5-HT2C receptors in the basolateral amygdala and dorsal striatum are a novel target for the anxiolytic and antidepressant effects of exercise

Abstract

Physical activity reduces the incidence and severity of psychiatric disorders such as anxiety and depression. Similarly, voluntary wheel running produces anxiolytic- and antidepressant-like effects in rodent models. The specific neurobiological mechanisms underlying the beneficial properties of exercise, however, remain unclear. One relevant pharmacological target in the treatment of psychiatric disorders is the 5-HT(2C) receptor (5-HT(2C)R). Consistent with data demonstrating the anxiogenic consequences of 5-HT(2C)R activation in humans and rodents, we have previously reported that site-specific administration of the selective 5-HT(2C)R agonist CP-809101 in the lateral/basolateral amygdala (BLA) increases shock-elicited fear while administration of CP-809101 in the dorsal striatum (DS) interferes with shuttle box escape learning. These findings suggest that activation of 5-HT(2C)R in discrete brain regions contributes to specific anxiety- and depression-like behaviors and may indicate potential brain sites involved in the anxiolytic and antidepressant effects of exercise. The current studies tested the hypothesis that voluntary wheel running reduces the behavioral consequences of 5-HT(2C)R activation in the BLA and DS, specifically enhanced shock-elicited fear and interference with shuttle box escape learning. After 6 weeks of voluntary wheel running or sedentary conditions, the selective 5-HT(2C)R agonist CP-809101 was microinjected into either the BLA or the DS of adult Fischer 344 rats, and shock-elicited fear and shuttle box escape learning was assessed. Additionally, in-situ hybridization was used to determine if 6 weeks of voluntary exercise changed levels of 5-HT(2C)R mRNA. We found that voluntary wheel running reduced the behavioral effects of CP-809101 and reduced levels of 5-HT(2C)R mRNA in both the BLA and the DS. The current data indicate that expression of 5-HT(2C)R mRNA in discrete brain sites is sensitive to physical activity status of the organism, and implicates the 5-HT(2C)R as a target for the beneficial effects of physical activity on mental health.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Intra-amygdala experimental design, body weights and running distance.

(A) Experimental timeline. Adult, male Fischer 344 rats were allowed voluntary access to running wheels for 6 weeks (Run) or remained sedentary. All rats had cannulae implanted into the region of the lateral/basolateral amygdala (BLA) between weeks 3 and 4. Three weeks later, rats were injected with either saline or increasing doses of the selective 5-HT2C receptor agonist CP-809101 (0.3 mM, 2.0 mM, or 6.0 mM) through the guide cannulae. Rats were placed into shuttle boxes 15 minutes following intra-DS injections and shock-elicited freezing and escape learning were tested sequentially. (B) Mean weekly body weight (grams) of physically active and sedentary rats. (C) The daily distance run (meters) pre- and post- cannula implantation surgery. Data represent means ± SEM.

Figure 2. Six weeks of voluntary wheel running reduces the increase in fear behavior produced by 5-HT2C receptor activation in the region of the lateral/basolateral amygdala (BLA).

Following 6 weeks of voluntary wheel running (Run) or no running (Sedentary), rats received intra-BLA microinjections of either saline or the selective 5-HT2C receptor agonist CP-809101 (0.3 mM, 2.0 mM, or 6.0 mM). Shockelicited freezing and shuttle box escape latency were measured sequentially in shuttle boxes 15 minutes later. (A) Cannula placement within the amygdala. Sedenatry rats are denoted with black triangles, physically active rats are denoted with gray triangles, and off-site placements are denoted with an X. Brain illustrations adapted from Paxinos and Watson (published in the Rat Brain in Stereotaxic Coordinates, 4th ed., Copyright Elsevier (1998)). Numbers left of illustrations refer to distance from Bregma (mm). (B) Mean freezing behavior presented in 2 minute blocks (pre-FR-1 scores are not different and therefore overlap). Error bars are ommited for clarity. (C) The mean percent shock-elicited freezing for the entire 20 minute observation period. (D) Shuttle box escape latencies for one block of 2 FR-1 trials (FR-1) and five blocks of 5 FR-2 trials (FR-2). Error bars are omitted for clarity. (E) The mean escape latency for all 25 FR-2 escape trials. Data represent group means ± SEM. * p<0.05 relative to Offsite Control, Sedentary/Saline, Sedentary/0.3 mM, Run/2.0 mM, and Run/6.0 mM groups. Φ p<0.05 relative to the Sedentary/2.0 mM group. θ p<0.05 relative to the Run/2.0 mM group.

