A competitive inhibitor of kainic acid binding from the goldfish nervous tissue

Absence of neurotoxic effects in leopard sharks, Triakis semifasciata, following domoic acid exposure

Domoic acid (DA), a potent neurotoxin produced by select species of algae and diatoms, kills neurons bearing kainic acid-type glutamate receptors. Studies have shown that DA bioaccumulates in invertebrates and fish that consume the diatoms. In every vertebrate species tested or observed in the wild, dietary or systemic DA causes neuronal damage or clinical signs of neurotoxicity. Sharks, like marine birds and mammals, are exposed to DA through their diet; however, no research has demonstrated the effect of DA on shark behavior or physiology. In this study, juvenile leopard sharks (Triakis semifasciata) were given DA by intracoelomic injection at doses of 0, 1, 3, 9, and 27 mg/kg and observed for 7 days. The sharks failed to demonstrate behavioral or histological changes in response to the toxin. We identified putative brain glutamate receptors by probing western blots with an antibody specific for kainic acid-type glutamate receptors and demonstrated receptor localization in the cerebellum with immunohistochemistry. Blood levels of DA in three sharks dosed at 9 mg/kg fell rapidly within 1.5h of injection. We show that leopard sharks possess the molecular target for DA but are resistant to doses of DA known to be toxic to other vertebrates.

6,7-Dinitroquinoxaline-2,3-dione but not MK-801 exerts a protective effect against kainic acid neurotoxicity in the goldfish retina

Recent findings indicated that the excitotoxicity of glutamate analogues was prevented in the mammalian nervous system by N-methyl-D-aspartate (NMDA) antagonists. The neurodegenerative effects of kainic acid, and the putative protection of MK-801 and 6,7-dinitroquinoxaline-2,3-dione (DNQX), were investigated by morphological studies showing the toxicity of kainic acid to the neurons of the inner nuclear layer, and measuring choline acetyltransferase and glutamate decarboxylase activities in the retina. In addition, the proliferation of Müller retinal cells was assumed as an index of neuronal degeneration and was quantified by counting glial fibrillary acidic protein immunopositive cells. Our observations suggest that the non-NMDA receptor antagonist DNQX exerted a protective effect on goldfish retinal neurons, while MK-801 did not prevent the neurotoxicity induced by kainic acid in the goldfish retina. This finding is in agreement with previous work on kainic acid toxicity in the goldfish optic tectum.

An endogenous ligand for the kainate-type binding sites from rat brain

Extracts from the rat brain were screened to identify a putative endogenous ligand for the binding sites of the neuroexcitant kainic acid (KA). The extracted substances were separated by chromatographic techniques and tested for their ability to inhibit KA binding to fish synaptosomes and to membranes from rat brain. A substance isolated in this way (rat kainate-binding inhibitor, RKBI) display a competitive interaction with KA for the low-affinity binding sites in rat brain membranes. According to the separation behavior in the purification step, RKBI is distinct from an inhibitor formerly isolated from fish nervous tissue (KBI). The substance exhibits positive co-operativity with KA for a very-low-affinity site population, particularly concentrated in the cerebellum, and could play a physiological role in this area.

Characterization of L-glutamate and kainate binding sites in the brain of a freshwater fish, Telapilia monsanbica

[3H]Kainate and L-[3H]glutamate binding sites in a rich source of kainate binding sites, fish brain, have been thoroughly analysed here for the purpose of studying the correlation between kainate binding sites and L-glutamate receptors in vertebrate CNS. The brain of a freshwater fish, Telapilia monsanbica, was found to contain three types of kainate binding sites: Type 1 sites (Kd = 1050 +/- 380 microM, Bmax = 4 +/- 4 pmol/mg), Type 2 sites (Kd = 133 +/- 20 nM, Bmax = 190 +/- 20 pmol/mg), and Type 3 sites (Kd = 23 +/- 15 nM, Bmax = 28 +/- 19 pmol/mg). The dissociation constants of L-glutamate to Type 1, 2 and 3 sites were, respectively, 0.28 +/- 0.04, 5.5 +/- 0.2 and 137 +/- 28 microM. Pharmacological characterization of these binding sites showed that Type 1 and 2 sites, respectively, corresponded to N-methyl-D-aspartate-subtype L-glutamate receptors and non-N-methyl-D-aspartate L-glutamate receptors. Autoradiographic studies showed that Type 1 and 2 sites were distributed widely in fish brain, indicating the involvement of L-glutamate receptors in various brain functions. Type 3 sites, on the other hand, were relatively insensitive to most endogenous amino acids and were only found in the molecular layer of cerebellum and torus longitudinalis. Type 3 sites possibly representing a distinctive class of receptor has been suggested by the results.

Glutamic acid-insensitive [3H]kainic acid binding in goldfish brain

Kainic acid is supposed to be a specific agonist for a subclass of excitatory glutamate receptors in the vertebrate CNS. An investigation of (2 nM) [3H]kainic acid binding sites in goldfish brain, using quantitative autoradiography, has revealed evidence for two types of kainic acid receptors which differ in sensitivity to glutamic acid. L-Glutamic acid (0.1-1 mM) displaced over 95% of specific [3H]kainic acid binding elsewhere in the brain but only 10-50% in the cerebellum and cerebellar crest. These structures apparently contain [3H]kainic acid binding sites that are extremely insensitive to glutamic acid. The glutamic acid-insensitive [3H]kainic acid binding was not displaced by quisqualic acid, kynurenic acid, alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA), or N-methyl-D-aspartatic acid, but was completely displaced by the kainic acid analogue domoic acid. The data indicate that two types of high affinity binding sites for [3H]kainic acid exist in the goldfish brain: glutamic acid-sensitive and glutamic acid-insensitive. High affinity [3H]kainic acid binding may therefore not always represent binding to subsets of glutamic acid receptors.