Fig. 1: GABA_A receptors mediate VTA_DA pauses in vivo.

A: DART technology. AAV expression of the ⁺HTP protein (pink) enables cell-specific covalent capture of gabazine^DART (black) and Alexa647^DART (cyan) ligands. Once tethered, gabazine^DART blocks native GABA_A receptors, while Alexa647^DART enables fluorescent visualization of target engagement.

B: Example histology. AAV expression of the active ⁺HTP or control ^ddHTP in VTA_DA neurons is indicated by dTomato (red). All mice receive an intracranial ligand infusion of 10 μM gabazine^DART + 1 μM Alexa647^DART, and are perfused 36 hr later for histology. Alexa647^DART (cyan) quantifies ligand target engagement.

C: Electrophysiology. Top: an electrode bundle targeting the medial VTA enables in vivo extracellular recordings. A nearby cannula permits ligand infusion. Bottom: sample putative dopamine-neuron spikes, recorded in head-fixed animals, shown for ⁺HTP and ^ddHTP mice.

D: Pauses in firing: Top: time course of recording, baseline 15-min (pre-gabazine^DART) followed by infusion and post-gabazine^DART recording. Bottom: pause metric, %PSI (percent of interspike intervals longer than twice the median interspike interval). Changes in %PSI compare a 15-min baseline (%PSI_pre) to a 15-min sliding window (%PSI_post) according to Δ%PSI_norm=(%PSI_post−%PSI_pre)/(%PSI_post+%PSI_pre). Left: Δ%PSI_norm time course, mean ± SEM over cells (n = 18 ^ddHTP cells, 3 mice; n = 39 ⁺HTP cells, 5 mice). Right: steady-state Δ%PSI_norm (1-hr post-gabazine^DART) with individual cells (circles), group means (thin horizontal lines), mean-difference bootstrap (pink distribution), and 95% CI of the two-sided permutation test (vertical black bar); ⁺HTP and ^ddHTP cells differ significantly (P=0.009).

E-F: Burst/tonic firing: Analysis of %SFB (percent of spikes fired in bursts) and FR (firing rate) from the same cells; format as above.