If carefully interpreted, this convergence of multiple signaling pathways on 
activation of c -fos may, in fact, prove to be advantageous in investigations of 
neural circuitry in that the particular neurotransmitter receptor types mediating 
the signal need not be known ahead of time. However, in analyzing the neural 
circuitry and receptor types involved with IEG activation, both synergy and 
cross-talk among second messenger pathways within cells must be taken into 
account. 
IF EVERYTHING ACTIVATES C -FOS, HOW CAN IT BE SIGNIFICANT? 
lEGs and their protein products can be conceptualized as components of a 
molecular cascade that transduces signals from cell surface receptors to the 
nucleus. Thus, lEGs such as c-fos have been termed "third messengers" in 
signal transduction cascades if neurotransmitters are designated as intercellular 
“first messengers" and small molecules such as cAMP as intracellular "second 
messengers" (Morgan and Curran 1991); alternatively, lEGs have been termed 
"fourth messengers" if second messenger-dependent protein kinases are 
designated as the intervening "third messengers” (Hyman and Nestler, in 
press). In either case, characterizing the biological actions of lEGs within 
particular neurons has many analogies with understanding the functions of 
other molecules within signal transduction cascades, such as the second 
messenger, cAMP, a ubiquitous molecule, the biological significance of 
which is rarely questioned. Thus, for example, in considering the biological 
specificity and function of cAMP within a cell, it is necessary to determine 
the threshold for activating cAMP-dependent protein kinase, the available 
substrates for this kinase within the cell, the basal phosphorylation status 
of these substrates, the activity of specific phosphodiesterases and 
phosphoprotein phosphatases, cross-talk with other second messenger 
systems, and subcellular compartmentalization of cAMP and protein kinase 
A. Thus, although many neurotransmitters activate cAMP, based on these 
parameters its cellular effects are specific as well as biologically critical to 
cellular function. 
Similarly, investigations of the biological specificity and function of lEGs must 
address such issues as what particular cells the IEG is induced in, which 
potential target genes are transcriptionally active in those cells, what partners 
exist within those cells for dimer formation with the protein product of the 
IEG, what other proteins in the cell will compete for binding to DNA, and what 
the effect is of the dimers formed on target gene expression. Experiments 
that would address these issues include colocalization of lEGs coinduced 
with putative target genes within identified neurons (Naranjo et al. 1991), 
demonstration that the temporal appearance of the activator protein-1 (AP-1) 
proteins of interest precede induction of the putative target gene in response to 
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