Duration- and sex-dependent neural circuit control of voluntary physical activity

Abstract

Rationale: Exercise participation remains low despite clear benefits. Rats engage in voluntary wheel running (VWR) that follows distinct phases of acquisition, during which VWR escalates, and maintenance, during which VWR remains stable. Understanding mechanisms driving acquisition and maintenance of VWR could lead to novel strategies to promote exercise. The two phases of VWR resemble those that occur during operant conditioning and, therefore, might involve similar neural substrates. The dorsomedial (DMS) dorsal striatum (DS) supports the acquisition of operant conditioning, whereas the dorsolateral striatum (DLS) supports its maintenance.

Objectives: Here we sought to characterize the roles of DS subregions in VWR. Females escalate VWR and operant conditioning faster than males. Thus, we also assessed for sex differences.

Methods: To determine the causal role of DS subregions in VWR, we pharmacologically inactivated the DMS or DLS of adult, male and female, Long-Evans rats during the two phases of VWR. The involvement of DA receptor 1 (D1)-expressing neurons in the DS was investigated by quantifying cfos mRNA within this neuronal population.

Results: We observed that, in males, the DMS and DLS are critical for VWR exclusively during acquisition and maintenance, respectively. In females, the DMS is also critical only during acquisition, but the DLS contributes to VWR during both VWR phases. DLS D1 neurons could be an important driver of VWR escalation during acquisition.

Conclusions: The acquisition and maintenance of VWR involve unique neural substrates in the DS that vary by sex. Results reveal targets for sex-specific strategies to promote exercise.

Keywords: Dopamine; Dorsal striatum; Dorsolateral striatum; Dorsomedial striatum; Exercise; Habit; Sex differences; Wheel running.

Fig. 1

Sex differences in voluntary wheel running (VWR) behavior. A Average daily distance run of rats assigned to VWR conditions for at least 28 d. Each data point in the violin plot represents a single subject’s data point, coded by sex. **Females different from males (p < 0.0001). B Average daily distance run of male and female rats during specific phases of the estrous cycle: proestrus (Pro), estrus (Est), and metestrus or diestrus (Met/Di). *Pro different from all other groups (p < 0.01); **Pro different from all other groups (p < 0.0001); θ Met/Di different from males (p < 0.05); Φ all estrous phases different from males (p < 0.01). C. Distance run during the 7 days (acquisition). *Females different from males (p < 0.01); **Females different from males (p < 0.0001). D Distance run during the last 7 days (maintenance). *Females different from males (p < 0.01). E Diurnal pattern of VWR during acquisition (week 1). *Pro different from all other groups (p < 0.05); θ Pro different from males (p < 0.05); Φ all estrous phases different from males (p < 0.05). F Diurnal pattern of VWR during maintenance (week 4). *Pro different from all other groups (p < 0.05); θ Pro different from males (p < 0.05); Φ all estrous phases different from males (p < 0.05). G Average distance run in the inactive cycle. *p < 0.05; **p < 0.01; ***p < 0.001. Bars and symbols points represent group means ± SEM

Fig. 2

Role of the dorsomedial (DMS) and dorsolateral (DLS) striatum in voluntary wheel running (VWR). A Experimental timeline. Rats received saline and muscimol/baclofen (Musc/Bac) injections into either the DMS or DLS during both the acquisition phase (days 4 and 5 after start of VWR) and maintenance phase (days 28 and 29 after start of VWR). Since VWR behavior peaks during the first few h after the start of the active cycle (Fig. 1E and F), saline or Musc/Bac was injected 15 min prior to the start of the active cycle and VWR was monitored for 2 h prior to wheels being locked. B Fluorescent muscimol depicts spread of injection within the DLS (scale bar = 500 μm). C Graphical reconstruction of smallest (dark red) and largest (bright red) muscimol spread within the DMS (top row) and DLS (bottom row) included in the experiment. Coronal sections modified from Paxinos and Watson (Paxinos 1998). D Running distance over a 2 h period following saline or muscimol/baclofen (Musc/Bac) injection into the DMS or DLS during the acquisition phase of VWR. E Running distance over a 2 h period following saline or muscimol/baclofen injection into the DMS or DLS during the maintenance phase of VWR. F Running distance of rats injected with Musc/Bas into the DMS or DLS during the acquisition phase expressed as a percent difference from the same rats injected with saline. G Running distance of rats injected with Musc/Bas into the DMS or DLS during the maintenance phase expressed as a percent difference from the same rats injected with saline. H Regression analysis showing the relationship between average daily distance run during the first 3 days of the experiment and the effect of intra-DLS Musc/Bac expressed as a % of saline. I Regression analysis showing the relationship between average daily distance run during the first 3 days of the experiment and the effect of intra-DMS Musc/Bac expressed as a % of saline. J Male (n = 7 after 1 exclusion due to misplaced cannulae) and female (n = 8) rats received intra-DLS saline or muscimol/baclofen (Musc/Bac) on alternating days for 4 days (2 saline and 2 Musc/Bac injections each) prior to placement into Med Associates locomotor activity chambers for 1 h. The data collected over the 2 d of injections were averaged to yield one value each for saline and Musc/Bac. K Rats were injected a 3^rd time with Musc/Bas into the DMS or DLS on day 32 and running distance was recorded for 90 min prior to perfusion for cFos immunohistochemistry (IR). L cFos IR expressed as a % of the saline group for rats injected in the DMS. (M) cFos IR expressed as a % of the saline group for rats injected in the DMS. Representative photomicrographs depicting cFos in the DLS of female rats injected with (N) saline or (O) Musc/Bac into the DLS. Arrows point to some of the many cFos-positive cells. Scale bar = 85 μm. Bars represent group means ± SEM. *p < 0.05

Fig. 3

Activity of dopamine receptor 1 (Drd1) mRNA-expressing striatal neurons during voluntary wheel running (VWR). A Experimental timeline. B Photomicrograph depicting double fluorescent in situ hybridization for Drd1 mRNA (red), cfos mRNA (green), double-labeled cells (yellow), and DAPI (blue) in the DLS. Scale bar = 50 um. The inset shows regions counted. Percentage of Drd1 mRNA-expressing neurons co-expressing cfos mRNA are shown in the C dorsomedial striatum (DMS), D dorsolateral striatum (DLS), E nucleus accumbens shell (NAcS), and F nucleus accumbens core (NAcC). G Number of single cfos mRNA-positive cells in the DLS, H plasma corticosterone following 30 min of VWR. Bars represent group means ± SEM. *p < 0.05

Fig. 4

Relationships between running distance and neural activity in dorsal striatum subregions. A In females, distance run during the first 3 days (acquisition) predicts the ratio of dorsolateral (DLS):dorsomedial (DMS)D1 neural activity, but not DLS:DMS putative D2 activity (measured using single cfos). B Also in females, distance run during the last 3 days (maintenance) predicts the ratio of DLS:DMS putative D2 activity, but not DLS:DMS D1 neural activity. Neither of these relationships were significant in males, either during C acquisition or D maintenance