Robert Schwarcz

Lesion of striatal neurons with kainic acid provides a model for Huntington's chorea

THE symptoms of Huntingtons chorea, an hereditary movement disorder, result from degeneration of neurones primarily in the basal ganglia1. Several neurochemical abnormalities have been identified in the brains of patients dying with this disorder2–5, but no animal system with similar neuropathological changes has been described. We now report that the injection of kainic acid into the rat striatum causes neuronal degeneration, neurochemical alterations and behavioural responses resembling Huntingtons chorea. This procedure could provide an animal model for the study of the disease.

Kynurenines in the mammalian brain: when physiology meets pathology

The essential amino acid tryptophan is not only a precursor of serotonin but is also degraded to several other neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine and quinolinic acid. The synthesis of these metabolites is regulated by an enzymatic cascade, known as the kynurenine pathway, that is tightly controlled by the immune system. Dysregulation of this pathway, resulting in hyper- or hypofunction of active metabolites, is associated with neurodegenerative and other neurological disorders, as well as with psychiatric diseases such as depression and schizophrenia. With recently developed pharmacological agents, it is now possible to restore metabolic equilibrium and envisage novel therapeutic interventions.

Striatal lesions with kainic acid: neurochemical characteristics

Stereotaxic injection of 2.5 microng of kainic acid, a rigid analogue of glutamate into the rat striatum caused a 70% reduction in the striatum of the cholinergic parameters, choline acetyltransferase, acetylcholine and synaptosomal uptake of choline and a similar reduction in the GABAergic parameters, glutamic acid decarboxylase, psi-aminobutyric acid (GABA) and synaptosomal uptake of GABA. In contrast, the striatal content of dopamine and the synaptosomal uptake of dopamine were unchanged, and the activity of tyrosine hydroxylase was significantly increased. Significant changes in the activity of neurotransmitter synthesizing enzymes were demonstrable within 6h after injection of 2.5 microng of kainic acid and maximal effects occurred at 48h; the activities of choline acetyltransferase and glutamic acid decarboxylase remained depressed up to 21 days after injection. The kinetic characteristics of striatal tyrosine hydroxylase were altered 48h after injection with a two-fold increase in the Vmax for tyrosine and a three-fold reduction in Km for the pteridine cofactor. In contrast to the effects of kainic acid, the injection of copper sulfate, a non-specific toxin, caused a proportionate reduction in the dopaminergic as well as the cholinergic and GABAergic presynaptic markers. The kainate lesion caused an 85% decrement in the activity of dopamine-sensitive adenylate cyclase, a 40% reduction in the specific binding of [3H]quinuclidinyl benzilate and a 195% increase in the specific binding of [3H]GABA in the striatum. The morphology of the kainate injected striatum was markedly altered with nearly a complete loss of intrinsic neurons, increased number of glial cells but intact internal capsule fibers. Intracerebral injection of nanomolar quantities of kainic acid appears to cause degeneration of neurons with cell bodies near the injection site while sparing axons terminating in or passing through the region.

II. Excitotoxic models for neurodegenerative disorders

Abstract In recent years, considerable interest has been shown in the neurotoxic properties of excitatory amino acids and their possible relevance for the study of human neurodegenerative disorders. The term “excitotoxin” has been coined for a family of acidic amino acids which are neuroexcitants and produce a characteristic type of “axon-sparing” neuronal lesion. Intracerebral infusions of kainic and ibotenic acids, the two most commonly used excitotoxins, result in a morphological and biochemical picture in experimental animals which resembles that observed in the brains of Huntingtons disease and epilepsy victims. The emergence of such animal models for neurodegenerative disorders has led to the hypothesis that endogenous excitotoxins may exist which are linked to the pathogenesis of human diseases. The most promising candidate discovered so far is quinolinic acid, a hepatic tryptophan metabolite which has recently also been found to occur in brain tissue. The particular excitotoxic properties of quinolinic acid warrant a thorough investigation of its metabolic and synaptic disposition in normal and abnormal brain function. While little is known about the mechanisms by which excitotoxins cause selective neuronal death, most current speculations propose the participation of specific synaptic receptors for acidic amino acids. The recent development of selective antagonists of such receptors has aided in the elucidation of excitotoxic mechanisms. Although a biochemical link between endogenous excitotoxins and human neurodegenerative disorders remains elusive at present, pharmacological blockade of excitotoxicity may constitute a novel therapeutic strategy for the treatment of these disease states.

