NMDA, Sensitization, and VTA Figure 3. LTP evoked by high frequency afferent stimulation is not observed in DA neurons from NR1DATCre mice. (A) Photomicrograph of a recorded VTA neuron. The left panel shows a biocytin labeled neuron, the middle panel shows TH immunohistochemistry in the same brain slice, and the right panel shows a merged image demonstrating the overlap of biocytin and TH staining. Scale bar = 40 mm. (B) Mean time course of EPSPs before and after LTP initiation (arrow) in both WT and NR1-KO cells. Scale bars are 5 mV and 25 ms. The spike timing protocol did not lead to LTP in any of the KO cells tested (n = 23, gray circles). However, all 10 WT DA cells demonstrated LTP (black circles). doi:10.1371/journal.pone.0012141.g003 period and examined their exploratory behavior in a novel environment. Interestingly, NR1DATCre mice showed a significant decrease in locomotor response to a novel environment as measured during the first 60 min in novel activity chambers (Figure 6A; p,0.01, two-tailed t-test). However, this could not be attributed to differences in body weight, as the distribution was similar between the groups (data not presented). In contrast to exploratory behavior, the cumulative 24 hr locomotor activity did not differ between the groups (Figure 6B; p = 0.519, two-tailed t- test), and both WT and NR1DATCre animals showed a similar pattern of activity in a single day-night cycle (Figure 6C). Together with the observed decrease in NAc DA release elicited by phasic stimulation, our data suggest that the compensatory changes in DA release observed in the FSCV experiments might be responsible for the observed changes in behavioral output in response to a novel environment. Context-dependent and home cage behavioral sensitization to cocaine does not require NMDAR activation in DA neurons of the VTA To determine if behavioral sensitization to cocaine requires the activation of NMDARs in DA neurons, we examined the progressive escalating locomotor response to a fixed cocaine dose given in a novel environment or in the home cage [23,24]. It has previously been suggested that the behavioral interaction between novelty and drug-induced sensitization may involve a greater recruitment of limbic, cortical and basal ganglia circuits, when compared to sensitization initiated in the home environment. For sensitization associated with a new environment, or context dependent sensitization, the same dose of cocaine (15 mg/kg, i.p.) was given to WT and KO mice for 5 consecutive days in an activity chamber that is distinct from the home cage. Cocaine injection induced an increase in locomotor activity in both WT and KO mice compared to saline injection (two way ANOVA, treatment, F1, 25 = 15.330, p,0.001) and there is no difference between the genotypes (two way ANOVA, genotype, F1, 25 = 0.295, p = 0.592). Consistent with previous observations [10,11], WT and NR1DATCre mice showed a significant and progressive increase in cocaine-induced activity (Fig. 7A, two way RM- ANOVA, days, F2, 26 = 14.849, p,0.001), and again, no statistical differences were observed between genotypes (two way RM- ANOVA, genotypes, F1,26 = 0.211, p = 0.654). We also measured the locomotor response to a challenge cocaine injection (15 mg/ kg) given 14 days after cocaine withdrawal, following the novel sensitization paradigm. The locomotor response to cocaine after a withdrawal period was not significantly different between WT and NR1DATCre mice (p = 0.585, two way ANOVA, Newman-Keuls post hoc for genotypes). PLoS ONE | www.plosone.org 7 August 2010 | Volume 5 | Issue 8 | e12141