The physiological significance of pulsatile LHRH secretion in man: gonadotrophin responses to physiological doses of pulsatile versus continuous LHRH administration

The significance of gene expression dynamics in neural stem cell regulation

Neural stem cells (NSCs) actively proliferate and generate neurons and glial cells (active state) in the embryonic brain, whereas they are mostly dormant (quiescent state) in the adult brain. The expression dynamics of Hes1 are different between active and quiescent NSCs. In active NSCs, Hes1 expression oscillates and periodically represses the expression of proneural genes such as Ascl1, thereby driving their oscillations. By contrast, in quiescent NSCs, Hes1 oscillations maintain expression at higher levels even at trough phases (thus continuous), thereby continuously suppressing proneural gene expression. High levels of Hes1 expression and the resultant suppression of Ascl1 promote the quiescent state of NSCs, whereas oscillatory Hes1 expression and the resultant oscillatory Ascl1 expression regulate their active state. Furthermore, in other developmental contexts, high, continuous Hes1 expression induces astrocyte differentiation or the formation of boundaries, which function as signaling centers. Thus, the expression dynamics of Hes1 are a key regulatory mechanism generating and maintaining various cell types in the nervous system.

Mechanisms of Disease: late-onset hypogonadism

Late-onset hypogonadism (formerly called the andropause) is a clinical and biochemical syndrome associated with advancing age, which is characterized by typical signs and symptoms and a deficiency in serum testosterone levels. Age-related hypoandrogenism in the male is a result of the interaction of hypothalamopituitary and testicular factors. The hypothalamic pulsatile secretion of gonadotropin-releasing hormone is blunted, due to increased hypothalamic sensitivity to inhibition by steroids, but the responsiveness of the pituitary gonadotrophs seems to be intact. In addition, testicular volume as well as Leydig cell mass and reserve function are diminished. Taken together, these mechanisms result in reduced testosterone secretion and the loss of nycthemeral variability.

Control of luteinizing hormone-releasing hormone pulse generation in nonhuman primates

1. The pulsatile release of luteinizing hormone-releasing hormone (LHRH) is critical for reproductive function. However, the exact mechanism of LHRH pulse generation is unclear. The purpose of this article is to review the current knowledge on LHRH pulse generation and to discuss a series of studies in our laboratory. 2. Using push-pull perfusion in the stalk-median eminence of the rhesus monkey several important facts have been revealed. There is evidence indicating that LHRH neurons themselves have endogenous pulse-generating mechanisms but that the pulsatility of LHRH release is also modulated by input from neuropeptide Y (NPY) and norepinephrine (NE) neurons. The release of NPY and NE is pulsatile, with their pulses preceding or occurring simultaneously with LHRH pulses, and the neuroligands NPY and NE and their agonists stimulate LHRH pulses, while the antagonists of the ligands suppress LHRH pulses. 3. The pulsatile release of LHRH increases during the estrogen-induced LH surge as well as the progesterone-induced LH surge. These increases are partly due to the stimulatory effects of estrogen and progesterone on NPY neurons. 4. An increase in pulsatile LHRH release occurs at the onset of puberty. This pubertal increase in LHRH release appears to be due to the removal of tonic inhibition from gamma aminobutyric acid (GABA) neurons and a subsequent increase in the inputs of NPY and NE neurons to LHRH neurons. 5. There are indications that additional neuromodulators are involved in the control of the LHRH pulse generation and that glia may play a role in coordinating pulses of the release of LHRH and neuromodulators. 6. It is concluded that the mechanism generating LHRH pulses appears to comprise highly complex cellular elements in the hypothalamus. The study of neuronal and nonneuronal elements of LHRH pulse generation may serve as a model to study the oscillatory behavior of neurosecretion.