An unconventional response of adenylate cyclase to morphine and naloxone in the chicken during early development

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.

mu-Opioids activate tyrosine kinase focal adhesion kinase and regulate cortical cytoskeleton proteins cortactin and vinculin in chick embryonic neurons

We have investigated the signal transduction pathway of the G-protein mu-opioid receptor upstream of phospholipase D (PLD) and protein kinase C-epsilon (PKC-epsilon) activation in postmitotic E6CH chick embryo cortical neurons. The mu-opioid receptor and PLD-PKC-epsilon functional coupling depends on upstream tyrosine kinase activation. We now report that the mu-opioid agonists specifically stimulated tyrosine phosphorylation and activation of the focal adhesion kinase (FAK) in a time-dependent manner. We also demonstrate that met-enkephalin, a mu-opioid agonist in E6CH cultures, significantly increases tyrosine phosphorylation of another Src kinase substrate, the cytoskeletal protein cortactin. Tyrosine phosphorylation of cortactin led to drastic changes in subcellular localization, an estimated 2-fold enrichment in the cytosol. Similarly, opioids stimulated a sustained tyrosine phosphorylation of vinculin, a protein enriched in focal adhesion sites. These data provide novel evidence that opioid receptor intracellular signaling engages the specific activation of tyrosine kinase FAK and regulates the neuronal cytoskeleton during central nervous system morphogenesis.

Opioid peptides activate phospholipase D and protein kinase C-epsilon in chicken embryo neuron cultures

The mu-opioid peptide morphiceptin stimulated a Ca(2+)-independent protein kinase C (PKC-epsilon) that is expressed both in embryonic day 6 chicken telencephalon and in derived neuronal cultures. This activation was seen as a 2-fold increase in the activity and level of cytosolic PKC-epsilon and as a transient increase in membrane-associated PKC-epsilon following morphiceptin treatment. Morphiceptin did not activate phospholipase C-mediated phosphatidylinositol hydrolysis but did transiently activate (2- to 3-fold) phospholipase D (PLD), as measured by phosphatidylethanol formation in neuron cultures derived from embryonic day 6 or day 7 cerebral hemispheres. This PLD activation could provide an alternative source of diacylglycerol for the activation of PKC-epsilon and was naloxone-reversible and at least partially blocked by the tyrosine kinase inhibitor herbimycin A. Addition of phorbol 12-myristate 13-acetate stimulated both PLD and PKC-epsilon activities to a greater extent than opioids. The phorbol ester and insulin stimulation of PLD was also blocked by herbimycin. Both morphiceptin (in a naloxone-reversible manner) and phorbol ester increased phosphorylation of similar cytosolic proteins in intact cells, demonstrating a functional role for the PKC-epsilon activation by opioids. This is evidence that opioid receptors are transiently coupled to tyrosine kinase, PLD and PKC-epsilon activation and, by implication, to neuronal cell growth during brain morphogenesis.

Effects of opiates on neuronal development in the rat cerebral cortex

Quantitative morphological characteristics of cells in the primary somatosensory cortex of 6-day-old rats were examined following continuous maternal administration of morphine (10 mg/kg/day), naltrexone (10 mg/kg/day), or saline vehicle from gestation day 12. Naltrexone reduced neuronal packing density and significantly increased cortical thickness but had not effect on neuronal number, while morphine reduced neuronal packing density and the number of neurons without affecting cortical thickness. These results suggest that blockade of endogenous opioid function during development enhances neuronal maturation in this brain region, while perinatal morphine administration might act to restrict cortical cell proliferation and maturation. Thus, the effect of ontogenetic activation of opioid receptors by endogenous opioid compounds could be similar to but less severe than the effect of exogenous opiate exposure on cortical cell development.

Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro

To determine whether one or more opioid receptor types might be preferentially involved in gliogenesis, primary mixed glial cultures derived from mouse cerebra were continuously treated with varying concentrations of opioid agonists selective for mu (mu), i.e., DAGO ([D-Ala2, MePhe4, Gly(ol)5]enkephalin), delta (delta), i.e., DPDPE ([D-PEN2,D-PEN5]enkephalin), or kappa (kappa), i.e., U69,593, opioid receptor types. In addition, a group of cultures was treated with [Met5]-enkephalin, an agonist for delta opioid receptors as well as putative zeta (zeta) opioid receptors. Opioid-dependent changes in growth were assessed by examining alterations in (1) the number of cells in mixed glial cultures at 3, 6, and 8 days in vitro (DIV), (2) [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes at 6 DIV, and (3) the area and form factor of GFAP-immunoreactive, flat (type 1) astrocytes. DPDPE at 10(-8) or 10(-10) M, as well as [Met5]-enkephalin at 10(-6), 10(-8), or 10(-10) M, significantly reduced the total number of glial cells in culture; but this effect was not observed with DAGO or U69,593 (both at 10(-6), 10(-8), or 10(-10) M). Equimolar concentrations (i.e., 10(-6) M) of [Met5]enkephalin or U69,593, but not DPDPE or DAGO, suppressed the rate of [3H]thymidine incorporation by GFAP-immunoreactive, flat (type 1) astrocytes. DAGO had no effect on growth, although in previous studies morphine was found to inhibit glial numbers and astrocyte DNA synthesis. [Met5]enkephalin (10(-6) M) was the only agonist to significantly influence astrocyte area. Collectively, these results indicate that delta (and perhaps mu) opioid receptor agonists reduce the total number of cells in mixed glial cultures; while [Met5]enkephalin-responsive (and perhaps kappa-responsive) opioid receptors mediate DNA synthesis in astrocytes. This implies that delta opioid receptors, as well as [Met5]enkephalin-sensitive, non-delta opioid receptors, mediate opioid-dependent regulation of astrocyte and astrocyte progenitor growth. These data support the concept that opioid-dependent changes in central nervous system growth are the result of endogenous opioid peptides acting through multiple opioid receptor types.

