Use of the opioid analgesic morphine results in profound tolerance and dependence, which may often lead to compulsive use and abuse. While it has been established that exposure to morphine, as with other drugs of abuse, causes modifications in brain circuits, the underlying mechanisms have not been completely elucidated. The broad objectives of this proposal are to investigate the molecular role of mu-opioid receptor (MOR) endocytosis and recycling in morphine dependence. Withdrawal from chronic morphine results in a cyclic adenosine monophosphate (cAMP)-dependent increased probability of gamma-aminobutyric acid (GABA) release in ventral tegmental area (VTA) dopamine neurons, part of the midbrain circuit strongly implicated in drug-induced synaptic plasticity. Based on preliminary data, this increased probability of GABA release after chronic morphine use is connected to its inability to promote substantial MOR endocytosis and recycling. Lack of substantial morphine-induced MOR endocytosis could be due to reduced expression or lack of recruitment of the G-protein coupled receptor kinase2 (GRK2). Therefore, in Specific Aim 1, the role of lentiviral GRK2 over-expression in VTA neurons in modulating morphine endocytosis will be evaluated. Subsequent chronic morphine-induced changes in GABA release and behavioral expression of withdrawal will also be evaluated.
In Specific Aim 2, the basis for adaptations in GABA transmission will be investigated at the cellular level using electrophysiological techniques. Withdrawal-induced changes in pre-synaptic G-protein coupled inwardly rectifying potassium (GIRK) conductance and the role of cAMP in mediating the same will be evaluated. The results from these experiments are likely to yield interesting and novel insights on the induction of synaptic plasticity and the role of MOR endocytosis during development of morphine dependence.
Opiate abuse and dependence has debilitating consequences on the individual and society and is a public health problem. Yet, the reason why opiate drugs, such as morphine and heroin, induce tolerance and dependence is not completely understood. In this proposal, we will identify the cellular substrates governing synaptic transmission and mu-opioid receptor endocytosis and recycling that are modified during chronic morphine use resulting in plasticity in the ventral tegmental area, a brain circuit that governs opiate reward and dependence.
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