A Direct Projection from Mouse Primary Visual Cortex to Dorsomedial Striatum

The mammalian striatum receives inputs from many cortical areas, but the existence of a direct axonal projection from the primary visual cortex (V1) is controversial. In this study we use anterograde and retrograde tracing techniques to demonstrate that V1 directly innervates a topographically defined longitudinal strip of dorsomedial striatum in mice. We find that this projection forms functional excitatory synapses with direct and indirect pathway striatal projection neurons (SPNs) and engages feed-forward inhibition onto these cells. Importantly, stimulation of V1 afferents is sufficient to evoke phasic firing in SPNs. These findings therefore identify a striatal region that is functionally innervated by V1 and suggest that early visual processing may play an important role in striatal-based behaviors

Dopaminergic neurons inhibit striatal output via non-canonical release of GABA

The substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) contain the two largest populations of dopamine (DA)-releasing neurons in the mammalian brain. These neurons extend elaborate projections in striatum, a large subcortical structure implicated in motor planning and reward-based learning. Phasic activation of dopaminergic neurons in response to salient or reward-predicting stimuli is thought to modulate striatal output via the release of DA to promote and reinforce motor action1–4. Here we show that activation of DA neurons in striatal slices rapidly inhibits action potential firing in both direct-and indirect-pathway striatal projection neurons (SPNs) through vesicular release of the inhibitory transmitter γ-aminobutyric acid (GABA). GABA is released directly from dopaminergic axons but in a manner that is independent of the vesicular GABA transporter VGAT. Instead GABA release requires activity of the vesicular monoamine transporter VMAT2, which is the vesicular transporter for DA. Furthermore, VMAT2 expression in GABAergic neurons lacking VGAT is sufficient to sustain GABA release. Thus, these findings expand the repertoire of synaptic mechanisms employed by DA neurons to influence basal ganglia circuits, reveal a novel substrate whose transport is dependent on VMAT2, and demonstrate that GABA can function as a bona fide co-transmitter in monoaminergic neurons

Midbrain dopamine neurons sustain inhibitory transmission using plasma membrane uptake of GABA, not synthesis

Synaptic transmission between midbrain dopamine neurons and target neurons in the striatum is essential for the selection and reinforcement of movements. Recent evidence indicates that nigrostriatal dopamine neurons inhibit striatal projection neurons by releasing a neurotransmitter that activates GABAA receptors. Here, we demonstrate that this phenomenon extends to mesolimbic afferents, and confirm that the released neurotransmitter is GABA. However, the GABA synthetic enzymes GAD65 and GAD67 are not detected in midbrain dopamine neurons. Instead, these cells express the membrane GABA transporters mGAT1 (Slc6a1) and mGAT4 (Slc6a11) and inhibition of these transporters prevents GABA co-release. These findings therefore indicate that GABA co-release is a general feature of midbrain dopaminergic neurons that relies on GABA uptake from the extracellular milieu as opposed to de novo synthesis. This atypical mechanism may confer dopaminergic neurons the flexibility to differentially control GABAergic transmission in a target-dependent manner across their extensive axonal arbors. DOI: http://dx.doi.org/10.7554/eLife.01936.00

Layer 5 neurons in primary visual cortex project to dorsomedial striatum.

<p><b>A–F</b>. Representative serial coronal sections from the brain of a <i>Rbp4</i>-Cre mouse virally injected in the primary visual cortex (V1) with an AAV encoding Cre-dependent EGFP to label the cell bodies and axonal processes of neurons in layer 5 (<b>A</b>). EGFP-labeled axons leave V1 in through the external capsule (<b>B</b>) and innervate various structures including several layers of the superior colliculus (SC; <b>B</b>) and dorsal lateral geniculate nucleus (DLG; <b>C</b>). V1 axons enter the posterior tail of the striatum (caudate/putamen, CPu; <b>D</b>) and course throughout the length of the striatum (<b>E</b>) before innervating the anterior dorsomedial quadrant (<b>F</b>). <i>Left</i>, entire hemisphere stained with DAPI to highlight different brain structures overlaid with the EGFP fluorescence. <i>Middle</i>, detailed view of regions outlined in white in the corresponding left panel. <i>Right</i>, corresponding images from Paxinos Mouse Brain Atlas highlighting the putative structures where EGFP fluorescence is detected. Similar results were observed eight different mouse brains. M1; primary motor cortex. S1BF; primary somatosensory cortex barrel field.</p

Retrograde labeling of V1 neurons innervating the DMS.