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.

Evidence for plasticity in neurotransmitter expression in neuronal cultures derived from 3-day-old chick embryo

We have previously reported the developmental profiles of glutamate decarboxylase (GAD) and choline acetyltransferase (ChAT) bio- and immunocytochemically, assessing GABAergic and cholinergic neuronal phenotypes respectively, in neuroblast-enriched cultures from 3-day-old chick embryo, plated on poly-L-lysine. We have also reported that collagen as culture substrate inhibits neuronal aggregation and neuritic fasciculation in this culture system. In this study we assessed the same parameters for cultures on collagen. In addition, we evaluated the effects of nerve growth factors (NGF) on cholinergic and GABAergic expression on neurons plated either on polylysine or collagen. We found that non-neuronal cells and NGF prolonged the survival of cholinergic and GABAergic neuronal populations and that both markedly stimulated GABAergic expression. In contrast, cholinergic expression was only enhanced by NGF. Immunostaining for GABA and ChAT reflected the biochemical findings. Glutamine synthetase and cyclic nucleotide phosphohydrolase, used as markers for astrocytes and oligodendrocytes respectively, showed very low activity in both substrata and were not related to GAD or ChAT peak activities. Our findings suggest that humoral factors and cell-cell contacts markedly influence neuronal phenotypic expression in culture. Moreover, it appears that during early neuronal differentiation GABAergic neurons are more responsive to microenvironmental regulation compared to cholinergic neurons.

Paradoxical and subtype-specific effects of opiate antagonists on the expression of opioid receptors in rat brain cultures

Cultures of aggregating fetal rat brain cells express mu, delta, and kappa opioid receptors. The potent and long-lasting opioid antagonist naltrexone was used to investigate whether different regulatory mechanisms are involved in the expression of the three receptor subtypes. In cultures treated for seven days, naltrexone increased dose-dependently the binding of 3H-diprenorphine to the three receptor subtypes, with the mu sites being affected at a lower concentration than the other two; A Scatchard analysis indicated that this receptor up-regulation was obtained by an increase in the BMax, with no significant change in the affinity of the ligand to the receptors. In contrast to these effects in cultures treated for 7 days, it was surprising to find that a 48 hr treatment with naltrexone had an apparent converse and subtype-specific influence; the antagonist decreased significantly the binding of 3H-diprenorphine to both mu and delta receptors but had no effect on kappa sites. Two other opioid antagonists, naloxone and levallorphan, had a similar effect. Further analysis of naltrexone's mode of action was obtained by studying its effect on the adenylate cyclase activity. Of several inducers of this enzyme, the beta-adrenergic inducer isoproterenol gave the highest increase in cyclic AMP. Naltrexone had no significant effect on the basal adenylate cyclase activity but it altered the pattern of cyclic AMP formation in isoproterenol-stimulated cultures. Overall, the results indicate that in addition to its classic antagonistic activity, naltrexone exhibits in fetal brain aggregates some properties associated with opiate agonists.

Differential maturation of mu and delta opioid receptors in the chick embryonic brain

The developmental profiles of the binding of mu and delta opiate receptors agonists was investigated using the chick embryo brain. Binding of opioids was performed at embryonic days 5, 6, 15, 18, and 20 in the developing chick embryo brain. [3H]dihyromorphine was used as a mu ligand and with 5 X 10(-7) M levorphanol for non-specific binding, and [3H](D-Ala2-D-Leu5)-enkephalin was used as a delta with 5 X 10(-7) M (D-Ser-Gly-Phe-Leu-Thr)-enkephalin for non-specific binding. Crude membranes were prepared from whole brain at days 5, 6 and cerebral hemispheres at days 15, 18, and 20 of embryonic age. Both mu and delta opiate receptors were present during early embryogenesis and as early as day 5. Analysis of binding sites revealed high and low affinity mu sites during early embryogenesis but only one delta site. By 18 days of embryonic age, only one mu site remained. This developmental change is interpreted as a transitory state of the receptor to the adult mu pattern. The presence of only one delta site is constant throughout embryonic age; it is high during early embryogenesis reaching a lower level by 18 days. The presence of a dual binding site pattern for the mu receptor in early embryogenesis is implicated to have a functional significance in the pluripotential role of the endogenous opioids in early development.

Effects of opiates on the growth of neuron-enriched cultures from chick embryonic brain

Neuron-enriched cultures derived from 6-day-old chick embryo cerebral hemispheres were treated with morphine or methadone, 10(-5) M or 10(-6) M, on days 4-6 or 6-8 in culture and were evaluated morphologically and biochemically at day 9 using phase contrast microscopy and choline acetyltransferase activity (ChAT) as a cholinergic marker. The treatment of the cultures with morphine markedly affected their growth pattern; specifically, we observed an increased number of flat cells presumptively glia, and aggregates sided by flat cells and devoid of thick bundles of neuritic processes that normally characterize neuron-enriched cultures. These morphologic changes were reflected in a drastic decrease of ChAT activity in cultures treated from day 4 to day 6 but not from 6 to 8. In contrast to morphine, exposure to 10(-6) M methadone from day 4 to day 6 resulted in reduced ChAT activity but the growth pattern of the cultures remained morphologically intact. We suggest that morphine exerts a general neurotoxic effect whereas methadone may affect some specific cholinergic function.