(A) TrkB mRNA is expressed in D1+ and D2+ MSNs FACS-purified from D1-GFP and D2-GFP transgenic mice, but is significantly enriched in D2+ MSNs (n = 4 per group); Student’s t-test, p < 0.05). (B) D1-Cre–flTrkB (n = 9) mice displayed enhanced cocaine conditioned place preference (CPP) relative to littermate controls (n = 10), whereas (C) D2-Cre–flTrkB mice (n = 14) exhibited decreased cocaine CPP compared to littermate controls (n = 16) (cocaine dose: 7.5 mg/kg ip; Student’s t-test, ^**p < 0.01, ^*p < 0.05). (D,E) D1-Cre–flTrkB and D2-Cre–flTrkB mice and littermate controls were treated with saline on day 0 and with cocaine (10 mg/kg) on days 1–7 and locomotor activity was assessed over a 30-min time period. (D) D1-Cre–flTrkB mice (n = 6) displayed enhanced cocaine-induced locomotor activity after repeated cocaine administration compared to littermate controls (n = 7) (Repeated Measures Two-Way ANOVA, genotype effect: F_(1,11) = 6.20, p < 0.05; day effect: F_(6,66) = 5.50, p < 0.01), while (E) D2-Cre–flTrkB mice (n = 10) showed decreased locomotor activity to acute and repeated cocaine relative to controls (n = 14) (Repeated Measures Two-Way ANOVA, genotype effect: F_(1,22) = 9.98, p < 0.01; day effect: F_(6,132)= 4.00, p < 0.01). Post-hoc analysis reveals significant differences on specific cocaine days (Student’s t-test, ^**p <0.01, ^*p < 0.05). Data represented as mean ± SEM. (F–I) c-Fos induction was examined 90 min after acute cocaine (20 mg/kg) by double immuno-labeling of c-Fos (green) and Cre (red) in the NAc. (F,H) D1-Cre–flTrkB mice exhibited a significant decrease in double-labeled c-Fos (green) and Cre (red) neurons in the NAc after cocaine exposure compared to D1-Cre control mice and this down-regulation is specific to the NAc shell. (G,I) In contrast, D2-Cre–flTrkB mice, relative to D2-Cre controls, displayed an increase in double-labeled c-Fos and Cre neurons in the NAc, an effect also specific to the NAc shell (n = 4 per group, Student’s t-test, ^**p < 0.01, ^*p < 0.05). Images displayed are from the NAc shell. Arrows represent neurons double labeled with c-Fos and Cre. Arrowheads represent c-Fos neurons that are not Cre positive. Scale bars, 20 µm. Data represented as mean ± SEM. (J) Sample traces obtained by 200 pA current injection (holding potential at −80 mV) in NAc shell MSNs in D1-Cre-flTrkB, D2-Cre-flTrkB, and their control mice injected with DIO-AAV-EYFP into the NAc for visualization of D1+ or D2+ MSNs. (K,L) D2+ MSNs in D2-Cre-flTrkB NAc (n=3 animals), but not from D1+ MSNs in D1-Cre-flTrkB NAc (n=4), display increased cell excitability following incremental steps in current injections (100, 150, and 200 pA) compared to respective controls, D2-Cre (n=5) and D1-Cre (n=8). Two-way ANOVA, F_(1,7) = 13.23, p = 0.002 (for D2+ MSNs), F_(1,11) = 4.04, p = 0.054 (for D1+ MSNs). Post hoc analysis reveals significant effects for 100 and 150 pA currents in D2+ MSNs, Student’s t-test, *p<0.05.

