Actin filaments and the opposing actions of CaM kinase II and calcineurin in regulating alpha7-containing nicotinic receptors on chick ciliary ganglion neurons.

Nicotinic acetylcholine receptors containing alpha7 subunits have a high relative permeability to calcium and influence numerous calcium-dependent cellular events. On chick ciliary ganglion neurons the receptors are concentrated on somatic spines containing actin filaments. Using conventional whole-cell patch-clamp recording from dissociated ciliary ganglion neurons, we show that responses from alpha7-containing receptors undergo substantial rundown when the receptors are repeatedly challenged with nicotine. Stabilization of actin filaments with phalloidin partially prevents the rundown, whereas collapse of actin filaments with latrunculin A exacerbates it. The rundown depends on calcium influx through the receptors because it requires receptor activation and can be prevented by replacing extracellular calcium with barium or by intracellular dialysis with BAPTA. Thapsigargin and ryanodine each inhibit the rundown, demonstrating further a requirement for calcium release from internal stores. Blockade of calmodulin by calmidazolium or blockade of CaM kinase II with either KN93 or autocamtide-2-related inhibitory peptide each prevents the rundown; blockade of the phosphatase calcineurin with either cyclosporin A or deltamethrin increases the rundown. The results indicate a balance of calcium-dependent kinase and phosphatase activities in regulating the function of alpha7-containing receptors. Manifestation of the rundown depends in part on the loss of intracellular components via dialysis because little rundown is seen if perforated patch-clamp recording is used to monitor receptor responses even in latrunculin A-treated cells. A membrane-permeable calcineurin inhibitor, however, still decreases the nicotinic response in a calcium-dependent manner, confirming that calcium-dependent phosphoregulation of alpha7-containing receptors occurs in the intact cell.