The octadecaneuropeptide (ODN; QATVGDVNTDRPG LLDLK) and its C-terminal octapeptide (OP; RPGLLDLK), which exert anxiogenic activity, have been previously shown to increase intracellular calcium concentration ([Ca 21 ] i ) in cultured rat astrocytes through activation of a metabotropic receptor positively coupled to phospholipase C. It has also been found that the [7].The mechanism of action of endozepines is not fully understood. It has been initially proposed that these peptides act as inverse agonists of central-type benzodiazepine receptors [6] thus inhibiting the activity of the GABA Achloride channel complex [8]. Subsequently, endozepines were found to interact with peripheral-type BZ receptors and to stimulate cholesterol transport into mitochondria [9]. More recently, it has been shown that, in rat astrocytes, ODN activates a metabotropic receptor positively coupled to phospholipase C, leading to an increase in cytosolic calcium concentration [10,11]. Structure-activity relationship studies have shown that the C-terminal octapeptide of ODN (OP; ODN 11218 ) is the minimum sequence retaining full calcium-mobilizing activity [12]. The Ala-scan of OP has revealed that replacement of the Leu6 residue suppresses the activity of the peptide. It has also been found that the

UII (urotensin II) and its paralogue URP (UII-related peptide) are two vasoactive neuropeptides whose respective central actions are currently unknown. In the present study, we have compared the mechanism of action of URP and UII on cultured astrocytes. Competition experiments performed with [125I]UII showed the presence of very-high- and high-affinity binding sites for UII, and a single high-affinity site for URP. Both UII and URP provoked a membrane depolarization accompanied by a decrease in input resistance, stimulated the release of endozepines, neuropeptides specifically produced by astroglial cells, and generated an increase in [Ca2+]c (cytosolic Ca2+ concentration). The UII/URP-induced [Ca2+]c elevation was PTX (pertussis toxin)-insensitive, and was blocked by the PLC (phospholipase C) inhibitor U73122 or the InsP3 channel blocker 2-APB (2-aminoethoxydiphenylborane). The addition of the Ca2+ chelator EGTA reduced the peak and abolished the plateau phase, whereas the T-type Ca2+ channel blocker mibefradil totally inhibited the Ca2+ response evoked by both peptides. However, URP and UII induced a mono- and bi-phasic dose-dependent increase in [Ca2+]c and provoked short- and long-lasting Ca2+ mobilization respectively. Similar mono- and bi-phasic dose-dependent increases in [3H]inositol incorporation into polyphosphoinositides in astrocytes was obtained, but the effect of UII was significantly reduced by PTX, although BRET (bioluminescence resonance energy transfer) experiments revealed that both UII and URP recruited Galphao-protein. Finally, UII, but not URP, exerted a dose-dependent mitogenic activity on astrocytes. Therefore we described that URP and UII exert not only similar, but also divergent actions on astrocyte activity, with UII exhibiting a broader range of activities at physiological peptide concentrations.