A comparison of the release of a vasoactive-intestinal-peptide-like peptide and acetylcholine in the giant axon-Schwann cell preparation of the tropical squid Sepioteuthis sepioidea

The mechanisms and possible sites of acetylcholine release during chick primary sensory neuron differentiation

AIMS

In this study, we evaluated the ability of differentiating embryonic chick DRG neurons to release and respond to acetylcholine (ACh). In particular, we investigated the neuronal soma and neurites as sites of ACh release, as well as the mechanism(s) underlying this release.

MAIN METHODS

ACh release from DRG explants in the Campenot chambers was measured by a chemiluminescent assay. Real-time PCR analysis was used to evaluate the expression of ChAT, VAChT, mediatophore and muscarinic receptor subtypes in DRGs at different developmental stages.

KEY FINDINGS

We found that ACh is released both within the central and lateral compartments of the Campenot chambers, indicating that ACh might be released from both the neuronal soma and fibers. Moreover, we observed that the expression of the ChAT and mediatophore increases during sensory neuron differentiation and during the post-hatching period, whereas VAChT expression decreases throughout development. Lastly, the kinetics of the m2 and m3 transcripts appeared to change differentially compared to the m4 transcript during the same developmental period.

SIGNIFICANCE

The data obtained demonstrate that the DRG sensory neurons are able to release ACh and to respond to ACh stimulation. ACh is released both by the soma and neurite compartments. The contribution of the mediatophore to ACh release appears to be more significant than that of VAChT, suggesting that the non-vesicular release of ACh might represent the preferential mechanism of ACh release in DRG neurons and possibly in non-cholinergic systems.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by <or=30%. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

N-acetylaspartylglutamate (NAAG) is the probable mediator of axon-to-glia signaling in the crayfish medial giant nerve fiber

Glial cell hyperpolarization previously has been reported to be induced by high frequency stimulation or glutamate. We now report that it also is produced by the glutamate-containing dipeptide N-acetylaspartylglutamate (NAAG), by its non-hydrolyzable analog beta-NAAG, and by NAAG in the presence of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent inhibitor of the NAAG degradative enzyme glutamate carboxypeptidase II. The results indicate that NAAG mimics the effect of nerve fiber stimulation on the glia. Although glutamate has a similar effect, the other presumed product of NAAG hydrolysis, N-acetylaspartate, is without effect on glial cell membrane potential, as is aspartylglutamate (in the presence of 2-PMPA). The hyperpolarization induced by stimulation, glutamate, NAAG, beta-NAAG, or NAAG plus 2-PMPA is completely blocked by the Group II metabotropic glutamate receptor antagonist (S)-alpha-ethylglutamate but is not altered by antagonists of Group I or III metabotropic glutamate receptors. The N-methyl-D-aspartate receptor antagonist MK801 reduces but does not eliminate the hyperpolarization generated by glutamate, NAAG or stimulation. These results, in combination with those of the preceding paper, are consistent with the premise that NAAG could be the primary axon-to-glia signaling agent. When the unstimulated nerve fiber is treated with cysteate, a glutamate reuptake blocker, there is a small hyperpolarization of the glial cell that can be substantially reduced by pretreatment with 2-PMPA before addition of cysteate. A similar effect of cysteate is seen during a 50 Hz/5 s stimulation. From these results we suggest that glutamate derived from NAAG hydrolysis appears in the periaxonal space under the conditions of these experiments and may contribute to the glial hyperpolarization.