Nociceptive stimuli enhance anesthetic-induced neuroapoptosis in the rat developing brain

Anesthetic-induced neurodegeneration in the developing brain has been well documented. However, the experiments carried out so far do not include surgical conditions. This proof of concept study was designed to investigate the impact of nociceptive stimuli on anesthetic induced neuroapoptosis in the rat developing brain. Separate cohorts of 7-day-old Sprague-Dawley rat pups were randomly assigned to six groups: Naïve (room air); Anesthesia alone (70% nitrous oxide and 0.75% isoflurane for 6 h); Formalin injection alone (subcutaneous injection with 10 microL 5% formalin into the left hind paw); Anesthesia+formalin injection; Surgical incision (to the left hind paw) alone; Anesthesia+surgical incision. Apoptosis (Caspase-3) and neuronal activation (c-Fos) in the brain and spinal cord section, and cortical TNF-α and IL-1β were measured with in situ immunostaining and western blot respectively. Cognition was tested using Trace Fear conditioning 40 days after the insult. Prolonged anesthesia caused widespread apoptosis in the central nervous system compared to naïve animals. Nociceptive stimulation with formalin (F) or surgical incision (S) increased the injury in the brain cortex (F: 60% or S: 40% increase) and spinal cord (F: 80% vs. S: 50% increase) respectively. Both nociceptive stimuli further augmented cognitive impairment induced by the anesthetics when assessed 40 days later. The activated pain pathway and the increased expression of the pro-inflammatory cytokine, IL-1β, in the cortex may be responsible for the enhanced neuroapoptosis. Nociceptive stimulation and prolonged anesthesia produced significantly more apoptosis than prolonged anesthesia alone when administered to neonates during the synaptogenic period.