Noradrenergic modulation of 4-aminopyridine-induced acetylcholine release from rat cerebral cortex

Characterization of neurotransmitters and related metabolites in Daphnia magna juveniles deficient in serotonin and exposed to neuroactive chemicals that affect its behavior: A targeted LC-MS/MS method

Neurotransmitters are endogenous metabolites that play a crucial role within an organism, at the chemical synapses. There is a growing interest in their analytical determination for understanding the neurotoxic effect of contaminants. Daphnia magna represents an excellent aquatic model for these environmental studies, due to its similarities with vertebrates in several neurotransmitters and related gene pathways and because of its wide application in ecotoxicological studies. Within this study, an accurate and sensible method of analysis of 17 neurotransmitters and related precursors and metabolites was developed. The method was validated in terms of sensitivity, reproducibility, precision, and accuracy, and also matrix effect was evaluated. As an independent probe of method validation and applicability, the method was applied to two different scenarios. First, it was used for the study of neurotransmitter levels in genetically mutated tryptophan hydrolase D. magna clones, confirming the absence of serotonin and its metabolite 5-HIAA. Additionally, the method was applied for determining the effects of chemical compounds known to affect different neurotransmitter systems and to alter Daphnia behavior. Significant changes were observed in 13 of the analyzed neurotransmitters across treatments, which were related to the neurotransmitter systems described as being affected by these neurochemicals. These two studies, which provide results on the ways in which the neurotransmitter systems in D. magna are affected, have corroborated the applicability of the presented method, of great importance due to the suitability of this organism for environmental neurotoxicity studies.

Interactions of 3,4-diaminopyridine and choline in stimulating acetylcholine release and protecting membrane phospholipids

We investigated the effects of 3,4-DAP on ACh release from rat striatal slices superfused with or without choline, at rest and during electrical stimulation. In a choline-free medium, 3,4-DAP increased basal and stimulated ACh release while lowering the net efflux of choline; thus while the sum of ACh plus choline released remained constant, the ratio of released ACh to that of choline was increased. The drug failed to affect tissue ACh, choline or membrane phospholipid levels (including those of phosphatidylcholine). In a choline-containing medium, 3,4-DAP potentiated the enhancement by choline of both basal and electrically stimulated ACh release. Electrical stimulation alone increased ACh release from the slices without altering choline efflux or depleting tissue choline or ACh stores; however, this treatment did deplete membranes of phosphatidylcholine and of other major phospholipids. Superfusion of the slices with 3,4-DAP protected the slices from stimulation-induced phospholipid depletion. Calcium-dependent activation of high-affinity choline uptake may underlie the observed effects of 3,4-DAP.

Purinergic modulation of hippocampal acetylcholine release involves alpha-dendrotoxin-sensitive potassium channels

Modulation of acetylcholine (ACh) release from superfused hippocampal slices was examined when the release of ACh was stimulated by exposure of slices to elevated K+ concentration. Evoked release was not sensitive to inhibition by 0.1 microM tetrodotoxin, but it could be inhibited in a dose-dependent manner by a muscarinic agonist (10-100 nM oxotremorine) and a purinergic agonist (10-100 nM 2-chloroadenosine). The alpha-dendrotoxin (100 nM), which selectively blocks voltage-gated inactivating K+ channels in nerve endings, did not affect the release of ACh under resting or depolarized conditions. However, alpha-dendrotoxin reduced the 2-chloroadenosine-induced inhibition of release, but did not alter the oxotremorine-induced inhibition. These results suggest that an alpha-dendrotoxin-sensitive K+ channel may be activated as an obligatory step in the modulation of ACh release by presynaptic purinergic receptor activation, but not in the modulation by presynaptic muscarinic receptors.