GABA_B, opioid and α₂ receptor inhibition of calcium channels in acutely-dissociated locus coeruleus neurones

1 The effects of GABA_B, opioid and α₂ receptor activation on different subtypes of calcium channels in acutely-dissociated rat locus coeruleus (LC) neurones were investigated using whole-cell patch clamping. 2 Barium currents through calcium channels could be fractionated into four classes: L-type (nimodipine-sensitive), N-type (ω-conotoxin GVIA-sensitive), P/Q-type (ω-agatoxin IVA-sensitive) and R-type (remaining in the presence of all three blockers). The percentage of each was, respectively, 25 ± 2, 34 ± 1, 28 ± 3 and 12 ± 1% (mean ± s.e. mean, n = 4). 3 The GABA_B receptor agonist, baclofen, and the opioid receptor agonist, enkephalin, partially inhibited the total barium current in a concentration-dependent manner with EC₅₀ values of 2 and 0.3 μM respectively. Maximal inhibition was 17 ± 1% (n = 38) for baclofen and 30 ± 2% (n = 20) for enkephalin. The α₂-adrenoceptor agonist, UK14304 (10 μM), also inhibited barium current in these neurones (28 ± 2%, n = 11). The agonists did not shift the current-voltage relationship along the voltage axis. 4 Maximal baclofen inhibition of different calcium channel subtypes was 9 ± 7% L-type, n = 4), 11 ± 8% (N-type, n = 4), 26 ± 6% (P/Q-type, n = 4): and 6 ± 5% (R-type, n = 5). The corresponding values for enkephalin inhibition were 5 ± 9% (L-type), 30 ± 11% (N-type), 37 ± 9% (P/Q-type), and 17 ± 8% (R-type). 5 In the presence of a saturating concentration of enkephalin, baclofen produced additional inhibition of the barium current. In contrast, in the presence of a saturating concentration of enkephalin, UK14304 produced no further inhibition of the barium current. 6 These results indicate that neuromodulation of calcium channels in LC neurones involves a complex pattern of overlapping and distinct second messenger pathways. Regulation of LC neuronal firing activity by the modulation of calcium channels may be important for LC-mediated behaviour such as alertness and vigilance.