The periaqueductal gray of the rat contains significant levels of the putative peptide neurotransmitter neurotensin. The profound anti-nociceptive effects of neurotensin injected into the periaqueductal gray may involve a population of periaqueductal gray neurons having descending projections to the rostral ventral medulla, including nucleus raphe magnus and adjacent reticular nuclei. In this study, electron microscopic immunocytochemistry was used to examine the ultrastructure of periaqueductal gray axon terminals containing neurotensin-like immunoreactive material and to obtain quantitative data regarding the relationship of such terminals to other elements of the neuropil. Of particular interest was the interaction between neurotensin-like immunoreactive terminals and retrogradely labeled neurons that project to nucleus raphe magnus and adjacent reticular nuclei. Within the periaqueductal gray, the sites of retrograde and immuno-labeling were consistent with previous reports. The neurotensin-immunoreactive structures were predominantly axon fibers and terminals. In the ventrocaudal periaqueductal gray, the mean diameter of neurotensin-containing terminals was 0.93 ± 0.02 μm and they comprised a volume fraction of 0.0010. Most of the neurotensin-positive terminals examined (74.2%) were in contact with or closely apposed to dendrites. The most common anatomical configuration observed was a single neurotensin-immunoreactive terminal juxtaposed to three dendrites. Only 2% of immunoreactive terminals were apposed to perikarya. Neurotensin-immunoreactive terminals were observed to form symmetrical synapses and 96.4% of such terminals were axodendritic. Occasional multiple neurotensin-immunoreactive terminals associated with single dendrites were observed. Although neurotensin-like immunoreactive terminals were quite prominent, only a small percentage made synaptic contact with periaqueductal gray neurons that project to the nucleus raphe magnus and adjacent reticular formation. Among the population of periaqueductal gray neurons retrogradely-labeled from nucleus raphe magnus and adjacent reticular nuclei, the frequency of direct synaptic contact by neurotensin-immunoreactive terminals was 2%. These data suggest that the periaqueductal gray circuitry by which neurotensin ultimately affects descending pathways is complex and may involve a population of local circuit neurons whose transmitters and connections remain to be elucidated.
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Acknowledgements-The authors wish to thank Mary Mullett, Lori Weiner and Joan Houtman for their excellent technical assistance, and Dr L. Jennes for the gift of anti-neurotensin antiserum. This work was supported by grant BNS-8607520 from the National Science Foundation and grants NS19208 and DE06682 from the National Institutes of Health.