MODULATORY ACTION OF ARACHIDONIC-ACID ON GABAA/CHLORIDE CHANNEL RECEPTOR FUNCTION IN ADULT AND AGED BRAIN CORTEX MEMBRANES

Effect of arachidonic acid on binding parameters of two binding sites oil the GABA(A) receptor and on GABA activated Cl- uptake was investigated in synaptic plasma membrane and in synaptoneurosomes from brain cortex of adult (4-months old) and aged (27-months old) rats. The ligands used were [H-3]muscimol, a GABA agonist and [S-35]-t-butylbicyclophospliorothionate ([S-35]TBPS). a convulsant that binds to the site near the chloride channel. Arachidonic acid increases significantly GABA(A) agonist binding and concomitantly decreases [S-35]TBPS binding in a concentration dependent manner. The analysis of binding parameters in adult brain showed a significant decrease by AA of K(D) value for low but not for high affinity of [H-3]muscimol binding. Concomitantly, AA enhances B(max) values for high affinity binding and has no effect on B(max) value for low affinity binding in synaptic plasma membrane (SPM) from adult brain. In synaptic plasma membrane from aged brain AA increases low and high affinity binding of agonist to GABA(A) receptor, modified significantly K(D) value by about 30 and 66%, respectively. On the other hand, AA significantly decreases of [S-35]TBPS binding to chloride channel recognition site. Scatchard's analysis indicates that this inhibition results from a decrease of total number of binding sites. Moreover, the affinity of [S-35]TBPS binding was increased (K(D) = 37.0 nM for AA-treated membrane, as compared to 69.3 nM K(D) in control membrane). GABA-dependent chloride uptake into synaptoneurosomes is also inhibited by AA in a concentration dependent manner in adult brain. In aged brain synaptoneurosomes AA has similar inhibitory effect on GABA-activated chloride uptake. These results indicate that arachidonate may be an important endogenous modulator of GABA/Cl- receptor function and in the case of its accumulation in the brain may be responsible for the dysfunction of GABA/Cl- channel receptor complex.