(A,B) DIO-AAV-ChR2-EYFP or DIO-AAV-EYFP was injected into the NAc of D1-Cre and D2-Cre mice resulting in ChR2-EYFP or EYFP expressing neurons (green) that also express Cre (red). Scale bars, 50 µm (low power images) and 20 µm (high power images). (C) Diagram of D1+ or D2+ ChR2 expressing MSNs and blue light emission from the optic fiber. (D) Control of neuronal firing when a NAc MSN expressing DIO-AAV-ChR2-EYFP is exposed to blue light at 1.4 or 1.0 Hz. (E) c-Fos (red) expression is induced in D1+ or D2+ MSNs expressing ChR2 (green) that have been activated with 10 Hz blue light stimulation but not EYFP expressing MSNs. Scale bars, 20 µm. (F) Quantification of E shows a significant increase in c-Fos expressing ChR2 expressing D1+ and D2+ MSNs compared to EYFP expressing controls after blue light exposure (n = 3 per group, Student’s t-test, ^*p < 0.01). (G) c-Fos mRNA is significantly upregulated in the NAc after blue light pulses in DIO-AAV-ChR2-EYFP expressing D1-Cre and D2-Cre mice (n = 4–5 per group, Student’s t-test, ^**p < 0.01, ^*p < 0.05).

(A) Illustration of the CPP cocaine/blue light paradigm. Mice were conditioned to a cocaine/blue light chamber and a saline/no light chamber for 30 min. Blue light pulses (10 Hz) were delivered for four 3-min periods during the 30-min conditioning. (B,C) Activating D1+ MSNs in D1-Cre DIO-AAV-ChR2-EYFP mice (n = 9) during cocaine (5 mg/kg)/blue light CPP enhances cocaine reward compared to D1-Cre DIO-AAV-EYFP controls (n = 10), whereas activating D2+ MSNs in D2-Cre DIO-AAV-ChR2-EYFP mice (n = 7) during cocaine (7.5 mg/kg)/ blue light CPP attenuates cocaine reward relative to D2-Cre DIO-AAV-EYFP controls (n = 4) (Students t-test, ^**p < 0.01, ^*p < 0.05). (D,E) Optical control of D1+ and D2+ MSNs in D1-Cre and D2-Cre mice expressing DIO-AAV-ChR2-EYFP or DIO-AAV-EYFP in a cocaine naïve state and 24 hours after 6 days of repeated cocaine (15 mg/kg) results in increased locomotor activity only when D1+ MSNs are activated after cocaine exposure (Two Way ANOVA, genotype effect: F_(1,22) = 4.37, p < 0.05; Student’s t-test ^*p<0.05)

(A) HSV-mCherry or HSV-ChR2-mCherry (red) were injected into the NAc of wildtype mice. Scale bar 50 µm. (B) Activation of ChR2 with blue light (10 Hz) stimulation induces c-Fos (green) expression in HSV-ChR2-mCherry neurons (red), but not in HSV-mCherry neurons (red), after 10 Hz blue light. Scale bar 25 µm. (C) Quantification of c-Fos positive neurons in B and c-Fos mRNA after 10 Hz blue light shows a significant increase in c-Fos in HSV-ChR2-mCherry expressing NAc compared to HSV-mCherry expressing controls (n = 3–5 per group, Student’s t-test, ^**p < 0.01, ^*p < 0.05). (D) HSV-ChR2-mCherry (red) injection into the NAc of D1-GFP and D2-GFP mice mediates transgene expression in both D1+ and D2+ MSNs (GFP, green) and (E) c-Fos (blue) is induced by light in each MSN (green). Scale bar 10 µm. (F) Quantification of D and E shows HSV-ChR2-mCherry and blue light induced c-Fos equally in D1+ and D2+ MSNs. (G) Mice expressing HSV-ChR2-mCherry (n = 9) in the NAc displayed enhanced preference for the cocaine (5 mg/kg)/blue light chamber compared to control mice expressing HSV-mCherry (n = 8) (Student’s t-test, p < 0.05). (H,I) Blue light simulation results in a significant decrease in phospho:total ERK levels (pERK42/ERK42 and pERK44/ERK44) in the NAc of D1-Cre DIO-AAV-ChR2-EYFP mice and mice expressing HSV-ChR2-mCherry relative to their controls (DIO-AAV-EYFP or HSV-mCherry) (n = 5–8 per group, Student’s t-test *p < 0.05).