c-Fos levels in Lewis and Fischer rat strains following a dose of morphine or cocaine shown to differentially induce conditioned taste aversions

Lewis (LEW) and Fischer (F344) rats differ in a variety of measures. For example, LEW rats are more sensitive to drug reward, as indicated by behavioral (Martin et al., 1999) and molecular assays (Selim & Bradberry, 1996). These strains are also differentially sensitive to drugs within conditioned taste aversion (CTA) learning, with LEW rats showing stronger cocaine- (Glowa, Shaw & Riley, 1994) and F344 rats demonstrating greater morphine- (Lancellotti, Bayer, Glowa, Houghtling & Riley, 2001) induced CTAs. Given that molecular differences between LEW and F344 rats parallel their differences in drug reward, Experiment 1 of the present study examined whether molecular differences between these strains may be related to differential CTA learning. LEW and F344 rats were injected with morphine, cocaine (32 mg/kg each) or their vehicles and tested for c-Fos expression in CTA-associated brain regions. Because these strains demonstrate differential reward (see above) and locomotion (Lipska & Weinberger, 1995) responsivities, Fos within the nucleus accumbens (NAc) and caudate putamen (CPu) was also assessed. The present results generally indicated that Fos in CTA-associated regions was greater for F344 rats following morphine and for LEW rats following cocaine, which paralleled the differences within CTA learning (see above). Fos levels within the NAc were greater in LEW rats following either drug (compared to saline levels), consistent with studies indicating that LEW (compared to F344) rats find morphine and cocaine more rewarding. In contrast, no strain differences were found in Fos levels within the CPu, indicating that differences in dopaminergic function were specific to the NAc. Because experiments have shown differential neurohormone responsivities in LEW and F344 rats (Simar, Sapier & Goeders, 1996), Experiment 2 examined corticosterone levels in animals from Experiment 1 to determine whether differential Fos levels were related to different stress responsivities to saline, morphine or cocaine. The present experiment indicated that differences in stress responsivities may have played a role in differential Fos levels following morphine, but not saline or cocaine.