Journal of Neuroscience, Vol 4, 2117-2132, Copyright © 1984 by Society for Neuroscience
Demonstration of postsynaptic opioid modulation of thalamic projection neurons by the combined techniques of retrograde horseradish peroxidase and enkephalin immunocytochemistry
MA Ruda, J Coffield and R Dubner
Thalamic projection neurons represent a major source of nociceptive
information from the dorsal horn to higher centers of the neuraxis. The
synaptic relationship between thalamic projection neurons and the opioid
peptide enkephalin (ENK) was examined at the light (LM) and ultrastructural
(EM) level using the combined techniques of retrograde transport of
horseradish peroxidase and ENK immunocytochemistry. Utilizing two different
chromogens to develop the peroxidase reaction product, the two labeled
neural elements could be readily distinguished at the LM level in the same
tissue section. In the medullary, cervical, and lumbar levels of the dorsal
horn of both the cat and monkey, at least 30% of the thalamic projection
neurons in lamina I were observed at the LM level to be contacted by
ENK-immunoreactive varicosities. In lamina V, approximately 50% of the
thalamic projection neurons received ENK contacts. Since some neurons were
not observed to receive a dense ENK innervation on their somata and
proximal dendrites, these data suggest that there may be different
functional types of thalamic projection neurons. At the EM level, the
ENK-immunoreactive varicosities were observed to form asymmetrical synaptic
contacts on the labeled somata and proximal dendrites of the projection
neurons. In all cases, the ENK varicosities were morphologically similar
and contained round or oval agranular vesicles and a few dense-core
vesicles. These observations suggest that ENK acts to a substantial degree
on postsynaptic opiate receptors located on thalamic projection neurons.