In vivo effects of morphine on neuronal fate and opioid receptor expression in zebrafish embryos

Abstract

Morphine remains one of the most potent analgesic compounds used to control chronic pain despite its known adverse effects. It binds to the opioid receptors mu, delta and kappa, which are involved in aspects of neuronal fate such as cell proliferation, neuroprotection and neuronal differentiation. However, the effect of morphine on these processes is controversial and in vitro studies, as well as in vivo studies on adults and neonates in mammalian models, have not been able to clarify the diverse roles of morphine in the central nervous system. We have used zebrafish embryos to determine in vivo how morphine affects neuronal fate and opioid receptor gene expression and to elucidate if there is a link between these processes. Our results show that at 24 and 48 h post fertilization (hpf) morphine enhances cell proliferation, although it has opposing effects as an inducer of neuronal differentiation at these two stages, increasing the number of certain neuronal populations at 24 hpf and decreasing it at 48 hpf. The present study also demonstrates that in 24-hpf embryos morphine acts as a neuroprotector against glutamate damage in motor neurons and Pax-6-positive neurons. Furthermore, the gene expression of the opioid receptors is altered by embryonic exposition to morphine. In conclusion, our study sheds new light on the in vivo roles of morphine, and it indicates for the first time that its implication in cell proliferation and neuroprotection might be related to changes in the gene expression of opioid receptors.