Effect of Activators and Blockers of Ligand-Regulated Ion Channels on the Activity of the Cl^--Stimulated Mg²⁺-ATPase of the Plasma Membrane Fraction from Bream (Abramis brama L.) Brain

S. A. Menzikov^* and O. V. Menzikova

Papanin Institute of Biology of Inland Waters, Russian Academy of Sciences, Borok, Yaroslavl Region, 152742 Russia; fax: (085) 472-4042; E-mail: sam@ibiw.yaroslavl.ru

^* To whom correspondence should be addressed.

Received September 29, 2000; Revision received April 25, 2001

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Effects of GABA, glycine, acetylcholine, and glutamate (agonists of the GABA_a/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 GABA_a/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 GABA_a/benzodiazepine and glycine receptors but not with m-choline and glutamate receptors.

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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

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