation of the hypothalamus-pituitary-adrenal axis. In contrast, two single nucleotide polymorphisms (SNPs) of the AGRP gene have been associated with anorexia nervosa.

Topography and associations of luteinizing hormone-releasing hormone and neuropeptide Y-immunoreactive neuronal systems in the human diencephalon

Neuropeptide Y (NPY) potentiates the effect of luteinizing hormone-releasing hormone (LHRH) on luteinizing hormone secretion in several species, including human. In addition to the pituitary sites, the interactions of the NPY and LHRH systems may involve diencephalic loci. However, the morphologic basis of this putative communication has not yet been elucidated in the human brain. To discover interaction sites, the distribution and connections of LHRH and NPY-immunoreactive (IR) neuronal elements in the human hypothalamus were investigated by means of light microscopic single- and double-label immunocytochemistry. NPY-IR perikarya and fibers were found to be widely distributed in the ventral diencephalon, with high densities in the preopticoseptal, periventricular, and tuberal regions. Small neuronal cell groups were infiltrated with a dense network of varicose NPY-IR fibers in the lateral preoptic area. The LHRH-IR perikarya were located mainly in the preopticoseptal region, diagonal band of Broca, lamina terminalis, and periventricular and infundibular nuclei. A few LHRH-IR neurons and fibers were scattered in the mamillary region. The overlap between the NPY and LHRH systems was apparent in the periventricular, paraventricular, and infundibular nuclei. Double-labeling immunohistochemistry showed NPY-IR axon varicosities in contact with LHRH-IR perikarya and main dendrites. The putative innervation of LHRH neurons by NPY-IR fibers was also seen in 1-microm-thick plastic sections and with confocal laser scanning microscope, thus further supporting the functional impact of NPY-IR terminals on LHRH-IR neurons. The present findings suggest that the hypophysiotropic LHRH-synthesizing neurons may be innervated by intrahypothalamic NPY-IR fibers. Confirmation by ultrastructural analysis would demonstrate that the LHRH system in the human hypothalamus is regulated by NPY, as has been demonstrated in nonhuman species.