Lewis and Fischer rat strains display differences in biochemical, electrophysiological and behavioral parameters: studies in the nucleus accumbens and locus coeruleus of drug naive and morphine-treated animals

Abstract

In previous studies, we demonstrated that tyrosine hydroxylase and neurofilament proteins are regulated by chronic morphine and chronic cocaine treatments in the ventral tegmental area in Sprague-Dawley rats and that the inbred Lewis and Fischer 344 rat strains, under drug-naive conditions, show different levels of these proteins specifically in this brain region. In the current study, we compared Lewis and Fischer rats with respect to levels of adenylate cyclase, cyclic AMP-dependent protein kinase and G-proteins in the nucleus accumbens (NAc) and locus coeruleus (LC), brain regions in Sprague-Dawley rats where these proteins are regulated by chronic exposure to morphine or to cocaine. We found that levels of adenylate cyclase and cyclic AMP-dependent protein kinase activity are higher in the NAc and LC of Lewis rats compared to Fischer rats, whereas levels of Gi alpha and G beta were lower. These strain differences were not seen in several other brain regions analyzed and no strain differences were detected in levels of other G-protein subunits. Lewis and Fischer rats also differed in the ability of chronic morphine to regulate adenylate cyclase and cyclic AMP-dependent protein kinase in the NAc and LC. In the NAc, chronic morphine increased levels of the two enzymes in the Fischer strain only, whereas in the LC chronic morphine increased levels of the enzymes in both strains, with more robust effects seen in the Lewis rat. To understand possible physiological consequences of these strain differences in the cyclic AMP pathway, we studied LC neuronal activity under basal and chronic morphine-treated conditions. LC neurons of Lewis rats showed higher spontaneous firing rates in brain slices in vitro than those of Fischer rats and also showed greater morphine-induced increases in responsiveness to bath-applied 8-bromo-cyclic AMP. These electrophysiological findings are generally consistent with the biochemical observations. Moreover, Lewis and Fischer rats displayed very different opiate withdrawal syndromes, with different types of behaviors elicited upon precipitation of opiate withdrawal with the opiate receptor antagonist, naltrexone. The possible relationship between these behavioral findings and the biochemical and electrophysiological data is discussed. These studies provide further support for the possibility that Lewis and Fischer rat strains provide a useful model system in which some of the genetic factors that contribute to drug-related behaviors can be investigated.