The ghrelin system: physiopathological involvement in the control of body growth and energy metabolism.

This short review will summarize some recent findings on the physiopathology of the endogenous ghrelin/obestatin system by focussing on experimental studies aiming at blocking the effects of endogenous ghrelin and clinical studies investigating genotype/phenotype correlations concerning the genes encoding for ghrelin and its cognate receptor.

Functional potencies of new antiparkinsonian drugs at recombinant human dopamine D1, D2 and D3 receptors.

We measured the affinities of bromocriptine, pramipexole, pergolide and ropinirole at human recombinant dopamine D1, D2 and D3 receptors in binding and functional tests. All four compounds bound with high affinity at the dopamine D3 receptor; bromocriptine and pergolide also had high affinity for the dopamine D2 receptor, while only pergolide had significant, although moderate, affinity for the dopamine D1 receptor. Only pergolide had high potency and intrinsic activity at the dopamine D1 receptor for stimulating cyclic AMP accumulation.

Two intracellular signaling pathways for the dopamine D3 receptor: opposite and synergistic interactions with cyclic AMP.

As cerebral neurons express the dopamine D1 receptor positively coupled with adenylyl cyclase, together with the D3 receptor, we have investigated in a heterologous cell expression system the relationships of cyclic AMP with D3 receptor signaling pathways. In NG108-15 cells transfected with the human D3 receptor cDNA, dopamine, quinpirole, and other dopamine receptor agonists inhibited cyclic AMP accumulation induced by forskolin. Quinpirole also increased mitogenesis, assessed by measuring [3H]thymidine incorporation.

A functional test identifies dopamine agonists selective for D3 versus D2 receptors.

The functional potency of a series of dopamine agonists for increasing mitogenesis, measured by incorporation of [3H]thymidine, was established in transfected cell lines expressing human D2 or D3 receptors. The functional selectivity of agonists markedly differs from their binding selectivity. (+)7-OH-DPAT, pramipexole, quinerolane and PD 128,907, the most D3 receptor-selective compounds in binding studies, were 7, 15, 21 and 54 times more potent, respectively, at the D3 than at the D2 receptor in the functional test.

Formation and inactivation of endogenous cannabinoid anandamide in central neurons.

Anandamide (N-arachidonoyl-ethanolamine) was recently identified as a brain arachidonate derivative that binds to and activates cannabinoid receptors, yet the mechanisms underlying formation, release and inactivation of this putative messenger molecule are still unclear. Here we report that anandamide is produced in and released from cultured brain neurons in a calcium ion-dependent manner when the neurons are stimulated with membrane-depolarizing agents. Anandamide formation occurs through phosphodiesterase-mediated cleavage of a novel phospholipid precursor, N-arachidonoyl-phosphatidylethanolamine.

3'-Phosphoadenosine 5'-phosphosulfate biosynthesis and the sulfation of cholecystokinin by the tyrosylprotein-sulfotransferase in rat brain tissue.

This article resumes the work we have accomplished in the past few years. Cholecystokinin sulfation is an important post-translational modification necessary for the biological activity of this peptide hormone. The tyrosyl protein sulfotransferase (TPST) activity from rat cerebral cortex was characterized.

Vip-induced cross-talk between G-proteins in membranes from rat anterior pituitary cells.

In order to study the activation mechanism of heterotrimeric G-proteins by agonist-liganded receptors, GTP gamma S binding to membranes was measured in rat adenohypophyseal cells after addition of dopamine (DA) or vasoactive intestinal peptide (VIP), which, respectively, inhibit and activate pituitary adenylyl cyclase. G-protein subunit present in anterior pituitary cells was characterized by either ADP-ribosylation catalysed by Bordetella pertussis and cholera toxins or by immunoblot using specific antisera. Binding of GTP gamma S was found to depend upon GTP gamma S and Mg2+ concentrations; it was sensitive to pretreatment of the cells with cholera and Bordetella pertussis toxins (IAP).

