CB1 Cannabinoid Receptors Modulate Kinase and Phosphatase Activity During Extinction of Conditioned Fear in Mice

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Abstract
Cannabinoid receptors type 1 (CB1) play a central role in both short-term and long-term extinction of auditory-cued fear memory. The molecular mechanisms underlying this function remain to be clarified. Several studies indicated extracellular signal-regulated kinases (ERKs), the phosphatidylinositol 3-kinase with its downstream effector AKT, and the phosphatase calcineurin as potential molecular substrates of extinction behavior. To test the involvement of these kinase and phosphatase activities in CB1-dependent extinction of conditioned fear behavior, conditioned CB1-deficient mice (CB1-/-) and wild-type littermates (CB1+/+) were sacrificed 30 min after recall of fear memory, and activation of ERKs, AKT, and calcineurin was examined by Western blot analysis in different brain regions. As compared with CB1+/+, the nonreinforced tone presentation 24 h after auditory-cued fear conditioning led to lower levels of phosphorylated ERKs and/or calcineurin in the basolateral amygdala complex, ventromedial prefrontal cortex, dorsal hippocampus, and ventral hippocampus of CB1-/-. In contrast, higher levels of phosphorylated p44 ERK and calcineurin were observed in the central nucleus of the amygdala of CB1-/-. Phosphorylation of AKT was more pronounced in the basolateral amygdala complex and the dorsal hippocampus of CB1-/-. We propose that the endogenous cannabinoid system modulates extinction of aversive memories, at least in part via regulation of the activity of kinases and phosphatases in a brain structure-dependent manner.