Developmental effects of wheel running on hippocampal glutamate receptor expression in young and mature adult rats

Abstract

Recent evidence suggests that the behavioral benefits associated with voluntary wheel running in rodents may be due to modulation of glutamatergic transmission in the hippocampus, a brain region implicated in learning and memory. However, the expression of the glutamatergic ionotropic N-methyl-d-aspartate receptor (GluN) in the hippocampus in response to chronic sustained voluntary wheel running has not yet been investigated. Further, the developmental effects during young and mature adulthood on wheel running output and GluN expression in hippocampal subregions has not been determined, and therefore is the main focus of this investigation. Eight-week-old and 16-week-old male Wistar rats were housed in home cages with free access to running wheels and running output was monitored for 4weeks. Wheel access was terminated and tissues from the dorsal and ventral hippocampi were processed for Western blot analysis of GluN subunit expression. Young adult runners demonstrated an escalation in running output but this behavior was not evident in mature adult runners. In parallel, young adult runners demonstrated a significant increase in total GluN (1 and 2A) subunit expression in the dorsal hippocampus (DH), and an opposing effect in the ventral hippocampus (VH) compared to age-matched sedentary controls; these changes in total protein expression were not associated with significant alterations in the phosphorylation of the GluN subunits. In contrast, mature adult runners demonstrated a reduction in total GluN2A expression in the DH, without producing alterations in the VH compared to age-matched sedentary controls. In conclusion, differential running activity-mediated modulation of GluN subunit expression in the hippocampal subregions was revealed to be associated with developmental effects on running activity, which may contribute to altered hippocampal synaptic activity and behavioral outcomes in young and mature adult subjects.

Keywords: GluN receptor; aging; dorsal; exercise; hippocampus; ventral.

Figure 1. Weight Gain during the Exercise Period

The weight gained by each animal was monitored during the exercise period (Weight on final day of running – weight on first day of running). Running and age were significant factors in the amount of weight gained by each animal. Asterisk denotes a significant difference between sedentary young adult rats and other groups, hatch denotes a significant difference between young adult and mature adult runners. ***p≤0.001, ****p≤0.0001, ^####p≤0.0001.

Figure 2. Running Activity in Young Adult and Mature Adult Rats

A) Daily running activity measured as number of wheel revolutions per day for young adult and mature adult rats. Asterisk denotes a significant difference between young adult and mature adult animals on a given running day, hatch denotes a significant difference of running activity between a given day of activity and the first day of wheel access. B) Percent change in running activity in young adult and mature adult rats from the first day of running to the last day of running. C) Daily wheel running patterns of young adult rats on day one (filled circle) and day 28 (open circle) of wheel access. Asterisk denotes a significant difference in activity between groups at a given time of day. D) Daily wheel running patterns of mature adult rats on day one (filled square) and day 28 (open square) of wheel access. Asterisk denotes a significant difference in activity between groups at a given time of day. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001, #p<0.05, ##p<0.01, ###p<0.001, ####p<0.0001

Figure 3. GluN Expression in the Dorsal and Ventral Hippocampus of Young Adult and Mature Adult Runners

Dorsal (A) and ventral (B) hippocampus tissue was collected from young and mature adult runners. i) Schematic representations of dorsal (AP −3.14 mm to −4.30 mm from bregma) and ventral hippocampus (AP −5.3 mm to −6.1 mm from bregma) sections adapted from (Paxinos and Watson, 2007). Tissue punches were collected from 500 μm thick sections of young adult and mature adult sedentary and running rats. Blue circles represent site of dorsal tissue collection and pink circles represent site of ventral tissue collection via tissue punch. ii) Representative western blots and associated coomassie staining in sedentary (S) and running (R) animals. iii) Summarized data for GluN subunit expression as percent of sedentary age-match controls. Asterisks denote significant differences between age groups of runners within hippocampal subregion. *p≤0.05, **p≤0.01