Ibotenic acid-induced neuronal degeneration: A morphological and neurochemical study

SummaryPossible neurotoxic actions of intracerebral injections of ibotenic acid, a conformationally restricted analogue of glutamic acid, have been evaluated in rat brain and compared with those of kainic acid.Light microscopical analysis revealed that ibotenic acid produced a marked disappearance of nerve cells in all areas studied, namely striatum, the hippocampal formation, substantia nigra and piriform cortex. Lesions in areas distant to the injection site were not seen. Axons of passage and nerve terminals of extrinsic origin did not seem to be damaged, since, e.g., no apparent degeneration of the dopaminergic terminals in the neostriatum was observed except for a small area surrounding the cannula. In the neostriatum, enkephalin immunoreactive neuronal cell bodies as well as nerve terminals disappeared after injection of ibotenic acid into this nucleus. After injection into the substantia nigra tyrosine hydroxylase immunoreactive cell bodies in the zona compacta disappeared, whereas no certain effect could be seen on the enkephalin immunoreactive nerve fibers.In vitro experiments, conducted with striatal synaptosomal and membrane preparations, showed that ibotenic acid differed from kainic acid by being devoid of a significant inhibitory effect on high affinity glutamate uptake and by having a low affinity for 3H-kainic acid binding sites. Furthermore, ibotenic acid did not interfere with the binding of a number of radioligands for other transmitter receptors.As compared to kainic acid, ibotenic acid has the advantage of being less toxic to the animals and of producing more discrete lesions, possibly due to faster metabolism and/or other fundamental biochemical differences. Because of these special features, ibotenic acid seems to represent a valuable new tool in the morphological and functional analysis of central neuronal systems.

Increased cortical kynurenate content in schizophrenia.

BACKGROUND Metabolites of the kynurenine pathway of tryptophan degradation may play a role in the pathogenesis of several human brain diseases. One of the key metabolites in this pathway, kynurenine, is either transaminated to form the glutamate receptor antagonist, kynurenate, or hydroxylated to 3-hydroxykynurenine, which in turn is further degraded to the excitotoxic N-methyl-D-aspartate receptor agonist quinolinate. Because a hypoglutamatergic tone may be involved in the pathophysiology of schizophrenia, it is conceivable that alterations in kynurenine pathway metabolism may play a role in the disease. METHODS The tissue levels of kynurenine, kynurenate, and 3-hydroxykynurenine were measured in brain tissue specimens obtained from the Maryland Brain Collection. All three metabolites were determined in the same samples from three cortical brain regions (Brodmann areas 9, 10, and 19), obtained from 30 schizophrenic and 31 matched control subjects. RESULTS Kynurenate levels were significantly increased in schizophrenic cases in Brodmann area 9 (2.9 +/- 2.2 vs. 1.9 +/- 1.3 pmol/mg protein, p <.05), but not in Brodmann areas 10 and 19. Kynurenine levels were elevated in schizophrenic cases in Brodmann areas 9 (35.2 +/- 28.0 vs. 22.4 +/- 14.3 pmol/mg protein; p <.05) and 19 (40.3 +/- 23.4 vs. 30.9 +/- 10.8; p <.05). No significant differences in 3-hydroxykynurenine content were observed between the two groups. In both groups, significant (p <.05) correlations were found in all three brain areas between kynurenine and kynurenate, but not between kynurenine and 3-hydroxykynurenine (p >.05). In rats, chronic (6-months) treatment with haloperidol did not cause an increase in kynurenate levels in the frontal cortex, indicating that the elevation observed in schizophrenia is not due to antipsychotic medication. CONCLUSIONS The data demonstrate an impairment of brain kynurenine pathway metabolism in schizophrenia, resulting in elevated kynurenate levels and suggesting a possible concomitant reduction in glutamate receptor function.

