Opioid peptides in the regulation of TSH and prolactin secretion in the rat

Thyroid hormones alterations during acute liver failure: possible underlying mechanisms and consequences

Thyroid hormones are now recognized to change in different disease states with important consequences on severity and prognosis of disease. However, little is known about thyroid hormones' alterations in acute liver failure (ALF). To study the changes in thyroid hormones and cardiac thyroid receptors during ALF, we subjected seven female pigs to surgical liver devascularization. Liver function biochemical markers, thyroid hormones, endogenous opioids, malondialdehyde (MDA), and interleukins 1 and 6 were measured in serum for 24 h postoperatively. Heart biopsies were harvested at the end of the experiment. Baseline heart biopsies were taken from five additional animals. Serum thyroxin (T(4)) and triiodothyronine (T(3)) levels markedly decreased, whereas free-triiodothyronine and thyroxin-stimulating hormone levels did not change. T(4) and T(3) levels correlated with the degree of liver failure and with MDA and interleukin-6 levels. Beta-endorphin levels initially increased, whereas levels of leucine-enkephalin did not change. Thyroid hormone receptor-alpha1 protein expression in the heart decreased 1.6-fold after ALF, whereas myocardial myosin isoform expression remained unchanged. The downregulation of T(4) and T(3) levels during ALF seems to correlate well with the severity of disease. This downregulation related to inflammation and oxidative stress and resulted in changes in myocardial thyroid receptors.

In vitro effects of mammalian leptin, neuropeptide-Y, β-endorphin and galanin on transcript levels of thyrotropin β and common α subunit mRNAs in the pituitary of bighead carp (aristichthys nobilis)

Thyrotropin (thyroid stimulating hormone, TSH) is a member of the pituitary glycoprotein hormones, consisting of two dissimilar subunits, alpha and beta. The two subunits are produced by different genes and are regulated independently. We have previously cloned a TSHbeta cDNA from bighead carp pituitary and investigated its gene regulation. We report here the direct effects of mammalian TSH-releasing hormone (TRH), leptin, neuropeptide-Y (NPY), beta-endorphin and galanin on mRNA levels of both TSHbeta and alpha-subunits in the pituitary of bighead carp in vitro. The dispersed pituitary cells of bighead carp were incubated at 25 degrees C for 6 h with different doses of these factors. The relative mRNA levels of TSHbeta and alpha-subunits were estimated by traditional polymerase chain reaction (PCR) analysis and fluorescence real-time PCR analysis. The results revealed that mammalian TRH, leptin and beta-endorphin produced dose-dependent stimulatory effects on mRNA levels of both TSHbeta and alpha-subunits while thyroxine (T4) and mammalian galanin suppressed mRNA levels of both TSHbeta and alpha-subunits. NPY suppressed TSHbeta mRNA level, but stimulated alpha-subunit mRNA level. This study has demonstrated that mammalian TRH, leptin, NPY, beta-endorphin and galanin were active in modulating the steady-state mRNA levels of TSHbeta and alpha-subunits of bighead carp pituitary in vitro. The results suggest that endogenous TRH, leptin, NPY, beta-endorphin and galanin may modulate transcript levels of TSHbeta and alpha-subunits in pituitary of bighead carp.

[Regulation of thyrotropin synthesis and secretion]

The set point of thyrotropin (TSH) secretion is determined by the balance of a positive regulation of thyrotropin releasing hormone (TRH) and the strong negative regulation exerted by thyroid hormones. In addition, there are other regulators superimposed on this main axis such as somatostatin and dopamine, which act as inhibitors of TSH secretion, and central alpha-adrenergic pathways that are predominantly stimulatory and involved in the cold-induced thyroid activation. Nutritional status and leptin also regulate TSH by stimulating TRH neurons through direct and indirect mechanisms. Stress is also involved in lowering TRH/TSH secretion possibly through glucocorticoids, cytokines and opioids. Recently, a new regulatory pathway has been proposed, via peptides produced in pituitary, acting in an autocrine/paracrine manner. Among those, more consistent data are available on neuromedin B, gastrin-releasing peptide and pituitary leptin, which act as local inhibitors of TSH release. Neonatal programming of TSH secretion set point is also discussed.

