Modulatory effects of [Met⁵]-enkephalin on interleukin-1β secretion from microglia in mixed brain cell cultures

In the present study, functional interactions between [Met⁵]-enkephalin (ME), naloxone and lipopolysaccharide (LPS) on interleukin-1β (IL-1β) immunostaining and secretion have been assessed in mixed brain cell cultures from embryonic day 17 mice. Adding ME alone or together with LPS to the culture increased the release of IL-1β after 48 h in a concentration-dependent fashion. In situ hybridization studies showed that LPS, but not ME, increased the abundance of IL-1β mRNA. The enhanced release of IL-1β caused by ME or LPS was partially blocked by naloxone. LPS induced concentration-dependent morphological changes in microglia in mixed brain cell cultures, identified by a monoclonal antibody F4/80 which is specific for macrophages/microglia. Despite increasing IL-1β release into the media, ME (10 ^-8 M) did not induce morphological changes in microglia. Naloxone alone also had no effect on glial morphology; however, the LPS-induced morphological changes were blocked by naloxone. Our data indicate that both exogenous and endogenous opioids regulate IL-1β production by microglial cells in the mixed brain cell cultures.