XK and major depressive disorder: Synaptic depression (Rubin et al., 2009; Guerrier et al., 2015; Kottick and Del Negro, 2015) and activity-dependent, Na+-linked mechanisms including Na+/K+ ATPase pumps, IK-Na, and IKATP (Del Negro et al., 2009; Krey et al., 2010) contribute to burst termination.