Neural or glial differentiation from neural stem cells induced by opioids

Recent findings indicate that neurogenesis continues throughout the life of the animal from neural stem cells (NSCs). NSCs are able to generate the three major cell types that constitute the central nervous system: neurons, astrocytes, oligodendrocytes. Cell-cell interaction between NSCs and the newly generated cells is considered to be essential for axonal conduction and synaptic transmission. The aim of present study was undertaken to evaluate whether opioids could affect either astrocytes or NSCs and promote cell differentiation from NSCs via neurons and astrocytes. Treatment with a mu-opioid receptor agonist morphine for 3 days did not produce morphological changes in the activation of purified cortical astrocytes. In contrast to morphine treatment in purified cortical astrocytes, treatment with morphine into the cortical neuron/glia co-cultures for 3 days caused the activation of astrocytes as detected by a stellation of morphology and an increase in levels of glial fibrillary acidic protein (GFAP). To ascertain whether morphine could induce astrogliogenesis, we performed the differentiation experiments using multipotent NSCs obtained from the mouse forebrain. Treatment of NSCs with morphine failed to induce the differentiation to GFAP-positive astrocyte. Moreover, treatment of NSCs with methamphetamine, cocaine or tridazolam also failed to induce the differentiation from NSCs to GFAP-positive astrocytes. Astrocytes affect neuronal function through the release of neurotransmitters, neurotrophic factors, cytokines, chemokines, and extracellular matrix. To examine the role of activated-astrocyte-related soluble factors in the differentiation from NSCs to GFAP-positive astrocytes, we next examined the effect of morphine-treated neuron/glia-conditioned medium (MRP-NACM) on NSCs. Here we show that treatment with MRP-NACM for 3 days resulted in astrogliogenesis differentiated from NSCs, as characterized by the increase in GFAP-positive cells. On the other hand, exposure to a selective delta-opioid receptor agonist [(+)-4-[(aR)-a-((2S, 5R)-4-Allyl-2, 5-dimethyl-l-piperazinyl)-3-methoxybenzyl]-N, N-dimethylbenzamide] (SNC80) resulted in neurogenesis differentiated from NSCs, whereas a mu-opioid receptor agonist or a K-opioid receptor agonist did not have such effect. This effect was blocked by a Trk-dependent tyrosine kinase inhibitor, K-252a. These findings constitute evidence that opioids may promote neural or glial differentiation from NSCs through directly or indirectly pathway.