Post by Shireen Parimoo
What's the science?
The nucleus accumbens (NAc), located in the midbrain, regulates motivational or reward-seeking behavior primarily through dopaminergic signaling. For example, dopaminergic activity increases in response to cues that signal an upcoming reward, such as a tone signaling that food will be arriving soon. Conversely, activation of cholinergic interneurons in the NAc is associated with the inhibition of reward-seeking behavior when this behavior might not be beneficial, like seeking food when already full. However, the causal role of these interneurons in regulating cue-motivated behavior is not well-understood. This week in Biological Psychiatry, Collins and colleagues used behavioral conditioning and a combination of optogenetic and chemogenetic techniques to investigate the causal role of cholinergic NAc interneurons in cue-motivated behavior.
How did they do it?
Adult transgenic rats were were injected with adeno-associated virus carrying channelrhodopsin (ChR2) or a control yellow fluorescent protein, and the human M4 muscarinic receptor (hM4D(Gi)) or the control mCherry, which were selectively expressed in the NAc interneurons. Optical stimulation of ChR2 increases interneuron activity, whereas clozapine N-oxide (CNO) injection inactivates interneurons by binding to hM4D(Gi). Cue-motivated behavior was measured using the Pavlovian-to-instrumental transfer (PIT) test, which consisted of a training and a test phase. In the training phase, the rats underwent 8 days each of Pavlovian and instrumental conditioning. For Pavlovian conditioning, a predictive tone cue was repeatedly paired with a chocolate pellet reward; for instrumental conditioning, pressing a lever resulted in a chocolate pellet reward, but no tone was present. Rats were also exposed to a neutral tone in the absence of any rewards during a single session.
In the PIT test phase, the rats were presented with the neutral and reward-predictive cues. They had access to a lever but pressing it did not result in a reward. The authors tested the effect of cholinergic interneuron inactivation on cue-motivated behavior (i.e. lever-pressing) by injecting the rats with CNO or a vehicle and examining behavior before (baseline) and after cue presentation. They optically stimulated the interneurons at cue presentation to test the effect of interneuron activation on behavior. Finally, they injected DhbE into the NAc, which is an acetylcholine antagonist that binds to b2-containing nicotinic receptors. They then optogenetically stimulated the interneurons and recorded cue-motivated behavior to determine whether acetylcholine released from the interneurons acts on nicotinic receptors to regulate cue-motivated behavior.
What did they find?
When the reward predictive cue was presented, both the hM4D(Gi) and control rats had more lever presses compared to baseline or when the neutral cue was presented. However, the hM4D(Gi) rats showed much greater lever-pressing behavior compared to controls when CNO was administered (i.e. when interneuron activity was reduced). Optogenetic activation of cholinergic interneurons led to greater acetylcholine release in ChR2-expressing rats, which was accompanied by a reduction in reward-seeking behavior in ChR2-expressing rats but not in control rats. This means that inhibiting cholinergic interneurons increases cue-motivated behavior, whereas activating the interneurons reduces cue-motivated behavior. Lastly, in the absence of any stimulation, rats infused with an acetylcholine antagonist or a vehicle (control condition) showed greater lever-pressing behavior in response to the predictive cue, but not at baseline or in response to the neutral cue. However, when the cholinergic interneurons were optically activated, there was a reduction in cue-motivated behavior among rats infused with the vehicle, but no change in response to the cue among rats infused with the acetylcholine antagonist. Thus, even though the activation of interneurons normally decreases cue-motivated behavior, an acetylcholine antagonist disrupts the downstream effects of these interneurons by blocking the interaction between acetylcholine and b2-containing nicotinic receptors.
What's the impact?
This study demonstrates that nucleus accumbens cholinergic interneurons have an inhibitory effect on cue-motivated behavior and regulate this behavior by acting on downstream nicotinic receptors. These findings provide further insight into our current understanding of psychiatric conditions with dysfunctional motivational behavior, such as eating disorders and addiction, and have important implications for developing treatments for such disorders.
Collins et al. Nucleus accumbens cholinergic interneurons oppose cue-motivated behavior. Biological Psychiatry (2019). Access the original scientific publication here.