Muscle-derived factors enhance cholinergic neuronal expression in the chick embryo--I. In ovo studies

Acetyl-L-carnitine has a neuromodulatory influence on neuronal phenotypes during early embryogenesis in the chick embryo

Studies from this laboratory and others have demonstrated that neuroblasts in early embryogenesis exhibit a high degree of plasticity with respect to neurotransmitter phenotype. The critical period within which these neuroblasts are sensitive to the effects of endogenous neurotrophins has been defined as 1-3 days of development in the chick embryo. In this study, we examined the influence of acetyl-L-carnitine (ALCAR) administered in ovo during embryonic days 1-3 (E1-E3) and sacrificed at embryonic day 8 (E8) on cholinergic and GABAergic neuronal phenotypes using as neuronal markers the activities of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD), respectively. Phenotypic expression was assessed in 3 distinct anatomical regions of the embryonic brain: cerebral hemispheres (CH), optic lobes (OL), and diencephalon-midbrain-brainstem (DMBS). A single administration of ALCAR at embryonic day 1 resulted in a dose-dependent increase in ChAT activity and decrease in GAD activity in CH. ChAT activity was again increased and GAD activity decreased in CH from embryos that were administered ALCAR (100 micrograms/50 microliters/day) daily from embryonic day E1 to E3. No change was observed in either ChAT or GAD activity in OL in response to ALCAR administration during the critical period (E1-E3) at doses ranging from 10 to 500 micrograms/day. In the DMBS, the activity of ChAT exhibited a marked increase at lower doses (10 micrograms) followed by a marked decrease at higher doses (500 micrograms) of ALCAR. The decrease in ChAT activity in DMBS was again observed at an ALCAR dose of 100 micrograms/day when administered from E1 to E3.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity of cholinergic and GABAergic neurons in chick embryos treated intracerebrally with ethanol at 8 days of embryonic age

We have shown that in embryos treated with ethanol in ovo during days 1-3, a critical period of neuroembryogenesis, cholinergic neuronal phenotypic expression is decreased whereas GABAergic and catecholaminergic neuronal populations are increased as assessed by neuronal markers choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD) and tyrosine hydroxylase (TH) respectively. In this study, ethanol was administered intracerebrally to embryos at embryonic day 8, embryos were sacrificed at day 9 and ChAT and GAD activities assayed separately in cerebral hemispheres and remaining brain (diencephalon-midbrain and optic lobes). We found that ChAT activity was enhanced in the cerebral hemispheres only, whereas GAD activity was decreased in both cerebral hemispheres and remaining brain. We have concluded that the differential responses of neuronal phenotypes to ethanol may reflect compensatory mechanisms to ethanol insult. Moreover, these findings emphasize the vulnerability of the GABAergic neuronal phenotypes to ethanol neurotoxicity during early brain development in the chick.

Muscle-derived factors reverse the cholinotoxic effects of ethanol during early neuroembryogenesis in the chick embryo

The interaction between muscle-derived factors and ethanol on cholinergic neuronal expression was studied in the chick embryo during early neuroembryogenesis using choline acetyltransferase (ChAT) as cholinergic neuronal marker. Ethanol (10 mg/50 microliters) and limb muscle extract (130 micrograms protein/50 microliters) (LME) were administered in ovo either alone or concomitantly at embryonic days 1-3 (E1-E3); or ethanol was given E1-E3 and followed by LME at E4-E7. All groups were sacrificed at embryonic day 8 (E8) and ChAT activity was assayed in homogenates of whole brain and of spinal cord. As previously reported, ethanol at E1-3 produced a 30% decrease in brain ChAT activity and 35% in spinal cord. Concomitant administration of ethanol and LME at embryonic days E1-E3 eliminated the decrease in choline acetyltransferase activity produced by ethanol in the brain, but not in the spinal cord. On the other hand, administration of LME at embryonic days E4-E7 to embryos pretreated with ethanol at embryonic days E1-E3, raised ChAT activity to control level in the spinal cord, but only partially restored ChAT activity in the brain. In view of the alleged neurotrophic effects of muscle-derived factors on neuronal survival and neuronal growth, we interpret these findings to suggest that LME in addition to its ability to decrease natural neuronal death, may prevent death resulting from neurotoxicity.

Muscle-derived factors enhance cholinergic neuronal expression in the chick embryo--II. In culture studies

The effects of muscle-derived factors on the cholinergic and GABAergic neuronal phenotypes were studied in cultures derived from 3-day-old whole chick embryo (E3WE) consisting of proliferating neuroblasts and 8-day-old chick embryo cerebral hemispheres (E8CH) consisting of differentiated neurons. The effects of limb muscle extract (LME) were examined either when added to the culture medium or to the polylysine coating substratum cultures. We also compared the effects of LME derived from 8- to 15-day-old chick embryos. We found that LME added to the medium not only increased choline acetyltransferase activity, a marker for cholinergic neurons throughout the development of neurons in E3WE or E8CH culture, but also delayed the decline in the activity observed in untreated cultures. The marked increase in choline acetyltransferase activity in E8CH cultures grown on substratum-bound LME as compared to those grown in medium-containing LME, suggests that LME factors may be adhesion-promoting substances stimulating neuronal growth. These findings provide evidence that muscle-derived factors may be important in early cholinergic phenotypic expression and support our previous in ovo studies indicating that target-derived factors in limb muscle extract have general cholinotrophic effect.