Everything Activates C-fos — How Can 
It Matter? 
Steven E. Hyman , Barry E. Kosofsky, Tuong V . Nguyen , Bruce 
M. Cohen , and Michael J . Comb 
INTRODUCTION 
This chapter addresses often-raised questions concerning the specificity and 
biological significance of drug-induced immediate early gene (IEG) expression, 
especially c-fos expression, in the nervous system. The definition of cellular 
lEGs and basic information about their induction are described elsewhere in 
this volume and are well reviewed in Morgan and Curran (1991) and Sheng 
and Greenberg (1990). 
Immunohistochemical detection of c-Fos protein has become a widely used 
tool to map activation of cells in the nervous system in response to many 
drugs and other stimuli. The widespread use of this approach has been 
facilitated by the availability of antisera recognizing c-Fos and, more particularly, 
by the properties of the c-fos gene, mRNA, and protein. lEGs, including c-fos , 
are activated rapidly by multiple intracellular second messenger systems and, 
therefore, by multiple primary neural stimuli. Under basal (unstimulated) 
conditions, many lEGs, including c-fos, have nearly undetectable levels of 
mRNA and protein; this lack of background expression contributes to the 
sensitivity of the technique, making inductions readily detectable by visual 
inspection. Given its simplicity, sensitivity, cellular resolution, and utility with 
respect to diverse stimuli, c-Fos immunohistochemistry has been applied to 
many paradigms, including several relevant to the field of substance abuse 
such as administration of opiates (Chang et al. 1988; Chang and Harlan 1990), 
cocaine (Graybiel et al. 1990; Young et al. 1991; Hope et al., in press), and 
amphetamine (Graybiel et al. 1990; Nguyen et al., in press). In addition, 
induction of c-fos mRNA has been studied in opiate withdrawal (Hayward 
et al. 1990). 
Given the many stimuli that activate c-fos expression, questions have 
often been raised about the biological specificity and significance of 
immunohistochemically detected c-Fos. Such concerns have been worsened 
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