(A) Chronic morphine decreased the cell surface area of TH+ cells in the mouse VTA. Student's t-test, **p<0.01, n=4, 6 mice/group (6-12 cells/mouse). (B) VTA DA neuron soma size is decreased in postmortem human heroin addicts compared to controls. Student's t-test, *p<0.05, n=3, 5 subjects/group (4-13 cells/subject). (C) VTA DA neurons in slices taken from chronic morphine-treated mice exhibited increased spontaneous firing rates compared to sham-treated mice as measured by extracellular single unit recording. One-way ANOVA, Tukey's post-hoc test, *p<0.05 compared to Sham-aCSF, **p<0.01 compared to Sham-naloxone (Nlx), n=6-8 mice per group (4-17 cells/mouse). Representative traces are shown above the graph. (D) Chronic morphine significantly decreased electrically-evoked DA release in NAc. Student's t-test, **p<0.01, n=5, 6 rats (50, 66 observations/condition). Representative traces are shown above the graph. All scale bars are 20μm.

(A) RT-PCR analysis revealed a significant decrease in levels of KCNAB2 and GIRK3 mRNA in the mouse VTA following chronic morphine. Student's t-test, *p<0.05, n=9-11 mice/group. (B) Epigenetic changes associated with increased gene expression were significantly decreased at the promoters of KCNAB2, KCNF1, GIRK1, and GIRK3 genes in VTA by chronic morphine. Antibodies to acH3, acH4, 3mK4H3, and POL2 were used for ChIP. Student's t-test, *p<0.05, **p<0.01, n=3-7 samples/group (4-5 rats pooled/sample). (C) An epigenetic change consistent with decreased expression (MeCP2 binding) was increased at the KCNF1 and GIRK2 gene promoters in VTA by chronic morphine. Student's t-test, *p<0.05, n=3-4 samples/group (4-5 rats pooled/sample).

(A/B) Chronic morphine decreased IRS2 and phospho-AKT in the mouse VTA, while mTORC1 signaling is increased (blue) and mTORC2 signaling is decreased (red). Student's t-test, *p<0.05, n=9-29 mice/group. Representative blots are shown in (A) and means are shown in (B). (C) mTORC1 activity was further assessed in the mouse VTA by immunohistochemical labeling of phospho-S6. VTA sections were labeled for phospho-S6 (red) and tyrosine hydroxylase (TH) (green). Phospho-S6 labeling was specific, as treatment with the mTORC1 inhibitor rapamycin (30 mg/kg daily, 6 days) eliminated the signal detected in VTA. (D) The proportion of TH and phospho-S6 colabeled cells increased in mice treated with chronic morphine. Student's t-test, *p<0.05, n=9 mice/group (9-29 colabeled cells/mouse). Representative scans are shown below the bar graph. (E) Soma size did not differ between phospho-S6-+ and - cells. n=9 mice, (3-9 cells/mouse). (F/G) Rapamycin (10 or 30 mg/kg daily, 6 days) decreased mTORC1 signaling in VTA without affecting mTORC2. One-way ANOVA, Tukey's post-hoc test, *p<0.05, **p<0.01, ***p<0.001 compared to vehicle treatment, n=8 mice/group. Representative blots are shown in (F). (H) Rapamycin treatment does not prevent the morphine-induced decrease in DA soma size. Mice were pretreated with rapamycin for 5 days before pelleting for morphine. One-way ANOVA, Tukey's post-hoc test, **p<0.01 compared to both Sham-Veh and Sham-Rap, n=2-4 mice/group (5-18 cells/group). Representative cells are shown below bars. All scale bars are 20μm.

Chronic morphine decreases VTA DA soma size yet increases neuronal excitability, while DA transmission to NAc is decreased. The net effect of morphine is a less responsive reward pathway, i.e., reward tolerance. Downregulation of IRS2-AKT signaling in VTA mediates the effects of chronic morphine on soma size and electrical excitability; the effect on excitability is mediated via decreased GABA_A currents and suppression of K⁺ channel expression. Morphine-induced downregulation of mTORC2 activity in VTA is crucial for these morphine-induced morphological and physiological adaptations as well as for reward tolerance. In contrast to mTORC2, chronic morphine increases mTORC1 activity, which does not influence these morphine-induced adaptations.

