PAC 1 Receptor Activation by PACAP-38 Mediates Ca 2؉ Release from a cAMP-dependent Pool in Human Fetal Adrenal Gland Chromaffin Cells* Downloaded from

International audiencePrevious studies have shown that human fetal adre-nal gland from 17-to 20-week-old fetuses expressed pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, which were localized on chromaf-fin cells. The aim of the present study was to identify PACAP receptor isoforms and to determine whether PACAP can affect intracellular calcium concentration ([Ca 2؉ ] i) and catecholamine secretion. Using primary cultures and specific stimulation of chromaffin cells, we demonstrate that PACAP-38 induced an increase in [Ca 2؉ ] i that was blocked by PACAP (6-38), was independent of external Ca 2؉ , and originated from thapsi-gargin-insensitive internal stores. The PACAP-triggered Ca 2؉ increase was not affected by inhibition of PLC␤ (preincubation with U-73122) or by pretreatment of cells with Xestospongin C, indicating that the inosi-tol 1,4,5-triphosphate-sensitive stores were not mobilized. However, forskolin (FSK), which raises cytosolic cAMP, induced an increase in Ca 2؉ similar to that recorded with PACAP-38. Blockage of PKA by H-89 or (R p)-cAMPS suppressed both PACAP-38 and FSK calcium responses. The effect of PACAP-38 was also abolished by emptying the caffeine/ryanodine-sensitive Ca 2؉ stores. Furthermore, treatment of cells with or-thovanadate (100 M) impaired Ca 2؉ reloading of PACAP-sensitive stores indicating that PACAP-38 can mobilize Ca 2؉ from secretory vesicles. Moreover, PACAP induced catecholamine secretion by chromaf-fin cells. It is concluded that PACAP-38, through the PAC 1 receptor, acts as a neurotransmitter in human fetal chromaffin cells inducing catecholamine secretion , through nonclassical, recently described, ryano-dine/caffeine-sensitive pools, involving a cAMP-and PKA-dependent phosphorylation mechanism. Pituitary adenylate cyclase-activating polypeptide is a 38-residue ␣-amidated neuropeptide (PACAP-38) 1 originally isolated from the ovine hypothalamus for its ability to stimulate cAMP formation in rat anterior pituitary cells. Processing of PACAP-38 can generate a 27-amino acid amidated peptide (PACAP-27) that exhibits 68% sequence identity with vasoac-tive intestinal polypeptide (VIP), thus identifying PACAP as a member of the VIP/secretin/glucagon superfamily of regulatory peptides (1, 2). The effects of PACAP are mediated through interaction with two types of high affinity receptors: type I receptors are selectively activated by PACAP, whereas type II receptors bind PACAP and VIP with similar affinity (3). Three isoforms of PACAP receptors have now been cloned and designated as PACAP-specific receptor I (PAC 1-R) (4, 5) and VIP/PACAP mutual receptors 1 and 2 (VPAC 1-R and VPAC 2-R) (6, 7). Both PAC 1-R (type 1 receptors) and VPAC 1-R/VPAC 2-R (type 2 receptors) belong to the seven-transmembrane domain, G-protein coupled receptor family, and are all positively coupled to adenylyl cyclase (2). Eight isoforms of PAC 1-R, resulting from alternative splicing, have been characterized to date. These variants display differential signal transduction properties with regard to adenylyl cyclase and phospholipase C (PLC) stimulation (1, 2). In addition to these classical signaling pathways , PACAP has been found to stimulate a Ca 2ϩ-calmodulin nitric oxide synthase (8) and mitogen-activated protein kinase activity (9). These various transduction mechanisms are involved in the neurotrophic activities exerted by PACAP (i.e. inhibition of apoptosis and stimulation of neurite outgrowth) during development (9-11). PACAP and its receptors are actively expressed in the adre-nal medulla (12-14). In particular, we have previously demonstrated the occurrence of PACAP-38 (15) and PACAP binding sites (16) in chromaffin cells from 16-to 20-week-old fetal human adrenal glands. Activation of these receptors by PACAP-38 causes stimulation of cAMP production and induces a modest increase in inositol 1,4,5-triphosphate (IP 3) formation (16), suggesting a role for the neuropeptide in the developin

Angiotensin II, a Neuropeptide at the Frontier between Endocrinology and Neuroscience: Is There a Link between the Angiotensin II Type 2 Receptor and Alzheimer’s Disease?

Amyloid-β peptide deposition, abnormal hyperphosphorylation of tau, as well as inflammation and vascular damage, are associated with the development of Alzheimer’s disease (AD). Angiotensin II (Ang II) is a peripheral hormone, as well as a neuropeptide, which binds two major receptors, namely the Ang II type 1 receptor (AT1R) and the type 2 receptor (AT2R). Activation of the AT2R counteracts most of the AT1R-mediated actions, promoting vasodilation, decreasing the expression of pro-inflammatory cytokines, both in the brain and in the cardiovascular system. There is evidence that treatment with AT1R blockers (ARBs) attenuates learning and memory deficits. Studies suggest that the therapeutic effects of ARBs may reflect this unopposed activation of the AT2R in addition to the inhibition of the AT1R. Within the context of AD, modulation of AT2R signaling could improve cognitive performance not only through its action on blood flow/brain microcirculation but also through more specific effects on neurons. This review summarizes the current state of knowledge and potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT2R activation by non-peptide and highly selective agonists, acting on neuronal plasticity, could represent new pharmacological tools that may help improve impaired cognitive performance in AD and other neurological cognitive disorders