<p><b>A</b>. Coronal striatal section showing the site of red retrobead (RRB) injection into the dorsomedial striatum (DMS). The section is counterstained with DAPI. <b>B</b>. Coronal brain section of V1 containing RRBs retrogradedly transported from the dorsomedial striatum. Secondary visual cortex (V2ML and V2L) and secondary auditory cortex (AuD), but not Au1 (primary auditory cortex) were also labeled. As expected, retrobeads were also detected in putative dopaminergic neurons within the substantia nigra pars compacta (SNc). <b>C</b>. Detailed view of V1 showing the relative distribution of retrobead-labeled cells bodies across cortical layers. <b>D</b>. Distribution of retrobead-labeled neurons in V1 across cortical layers. The majority (81%) of labeled cells are situated within layer 5, while only 19% distribute to layers 2/3. <b>E</b>. Representative coronal section from a <i>Rbp4</i>-Cre;ZsGreen1 brain showing EGFP fluorescence concentrated in the cell bodies of Cre-containing cortical neurons in layer 5. The section is counterstained with DAPI. <b>F</b>. Detailed view of V1 in a <i>Rbp4</i>-Cre;ZsGreen1 brain injected with RRBs in the DMS illustrating the extensive overlap between retrobead- and Cre-containing (EGFP-positive) cells in upper layer 5. <b>G</b>. Percentage of all retrobead-labeled layer 5 neurons that also contain Cre (RRB/EGFP⁺; 65%, shown in yellow) vs. cells that are only RRB⁺ (35%, red). <b>H</b>. Coronal brain section showing red retrobead labeled cells in thalamus. ILT: Intralaminar thalamic nuclei. Scale: 1 mm for panels A, B, E and H; 100 µm for panels C and F.</p

Stimulation of V1 axons engages glutamatergic and GABAergic synaptic transmission onto SPNs.

<p><b>A</b>. Representative high magnification view of the dorsomedial striatum in a <i>Rbp4</i>-Cre; <i>Drd2-</i>EGFP mouse virally injected in V1 with an AAV encoding Cre-dependent ChR2-mCherry. Axonal projections from V1 (red) are clearly seen within the DMS. The section is stained with DAPI (blue) to label cell bodies. iSPNs are easily identified using EGFP fluorescence (green). A presumptive dSPN (DAPI⁺; EGFP⁻) is also shown. <b>B</b>. Cell-attached recording from a SPN showing that a 1 ms 473 nm light flash (blue bar) reliably evokes a single action potential. Three consecutive extracellular waveforms are overlaid. <b>C</b>. As in <b>B</b> for another SPN recorded in the whole-cell current-clamp configuration. <b>D</b>. Whole-cell voltage-clamp traces from a SPN upon optogenetic stimulation (1 ms, blue bar) of ChR2-expressing V1 axons. EPSCs (black) were recorded at <i>E</i>_Cl = −70 mV, while IPSCs (red) were recorded at 0 mV (the reversal potential for ionotropic glutamate receptor mediated currents). Dashed gray lines mark the onset of both currents and highlight the delayed onset of IPSCs relative to EPSCs. Both EPSC and IPSC were eliminated after bath application of the glutamate receptor antagonists NBQX and CPP (both at 10 µM; gray and pink lines, respectively), confirming the disynaptic origin of IPSCs. <b>E</b>. Mean (± s.e.m) peak EPSC amplitude in dSPNs (blue) and iSPNs (green). <b>F</b>. Mean (± s.e.m) latency from flash onset to current onset of EPSCs (black) and IPSCs (red). <b>G</b>. Mean (± s.e.m) EPSC (black) and IPSC (red) amplitudes. Asterisk represents statistical significance, <i>P</i><0.05 vs. baseline amplitude. Data in E–G represent mean ± s.e.m. Number of recordings are indicated in parentheses.</p

GABA co-released from striatal dopamine axons dampens phasic dopamine release through autoregulatory GABAA receptors

Summary: Striatal dopamine axons co-release dopamine and gamma-aminobutyric acid (GABA), using GABA provided by uptake via GABA transporter-1 (GAT1). Functions of GABA co-release are poorly understood. We asked whether co-released GABA autoinhibits dopamine release via axonal GABA type A receptors (GABAARs), complementing established inhibition by dopamine acting at axonal D2 autoreceptors. We show that dopamine axons express α3-GABAAR subunits in mouse striatum. Enhanced dopamine release evoked by single-pulse optical stimulation in striatal slices with GABAAR antagonism confirms that an endogenous GABA tone limits dopamine release. Strikingly, an additional inhibitory component is seen when multiple pulses are used to mimic phasic axonal activity, revealing the role of GABAAR-mediated autoinhibition of dopamine release. This autoregulation is lost in conditional GAT1-knockout mice lacking GABA co-release. Given the faster kinetics of ionotropic GABAARs than G-protein-coupled D2 autoreceptors, our data reveal a mechanism whereby co-released GABA acts as a first responder to dampen phasic-to-tonic dopamine signaling

Calcium action potentials in hair cells pattern auditory neuron activity before hearing onset

We found rat central auditory neurons to fire action potentials in a precise sequence of mini-bursts before the age of hearing onset. This stereotyped pattern was initiated by hair cells in the cochlea, which trigger brief bursts of action potentials in auditory neurons each time they fire a Ca2+spike. By generating theta-like activity, hair cells may limit the influence of synaptic depression in developing auditory circuits and promote consolidation of synapses