The nature of the defect in congenital myasthenia was investigated in biopsy specimens of intercostal muscle from 5 male patients whose symptoms presented between birth and 2 years of age. Miniature end-plate potentials were reduced in amplitude in all 5 patients. The number of acetylcholine receptors as determined by alpha-bungarotoxin binding was normal in case 1 and reduced in cases, 2, 4, and 5. The shape of the end-plates as shown by autoradiography and cholinesterase staining was normal in case 1 and elongated in cases 2, 4, and 5. In cases 3, alpha-bungarotoxin binding was slowly reversible, and there were some muscle fibers with multiple end-plate regions. The acetylcholine content of the muscle was normal in all 5 cases. None of the patients had serum antibody to human acetylcholine receptor as measured by immunoprecipitation or inhibition of alpha-bungarotoxin binding. We conclude that congenital myasthenia is a heterogeneous condition of nonimmune etiology in which both presynaptic and postsynaptic defects can be found.

Acetylcholine↗

PubMedDate not supplied

NMDA receptor diversity in the cerebellum: identification of subunits contributing to functional receptors.

Recent studies of N-methyl-D-aspartate (NMDA) receptors have led to the suggestion that there are two distinct classes of native NMDA receptors, identifiable from their single-channel conductance properties. 'High-conductance' openings arise from NR2A- or NR2B-containing receptors, and 'low-conductance' openings arise from NR2C- or NR2D-containing receptors. In addition, the low-conductance channels show reduced sensitivity to block by Mg2+. The readily identified cell types and simple architecture of the cerebellum make it an ideal model system in which to determine the contribution of specific subunits to functional NMDA receptors. Furthermore, mRNA for all of these four NR2 subunits are represented in this brain region. We have examined NMDA channels in Purkinje cells, deep cerebellar nuclei (DCN) neurons and Golgi cells. First we find that NR2D-containing NMDA receptors give rise to low-conductance openings in cell-attached recordings from Purkinje cells. The characteristic conductance of these events cannot, therefore, be ascribed to patch excision. Second, patches from some DCN neurons exhibit mixed populations of high- and low-conductance openings. Third, Golgi cells also exhibit a mixed population of high- and low-conductance NMDA receptor openings. The features of these low-conductance openings are consistent with the presence of NR2D-containing NMDA receptors, as suggested by in situ hybridization data. On the other hand the existence of high-conductance channels, with properties typical of NR2B-containing receptors, was not expected. Our results provide new evidence about the subunit composition of NMDA receptors in identified cerebellar cells, and suggest that examination of single-channel properties is a potentially powerful approach for determining the possible subunit composition of native NMDA receptors.

Animals↗

PubMedDate not supplied

Multiple-conductance channels activated by excitatory amino acids in cerebellar neurons.

In the mammalian central nervous system amino acids such as L-glutamate and L-aspartate are thought to act as fast synaptic transmitters. It has been suggested that at least three pharmacologically-distinguishable types of glutamate receptor occur in central neurons and that these are selectively activated by the glutamate analogues N-methyl-D-aspartate (NMDA), quisqualate and kainate. These three receptor types would be expected to open ion channels with different conductances. Hence if agonists produce similar channel conductances this would suggest they are acting on the same receptor. Another possibility is suggested by experiments on spinal neurons, where GABA (gamma-amino butyric acid) and glycine appear to open different sub-conductance levels of one class of channel while acting on different receptors. By analogy, several types of glutamate receptor could also be linked to a single type of channel with several sub-conductance states. We have examined these possibilities in cerebellar neurons by analysing the single-channel currents activated by L-glutamate, L-aspartate, NMDA, quisqualate and kainate in excised membrane patches. All of these agonists are capable of opening channels with at least five different conductance levels, the largest being about 45-50 pS. NMDA predominantly activated conductance levels above 30 pS while quisqualate and kainate mainly activated ones below 20 pS. The presence of clear transitions between levels favours the idea that the five main levels are all sub-states of the same type of channel.

Action Potentials↗