A role for potassium channels in the regulation of cortical muscarinic acetylcholine receptors in an in vitro slice preparation

Abstract

The rules underlying muscarinic acetylcholine receptor (mAChR) regulation in an in vitro cortical slice preparation of adult rats were examined following various alterations of bioelectric activity and following agonist stimulation. Muscarinic ACh antagonists [ 3 H]N- methyl scopolamine ([ 3H]NMS) or [ 3H]quinuclidinyl benzylate ([ 3H]QNB) were used to label cell surface vs total (i.e. surface and internal) receptors, respectively. Depolarization of neural membranes for 4 h at 22–37 °C using veratridine or high external potassium (K +) led to a temperature-dependent down-regulation of surface mAChR of 26.2% and 11.3%. Total mAChRs decreased by 37.6% and 8.1%. Addition of picrotoxin and glutamic acid also led to decreases in mAChRs. Increases in inward chloride ion current induced by γ-aminobutyric acid (GABA) or gold chloride had no significant effect on mAChRs. Blockade of calcium channels and synaptic transmission by magnesium or cobalt and postsynaptic calcium channels with nifedipine showed a significant effect on mAChRs only in the latter case. In contrast, agonist stimulation using carbachol led to a large down-regulation for both [ 3H]NMS and [ 3H]QNB (26.1%, 35.9%). ACh decreased [ 3H]QNB binding by 33.9%, but had little effect on [ 3H]NMS binding (6.3%). For [ 3H]QNB binding sites the effects of carbachol appeared to summate with those of veratridine. Down-regulation of [ 3H]NMS labelled mAChRs by carbachol and veratridine had an estimated half-time of 30 min and 2 h, respectively. Neither the effects of veratridine nor carbachol could be antagonized by tetrodotoxin (TTX), showing that the effects were not due to an increase in sodium ion currents. However, a common thread linking the various agents which induce mAChR down-regulation appears to involve changes in potassium (K +) current. Potassium channel blockers tetraethylammonium chloride (TEA), 4-aminopyridine (4-AP) and apamin had little independent effect on mAChR number, but prevented veratridine-induced down-regulation, presumably through a blockade of K +- and Ca 2+-dependent K +-channels. Only TEA and 4-AP diminished carbachol-induced down-regulation suggesting that this effect involves only the non Ca 2+-dependent K +-channels. It thus appears that mAChR regulation in the rat cerebral cortex is linked to changes in active K +-channel currents: activation of the K +-channel by depolarization-induced changes in K + current or by agonist stimulation leading to changes in the selective K + currents stimulate mAChR down-regulation; blockage of the K +-channels prevents this down-regulation.