Abstract
In our study, evidence is provided that strychnine, a competitive antagonist at glycine-gated Cl− channels, is also a potent competitive antagonist at native α-7-containing, α-bungarotoxin-sensitive nicotinic acetylcholine receptor (nAChRs). To address the effects of strychnine on two types of nicotinic responses, the whole-cell mode of the patch-clamp technique was applied to rat hippocampal neurons in culture. Type IA and type II nicotinic currents evoked by acetylcholine (ACh) were inhibited by strychnine in a concentration-dependent manner with IC50s of 1.2 and 38 μM, respectively. Strychnine (2 μM) decreased the peak amplitude of the α-bungarotoxin-sensitive type IA current in a voltage-independent manner and prolonged the decay phase of this current. The concentration-response curve for ACh in evoking type IA current showed a parallel shift to the right in the presence of strychnine (2 μM); the EC50 for ACh was increased from 0.4 to 0.8 mM. These findings suggest that strychnine acts as a competitive antagonist of ACh at the α7 nAChRs that subserve type IA current. In contrast, the inhibition by strychnine of type II current was strongly voltage dependent, and the decay phase of this current was markedly accelerated by the toxin, suggesting an open-channel blockade by strychnine of the α4β2 nAChRs subserving type II currents. Preexposure of the neurons to strychnine enhanced its ability to decrease the peak amplitude of type II currents, indicating that the toxin may also act on α4β2 nAChR channels that are not open. It is concluded that strychnine is a potent competitive antagonist of ACh at neuronal α7 nAChRs and a noncompetitive antagonist at the α4β2 nAChR.
Footnotes
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Send reprint requests to: Dr. Edson X. Albuquerque, Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, 655 West Baltimore Street, Baltimore, MD 21201.
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↵1 This study was supported by United States Public Health Service Grants NS25296 and ES05730 and by PRONEX (Brazil). Some of the data have appeared previously in abstract form (Matsubayashi et al., 1996).
- Abbreviations:
- ACh
- acetylcholine
- nAChR
- nicotinic acetylcholine receptor
- α-BGT
- α-bungarotoxin
- CNS
- central nervous system
- GABA
- γ-amino butyric acid
- MEM
- minimum essential medium
- HEPES
- (N-[2-hydroxyethyl]piperazine-N′-[2-ethane sulfonic acid])
- DHβE
- dihydro-β-erythroidine
- ATP-RS
- adenosine 5′-trisphosphate-regenerating solution
- EGTA
- ethyleneglycol-bis-(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
- τ
- decay-time constant
- EC50
- agonist concentration that evokes 50% of the maximal response
- IC50
- antagonist concentration that reduces by 50% the maximal response
- nH
- Hill coefficient
- ANOVA
- analysis of variance
- Ki
- apparent affinity of an antagonist for the receptor
- Kd
- apparent affinity of an agonist for the receptor
- Received August 14, 1997.
- Accepted November 17, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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