Neuronal morphological changes, including increases in spine density or dendritic length, have been reported following environmental enrichment or drug administration (Johansson and Belichenko, 2002; Leggio et al., 2005) and are thought to reflect structural reorganization of neural circuits manifested by molecular events discussed thus far in this review. In the NAc, long-lasting structural plasticity has been demonstrated in MSN following nicotine administration; however, these changes may be dependent on treatment schedule and cohort age. One study found an increase in dendritic length and spine density in the NAc and PFC of adult rats after intermittent subcutaneous injections of nicotine (Brown and Kolb, 2001) while another reported significant increases in dendritic length and branch number in the NAc shell of adolescent but not adult rats after continuous nicotine administration (Table 1) (McDonald et al., 2007).
