Isolation of endogenous modulators for the GABAA and taurine receptors

Polyhydroxylated C(60), fullerenols, as glutamate receptor antagonists and neuroprotective agents

Derivatives of C(60) have been shown to be effective free radical scavengers. Hence, many of the biological functions of fullerene are believed to be due to their antioxidant properties. Here we present evidence to show that fullerenols, that are caged fullerene oxides, exert their neuroprotective functions by blocking glutamate receptors and lowering the intracellular calcium, [Ca(2+)](i). In neuronal cultures, fullerenols reduce glutamate-induced neurotoxicity by about 80% at 50microM. No significant effect was observed on H(2)O(2)/Fe(2+)-induced neurotoxicity under the same conditions. Fullerenols were found to inhibit glutamate receptor binding in a dose-dependent manner inhibiting 50% of glutamate binding at 50 microM. Furthermore, AMPA receptors were found to be more sensitive to fullerenols than NMDA and KA receptors. On the other hand, GABA(A) receptors and taurine receptors were not significantly affected by fullerenols at the same concentrations used, suggesting that fullerenols inhibit primarily the glutamate receptors. In addition, fullerenols were also found to lower glutamate (Glu) receptor-induced elevation of [Ca(2+)](i), suggesting that the underlying mechanism of neuronal protective function of fullerenols is likely due to its ability to block the glutamate receptors and to reduce the level of [Ca(2+)](i).

Identification of inosine as an endogenous modulator for the benzodiazepine binding site of the GABAA receptors

Previously we have reported the presence of endogenous ligands that are involved in the regulation of the binding of muscimol to the GABA binding site of the GABAA receptors. Here, we report the presence of multiple forms of endogenous ligands in the brain which modulate the binding of flunitrazepam (FNZP) to the benzodiazepine (BZ) binding site of the GABAA receptor. Furthermore, one of the endogenous ligands for the BZ receptors, referred to as EBZ, has been identified as inosine based on the following observations: (1) standard inosine and the EBZ have identical NMR and UV spectra; (2) the elution profile of inosine and the EBZ from a HPLC column are indistinguishable, and (3) inosine and the EBZ show identical activity in inhibiting [3H]FNZP binding.

Effects of taurine and light on retinal GABA content and the efflux of 14C-GABA and 14C-aspartate from frog retina

GABA content of isolated, dark adapted frog retina was found to be 3.15 +/- 0.28 mM. After 30 minutes of exposure to intense light (200 lx), retinal GABA levels increased about 70%. Interestingly, incubation of dark adapted retina for 30 minutes with medium containing 0.4 mM taurine also led to a 70% increase in GABA levels. Since the light-induced elevation in GABA content was reduced over 50% by a simultaneous injection of 0.02 mM strychinine, it is likely that the light-induced GABA change is partly mediated by the release of taurine from the retina seen after light exposure. However, incubation of isolated retina with medium containing increasing concentrations of taurine (1, 2 and 20 mM), caused a progressive rise in 14C-GABA efflux from retina that was preloaded with 2.2 microM GABA and exposed to dim light (0.05 lx). It was also shown that taurine (1 and 5 mM) dramatically reduced 14C-aspartate efflux from retina preloaded with radioactive aspartate and exposed to dim light conditions. By comparison, intense light stimulation (40 lx) reduced basal 14C-aspartate efflux while dark exposure increased 14C-aspartate loss from the isolated retina. We found that taurine depressed the b-wave signal of frog retina, with the maximum effect occurring at a concentration of 1 mM. Addition of strychnine (0.4 mM) reversed the taurine effect on the b-wave, indicating that taurine receptors must be present in the inner retina. By contrast, taurine (0.1-20 mM) had no effect on the P111 component of the ERG initiated by either aspartate or cobalt. However, taurine exerted a modest depressant activity on P111 initiated by glutamate. The significance of these data relative to the putative neurotransmitter function of taurine in the inner retina is discussed.

Multiplicity of Brain Cysteine Sulfinic Acid Decarboxylase - Purification, Characterization and Subunit Structure

Cysteine sulfinic acid decarboxylase (CSAD), the rate-limiting enzyme in taurine biosynthesis, appears to be present in the brain in multiple isoforms. Two distinct forms of CSAD, referred to as CSAD I and CSAD II, were obtained on Sephadex G-100 column. CSAD I and CSAD II differ in: (1) the elution profile on Sephadex G-100 column; (2) the sensitivity towards Mn(2+), methione, and other sulfur-containing amino acids, and (3) their immunologic properties. CSAD II has been purified to about 2,500-fold by a combination of column chromatographies and polyacrylamide gel electrophoresis (PAGE). The purity of the enzyme preparation was established as judged from the following observations: (1) a single protein band was observed under various electrophoretic conditions, e.g., 5-20% nondenaturing PAGE, 7% nondenaturing PAGE and 10% SDS-PAGE, and (2) in nondenaturing PAGE, the protein band comigrated with CSAD activity. CSAD II has a molecular weight of 90 kDa and is a homodimer consisting of two 43 +/- 2 kDa subunits. CSAD appears to require Mn(2+) for its maximum activity. Other divalent cations fail to replace Mn(2+) in activation of CSAD activity. However, the precise role of Mn(2+) in the action of CSAD remains to be determined. Copyright 1996 S. Karger AG, Basel

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.