Joshua L. Plotkin

Striatal cholinergic interneurons (CINs) can drive local dopamine release via nicotinic acetylcholine receptors (nAChRs) expressed on dopaminergic axons, but their role in modulating serotonin (5-HT) signaling is poorly understood. Here, we show that synchronous activation of CINs directly triggers local 5-HT release in the dorsal striatum via nAChRs expressed on serotonergic axons. This CIN–5-HT coupling is not detectable in the ventral striatum, despite its substantially denser serotonergic innervation. The nAChR-dependent release not only increases 5-HT levels in the dorsal striatum, but also expands the spatial footprint of serotonergic signaling. In Sapap3^-/- mice, a model of obsessive-compulsive disorder (OCD)-like behavior, this mechanism is exaggerated due to a hypercholinergic state, selectively amplifying the nAChR-dependent component of monoamine release. These findings demonstrate a regionally confined form of acetylcholine–5-HT crosstalk in the striatum and identify CINs as regulators of 5-HT dynamics in both healthy and pathological states.

Striatal output is dynamically modulated by cholinergic interneurons (CINs), the primary source of acetylcholine in the striatum. CINs have been classically viewed as a random and homogeneous population, but recent evidence suggests heterogeneity in their anatomical and functional organization. Here, using systematic mapping and quantitative spatial analyses, we found that—contrary to current dogma—CINs exhibited striking enrichment and nonrandom clustering in the striosome compartment, particularly in the lateral striatum. Similar analyses carried out for parvalbumin- and somatostatin-expressing interneurons revealed that compartmental organization is interneuron specific. The strong “striosome preference” exhibited by CINs was confined within striosome borders, not extending to the surrounding matrix. We further found that striosome and matrix CINs differed in their expression levels of phospho-S6 ribosomal protein-Ser240/244 and choline acetyltransferase, suggesting functional differences, and clustered CINs differed from unclustered CINs in their intrinsic membrane properties. Finally, CINs expressing Lhx6, which defines a distinct γ-aminobutyric acid (GABA) coreleasing population, were notably absent from regions where highly clustered striosomal CINs appeared. Collectively, our findings uncover important dimensions of CIN organization, suggesting that modulation of regional and compartmental striatal output may depend upon the spatial–functional heterogeneity of CINs.

Striatal cholinergic interneurons (CINs) activate nicotinic acetylcholine receptors on dopamine axons to extend the range of dopamine release. Here we show that synchronous activation of CINs induces and extends the range of local serotonin release via a similar mechanism. This process is exaggerated in the hypercholinergic striatum of a mouse model of OCD-like behavior, implicating CINs as critical regulators of serotonin levels in the healthy and pathological striatum.

Striatal cholinergic interneurons (CINs) activate nicotinic acetylcholine receptors on dopamine axons to extend the range of dopamine release. Here we show that synchronous activation of CINs induces and extends the range of local serotonin release via a similar mechanism. This process is exaggerated in the hypercholinergic striatum of a mouse model of OCD-like behavior, implicating CINs as critical regulators of serotonin levels in the healthy and pathological striatum.

The auditory striatum, the tail portion of dorsal striatum in basal ganglia, is implicated in perceptual decision-making, transforming auditory stimuli to action outcomes. Despite its known connections to diverse neurological conditions, the dopaminergic modulation of sensory striatal neuronal activity and its behavioral influences remain unknown. We demonstrated that the optogenetic inhibition of dopaminergic projections from the substantia nigra pars compacta to the auditory striatum specifically impairs mouse choice performance but not movement in an auditory frequency discrimination task. In vivo dopamine and calcium imaging in freely behaving mice revealed that this dopaminergic projection modulates striatal tone representations, and tone-evoked striatal dopamine release inversely correlated with the evidence strength of tones. Optogenetic inhibition of D1-receptor expressing neurons and pharmacological inhibition of D1 receptors in the auditory striatum dampened choice performance accuracy. Our study uncovers a phasic mechanism within the nigrostriatal system that regulates auditory decisions by modulating ongoing auditory perception.

Striatal spiny projection neurons (SPNs) transform convergent excitatory corticostriatal inputs into an inhibitory signal that shapes basal ganglia output. This process is fine-tuned by striatal GABAergic interneurons (GINs), which receive overlapping cortical inputs and mediate rapid corticostriatal feedforward inhibition of SPNs. Adding another level of control, cholinergic interneurons (CINs), which are also vigorously activated by corticostriatal excitation, can disynaptically inhibit SPNs by activating α4β2 nicotinic acetylcholine receptors (nAChRs) on various GINs. Measurements of this disynaptic inhibitory pathway, however, indicate that it is too slow to compete with direct GIN-mediated feedforward inhibition. Moreover, functional nAChRs are also present on populations of GINs that respond only weakly to phasic activation of CINs, such as parvalbumin-positive fast-spiking interneurons (PV-FSIs), making the overall role of nAChRs in shaping striatal synaptic integration unclear. Using acute striatal slices from mice we show that upon synchronous optogenetic activation of corticostriatal projections blockade of α4β2 nAChRs shortened SPN spike latencies and increased postsynaptic depolarizations. The nAChR-dependent inhibition was mediated by downstream GABA release, and data suggest that the GABA source was not limited to GINs that respond strongly to phasic CIN activation. In particular, the observed decrease in spike latency caused by nAChR blockade was associated with a diminished frequency of spontaneous inhibitory postsynaptic currents in SPNs, a parallel hyperpolarization of PV-FSIs, and was occluded by pharmacologically preventing cortical activation of PV-FSIs. Taken together, we describe a role for tonic (as opposed to phasic) activation of nAChRs in striatal function. We conclude that tonic activation of nAChRs by CINs maintains a GABAergic brake on cortically-driven striatal output by ‘priming’ feedforward inhibition, a process that may shape SPN spike timing, striatal processing, and synaptic plasticity.

Summary: Deletion of the OCD-associated gene SAP90/PSD-95-associated protein 3 ( Sapap3 ), which encodes a postsynaptic anchoring protein at corticostriatal synapses, causes OCD-like motor behaviors in mice. While corticostriatal synaptic dysfunction is central to this phenotype, the striatum efficiently adapts to pathological changes, often in ways that expand upon the original circuit impairment. Here we show that SAPAP3 deletion causes non-synaptic and pathwayspecific alterations in dorsolateral striatum circuit function. While somatic excitability was elevated in striatal projection neurons (SPNs), dendritic excitability was exclusively enhanced in direct pathway SPNs. Layered on top of this, cholinergic modulation was altered in opposing ways: striatal cholinergic interneuron density and evoked acetylcholine release were elevated, while basal muscarinic modulation of SPNs was reduced. These data describe how SAPAP3 deletion alters the striatal landscape upon which impaired corticostriatal inputs will act, offering a basis for how pathological synaptic integration and unbalanced striatal output underlying OCD-like behaviors may be shaped.