Subregional Differences in Medium Spiny Neuron Intrinsic Excitability Properties between Nucleus Accumbens Core and Shell in Male Rats.

The nucleus accumbens (NAc) is known for its central role in reward and motivation (Day and Carelli, 2007; Floresco, 2015; Salgado and Kaplitt, 2015). Decades of research on the cellular arrangement, density, and connectivity of the NAc have identified two main subregions known as the core and shell (Záborszky et al., 1985; Berendse and Groenewegen, 1990; Zahm and Heimer, 1990). Although anatomically and functionally different, both the NAc core and shell are mainly comprised of GABAergic projection neurons known as medium spiny neurons (MSNs) (Matamales et al., 2009). Several studies have identified key morphologic differences between core and shell MSNs (Meredith et al., 1992; Forlano and Woolley, 2010) but few studies have directly addressed how core and shell MSNs differ in their intrinsic excitability (Pennartz et al., 1992; O'Donnell and Grace, 1993). Using whole-cell patch-clamp recordings in slices prepared from naive and rewarded male rats, we found that MSNs in the NAc shell were significantly more excitable than MSNs in the NAc core in both groups. In the shell, MSNs had significantly greater input resistance, lower cell capacitance, and a greater sag. This was accompanied by a lower action potential current threshold, a greater number of action potentials, and faster firing frequency compared with core MSNs. These subregional differences in intrinsic excitability could provide a potential physiological link to the distinct anatomic characteristics of core and shell MSNs and to their distinct functional roles in reward learning (Zahm, 1999; Ito and Hayen, 2011; Saddoris et al., 2015; West and Carelli, 2016).