Thermal pain sensitivity in late terms of prolonged social isolation in rats

Objective. To analyze pain sensitivity in hot-plate test in male and female rats undergoing social isolation for a year. Material and methods. The study was performed on 69 Wistar rats and approved by local ethics committee. Rats were separated into four groups after being weaned from their mothers on the 29th postnatal day: socially isolated females (n=17) and males (n=18) who were housed separately throughout the entire experiment; control females (n=17) and males (n=17) who were housed in cages with four to five animals each. Pain thresholds were assessed in hot-plate test at 55±0.3°C by measuring the latency period of escape reactions of licking the forepaw, hind paw, and jumping. Half of the rats were tested at the age of 10.5 months, all rats — at the age of 12.5 months. Statistical analysis was performed using Factorial and Repeated Measures ANOVA. Results. Rats kept in social isolation had higher pain thresholds than rats kept in groups as seen by increased latency of licking the forepaw (F(1.62)=11.517, p=0.001) and hind paw (F(1.60)=5.005, p=0.029). Pain thresholds for licking the hind paw were higher in males of the control group and socially isolated females than in females of the control group. In rats of both sexes, pain thresholds for jumping and hind paw licking dropped with age independent of housing conditions. Conclusion. Higher pain thresholds in rats at late terms of social isolation indicate hypoalgesia possibly associated with processes of adaptation to long-term psychosocial stress. Compared to males, females showed more noticeable decrease in thermal pain sensitivity.