Modulation of Muscle Nicotinic Acetylcholine Receptors by the Glucocorticoid Hydrocortisone

Abstract

Mechanisms of ion channel blockade by noncompetitive inhibitors of the nicotinic acetylcholine receptor (AChR) have been particularly difficult to elucidate. We have combined here transient expression of embryonic, adult, and a mutated adult muscle AChR associated with a slow channel syndrome (Ohno, K., Hutchinson, D. O., Milone, M., Brengman, J. M., Bouzat, C., Sine, S., and Engel, A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 758-762) with single channel recordings to determine subunit specificity and mechanisms of action of the prototype glucocorticoid hydrocortisone (HC). HC affected in a similar manner the gating kinetics of all types of muscle AChR, producing briefer openings with normal amplitudes. We postulate that this steroid acts as a noncompetitive inhibitor of the AChR and that its mechanism of action can be interpreted in terms of blocking models. The forward rate constant for the blocking process was also similar for all channel types, indicating that the structural differences between them are not responsible for the effect. The reduction in the channel open time was not dependent on agonist concentration; it was slightly voltage dependent, suggesting that HC binds to a site located inside the membrane that senses the electric field. Recordings at high acetylcholine concentration in the presence of HC showed a reduced number of openings per activation period and the long closed times typically observed in the desensitization phenomenon. In competition studies with the classical open channel blocker QX-222, HC induced an early termination of the burst, suggesting that the two act at different sites. Taken together the results support the existence of specific sites sensed by the membrane field, different from those of open channel blockers and probably located at the lipid-protein interface. From this site(s), glucocorticoids and other hydrophobic noncompetitive inhibitors could allosterically mediate channel blockade.

Unique Effects of Different Fatty Acid Species on the Physical Properties of the Torpedo Acetylcholine Receptor Membrane

“…In living BC3H-1 cells in culture, fatty acids exert inhibitory effects on AChR channel activity (31). These effects are also observed in membrane patches excised from the cell and are therefore not mediated by signal transduction pathways that require soluble factors such as nucleotides and Ca 2ϩ .…”

“…Previous studies from several laboratories demonstrate that free fatty acids (FFA) inhibit the ion flux mediated by the AChR in vitro (29,30) or in vivo (31). Analysis of single-channel electrophysiological data argues for a mechanism compatible with noncompetitive inhibition of the AChR.…”

Anaesthetic Effects on the Hepatic Portal Vein and on the Vascular Resistance of the Tail of the Chinook Salmon (Oncorhynchus tshawytscha)

“…Cortisol acts as a non-competitive inhibitor of acetylcholine channels, reducing mean open times in vitro (Bouzat and Barrantes 1996;Nurowska and Ruzzier 2002). Even in the presence of high acetylcholine concentrations, cortisol reduced the number of channel openings and caused long closed times, mimicking those typically observed during desensitisation (Bouzat and Barrantes 1996). High levels of cortisol in Ônew' fish may have desensitised acetylcholine channels, reducing the effects of AQUI-S TM in this group.…”

“…There is recent evidence to suggest AQUI-S TM acts as a depolarising, competitive blocker of nicotinic acetylcholine channels (Ingvast-Larsson et al 2003), and this may be the mechanism of action for this anaesthetic. Cortisol acts as a non-competitive inhibitor of acetylcholine channels, reducing mean open times in vitro (Bouzat and Barrantes 1996;Nurowska and Ruzzier 2002). Even in the presence of high acetylcholine concentrations, cortisol reduced the number of channel openings and caused long closed times, mimicking those typically observed during desensitisation (Bouzat and Barrantes 1996).…”

Nicotinic acetylcholine receptors: from structure to brain function

“…The glucocorticoid hydrocortizone and mono-hydroxylated steroids such as progesterone shortened the open times of the muscle nAChR by up to 60% and displayed an inverse relationship between lipophilicity and their inhibitory potency, suggesting that their effects are not caused by a perturbation of the lipid membrane (Bouzat and Barrantes 1996;Garbus et al 2001). Steroids did not affect binding of radio- labeled cytisine to cell homogenates containing neuronal a4b2 receptors (Sabey et al 1999) and steroid effects at the nAChR were not affected by the presence of agonists, indicating a noncompetitive block mechanism.…”