Morphine‐induced Changes of Gene Expression in the Brain

Repeated opiate administration alters gene expression in different brain regions of rodents, an effect which may contribute to plastic changes associated with addictive behaviour. There is increasing evidence that multiple transcription factors are induced in morphine tolerance, sensitization and during morphine withdrawal. Whereas morphine treatment does not lead to major alterations in the expression of μ‐opioid receptors (MOR), there is transcriptional regulation of proteins involved in MOR trafficking such as GRK2 or beta arrestin 2 as well as altered expression of other receptors such as dopamine receptors, NMDA receptors, GABA A receptor and alpha 2A adrenoceptor. Recent gene expression profiling studies reveal additional clusters of morphine‐responsive genes: whereas single dose administration has been shown to predominantly reduce expression of genes involved in metabolic function, ascending morphine doses leading to morphine tolerance revealed induction of genes which alter patterns of synaptic connectivity such as arc or ania‐3. These genes remained elevated after precipitated withdrawal, which also triggered the expression of several transcriptional activators and repressors. In addition, morphine has been shown to be a strong inducer of heat shock protein 70, a cell protective protein which might counter‐regulate opiate‐induced neurotoxicity. Temporal expression profiles during a chronic morphine application schedule revealed discrete and fluctuating expression of gene clusters such as transcription factors, G‐protein‐coupled receptors and neuropeptides. Prolonged abstinence seems to be characterized by up‐regulation of several transcription factors and persistent down‐regulation of ligand gated ion channels such as glutamatergic and GABA‐ergic receptor subunits. These long‐term changes in receptor expression suggest a persistent alteration of synaptic signalling after morphine treatment.