, 2004). These mice were also bred to carry a Gt(ROSA)26Sor-YFP (R26-YFP) reporter cassette in which Cre-mediated ALK inhibitor recombination of a transcriptional STOP promotes YFP expression as a marker of Cre activity ( Figure 1B). p38α-ir was absent in AAV1-CreGFP transduced cells that coexpressed the YFP reporter ( Figure 1C). In contrast, injection of AAV1-CreΔGFP vector expressing an inactive, mutated form of the Cre-recombinase (CreΔ) did not affect p38α MAPK expression in DRN ( Figure 1C). Prior reports established that stress causes relapse to drug seeking (Nestler and Hyman, 2010 and Krishnan et al., 2007), and

in particular, social defeat stress (SDS) represents an ethologically relevant stressor for evoking R428 research buy dysphoria-like behavioral

states (Miczek et al., 2008). The Mapk14lox/lox mice were injected in the DRN with AAV1-CreGFP to determine whether p38α MAPK was required for SDS induced reinstatement. We followed this injection with a conditioning paradigm for cocaine place preference ( Figure 1D). Both AAV1-CreGFP and AAV1-CreΔGFP injected mice developed normal place preference to cocaine ( Figure 1E), suggesting that deletion of p38α in DRN cells does not disrupt associative learning components required for initial acquisition of cocaine place preference. We then extinguished the conditioned preference by substituting saline for cocaine in the drug-paired chamber ( Figure 1D). After mice met extinction criteria (≤15% of their initial preference score; Figure 1E), mice were exposed to social defeat stress (20 min session) and then place preference was reassessed. Importantly, AAV1-CreGFP-induced PAK6 deletion of p38α in the DRN completely blocked SDS-induced reinstatement of cocaine CPP, whereas floxed p38α mice injected with the virus expressing the inactive form of Cre recombinase still showed robust SDS-induced reinstatement of cocaine CPP ( Figure 1E). These data suggest that expression of p38α in the DRN is required for stress-induced reinstatement of reward seeking behavior. To expand on this concept and to parallel

other studies showing that stress negatively modulates reward to initiate the drive for reward seeking (Koob, 2008), we injected Mapk14lox/lox (floxed p38α) mice with either AAV1-CreGFP or AAV1-CreΔGFP in either the DRN or nucleus accumbens (NAc), and then assessed conditioned avoidance of a context paired with an aversive stimulus. Since KOR activation results from stress and is known to produce aversive behavioral responses in stress-paired contexts ( Land et al., 2008, Land et al., 2009, Bruchas et al., 2010, Carlezon et al., 1998 and Shippenberg et al., 1986) we conditioned mice with the KOR agonist U50,488 (2.5 mg/kg, i.p.) over 2 days and then assessed their avoidance of the drug-paired context.