Adult NAc core D1 MSNs are more excitable than D2 MSNs. A, Typical membrane responses from NAc core D1 or D2 MSNs in reaction to a series of increasing somatic current injections. Sample spike trains in response to depolarizing current from D1 or D2 MSN. B, Summary of all the I–V curves recorded in D1 (black, n = 30) and D2 (white, n = 26) MSNs (F_{(interaction 26,1404)} = 8.03, p < 0.0001, F_{(cell type 1,54)} = 2.893, p < 0.0001, two-way repeated-measures ANOVA). C, The resting membrane potential of D2 MSNS was significantly hyperpolarized compared with that of D1 MSNs (p = 0.0051, Mann–Whitney U test). D, The rheobase, the minimal current required to trigger an action potential, was much lower in D1 MSNs (p = 0.0025, Mann–Whitney U test). E, The number of evoked action potentials in response to increasing depolarizing current steps was larger in D1 MSNs compared with D2 MSNs (F_{(interaction 12,648)} = 5.927, p < 0.0001, F_{(cell type 1,54)} = 27.38, p < 0.0001, two-way repeated measure ANOVA). F, Example of individual AP evoked by depolarizing current injection, indicating delay to the first spike, AP threshold, AP amplitude, AP duration, fAHP amplitude and fAHP duration metrics. G, AP duration is shorter in D1 MSNs compared with D2 MSNs (D1 MSNs n = 30; D2 MSNs n = 26, p = 0.0354, Mann–Whitney U test). The following action potential properties did not vary between the MSN subtype: H) delay to the first spike, I) AP threshold, J) AP amplitude and K) fAHP amplitude. L, fAHP duration is increased in D2 MSNs compared with D1 MSNs (p = 0.0009 Mann–Whitney U test). Individual point in scatter dot plots represents one individual neuron. All data are shown as mean ± SEM. *p < 0.05.