Apelin, a recently isolated neuropeptide that is expressed in the supraoptic and the paraventricular nuclei, acts on specific receptors located on vasopressinergic neurons. The increased phasic pattern of these neurons facilitates sustained antidiuresis during dehydration or lactation. Here, we investigated whether apelin interacts with arginine vasopressin (AVP) to maintain body fluid homeostasis. We first characterized the predominant molecular forms of endogenous hypothalamic and plasma apelin as corresponding to apelin 13 and, to a lesser extent, to apelin 17. We then demonstrated that, in lactating rats, apelin was colocalized with AVP in supraoptic nucleus magnocellular neurons and given intracerebroventricularly inhibited the phasic electrical activity of AVP neurons. In lactating mice, intracerebroventricular administration of apelin 17 reduced plasma AVP levels and increased diuresis. Moreover, water deprivation, which increases systemic AVP release and causes depletion of hypothalamic AVP stores, decreased plasma apelin concentrations and induced hypothalamic accumulation of the peptide, indicating that AVP and apelin are conversely regulated to facilitate systemic AVP release and suppress diuresis. Opposite effects of AVP and apelin are likely to occur at the hypothalamic level through autocrine modulation of the phasic electrical activity of AVP neurons. Altogether, these data demonstrate that apelin acts as a potent diuretic neuropeptide counteracting AVP actions through inhibition of AVP neuron activity and AVP release. The coexistence of apelin and AVP in magnocellular neurons, their opposite biological effects, and regulation are likely to play a key role for maintaining body fluid homeostasis.A pelin is a bioactive peptide recently isolated from bovine stomach extracts (1). It was identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ (1, 2), reported to act as a coreceptor of CD4 for human and simian immunodeficiency viruses (3, 4). Apelin is a 36-aa peptide derived from a 77-aa precursor, preproapelin, for which cDNAs have been cloned from humans, cattle, rats, and mice (1, 5, 6). The apelin precursor has a fully conserved C-terminal sequence between Trp-55 to Phe-77, including the C-terminal 17 (Lys-Phe-Arg-ArgGln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe; K17F) and 13 (Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe; Q13F) amino acid sequences. These molecular species, and the pyroglutamyl form of Q13F (pE13F), exhibit the highest activities on extracellular acidification rate (1) and strongly inhibit forskolinstimulated cAMP production in Chinese hamster ovary (CHO) cells expressing the human (5,7,8) or the rat (9) apelin receptor. These peptides also are highly potent inducers of rat apelin receptor internalization (10,11).In situ hybridization and RT-PCR studies have shown that the apelin precursor and apelin receptor mRNAs are expressed in various rat brain structures (6,8,9,12,13). Apelin-immunoreactive (IR) neurons are particularly abundant in ...
CCL2 chemokine and its receptor CCR2 may contribute to neuropathic pain development. We tested the hypothesis that injury to peripheral nerves triggers CCL2 release from afferents in the dorsal horn spinal cord (DHSC), leading to pronociceptive effects, involving the production of proinflammatory factors, in particular. Consistent with the release of CCL2 from primary afferents, electron microscopy showed the CCL2 immunoreactivity in glomerular boutons and secretory vesicles in the DHSC of naive rats. Through the ex vivo superfusion of DHSC slices, we demonstrated that the rate of CCL2 secretion was much lower in neonatal capsaicin-treated rats than in controls. Thus, much of the CCL2 released in the DHSC originates from nociceptive fibers bearing TRPV1 ( . These pathological pain-associated changes in the DHSC were mimicked by the intrathecal injection of exogenous CCL2 in naive rats and were prevented by the administration of INCB3344 or ERK inhibitor (PD98059). Finally, mechanical allodynia, which was fully developed 2 weeks after SN-CCI in rats, was attenuated by the intrathecal injection of INCB3344. Our data demonstrate that CCL2 has the typical characteristics of a neuronal mediator involved in nociceptive signal processing and that antagonists of its receptor are promising agents from treating neuropathic pain.
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