In Situ-Produced 7-Chlorokynurenate Has Different Effects on Evoked Responses in Rats with Limbic Epilepsy in Comparison to Naive Controls

Astroglial d-serine is the endogenous co-agonist at the presynaptic NMDA receptor in rat entorhinal cortex

Presynaptic NMDA receptors facilitate the release of glutamate at excitatory cortical synapses and are involved in regulation of synaptic dynamics and plasticity. At synapses in the entorhinal cortex these receptors are tonically activated and provide a positive feedback modulation of the level of background excitation. NMDA receptor activation requires obligatory occupation of a co-agonist binding site, and in the present investigation we have examined whether this site on the presynaptic receptor is activated by endogenous glycine or d-serine. We used whole-cell patch clamp recordings of spontaneous AMPA receptor-mediated synaptic currents from rat entorhinal cortex neurones in vitro as a monitor of presynaptic glutamate release. Addition of exogenous glycine or d-serine had minimal effects on spontaneous release, suggesting that the co-agonist site was endogenously activated and likely to be saturated in our slices. This was supported by the observation that a co-agonist site antagonist reduced the frequency of spontaneous currents. Depletion of endogenous glycine by enzymatic breakdown with a bacterial glycine oxidase had little effect on glutamate release, whereas d-serine depletion with a yeast d-amino acid oxidase significantly reduced glutamate release, suggesting that d-serine is the endogenous agonist. Finally, the effects of d-serine depletion were mimicked by compromising astroglial cell function, and this was rescued by exogenous d-serine, indicating that astroglial cells are the provider of the d-serine that tonically activates the presynaptic NMDA receptor. We discuss the significance of these observations for the aetiology of epilepsy and possible targeting of the presynaptic NMDA receptor in anticonvulsant therapy.

α-methyl-L-tryptophan: mechanisms for tracer localization of epileptogenic brain regions

The purpose of this paper is to discuss the mechanisms of α-[(11)C]methyl-L-tryptophan (AMT) PET as an in vivo biomarker for detection of epileptogenic cortex. AMT was originally designed as a tracer to measure the serotonin synthesis rate. This tracer was first applied in patients with medically refractory epilepsy in an attempt to detect changes in serotonin synthesis based upon reports of increased serotonergic innervation in cortical specimens obtained following epilepsy surgery. The first group of epilepsy patients undergoing AMT PET scans were patients with tuberous sclerosis complex. Studies of brain tissue subsequent to epilepsy surgery in these patients with tuberous sclerosis complex implicated the kynurenine pathway of tryptophan metabolism as a primary mechanism of increased brain tissue retention of AMT in epileptogenic brain regions, rather than alterations in serotonin synthesis. Kinetic analyses of AMT in brain tumors indicate changes in tryptophan transport and tissue retention in other pools as well. These studies indicate that AMT PET may be a biomarker of immune activation in the epileptogenic process.

Clinical and histopathologic correlates of 11C-alpha-methyl-L-tryptophan (AMT) PET abnormalities in children with intractable epilepsy

PURPOSE

Interictal increase of (11) C-alpha-methyl-l-tryptophan (AMT) on positron emission tomography (PET) can be seen in cortical epileptic foci, and is particularly common in cortical developmental malformations. Therefore, in the present study, we evaluated the clinical and histopathologic correlates of AMT-PET abnormalities in children with intractable epilepsy undergoing resective surgery.

METHODS

Thirty children (mean age: 6.7 ± 3.2 years) were included in this study. All patients received AMT-PET as part of their presurgical evaluation and subsequently underwent epilepsy surgery. Magnetic resonance imaging (MRI) scans were normal in 15, showed nonspecific changes in 8, and suggested malformations of cortical development (MCDs) in nine children. Asymmetry indices (AIs) were calculated to determine increased AMT uptake.

KEY FINDINGS

Histopathology revealed MCDs in 16 (53%) children, including 12 with cortical dysplasia (CD) [mild MCD = 3; CD type IA = 2; CD type IIA = 2 and CD type IIB (severe CD with balloon cells) = 5]. Polymicrogyria and heterotopias (P&Hs) were seen in three cases and subependymal heterotopias (SEHs) in one child. The remaining 14 cases showed normal histopathology with varying degrees of gliosis. Increased AMT uptake was found in all five with CD type IIB, and all three with P&H, but in none with mild MCD and types IA-IIA CD or SEH. Whereas all five children with CD IIB and two with P&H had excellent surgical outcome (class I); children with milder CD or SEH had variable surgical outcome. The 14 patients with normal histopathology included seven patients with focally increased and seven with normal AMT uptake. Although patients with normal pathology and normal AMT-PET had better surgical outcome (class I = 5; II = 2), those with normal pathology, normal MRI, but abnormal AMT-PET had poor surgical outcome (class III = 4; IV = 3).

SIGNIFICANCE

Increased AMT uptake in children with CD may predict type IIB dysplasia (with balloon cells) and good surgical outcome. Histopathologic similarities between CD type IIB and epileptogenic cortical tubers may imply a common role of the inflammatory kynurenine pathway of tryptophan metabolism in these lesions. In children with normal histopathology, there is a subgroup with increased AMT uptake and poor surgical outcome.

Activation of rat ventral tegmental area dopamine neurons by endogenous kynurenic acid: a pharmacological analysis

Kynurenic acid (KYNA) is an endogenous NMDA receptor antagonist as well as a blocker of the alpha7* nicotinic receptor and mounting evidence suggests that the compound participates in the pathophysiology of schizophrenia. Previous studies have shown that elevated levels of endogenous KYNA are associated with an increased firing of midbrain dopamine (DA) neurons. In the present study, utilizing extracellular single unit cell recording techniques, the mechanism involved in this excitatory action of the compound was analyzed in male Sprague-Dawley rats. Administration of 4-chlorokynurenine (4-Cl-KYN; 25mg/kg, i.p.), which is converted to the selective NMDA glycine-site antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), was found to increase firing rate and per cent burst firing activity of ventral tegmental area (VTA) DA neurons to the same magnitude as pretreatment of kynurenine (causing a 25-fold elevation in extracellular brain KYNA). Intravenous administration of the selective antagonist at the alpha7* nicotinic receptor methyllycaconitine (MLA; 1-4mg/kg) did not affect firing of VTA DA neurons, whereas intraperitoneal administration of this drug in a high dose (6mg/kg) was associated with a decreased firing rate and per cent burst firing activity. Administration of SDZ 220-581 (10mg/kg, i.v.), a competitive antagonist at the glutamate recognition-site of the NMDA receptor, was found to increase firing rate and per cent burst firing. Present results have potential implications for the treatment of schizophrenia, and indicate that the increased activity of VTA DA neurons following elevation of brain KYNA is mediated through glutamatergic rather than by nicotinergic mechanisms.