Cell type‐specific distribution of GABA_A receptor subtypes in the mouse dorsal striatum

Abstract

The striatum is the main input nucleus of the basal ganglia, mediating motor and cognitive functions. Striatal projection neurons are GABAergic medium spiny neurons (MSN), expressing either the dopamine receptor type 1 (D1‐R MSN) and forming the direct, movement‐promoting pathway, or dopamine receptor type 2 (D2‐R MSN), forming the indirect movement‐suppressing pathway. Locally, activity and synchronization of MSN are modulated by several subtypes of GABAergic and cholinergic interneurons. Overall, GABAergic circuits in the striatum remain poorly characterized, and little is known about the intrastriatal connectivity of interneurons and the distribution of GABAA receptor (GABAAR) subtypes, distinguished by their subunit composition, in striatal synapses. Here, by using immunofluorescence in mouse tissue, we investigated the distribution of GABAARs containing the α1, α2, or α3 subunit in perisomatic synapses of striatal MSN and interneurons, as well as the innervation pattern of D1R‐ and D2R‐MSN soma and axonal initial segment (AIS) by GABAergic and cholinergic interneurons. Our results show that perisomatic GABAergic synapses of D1R‐ and D2R‐MSN contain the GABAAR α1 and/or α2 subunits, but not the α3 subunit; D2R‐MSN have significantly more α1‐GABAARs on their soma than D1R‐MSN. Further, interneurons have few perisomatic synapses containing α2‐GABAARs, whereas α3‐GABAARs (along with the α1‐GABAARs) are abundant in perisomatic synapses of CCK+, NPY+/SOM+, and vAChT+ interneurons. Each MSN and interneuron population analyzed received a distinct pattern of GABAergic and cholinergic innervation, complementing this postsynaptic heterogeneity. In conclusion, intra‐striatal GABAergic circuits are distinguished by cell‐type specific innervation patterns, differential expression and postsynaptic targeting of GABAAR subtypes.

Dopamine depletion induces neuron‐specific alterations of GABAergic transmission in the mouse striatum

“…The analysis had to be restricted to perisomatic synapses, as the dendrites of individual MSN could not be resolved by GFP staining. Data acquired from 6‐OHDA tissue were compared to data reported previously on naïve tissue (Boccalaro et al., 2019).…”

“…Such false‐positive results lead to an overestimation of perisomatic synaptic density, which is mainly relevant when the density is low, such as for the α 3 subunit. In our previous study (Boccalaro et al., 2019), we arbitrarily set the threshold at 0.05 clusters/μm for a cell to be considered to express a given GABA A R subtype. Considerably more GABA A R clusters labeled for the α 1 or α 2 subunit were present on the soma of D 1 R‐MSN than α 3 clusters, which were either not present or only in very small numbers (Figure ).…”

“…Electrophysiological experiments were designed to compare a vehicle‐injected group with a 6‐OHDA‐injected group. After validating that the injection of vehicle did not result in morphological changes, immunofluorescence experiments were designed to compare morphological results from naïve control animals, which were published before (Boccalaro et al., 2019), to a 6‐OHDA‐injected group, to reduce the amount of data analysis needed for this study. In some experiments, data from the contralateral side were also included in the analysis.…”

“…In a previous study, we have investigated the distribution of three GABA A R subtypes (containing the α 1 , α 2 , or α 3 subunit)—known to be predominantly localized postsynaptically along with the clustering protein gephyrin (Sassoè‐Pognetto, Panzanelli, Sieghart, & Fritschy, 2000)—at the soma of different types of neurons in the dorsolateral striatum of naïve mice to determine whether they are distributed in distinct striatal microcircuits. We showed that GABA A R containing the subunit α 1 are the most widely distributed, being present in perisomatic synapses of all cell types investigated, whereas the α 2 subunit is found primarily in D 1 R‐ and D 2 R‐MSN and the α 3 subunit is specific for certain types of GABAergic and cholinergic interneurons (Boccalaro, Cristiá‐Lara, Schwerdel, Fritschy, & Rubi, 2019). Here, we investigated whether GABAergic synapses undergo a circuit‐specific remodeling after depletion of striatal DA as a model of PD.…”

“…To this end, we characterized inhibitory synaptic inputs onto D 1 R‐ and D 2 R‐MSN by whole‐cell patch‐clamp recordings ex vivo and determined immunohistochemically the cell type‐specific distribution of GABA A Rs containing the subunits α 1 , α 2 or α 3 in brain sections from 6‐OHDA‐injected mice, using specific markers of MSN and interneuron subpopulations. For control, we used data from naïve mice published in (Boccalaro et al., 2019) as well as the contralateral side of 6‐OHDA‐injected mice in some experiments.…”