Proenkephalin opioid peptide product in the sensory ganglia of the rat: a developmental immunohistochemical study.

The distribution and development of Met-enkephalin-Arg6-Gly7-Leu8 (Enk-8)-containing neurons in the sensory ganglia of the rat were investigated by means of immunocytochemistry using specific antiserum to this octapeptide. Enk-8-like immunoreactivity first appeared in neurons of the trigeminal ganglia of the 18-day embryo, then in the dorsal root ganglia of the 21-day embryo, thus exhibiting a rostrocaudal gradient in terms of appearance and abundance. The number of immunoreactive neurons in these sensory ganglia peaked on the 5th-7th postnatal days, with several small ones observed in each section (1.0-1.4% of total cell number). About 30-40% of these Enk-8-like immunoreactive neurons were also immunoreactive to substance P. Subsequently, Enk-8-like immunoreactivity in the sensory ganglia was decreased and was rarely detected in adult animals. However, colchicine treatment revealed the presence of several Enk-8-containing neurons per section prepared from mature rat. All these neurons were small (12.5-25 microns; mean +/- S.E.M., 19.86 +/- 3.26 microns). Some of these were also immunoreactive to substance P. These results strongly suggest that the preproenkephalin A system exists in subpopulations of both developing and matured sensory cells in the rat. Functional significance of this is discussed.