C-Fos and Fos-Related Antigens 
as Markers for Neuronal Activity: 
Perspectives From Neuroendocrine 
Systems 
Gloria E. Hoffman, Wen-Sen Lee, M. Susan Smith, Rula Abbud, 
Michelle M. Roberts, Alan G. Robinson, and Joseph G. Verbalis 
INTRODUCTION 
The alterations in neuronal function underlying drug abuse are complex, yet 
important. Several specific neuronal systems have been implicated in drug- 
seeking behavior, addiction, and withdrawal, but none has emerged as key. 
What may be needed is an approach to mark neurons throughout the brain that 
are influenced by drugs of abuse. Once targeted neurons are identified, studies 
can be extended to define their phenotype and projections and to determine 
their functions in drug-related processes. The use of immediate early gene 
(I EG) products may provide an important tool for labeling neurons whose 
activity has changed as a result of drug treatment. As is detailed below, c-Fos 
and Fos-related antigens (FRAs) serve as markers for identifying neuronal 
systems whose activity is increased or decreased in response to a stimulus. 
In situ hybridization of I EG messenger RNAs (mRNAs) as well as 
immunocytochemical localization of protein products are useful for addressing 
this issue. The use of the protein products in examining specific systems can 
have advantages over the study of mRNA. First, molecular probes are not 
available for all lEGs, whereas antisera have been generated that recognize 
the protein products of some genes not yet cloned. Second, IEG products 
such as c-Fos, c-Jun, and Zif/268, when present, concentrate within the 
cell nucleus (Sheng and Greenberg 1990). This feature makes it possible to 
label these proteins along with cytoplasmic markers for the neurotransmitter 
(Ceccatelli et al. 1989; Hoffman et al. 1990; Jacobson et al. 1990) and/or 
retrogradely transported tracers (Menetrey et al. 1989) using standard double 
immunocytochemical techniques. For heterogenous neuronal populations, or 
diffusely organized neuronal systems, this capability is imperative. In contrast, 
mRNA is located in the cytoplasm. Techniques for double labeling of either two 
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