Unilateral optogenetic inhibition and excitation of basal ganglia output show opposing effects on directional lick choices and movement initiation in mice

Models of basal ganglia function predict that tonic inhibitory output to motor thalamus suppresses unwanted movements, and that a decrease in such activity leads to action selection. A direct test of these outcomes of thalamic inhibition has not been performed, however. To conduct such a direct test, we utilized rapid optogenetic activation and inactivation of the GABAergic output of the substantia nigra pars reticulata (SNr) to motor thalamus in mice that were trained in a sensory cued left/right licking task. Directional licking tasks have previously been shown to depend on a thalamocortical feedback loop between ventromedial motor thalamus and antero-lateral premotor cortex (Li et al., 2015; Guo et al., 2017). In confirmation of model predictions, we found that 1s of unilateral optogenetic inhibition of GABAergic output from the SNr biased decision making towards the contralateral lick spout with ipsilaterally cued trials while leaving motor performance intact. In contrast, 1s of optogenetic excitation of SNr terminals in motor thalamus resulted in an opposite bias towards the ipsilateral direction confirming a bidirectional effect of tonic nigral output on directional decision making. In a second variant of the task we disallowed anticipatory licking and found that successful suppression of anticipatory licking was also impacted by our optogenetic manipulations in agreement with the suppressive effect of tonic nigral output. Nevertheless, direct unilateral excitation of SNr cell bodies resulted in bilateral movement suppression, suggesting that descending motor pathways from the SNr to superior colliculus also play an important role in the control of licking behavior.\n\nSignificance StatementThis study provides the first evidence that basal ganglia output to motor thalamus can control decision making in left/right licking choices and suppress anticipatory movement initiation. Unilateral optogenetic inhibition or excitation of basal ganglia output via the substantia nigra resulted in opposite changes of directional lick choices and could override the sensory information on lick direction provided by a whisker stimulus. These results suggest that basal ganglia output gates activity in a thalamo-cortical feedback loop previously shown to underlie the control of forced choice directional licking behavior. The results substantiate models stating that tonic inhibition of motor thalamus from the basal ganglia directs action selection and suppresses unwanted movements.