Demonstration of postsynaptic opioid modulation of thalamic projection neurons by the combined techniques of retrograde horseradish peroxidase and enkephalin immunocytochemistry.

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.