Two mutations linked to nocturnal frontal lobe epilepsy cause use-dependent potentiation of the nicotinic ACh response.
1. We constructed rat homologues (S252F and +L264) of two human alpha4 nicotinic mutations - alpha4(S248F) and alpha4(777ins3) - that have been linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and co-expressed them with wild-type rat beta2 subunits in Xenopus oocytes. 2. The S252F and +L264 mutations had three common effects on the ACh response. First, they caused use-dependent potentiation of the response during a train of brief 100 nM ACh pulses. Second, they delayed the rise times of the 5-15 nM (+L264) and 30 nM (S252F) ACh responses. Third, they reduced extracellular Ca2+-induced increases in the 30 microM ACh response. 3. Beside these shared effects, the S252F mutation also reduced the channel burst duration measured from voltage-jump relaxations, enhanced steady-state desensitization and reduced the single-channel conductance. In contrast, the +L264 mutation prolonged the channel burst duration, did not affect desensitization and slightly increased single-channel conductance. Neither mutation affected the number of surface receptors measured by antibody binding but the S252F mutation reduced the maximum ACh response. 4. The ACh concentration dependence of use-dependent potentiation and the delay in the rising phase of the mutant ACh response suggest that these effects are caused by a slow unblocking of the closed mutant receptors. Use-dependent potentiation of the mutant response during a series of high-frequency cholinergic inputs to the presynaptic terminal could trigger ADNFLE seizures by suddenly increasing nicotinic-mediated transmitter release.
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Division of Biomedical Sciences, University of California, Riverside, CA 92521-0121, USA.
Viseshakul
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Shafaee
N
N
Forsayeth
J
J
Cohen
B N
BN
D000465:Algorithms; D000818:Animals; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D017034:Epilepsy, Frontal Lobe / Q000235:genetics*; D006801:Humans; D007457:Iodine Radioisotopes; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D009154:Mutation / Q000502:physiology*; D018722:Nicotinic Agonists / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011725:Pyridines / Q000378:metabolism; D051381:Rats; D011978:Receptors, Nicotinic / Q000235:genetics*; D014981:Xenopus
D019086:Bridged Bicyclo Compounds, Heterocyclic; D007457:Iodine Radioisotopes; D007473:Ion Channels; D018722:Nicotinic Agonists; D011725:Pyridines; D011978:Receptors, Nicotinic; C082748:epibatidine; D002118:Calcium