Viral labeling of neurons synaptically connected to nucleus accumbens somatostatin interneurons.

Efrain A Ribeiro,Ja Wook Koo,Eric J Nestler,Lisa E Pomeranz,Alexander R Nectow,Mats I Ekstrand

doi:10.1371/journal.pone.0213476

Abstract

The nucleus accumbens, a key brain reward region, receives synaptic inputs from a range of forebrain and brainstem regions. Many of these projections have been established using electrophysiology or fluorescent tract tracing. However, more recently developed viral tracing techniques have allowed for fluorescent labeling of synaptic afferents in a cell type-specific manner. Since the NAc is comprised of multiple cell types, these methods have enabled the delineation of the cell type-specific connectivity of principal medium spiny neurons in the region. The synaptic connectivity of somatostatin interneurons, which account for <5% of the neurons in the region, has been inferred from electrophysiological and immunohistochemical data, but has not yet been visualized using modern viral tracing techniques. Here, we use the pseudorabies virus (PRV)-Introvert-GFP virus, an alphaherpes virus previously shown to label synaptic afferents in a cell type-specific manner, to label first order afferents to NAc somatostatin interneurons. While we find GFP(+) labeling in several well established projections to the NAc, we also observe that several known projections to NAc did not contain GFP(+) cells, suggesting they do not innervate somatostatin interneurons in the region. A subset of the GFP(+) afferents are c-FOS(+) following acute administration of cocaine, showing that NAc somatostatin interneurons are innervated by some cells that respond to rewarding stimuli. These results provide a foundation for future studies aimed toward elucidating the cell type-specific connectivity of the NAc, and identify specific circuits that warrant future functional characterization.

Highlights

The nucleus accumbens, part of the ventral striatum, regulates motivation, reward, and aversion by integrating information from several distinct input pathways including glutamatergic inputs from the prefrontal cortex, ventral subiculum, amygdala, thalamus, and dopaminergic inputs from the ventral tegmental area (VTA) [1,2,3]
In SST-Cre mice, we found that at 24–36 hr after infection there were a small number of GFP(+) cells in the NAc, and no GFP(+) cells in any other regions
We injected the virus into the NAc of SST-Cre mice to identify synaptic afferents to somatostatin interneurons, a sparse cell type in this brain region

Summary

Introduction

The nucleus accumbens, part of the ventral striatum, regulates motivation, reward, and aversion by integrating information from several distinct input pathways including glutamatergic inputs from the prefrontal cortex, ventral subiculum, amygdala, thalamus, and dopaminergic inputs from the ventral tegmental area (VTA) [1,2,3]. The latter study used a modified rabies virus to trace inputs to D1 or D2 expressing MSNs in the dorsal striatum using Cre-expressing transgenic mice. In this important paper, the group showed that, while D1 and D2 expressing neurons in the NAc receive the same set of inputs, certain inputs preferentially innervate one population of MSNs over the other. The use of distinct viral tracing systems combined with optogenetics has added a layer of complexity to the NAc circuitry, which has led to the formation of novel hypotheses regarding the function of specific neural projections to the NAc (e.g., [6,7,8,9,10,11,12,13,14])

Methods

Results

Conclusion