Resistance to somatostatin (SRIH) analog therapy in acromegaly. Re-evaluation of the correlation between the SRIH receptor status of the pituitary tumor and the in vivo inhibition of GH secretion in response to SRIH analog.

The development of a long-acting somatostatin (SRIH) analog (octreotide, Sandoz) has been a major breakthrough in the treatment of acromegaly. However, in 20-30% of the patients, growth hormone (GH) plasma levels remain elevated (> 10 micrograms/l) despite treatment with octreotide. This raised the concept of resistance to SRIH analog therapy in acromegaly.

A novel potential metallopeptidase derived from the enkephalinase gene by alternative splicing.

Amplification of rat intestine mRNAs was performed by the reverse transcriptase-polymerase chain reaction (RT-PCR) using various oligonucleotide primers mainly corresponding to the translated region of the enkephalinase (EC 3.4.24.

Estradiol modulates protein kinase C activity in the rat pituitary in vivo and in vitro.

17 beta-Estradiol (E2) alters different functions of pituitary cells, including cell sensitivity to several neurohormones such as LHRH, TRH, somatostatin, or dopamine, presumably by affecting receptor coupling mechanisms. Attempting to pinpoint the membrane processes underlying this modulation, we studied the effect of E2 on pituitary kinase-C (PKC) activity, a major signal transduction enzyme. The distribution of calcium- and phospholipid-dependent partially purified PKC (chromatography on DEAE-52 cellulose columns) was evaluated in membrane and cytosol fractions from anterior pituitaries of ovariectomized (OVX) or OVX plus E2-treated rats.

Multiple transduction mechanisms of dopamine, somatostatin and angiotensin II receptors in anterior pituitary cells.

The concept of multifactorial pituitary control is now well established. As in other cell systems, integration of complex messages involves dynamic interactions of receptors and coupling mechanisms. Regulation of adenohypophyseal secretions has been shown to involve cyclic AMP production, the modulation of phosphatidylinositol phosphate breakdown and Ca2+ mobilization.

Dihydropyridine-sensitive calcium channel activity related to prolactin, growth hormone, and luteinizing hormone release from anterior pituitary cells in culture: interactions with somatostatin, dopamine, and estrogens.

In the present work, we determined the activity of voltage-dependent dihydropyridine (DHP)-sensitive Ca2+ channels related to PRL, GH, and LH secretion in primary cultures of pituitary cells from male or female rats. We investigated their modulation by 17 beta-estradiol (E2) and their involvement in dopamine (DA) and somatostatin (SRIF) inhibition of PRL and GH release. BAY-K-8644 (BAYK), a DHP agonist which increases the opening time of already activated channels, stimulated PRL and GH secretion in a dose-dependent manner.

[Potential interest in powerful and specific ligands for the histamine H3 receptor].

In contrast to cerebral histamine H1 and H2-receptors, histamine H3-receptors are presynaptically located on histamine-synthesizing nerve terminals (autoreceptors) and control the synthesis and release of the amine in cerebral neurons. Two imidazole derivatives were designed to interact at H3-receptors: (R) alpha-methylhistamine (alpha-MeHA), a chiral agonist, and thioperamide, a competitive antagonist derived from imidazolyl piperidine, both display high selectivity and potency at nanomolar concentrations in vitro. (R) alpha-MeHA, being about 15 times as potent as histamine itself, constitutes, when tritiated, a suitable probe for the radioassay of H3-receptors.

Potent inhibition of cerebral aminopeptidases by carbaphethiol, a parenterally active compound.

We designed phethiol (1-amino-1-benzyl-2-mercaptoethane) as a potent and selective inhibitor of Zn-containing aminopeptidases. This compound inhibited purified aminopeptidase M (EC.3.

Inhibition of enkephalin degradation by phelorphan: effects on striatal [Met5]enkephalin levels and jump latency in mouse hot plate test.

