ze the progress. Project 1: How does chronic cocaine treatment affect synaptic function and morphology? During this first year we addressed two unresolved questions regarding the actions of cocaine in the nucleus accumbens: a) what is the role of abstinence or withdrawal in the cocaine-induced plasticity, and b) whether cocaine actions in the accumbens are cell-specific or rather affect medium spiny neurons from both pathways (those with direct and indirect projections to substantia nigra). a) While just a few hours after a single cocaine injection are enough to detect changes in the glutamatergic transmission in the ventral tegmental area (Ungless et al., Nature 2001), an increase in transmission in the nucleus accumbens is seen only after repeated cocaine injections and 2 weeks of withdrawal (Kourrich et al., J Neurosci 2007). Similarly, increases in spine density in the accumbens were observed after one month of the last injection (Robinson and Kolb, Eur J Neurosci 1999). These observations together seem to indicate that drug withdrawal is a critical factor in the developing of plastic changes in the accumbens. We tested this hypothesis by comparing synaptic transmission after cocaine treatment with two protocols that differ in the extent of the withdrawal period. We found that prolonged cocaine withdrawal is not required for the cocaine induced plasticity in glutamatergic synapses. Furthermore, the increase in dendritic spine density develops without prolonged withdrawal from cocaine. b) To address whether cocaine induced plasticity affects a subpopulation of medium spiny neurons, we use BAC transgenic mice that express GFP under the D1-dopamine receptor promoter and then label with fluorescence the subpopulation of medium spiny neurons of the striatonigral pathway. We found that chronic cocaine triggers plasticity of glutamatergic synapses onto D1 expressing medium spiny neurons and that morphological changes were also restricted to the neurons of the striatonigral pathway and not present in D2-expressing medium spiny neurons. Project 2: Only a fraction of cocaine users become compulsive drug-takers. This also seems to be the case in animal models of addiction (Deroche-Gamonet et al., 2004 Science 305:1014-1017). What is different in the brain of subjects exposed and addicted in comparison to those exposed but non-addicted? Is there a difference in synaptic function and morphology between mice that display addictive-like behaviors and mice that do not after they all have been chronically exposed to cocaine? In order to answer these questions, we must first be able to identify addicted animals within the population of animals exposed to the drug of abuse. With this purpose, we use a set of behavioral tests to score animals for the development of addictive like behaviors and proceed to study and compare the synaptic properties of animals with high and low addictive scores. Mice are trained to administer cocaine voluntarily by poking the nose into a hole that results in the delivery of a cocaine infusion through a cannula implanted into the jugular vein. After training, mice are allowed to self-administer in daily sessions for 40 days and 3 behavioral characteristics are measured: - Perseverance of drug seeking (active nose pokes during periods of no drug availability) - Motivation for the drug (break point reached during sessions with progressive ratio) - Aversive consequences (perseverance of drug seeking when drug paired to aversive stimulus) Experiments are underway to study the electrophysiological properties of glutamatergic transmission and calcium signaling in dendrites from animals with high addiction scores in comparison to those with a low addiction score. Note that all animals in this study actively self-administered cocaine and so total levels of cocaine intake are not expected to vary much across the population but rather the compulsive nature of the drug seeking behavior. Project 3: What is the effect of voluntary ethanol exposure on neuronal function and morphology in specific brain regions? In collaboration with the laboratory of Dr. Kathleen Grant, we are studying the changes in neuronal and synaptic morphology that are associated with light and heavy ethanol drinking. This study involves non-human primates that undergo 2 years of voluntarily ethanol drinking in a schedule that includes repeated withdrawal. The average daily ethanol consumption was 2.8 g/kg for all the subjects in the study but actually varied from 5.6 to 1.9 g/kg. Blood ethanol levels varied accordingly showing a skewed distribution where a few subjects consumed the vast majority of total ethanol. Upon necropsy, brain tissue was collected from the caudate and putamen of these subjects and processed for diolistic staining and morphological analysis. Confocal images are being acquired and dendrite and spine analysis will be performed to investigate changes in spine density, spine morphology and dendritic branching between light and heavy ethanol drinkers.
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