RESUMEN
We have identified a zebrafish mutant line, bajan, in which compromised motility and fatigue result from a point mutation in the gene coding choline acetyltransferase (ChAT), the enzyme responsible for acetylcholine (ACh) synthesis. Although the mutation predicts loss of ChAT function, bajan inexplicably retains low levels of neuromuscular transmission. We exploited this residual activity and determined the consequences for synaptic function. The attenuated synaptic responses were a direct consequence of a decrease in both resting mean quantal size and quantal content. To replicate behavioral fatigue in swimming, motorneurons were stimulated at high frequencies. A prominent reduction in quantal content, reflecting vesicle depletion, was coincident with a small additional reduction in quantal size. In humans, defective ChAT leads to episodic apnea, a form of congenital myasthenic syndrome characterized by use-dependent fatigue. In contrast to bajan, however, afflicted individuals exhibit a normal resting quantal size and quantal content. The fatigue in humans results from a pronounced long-lasting drop in quantal size with little or no change in quantal content. These differences have important implications for interpreting fatigue as well as on understanding the impact of ACh availability on vesicle filling and recycling.
Asunto(s)
Colina O-Acetiltransferasa/genética , Colina O-Acetiltransferasa/fisiología , Unión Neuromuscular/fisiología , Pez Cebra/genética , Pez Cebra/fisiología , Animales , ADN Complementario/biosíntesis , ADN Complementario/genética , Electrofisiología , Inmunohistoquímica , Técnicas In Vitro , Larva , Microscopía Confocal , Placa Motora/efectos de los fármacos , Placa Motora/fisiología , Mutación/genética , Mutación/fisiología , Unión Neuromuscular/enzimología , Unión Neuromuscular/genética , Neurotransmisores/metabolismo , Neurotransmisores/fisiología , Oligonucleótidos/genética , Técnicas de Placa-Clamp , Receptores Presinapticos/genética , Receptores Presinapticos/fisiología , Conducta Estereotipada , Transmisión Sináptica/genética , Transmisión Sináptica/fisiologíaRESUMEN
Glutamate receptors mediate excitatory neurotransmission in the brain and are important in the formation of memory and in some neurodegenerative disorders. A complementary DNA clone that encoded a 33-kilodalton protein (GR33) was obtained by screening a library with an antibody generated against glutamate binding proteins. The sequence of GR33 is identical to that of the recently reported presynaptic protein syntaxin. When GR33 was expressed in Xenopus oocytes, it formed glutamate-activated ion channels that are pharmacologically similar to those of N-methyl-D-aspartate receptors but with different electrophysiological properties. Mutation of the leucine 278 residue in the single putative transmembrane segment of GR33 affects the properties of the channel. Thus, in vivo GR33 may be a presynaptic glutamate receptor.