Release of homocysteic acid from rat thalamus following stimulation of somatosensory afferents in vivo: feasibility of glial participation in synaptic transmission

doi:10.1016/j.neuroscience.2003.08.068

Neuroscience

Volume 124, Issue 2, 2004, Pages 387-393

https://doi.org/10.1016/j.neuroscience.2003.08.068 Get rights and content

Abstract

The sulphur-containing amino acid homocysteic acid (HCA) is present in and released in vitro from nervous tissue and is a potent neuronal excitant, predominantly activating N-methyl-d-aspartate (NMDA) receptors. However, HCA is localised not in neurones but in glial cells [Eur J Neurosci 3 (1991) 1370], and we have shown that it is released from astrocytes in culture upon glutamate receptor activation [Neuroscience 124 (2004) 377]. We now report the in vivo release of HCA from ventrobasal (VB) thalamus following natural stimulation of somatosensory afferents arising from the facial vibrissae of the rat. Simultaneously with multi-unit recording, [³⁵S]-methionine, a HCA precursor, was perfused through a push-pull cannula in VB thalamus of anaesthetized rats. Perfusates were collected before, during and after 4 min stimulation of the vibrissal afferents with an air jet. A marked release of radiolabeled HCA was observed during and after the stimulation. Furthermore, the β-adrenoreceptor agonist isoproterenol, which is known to evoke HCA release from glia in vitro, was found to increase the efflux of HCA in the perfusate in vivo. In separate experiments, the excitatory actions of iontophoretically applied HCA on VB neurones were inhibited by the NMDA receptor antagonist CPP, but not by the non-NMDA antagonist CNQX. These results suggest a possible “gliotransmitter” role for HCA in VB thalamus. The release of HCA from glia might exert a direct response or modulate responses to other neurotransmitters in postsynaptic neurons, thus enhancing excitatory processes.

Section snippets

Push–pull perfusion

All experiments were carried out in adult male Wistar rats which were deeply anaesthetised with urethane (1.2 g/kg, i.p.) and surgically prepared for recording as detailed previously (Salt, 1987). All animal experiments were carried out in accordance with the U.K. Animals (Scientific Procedures) Act, 1986 and international guidelines on the ethical use of animals. Efforts have been made to minimize the number of animals used and their suffering. The two channel push–pull cannulae were inserted

Release of HCA from the VB thalamus

Endogenous amino acids were detected in the collected material through the use of reversed phase HPLC combined with fluorescence detection. Twenty-three amino acids including the sulphur containing analogues, HCA, cysteic acid and cysteine sulphinic acid, were resolved (Do et al., 1997). By coupling the flow scintillation analyser to the HPLC, it was possible to analyse simultaneously [³⁵S]-labelled compounds with the endogenous amino acids. In fractions collected before the stimulation,

Discussion

The present paper describes the release of HCA from rat VB thalamus in vivo. This release is significantly increased during and after sensory afferent stimulation. As discussed in the companion paper (Benz et al., 2004) and in Do et al. (1997), we were not able to detect endogenous HCA in perfusates, as its concentration was below the detection level using the fluorescence detection method. However, using pre-incubation with the radiolabelled HCA precursor, [³⁵S]-methionine (Do et al., 1988, Do

Acknowledgements

We thank Prof. Michel Cuénod for helpful discussions and constant support. This work was supported by grants from the Swiss National foundation to K. Q. Do (31-55924.98) and the Human Frontier Science Programme to T. E. Salt (RG95/2).

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Release of homocysteic acid from rat thalamus following stimulation of somatosensory afferents in vivo: feasibility of glial participation in synaptic transmission

Abstract

Section snippets

Push–pull perfusion

Release of HCA from the VB thalamus

Discussion

Acknowledgements

Neuroscience

Neuron

Neuroscience

Neuroscience

Neurosci Res

Neurosci Lett

Brain Res

Neurochem Int

Neuroscience

Neurosci Lett

Brain Res Bull

Neurosci Lett

Prog Neurobiol

Prog Neurobiol

Neurosci Lett

Brain Res

Immunohistochemical localization of homocysteate in human primary visual cortex

Neuroreport