The Tail of the Mouse Striatum Contains a Novel Large Type of GABAergic Neuron Incorporated in a Unique Disinhibitory Pathway That Relays Auditory Signals to Subcortical Nuclei.

The most caudal part of the striatum in rodents, the tail of the striatum (TS), has many features that distinguish it from the rostral striatum, such as its biased distributions of dopamine receptor subtypes, lack of striosomes and matrix compartmentalization, and involvement in sound-driven behaviors. However, information regarding the TS is still limited. We demonstrate in this article that the TS of the male mouse contains GABAergic neurons of a novel type that were detected immunohistochemically with the neurofilament marker SMI-32. Their somata were larger than cholinergic giant aspiny neurons, were located in a narrow space adjacent to the globus pallidus (GP), and extended long dendrites laterally toward the intermediate division (ID) of the trilaminar part of the TS, the region targeted by axons from the primary auditory cortex (A1). Although vesicular glutamate transporter 1-positive cortical axon terminals rarely contacted these TS large (TSL) neurons, glutamic acid decarboxylase-immunoreactive and enkephalin-immunoreactive boutons densely covered somata and dendrites of TSL neurons, forming symmetrical synapses. Analyses of GAD67-CrePR knock-in mice revealed that these axonal boutons originated from nearby medium spiny neurons (MSNs) in the ID. All MSNs examined in the ID in turn received inputs from the A1. Retrograde tracers injected into the rostral zona incerta and ventral medial nucleus of the thalamus labeled somata of TSL neurons. TSL neurons share many morphological features with GP neurons, but their strategically located dendrites receive inputs from closely located MSNs in the ID, suggesting faster responses than distant GP neurons to facilitate auditory-evoked, prompt disinhibition in their targets.