Rat Nucleus Accumbens Core Astrocytes Modulate Reward and the Motivation to Self-Administer Ethanol after Abstinence

Abstract

Our understanding of the active role that astrocytes play in modulating neuronal function and behavior is rapidly expanding, but little is known about the role that astrocytes may play in drug-seeking behavior for commonly abused substances. Given that the nucleus accumbens is critically involved in substance abuse and motivation, we sought to determine whether nucleus accumbens astrocytes influence the motivation to self-administer ethanol following abstinence. We found that the packing density of astrocytes that were expressing glial fibrillary acidic protein increased in the nucleus accumbens core (NAcore) during abstinence from EtOH selfadministration. No change was observed in the nucleus accumbens shell. This increased NAcore astrocyte density positively correlated with the motivation for ethanol. Astrocytes can communicate with one another and influence neuronal activity through gap-junction hemichannels. Because of this, the effect of blocking gap-junction hemichannels on the motivation for ethanol was examined. The motivation to self-administer ethanol after 3 weeks abstinence was increased following microinjection of gap-junction hemichannel blockers into the NAcore at doses that block both neuronal and astrocytic channels. In contrast, no effect was observed following microinjection of doses that are not thought to block astrocytic channels or following microinjection of either dose into the nucleus accumbens shell. Additionally, the motivation for sucrose after 3 weeks abstinence was unaffected by NAcore gap-junction hemichannel blockers. Next, Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) were selectively expressed in NAcore astrocytes to test the effect of astrocyte stimulation. DREADD activation increased cytosolic calcium in primary astrocytes, facilitated responding for rewarding brain stimulation, and reduced the motivation for ethanol after 3 weeks abstinence. This is the first work to modulate drug-seeking behavior with astrocyte-specific DREADDs. Taken together, our findings demonstrate that NAcore astrocytes can shape the motivation to self-administer ethanol; suggesting that the development of ligands which selectively stimulate astrocytes may be a successful strategy to abate ethanol-seeking behavior.

Gq-DREADD Selectively Initiates Glial Glutamate Release and Inhibits Cue-induced Cocaine Seeking

“…3D). Consistent with a Gq mechanism and the recent report that GFAP driven Gq DREADD activation elevated [Ca 2+ ] i in astroglia (34), these data indicate that CNO-mediated release of glutamate relies on internal Ca 2+ stores. Also consistent with this interpretation, previous reports show minor involvement of plasmalemmal Ca 2+ channel conductances on vescicular glial glutamate release (13, 35).…”

“…6). Further supporting an involvement of glutamate gliotransmission in the regulation of addictive behavior, a recent report from the Bowers lab demonstrated that stimulating glial Gq-DREADD in the NAcore inhibited the motivation to seek ethanol (34). While we used cell morphology and characteristics of glutamate release by CNO (dnSNARE mice and relative insensitivity to N-channel blockers) to show that Gq-DREADD was expressed in glia, a very minor contribution by neuronal Gq-DREADD cannot be 100% ruled out.…”

“…Several groups have shown that, activation of Gq-DREADD elevates [Ca 2+ ] i , potentially leading to enhanced gliotransmission (21, 34). Here we show that activating Gq-DREADD in NAcore astrocytes promotes the release of glutamate.…”

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

“…Bull et al (2014) demonstrate that activation of Gq signaling with the hM3D DREADD in NAcore astrocytes enhances internal calcium concentration, facilitates intracranial self-stimulation, and reduces motivation to seek ethanol after 3 weeks of abstinence. Furthermore, activation of astroglial Gq-DREADD promotes glutamate release and inhibits cue-induced cocaine seeking, likely through restoration of glutamate tone on mGluR2/3 receptors in the NAcore similar to what is described above for NAC .…”

DREADDs for Neuroscientists

“…Since then, hM3Dq has been widely used to enhance neuronal firing, and I here cite only representative examples in which feeding (Krashes et al, 2011; Atasoy et al, 2012), energy expenditure (Kong et al, 2012), locomotion and striatal synaptogenesis (Kozorovitskiy et al, 2012), memory (Garner et al, 2012), and social behaviors (Peñagarikano et al, 2015) have been modulated by hM3Dq in vivo. Because hM3Dq activation induces intracellular calcium release, it has also been used to “activate” astrocytes (Agulhon et al, 2013; Bull et al, 2014; Scofield et al, 2015), hepatocytes (Li et al, 2013), pancreatic βcells (Guettier et al, 2009; Jain et al, 2013), vascular smooth muscle cells (Armbruster et al, 2007), and iPS-derived neuroblasts (Dell’Anno et al, 2014). …”