The impact of early repeated pain experiences on stress responsiveness and emotionality at maturity in rats Gayle G. Page, a, * Wendy P. Blakely, b and Miyong Kim a a School of Nursing, Johns Hopkins University, Baltimore, MD, USA b College of Nursing, Ohio State University, Columbus, OH, USA Received 17 January 2004; received in revised form 25 March 2004; accepted 17 May 2004 Available online 28 July 2004 Abstract The intensive care necessary for premature newborns is characterized by multiple procedures, many of which are painful. Given emerging evidence that such early pain during this time of high brain plasticity may affect long-term neurodevelopmental and social– emotional functioning, this study explored the impact of early repeated pain on emotionality and stress responsivity at maturity. From birth through postnatal day 7, Fischer 344 pups underwent either paw needle prick every day versus every other day or daily paw touch, or were left unperturbed. Each paw received the designated perturbation once per day. At maturity, some animals underwent emotionality testing: either a 4-day series of open field exposures or a single elevated plus-maze (EPM) exposure. The paw prick groups exhibited less open field habituation and occupied the EPM open arms more. Two weeks later, all animals were either subjected to forced swim or not. At 1 h post-swim, animals underwent either blood withdrawal for plasma corticosterone (CS) levels and ex vivo natural killer cell activity (NKCA) or were injected intravenously with radiolabeled NK-sensitive syngeneic MADB106 tumor cells and assessed for lung tumor retention. Sex was a major factor in the manifestation of perturbation-related differences in the biologic outcomes. Whereas postnatal pain differentially affected baseline tumor retention between males and females, only males exhibited perturbation-related differences in swim stress-induced increases in tumor retention and CS. Finally, male-female differences were evident in CS, NKCA, and tumor responses to swim stress. These findings suggest that early pain affects neurodevelopmental function in the mature organism; however, these relationships are complicated by sex differences, the postnatal pain schedule, and the outcome measured. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Natural killer cell activity; Tumor; Corticosterone; MADB106; Fischer 344; Neonatal 1. Introduction Premature and critically ill newborns are among the most vulnerable of populations, requiring intensive care characterized by multiple procedures, many of which are painful (Johnston et al., 1997; Porter et al., 1999; Ste- vens et al., 2003). Pain at this very young age is a pro- found physiologic stressor (e.g., Anand, 2000; Goldman and Koren, 2002), and emerging evidence suggests such repeated early pain experiences in premature newborns at this time of high plasticity in the developing brain may affect long-term neurodevelopmental and social– emotional functioning (Bhutta et al., 2002; Grunau, 2002). By postnatal day (PND) 10, the somatosensory function of the rat approximates that of the full term human infant (Fitzgerald, 1995; Fitzgerald et al., 1988) and pain experiences through the first postnatal week have been suggested to correspond to those experienced by extremely low birthweight infants during their stay in the neonatal intensive care unit (Johnston et al., 2002; Lidow, 2002). Although animal models can only ap- proach in a very limited way the human experience, this study sought to explore in the rat the effects of postnatal repeated pain experiences during the first week of life on emotionality and stress responsiveness in the mature organism. Emotionality was assessed as elevated plus- maze (EPM) behavior and habituation to an open field. Stress responsivity was assessed as changes in plasma * Corresponding author. Fax: 1-410-955-7463. E-mail address: gpage@son.jhmi.edu (G.G. Page). 0889-1591/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bbi.2004.05.002 Brain, Behavior, and Immunity 19 (2005) 78–87 BRAIN, BEHAVIOR, and IMMUNITY www.elsevier.com/locate/ybrbi