Figure 3. Intra-striatum experimental design, body weights and running distance.

(A) Experimental timeline. Adult, male Fischer 344 rats were allowed voluntary access to running wheels for 6 weeks (Run) or remained sedentary. All rats had cannulae implanted into the region of the dorsal striatum (DS) between weeks 3 and 4. Three weeks later, rats were injected with the selective 5-HT2C receptor agonist CP-809101 (0.3 mM, 2.0 mM, or 6.0 mM) through the guide cannulae. Rats were placed into shuttle boxes 15 minutes following intra-DS injections and shock-elicited freezing and escape learning were tested sequentially. (B) Mean weekly body weight (grams) of physically active and sedentary rats. (C) The daily distance run (meters) pre- and post- cannula implantation surgery. Data represent group means ± SEM.

Figure 4. Six weeks of voluntary wheel running reduces the deficit in instrumental escape learning produced by 5-HT2C receptor activation in the dorsal striatum.

Fifteen minutes prior to behavioral testing, sedentary and physically active (Run) rats received intra-DS microinjections of the 5-HT2C receptor agonist CP-809101 (0.3 mM, 2.0 mM or 6.0 mM). (A) Cannula tip placement within the DS. Sedentary rats are denoted with black triangles, physically active rats are denoted with gray triangles, and off-site placements are denoted with an X. Brain illustrations adapted from Paxinos and Watson (published in the Rat Brain in Stereotaxic Coordinates, 4th ed., Copyright Elsevier (1998)). Numbers left of illustrations refer to distance from Bregma (mm). (B) Freezing behavior over the duration of the post-FR-1 freezing observation period presented in 2 minute blocks (pre-shock scores are not different and therefore overlap). (C) The mean percent time spent freezing during the 20 minute observation period. (D) Shuttle box escape latencies for one block of 2 FR-1 trials (FR-1) and five blocks of 5 FR-2 trials (FR-2). (E) The mean escape latency for all 25 FR-2 escape trials. Data represent group means ± SEM. * p<0.05 relative to 0.3 mM groups and off-site control group; Φ p<0.05 relative to 2.0 mM sedentary group.

Figure 5. Six weeks of voluntary wheel running decreases 5-HT2C mRNA expression in the amygdala.

(A) The region of the amygdala as shown by Paxinos and Watson (published in the Rat Brain in Stereotaxic Coordinates, 4th ed., Copyright Elsevier (1998)). (B) Representative autoradiographic coronal section through the region of the striatum (Bregma – 3.14 mm) in a sedentary rat processed with in situ hybridization for 5-HT2CR messenger ribonucleic acid (mRNA). (C) Representative autoradiographic coronal section through the region of the amygdala (Bregma – 3.14 mm) in a physically active rat (Run) processed with in situ hybridization for 5-HT2CR mRNA. (D) Relative levels of 5-HT2C receptor (mRNA) in the lateral amygdala (LA), basolateral amygdala (BLA), central amygdala (CeA), and lateral ventricle (LV) of sedentary rats or rats allowed voluntary access to running wheels for 6 weeks (Run). Values represent mean integrated density ± SEM. * p≤0.05 relative to respective sedentary groups.

Figure 6. Six weeks of voluntary wheel running decreases 5-HT2C mRNA expression in the medial dorsal striatum.

(A) The region of the striatum as shown by Paxinos and Watson (published in the Rat Brain in Stereotaxic Coordinates, 4th ed., Copyright Elsevier (1998). (B) Representative autoradiographic coronal section through the region of the striatum (Bregma 1.70 mm) in a sedentary rat processed with in situ hybridization for 5-HT2CR messenger ribonucleic acid (mRNA). (C) Representative autoradiographic coronal section through the region of the striatum (Bregma 1.70 mm) in a physically active rat (Run) processed with in situ hybridization for 5-HT2CR mRNA. (D) Relative levels of 5-HT2C receptor (mRNA) in the medial and lateral dorsal striatum (DS) of sedentary rats or rats allowed voluntary access to running wheels for 6 weeks (Run). Values represent mean integrated density ± SEM. * p≤0.05 relative to respective sedentary group.