Figure 6.

Determination of MOR reserve for the inhibition of Ca²⁺ events by ME. A, Slice average (mean ± SEM) plotting the frequency of GCaMP6f events in response to increasing concentrations of ME (300 nm, 1 μm, and 10 μm) following a 2 min β-CNA (50 nm) pretreatment. GABA_A and AMPA receptor blockers (DNQX and picrotoxin, respectively) were included throughout the recording to occlude the potential effects of presynaptic MORs. B, Dose–response curves for the inhibition of Ca²⁺ events by ME under control conditions or after pretreatment with 50 or 100 nm β-CNA. Treatment with 50 nm β-CNA caused a significant rightward shift of the EC₅₀ concentration (control, 95 nm; 50 nm β-CNA, 1260 nm), without a concomitant decrease in the maximal inhibition, suggesting the presence of a large receptor reserve. Treatment with 100 nm β-CNA did not significantly shift the EC₅₀ concentration (1410 nm) compared with 50 nm, but there was a decrease in the maximal inhibition to 80%. C, Double reciprocal plot of the concentration of agonist required for equally effective points on the dose–response curve between control (y-axis) and β-CNA treatments (x-axis) as used for the Furchgott (1966) analysis, to calculate q, the proportion of receptors still functional following β-CNA treatment, from the slope of the linear fits.