Comparison of met‐enkephalin‐, dynorphin A‐, and neurotensin‐immunoreactive neurons in the cat and rat spinal cords: I. Lumbar cord

This study compared the distribution of methionine enkephalin‐, dynorphin A 1–8‐, and neurotensin‐immunoreactive (IR) perikarya in laminae I and IV–VII of selected segments of lumbar spinal cord of cat(L5) and rat(4). Immunoreactive neurons for each peptide were found throughout the dorsal horn and dorsal lamina VII but were quantified only within laminae I and IV–VII. In lamina I, both large (>20 μm) and small (<20 μm) IR neurons were identified. Large IR neurons for each peptide in both species resembled Waldeyer neurons studied by Golgi stain and were outnumbered by small IR neurons. Comparison among the laminae of the distribution of metenkephalin IR neurons showed a similar pattern in the two species with the majority of IR neurons (>65%) in laminae V and VI. Differences in laminar distribution occurred between species for the other peptides. Dynorphin IR neurons were greatest in number in lamina V in rat but greatest in number in laminae I and V in cat. Neurotensin IR neurons occurred predominately in cat lamina I but were nearly equal in density in rat laminae I and VI. The topographic distribution of each peptide in laminae V and VI was similar between the two species with IR neurons occuring laterally in lamina V and more medially in lamina VI. Comparisons between species of the numbers of IR neurons/segment indicated distinct relationships for each peptide. The number of met‐enkephalin IR neurons in laminae of cat L5 was generally two times greater than the number of IR neurons in the same laminae of rat L4, except in laminae I and IV, where the numbers were nearly equal. In contrast, the number of dynorphin IR neurons in cat laminae was generally one‐half the number in rat, except in lamina I, where the number in cat was two times greater than rat. A high degree of variability occurred in laminar comparisons of neurotensin IR neurons. Neurotensin IR neurons in lamina I of cat outnumbered those of rat 2:1, but in laminae IV–VII, the ratio of cat to rat IR neurons varied from 1:1 to 1:20. The metenkephalin, dynorphin, and neurotensin IR neurons quantified in this study may be interneurons or may serve as projection neurons to brainstem and/or thalamic nuclei. The observed differences in distribution may be relevant to differences in spinal cord physiology in the two species.