burst of mRNA synthesis. These data argue that the signal transduction 
pathway for cAMP-inducible proenkephalin gene expression has become 
desensitized. 
With these problems in mind, the authors have begun studying lEG-target 
interactions in chronic drug treatment paradigms, which are critical to the field 
of substance abuse research. Although studies of lEG-target interactions 
in response to cocaine are in progress, we have begun with the study of 
haloperidol because it is known to regulate the proenkephalin gene, whereas 
other target genes are likely to be relevant to cocaine administration. In rats 
given daily intraperitoneal injections of haloperidol, 2 mg/kg for 2 weeks, the 
proenkephalin gene is induced twofold to threefold, as previously reported 
(Sabol et al. 1983; Tang et al. 1983). However, at this time point c-/bs mRNA is 
undetectable, and jun - D mRNA has diminished 20 to 40 percent compared with 
saline-treated animals (Nguyen et al., unpublished data). The phosphorylation 
status of constitutively expressed AP-1 proteins in this paradigm are currently 
being investigated; however, at the present time the mechanism by which 
proenkephalin gene expression is activated in brain by a chronic drug stimulus 
remains unknown. 
SUMMARY 
It is probably safe to say that the regulation of c -fos expression by drug 
treatment and other stimulus paradigms has biological specificity and 
mechanistic significance. However, as this brief essay makes clear, the 
immunohistochemical detection of c-fos is only the tip of the biological iceberg. 
lEGs deserve to be studied as critical components of the intracellular signaling 
machinery that may transduce intercellular signals, including those produced 
by drugs of abuse, into longer term changes in cellular function. However, the 
study of IEG expression must be related to biologically significant target genes 
and must be complemented by the study of phosphorylation of constitutively 
expressed transcription factors. Moreover, as illustrated by the proenkephalin 
gene, the regulation of genes that play important roles in differentiated cell 
function may differ depending on which cell types and stimulus conditions 
are investigated. Finally, attention must be paid to the complexities of gene 
regulation under conditions of repetitive and chronic stimulation if phenomena 
such as the neural substrates of drug dependence are to be understood. 
REFERENCES 
Chang, S.L., and Harlan, R.E. The fos proto-oncogene protein: Regulation by 
morphine in the rat hypothalamus. Life Sci 46: 1825-1 832, 1990. 
Chang, S.L.; Squinto, S.P.; and Harlan, R.E. Morphine activation of c-fos 
expression in rat brain. Biochem Biophys Res Commun 157:698-704, 1988. 
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