The autocrine/paracrine regulation of thyrotropin secretion

Peptides originally described in other tissues have been located in the anterior pituitary gland. Detection of their encoding mRNAs and specific receptors, together with demonstration of peptide local action led to the postulation of the existence of a paracrine/autocrine regulation of pituitary function. Direct evidence for the role of endogenous peptides has come from studies aiming to block their action through immunoneutralization or pharmacologic blockade. Here we review evidence of pituitary produced peptides as potential candidates as local regulators of thyrotropin secretion. Few studies have approached the subject and most data are not conclusive. Until now, the most consistent data relate to neuromedin B, a bombesin-like peptide. The combined observation of high peptide concentration in rat thyrotrophs, the ability of the exogenous peptide to inhibit thyrotropin (TSH) release in physiologic doses plus the effect of the specific neuromedin B antiserum to increase basal TSH release from isolated pituitaries suggest that neuromedin B acts as a constitutive autocrine TSH-release inhibitor. Neuromedin B is upregulated by thyroid hormones and downregulated by thyrotropin-releasing hormone (TRH) that is consistent with proposed role of local factors, namely to partially mediate or modulate the effects of hormones on pituitary function. However, future studies will certainly confirm other candidates as local regulators of TSH secretion, as well as their relevance at physiologic and pathologic conditions.

Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Recently, mu-, delta- and kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major opioid receptor types, more extensive studies have suggested the possible existence of other opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, beta-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative epsilon-opioid receptor. Many epsilon-opioid receptors are functionally coupled to G-proteins. The functional epsilon-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as beta-endorphin, but not by selective mu-, delta- or kappa-opioid receptor agonists. Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the epsilon-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no epsilon-selective ligand currently available, these findings provide strong evidence for the existence of the putative epsilon-opioid receptor and its unique function in the brain.

Activation of mu opioid receptors in the pre-optic-anterior hypothalamus releases prolactin in the conscious male rat

Microinjection of opioid agonists into the pre-optic-anterior hypothalamus (PO/AHA) was used to determine the identity of the opioid receptor subtype(s) involved in the stimulation of prolactin release. The mu agonist DAGO [(D-Ala2, NMe-Phe4, Gly-o15)-enkephalin] was the only opioid agonist to show dose-dependent release of prolactin, the lowest significant dose being 0.001 nmoles. Neither the specific delta agonist DPDPE [(D-Pen2, D-Pen5)-enkephalin] nor the specific kappa agonist U50,488H [(trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrodinyl)-cyclohexyl)-benz ene acetamide] showed dose-dependent increase of prolactin secretion, or indeed any significant increase in prolactin secretion in the dose range 0.01-1 nmoles and 0.01-10 nmoles respectively. We suggest that mu (and not kappa or delta) opioid receptors in the PO/AHA are involved in the opioid stimulated release of prolactin in the conscious male rat.

Effects of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on GH, LH, prolactin, FSH, and TSH secretion in rats with and without morphine

Plasma concentrations of GH, LH, prolactin, FSH, and TSH were evaluated in adult rats after administration of Tyr-MIF-1. Male rats were killed 0, 15, 30, and 60 min after Tyr-MIF-1 (0.02, 0.2 and 2.0 mg/kg) and ovariectomized females 15 min after injection of the peptide (0.2 and 2.0 mg/kg). The effect of Tyr-MIF-1 on pituitary hormonal secretion in morphine-treated ovariectomized rats also was studied. After 15 min, Tyr-MIF-1 (0.2 mg/kg) increased plasma concentrations of LH in males (p less than 0.05) and, at 2.0 mg/kg, in ovariectomized rats (p less than 0.05). Tyr-MIF-1 (0.2 mg/kg) decreased plasma concentrations of GH as compared with diluent at 15 min in males (p less than 0.05) but was ineffective in ovariectomized females not receiving morphine. Plasma concentrations of prolactin, FSH, and TSH remained unchanged both in males and in ovariectomized females by any of the administered doses of the peptide at any of the times tested. When administered to ovariectomized rats injected earlier with morphine sulfate, Tyr-MIF-1 (0.2 and 2.0 mg/kg) reduced (p less than 0.05) the effect of morphine (5 mg/kg) on GH secretion and tended (p = 0.061) to partially inhibit the effect of morphine (10 mg/kg) on prolactin secretion at a dose of 2.0 mg/kg. The decrease in plasma concentrations of TSH after morphine at a dose of 10 mg/kg (p less than 0.001) remained unaffected by any of administered doses of Tyr-MIF-1. The results suggest that Tyr-MIF-1 may affect the regulation of the secretion of some anterior pituitary hormones.

Blockade of opioid receptors with naltrexone inhibits thyrotropin increase after noise stress but does not prevent the decrease caused by immobilization

The influence of naltrexone-induced opioid receptor blockade on the response of thyrotropin to two different acute stressors was studied in adult male rats. Naltrexone slightly but significantly reduced basal thyrotropin levels and abolished the increase in serum thyrotropin caused by acute noise stress. In contrast, the opioid antagonist did not prevent the decrease in serum thyrotropin caused by another much more severe stressor such as immobilization. The present data offer the first evidence that endogenous opioids could play a stimulatory role in the control of thyrotropin secretion in a presumably physiological condition such as the response to a mild stressor. In addition, factors other than opioids could be involved in the inhibition of thyrotropin secretion under severe stress.