Adenosine inhibits voltage-dependent Ca2+ currents in rat dissociated supraoptic neurones via A1 receptors

1. The modulation of voltage-dependent Ca2+ currents (ICa) by adenosine was investigated in magnocellular neurones acutely dissociated from the rat hypothalamic supraoptic nucleus (SON) by using the whole-cell patch-clamp technique. 2. Adenosine dose dependently and reversibly inhibited ICa elicited by depolarizing voltage steps from a holding potential of -80 mV to potentials ranging from -30 to +20 mV. The mean (+/- s.e.m.) maximum inhibition rate was 36.1 +/- 4.1 % (n = 6) at -20 mV and the EC50 was 9.8 x 10-7 M (n = 6). 3. The inhibition of ICa by adenosine was completely reversed by the selective A1 receptor antagonist 8-cyclopentyl theophylline (CPT), and was mimicked by the selective A1 receptor agonist N 6-cyclohexyladenosine (CHA). 4. The inhibition by CHA was strongly reduced when ICa was inhibited by omega-conotoxin GVIA, a blocker of N-type Ca2+ channels. 5. The adenosine-induced inhibition of ICa was largely reversed by a depolarizing prepulse to +150 mV for 100 ms, which is known to reverse the inhibition of Ca2+ channels mediated by G-protein betagamma subunits. 6. The adenosine receptor-mediated inhibition of ICa was not abolished by intracellularly applied preactivated pertussis toxin (PTX). 7. Using immunohistochemistry, Gzalpha-like immunoreactivity (a PTX-resistant inhibitory G-protein) was observed throughout the SON. 8. These results suggest that adenosine modulates the neuronal activity of SON neurones by inhibiting N-type voltage-dependent Ca2+ channels via A1 receptors which are coupled to PTX-resistant G-proteins.