Aspects of acetylcholine receptor immunology and circulating receptor antibodies are reviewed with regard to both human and experimental myasthenia gravis. Receptor antibodies that have negligible cross-reactivity with skeletal muscle receptor can nonetheless cause destruction of the postsynaptic motor endplate area. Antibodies to mammalian skeletal muscle receptor can bind in-situ receptors. Transfer of myasthenic IgG increases neurophysiologic symptoms in rabbits showing slight signs of experimental myasthenia gravis, suggesting a block of in-situ receptors. Determination of receptor antibodies using RIA tests and partially purified human skeletal muscle receptor has been evaluated in the diagnosis of myasthenia gravis. Ninety percent of Swedish myasthenic patients in one study were found to have receptor antibodies. A rough correlation has been found between antibody titer and the severity of disease. Immunosuppressive treatment and thymectomy decrease the titer. In patients with thymoma, high antibody titers remain. When taken together, antibody-titer determination and electrophysiologic tests--particularly single-fiber electromyography--are very valuable diagnostic tools.

Animals↗

PubMedDate not supplied

Cholinergic and purinergic signalling in outer hair cells of mammalian cochlea.

Outer (OHC) and inner (IHC) hair cells in the organ of Corti of the mammalian cochlea process sound. OHC and their efferent synapse are part of a feedback system assumed to control and modulate information carried by afferent neurons passing from IHC to the brain. Underlying mechanisms are not well understood. This paper discusses recent progress. In vivo and in vitro information is presented on structure, pharmacology, function and localization of the pre- and postsynaptic acetylcholine receptors (AChRs) at the efferent synapse. Recent data are given on a presynaptic M3 AChR subtype, probably an autoreceptor involved in transmitter release. Data from our lab on specific binding of [3H]3-quinuclidinyl benzilate ([3H]3-QNB) to non-enzymatically isolated guinea pig OHC reveal a KD several 100 x higher than that for any known muscarinic receptor subtype, including the above-mentioned presynaptic muscarinic AChR of the OHC efferent synapse. The extremely high concentrations of [3H]3-QNB needed for any binding at all to OHC thus rule out presynaptic membrane impurities as the cause of such binding, and also the presence of a typical mAChR subtype on OHC. The number of [3H]3-QNB binding sites (approximately 10(6)/OHC) we found on OHC was 1/10th of that we found for binding of nicotinic ligands to OHC, further making it questionable that an ACh-binding site on OHC binds [3H]3-QNB. Observations may instead point to the possibility of another binding site, e.g. an (allosteric) site involved with the as yet not understood 'weak' muscarinic properties of the OHC AChR. Further new data on the OHC AChR confirm reversible alpha-bungarotoxin, nicotine and d-tubocurarine binding. [3H]alpha-Bungarotoxin and [3H]-nicotine binding sites are estimated at approximately 6.10(7) sites/OHC. Strychnine, a glycine receptor blocker suggested to interfere with cholinergic sites of the efferent OHC synapse, was found to bind to OHC (cold strychnine for unspecific binding). This binding, not seen in the presence of high [glycine], increased in the presence of depolarizing [K+], while ACh (100 microM) had no significant effect. Results suggest strychnine binding to the outside of OHC, but also sites accessible only after cell depolarization, possibly to the hyperpolarizing Ca2(+)-dependent K+ channel. Recent molecular cloning of the OHC AChR indicates a novel alpha-subunit. An often observed ACh-activated Ca(2+)-influx close to zero into OHC leaves an unanswered question. OHC also carry P2-purinergic receptors (P2Rs), a more rapid ionotropic P2zR-like subtype and a quantitatively dominating slow metabotropic P2yR subtype coupled to a G protein-phospholipase C cascade and not desensitized. Both contribute to increased cytoplasmic [Ca2+], from respectively external and internal sources. Whether or not such receptors are part of efferent synaptic activity is unknown; their localization on the OHC plasma membrane is so far only indirect and synaptic vesicles of the efferent nerve endings have not yet been analyzed for their ATP content. Localization, function and interaction of [Ca2+] increases triggered by, respectively, ATP and ACh are currently studied in this laboratory.

