The co-agonist concept: is the NMDA-associated glycine receptor saturated in vivo?

High extracellular glycine does not potentiate N-methyl-D-aspartate-evoked depolarization in vivo

As N-methyl-D-aspartate receptor (NMDA) ionophore complexes have a distinct positive, allosteric regulatory site for glycine, it has been proposed that elevated extracellular glycine during or after cerebral ischaemia may induce excessive NMDA/glutamate receptor activation and, thereby, excitotoxicity. To test this hypothesis, we have perfused increasing concentrations of glycine, either alone or with co-application of NMDA, through a microdialysis probe implanted in the striatum of halothane anaesthetized rats. Changes in the extracellular field (DC) potential indicative of depolarization were recorded precisely at the site of drug application by an electrode incorporated within dialysis fibre. Microdialysis application of up to 1 mM of glycine had no effect on the basal DC potential. Above 10 mM, glycine produced concentration-dependent depolarizations, but the amplitude of these responses remained very small (e.g. 0.52 +/- 0.05 mV for 100 mM glycine, n = 10, i.e. around 30-fold smaller than that of a wave of spreading depression). Application of 200 microM NMDA via the microdialysis probe produced consistent short-lasting depolarizations (around 2.5 mV amplitude), but these were not potentiated by co-application of up to 100 mM glycine. These data do not support the view that increased extracellular concentrations of glycine, such as those observed in ischaemia, may be potentially excitotoxic. Nevertheless, as occupation of the glycine site coupled to the NMDA-receptor is required for NMDA/glutamate receptor activation, this site remains an attractive target for potential neuroprotective agents.

Familial and developmental abnormalities of front lobe function and neurochemistry in schizophrenia

structural abnormalities of the cerebral cortex in schizophrenia have been revealed by magnetic resonance imaging, although it is not clear whether these abnormalities are diffuse or local. We predicted that changes in cortical structure would result in abnormalities in biochemical markers for the glutamate system in post-mortem brain, and that the pattern of neurochemical abnormalities would be a clue to the distribution and extent of pathology. A number of studies have now reported increases in biochemical and other markers of glutamatergic cell bodies and terminals in the frontal cortex in schizophrenia. These findings are consistent with the presence of an abnormally abundant glutamatergic innervation, which may be due to an arrest in the normal developmental process of synaptic elimination. In the anterior temporal cortex and hippocampus there is evidence of an asymmetric loss of glutamate terminals, and of reduced GABA function, which may be secondary to the glutamatergic deficit. Glutamate cell body markers are spared in the temporal lobe; we argue that the loss of glutamate uptake sites may reflect the loss of an extrinsic glutamatergic innervation of the polar temporal cortex which arises from the frontal cortex. These fronto-temporal projections may be vulnerable because they arise from a cytoarchitecture which has not been stabilized by remodelling during early post-natal life. There have been several therapeutic studies of drugs with actions on brain glutamate systems. Based on the glutamate deficiency theories, one approach has been to enhance glutamatergic function using agonists of the N-methyl-D-aspartate-linked glycine site. However, there are no clear therapeutic effects, and some studies report aggravation of positive symptoms. This might be expected if, as part of our post-mortem studies suggested, there is excess glutamatergic innervation in some brain regions in schizophrenia. There is neuropsychological evidence that frontal abnormalities in schizophrenia may be genetically determined. We found that first degree relatives of schizophrenic patients were selectively impaired in tests of frontal lobe function, whereas both frontal and temporal function is impaired in patients We conclude that the genetic predisposition to schizophrenia involves impaired frontal lobe function. Psychotic symptoms develop only when a second process results in a loss of fronto-temporal projections and leads to temporal lobe dysfunction.

Chronic, post-injury administration of D-cycloserine, an NMDA partial agonist, enhances cognitive performance following experimental brain injury

The purpose of this study was to determine the effect of augmenting NMDA receptor activation on cognitive deficits produced by traumatic brain injury (TBI). Specifically, D-cycloserine (DCS), a partial agonist of the NMDA-associated glycine site, was tested as a potential cognitive enhancer. Rats were injured using lateral fluid percussion TBI (2.8 +/- .10 atm). On days 1-15 post-injury, animals were injected (i.p.) with vehicle (n = 8), 10 mg/kg (n = 9), or 30 mg/kg (n = 8) of DCS. Sham-injured animals treated with either vehicle (n = 8) or 30 mg/kg of DCS (n = 8) were used for comparison. On days 11-15 post-injury, cognitive function was assessed using the Morris water maze (MWM). Results indicate that the 30 mg/kg dose of DCS significantly attenuated memory deficits as compared to injured vehicle-treated animals (P < 0.01). Analysis also revealed that performance of the injured-DCS (30 mg/kg) group was not significantly different from sham-injured animals treated with vehicle (P > 0.10). In contrast, the 10 mg/kg dose of DCS was ineffective in reducing injury-induced memory deficits. DCS (30 mg/kg) also significantly improved the spatial memory of sham-injured animals when compared with sham-injured animals treated with vehicle (P < 0.05). In conclusion, chronic, post-injury enhancement of the NMDA receptor is an effective strategy for ameliorating TBI-associated cognitive deficits.

The metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionic acid inhibits phosphoserine phosphatase

Phosphoserine phosphatase catalyzes the final step in the major pathway of L-serine biosynthesis in brain. Using D-phosphoserine as substrate, the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionic acid (L-AP3) inhibits phosphoserine phosphatase partially purified from rat brain with a Ki of 151 microM. In contrast to AP3 enantioselectivity at metabotropic receptors, D-AP3 (Ki 48 microM) is more potent as an inhibitor of phosphoserine phosphatase than L-AP3, whereas DL-AP3 has intermediate potency. D-, L-, and DL-AP3 are 6- to 8-fold more potent inhibitors using D-phosphoserine rather than L-phosphoserine as substrate, suggesting that AP3 may have selectivity for isoforms of phosphoserine phosphatase which preferentially cleave D-phosphoserine. D-AP3 decreases the apparent affinity of D- and L-phosphoserine with little or no change in maximal velocity indicating that it is a competitive inhibitor of the enzyme. Whereas L-AP3 has similar potency at metabotropic glutamate receptors and phosphoserine phosphatase, D-AP3 is selective for phosphoserine phosphatase and is the most potent and only known competitive inhibitor of this enzyme.