Long-term gender behavioral vulnerability after nociceptive neonatal formalin stimulation in rats

The role of sex and gender in accounting for individual pain behaviors is poorly understood. The present study was conducted to determine whether neonatal nociceptive stimuli at postnatal day 1 (PD1) in rats would lead to a differential behavioral impact based on gender. Animals were divided in 4 groups according to treatment (two injections of 4% formalin into the pad of right paws at PD1 or control) and gender. The sensory threshold and cognition tests were performed in adult rats using the hot plate, open field, elevated plus maze and forced swim tests. The number of paw licks was higher in females and in formalin-treated rats (P=0.02), but without interaction between gender and treatment. Exploratory activity was reduced in males (P<0.01), especially in the nociceptive group (P<0.01). Anxiety levels were higher in the female-nociceptive group (P<0.05). Depression-like behavior was more evident among females, independent of treatment. We concluded that a single acute nociceptive stimulation early in development does not affect nociception and depressive behaviors, but is able to alter the exploratory behavior and anxiety levels in adulthood in a gender specific manner.