Fyn Kinase and NR2B-Containing NMDA Receptors Regulate Acute Ethanol Sensitivity But Not Ethanol Intake or Conditioned Reward

FYN KINASE IS a member of the Src family of protein tyrosine kinases and is expressed in the adult brain (Bare et al., 1993; Zhao et al., 2000). Fyn plays an important role in processes linked to synaptic plasticity via its interactions with the ionotrophic glutamate receptor channel, the N-methyl-d-aspartate receptor (NMDAR). NMDAR-dependent long-term potentiation (LTP) in the CA1 region of the hippocampus is blunted in mice in which the Fyn gene is deleted (Fyn−/−) (Grant et al., 1992); however, LTP was rescued when the wild-type Fyn gene was introduced into the Fyn−/− mice (Kojima et al., 1997). Subsequent studies further suggested that Fyn is important for the induction of LTP (Lu et al., 1998, 1999). NMDAR is a major target for ethanol (Allgaier, 2002; Kumari and Ticku, 2000). It has been shown that deletion of the Fyn gene eliminates the development of acute tolerance to ethanol inhibition in hippocampal slices taken from Fyn−/−mice compared with Fyn+/−littermates, and the duration of the loss of righting reflex (LORR) after ethanol administration was longer for Fyn−/−mice than for Fyn+/− mice (Miyakawa et al., 1997). These findings suggest that the duration of the LORR may depend on the degree of acute tolerance that occurs after ethanol administration. Furthermore, these results suggest that the interaction between Fyn and the NMDAR is important for mediating ethanol-induced behaviors. Among the processes that regulate NMDAR channel function is tyrosine phosphorylation of the long intracellular tail of the NR2B subunit by Src family protein tyrosine kinases, including Fyn (Nakazawa et al., 2001; Yaka et al. 2002, 2003b), and this phosphorylation enhances channel function (Yaka et al., 2002, 2003b). We and others found that acute exposure of slices to ethanol, as well as systemic injection of ethanol to mice, results in increased NR2B phosphorylation in the hippocampus that is mediated by Fyn (Miyakawa et al., 1997; Yaka et al., 2003a). Recently, we found that compartmentalization of Fyn to the NR2B subunit of the NMDAR in the hippocampus regulates the enhanced Fyn phosphorylation of NR2B in response to acute ethanol administration. We also found that the Fyn-dependent increase in NR2B phosphorylation induced by ethanol mediates the acute tolerance to ethanol inhibition in hippocampal slice preparation (Yaka et al., 2003a). Our results suggest that the absence of Fyn-mediated phosphorylation of the NR2B subunit, and hence an absence of acute tolerance to ethanol’s effects at this receptor subtype, mediate the increase in ethanol-induced sedation found in Fyn−/− mice. If this is so, then blockade of acute tolerance at this receptor with the NR2B-selective antagonist, ifenprodil, should alter the LORR in mice expressing the Fyn gene (and hence expressing acute tolerance) but not in mice with no Fyn expression. Therefore, in the present study we tested whether blockade of NR2B-containing NMDARs alters ethanol’s acute effects in the Fyn+/+ and Fyn−/− mice. Previous studies have demonstrated that deletion of specific genes increases acute sensitivity to ethanol and that this increase was associated with a decrease in ethanol consumption (Hodge et al., 1999; Phillips et al., 1998) and vice versa (Thiele et al., 2002). These studies suggest that alteration in acute sensitivity to ethanol can predict ethanol intake. Because Fyn−/− mice are more sensitive to ethanol’s acute effects, we hypothesized that deletion of the Fyn gene will decrease ethanol intake and ethanol’s rewarding properties. Therefore, we compared voluntary ethanol intake and ethanol-induced conditioned place preference (CPP) in Fyn−/− mice and Fyn+/+ littermate controls. We found that inhibition of NR2B-containing NMDARs resulted in an increased sensitivity to the acute sedative effects of ethanol in Fyn+/+ but not in Fyn−/− mice. However, Fyn+/+ and Fyn−/− mice did not differ in their voluntary ethanol consumption and preference or in the conditioned rewarding properties of ethanol.