Figure 4. Mechanisms underlying strengthened BLA glutamatergic input to NAcs following chronic Δ⁹-THC.

A. oEPSC I-O curves evoked by ChR2 activation of BLA axons projecting to NAcs, after chronic vehicle, Δ⁹-THC, or Δ⁹-THC + AM251 injections. Representative mean oEPSCs are shown at 5 laser intensities in neurons from vehicle (gray traces), Δ⁹-THC (green traces), or Δ⁹-THC + AM-251-injected (blue traces) rats. The strength of BLA glutamate input to NAcs neurons was significantly increased following chronic Δ⁹-THC and this was prevented by AM251 (** = F_18,392 = 2.6, p < 0.001, intensity x treatment interaction, 2-way repeated measures ANOVA). B. Mean PPR of BLA-evoked oEPSCs at different inter-laser pulse intervals (ISI) in NAc neurons from chronic Δ⁹-THC- and vehicle injected rats. Mean representative traces from individual cells in both groups are shown above. The PPR was not significantly altered by chronic Δ⁹-THC (* = F_12,255 = 0.3, p = 0.98, ISI x treatment interaction, 2-way repeated measures ANOVA). Note that paired oEPSCs overlapped at the 20 ms ISI, thereby yielding a ratio = 0. C. The rate of block of ChR2-evoked oNMDA receptor oEPSCs by MK801, was significantly faster following chronic Δ⁹-THC in the BLA → NAcs pathway (time-constant of oNMDA block by MK801: chronic vehicle = 34.3 stimuli, 95% confidence interval = 32.2–36.8 stimuli, n = 8 neurons, gray line; chronic Δ⁹-THC = 27.9 stimuli, 95% confidence interval = 26.1–29.9 stimuli, n = 8, green line). Exponential decay time constants (τ) were obtained by best fit (dashed lines). Representative oNMDA waveforms from each group obtained before MK801 application (control, C), and at 30 and 140 stimuli are shown above. D. AMPA/NMDA ratios show potentiated transmission at BLA → NAcs synapses. Left, representative mean waveforms of laser-evoked oEPSCs collected at −70 mV (solid line) and +40 mV (dashed line) holding potentials from chronic vehicle (gray), chronic Δ⁹-THC (green), or chronic Δ⁹-THC + AM251 (blue) groups. AMPA/NMDA = peak AMPA response at −70 mV divided by the peak NMDA response at +40 mV. Right, AMPA/NMDA ration group means (± s.e.m.). ** = p < 0.01, F_2,47 = 5.8, one-way ANOVA and Holm-Sidak post-hoc comparison). E. I-O curves of ChR2-evoked oNMDA currents at BLA → NAcs synapses in neurons from chronic vehicle, and chronic Δ⁹-THC treated rats. Representative mean oNMDA wave forms are shown above for each condition. The treatment x laser power interaction was significant (F_5,70 = 6.66, p <0.0001, 2-way repeated measures ANOVA; ** = p < 0.01, Holm-Sidak comparison), indicating a decrease in oNMDA current amplitude after chronic Δ⁹-THC. F. Lack of chronic Δ⁹-THC effect on the voltage-dependence of oNMDA currents. Representative mean traces from cells in chronic vehicle (gray), and Δ⁹-THC-treated (green) groups are shown above. G. Δ⁹-THC does not change inward rectification of ChR2-evoked AMPA receptor oEPSCs at BLA → NAcs synapses. Ga. Representative I-V relationships and mean AMPA oEPSCs evoked by ChR2 in the BLA → NAcs pathway at holding potentials of −70, 0, and +40 from each of the 3 groups (chronic vehicle, gray, chronic Δ⁹-THC, green, and chronic Δ⁹-THC + AM251, blue). A hypothetical slope = 1.0 is indicated by a dashed line. Gb. Mean oEPSC RI in each group. The cells from chronic Δ⁹-THC-treated animals showed no significant change in RI compared to chronic vehicle-treated rats (F_2,44 = 2.4, p = 0.10, 1-way ANOVA). H. Mean time course of NASPM, on oEPSCs evoked by ChR2-activation of BLA inputs to NAcs neurons. Mean wave forms shown above indicate peak effect of NASPM (solid line), compared to baseline (dashed line) in representative cells from chronic vehicle (gray) or chronic Δ⁹-THC-treated (green) animals. The effect of NASPM on BLA → NAcs oEPSCs was not different between chronic vehicle and chronic Δ⁹-THC groups (F_2,20 = 1.6, p = 0.23, 1-way ANOVA and Holm-Sidak’s post-hoc test). Number of cells/Rats: A. Veh: 23/9, THC: 16/8, THC+AM: 11/6. B. Veh: 20/9, THC: 16/8. C. Veh: 8/7, THC: 8/5. D. Veh: 17/8, THC: 17/7, THC+AM: 16/6. E. Veh: 9/7, THC: 7/5 F. Veh: 5/5, THC: 7/5 Gb. Veh: 19/8, THC: 18/7, THC+AM: 10/6 H. Veh: 9/6, THC: 10/7.