I and outer II (Light and Perl 1979; Sugiura et al. 1987). Consistent with the 
anatomical projection of the primary afferent fibers, electrophysiological studies 
have established that neurons in laminae I and llo respond predominantly to 
noxious input; lamina V neurons respond to both noxious and nonnoxious 
stimuli and are thus referred to as wide-dynamic range neurons (for a review, 
see Besson and Chaouch 1987). Cells in the nucleus proprius (laminae III, IV) 
and in lamina VI are predominantly driven by nonnoxious stimulation. Finally, 
many neurons in laminae VII and VIII of the ventral horn and in lamina X, 
around the central canal, also respond to noxious stimulation. 
Although they provide important information about the functional properties of 
spinal neurons, electrophysiological techniques have limitations. Specifically, 
since only one cell is analyzed at a time, it is not possible to monitor the 
responses of large populations of neurons to noxious stimulation. Thus, 
sample size is typically small. Furthermore, since the animals are almost 
always anesthetized and/or spinalized, it is not possible to correlate physiology 
with behavioral responses to stimulation. Even when recording is done in 
awake animals (e.g., Collins 1 987), one is still limited by the number of cells 
that can be recorded. 
THE USE OF FOS EXPRESSION TO ASSESS THE ACTIVITY OF NEURONS 
IN THE CNS 
Hunt and colleagues (1 987) were the first to show that immunocytochemical 
localization of the Fos protein product of the c -fos protooncogene can be 
used to monitor the “activity” of large populations of neurons in the spinal cord 
dorsal horn following either noxious or nonnoxious stimulation. As discussed in 
detail elsewhere in this monograph, c -fos is the cellular homolog of a retroviral 
transforming factor that induces osteosarcoma in susceptible strains of mice. 
Transcription of the c -fos gene is among the earliest nuclear events that occur 
when a cell is stimulated (Curran and Morgan 1985). Activation of transcription 
can be produced by a variety of external stimuli, including neurotransmitters 
and membrane depolarization (Curran and Morgan 1985; Greenberg et al. 
1986; Morgan and Curran 1986). The latter stimuli in turn increase intracellular 
calcium, cyclic AMP (cAMP), diacylglycerol, and other second messengers, 
which, either directly or indirectly, stimulate transcription of the c -fos gene. 
Although increased Fos expression is associated with cellular proliferation 
and differentiation, its function in postmitotic neurons has not been established. 
However, it is clear that the expression of c -fos as well as other lEGs (e.g., 
c-juri) acts as a link between the excitation of neurons and long-term adaptive 
responses in the cell. Since the c -fos message appears within 5 to 1 0 minutes 
of stimulation and the Fos protein within another 10 to 20 minutes, it has been 
135 
