Taurine Receptor Research Articles

Taurine modulation of the benzodiazepine-gamma-aminobutyric acid receptor complex in brain membranes.

In unwashed brain membranes taurine produced an inhibition of [3H]flunitrazepam [( 3H]FNZ) binding with IC50 ranging between 31.5 and 11.9 microM; the IC20 varied between 18 and 26 nM. This inhibitory effect was of a mixed type, with a reduction in Bmax and an increase in KD. Various precursors and metabolites of taurine have a less inhibitory effect. Taurine also has little inhibitory effect (IC50 above 500 microM) on the binding of [3H]ethyl-beta-carboline-3-carboxylate. In extensively washed membranes, 10(-5) M taurine produces a 16-21% increase in the binding of [3H]FNZ while 10(-5) M gamma-aminobutyric acid (GABA) increases it between 31 and 42%. However, if 10(-5) M GABA plus 10(-5) M taurine is included in the assay there is a dramatic inhibitory effect. Taurine causes an inhibition of the GABAergic enhancement of [3H]FNZ binding with an IC50 between 7.3 and 7.8 microM. Binding experiments with [3H]taurine done under different conditions failed to detect a Na+-independent and specific [3H]taurine receptor. These results suggest that endogenous taurine, the second most abundant free amino acid in brain, may play an important modulatory role in the GABA-benzodiazepine receptor complex.

Narrow-spectrum chemoreceptor cells in the antenules of the american lobster, Homarus americanus

The present study shows that smell (antennular) receptor cells are as narrowly tuned to single compounds as taste (leg) receptor cells in the lobster. Antennular receptors responded best to hydroxy- l-proline (57% of the 30 cells sampled) and taurine (24%). In the presence of 14 other compounds in equimolar concentrations, the hydroxy- l-proline and taurine receptors showed a suppressed response to their best stimulus. Other cells had best responses to ammonium chloride, betaine, L-glutamate or L-proline. The results have implications for molecular receptor processes and for the neural basis of feeding behavior.

The role of taurine in nervous tissue: its effects on ionic fluxes.

The recognition of taurine and GABA, two neuroactive amino acids, as normal constituents of nervous tissue, occurred almost at the same time (1,43). Their structural similarity and the resemblance of their effects on neuronal activity had suggested similar roles for them in nervous function. However, whereas during the last few years an impressive amount of evidence has accumulated supporting a role for GABA as an inhibitory neurotransmitter (44), the elucidation of the role of taurine in nervous function has been particularly difficult. Some experimental evidence exists supporting a role for taurine as a neurotransmitter. It has been demonstrated that taurine, when applied via microiontophoresis, excerts a depressant effect on neuronal activity in several regions of the central nervous system (10–12). However, the unsuccessful search for specific blockers of this action of taurine, together with the antagonism shown by strychnine and bicuculline on inhibitory effects of taurine (10–12), have raised some doubts about the specificity of taurine as an inhibitory neurotransmitter itself. Since strychnine and bicuculline are likely to be specific antagonists of GABA and glycine receptors, the effect of taurine on post-synaptic neurons blocked by these drugs could be mediated through an interaction with GABA and glycine receptors. Studies in vitro using biochemical techniques to characterize GABA and glycine postsynaptic receptors in several regions of the CNS, have clearly shown a significant interaction of taurine with GABA and glycine binding to their specific receptors (15,51). Furthermore, the identification of taurine receptors by this technique, has proved unsuccessful (Lopez-Colome, this volume).

ANTENNULAR CHEMOSENSITIVITY IN THE SPINY LOBSTER,PANULIRUS ARGUS: STUDIES OF TAURINE SENSITIVE RECEPTORS

1. Taurine sensitive receptors in the antennules of the spiny lobster, Panulirus argus, were identified electrophysiologically.2. Recordings from single receptors revealed a narrow and consistent specificity when tested with taurine, taurine analogs and derivatives, and structurally related compounds.3. Taurine was the most stimulatory compound tested. Threshold concentrations for 36 individual receptors ranged from 10-8 to 10-10 M.4. The taurine analogs, hypotaurine and β-alanine, were also very effective but the phosphonic acid analog of taurine was ineffective.5. Regarding receptor specificity, receptor stimulation was greatest with compounds having single terminal basic (amine) and acidic groups separated by two carbon atoms. Compounds having terminal basic and acidic groups separated by three to five carbon atoms were also active. However, activity decreased with the distance of separation of charged groups.6. Alpha-amino acids and compounds with terminal basic and acidic groups separated by only one carbon atom were virtually ineffective.7. Receptor stimulation was markedly less with structurally related compounds that either lacked a terminal amine group, had additional amine or acidic groups, or had neutral side chains.8. Dose/response relationships of four differentially stimulatory compounds (taurine, hypotaurine, β-alanine and υ-aminobutyric acid) applied to single receptors were compared and found to describe a series of roughly parallel lines. This implies that a less stimulatory compound effects receptor response in the same manner as a more stimulatory compound applied at a lower concentration.9. The possible role of taurine in food finding, and the similarity of the specificity of antennular taurine receptors and taurine endoreceptors identified in various organisms are discussed.

The identification of taurine receptors from rat heart sarcolemma

Two sets of taurine receptors on rat heart sarcolemma have been identified. The high affinity taurine receptors (Kd=3.5×10 −4M) show a non-cooperative binding profile while the low affinity taurine receptors exhibit positive cooperativity. Taurine binding to the membrane exhibits a typical bell shaped pH profile with maximum binding occurring at pH 8.0. The maximum temperature for binding is 24°C. The effect of various taurine analogues on the receptors was investigated. It was found that binding is prevented by hypotaurine and inhibited to a lesser degree by isethionic acid and cysteine sulfinic acid, while β-alanine was found to increase taurine binding. The effect of several hydrolytic enzymes was also examined and it was shown that several proteases and phospholipase C inhibit binding. The results indicate that the taurine receptors are membrane bound proteins in a phospholipid environment.