* To whom correspondence should be addressed.
Received September 29, 2000; Revision received April 25, 2001
Effects of GABA, glycine, acetylcholine, and glutamate (agonists of the GABAa/benzodiazepine, glycine, choline, and glutamate receptors, respectively) at concentrations in the range 10-8-10-4 M on the activity of basal Mg2+-ATPase of the plasma membrane fraction from bream brain and on its activation by Cl- were investigated. GABA and glycine activated basal Mg2+-ATPase activity and suppressed its activation by Cl-. Acetylcholine and glutamate activated basal Mg2+-ATPase to a lesser extent and did not suppress the activation of the enzyme by Cl-. The activation of basal Mg2+-ATPase by neuromediators was decreased by blockers of the corresponding receptors (picrotoxin, strychnine, benztropine mesylate, and D-2-amino-5-phosphonovaleric acid). In addition, picrotoxin and strychnine eliminated the inhibiting effect of GABA and glycine, respectively, on the Cl--stimulated Mg2+-ATPase activity. Agonists of the GABAa/benzodiazepine receptor--phenazepam (10-8-10-4 M) and pentobarbital (10-6-10-3 M)--activated the basal Mg2+-ATPase activity and decreased the Cl--stimulated Mg2+-ATPase activity. The dependence of both enzyme activities on ligand concentration is bell-shaped. Moreover, phenazepam and pentobarbital increased the basal Mg2+-ATPase activity in the presence of 10-7 M GABA and did not influence it in the presence of 10-4 M GABA and 10-6 M glycine. The data suggest that in the fish brain membranes the Cl--stimulated Mg2+-ATPase interacts with GABAa/benzodiazepine and glycine receptors but not with m-choline and glutamate receptors.
KEY WORDS: fish, brain, plasma membranes, Mg2+-ATPase, chloride, GABA, glycine, acetylcholine, glutamate, picrotoxin, strychnine, benztropine mesylate, pentobarbital, phenazepam, D-2-amino-5-phosphonovaleric acid