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 alpha 4 nic otinic mutations - alpha 4(S248F) and alpha 4(777ins3) - that have been lin ked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and co-e xpressed them with wild-type rat beta 2 subunits in Xenopus oocytes. 2. The S252F and +L264 mutations had three common effects on the ACh respon se. 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 th e 5-15 nM (+L264) and 30 nM (S252F) ACh responses. Third, they reduced extr acellular Ca2+-induced increases in the 30 mu M ACh response. 3. Beside these shared effects, the S252F mutation also reduced the channel burst duration measured from voltage-jump relaxations, enhanced steady-sta te desensitization and reduced the single-channel conductance. In contrast, the +L264 mutation prolonged the channel burst duration, did not affect de sensitization and slightly increased single-channel conductance. Neither mu tation affected the number of surface receptors measured by antibody bindin g but the S252F mutation reduced the maximum ACh response. 4. The ACh concentration dependence of use-dependent potentiation and the d elay in the rising phase of the mutant ACh response suggest that these effe cts are caused by a slow unblocking of the closed mutant receptors. Use-dep endent potentiation of the mutant response during a series of high-frequenc y cholinergic inputs to the presynaptic terminal could trigger ADNFLE seizu res by suddenly increasing nicotinic-mediated transmitter release.