Original Report| Volume 12, ISSUE 2, P263-271, February 2011

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Formalin-Induced c-fos Expression in the Brain of Infant Rats

  • Gordon A. Barr
    Address reprint requests to Dr Gordon A. Barr, Department of Anesthesiology and Critical Care Medicine, Children’s Hospital of Philadelphia, 3615 Civic Center Blvd, ARC 716D, Philadelphia, PA 19104.
    Department of Psychology, Biopsychology Doctoral Program, City University of New York, Hunter College, New York, NY
    Department of Developmental Psychobiology, New York State Psychiatric Institute, Columbia University College of Physicians and Surgeons, New York, NY
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Published:December 13, 2010DOI:


      In the fetal, infant, and adult rat, injury induces a well-defined behavioral response and induces c-fos expression in the spinal cord dorsal horn. There is more limited information about the processing of noxious stimulation in the infant brain. We describe here the appearance of the Fos protein in the brain of fetal and infant rats following formalin-induced injury. Regions were chosen for analysis with a special focus on brain loci that express c-fos in the adult. No Fos positive cells were found in the brains of fetuses; newborns did not show increased Fos expression after formalin injection in any structure examined. At 3 and 14 days of age, there was a significant increase in Fos staining induced by formalin in the ventral lateral medulla. In contrast, paraventricular and medial dorsal nuclei of the thalamus, the paraventricular nucleus of the hypothalamus, and periaqueductal gray of the midbrain showed increased levels of Fos protein only at 14 days of age. We hypothesize that this developmental pattern is related not only to the maturation of pain perception but also to development of autonomic and defensive reactions to pain in the infant.


      Because the infant processes pain differently than the adult, knowledge of those differences informs pediatric clinical practice. Using Fos expression as a marker of neural activity in the rat, we show that the pattern of brain activation is immature at birth but is in place by 14 days of age.

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