Long-term changes in adaptive behavior of rats after inflammatory pain stimulation during neonatal development

Abstract

This study is devoted to the investigation of the functional activity of the tonic nociceptive system using the formalin test. We also studied the anxiety level indices, proneness to depression-like behavior and spatial learning in adolescent male rats exposed in the neonatal development to repeated pain stimulation induced by peripheral inflammation. The following groups of 25-day-old rats were used being exposed at the age of 7 and 8 days to: (1) pain stimulation by formalin injection with maternal deprivation for 1 h (FD), (2) same formalin-induced pain stimulation without maternal deprivation (FWD), (3) physiological saline injection with maternal deprivation for 1 h (PD), (4) physiological saline injection without maternal deprivation (PWD) and (5) no stimulation (intact rats). Our findings indicate that pain induced in 7–8-day-old rat pups by formalin injection into the hind leg plantar pad manifests itself at adolescence (25 days) in an inflammatory pain response enhancement under the same pain stimulation as in the formalin test, adaptive behavior disorder in the forced swim test, and spatial learning disability. Also of particular importance is our observation that short-term repeated maternal deprivation of the 7–8-day-old rat pups in the absence of inflammatory pain increases the depression-like behavior proneness index. Thus, the repeated inflammatory pain stimulation during the neonatal development brings about significant changes in the studied types of adaptive behavior as well as in spatial learning in adolescent rats.