limitations, Fos expression can be used as a marker to tell whether and when 
something happens to a particular neuron. Importantly, the findings (1) that 
the distribution of noxious stimulus-evoked Fos immunoreactivity closely 
parallels the known distribution of spinal nociceptors, (2) that there is a 
significant correlation between the expression of noxious stimulus-evoked 
pain behaviors and spinal cord Fos expression, and (3) that noxious stimulus- 
evoked Fos immunoreactivity is suppressed by systemic as well as supraspinal 
administration of opioid analgesics, such as morphine and DAMGO, strongly 
suggest that this approach can be used to study the mechanisms by which 
drugs produce analgesia. 
REFERENCES 
Barton, C.; Basbaum, A.I.; and Fields, H.L. Dissociation of supraspinal and 
spinal actions of morphine: A quantitative evaluation. Brain Res 188:487- 
498, 1980. 
Basbaum, A.I.; Clanton, C.H.; and Fields, H.L. Opiate and stimulus-produced 
analgesia: Functional anatomy of a medullospinal pathway. Proc Natl Acad 
Sci USA 73:4685-4688, 1 976. 
Basbaum, A.I., and Fields, H.L. Endogenous pain control mechanisms: 
Review and hypothesis. Ann Neurol 4:451-462, 1978. 
Basbaum, A.I., and Fields, H.L. Endogenous pain control systems: Brainstem 
spinal pathways and endorphin circuitry. Annu Rev Neurosci 7:309-338, 
1984. 
Basbaum, A.I.; Marley, N.; O’Keefe, J.; and Clanton, C.H. Reversal of 
morphine and stimulus-produced analgesia by subtotal spinal cord lesions. 
Pain 3:43-56, 1977. 
Bennett, G.J.; Abdelmoumene, M.; Hayashi, H.; and Dubner, R. Physiology 
and morphology of substantia gelatinosa neurons intracellularly stained with 
horseradish peroxidase. J Comp Neurol 194:809-827, 1980. 
Bennett, G.J., and Mayer, D.J. Inhibition of spinal cord interneurons by narcotic 
injections. Brain Res 172:243-257, 1979. 
Besson, J.M., and Chaouch, A. Peripheral and spinal mechanisms of 
nociception. Physiol Rev 67:67-1 86, 1 987. 
Bouhassira, D.; Villanueva, L.; and Le Bars, D. Intracerebroventricular 
morphine decreases descending inhibitions acting on lumbar dorsal horn 
neuronal activities related to pain in the rat. J Pharmacol Exp Ther 
247(1):332-342, 1988. 
Brown, A.G., and Fyffe, R.E.W. The morphology of Group la afferent fibre 
collaterals in the spinal cord of the cat. J Physiol 274: 1 1 1 -1 27, 1 978. 
Brown, A.G., and Fyffe, R.E.W. The morphology of Group lb afferent fibre 
collaterals in the spinal cord of the cat. J Physiol 296:215-228, 1979. 
Cervero, F. Visceral nociception: Peripheral and central aspects of visceral 
nociceptive systems. PhilosTrans R Soc Lond Biol 308:325-337, 1 985. 
156 
