Psychosocial stress plays a crucial role in the etiology of substance abuse, but the neurobiological bases of this phenomenon are still unknown. Previous evidence has shown that the two best-characterized endocannabinoids (eCB) - anandamide and 2-arachidonoylglycerol (2-AG) - and their receptors serve key functions in the modulation of stress-coping behaviors. The goal of the present application is to examine whether changes in eCB signaling contribute to the behavioral alterations induced by social isolation in rats. Rats reared in isolation since weaning develop a set of behavioral abnormalities, including hyperactivity, mood and sensorimotor gating deficits, as well as increased responsiveness to psychostimulants. Based on preliminary results obtained in this model, we hypothesize that post-weaning isolation rearing causes in rats permanent deficits in brain eCB signaling, which contribute to the behavioral abnormalities observed in adulthood. We have three specific aims that are relevant to a test of this hypothesis:
Aim 1 is to characterize the effects of isolation rearing on brain eCB signaling. In initial experiments, we found that anandamide levels (AEA) are reduced in the prefrontal cortex and striatum of isolation-reared rats compared to group-reared controls, suggesting that isolation rearing may cause permanent region-specific deficits in brain anandamide signaling. We will use a multidisciplinary approach to identify the mechanisms underlying these deficits and determine whether they are associated with changes in the expression of brain cannabinoid receptors.
Aim 2 is to determine the role of eCB signaling in the behavioral disturbances induced by isolation rearing. Isolation rearing disrupts adult behavior in rats, enhancing reactions to novel stimuli and reducing the ability to cope with stress. Based on our initial studies, we hypothesize that these disturbances may be due to a deficit in brain anandamide signaling. We will examine whether URB597, a selective fatty- acid amide hydrolase (FAAH) inhibitor, corrects isolation-induced behavioral abnormalities in three tests: open field, elevated plus maze, and startle reflex and prepulse inhibition of this reflex.
Aim 3 is to determine the role of eCB signaling in the sensitization to d-amphetamine induced by isolation rearing. Isolation-reared rats are exquisitely sensitive to the effects of d-amphetamine and other psychostimulants. Preliminary results indicate that d-amphetamine stimulates anandamide mobilization in the striatum of group- reared rats, suggesting that defective anandamide signaling may contribute to the hypersensitivity to d- amphetamine induced by isolation rearing. We will examine whether (i) administration of d-amphetamine differentially affects anandamide mobilization in the brain of group- and isolation-reared rats; and (ii) agents that either increase anandamide levels, such as the FAAH inhibitor URB597, or block cannabinoid receptors, such as rimonabant, influence d-amphetamine hypersensitivity in isolation-reared rats. ? ? ?
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