Medium spiny neurons (MSNsϩ MSNs may be a potential target for therapeutic intervention.

The development of BAC transgenic mice expressing promoter-specific fluorescent reporter proteins has been a great asset for neuroscience by enabling detection of neuronal subsets in live tissue. For the study of basal ganglia physiology, reporters driven by type 1 and 2 dopamine receptors have been particularly useful for distinguishing the two classes of striatal projection neurons – striatonigral and striatopallidal. However, emerging evidence suggests that some of the transgenic reporter lines may have suboptimal features. The ideal transgenic reporter line should (1) express a reporter with high sensitivity and specificity for detecting the cellular subset of interest and that does not otherwise alter the biology of the cells in which it is expressed, and (2) involve a genetic manipulation that does not cause any additional genetic effects other than expression of the reporter. Here we introduce a new BAC transgenic reporter line, Drd1a-tdTomato line 6, with features that approximate these ideals, offering substantial benefits over existing lines. In this study, we investigate the integrity of dopamine-sensitive behaviors and test the sensitivity and specificity of tdTomato fluorescence for identifying striatonigral projection neurons in mice. Behaviorally, hemizygous Drd1a-tdTomato line 6 mice are similar to littermate controls; while hemizygous Drd2-EGFP mice are not. In characterizing the sensitivity and specificity of line 6 mice, we find that both are high. The results of this characterization indicate that line 6 Drd1a-tdTomato+/− mice offer a useful alternative approach to identify both striatonigral and striatopallidal neurons in a single transgenic line with a high degree of accuracy.

The dorsolateral striatum (DLS) is implicated in habit formation. However, the DLS circuit mechanisms underlying habit remain unclear. A key role for DLS is to transform sensorimotor cortical input into firing of output neurons that project to the mutually antagonistic direct and indirect basal ganglia pathways. Here we examine whether habit alters this input-output function. By imaging cortically-evoked firing in large populations of pathway-defined striatal projection neurons (SPNs), we identify features that strongly correlate with habitual behavior on a subject-by-subject basis. Habitual behavior correlated with strengthened DLS output to both pathways as well as a tendency for action-promoting direct pathway SPNs to fire before indirect pathway SPNs. In contrast, habit suppression correlated solely with a weakened direct pathway output. Surprisingly, all effects were broadly distributed in space. Together, these findings indicate that the striatum imposes broad, pathway-specific modulations of incoming activity to render learned motor behaviors habitual.

We previously reported greater GABA A receptor-mediated tonic currents in D 2 ϩ striatopallidal than D 1 ϩ striatonigral medium spiny neurons (MSNs) are mediated by ␣5-subunit-containing receptors. Here, we used whole-cell recordings in slices from bacterial artificial chromosome transgenic mice to investigate the link between subunit composition, phosphorylation, and dopamine receptor activation. Whole-cell recordings in slices from ␦-subunit knock-out mice demonstrate that while MSNs in wild-type mice do express ␦-subunitcontaining receptors, this receptor subtype is not responsible for tonic conductance observed in the acute slice preparation. We assessed the contribution of the ␤1-and ␤3-subunits expressed in MSNs by their sensitivity to etomidate, an agonist selective for ␤2-or ␤3-subunit-containing GABA A receptors. Although etomidate produced substantial tonic current in D 2 ϩ neurons, there was no effect in D 1 ϩ neurons. However, with internal PKA application or dopamine modulation, D 1 ϩ neurons expressed tonic conductance and responded to etomidate application. Our results suggest that distinct phosphorylation of ␤3-subunits may cause larger tonic current in D 2 ϩ striatopallidal MSNs, and proper intracellular conditions can reveal tonic current in D 1 ϩ cells.