Optogenetic, voltammetric, and histochemical characterization of striatal tyrosine hydroxylase interneurons

The traditional view of striatal dopamine originating from the midbrain remained unchallenged until a publication by Dubach et al. (1987) showed that in primates there are striatal interneurons immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis. It has generally been assumed in subsequent publications that striatal TH interneurons (THINs) are an intrinsic striatal source of dopamine (DA). The possibility that THINs could be DAergic has deep implications for both normal and pathological states, such as Parkinson’s disease. However, despite nearly three decades since the first report of striatal TH+ neurons, no direct evidence that THINs are DAergic or can compensate for the loss of nigrostriatal DA has been reported. To examine if THINs contain and release DA, two bacterial artificial chromosome (BAC) transgenic mouse lines were used that express either the Cre recombinase enzyme or the enhanced green fluorescent protein (EGFP) reporter. Both transgenes are under the control of the regulatory elements of TH and each respective mouse line is referred to as TH-Cre or EGFP-TH. Immunofluorescent cytochemistry revealed no colocalization of EGFP+ striatal cells with DA in TH-EGFP mice. Neither was there any colocalization of EGFP+ striatal cells with aromatic L-amino acid decarboxylase (AADC), the vesicular monoamine transporter-2 (VMAT2), or the dopamine transporter (DAT). Additionally, TH-Cre mice that were unilaterally lesioned by 6-OHDA in the midbrain were also bilaterally injected into the striatum with Cre-dependent recombinant adeno-associated virus (AAV), encoding the opsin protein channelrhodopsin-2 (ChR2) fused to the enhanced yellow fluorescent protein (EYFP) to transduce THINs to express ChR2. THINs were then light-activated by 2-5 ms blue light pulses during simultaneous fast-scan cyclic voltammetry (FSCV) and found to not release any detectable amounts of DA, even in the presence of nomifensine, a potent DA reuptake inhibitor. Lastly, it was found that THINs are powerfully modulated by DA and provide a widespread and powerful inhibition on SPNs. THINs then, despite containing TH, are not DAergic but a novel class of GABAergic interneuron that regulates the timing of the output responses of SPNs and add to the complexity of the striatal microcircuitry.Ph.D.Includes bibliographical referencesIncludes vitaby Harry S. Xenia