The brainstem origin of enkephalin- and substance-P-like immunoreactive axons in the spinal cord of the North American opossum.

We have used the retrograde transport of True Blue (TB) in conjunction with immunofluorescence to determine if any of the enkephalin (ENK)- or substance P (SP)-immunoreactive axons in the spinal cord of the North American opossum originate within the brainstem. Neurons containing ENK-like immunoreactivity and TB after injections of the latter into the second or third cervical segment of the spinal cord were found within the locus coeruleus proper, the locus coeruleus pars alpha, the raphe magnus, pallidus and obscurus, and within several nuclei of the medullary reticular formation. Neurons containing SP-like immunoreactivity and TB were found in many of the same nuclei but not within the locus coeruleus proper. In general, the brainstem areas that provide ENK and SP projections to the spinal cord in marsupial opossums are comparable to those which provide similar projections in placental mammals, but differences exist. Our results are discussed in light of those differences and the probable origins of ENK and SP projections from the brainstem to different spinal targets.