Evidence for synergism between copper and prostaglandin E2 in stimulating the release of gonadotropin-releasing hormone from median eminence explants: Na+/Cl- requirements

Copper(II) Binding by the Earliest Vertebrate Gonadotropin-Releasing Hormone, the Type II Isoform, Suggests an Ancient Role for the Metal

In vertebrate reproductive biology copper can influence peptide and protein function both in the pituitary and in the gonads. In the pituitary, copper binds to the key reproductive peptides gonadotropin-releasing hormone I (GnRH-I) and neurokinin B, to modify their structure and function, and in the male gonads, copper plays a role in testosterone production, sperm morphology and, thus, fertility. In addition to GnRH-I, most vertebrates express a second isoform, GnRH-II. GnRH-II can promote testosterone release in some species and has other non-reproductive roles. The primary sequence of GnRH-II has remained largely invariant over millennia, and it is considered the ancestral GnRH peptide in vertebrates. In this work, we use a range of spectroscopic techniques to show that, like GnRH-I, GnRH-II can bind copper. Phylogenetic analysis shows that the proposed copper-binding ligands are retained in GnRH-II peptides from all vertebrates, suggesting that copper-binding is an ancient feature of GnRH peptides.

Chronic effects of copper exposure versus endocrine toxicity: two sides of the same toxicological process?

Chronic sub-lethal exposure to copper (Cu) causes a series of cellular and physiological changes in fish that enable the animal to survive. Copper is also an endocrine disrupting metal in the aquatic environment, and has a number of normal neuro-endocrine roles in vertebrates. This paper explores whether the chronic effects of Cu exposure can be explained by the effects of Cu on neuro-endocrine functions in fish. Chronic Cu exposure involves complex physiological adjustments in many body systems, including increased oxygen consumption, reduced mean swimming speed, up-regulation of ionic regulation, decreasing lymphocyte levels and increasing neutrophils, altered immunity, modulation of Cu-dependent and independent enzyme activities, and proliferation of epithelial cells in gills or intestine. These responses can occur with exposure via the food or the water and can be rationalised into three major categories: (1) up-regulation of enzymes/metabolism (2) altered haematopoietic responses and (3) altered cellularity (cell type, turnover or size) in tissues. Some of these responses can be explained by stimulation of general stress responses, including the adrenergic response and stimulated cortisol release via the hypothalamic-pituitary-interrenal axis. This can occur despite evidence of vacuolation and foci of necrosis in the brain, and increased macrophage activity, in the kidney of fish exposed to dietary Cu. In addition to generic stress responses, Cu regulates specific neuro-endocrine functions, including the loss of circadian rhythm during dietary Cu exposure that involves the failure to respond to circulating melatonin and a loss of circulating serotonin. We conclude that the chronic physiological effects of Cu and apparent endocrine disrupting effects of Cu are two sides of the same toxicological process.

Muscarinic involvement in the regulation of gonadotropin-releasing hormone in the cyclic rat

The release of gonadotropin-releasing hormone (GnRH) from the median eminence (ME) in cyclic rats was stimulated to a significant extent by the selective muscarinic antagonists 11[(2)(diethylamino)methyl][-1-piperidinyl]-acetyl-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX-116) and methoctramine, and to a lesser extent also by other ligands selectively antagonistic to m1 and m3 receptors. Such stimulation was estrous-cycle-dependent and was not achieved by muscarinic agonists. We suggest that the effect is induced via the m4 receptor subtype. Attempts to block the muscarinic-antagonist-induced stimulation of GnRH release with a variety of drugs were successful only in the presence of prazosin, an antagonist to alpha 1-adrenergic receptors. One possible explanation for this muscarinically mediated stimulation of GnRH release is that it results from cross-talk between the muscarinic and the alpha 1-adrenergic receptors, i.e., muscarinic agonists might inhibit the release induced by alpha 1-agonists, and muscarinic antagonists, by cancelling this inhibitory effect, might thus allow the endogenous alpha 1-agent, norepinephrine, to induce the release of GnRH.

A potential mechanism for copper amplification of prostaglandin E2 action: attenuation of prostaglandin E2-induced efflux of cyclic AMP from median eminence explants

It is known that prostaglandin E2 (PGE2) stimulation of LHRH release from the median eminence-arcuate nucleus (MEA) is mediated by the cAMP pathway, and a short pretreatment with copper markedly amplifies this release process. Because stimulation of cAMP accumulation is accompanied by cAMP efflux in many tissues, we considered the possibility that attenuation of cAMP efflux is one mechanism by which copper can enhance PGE2 action. When rat MEA explants were incubated in vitro, PGE2 induced a rapid (less than 2.5 min) and sustained (15 min) increase in cAMP efflux, the degree of which was a function of [PGE2]: by 5 min exposure to 10 microM PGEs2, efflux was 8-fold greater than the control (no PGE2) and it accounted for 12.4% of the total (tissue + medium) cAMP. Unlike the dramatic increase in cAMP efflux, PGE2 induced a moderate increase in cAMP content (49%) and in the incorporation of [3H] adenine into [3H] cAMP (78%); this increase was transient: it was evident after a 2.5 but not after a 5 min period of PGE2 exposure. Copper pretreatment did not alter this PGE2-induced increase in tissue cAMP content. In contrast, copper markedly inhibited (by 49%-66%) PGE2-induced cAMP efflux and this inhibition was noted regardless of the length of PGE2 exposure and PGE2 concentration. There was no evidence for hydrolysis of [3H]3',5'-cAMP included in the medium during the incubation with PGE2 with and without copper pretreatment. In summary, copper attenuated PGE2-induced cAMP efflux from MEA explants and this attenuation is not a consequence of a reduction in the availability of intracellular cAMP nor of hydrolysis of cAMP extracellularly.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.

A putative role for extracellular ATP: facilitation of 67copper uptake and of copper stimulation of the release of luteinizing hormone-releasing hormone from median eminence explants

We have previously shown that extracellular copper stimulates the release of the luteinizing hormone-releasing hormone (LH-RH) from explants of the median eminence area (MEA), that chelated copper (Cu) but not ionic Cu is the active form of the metal, and that there is a direct correlation between the ligand specificity for 67Cu uptake and Cu action. In this study, we examined the possibility that extracellular ATP can serve as a ligand facilitating Cu action on the LH-RH neuron. Hypothalamic slices or MEA explants of adult male rats were used. It was found that ATP facilitates 67Cu uptake by hypothalamic slices when Cu:ATP molar ratio was 1:2000 but not 1:2. Keeping the [Cu] constant (150 microM) and varying [ATP], ATP facilitation of Cu stimulation of LH-RH release from MEA explants was found to be a saturable function of [ATP]; maximal facilitation occurred with 2.5 mM ATP. When the nucleotide phosphate specificity for facilitation of Cu action was assessed, ADP, ATP, alpha, beta-methylene-ATP (the non-hydrolyzable analogue of ATP) and GTP were equally effective, whereas AMP and adenosine were ineffective. These results indicate that extracellular ATP can facilitate Cu action on the LH-RH neuron and they are consistent with two mechanisms: (1) ATP facilitating Cu uptake and hence, Cu action and (2) ATP facilitating Cu action via an interaction with a purinergic receptor.