Neonatal nociception elevated baseline blood pressure and attenuated cardiovascular responsiveness to noxious stress in adult rats.

Neonatal nociception has significant long-term effects on sensory perception in adult animals. Although neonatal adverse experience affect future responsiveness to stressors is documented, little is known about the involvement of early nociceptive experiences in the susceptibility to subsequent nociceptive stress exposure during adulthood. The aim of this study is to explore the developmental change in cardiovascular regulating activity in adult rats that had been subjected to neonatal nociceptive insults. To address this question, we treated neonatal rats with an intraplantar injection of saline (control) or carrageenan at postnatal day 1. The carrageenan-treated rats exhibited generalized hypoalgesia at basal state, and localized hyperalgesia after re-nociceptive challenge induced by intraplantar injections of complete Freund's adjuvant (CFA) as adults. Then we recorded baseline cardiovascular variables and 24-h responsiveness to an injection of CFA in the free-moving adult rats with telemetric technique. The carrageenan-treated rats showed significantly higher basal blood pressures (110.3±3.16 vs. control 97.0±4.28 mmHg). In control animals, baroreceptor reflex sensitivity (BRS) decreased, sympathetic vasomotor activity increased, and parasympathetic activity was inhibited after CFA injection. Blood pressure elevation was evident (107.0±2.75 vs. pre-injection 97.0±4.28 mmHg). Comparatively, the carrageenan-treated rats showed a higher BRS (BrrLF 1.03±0.09 vs. control 0.70±0.06 ms/mmHg) and higher parasympathetic activity [0.93±0.17 vs. control 0.32±0.02 ln(ms²)] after CFA injection. The change in blood pressure is negligible (111.9±4.05 vs. pre-injection 110.3±3.16 mmHg). Our research has shown that neonatal nociception alters future pain sensation, raises basal blood pressure level, and attenuates cardiovascular responsiveness to nociceptive stress in adult rats.