Abnormalities in brain function contribute to dependence on drugs of abuse. This project aims to identify these abnormalities, and to offer leads in developing new treatment modalities. In a study using positron emission tomography (PET), the presentation of drug-related, but not neutral, visual stimuli induced drug craving and increased glucose metabolism in cortical and limbic areas in cocaine users but not in normal control subjects. Correlations of metabolic increases in dorsolateral prefrontal cortex, medial temporal lobe, and cerebellum with reports of craving suggest that environmental stimuli related to past cocaine use activate components of a neural network, which integrates emotional and cognitive aspects of memory, thereby linking craving to stimuli. Because drug abuse is a chronic disease, characterized by long-term changes in behavior, this project is geared at elucidating persistent abnormalities in brain function induced by drugs of abuse, including opiates and stimulants. In rats, prior drug exposure to cocaine and morphine enhances their rewarding and stimulant effects. Such sensitization, which is related to changes in mesolimbic dopaminergic activity as measured by in vivo microdialysis, increases during the first few days of abstinence, and is blocked by administration of kappa opioid agonists, including dynorphin fragments. Repeated kappa-opioid agonist administration produces long-term changes in dopamine (DA) D1 and D2 receptors, DA uptake, and DA release in the mesolimbic/mesostriatal pathways. Ongoing studies are testing if these changes mediate the antagonistic action of kappa-opioid agonists on the behavioral effects of cocaine and/or morphine. Delta-opioid receptors are also important because delta receptor antagonists block the development of behavioral sensitization to morphine (delta2 receptor- specific) and cocaine, and exacerbate withdrawal symptoms. In vivo methods to evaluate mesolimbic DA neurochemistry in animal models of drug abuse are under development. In the mouse, we are validating the application of microdialysis, and in the primate, we are developing a model to evaluate transient changes in DA neurochemistry using positron emission tomography. Other studies evaluated factors that modulate morphine withdrawal in the rat. During opiate withdrawal the accelerated firing of locus coeruleus (LC) neurons has suggested a functional role of the LC. Using the brain slice model of morphine withdrawal in which the LC lacks neuronal inputs, LC firing rates were not accelerated by naloxone, demonstrating a need for activation of extrinsic pathways. This observation challenges the concept that LC cells initiate excitatory activity in withdrawal, supported by observations that microinjection of methylnaloxonium, a quaternary opioid antagonist, directly into the LC but not in other brain areas of morphine-dependent rats increases cerebral glucose metabolism. The discrepancy between the in vitro and in vivo assays of LC activity indicates the importance of innervation of this brain region in the expression of opiate dependence. Studies with nitric oxide synthase (NOS) inhibitors demonstrated that the nitric oxide produced by constitutive and not inducible NOS modulates opiate withdrawal. 3- Bromo-7-nitroindazole, a selective inhibitor of constitutive NOS, attenuates withdrawal without affecting cardiovascular activity. Together, these studies of long-term drug use have characterized novel opioid and non-opioid mechanisms that modulate animal behavior and offer new perspectives for the pharmacologic treatment of drug abuse.
