Activity of D1/2 Receptor Expressing Neurons in the Nucleus Accumbens Regulates Running, Locomotion, and Food Intake

Abstract

While weight gain is clearly promoted by excessive energy intake and reduced expenditure, the underlying neural mechanisms of energy balance remain unclear. The nucleus accumbens (NAc) is one brain region that has received attention for its role in the regulation of energy balance; its D1 and D2 receptor containing neurons have distinct functions in regulating reward behavior and require further examination. The goal of the present study is to investigate how activation and inhibition of D1 and D2 neurons in the NAc influences behaviors related to energy intake and expenditure. Specific manipulation of D1 vs. D2 neurons was done in both low expenditure and high expenditure (wheel running) conditions to assess behavioral effects in these different states. Direct control of neural activity was achieved using a designer receptors exclusively activated by designer drugs (DREADD) strategy. Activation of NAc D1 neurons increased food intake, wheel running and locomotor activity. In contrast, activation of D2 neurons in the NAc reduced running and locomotion while D2 neuron inhibition had opposite effects. These results highlight the importance of considering both intake and expenditure in the analysis of D1 and D2 neuronal manipulations. Moreover, the behavioral outcomes from NAc D1 neuronal manipulations depend upon the activity state of the animals (wheel running vs. non-running). The data support and complement the hypothesis of specific NAc dopamine pathways facilitating energy expenditure and suggest a potential strategy for human weight control.

Keywords: designer receptors exclusively activated by designer drugs (DREADD); energy expenditure; exercise; food intake; locomotion; medium spiny neurons; nucleus accumbens; running.

Figure 1

Running distance and food intake increase and stabilize during the 14-day exposure to running wheels. (A) All four groups of animals in this study rapidly adapted to wheel running and average daily running distance was 2.71 ± 0.14 km on Day 1 and 9.72 ± 0.16 km on Day 14. (B) Average daily food intake across four groups of animals was 4.38 ± 0.73 g on Day 0 (the day before onset of wheel access), 5.48 ± 0.97 g on Day 1 and 6.10 ± 0.26 g on Day 14. D1 Gq stands for the cohort of D1-Cre mice with Gq-coupled DREADD expression in the NAc (n = 7). Similar abbreviations are used for D1 Gi (n = 6), D2 Gq (n = 5), and D2 Gi (n = 14).

Figure 2

Activation of D1 neurons in the nucleus accumbens (NAc) increases food intake and running distance, while inhibition of D1 neurons suppresses food intake without effecting running distance. D1-Cre mice with Cre-inducible Gq-DREADD expression in the NAc were given access to wheels for 2 weeks of training. Saline or clozapine-N-oxide (CNO) was injected i.p. 2 h before the onset of the dark cycle on two consecutive days, and running distance and food intake over 24 h after injection were assessed. (A) Activation of D1 neurons in the NAc increased running distance (paired t test *P = 0.0007, n = 7) and food intake (paired t test *P = 0.0008, n = 7) during 24 h after injection (B). (C) Inhibition of D1 neurons in the NAc did not change running distance but decreased food intake (D) over 24 h (paired t test *P = 0.0026, n = 8).

Figure 3

Activation of D2 neurons in the NAc suppresses running without affecting food intake, while inhibition of D2 neurons increases running without effects on intake. The same experimental procedures in Figure 2 were performed on D2-Cre mice with Cre-inducible Gq/Gi-DREADD expression in the NAc. (A) 24 h running distance was suppressed by activation of D2 neurons in the NAc (paired t test *P = 0.0054, n = 4) while food intake was not affected (B). (C) 24 h running distance was increased by inhibition of D2 neurons in the NAc (paired t test *P = 0.0014, n = 14) with no effects on food intake (D).

Figure 4

Activation of D1 neurons in the NAc decreases intake accompanying a mild increase in locomotion, while inhibition of D1 neurons in the NAc has no effects on locomotion and intake. (A) CNO activation of Gq-DREADD in the D1 neurons in the NAc tended to induce locomotion (paired t test P = 0.06, n = 7) over 24 h after injection and locomotor elevation (B) was robust during 0.5~3.5 h after injection (F_(47,564) = 5.877, *P < 0.0001, Bonferroni’s post hoc: *P < 0.05 over bin 2~7, saline baseline vs. CNO treatment). (C) Food intake was decreased during 24 h after injection (paired t test *P = 0.0007, n = 7). (D,E) CNO activation of Gi-DREADD in the D1 neurons in the NAc (n = 7) did not change locomotor activity over 24 h, and did not affect food intake (F). Black bars represent dark period and white bars represent light period.

Figure 5

Activation of D2 neurons in the NAc suppresses locomotion without affecting food intake, while inhibition of D2 neurons increases locomotion without effects on intake. (A) CNO activation of Gq-DREADD in the D2 neurons in the NAc reduced total locomotor activity over 24 h (paired t test *P = 0.0064, n = 5), with the most robust effect during 2.5~9 h after injection (F_(47,376) = 7.474, *P < 0.0001, Bonferroni’s post hoc: *P < 0.05 over bin 5~18, saline baseline vs. CNO treatment) (B). (C) Food intake during 24 h after activation of D2 neurons was not affected. (D) CNO activation of Gi-DREADD in the D2 neurons in the NAc robustly induced locomotion over 24 h (paired t test *P = 0.0005, n = 11), with the most robust effect during 2.5~6.5 h after injection (F_(47,940) = 3.789, *P < 0.0001, Bonferroni’s post hoc: *P < 0.05 in bins 5–11) (E). (F) Food intake was not affected during 24 h after CNO injection. Black bars represent dark period and white bars represent light period.

Figure 6

Assessments of placements and viral expression in the NAc. (A) Representative micrograph shows the expression and distribution of Cre-inducible DREADD virus with mCherry (red). DAPI (blue) depicts the brain coronal section containing the NAc. (B) Schematic drawing of the viral expression distribution across all four cohorts of animals. Red areas circled by solid lines indicates the common region with expression in each individual while the area circled by dashed lines shows the largest viral expression area among all animals used in this study. AcbSh and AcbC, shell and core regions of the NAc, respectively; Aca, anterior commissure.