Topographical rearrangement of acetylcholine receptors alters channel kinetics

Plasma membranes are dynamic structures of proteins and lipids. Protein-protein or protein-lipid interactions within the membrane are believed to have important roles in many membrane functions, including ion transport, enzyme activity and signal reception. The acetylcholine (ACh) receptor-channel complex in skeletal muscle membrane is one of the best known integral membrane proteins. Its ion transport function is accessible to direct measurement at the single-channel level by the use of the 'giga-seal' patch recording technique. Here we used an in situ electrophoresis technique to rearrange the topography of pre-existing ACh receptor-channels in the muscle membrane, and measured the single-channel kinetics of ACh-activated channels in two different molecular environments within the membrane: those in the diffusely distributed region and those in the ACh receptor clusters induced by the applied field. We found that the channel kinetics are significantly prolonged in the ACh receptor cluster compared with the non-clustered region of the same cell. This result strongly supports the notion that the function of a membrane ionic channel depends on the local molecular environment.     

Embryonic acetylcholine receptors guarantee spontaneous contractions in rat developing muscle

Many proteins are expressed in distinct embryonic and adult forms. However, in most cases we do not know why the embryonic form of proteins is required. This question can be readily addressed for the acetylcholine receptor (AChR) because developmentally specified modifications of this ligand-gated ion channel can be directly related to changes in membrane currents. In developing rat soleus muscle, spontaneous transmitter release causes miniature end-plate currents (m.e.p.cs) to flow into the muscle cell. We show here that these m.e.p.cs in neonatal soleus trigger spontaneous contractions. By injecting m.e.p.cs into young fibres, we showed that only embryonic m.e.p.cs can trigger such contractions; adult m.e.p.cs do not last long enough. Developing muscle fibres must be active for synapse and muscle differentiation. Our experiments indicate that the embryonic form of the AChR is essential for spontaneous contractile activity and may therefore be required for normal neuromuscular development.     

5-HT3 receptors mediate inhibition of acetylcholine release in cortical tissue

The release of cerebral acetylcholine from terminals in the cerebral cortex has been shown to be regulated by 5-hydroxytryptamine (5-HT) but it is not known which subtype of the 5-HT receptor is involved. 5-HT receptor agonists increase acetylcholine levels in vivo, indicating a reduced turnover, and reduce release of acetylcholine from striatal slices in vitro. Depleting 5-HT by inhibiting synthesis or by destroying the neurons containing 5-HT potentiates acetylcholine release, and increases acetylcholine turnover in the cerebral cortex and hippocampus. Selective antagonists for the 5-HT3 receptor subtypes which seem to have effects on mood and activity may exert their effect through the regulation of acetylcholine release in the cortex and limbic system. Radioligand binding studies show a high density of 5-HT3 receptors in the cholinergic-rich entorhinal cortex and we provide evidence that a reduction in cortical cholinergic function can be effected in vitro by 5-HT3 receptors.     

中枢烟碱乙酰胆碱受体与Alzheimer病的研究

中枢烟碱乙酰胆碱受体（ｎＡｃｈＲ）和Ａｌｚｈｅｉｍｅｒ病（ＡＤ）的研究进展迅猛．ＡＤ与胆碱能神经系统的功能缺陷密切相关,分子遗传学和药理学研究已经认为中枢神经系统存在多种ｎＡｃｈＲ的亚型,目前在ＡＤ病人发现只是某些ｎＡｃｈＲ的亚型发生变化,因此．研究中枢ｎＡｃｈＲ的多样性和药理反应特性．为揭示ＡＤ的发病机理和治疗有着重要意义．     

The strychnine-binding subunit of the glycine receptor shows homology with nicotinic acetylcholine receptors

We have cloned and sequenced cDNAs of the strychnine-binding subunit of the rat glycine receptor, a neurotransmitter-gated chloride channel protein of the CNS. The deduced polypeptide shows significant structural and amino-acid sequence homology with nicotinic acetylcholine receptor proteins, indicating that there is a family of genes encoding neurotransmitter-gated ion channels.     

Electrophoretic movement and localisation of acetylcholine receptors in the embryonic muscle cell membrane

A steady electric field of 30 mV across a single embryonic muscle cell produces accumulation of acetylcholine receptors toward one pole of the cell within 1 h. The movement is electrophoretic in nature and the accumulation results in the formation of stable, metabolically independent receptor aggregates.     

A single GTP-binding protein regulates K+-channels coupled with dopamine, histamine and acetylcholine receptors

Recently, a GTP-binding protein sensitive to islet activating protein (IAP) has been suggested to be important in producing K+-currents when the muscarinic receptor of the atrial muscle is activated by acetylcholine (ACh). Here we confirm the blocking effects of IAP and GTP gamma S (a nonhydrolysable analogue of GTP) on the ACh-induced K+-current recorded from the ganglion cells of the sea slug Aplysia and compare their effects on histamine (HA)-induced and dopamine (DA)-induced K+-currents. Intracellular injections of IAP irreversibly and selectively block the openings of K+-channels activated by either ACh, HA, or DA without affecting the resting potential or conductance states of the membranes. Intracellular application of GTP gamma S alone caused extremely slow, irreversible opening of K+-channels; however, repetitive receptor activations significantly increase the rate of the GTP gamma S effect. These results strongly suggest that a GTP-binding protein such as Gi regulates the opening of K+-channels coupled with these receptors.     

Metabolic stabilization of endplate acetylcholine receptors regulated by Ca2+ influx associated with muscle activity.

During formation of the neuromuscular junction, acetylcholine receptors in the endplate membrane become metabolically stabilized under neural control, their half-life increasing from about 1 day to about 10 days. The metabolic stability of the receptors is regulated by the electrical activity induced in the muscle by innervation. We report here that metabolic stabilization of endplate receptors but not of extrajunctional receptors can be induced in the absence of muscle activity if muscles are treated with the calcium ionophore A23187. Acetylcholine receptor stabilization was also induced by culturing non-stimulated muscle in elevated K+ with the Ca2+ channel activator (+)-SDZ202-791. Conversely, activity-dependent receptor stabilization is prevented in muscle stimulated in the presence of the Ca2+ channel blockers (+)-PN200-110 or D-600. Treatment of muscles with ryanodine, which induces Ca2+ release from the sarcoplasmic reticulum in the absence of activity, does not cause stabilization of junctional receptors. Evidently, muscle activity induces metabolic acetylcholine receptor stabilization by way of an influx of Ca2+ ions through dihydropyridine-sensitive Ca2+ channels in the endplate membrane, whereas Ca2+ released from the sarcoplasmic reticulum is ineffective in this developmental process.