Opiate addiction and neural plasticity: Regulation of neurotrophic intracellular signaling pathways by chronic morphin

Persistent changes in the structure and function of the mesolimbic dopamine system play an important role in mediating the addictive properties of opiates and other drugs of abuse. Given their importance to many forms of neural plasticity, neurotrophic factors (NTFs) and their signaling pathways are likely to be involved in these Persistent changes. The results presented here demonstrate that chronic morphine differentially regulates the protein levels of NTF signaling elements specifically within the ventral tegmental area (VTA). Phospholipase Cgammal (PLCgamma1) is upregulated and insulin receptor substrate (IRS) proteins are downregulated, while NTFs, NTF receptors, and elements of the Ras-Raf-ERK pathway are unchanged. A phospho-specific antibody recognizing activated (PLCgamma1) was generated and used to demonstrate that chronic morphine also increases (PLCgamma1) activity within the VTA, as well as in the nucleus accumbens and hippocampus. Both (PLCgamma1) and IRS proteins control multiple downstream signaling pathways, and their combined regulation may thus have a complex effect on NTF function, increasing the strength of some downstream pathways while decreasing others. Some of these changes may represent a counter-regulatory mechanism, opposing the effects of continuous opiate receptor stimulation.;In order to study the functional consequences of the observed alterations, Herpes Simplex Virus (HSV) vectors were engineered to overexpress PLCgamma1 or dominant negative IRS-2. HSV-mediated overexpression of (PLCgamma1) in PC 12 cells increased ERK activity via a pathway involving protein kinase C and MEK, suggesting that increased (PLCgamma1) signaling in the VTA may represent a novel mechanism contributing to the increase in ERK activity seen following chronic morphine treatment.;These HSV vectors can be targeted to specific brain regions by stereotaxic injection, which will facilitate in vivo studies of the roles (PLCgamma1) and IRS play in NTF signaling and in the biochemical, morphological, electrophysiological, and behavioral changes induced by chronic opiate treatment.