Natural peptides displaying agonist activity on the orphan G protein-coupled receptor GPR54 were isolated from human placenta. These 54-, 14,-and 13-amino acid peptides, with a common RF-amide C terminus, derive from the product of KiSS-1, a metastasis suppressor gene for melanoma cells, and were therefore designated kisspeptins. They bound with low nanomolar affinities to rat and human GPR54 expressed in Chinese hamster ovary K1 cells and stimulated PIP 2 hydrolysis, Ca 2؉ mobilization, arachidonic acid release, ERK1/2 and p38 MAP kinase phosphorylation, and stress fiber formation but inhibited cell proliferation. Human GPR54 was highly expressed in placenta, pituitary, pancreas, and spinal cord, suggesting a role in the regulation of endocrine function. Stimulation of oxytocin secretion after kisspeptin administration to rats confirmed this hypothesis.

Short chain fatty acids (SCFAs), including acetate, propionate, and butyrate, are produced at high concentration by bacteria in the gut and subsequently released in the bloodstream. Basal acetate concentrations in the blood (about 100 M) can further increase to millimolar concentrations following alcohol intake. It was known previously that SCFAs can activate leukocytes, particularly neutrophils. In the present work, we have identified two previously orphan G protein-coupled receptors, GPR41 and GPR43, as receptors for SCFAs. Propionate was the most potent agonist for both GPR41 and GPR43. Acetate was more selective for GPR43, whereas butyrate and isobutyrate were more active on GPR41. The two receptors were coupled to inositol 1,4,5-trisphosphate formation, intracellular Ca 2؉ release, ERK1/2 activation, and inhibition of cAMP accumulation. They exhibited, however, a differential coupling to G proteins; GPR41 coupled exclusively though the Pertussis toxinsensitive G i/o family, whereas GPR43 displayed a dual coupling through G i/o and Pertussis toxin-insensitive G q protein families. The broad expression profile of GPR41 in a number of tissues does not allow us to infer clear hypotheses regarding its biological functions. In contrast, the highly selective expression of GPR43 in leukocytes, particularly polymorphonuclear cells, suggests a role in the recruitment of these cell populations toward sites of bacterial infection. The pharmacology of GPR43 matches indeed the effects of SCFAs on neutrophils, in terms of intracellular Ca 2؉ release and chemotaxis. Such a neutrophil-specific SCFA receptor is potentially involved in the development of a variety of diseases characterized by either excessive or inefficient neutrophil recruitment and activation, such as inflammatory bowel diseases or alcoholism-associated immune depression. GPR43 might therefore constitute a target allowing us to modulate immune responses in these pathological situations.

Dendritic cells (DCs) and macrophages are professional antigen-presenting cells (APCs) that play key roles in both innate and adaptive immunity. ChemR23 is an orphan G protein–coupled receptor related to chemokine receptors, which is expressed specifically in these cell types. Here we present the characterization of chemerin, a novel chemoattractant protein, which acts through ChemR23 and is abundant in a diverse set of human inflammatory fluids. Chemerin is secreted as a precursor of low biological activity, which upon proteolytic cleavage of its COOH-terminal domain, is converted into a potent and highly specific agonist of ChemR23, the chemerin receptor. Activation of chemerin receptor results in intracellular calcium release, inhibition of cAMP accumulation, and phosphorylation of p42–p44 MAP kinases, through the Gi class of heterotrimeric G proteins. Chemerin is structurally and evolutionary related to the cathelicidin precursors (antibacterial peptides), cystatins (cysteine protease inhibitors), and kininogens. Chemerin was shown to promote calcium mobilization and chemotaxis of immature DCs and macrophages in a ChemR23-dependent manner. Therefore, chemerin appears as a potent chemoattractant protein of a novel class, which requires proteolytic activation and is specific for APCs.

The P2Y 13 receptor has recently been identified as a new P2Y receptor sharing a high sequence homology with the P2Y 12 receptor as well as similar functional properties: coupling to G i and responsiveness to ADP (Communi et al., 2001). In the present study, the pharmacology of the P2Y 13 receptor and its differences with that of the P2Y 12 Similarly, 2MeSADP was more potent than ADP in stimulating IP 3 accumulation after 10 min in AG32 cells and increasing cAMP in pertussis toxin-treated CHO-K1 cells stimulated by forskolin. On the other hand, ADP and 2MeSADP were equipotent at stimulating IP 3 formation in AG32 cells after 30 s and inhibiting forskolininduced cAMP accumulation in CHO-K1 cells. These differences in potency cannot be explained by differences in degradation rate, which in AG32 cells was similar for the two nucleotides. When contaminating diphosphates were enzymatically removed and assay of IP 3 was performed after 30 s, ATP and 2MeSATP seemed to be weak partial agonists of the P2Y 13 receptor expressed in AG32 cells. The stimulatory effect of ADP on the P2Y 13 receptor in AG32 cells was antagonized by reactive blue 2, suramin, pyridoxal-phosphate-6-azophenyl-2Ј,4Јdisulfonic acid, diadenosine tetraphosphate, and 2-(propylthio)-5Ј-adenylic acid, monoanhydride with dichloromethylenebis (phosphonic acid) (AR-C67085MX), but not by N 6

Electrophysiological and autoradiographic approaches were used to assess possible changes in 5-hydroxytryptamine (serotonin) 5-HT1A receptors in the rat dorsal raphe nucleus after a subchronic treatment with fluoxetine or paroxetine, two specific serotonin reuptake inhibitors with antidepressant properties. Fluoxetine or paroxetine were injected daily (5 mg/kg, i.p.) for various time periods up to 21 days. Electrophysiological recordings performed 24 h after the last injection showed that the potency of the 5-HT1A receptor agonist, 8-OH-DPAT, to depress the firing of serotoninergic neurons in the dorsal raphe nucleus within brain stem slices was significantly reduced as early as after a 3-day treatment with either drug. The proportion of recorded neurons showing desensitization of somatodendritic 5-HT1A autoreceptors increased along the treatment from approximately 40% on the 3rd day to 60-80% on the 21st day. At no time during the treatment, was the specific binding of [3H]8-OH-DPAT (agonist radioligand) or [3H]WAY-100 635 (antagonist radioligand) to 5-HT1A receptors modified in the dorsal raphe nucleus or in other brain areas, suggesting that neither the density nor the coupling of these receptors to G-proteins were probably altered in rats injected with fluoxetine or paroxetine for up to 21 days. These results show that adaptive desensitization of somatodendritic 5-HT1A autoreceptors within the dorsal raphe nucleus can already be detected after a 3-day treatment with selective serotonin reuptake inhibitors. Rather than the desensitization per se, it may be the progressive increase in the number of serotoninergic neurons with desensitized 5-HT1A autoreceptors which plays a critical role in the (slowly developing) antidepressant action of these drugs.