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PubMedDate not supplied

Cholinergic modulation of extracellular ATP-induced cytoplasmic calcium concentrations in cochlear outer hair cells.

Outer hair cells (OHC) of the mammalian cochlea modulate the inner hair cell (IHC) mechanoelectrical transduction of sound. They are contacted by synapsing efferent neurons from the CNS, their main efferent neurotransmitter being acetylcholine (ACh). OHC function and in particular their control of [Ca2+]i is highly important and is modulated by ACh and also by other substances including extracellular (EC) ATP. OHC carry at their efferent synapse a not yet completely identified neuronal type of ionotropic ACh receptor (AChR), with an unusual pharmacology, which is, in vivo and in vitro, reversibly blocked by alpha-bungarotoxin (alpha-bgtx). The AChR mediates a fast influx of Ca2+ into OHC which, in turn, activates a closeby located outwardly-directed Ca(2+)-dependent K(+)-channel, thus shortly hyperpolarizing the cell. A cloned homomeric alpha 9 nAChR mimicks the function and pharmacology of this receptor. We here report results from a study designed to observe only slower effects triggered by EC ATP and the ACh-AChR system. EC presence of ATP at OHC increases [Ca2+]i by activating both P2x and P2y purinoceptors and also by indirect activation of OHC L-type Ca(2+)-channels. The L-type channel activation is responsible for a large part of the [Ca2+]i increase. Simultaneous EC presence of ACh and ATP at OHC was found to depress ATP-induced effects on OHC [Ca2+]i, an effect that is completely blocked in the presence of alpha-bgtx. Our observations suggest that the ACh-AChR system is involved in the modulation of the observed EC ATP-triggered events; possibly the OHC AChR is able to act both in its well known rapid ionotropic way, but also, perhaps after modification in a slower, metabotropic way interfering with the EC ATP-induced [Ca2+]i increase.

Acetylcholine↗

PubMedDate not supplied

Alpha-bungarotoxin inhibits outer hair cell motility in situ.

The effect of two substances (alpha-bungarotoxin, alpha-BGTX, a small protein, and the local anesthetic bupivacaine hydrochloride) with an assumed effect on outer hair cell (OHC) motility were analyzed after exposing the cochlea via the round window membrane. Electrophysiological measurements were performed with a very narrow frequency-specific gating (+/- 100 Hz) technique to determine auditory brainstem response (ABR) thresholds, including ABR-based frequency tuning curves. Exposure to alpha-BGTX gave a minor improvement in thresholds, interpreted as a facilitation of OHCs, i.e. releasing their efferent inhibitory control, whereas exposure to bupivacaine hydrochloride impaired ABR thresholds, possibly due to immobilization of OHC motility via the lateral cell membrane. Our results are consistent with the hypothesis that efferent influence on the cochlea may be linked with a modulation of the mechanical function of OHCs. We can now postulate that there is in vivo evidence that acetylcholine exerts its effect at the OHCs via an alpha-BGTX alpha-BGTX binding acetylcholine receptor.

Acetylcholine↗

PubMedDate not supplied

Neurotoxic effects of manganese: studies on cell cultures, tissue homogenates and intact animals.

This paper gives a short summary of some neurotoxic effects of manganese. Further, a model of toxicological action of manganese is discussed. Such a model should probably involve influences via redox reactions. In this study results revealed that the lipid peroxidation was reduced when neuroblastoma cells in culture were treated with manganese chloride. Additionally, the thiol content of proteins as well as non-protein thiols were reduced in the same experimental system. Due to the essential function of thiol groups in life processes, the observed drop in the amount of thiols must be of some importance for the toxicology of manganese. In relation to this, previously known physiological and biochemical effects of manganese are discussed.

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