Light microscopic comparison of the patterns of glutamate-like and homocysteate-like immunoreactivities in rat dorsal lateral geniculate after combined visual cortical and retinal ablations

Glutamate-induced homocysteic acid release from astrocytes: possible implication in glia-neuron signaling

Glial cells synthesise neuroactive substances and release them upon neurotransmitter receptor activation. Homocysteic acid (HCA), an endogenous agonist for glutamatergic N-methyl-D-aspartate (NMDA) receptors, is predominantly localised in glial cells. We have previously demonstrated the release of HCA from mouse astrocytes in culture following activation of beta-adrenergic receptors. Moreover, a release of HCA has also been observed in vivo upon physiological stimulation of sensory afferents in the thalamus. Here we report the glutamate-induced release of HCA from astrocytes. The effect of glutamate was mediated by the activation of ionotropic (NMDA and non-NMDA) as well as by metabotropic receptors. In addition, the release of HCA was Ca(2+)- and Na(+)-dependent, and its mechanism involved the activation of the Na+/Ca(2+)-exchanger. Furthermore, we provide evidence for the presence of functional NMDA receptors on astrocytes, which are coupled to an intracellular Ca2+ increase via stimulation of the Na+/Ca(2+)-exchanger. Our data thus favour a participation of glial cells in excitatory neurotransmission and corroborate the role of HCA as a "gliotransmitter."

Local gamma-aminobutyric acid and glutamate circuit control of hypophyseotrophic corticotropin-releasing factor neuron activity in the paraventricular nucleus of the hypothalamus

Paraventricular corticotropin-releasing factor (CRF) neurons play a pivotal role in regulating neuroendocrine responses to stress. The mechanisms by which synaptic inputs control the activity of these neurons are not well understood. The present study was undertaken to determine the role of the intrinsic gamma-aminobutyric acid (GABA)- and glutamatergic neural circuits of the hypothalamic paraventricular nucleus (PVN) in the control of CRF neural activity. We show that in organotypic cultures of the PVN, blockade of the intrinsic GABAergic neurotransmission by the GABAA receptor antagonist bicuculline resulted in a significant increase in CRF secretion. The bicuculline-induced CRF secretory activity was abolished by the coadministration of the selective alpha-amino-3-hydroxy-5-methyl-4-isoxazoleprionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Electrical stimulation of the CRF cell division elicited glutamatergic extracellular field potentials that were dramatically enhanced by bicuculline and were suppressed by CNQX. These results show that the functional activity of CRF neurons in organotypic cultures of the PVN is under a tonic inhibitory influence of an intrinsic GABAergic circuit. Suppression of GABAergic transmission appears to have a permissive role for inducing an increased secretory activity of CRF neurons that is driven by an excitatory glutamatergic network via AMPA/kainate receptors.

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

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 beta-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.

Astrocytes, spontaneity, and the developing thalamus

Recent studies in the ventrobasal (VB) thalamus have shown that astrocytes display spontaneous intracellular calcium [Ca(2+)](i) oscillations early postnatally. [Ca(2+)](i) oscillations are correlated in groups of up to five astrocytes, and propagate between cells. NMDA receptor-mediated, long lasting inward currents in thalamocortical (TC) neurons of the VB complex are correlated to [Ca(2+)](i) increases in neighbouring astrocytes, and stimulation of astrocytic [Ca(2+)](i) increases also lead to inward currents in neurons. These findings suggest that astrocytes are spontaneously active and can induce neuronal activity, a reversal of the previously held view of neuron-glia interactions in the central nervous system. This activity occurs at an important period in the development of the thalamus and therefore suggests a potential functional role in a variety of processes. Along with data on the neurotransmitter receptor repertoire of thalamic astrocytes these findings enlarge the body of knowledge on astrocytes in the thalamus, and further contribute to the emerging field of astrocyte-neuron and neuron-astrocyte interactions in the central nervous system.