Fig. 6. Repeated cocaine enhanced Tat-induced increases in Ca²⁺ potentials.

a. Recording traces from baseline (BlCtr) illustrating that the duration of Ca²⁺ potentials was remarkably prolonged in a mPFC pyramidal neuron from a COC-treated rat as compared to that from a SAL-treated rat (trace #3 COC-BlCtr vs. #1 SAL-BlCtr). A low concentration of bath-applied Tat (10 nM) induced an additional increase in the Ca²⁺ potential duration in a neuron from a COC-treated rat (trace #4 COC-Tat vs. #3 COC-BlCtr), the magnitude of which was greater than that recorded from a SAL-treated rat (trace #2 SAL-Tat vs. #1 SAL-BlCtr). Note: The rheobase (i.e., the current used to evoked these potentials) for the neuron from the COC-treated rat was slightly lower than that from the SAL-treated rat (0.2 and 0.25 nA, respectively). b. Concentration-response relationships illustrating the effects of bath-applied Tat (10–40 nM) on the integrated Ca²⁺ potential area in neurons from both SAL-treated (n=8 neurons from 8 rats; filled circles) and COC-treated (n=9 neurons from 6 rats; filled squares) rats. There was a significant difference in the effects of treatment, Tat concentration, and an interaction on the area of evoked Ca²⁺ potentials (two-way rmANOVA, p=0.013, <0.001, 0.004, respectively; the signs and vertical bars represent the mean±S.E.M., respectively). A post hoc Newman-Keuls test indicated that there was a significant difference in the Ca²⁺ potential area between the neurons from SAL-treated and COC-treated rats at baseline (0 nM) and 10–40 nM Tat (*p<0.05, **p<0.01). To assess the effects of Tat concentration, independent from the treatment of SAL or COC, a simple main effect analysis (one-way rmANOVA) with post hoc Dunnett’s was used. In the neurons from SAL-treated rats, bath-applied Tat (i.e., SAL-Tat) significantly increased the Ca²⁺ potential area at 20 and 40 nM compared to SAL-BlCtr (++p<0.01). In the neurons from COC-treated rats, the Ca²⁺ potential area reached its maximum level with perfusion of Tat at 10 nM as compared to the COC-BlCtr (p=0.006; with post hoc Dunnett’s test, ++p<0.01). There was no significant difference in the Ca²⁺ potential area between COC-Tat (40 nM) and COC-BlCtr (p>0.05), consistent with response blunting per ‘overactivation’. It is worth noting that a subthreshold concentration of Tat (10 nM), which did not induce a significant increase in Ca²⁺ influx in the neurons from SAL-treated rats, enhanced Ca²⁺ influx to the level which was greater than the additive effects of acute Tat and chronic COC without Tat, This suggested that there might be a synergistic effect of cocaine and Tat. The signes and vertical bars represent the mean±S.E.M., respectively