The effect of phelorphan (mercaptoacetyl-Phe-Phe), an inhibitor of various enkephalin-degrading enzymes, was tested in the mouse hot plate test and its efficacy to prevent endogenous enkephalin degradation in mouse striatum was determined. The i.c.

Highly potent and selective ligands for a new class H3 of histamine receptor.

The third histamine receptor was first indentified on brain neurons and seems to be present in other cells such as lung mast cells. Hence the novel and potent H3-receptor agonist (R) alpha-methylhistamine might find therapeutic applications in allergic diseases.

H3-receptors control histamine release in human brain.

The regulation of histamine release was studied on slices prepared from pieces of human cerebral cortex removed during neurosurgery and labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration induced [3H]histamine release, although to a lesser extent than from rat brain slices. Exogenous histamine reduced by up to 60% the K+-evoked release, with an EC50 of 3.

[3H]glycogenolysis in brain slices mediated by beta-adrenoceptors: comparison of physiological response and [3H]dihydroalprenolol binding parameters.

The subclass of beta-adrenergic receptors mediating glycogenolysis in slices from cerebral cortex of the mouse, incubated in the presence of [3H]glucose, was identified by comparing the relative potencies of agonists and the inhibition constants of antagonists to those found on reference systems. (+/-)Isoprenaline, (-)adrenaline and (-)noradrenaline produced a concentration-related glycogenolysis with Kact values of 2.2 x 10(-8) M, 2.

Labelling and exploration of the active site of enkephalinase (EC 3.4.24.11) in kidney membranes with [3H]thiorphan as ligand.

[3H]Thiorphan, a potent inhibitor of enkephalinase (EC 3.4.24.

Synergism between histamine H1- and H2-receptors in the cAMP response in guinea pig brain slices: effects of phorbol esters and calcium.

The synergism between H1- and H2-receptors in the histamine-induced stimulation of cAMP accumulation was studied in slices from guinea pig hippocampus. Since H1-receptors appear to be coupled to the phosphatidylinositol cycle, the participation of the two branches of the cycle in this synergism was assessed by using phorbol esters and/or by removing Ca2+ from the external medium. The protein kinase C activator, 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB), strongly potentiated, with an EC50 of 0.

Autoinhibition of histamine synthesis mediated by presynaptic H3-receptors.

The regulation of histamine synthesis was studied on rat brain slices or synaptosomes labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration enhanced by about twofold the [3H]histamine formation in slices of cerebral cortex. This stimulation was also observed, although to a lesser extent, in synaptosomes from cerebral cortex and slices from the posterior hypothalamus where most histaminergic cell-bodies are located, suggesting that it may occur in nerve endings as well as in perikarya.

Evaluation of striatal enkephalin release in vivo from steady state levels and turnover rates of the tripeptide Tyr-Gly-Gly.

The tripeptide Tyr-Gly-Gly (YGG), an extraneuronal metabolite of opioid peptides (OP) derived from proenkephalin A is in a highly dynamic state in mouse striatum. Inhibition of its synthesis by Thiorphan reduced its levels with a t 1/2 of 12 min. Inhibition of its degradation by bestatin elicited its rapid accumulation consistent with a t 1/2 of enkephalins in the one-hour range.

The endogenous tripeptide Tyr-Gly-Gly as an extracellular metabolite of enkephalins in rat brain: origin and metabolism.

The tripeptide Tyr-Gly-Gly (YGG) was established as an endogenous constituent in rat brain. Its origin from enkephalin neurons is suggested by its regional distribution paralleling that of (Met5)-enkephalin (YGGFM), its decrease following kainate-induced ablation of the striato-pallidal neurons and its enhanced formation following depolarization of pallidal slices. Enkephalinase (EC 3.

Immunohistochemical and subcellular studies of aminopeptidase M localization in rat brain: microvessels and synaptic membranes.

A predominantly microvascular localization of aminopeptidase M (APM) was established both by immunohistochemistry and biochemical studies of isolated microvessels. In addition a high relative specific activity of APM was evidenced in fractions enriched in synaptic membranes.