Opioids influence neurotransmitter phenotypic expression in chick embryonic neuronal cultures

There is considerable interest in the role of endogenous opioid peptides in neural growth and differentiation. In this study we used neuron-enriched cultures derived from 3-day-old chick embryos to test the effects of endogenous enkephalins on neurotransmitter phenotypic expression. Cultures were grown in serum-free chemically defined medium and were treated with either Met-enkephalin antiserum (anti-Met) to immunoneutralize enkephalins, or with naloxone, a universal opioid receptor antagonist, to block receptor-mediated actions of released endogenous opioids. The enzyme activities of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) were used as markers for cholinergic and catecholaminergic phenotypic expression, respectively. We found that cultures treated with anti-Met or naloxone exhibited strikingly different neuronal growth patterns as compared to controls. In addition, ChAT activity was enhanced by anti-Met, and TH activity by both anti-Met and naloxone. These findings lend support to the possibility that neuropeptides may be co-localized with neurotransmitters and that peptides released into the microenvironment affect neuronal phenotypic expression by differential receptor subtypes.