Cloning of a mouse adrenocorticotropin receptor-encoding gene

To eat or not to eat; regulation by the melanocortin system

The central melanocortin (MC) system is one of the best-characterized neuropeptidergic systems involved in the regulation of energy balance. This short review describes the role of the central MC system in feeding behavior. Pharmacological, anatomical and genetic studies show that activation of the MC system reduces meal size, whereas de-activation of the MC system increases meal size. Several brain regions, including distinct hypothalamic nuclei and the hindbrain, are involved in this process. Further dissection of MC pathways in feeding behavior is the subject of recent and probably future studies. As the MC system is involved in animal models of obesity and (possibly) anorexia, it appears that this is a target system for development of drugs for the treatment of disturbed human eating behavior.

Response of rat fasciculata-reticularis cells in primary culture to bacterial lipopolysaccharide

The aim of this study was to determine the direct effect of a wide range of concentrations of lipopolysaccharide (LPS) of Escherichia coli O111:B4 on fasciculata-reticularis cells in primary cultures. In short-term cultures of fasciculata-reticularis cells, the presence of low (1-10 microg/ml) doses of LPS in the medium produced a decrease in ACTH-induced corticosterone secretion, in a dose-dependent manner and independent of the culture medium. The corticosterone production stimulated by db-cAMP was slightly decreased by the presence of LPS in culture medium, while the pregnenolone induced corticosterone biosynthesis was not modified. LPS modified the binding of [125I]-Tyr23-ACTH to the fasciculata-reticularis cell membrane and the signal transduction pathway, as LPS reduced ACTH-induced cAMP production. In long-term cultures, the presence of LPS in the medium produces a decrease in the specific binding of [125I]-Tyr23-ACTH, while the presence of ACTH in the culture medium produced an increase in its specific binding. The use of high doses of LPS (100-250 microg/ml) has helped to clarify some aspects of the LPS action. These doses of LPS severely inhibited ACTH-induced corticosterone production, and clearly reduced the corticosterone production stimulated by db-cAMP and the binding of ACTH to its receptors. In long-term cultures, LPS decreased the number of ACTH receptors, an effect that was reversed by subsequent exposure to ACTH. These results indicate that LPS exerts a direct action on fasciculata-reticularis cells and a model of the mechanism of LPS action is proposed.

Neuropeptides, food intake and body weight regulation: a hypothalamic focus

Energy homeostasis is controlled by a complex neuroendocrine system consisting of peripheral signals like leptin and central signals, in particular, neuropeptides. Several neuropeptides with anorexigenic (POMC, CART, and CRH) as well as orexigenic (NPY, AgRP, and MCH) actions are involved in this complex (partly redundant) controlling system. Starvation as well as overfeeding lead to changes in expression levels of these neuropeptides, which act downstream of leptin, resulting in a physiological response. In this review the role of several anorexigenic and orexigenic (hypothalamic) neuropeptides on food intake and body weight regulation is summarized.

