neck and ventral cord had almost no effect in the superficial dorsal horn. We 
hypothesize that the more profound suppression of Fos immu noreactivity in the 
superficial dorsal horn seen following systemic doses of morphine results from 
the fact that after systemic administration there is both a supraspinal and spinal 
contribution to analgesia. 
Since DAMGO is a potent and selective pi-opioid receptor agonist (Handa 
et al. 1981), and since morphine, when given supraspinally, is also thought to 
act preferentially at the ^-receptor (Fang et al. 1986; Roerig and Fujimoto 
1989), the contrast between the levels of superficial suppression following 
ICV morphine and DAMGO is particularly interesting. Thus, morphine (1C V) 
produced a maximal inhibition (i.e., 65 percent) of superficial dorsal horn 
Fos immunoreactivity at the lowest dose tested (2.5 ng); there was no greater 
inhibition when higher doses were tested. By contrast, the dose-response 
curve for ICV DAMGO-mediated Fos suppression in the superficial cord 
was shallow. In fact, significant inhibition occurred only at the highest dose 
tested (0.60 \xg). If one considers this difference in terms of the relationship 
between inhibition of superficial dorsal horn neuronal activity and behavioral 
antinociception, it can be seen that doses of ICV morphine and DAMGO that 
produce 50-percent inhibition of behavioral nociception produce 65-percent and 
<1 0-percent inhibition of superficial Fos immunoreactivity, respectively. Since 
DAMGO is thought to be a much more efficacious agonist than morphine, 
it is highly unlikely that the differences between the two compounds relate to 
differences in activity at the ^-receptor. Rather, these differences suggest 
that ICV morphine acts at another site in addition to the ^-receptor, possibly 
the 6-receptor. Alternatively, since some researchers have provided evidence 
for the existence of subpopulations of ^i-opioid receptors in brain (Wolozin 
and Pasternak 1981 ; Rothman et al. 1983), it is possible that the ^-receptor 
populations with which morphine and DAMGO interact are different. 
Regardless of the receptor(s) through which morphine acts to produce its 
effects, it is clear that by monitoring the regulation of Fos protein expression 
by these compounds, in addition to monitoring behavioral effects, much new 
information on the mechanisms through which these drugs work can be 
obtained. 
THE EFFECT OF DORSOLATERAL FUNICULUS LESIONS ON OPIOID- 
MEDIATED INHIBITION OF NOXIOUS STIMULUS-EVOKED FOS 
IMMUNOREACTIVITY 
It is commonly believed that analgesic compounds, such as morphine, 
activate descending systems within the brain stem, which in turn inhibit spinal 
cord nociresponsive neurons (Basbaum and Fields 1978, 1984). For example, 
supraspinal microinjection of morphine attenuates behavioral responses in 
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