The findings of reduced selleck compound DA soma size and DA output are consistent with earlier reports that chronic morphine decreases levels of neurofilament proteins in VTA and impairs axoplasmic transport from VTA to NAc (Beitner-Johnson et al.,

1992 and Beitner-Johnson and Nestler, 1993). Some of these neuroadaptations may also contribute to withdrawal symptoms from chronic morphine, as decreased VTA DA soma size (Spiga et al., 2003), neuronal activity (Diana et al., 1995), and output to NAc (Pothos et al., 1991, Rossetti et al., 1992a and Rossetti et al., 1992b) are reported in morphine-withdrawn rats. Our observation that chronic morphine increases the intrinsic excitability of VTA DA neurons in brain slices is consistent with previous findings of increased VTA neuronal firing rate in morphine-dependent rats in vivo (Georges et al., 2006). Previous data and our current findings suggest that chronic morphine induces this increased

excitability of VTA DA neurons by at least two mechanisms: downregulation of AKT which reduces GABAA currents in these neurons (Krishnan et al., 2008), and repression of KCNAB2 and perhaps other K+ channel subunits ( Figure 3). The reduced expression of K+ channel genes, which PF-02341066 cost reflects a transcriptional effect based on our ChIP assays, appears to be mediated by reduced AKT signaling, as overexpression of IRS2dn was sufficient to decrease expression of several K+ channel subunits. Downregulation of mTORC2 is also required for the morphine-induced increase in VTA excitability, since Rictor overexpression, which prevented morphine many downregulation of AKT activity, was sufficient to rescue this morphine effect, although whether

this action was through AKT modulation of GABAA channels, K+ channels, or another mTORC2 target has yet to be determined. Since we only observed the rescue of firing rate in VTA DA neurons that overexpressed Rictor, and not in nearby GFP-negative DA neurons, we believe that restoration of AKT/mTORC2 activity within DA neurons is sufficient to rescue opiate-induced changes. However, this does not preclude the possible influence of VTA GABA neurons in morphine-induced changes, as our viral manipulations were not specific for DA neurons. For example, HSV-dnK might also increase the activity of VTA GABA neurons, which would then decrease the activity of nearby DA neurons. However, we see a decrease in DA soma size with morphine, which is known to decrease GABA activity, making dnK activation of GABA neurons an unlikely contributor. A direct test of this hypothesis awaits the development of viral vectors that target specific neuronal subpopulations.