Role of AMPA receptors in the nucleus accumbens shell in morphine depedence

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DESCRIPTION (provided by applicant): The purpose of this R03 proposal is to investigate how synaptic plasticity of AMPA glutamate receptors in the nucleus accumbens shell (NASh) mediates morphine dependence. Chronic administration of opiates such as morphine can lead to dependence, characterized by physical (short-lasting) and affective (long-lasting) withdrawal signs. Alleviation of these aversive states associated with drug withdrawal is considered a major motivation for relapse to drug taking (Koob and Le Moal, 1997). AMPA glutamate receptors are responsible for the majority of fast excitatory synaptic transmission in the brain. They are heteromeric tetramers composed of GluR1-4 subunits that confer distinct physiological and functional properties to the receptor (Palmer et al., 2005). Prior work has demonstrated that elevating GluR1 levels in the NASh decreases, and GluR2 increases, the rewarding effects of cocaine and lateral hypothalamic brain stimulation (Kelz et al., 1999; Todtenkopf et al., submitted), suggesting that AMPA receptor subunits can impact motivational state. In morphine-dependent rats, the opiate antagonist naloxone elicits withdrawal signs (Schulteis et al., 1994; Chartoff et al., 2006) and increases extracellular glutamate levels in the nucleus accumbens (Sepulveda et al., 1998), suggesting that enhanced AMPA transmission might contribute to withdrawal signs. Consistent with this, systemic administration of an AMPA receptor antagonist attenuates somatic withdrawal signs (Rasmussen et al., 1996). Furthermore, we found that naloxone increases the phosphorylation of GluR1 in morphine-dependent rats. GluR1 phosphorylation can increase AMPA currents and facilitate synaptic insertion of receptor subunits (Malinow and Malenka, 2002). Based on these observations, we hypothesize that activation of AMPARs and modulation of GluR subunit function contribute to morphine withdrawal signs. We will test our hypothesis using two independent (but complementary) approaches. First we will microinject AMPA or the AMPA receptor antagonist NBQX into the NASh and measure the effects on naloxone-induced conditioned place aversions and somatic withdrawal signs in morphine-dependent rats. Second, we will use a protein cross-linking assay to determine how morphine dependence and withdrawal regulate the cell surface versus intracellular trafficking of GluR1 and GluR2 AMPAR subunits. Data from these studies will guide the formulation of future grant applications in which we will propose to use viral mediated gene transfer to determine the effects of manipulating either GluR1 or GluR2 subunit levels in the NASh on morphine dependence as well as determine molecular and cellular mechanisms underlying morphine-induced synaptic plasticity. The proposed studies will contribute to our understanding of the neurobiological mechanisms underlying aversive states associated with withdrawal from chronic morphine, which is important because the desire to alleviate withdrawal symptoms is a major factor in relapse to drug taking. Our goal is to determine whether synaptic plasticity of AMPA glutamate receptor function in the nucleus accumbens shell-a brain region critical for regulating motivational states-contributes to morphine dependence and hence aversive signs of morphine withdrawal. Results from these studies might identify brain region-specific functions of AMPA receptors that can be targeted in future treatments for addiction.