Activation of muscarinic acetylcholine receptors elevates intracellular Ca²⁺ concentrations in accessory lobe neurons of the chick

Abstract

Accessory lobes are protrusions located at the lateral sides of the spinal cord of chicks and it has been proposed that they play a role as a sensory organ for equilibrium during walking. We have reported that functional neurons exist in the accessory lobe. As there is histological evidence that synaptic terminals of cholinergic nerves exist near the somata of accessory lobe neurons, we examined the effects of acetylcholine on changes in intracellular Ca²⁺ concentrations ([Ca²⁺]_i), as an index of cellular activities. Acetylcholine (0.1–100 µM) caused a transient rise in the [Ca²⁺]_i. Acetylcholine-evoked [Ca²⁺]_i rises were observed in the absence of extracellular Ca²⁺, and they were abolished in the presence of cyclopiazonic acid, an inhibitor of Ca²⁺-ATPase of intracellular Ca²⁺ stores or atropine, a muscarinic receptor antagonist. mRNAs coding M₃ and M₅ isoforms of the muscarinic receptors were detected in accessory lobes by the RT-PCR. These results indicate that chick accessory lobe neurons express functional muscarinic acetylcholine receptors, and that acetylcholine stimulates Ca²⁺ mobilization from intracellular Ca²⁺ stores, which elevates the [Ca²⁺]_i in the somata of accessory lobe neurons, through activation of these receptors. Cholinergic synaptic transmission to the accessory lobe neurons may regulate some cellular functions through muscarinic receptors.