Development of Met-Enkephalin Immunoreactivity in Organotypic Explants of Fetal Mouse Spinal Cord and Attached Dorsal Root Ganglia

Abstract

Radioimmunoassays of methionine-enkephalin (Met-Enk) in organotypic cultures of 13-day fetal mouse spinal cord explants with attached dorsal root ganglia (DRG) demonstrate a progressive development of immunoreactivity (IR) during 5 weeks in vitro. Met-Enk IR in these cultures increased to levels observed in adult rodent spinal cord. Most of the Met-Enk IR assays were made on cord explants excised from cord-DRG cultures. In smaller numbers of assays performed on entire DRG-cord cultures or on cord cultured in the absence of DRGs, similar levels of Met-Enk IR were obtained. Thus most of the Met-Enk IR appeared to be located within the cord tissue. No Met-Enk IR was detected in DRGs cultured in the absence of cord. In contrast, low levels of Met-Enk IR were present in about 50% of the assays of DRGs cultured attached to the cord. Since these assays included the neuritic outgrowths of the cultures, our data do not preclude possible contamination by Met-Enk immunoreactive cord neurites that may have aberrantly projected into the outgrowth zones. Nevertheless, the data raise the possibility of a trophic influence of cord tissue on the development of Met-Enk IR in DRG neurons. The development of Met-Enk IR in cord regions of cord-DRG explants extends previous binding assays demonstrating development of opiate receptors in these cultures 13,14 and provides further support to electrophysiological analyses suggesting tonic opioid inhibitory networks in these explants 7.