Date Published: March 7, 2019
Publisher: Public Library of Science
Author(s): Efrain A. Ribeiro, Alexander R. Nectow, Lisa E. Pomeranz, Mats I. Ekstrand, Ja Wook Koo, Eric J. Nestler, Juan M. Dominguez.
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.
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) [1–3]. The NAc is primarily (~95%) comprised of principal GABAergic medium spiny neurons (MSNs) that express predominantly either D1 or D2 dopamine receptors, while the remaining neurons are GABAergic interneurons expressing somatostatin (SST)/nitric oxide synthetase-1 (NOS1), parvalbumin (PV), or the calcium-binding protein calretinin, and cholinergic interneurons expressing choline acetyltransferase. MSNs form the primary output pathways from the NAc that project to the ventral pallidum or midbrain.
In this study, we used PRV-Introvert-GFP, a modified alpha-Herpes Bartha virus containing an introvert coding sequence that improves cell type-specificity of transneuronal tracing. 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. We found that retrograde tracing was successful in identifying synaptic afferents to these cells. While every region that projected to somatostatin interneurons was among the global afferents to the NAc as reported by the Allen Brain Atlas, several global afferents showed no detectable connections with somatostatin interneurons and, conversely, several connections to these interneurons were quantitatively stronger than that reported for the NAc overall. These results are consistent with the view that a cell type-specific connectome is necessary to fully elucidate the connectivity of the NAc and that some projections selectively innervate subsets of neuronal subtypes, including interneurons, in this brain region.