Ž . European Journal of Pharmacology 340 1997 301–310 Inhibition of N-, PrQ- and other types of Ca 2q channels in rat hippocampal nerve terminals by the adenosine A receptor 1 Antonio F. Ambrosio a , Joao O. Malva a,b , Arselio P. Carvalho a , Caetana M. Carvalho a, ) ´ ´ ˜ ´ a Center for Neuroscience of Coimbra, Department of Zoology and Faculty of Medicine, UniÕersity of Coimbra, 3000 Coimbra, Portugal b Department of Biology, UniÕersity of Minho, Braga, Portugal Received 22 May 1997; revised 16 October 1997; accepted 21 October 1997 Abstract 6 Ž . 2q The effects of the adenosine A receptor agonist, N -cyclopentyladenosine CPA , on both the increase in intracellular free Ca 1 Žw 2q x . concentration Ca and on the release of endogenous glutamate in rat hippocampal synaptosomes were studied. The inhibitory effect i w 2q x of CPA on the increase in Ca stimulated with 4-aminopyridine was neutralized by the adenosine A receptor antagonist, i 1 Ž . 8-cyclopentyl-1,3-dipropylxanthine DPCPX . The inhibitory effect of CPA was greater in synaptosomes from the CA1 subregion than in whole hippocampal synaptosomes. The inhibitory effects of both CPA and of the Ca 2q channel blockers, v-conotoxin GVIA, v-conotoxin MVIIC or v-conotoxin GVIA plus v-conotoxin MVIIC, were greater than those caused by the Ca 2q channel blockers. The release of endogenous glutamate was inhibited by 41% by CPA. The inhibition observed when CPA and v-conotoxin GVIA or CPA and v-conotoxin MVIIC were present was also greater than the inhibition by the Ca 2q channel blockers alone. The presence of both v-conotoxin GVIA and v-conotoxin MVIIC did not completely inhibit the release of glutamate, and CPA significantly enhanced this w 3 x inhibition. The membrane potential and the accumulation of H tetraphenylphosphonium of polarized or depolarized synaptosomes was not affected by CPA, suggesting that adenosine did not increase potassium conductances. The present results suggest that, in hippocampal glutamatergic nerve terminals, adenosine A receptor activation partly inhibits PrQ- and other non-identified types of Ca 2q channels. 1 q 1997 Elsevier Science B.V. 2q Žw 2q x . 6 Ž . Keywords: Glutamate release; Ca concentration Ca , intracellular free; N -Cyclopentyladenosine CPA ; 4-Aminopyridine i 1. Introduction Adenosine is a neuromodulator of the central nervous Ž . system CNS that exerts its inhibitory effects by the Ž . activation of adenosine A receptors Ribeiro, 1995 . 1 However, the mechanisms which couple the activation of adenosine A receptors to the inhibition of neurotrans- 1 mitter release are not well understood. It has been argued that adenosine probably acts through several mechanisms, namely inhibition of Ca 2q currents, activation of K q currents, or even inhibition of the exocytotic machinery, and the importance of such mechanisms may vary among nerve terminals, animal species, with age and the mecha- Ž nism of stimulation Fredholm and Dunwiddie, 1988; . Ribeiro, 1995 . ) Corresponding author. Tel.: 351-39-25127; fax: 351-39-22776; e- mail: cmcarvalho@gemini.ci.uc.pt. Adenosine decreases the entry of 45 Ca 2q into synapto- Ž . somes Wu et al., 1982; Gonc ¸alves et al., 1991 , and 2q Ž decreases Ca currents in various cell types Dolphin et al., 1986; MacDonald et al., 1986; Scholz and Miller, . 1991 . The inhibitory effect of adenosine may also result q Ž from an increase in K conductances Okada and Ozawa, 1980; Trussel and Jackson, 1985; Gerber et al., 1989; . Zoltay and Cooper, 1990 , which causes hyperpolarization of the membrane potential. Glutamate is the most abundant excitatory neurotrans- mitter in the brain that can be released from an exocytotic Ž . pool Nicholls et al., 1987 , and adenosine inhibits gluta- Ž mate release Poli et al., 1991, 1993; Barrie and Nicholls, . 2q 1993 , probably by inhibiting Ca channels. Various lines of evidence suggest that adenosine reduces Ca 2q currents 2q Ž primarily by inhibiting N-type Ca channels Mogul et al., 1993; Yawo and Chuhma, 1993; Mynlieff and Beam, . 1994; Wu and Saggau, 1994 . Nevertheless, Wu and Sag- Ž . gau 1994 found that the inhibition produced by adeno- sine in presynaptic Ca 2q transients in hippocampal slices 0014-2999r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.