Development of Met-enkephalin immunoreactivity in organotypic explants of fetal mouse spinal cord and attached dorsal root ganglia.

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 and provides further support to electrophysiological analyses suggesting tonic opioid inhibitory networks in these explants.