1. ¹Department of Physiology and Biophysics, Faculty of Medicine, Hotchkiss Brain Institute, University of Calgary, Calgary, AB, Canada
2. ²Department of Biology, Mt St Vincent University, Halifax, NS, Canada

Correspondence: Dr SJ Spencer, Department of Physiology and Biophysics, Faculty of Medicine, Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Dr. N.W., Calgary, AB, Canada T2N 4N1. Tel: 403 220 4497, Fax: 403 283 2700, E-mail: spences@ucalgary.ca

Received 25 August 2005; Accepted 8 November 2005; Published online 4 January 2006.

Abstract

Many aspects of mammalian physiology are functionally immature at birth and continue to develop throughout at least the first few weeks of life. Animals are therefore vulnerable during this time to environmental influences such as stress and challenges to the immune system that may permanently affect adult function. The adult immune system is uniquely sensitive to immune challenges encountered during the neonatal period, but it is unknown where the critical window for this programming lies. We subjected male Sprague–Dawley rats at postnatal day (P)7, P14, P21, and P28 to either a saline or lipopolysaccharide (LPS) injection and examined them in adulthood for differences in responses to a further LPS injection. Adult febrile and cyclooxygenase-2 responses to LPS were attenuated in rats given LPS at P14 and P21, but not in those treated at P7 or P28, while P7-LPS rats displayed lower adult body weights than those treated at other times. P28-LPS rats also tended to display enhanced anxiety in the elevated plus maze. In further experiments, we examined maternal–pup interactions, looking at the mothers' preference in two pup-retrieval tasks, and found no differences in maternal attention to LPS-treated pups. We therefore demonstrate a 'critical window' for the effects of a neonatal immune challenge on adult febrile responses to inflammation and suggest that there are other critical time points during development for the programming of adult physiology. We also show that the neonatal LPS effects on the adult immune system are not likely due to overt differences in maternal attention.