The concentration-response relationships were determined by the sequential application of the MOR agonists at different concentrations in ascending order, and full recovery of the Ca2+ current amplitude was required before exposure to another concentration. Furthermore, to minimize desensitization, no more than three concentrations of agonists were applied to each neuron. Percent Ca2+ current inhibition was determined as [1-(peak current in the presence of drug/peak current before drug)] * 100%. The concentration-response curves were generated by pooling the results for each concentration and fitting the points to the Hill equation: I = IMAX/{1 + (IC50/[ligand])nH}, where I is the percentage inhibition, IMAX is the maximum inhibition of the Ca2+ current, IC50 is the half-inhibition concentration, [ligand] is the agonist concentration and nH is the Hill coefficient. It should be noted that not all TG neurons tested exhibited Ca2+ current inhibition in the presence of agonists. Overall, approximately 50-60% of neurons tested in both groups exhibited coupling between MOR and Ca2+ channels.
