Accumbal Cholinergic Interneurons Differentially Influence Motivation Related to Satiety Signaling

Abstract

Satiety, rather than all or none, can instead be viewed as a cumulative decrease in the drive to eat that develops over the course of a meal. The nucleus accumbens (NAc) is known to play a critical role in this type of value reappraisal, but the underlying circuits that influence such processes are unclear. Although NAc cholinergic interneurons (CINs) comprise only a small proportion of NAc neurons, their local impact on reward-based processes provides a candidate cell population for investigating the neural underpinnings of satiety. The present research therefore aimed to determine the role of NAc-CINs in motivation for food reinforcers in relation to satiety signaling. Through bidirectional control of CIN activity in mice, we show that when motivated by food restriction, increasing CIN activity led to a reduction in palatable food consumption while reducing CIN excitability enhanced food intake. These activity-dependent changes developed only late in the session and were unlikely to be driven by the innate reinforcer strength, suggesting that CIN modulation was instead impacting the cumulative change in motivation underlying satiety signaling. We propose that on a circuit level, an overall increase in inhibitory tone onto NAc output neurons played a role in the behavioral results, as activating NAc-CINs led to an inhibition of medium spiny neurons that was dependent on nicotinic receptor activation. Our results reveal an important role for NAc-CINs in controlling motivation for food intake and additionally provide a circuit-level framework for investigating the endogenous cholinergic circuits that signal satiety.

Figure 1.

Selective expression of viral constructs in NAc cholinergic interneurons. Viral targeting of NAc-CINs in ChAT-cre mice resulted in selective expression of MCY (B2) in neurons expressing the vesicular ACh transporter VAChT (B1, B3) in the medial core and shell region of the NAc (n = 10; A, C). C, Darker shading corresponds to the qualitative greater density of the dendritic and somatic extent of ACh-CIN labeling. ac, anterior commissure; ms, medial septum; NAcC, nucleus accumbens core; NAcS, nucleus accumbens shell. White arrows, VAChT-expressing neurons; yellow arrowheads, neurons expressing both VAChT and MCY. Scale bars: A, 100 μm; B, 0.5 mm.

Figure 2.

Designer receptor expression was sufficient for modulating CIN excitability in vitro. A, Recorded neurons were regular spiking and mostly spontaneously active at rest and showed signature CIN intrinsic currents. B, Bath application of 10 μM CNO reliably depolarized the membrane potential of neurons expression Gq-coupled (n = 10; B1, C) and Gs-coupled (n = 11; B3, C) designer receptors, whereas neurons expressing Gi-coupled (n = 9) receptors were inhibited (B2). Black arrowhead (left), baseline current ramp injection; colored arrow (right), post-CNO current ramp injection. C, Results summary for CNO-mediated response from left to right in Gq-expressing (green), Gi-expressing (red), Gs-expressing (blue), and MCY-only (black; n = 6) CINs plotted as the change in membrane potential. Synaptic antagonists (in μm) = CNQX 20, AP5 100, CGP-52432 10, gabazine 10.

Figure 3.

Impact of CIN activity modulation on food intake after free food access and a 24-h fast. A, MCY, Gs, Gq, and Gi NAc-CIN–expressing mice with ad libitum food access were given an injection of either VEH or CNO 30 min before the dark cycle (active phase), and food consumption was measured 150 min later. B, CNO injections 30 min before the active phase in mice fasted for 24 h resulted in a significant reduction in food intake for Gq-expressing compared with both MCY- and Gi-expressing mice at 2 h after reintroducing home-cage food. Gq, n = 10; Gs, n = 14; Gi, n = 10; MCY, n = 15. *, p < 0.05.

Figure 4.

Differential modulation of CIN excitability impacts food-seeking behavior in a time-dependent manner. A, Mice were trained on a FR5 schedule of reinforcement for a palatable food reward. B, Summary of trials completed after CNO injection in DREADD-expressing and MCY mice. C, Between-group comparison of response rate over time. D, Within-group comparisons (VEH vs. CNO) of response rate over time for all groups. MCY (D1), n = 12; Gs (D2), n = 14; Gi (D3), n = 10; Gq (D4), n = 10. *, p < 0.05; **, p < 0.01.

Figure 5.

Modulation of CINs impacts measure of satiety. A, In the FR5 operant task, all groups in VEH and CNO conditions showed similar total locomotor activity. B, Within-subject comparisons for the time taken, after reward consumption, for all groups to start a new trial (PRP). MCY (B2), n = 12; Gi (B3), n = 10; Gs (B4), n = 14; Gq (B5), n = 10. *, p < 0.05.

Figure 6.

Reinforcer strength largely independent of satiety signaling is unaffected by CIN modulation. A, Mice were trained on a PR schedule of reinforcement in which correct touches for a palatable food reward consecutively increased by four. B, All groups completed an equal amount of trials, thus consuming a similar amount of food reward. C, Within group comparisons (VEH vs. CNO) of response rate over time or all groups. MCY (C1), n = 12; Gi (C2), n = 10; Gs (C3), n = 14; Gq (C4), n = 10.

Figure 7.

Designer receptor activation of CINs inhibits MSNs through GABA_A and nicotinic receptor activation. A1, Setup schematic: in vitro whole-cell recordings from MSNs while bath-applying CNO onto Gq-expressing NAc-CINs. A2, Example of a typical MSN recording showing changes in membrane potential in response to current injections. B, Example trace showing a reduction of MSN activity after bath application of CNO and corresponding I-V relationship taken before (black trace, arrowhead) and after (orange trace, arrowhead) CNO application. In this example, the membrane potential was held at above threshold with current injections through the intracellular electrode. C, Group summary of CNO-evoked changes in membrane potential in control conditions (n = 9; left), when pretreated with the GABA_A receptor antagonist gabazine (GBZ, 10 μM; n = 6; middle), or with the nicotinic receptor antagonist mecamylamine (MECA, 10 μM; n = 6; right).