(A) Increasing DA neuron firing rate via HSV-dnK decreased VTA DA soma size. Student's t-test, *p<0.05, n=3-4 mice per group (2-5 cells/mouse). Representative cells are shown below bars, TH-blue, GFP-red. (B) Preventing the morphine-induced increase in DA neuron firing rate via HSV-Kir2.1 prevented the morphine-induced decrease in soma size. One-way ANOVA followed by Tukey's post-hoc test, *p<0.05 compared to Kir-Mor, n=5-12 mice/group (2-9 cells/mouse). Representative cells are shown below bars, TH-red, GFP-green. (C) HSV-Kir2.1 blocked the morphine-induced increase in DA firing rate. One-way ANOVA, Tukey's post-hoc test, *p<0.05 sham compared to sham-Kir2.1, **p<0.01 morphine compared to morphine-Kir2.1, n=2 mice/group (13-15 cells/mouse). All scale bars are 20μm.

(A) Decreased AKT signaling in VTA via HSV-IRS2dn induced a significant decrease in levels of KCNF1, KCNJ2, and GIRK3 by RT-PCR. Student's t-test, *p<0.05, n=8-10 mice/group. (B) HSV-IRS2dn in VTA, which reduces VTA DA soma size (Russo et al., 2007), significantly decreased electrically-evoked DA release in NAc. Student's t-test, **p<0.01, n=6 rats (66 observations/condition). Representative traces are shown above graph. (C) Modulation of AKT activity in the mouse VTA regulated DA soma size. Intra-VTA injections of HSV-AKTdn significantly decreased, while HSV-AKTca increased soma size. One-way ANOVA, Dunnett's post-hoc test. *p<0.05, **p<0.01 compared to HSV-Con, n=4-6 mice/group (23-28 cells/group). All scale bars are 20μm.

(A) Viral-mediated overexpression was used to target the mouse VTA. Inset shows DA neurons of VTA (red) and AAV-Cre (green). (B) Local knockout of Rictor in floxed-Rictor mice, achieved via AAV-Cre injection into VTA, decreased VTA DA soma size compared to wild-type injected mice. Student's t-test *p<0.05 compared to wild-type mice, n=2 (wild-type) or 4 (floxed-Rictor) mice/group (6-13 cells/mouse). Representative cells are shown below bars. HSV-Rictor-T1135A blocked the decrease in VTA DA soma size induced by morphine. One-way ANOVA, Dunnett's post-hoc *p<0.05 GFP-sham compared to GFP-morphine, n=4-6 mice/group (2-9 cells/mouse). (C) HSVRictor-T1135A also blocked the morphine-induced increase in DA neuron firing rate. One-way ANOVA, Tukey's post-hoc test, **p<0.01, n= 4-7 mice/group (3-14 cells/mouse). (D) Rictor-T1135A in VTA induced morphine place conditioning to a low dose of morphine (5 mg/kg, i.p.) that did not induce preference in GFP-treated mice. Rictor-T1135A also increased morphine-induced locomotor activity. Top panel, mean locomotor activity. Two-way repeated measures ANOVA, treatment variable significant, Bonferroni post-test, day 3 Rictor vs. GFP, *p<0.05, n=12-13 mice/group. Bottom panel, place preference, n= 9-10 mice/group, Student's t-test, *p<0.01. (E) Local knockout of Rictor in floxed-Rictor mice significantly decreased morphine CPP without affecting morphine locomotor activity (15 mg/kg i.p.). Top panel, mean locomotor activity, Two-way repeated measures ANOVA p>0.05, n=12-13 mice/group, bottom panel, CPP, n=12-13 mice/group, Student's t-test, *p<0.05. All scale bars are 20 μm.