Analysis of mRNA Expression Using Double In Situ Hybridization Labeling with Isotopic and Nonisotopic Probes

A large number of in vivo studies in the last decade have confirmed that abused drugs are able to up-regulate gene expression in striatal neurons. For example, acute or chronic exposures to psychostimulants, cocaine and amphetamine, increased basal levels of mRNA and protein products of immediate early genes (c-fos , zif/268 , ΔFosB , etc.) in the rodent striatum (1 –3 ). Similarly, acute administration of cocaine and amphetamines induced robust mRNA expression of preprodynorphin (PPD) and substance P (SP) in striatonigral projection neurons and preproenkaphalin (PPE) in striatopallidal projection neurons (4 –6 ). The increased immediate early gene and neuropeptide expression is mediated via selective activation of dopamine receptors (primary D₁ subtype) (7 ,8 ). Glutamatergic transmission also significantly contributes to the genomic effect of stimulants (9 ,10 ). The altered gene expression has been considered to be an essential molecular step for the development of psychoplasticity in the striatum related to addictive properties of drugs of abuse (11 ,12 ).