Osmosensitive response of glutamate in the substantia nigra

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.

Striatal glutamate degenerates thalamic neurons

Glutamate (GLU)-induced excitotoxicity is considered to be a frequent cause of cell degeneration in basal ganglia disorders; it is normally prevented by uptake of GLU by astrocytes. We recently found that transient perfusion of GLU in the striatum induces persistent accumulation of GLU in striatal astrocytes that could be from the initial administration or caused by the slow release from neurons or astrocytes in response to it. Endogenous production of GLU, that is, "self-induced GLU accumulation" (SIGA), may occur under physiological and pathological conditions. Here we studied the possible induction of SIGA after injury induced by perfusion of GLU receptor agonists into the striatum of rats. The agonists induced local degeneration in neurons and myelinated axons and microgliosis and astrocytosis; there was also gliosis and remote degeneration of neurons in the ventral-posterior complex of the thalamus that project to the cerebral cortex across the striatum. Reactive astrocytes showed persistent GLU accumulation in the striatum (local SIGA) and thalamus (remote SIGA) that persisted for at least 6 weeks after the injury. Thus, SIGA can be induced by neuronal degeneration retrogradely triggered from a remote brain region after excessive release of endogenous GLU from astrocytes. This may be an additional factor to be considered in basal ganglia disorders with glutamatergic excitotoxicity.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Self-induced accumulation of glutamate in striatal astrocytes and basal ganglia excitotoxicity

Excitotoxicity induced by high levels of extracellular glutamate (GLU) has been proposed as a cause of cell degeneration in basal ganglia disorders. This phenomenon is normally prevented by the astrocytic GLU-uptake and the GLU-catabolization to less dangerous molecules. However, high-GLU can induce reactive gliosis which could change the neuroprotective role of astrocytes. The striatal astrocyte response to high GLU was studied here in an in vivo rat preparation. The transient striatal perfusion of GLU (1 h) by reverse microdialysis induced complex reactive gliosis which persisted for weeks and which was different for radial-like glia, protoplasmic astrocytes and fibrous astrocytes. This gliosis was accompanied by a persistent cytosolic accumulation of GLU (immunofluorescence quantified by confocal microscope), which persisted for weeks (self-induced glutamate accumulation), and which was associated to a selective decrease of glutamine synthetase activity. This massive and persistent self-induced glutamate accumulation in striatal astrocytes could be an additional factor for the GLU-induced excitotoxicity, which has been implicated in the progression of different basal ganglia disorders.