Melanocortins and cardiovascular regulation

The melanocortins form a family of pro-opiomelanocortin-derived peptides that have the melanocyte-stimulating hormone (MSH) core sequence, His-Phe-Arg-Trp, in common. Melanocortins have been described as having a variety of cardiovascular effects. We review here what is known about the sites and mechanisms of action of the melanocortins with respect to their effects on cardiovascular function, with special attention to the effects of the gamma-melanocyte-stimulating hormones (gamma-MSHs). This is done in the context of present knowledge about agonist selectivity and localisation of the five melanocortin receptor subtypes cloned so far. gamma2-MSH, its des-Gly12 analog (= gamma1-MSH) and Lys-gamma2-MSH are 5-10 times more potent than adrenocorticotropic hormone-(4-10)(ACTH-(4-10)) to induce a pressor and tachycardiac effect following intravenous administration. The Arg-Phe sequence near the C-terminal seems to be important for full in vivo intrinsic activity. Related peptides with a C-terminal extension with (gamma3-MSH) or without the Arg-Phe sequence (alpha-MSH, as well as the potent alpha-MSH analog, [Nle4,D-Phe7]alpha-MSH), are, however, devoid of these effects. In contrast, ACTH-(1-24) has a depressor effect combined with a tachycardiac effect, effects which are not dependent on the presence of the adrenals. Although the melanocortin MC3 receptor is the only melanocortin receptor subtype for which gamma2-MSH is selective, in vivo and in vitro structure-activity data indicate that it is not via this receptor that this peptide and related peptides exert either their pressor and tachycardiac effects or their extra- and intracranial blood flow increasing effect. We review evidence that the pressor and tachycardiac effects of the gamma-MSHs are due to an increase of sympathetic outflow to the vasculature and the heart, secondary to activation of centrally located receptors. These receptors are most likely localised in the anteroventral third ventricle (AV3V) region, a brain region situated outside the blood-brain barrier, and to which circulating peptides have access. These receptors might be melanocortin receptors of a subtype yet to be identified. Alternatively, they might be related to other receptors for which peptides with a C-terminal Arg-Phe sequence have affinity, such as the neuropeptide FF receptor and the recently discovered FMRFamide receptor. Melanocortin MC4 receptors and still unidentified receptors are part of the circuitry in the medulla oblongata which is involved in the depressor and bradycardiac effect of the melanocortins, probably via interference with autonomic outflow. Regarding the effects of the gamma-MSHs on cortical cerebral blood flow, it is not yet clear whether they involve activation of the sympathetic nervous system or activation of melanocortin receptors located on the cerebral vasculature. The depressor effect observed following intravenous administration of ACTH-(1-24) is thought to be due to activation of melanocortin MC2 receptors whose location may be within the peripheral vasculature. Melanocortins have been observed to improve cardiovascular function and survival time in experimental hemorrhagic shock in various species. Though ACTH-(1-24) is the most potent melanocortin in this model, alpha-MSH and [Nle4,D-Phe7]alpha-MSH and ACTH-(4-10) are quite effective as well. As ACTH-(4-10) is a rather weak agonist of all melanocortin receptors, it is difficult to determine via which of the melanocortin receptors the melanocortins bring about this effect. Research into the nature of the receptors involved in the various cardiovascular effects of the melanocortins would greatly benefit from the availability of selective melanocortin receptor antagonists.

Molecular cloning of the chicken melanocortin 2 (ACTH)-receptor gene

The chicken melanocortin 2-receptor (MC2-R) gene was isolated. It is found to be a single copy gene encoding a 357 amino acid protein, sharing 65.8-68.7% identity with mammalian counterparts. The chicken MC2-R mRNA is expressed in the adrenal and spleen, suggesting that the receptor mediates both endocrine and immunoregulatory functions of ACTH in the chicken. The amino acid sequence of the chicken MC2-R is collinear with those of other subtypes of MC-R, whereas all cloned mammalian MC2-Rs contain a gap in the third intracellular loop, suggesting that mammalian MC2-R molecules have evolved by lacking a part of the domain which determines the specificity of signal transduction in G-protein coupled receptors. Interestingly, the codon usage differs dramatically between MC1-R and MC2-R in the chicken; the GC-contents at the third codon position in MC1-R and MC2-R are 94.6 and 50.6%, respectively. It may reflect selective constraints on the usage of synonymous codons.

Acute effects of thyroid hormones on the production of adrenal cAMP and corticosterone in male rats

