Inhibition of calcium channels by opioid- and adenosine-receptor agonists in neurons of the nucleus accumbens

1. The pharmacological effects of opioid- and adenosine-receptor agonists on neural signalling were investigated by measuring drug actions on barium current flowing through calcium channels in acutely-dissociated neurons of the rat nucleus accumbens (NAc). 2. Under whole-cell voltage clamp, opioids acted via μ, but not δ or κ, receptors to partially inhibit barium current. Mean inhibition was 35±2% (± s.e.mean, n= 33) for methionine-enkephalin and 37 ± 1% (n = 65) for the selective μ receptor agonist DAMGO, both measured at saturating agonist concentrations in neurons with diameter ≥ 20 μm. EC₅₀ for DAMGO was 100 nM. Perfusion of naloxone reversed the current inhibition by DAMGO. 3. Adenosine also partially inhibited barium current in these neurons. Mean inhibition was 28 ± 2% (n=29) for adenosine and 33±3% (n=27) for the selective A₁ receptor agonist N⁶CPA, both at saturating concentrations in neurons with diameter ≥20 μm. EC₅₀ for N₆CPA was 34 nM. Adenosine inhibition was reversed by perfusion of an A₁ receptor antagonist, 8-cyclopentyl-l,3-dipropylxanthine, while the selective A_2A receptor agonist, CGS 21680, had no effect. 4. Inhibition by opioids and adenosine was mutually occlusive, suggesting a converging pathway onto calcium channels. 5. These actions involved a G-protein-coupled mechanism, as demonstrated by the partial relief of inhibition by strong depolarization and by the application of N-ethylmaleimide or GTP-γ-S. 6. Inhibition of barium current by opioids had their greatest effect in large neurons, that is, in presumed interneurons. In contrast, opioid inhibition in neurons with diameter ≤15 μm was 11±2% (n=26) for methionine-enkephalin and 11±4% (n= 17) for DAMGO, both measured at saturating agonist concentrations. Adenosine inhibition in neurons with diameter ≤ 15 μm was 22±5% (n = 9). 7. These results implicate the interneurons as a locus for the modulation of the excitability of projection neurons in the NAc during the processes of addiction and withdrawal.