STUDIES ON THE EFFECTS OF STIMULANT EXPOSURE DURING ADOLESCENCE ON D2 RECEPTORS AND REINFORCING RESPONSES TO DRUGS DURING ADULTHOOD. ? ? Methylphenidate (MP) and amphetamine, which are the mainstay for treatment of ADHD, have raised concerns that their use during childhood or adolescence could facilitate drug abuse in adulthood. Methods: Here we measured the effects of chronic treatment (8 months) with oral MP (1 or 2 mg/kg), initiated in periadolescent rats (p 30). Following this treatment, rats were tested on cocaine self-administration and we measured dopamine D2 receptor (D2R) availability in striatum using 11Craclopride microPET (PET). Results: Animals treated 8 months with MP showed reduced rates cocaine self-administration at adulthood. D2R availability was significantly lower in rats after 2 months of treatment with MP but significantly higher after 8 months of treatment. In vehicle treated rats D2R decreased with age whereas it increased in rats treated with MP. Conclusions: Chronic oral MP treatment beginning in adolescence decreased cocaine-self administration during adulthood. D2R availability was decreased at 2 months of treatment with MP, which could raise a concern that shorter treatment in adolescence could possibly increase vulnerability to drug abuse during adulthood. ? ? ? STUDIES ON THE INVOLVEMENT OF DOPAMINE D2 RECEPTORS AND TRANSPORTERS ON THE RESPONSES TO COCAINE.? ? 1. Dopamine D2 receptors: Prior studies had shown that D2R upregulation in the nucleus accumbens (NAc) on rodents trained to self-administer alcohol markedly attenuated alcohol intake. Here we assess the effects of D2R upregulation on cocaine intake in rats. Methods: Following 2 weeks of intravenous cocaine self-administration, rats were stereotaxically treated with an adenovirus that carried the D2R gene to upregulate D2R in the NAc. Results: D2R vector treatment resulted in a significant decrease (75 %) in cocaine infusions. This effect lasted 6 days corresponding to the time for D2R to return to baseline levels. Conclusion: These findings suggest that strategies aimed at increasing D2R expression in NAc may be beneficial in treating cocaine addiction.? ? 2. Dopamine transporters: Cocaines ability to block the dopamine transporter (DAT) is crucial for its reinforcing effects but its ability to blocks norepinephrine and serotonin transporters may also be relevant to its pharmacology. To separate the dopaminergic from the non-dopaminergic effects of cocaine on brain function we compared the regional brain glucose metabolic responses to cocaine between DAT deficient (DAT-/-) mice with that of their DAT +/+ littermates. Methods: We measured regional brain metabolism (marker of brain function) with FDG and PET before and after cocaine (10 mg/kg ip). Results: At baseline DAT-/- mice had significantly greater metabolism in thalamus and cerebellum than DAT +/+. Acute cocaine decreased whole brain metabolism and this effect was greater in DAT +/+(15%) than in DAT -/- mice (5%). DAT +/+ mice showed regional decreases in multiple limbic regions whereas DAT -/- mice showed decreases only in thalamus. Conclusion Cocaine-induced decreases in metabolism in thalamus in DAT -/- suggest that these were mediated by cocaines blockade of norepinephrine transporters. The greater baseline metabolism in DAT-/- than DAT+/+ mice in cerebellum (brain region mostly devoid of DAT) suggests that DA indirectly regulates activity of this region.? ? ? STUDIES ON BRAIN MOLECULAR TARGETS IN OBESITY? ? 1. Leptin and cannabinoid receptors (CB1): Improper function of leptin receptors results in overeating and obesity in obese (Ob) Zucker rats. Leptin deficiency results in hyperphagia but the downstream mechanisms underlying overeating are poorly understood. Here we evaluate the role that CB1 receptors in obese Zucker rats. Methods: One month old Zucker rats were divided into unrestricted and restricted (70% of unrestricted rats) groups and maintained until adulthood (4 months). Brain CB1R levels were assessed (in-vitro autoradiography with 3HSR141716A). Results: (1) CB1R levels were significantly higher at 4 months than at 1 month in all animals; (2) at 1 month CB1R levels did not differ between Ob and Lean (Le) animals (except in frontal regions e); (3) at 4 months Ob rats had higher CB1R levels; and (4) diet restriction resulted in higher CB1R levels in Ob but not in Le. Conclusion: The high levels of CB1R in obese rats suggest that leptins inhibition of food-intake is in part mediated by downregulation of CB1R and that leptin interferes with CB1R upregulation under food deprivations conditions. ? ? 2. Leptin and Dopamine D2 receptors: Dopamine (DA) is one of the neurotransmitters that modulates the motivation to eat and leptin may modulate eating behavior in part by its effects on DA. Here we assess the interaction between leptin and DA by monitoring D2R in Zucker Obese rats (deficient in leptin receptor function). Methods: We compared D2R levels between Zucker Obese (Ob) and Lean (Le) rats at one and four months of age with restricted (R) and unrestricted (U) food access using PET and 11C raclopride (radiotracer sensitive to competition with endogenous DA) and with ARG (3H spiperone. Results: D2R were higher at 1 month than at 4 months of age and R animals had higher D2R than U animals. ARG showed that at 1 month and at 4 months Le U had significantly higher D2R than Ob U and that decrease in D2R binding between 1 and 4 months of age was significantly attenuated in R rats obese and lean. In contrast PET showed that at 4 months of age, Ob U showed greater D2R availability than Le U rats. Conclusion: The lower D2R in Ob U than Le U rats observed with ARG most likely reflects decreases in D2R levels whereas the increased availability observed with PET is likely to reflect reduced DA release. Lack of a significant difference between OB R and Le R suggests that the differences in DA between OB and Le are modulated by access to food. ? ? 3. Leptins role in response to conditioning to food stimuli: The mechanisms resulting the enhanced motivation to eat in leptins absence are poorly understood. Here we evaluate the role of leptin on the response to conditioned food stimuli. Methods: We measured brain glucose utilization before and after conditioning to an olfactory stimulus (bacon scent) as a function of food restriction and of leptin genotype (using the Ob rats). Rats were scanned withPET using 2FDG under two conditions: a) preconditioning and b) post conditioning. Results: (1) Conditioning resulted in deactivation of hippocampus. (2) Ob rats showed greater changes with conditioning than Le rats and Ob but not Le animals deactivated the frontal cortex. (3) Access to food resulted in an opposite pattern of changes to the conditioned stimuli in olfactory nucleus (deactivated in U and activated in R) and in right insular/parietal cortex (activated in U and deactivated in R). Conclusion: Leptin-receptor deficiency resulted in an enhanced response to conditioned cues that included deactivation of the frontal cortex. Inasmuch as the frontal cortex enables inhibitory control to intentional action this may be one of the mechanisms that results in compulsive overeating in the Ob rats.
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