Abstract
The role of cannabinoid systems in conditioned fear memory was investigated in streptozotocin (STZ)-induced diabetic mice. The cannabinoid receptor agonist WIN-55,212-2 (1mg/kg, i.p.), when injected into normal mice after conditioning, significantly prolonged the duration of freezing behavior. This effect was significantly inhibited by the cannabinoid CB1 receptor antagonist AM 251 (3mg/kg, s.c.), but not by the cannabinoid CB2 receptor antagonist AM 630 (1mg/kg, s.c.). The duration of freezing in STZ-induced diabetic mice was significantly longer than that in non-diabetic mice. The injection of WIN-55,212-2 (1mg/kg, i.p.) after conditioning significantly prolonged the duration of freezing in non-diabetic mice, but not in STZ-induced diabetic mice. In contrast, the injection of AM 251 (3mg/kg, s.c.) after conditioning significantly shortened the duration of freezing in STZ-induced diabetic mice, but not in non-diabetic mice. The injection of AM 251 (3mg/kg, s.c.) before conditioning or before testing did not significantly affect the duration of freezing in STZ-induced diabetic mice. The protein levels of cannabinoid CB1 receptors in the amygdala were increased in STZ-induced diabetic mice. In contrast, the protein levels of cannabinoid CB2 receptors and diacylglycerol lipase α, the enzyme that synthesizes endocannabinoid 2-arachidonoylglycerol, in the amygdala did not differ between non-diabetic and STZ-induced diabetic mice. None of these proteins in the hippocampus was different between non-diabetic and STZ-induced diabetic mice. The injection of AM 251 (50 ng/side) into the basolateral amygdala significantly inhibited the duration of freezing in STZ-induced diabetic mice. Since endocannabinoid is controlled by glutamatergic function, we further examined the role of glutamatergic function in the increased fear memory in STZ-induced diabetic mice. The amounts of glutamine and glutamic acid in the amygdala of STZ-induced diabetic mice were significantly increased compared to those in non-diabetic mice. The AMPA receptor antagonist NBQX (4 0ng/side), when injected into the basolateral amygdala, significantly inhibited the duration of freezing in STZ-induced diabetic mice. Finally, AMPA (40 ng, i.c.v.) significantly prolonged the duration of freezing in normal mice, and this effect was inhibited by AM 251 (3mg/kg, s.c.). These results suggest that cannabinoid functions in the amygdala are increased in diabetic mice and that enhanced glutamatergic function in the amygdala of diabetic mice activates the endocannabinoid system, which enhances fear memory via cannabinoid CB1 receptors.