Resposta do sistema serotonérgico central ao treinamento físico em ratos alimentados com dieta de alta palatabilidade

Abstract

The present study aimed to determine the effect of physical training and chronic administration of L-tryptophan (TRP) intraperitoneal (ip.) on the central serotonergic system and energy balance in rats fed a high palatability diet. 56 male Wistar rats weighing between 60 and 80 grams (weaning) were used in this study. Rats were kept in individual cages in a room with controlled light and temperature with free access to water and chow for 17 weeks. Food intake and body weight of rats were measured every two days throughout the experiment. Initially, the rats were divided into two groups: standard diet (DP) or a high palatability diet (DAP). After the treadmill running familiarization, rats were submitted to a progressive exercise test until voluntary interruption of effort (TP) to establish the intensity of physical training. Then, the two groups were divided in untrained (DP-NT and DAP-NT) and trained (DP-T and DAP-T) rats to the beginning of physical training which have duration for eight weeks. At the end of 4th week of training, rats were submitted to a 2nd TP to verify the performance before the chronic administration of TRP or saline (SAL) ip. From the 5th week of training, rats were again divided in two groups: that had received three weekly injections of TRP or SAL ip. by the end of physical training 48 h after the last training session, rats were subjected to a 3rd TP to determine maximal physical performance attained. 48 h after 3rd TP, rats were euthanized; blood was collected and tissues were removed. During the nine weeks prior to the beginning of the training, food intake, body mass and energy efficiency were 40, 10 and 56% higher in DAP compared to DP. TRP intraperitoneal administration did not alter any of the variables measured in this study. As expected, physical training increased the performance (DP-NT: 22.9 ± 1.4 vs. DP-T: 54.3 ± 2.6 kgm and DAP-NT: 20.1 ± 1.8 vs. DAP-T: 49.6 ± 4.3 kgm). Moreover, the training reduced food intake and body weight of rats, independently of diet ingested (DP-NT: 0.09 ± 0.6 vs. DP-T: -2.2 ± 0.4 g.day-1, p = 0.007 and DAP-NT: 1.3 ± 0.6 vs. DAP-T: -2.6 ± 0.6 g.day-1, p <0.001). However, no difference was observed in adipose tissue mass between groups. In addition, the training did not prevent the increase of the plasma concentration of leptin (DAP-NT: 29.9 ± 6.0 vs. DAP-T: 25.7 ± 6.2 ng.mL-1) and insulin (NT-DAP: 5.0 ± 0.6 vs. DAP-T: 4.8 ± 0.5 ng.mL-1) induced by high palatability diet. 5- HT and 5-HIAA concentrations and the ratio 5-HIAA/5-HT into the hippocampus or hypothalamus were not different between groups. Nevertheless, the ratio of 5-HIAA/5- HT into the hypothalamus was positively correlated with food intake (r = 0.76, p = 0.007) and body mass (r = 0.64, p = 0.017) in DAP-NT. Moreover, the ratio of 5-HIAA/5-HT was correlated with body mass only in the DAP-T (r = 0.65, p = 0.020). There was also a direct correlation between the ratio 5-HIAA/5-HT into the hippocampus and food intake in DAP-NT (r = 0.61, p = 0.027) and DAP-T (r = 0.68; p = 0.016). And body weight was positive correlated with the ratio 5-HIAA/5-HT into the hippocampus in trained groups, independent of diet ingested (DP-T: r = 0.67, p = 0.017 and DAP-T: r = 0.80 p = 0.002). We also found that training had prevented the increased in ratio 5- HT/DA induced by high palatability diet (DAP-NT: 2.22 ± 0.22 vs. DAP-T: 1.47 ± 0.10). From these results, we can conclude that physical training modified the energy balance, independent of diet ingested. This response was attained without modifying the serotonergic system in resting rats. However, it is likely that the training can interfere with the serotonergic transmission due to abolishment of the increase in the ratio 5- HT/DA induced by high palatability diet.