The acute effects of thyroid hormones on glucocorticoid secretion were studied. Venous blood samples were collected from male rats after they received intravenous 3,5,3'-triiodothyronine (T3) or thyroxine (T4). Zona fasciculata-reticularis (ZFR) cells were treated with adrenocorticotropic hormone (ACTH), T3, T4, ACTH plus T3, or ACTH plus T4 at 37 degrees C for 2 h. Corticosterone concentrations in plasma and cell media, and also adenosine 3',5'-cyclic monophosphate (cAMP) production in ZFR cells in the presence of 3-isobutyl-1-methylxanthine, were determined. The effects of thyroid hormones on the activities of steroidogenic enzymes of ZFR cells were measured by the amounts of intermediate steroidal products separated by thin-layer chromatography. Administration of T3 and T4 suppressed the basal and the ACTH-stimulated levels of plasma corticosterone. In ZFR cells, both thyroid hormones inhibited ACTH-stimulated corticosterone secretion, but the basal corticosterone was inhibited only with T3 > 10(-10) M or T4 > 10(-8) M. Likewise, T3 or T4 at 10(-7) M inhibited the basal- and ACTH-stimulated levels of intracellular cAMP. Physiological doses of T3 and T4 decreased the activities of 3 beta-hydroxysteroid dehydrogenase, 21-hydroxylase, and 11 beta-hydroxylase. These results suggest that thyroid hormones counteract ACTH in adrenal steroidogenesis through their inhibition of cAMP production in ZFR cells.

Genomic organization of the mouse adrenocorticotropin receptor

As a step toward understanding the transcriptional regulation of the adrenocorticotropin receptor (ACTH-R) gene, we examined the full length cDNA sequence of the mouse ACTH-R by rapid amplification of cDNA ends, and the organization of the gene. Mouse ACTH-R mRNA consists of 374 bp in the 5'-untranslated region (UTR), 888 bp in the coding sequence, and 445 bp in the 3'-UTR, the 1707 bp being fairly compatible with the 1.8-kb adrenal mRNA detected by Northern analysis. The mouse ACTH-R gene consists of at least four exons; the first three exons encode 5'-UTR and the fourth exon encodes part of 5'-UTR, the entire coding region, and the whole of 3'-UTR. We also defined two mRNA species, one with and one without the 57-bp exon 2, produced by alternative splicing.

Melanocortins and opiate addiction

Adrenocorticotropic hormone (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) are centrally acting melanocortin peptides with numerous reported functions, including induction of excessive grooming and antipyresis, among others. Also reported is a role for melanocortins in aspects of opiate action. Although early work examined the effects of ACTH and MSH on opiate-induced behaviors, further progress has been limited. Recently, however, advances in the identification and characterization of melanocortin receptor (MC-R) subtypes have provided novel tools with which to study interactions between melanocortins and addiction. The present review discusses the effects of ACTH and MSH on opiate-induced behaviors and relates these findings to more recent reports on the regulation of melanocortin systems by exogenous opiates. Emerging from these data is the possibility that melanocortin receptor activation, specifically at the MC4-R subtype, may act to antagonize certain properties of exogenous opiates, including perhaps addiction.

Major pharmacological distinction of the ACTH receptor from other melanocortin receptors

The mouse adrenocortical cell line Y1, that expresses ACTH receptors (MC2R), was used to probe the binding of ACTH and MSH peptides by using radio-labelled ACTH (1-39). The Y1 cells were found to bind [125I]-labelled ACTH (1-39) with high affinity (Kd approximately 130 pM). However, none of the melanocortin peptides NDP-MSH, alpha-MSH, beta-MSH or gamma 1-MSH could compete with the binding of the labelled ACTH(1-39). When other MC receptor subtype DNAs (MC1, MC3 and MC4) were transfected into the Y1 cells, characteristic binding of the [125I]NDP-MSH appeared for each of the receptor subtype, but no specific binding was present in non-transfected cells. This is the first report clearly demonstrating that the ACTH receptor binds only ACTH, but not other melanocortin peptides.

The ACTH receptor

The ACTH-R is a receptor that has a prominent role in mammalian physiology, but which has been notoriously difficulty to study. The cloning of the gene encoding this receptor in 1992 should permit significant advances in the understanding of the physiology, pharmacology and pathophysiology of ACTH. Areas of particular interest that should be clarified in the next few years are the control of the tissue specific expression of the gene, and an understanding of the molecular determinants of the ligand-receptor interaction. This latter area raises the prospect of clinically useful, orally active ACTH agonists and antagonists.