title: Veratridine and other depolarizing agents counteract the inhibitory effect of Mg2+ ions on N-methyl-D-aspartate (NMDA)-induced noradrenaline release in vitro.
abstract: Rat brain cortex slices preincubated with 3H-noradrenaline were superfused with Krebs-Henseleit solution with or without Mg2+. In the absence of Mg2+ ions, NMDA evoked 3H-noradrenaline overflow above basal efflux; this effect was concentration-dependently inhibited by Mg2+ (IC50: 19 mumol/l). Despite the presence of 1.2 mmol/l Mg2+, which is known to block cation influx through the ion channel coupled to the NMDA receptor, NMDA evoked 3H-noradrenaline release if the membrane was permanently kept depolarized by 20 or 25 mmol/l K+, 1 mumol/l veratridine or 200 mumol/1 3,4-diaminopyridine; the stimulant effect of NMDA was counteracted by 2-amino-5-phosphonovaleric acid (2-APV), a competitive antagonist at the NMDA receptor and by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK 801), an antagonist acting at the cation channel associated with the NMDA receptor. In contrast, no stimulatory effect of NMDA in the presence of 1.2 mmol/l Mg2+ was observed when the membrane of the nerve terminals was intermittently depolarized by electrical impulses of 2 ms duration at a frequency of 1-3 Hz. It is concluded that continuous depolarization of the nerve membrane counteracts the blocking effect of Mg2+ on cation influx through the NMDA receptor-associated ion channel. Under this condition, noradrenaline release can be stimulated by NMDA receptor activation even in the presence of physiological Mg2+ concentrations.
authors.affiliation: Institut für Pharmakologie und Toxikologie der Rheinischen Friedrich-Wilhelms-Universität Bonn, Federal Republic of Germany.
authors.forename: K
authors.initials: K
authors.lastname: Fink
authors.forename: M
authors.initials: M
authors.lastname: Göthert
authors.forename: E
authors.initials: E
authors.lastname: Schlicker
mesh_terms: D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D015761:4-Aminopyridine / Q000031:analogs & derivatives / Q000494:pharmacology; D000077770:Amifampridine; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D003986:Dibenzocycloheptenes / Q000494:pharmacology; D016291:Dizocilpine Maleate; D004558:Electric Stimulation; D006859:Hydrogen / Q000378:metabolism; D066298:In Vitro Techniques; D008274:Magnesium / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D008297:Male; D016202:N-Methylaspartate; D009467:Neuromuscular Depolarizing Agents / Q000494:pharmacology*; D009638:Norepinephrine / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014701:Veratridine / Q000494:pharmacology*; D014702:Veratrine / Q000031:analogs & derivatives*
chemical_list: D003986:Dibenzocycloheptenes; D009467:Neuromuscular Depolarizing Agents; D001224:Aspartic Acid; D016202:N-Methylaspartate; D016291:Dizocilpine Maleate; D014701:Veratridine; D015763:2-Amino-5-phosphonovalerate; D006859:Hydrogen; D015761:4-Aminopyridine; D014702:Veratrine; D008274:Magnesium; D000077770:Amifampridine; D011188:Potassium; D009638:Norepinephrine