Postnatal NGF administration causes adult hyperalgesia and overreactivity to social stimuli but does not reverse capsaicin induced hypoalgesia

The present longitudinal analysis was aimed at assessing (i) the effects of developmental capsaicin (CAPS) administration on nociceptive responsivity and on the response of adult mice to social stimuli; (ii) the action of NGF on the ontogeny of the same nociceptive response and social stimuli; (iii) whether capsaicine treatment could be reversed by subsequent treatment with NGF. CD-1 mouse pups were treated with either capsaicin (50 mg/kg, sc) or vehicle on postnatal days (PNDs) 5 and 8. Every other day from PND 9 to PND 21 the same pups received a daily injection of NGF (0.75 mg/kg, sc). During both the prepuberal stage (PNDs 14, 21, and 28) and adulthood, mice were repeatedly tested in a hot-plate apparatus (52±0.1°C for 1 min). At adulthood they also underwent an aggressive behaviour test. NGF-treated mice showed a shorter latency to hindlimb licking response in the hot plate compared to both controls and NGF-CAPS groups. CAPS-treated subjects showed a long-lasting hypoalgesia at both prepuberal and adult stages that was not modified by subsequent NGF treatment. Finally, NGF-treated mice were more aggressive than both controls and CAPS-NGF animals.