Neural regulation of acetylcholine receptors in rat neonatal muscle

Diversification of Intrinsic Motoneuron Electrical Properties During Normal Development and Botulinum Toxin–Induced Muscle Paralysis in Early Postnatal Mice

During early postnatal development, between birth and postnatal days 8–11, mice start to achieve weight-bearing locomotion. In association with the progression of weight-bearing locomotion there are presumed developmental changes in the intrinsic electrical properties of spinal α-motoneurons. However, these developmental changes in the properties of α-motoneuron properties have not been systematically explored in mice. Here, data are presented documenting the developmental changes of selected intrinsic motoneuron electrical properties, including statistically significant changes in action potential half-width, intrinsic excitability and diversity (quantified as coefficient of variation) of rheobase current, afterhyperpolarization half-decay time, and input resistance. In various adult mammalian preparations, the maintenance of intrinsic motoneuron electrical properties is dependent on activity and/or transmission-sensitive motoneuron–muscle interactions. In this study, we show that botulinum toxin–induced muscle paralysis led to statistically significant changes in the normal development of intrinsic motoneuron electrical properties in the postnatal mouse. This suggests that muscle activity during early neonatal life contributes to the development of normal motoneuron electrical properties.

Neurotoxins in the study of neural regulation of membrane proteins in skeletal muscle

The discovery and purification of several neurotoxins, including alpha-bungarotoxin and tetrodotoxin has provided very high-affinity ligands which have proved to be central to the elucidation of the neural control of skeletal muscle membrane proteins and to the purification and characterization of the nicotinic acetylcholine receptor (AChR) and the Na+ channel, respectively. This review describes the use of neurotoxins for quantification and localization of receptors and ion channels in normal and denervated skeletal muscles with particular emphasis on the appropriateness of the muscle preparation and ligand used in the studies. It is now clear that the nerve controls the synthesis and spatial distribution of AChRs and Na+ channels by regulating gene expression in extrajunctional and subjunctional nuclei. The down-regulation of extrajunctional AChRs is primarily mediated by neuromuscular activity and the concentration of AChRs and Na+ channels in specific membrane domains at the neuromuscular junction is controlled by a number of neurotrophic substances at the neuromuscular junction. These include agrin, ARIA, and CGRP.

Cyclic AMP-regulated AChR assembly is independent of AChR subunit phosphorylation by PKA

Forskolin treatment of cells expressing Torpedo acetylcholine receptors leads to enhanced assembly efficiency of subunits, which correlates with increased phosphorylation of the gamma subunit. To determine the role of the two potential protein kinase A sites of the gamma subunit in receptor assembly, cell lines expressing different mutant receptors were established. Mouse fibroblast cell lines stably expressing wild-type Torpedo acetylcholine receptor alpha, beta, delta subunits plus one of three gamma subunit mutations (S353A, S354A, or S353,354A) were established to identify the protein kinase A phosphorylation sites of gamma in vivo, and to determine if increased phosphorylation of the gamma subunit leads to enhanced expression of receptors. We found that both serines (353, 354) in gamma are phosphorylated in vivo by protein kinase A, however, phosphorylation of either or both of these sites does not lead to increased assembly efficiency. We established a cell line expressing alpha, beta, and gamma(S353,354A) subunits only (no delta), and found that the presence of delta (or its phosphorylation) is also not necessary for the observed stimulation by forskolin. alpha beta gamma, alpha gamma, and beta gamma associations were stimulated by forskolin but alpha beta and alpha delta interactions were not. These data imply that the presence of gamma is necessary for forskolin action. We postulate that forskolin may stimulate acetylcholine receptor expression through a cellular protein that is involved in the folding and/or assembly of protein complexes, and that forskolin may regulate the action of such a protein through phosphorylation.

Synaptogenesis in Drosophila: coupling genetics and electrophysiology

Drosophila is one of the most fully described eukaryotic organisms and, as a system, offers the most advanced genetic and molecular techniques. In particular, Drosophila embryonic development has been subject to intensive genetic and molecular examination. Drosophila is also one of the few genetically malleable organisms to permit electrophysiological investigation and so allow detailed physiological characterization of specific molecular lesions. These two fields, the developmental and electrophysiological, are being coupled for the first time to examine a key aspect of neural development, synaptogenesis. Here, I describe synaptogenesis in the Drosophila embryo at the identified neuromuscular junction. I focus particular attention on the use of known genetic mutations to dissect the mechanisms of synapse formation. This simple, well-characterized synapse is already proving valuable in describing the defects of mutations in genes essential for synaptic development and function. In the long term, this system will allow us to systematically mutate the Drosophila genome to identify and describe the genetic and molecular pathways directing the construction of a synapse.