Blood–Brain Barrier Transport of Kynurenines: Implications for Brain Synthesis and Metabolism

Abstract: To evaluate the potential contribution of circulating kynurenines to brain kynurenine pools, the rates of cerebral uptake and mechanisms of blood–brain barrier transport were determined for several kynurenine metabolites of tryptophan, including L‐kynurenine (L‐KYN), 3‐hydroxykynurenine (3‐HKYN), 3‐hydroxyanthranilic acid (3‐HANA), anthranilic acid (ANA), kynurenic acid (KYNA), and quinolinic acid (QUIN), in pentobarbital‐anesthetized rats using an in situ brain perfusion technique. L‐KYN was found to be taken up into brain at a significant rate [permeability—surface area product (PA) = 2–3 × 10−3 ml/s/g] by the large neutral amino acid carrier (L‐system) of the blood–brain barrier. Best‐fit estimates of the Vmax and Km of saturable L‐KYN transfer equalled 4.5 × 10−4μmol/s/g and 0.16 μmol/ml, respectively. The same carrier may also mediate the brain uptake of 3‐HKYN as D,L‐3‐HKYN competitively inhibited the brain transfer of the large neutral amino acid L‐leucine. For the other metabolites, uptake appeared mediated by passive diffusion. This occurred at a significant rate for ANA (PA, 0.7–1.6 × 10−3 ml/s/g), and at far lower rates (PA, 2–7 × 10−5 ml/s/g) for 3‐HANA, KYNA, and QUIN. Transfer for KYNA, 3‐HANA, and ANA also appeared to be limited by plasma protein binding. The results demonstrate the saturable transfer of L‐KYN across the blood–brain barrier and suggest that circulating L‐KYN, 3‐HKYN, and ANA may each contribute significantly to respective cerebral pools. In contrast, QUIN, KYNA, and 3‐HANA cross the blood–brain barrier poorly, and therefore are not expected to contribute significantly to brain pools under normal conditions.

Dopamine receptors localised on cerebral cortical afferents to rat corpus striatum.

STRIATAL dopamine receptors, monitored by dopamine-sensitive adenylate cyclase activity1,2 or binding of 3H-haloperidol3–5 apparently represent distinct entities because of differences in drug sensitivity and the pattern of their ontogenetic development6,7. Dopamine also elicits both excitatory and inhibitory effects on striatal neurones8,9. We have examined the effects of selective degeneration of striatal intrinsic neurones with the neurotoxin. kainic acid10–12, and elimination of cortico-striate afferents by cortical ablation13 on the dopamine receptors in rat striatum. We present evidence that a substantial portion of striatal 3H-haloperidol receptor sites are localised to axons of cerebral cortical afferents whereas dopamine-sensitive adenylate cyclase is confined to neurones intrinsic to the striatum.

Kynurenic acid blocks neurotoxicity and seizures induced in rats by the related brain metabolite quinolinic acid

Kynurenic acid (KYNA) was tested as an antagonist of the neurotoxic and epileptogenic effects of the metabolically related brain constituent quinolinic acid (QUIN). In the rat striatum, KYNA blocked the neurotoxic effects of QUIN in preference to those of other excitotoxins. In the hippocampus, KYNA antagonized both the neurodegeneration and seizures caused by the local application of QUIN. These properties of KYNA raise the possibility of a functional link between KYNA and QUIN in the brain which may be of relevance for an understanding of human neurodegenerative disorders.

Preferential neuronal loss in layer III of the entorhinal cortex in patients with temporal lobe epilepsy

We report a characteristic pattern of neuropathological change in the entorhinal cortex (EC) from four patients with temporal lobe epilepsy. Specimens of the EC were obtained during the surgical treatment of intractable partial seizures and were studied by light microscopy in Nissl-stained sections. A distinct loss of neurons was observed in the anterior portion of the medial EC in the absence of apparent damage to temporal neocortical gyri. Cell loss was most pronounced in layer III, but also noticed in layer II, particularly in the rostral field. A similar pattern of neurodegeneration in the EC was found in all specimens examined though the degree of neuronal loss varied between cases. These observations provide neuropathological evidence for an involvement of the EC in temporal lobe epilepsy. Since the EC occupies a pivotal position in gating hippocampal input and output, our results further support previous suggestions that dysfunction of this region may contribute, either independently or in concert with Ammons horn sclerosis, to epileptogenesis in humans.