Release of glutamate by the embryonic spinal motoneurons of rat positively regulated by acetylcholine through the nicotinic and muscarinic receptors.

It has been shown that mature neurons in adult vertebrates can co-express glutamate and acetylcholine. Furthermore, interactions at the synaptic level have been demonstrated. In a previous study we found that also motoneurons at early embryonic stages, thus well prior to synapse formation, release acetylcholine, and that glutamate increases this release. We now report the existence of a glutamate release from embryonic motoneurons and the increase of glutamate release by acetylcholine. This effect is mediated by nicotinic and muscarinic cholinergic receptors present on embryonic motoneurons. Using conditions of partial or total depletion of calcium, we show that the glutamate release has two components: one is calcium-dependent and the other calcium-independent. Furthermore, we show that extracellular glutamate can be taken up by motoneurons, probably via the neuronal glutamate transporter EAAC1, which we find to be expressed at this stage. Monitoring of the glutamate release kinetics showed that extracellular glutamate concentration reached a steady-state level, strongly suggesting the establishment of equilibrium between glutamate release and uptake. Altogether, these results support the idea that glutamate can act as a neurotransmitter in embryonic motoneurons. We hypothesise that, glutamate acts as a regulator of motoneuron maturation and spinal cord development.

Acetylcholine secretion enhanced by glutamate in rat embryonic spinal motoneurons: respective involvement of NMDA and AMPA receptors.

The spontaneous acetylcholine secretion and endogenous acetylcholine content were measured by means of chemiluminescent assay from isolated embryonic rat spinal motoneurons. The sensitivity of the detection allows to study the kinetics of the acetylcholine secretion with short time intervals. Following the demonstration of the presence of acetylcholine and glutamate in embryonic motoneurons, the aim of this work was to study the characteristics of acetylcholine secretion and the effect of glutamate in its modulation. The involvement of NMDA and AMPA glutamatergic receptors was mainly studied. Our data show that spontaneously acetylcholine secretion, is not calcium-dependent and is significantly enhanced by glutamate (1 mM). Pharmacological approaches show that glutamate effect on acetylcholine secretion is decreased in presence of APV (50 microM and 100 microM), or in presence of GYKI 53655 (10 microM), demonstrating that both NMDA and AMPA receptors are present at the membrane of embryonic spinal motoneurons and involved in the modulation of acetylcholine secretion. Presence of glutamate in the embryonic motoneuron and secretion may represent a mechanism of control of extracellular acetylcholine concentration, which was shown to control neuritic growth at early embryonic stage.

Effect of nitrogen narcosis on extracellular levels of dopamine and its metabolites in the rat striatum, using intracerebral microdialysis.

In man, nitrogen narcosis is characterized by euphoria, impaired cognitive function, neuromuscular incoordination and, ultimately, loss of consciousness. Because of the motor movement disorders, we chose to study the nigrostriatal system, whose major function is to regulate the extrapyramidal nervous system. The purpose of this investigation was to monitor changes in extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum of conscious rats, using intracerebral microdialysis. Results show a 40% decrease in extracellular DA concentration, a 59% increase in extracellular DOPAC and an increase in HVA starting with exposure to the nitrogen mixture. Thirty minutes after the beginning of the exposure, a compensation phase took place. HVA returns to its initial basal value, and levels of DOPAC and DA returned towards normal but never reached their initial values. These results contrast with those observed during the High Pressure Neurological Syndrome (HPNS, 5.1 MPa of helium pressure) in which there is a significant increase in extracellular DA. Therefore, some of the symptoms of nitrogen narcosis may be linked with the decrease in the extracellular DA levels.