The action of luteinizing hormone on the testis

Open conformers: the hidden face of MHC-I molecules

A pool of MHC-I molecules present at the plasma membrane can dissociate from the peptide and/or the light chain, becoming open MHC-I conformers. Whereas peptide-bound MHC-I molecules have an important role in regulating adaptive and innate immune responses, through trans-interactions with T cell and NK cell receptors, the function of the open MHC-I conformers is less clear but seems to be related to their inherent ability to cis-associate, both with themselves and with other receptors. Here, we review data indicating the open MHC-I conformers as regulators of ligand-receptor interactions and discuss the biological implications for immune and non-immune cells. The likelihood that the MHC-I heavy chains have hidden functions that are determined by the amino acid sequence of the alpha1 and alpha2 domains are discussed.

Castration differentially regulates nitric oxide synthase in the hypothalamus and pituitary

Mammalian reproductive function is under control of the integrated hypothalamic-pituitary-gonadal (HPG) axis. Castration in male rats has been utilized as an effective tool to investigate hormonal interactions in the mammalian HPG axis. Recently, nitric oxide (NO) has been suggested to play a role in HPG hormonal regulation. In order to gain further insight into the function of the NO-NOS system in reproductive neuroendocrine control, particularly in the gonadal feedback regulation of the hypothalamic-pituitary unit, we examined steady state levels of nNOS mRNA, nNOS protein, and the important physiological index, NOS enzyme activity, of the intrinsic NOergic system in both hypothalamus and pituitary in castrated male rats and their sham-operated counterparts one week after surgery. In the pituitary, we found a significant four-fold increase in nNOS mRNA, p < 0.0003 compared to sham. Castration also resulted in a four-fold rise in pituitary nNOS protein, p < 0.02 compared to sham. Pituitary NOS enzyme activity was stimulated 2 fold, p < 0.003 after castration. In the hypothalamus, conversely, we observed no significant castration-modulated difference in either nNOS mRNA, nNOS protein or NOS enzyme activity. Thus, it appears that the hypothalamic NO-NOS system is either not required for hypothalamic adaptations to castration, although important in the release of LHRH under normal physiological conditions, or alternatively, the hypothalamus may become more sensitive to the effects of NO in the castrated state. In the pituitary, NO may attenuate the gonadotropin response to castration as a local balancing mediator.

Sustained effects of a single injection of ethanol on the hypothalamic-pituitary-gonadal axis in the male rat

The hormones responsible for regulating the hypothalamic-pituitary-gonadal axis are essential for proper reproductive function. Ethanol (EtOH) has been shown to exert its effect at all three levels of this axis. The present study defines striking differences in the time course of recovery of luteinizing hormone (LH) in gonadally intact, compared with, castrated male rats after acute EtOH administration. Serum levels of LH and testosterone were measured at various time points up to 2 weeks (1.5, 3, 24, 48, 72, 96, 168, and 336 hr) after a single intraperitoneal injection of either saline or 3 g/kg of EtOH in intact adult male rats. One EtOH injection significantly suppressed testosterone levels as low as 20% (p < 0.01) of saline-injected intact rats. This occurred as early as 1.5 hr after EtOH administration (the first measured time point), and statistically significant suppression was sustained for 96 hr. Similarly, LH levels showed a significant decrease. However, this significant fall in LH did not begin until 3 hr (p < 0.05) and continued up to 96 hr (p < 0.01), with a gradual return to control levels at 168 and 336 hr after treatment. Despite the significant and prolonged fall in testosterone levels in the EtOH-treated intact rats, beta-LH mRNA levels were inappropriately not elevated, as would be expected in the context of low circulating testosterone. However, at 168 and 336 hr, steady-state levels of beta-LH mRNA were significantly higher than seen in saline-injected controls (p < 0.05 and p < 0.01, respectively), temporally correlating with the return of serum LH to control. LH levels in the castrated animals were significantly suppressed at 1.5 hr (p < 0.05) and 3 hr (p < 0.01) after EtOH treatment, compared with controls, yet they returned much more quickly by 24 hr after treatment. beta-LH mRNA levels of castrated animals also showed a significant depression at 1.5 and 3 hr, and returned to control levels by 24 hr. In these rats, the hypothalamic LH-releasing hormone mRNA levels were not altered by a single EtOH injection at any time point. However, in the intact animals, there was a transient increase in LH-releasing hormone mRNA at 72 and 96 hr (p < 0.01 and p < 0.05, respectively) that may lead to the upregulation of beta-LH mRNA expression. These studies indicate that EtOH causes prolonged decreases in important serum hormones that are essential to the reproductive axis of the adult male rat.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.