morphine is acting on to produce its effects. There is considerabie controversy 
concerning the specific circuitry by which opioids such as morphine produce 
antinociception (e.g., Fang et ai. 1986; Roerig and Fujimoto 1989; Heyman et 
al. 1987). In addition, it is not clear whether activation of ^i-, 6-, or K-receptors 
produces analgesia. 
This pharmacological question is ideally suited to analysis with Fos 
immunocytochemistry. Specifically, by studying the effects of selective 
agonists for either n-, 6-, or K-opioid receptors on both formalin-evoked pain 
behavior and on spinal cord Fos immunoreactivity, not only can it be determined 
whether these compounds are analgesic, but also the circuitry through which 
they produce their analgesic effects can be examined. We started this series 
of experiments by studying the effects of supraspinal administration of the [i - 
selective agonist DAMGO on pain behaviors and on spinal cord Fos expression 
evoked by noxious stimulation in rats. For ICV drug infusions, guide cannulae 
were stereotaxically implanted in the third ventricle of rats. After surgery, the 
animals were allowed to recover for 7 to 1 0 days prior to behavioral testing. 
For dose-response studies, ICV injections were made 10 minutes prior to 
injection of formalin into the hindpaw. Immediately thereafter, the animal 
was placed into a Plexiglas chamber so that the animal’s movements could 
be viewed and its behavior recorded over a 1-hour period as discussed above. 
The laminar distribution of formalin-evoked Fos expression in the spinal 
cord was similar to that seen previously (figure 9, panel A). DAMGO (ICV) 
produced a dose-related, naloxone-reversible suppression of both the 
nociceptive behaviors and spinal cord Fos immunoreactivity evoked by 
formalin (figure 9). Since the dose-response curve for ICV DAMGO spanned 
two orders of magnitude (0.006 to 0.60 ng), the authors were able to calculate 
estimates of the dose required to produce a 50-percent reduction (i.e., ED 50 ) 
in both pain behaviors and Fos immunoreactivity. It was found that ICV 
DAMGO exhibits similar potencies for inhibition of pain behavior (ED 50 
=0.06 ng) and overall Fos immunoreactivity (ED^O.OS ng); this suggests 
that there is a similar mechanism of action for the two measures. There was 
a significant correlation between the expression of behavioral nociception and 
overall Fos immunoreactivity produced by formalin (figure 6), again supporting 
the contention that noxious stimulus-evoked Fos immunoreactivity can be used 
as a marker for nociceptive neurons. 
Although the potencies for DAMGO-mediated inhibition of formalin-evoked 
pain behaviors and Fos immunoreactivity were similar, the maximal levels of 
inhibition (i.e., Emax) for each measure were different. Specifically, a dose 
of DAMGO that produced 100-percent inhibition of behavioral nociception 
produced only 81 percent overall inhibition of Fos immunoreactivity. When the 
149 
