Two distinct calcium‐sensitive and ‐insensitive PKC up‐ and down‐regulate an α‐bungarotoxin‐resistant nAChR1 in insect neurosecretory cells (DUM neurons)

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Abstract
While there is mounting knowledge about the structure and diversity of insect neuronal nicotinic acetylcholine receptors, less attention has been directed towards their intracellular regulation by calcium-mediated activation or inhibition of protein phosphorylation. The main goal of this work was to delineate the chain of molecular events that lead to the up- and down-regulation by two protein kinase Cs of an insect neuronal alpha-bungarotoxin-resistant nicotinic acetylcholine receptor (called nAChR1). The native nicotinic acetylcholine receptor intracellular regulation was studied on dissociated adult dorsal unpaired median neurons isolated from the terminal abdominal ganglion of the cockroach Periplaneta americana using whole-cell patch-clamp technique and calcium imaging. We report that under 0.5 micro malpha-bungarotoxin treatment, the inward current produced by pressure ejection application of nicotine onto the cell body was differentially sensitive to specific protein kinase C activators and inhibitors. The phorbol ester PMA produced a calcium-dependent increase in current amplitude blocked by chelerythrine. By contrast, the diacylglycerol analogue 1,2-dioctanoyl-sn-glycerol produced a calcium-independent reduction of the nicotinic response, reversed by rottlerin and chelerythrine. This indicated that two protein kinase C isozymes ('classical' and 'novel' protein kinase C, named PKC1 and PKC2, respectively) up- and down-regulated nicotinic acetylcholine receptor function. PMA and 1,2-dioctanoyl-sn-glycerol effects were mimicked by pirenzepine-sensitive M1 muscarinic receptor subtype coupled to phospholipase C second messenger pathway. Low concentration of muscarine elevated internal calcium levels, which thereby activated PKC1. By contrast, a high concentration of muscarine strongly increased [Ca 2+]i, which induced inhibition of PKC1. This effect was reversed by FK506, suggesting the implication of PP2B which unmasked PKC2 activity mediating down-regulation of nicotinic acetylcholine receptor.