Cholinergic Currents in Leg Motoneurons ofCarausius morosus

Abstract

We used patch-clamp recordings and fast optical Ca2+imaging to characterize an acetylcholine-induced current ( IACh) in leg motoneurons of the stick insect Carausius morosus. Our long-term goal is to better understand the synaptic and integrative properties of the leg sensory-motor system, which has served extremely successfully as a model to study basic principles of walking and locomotion on the network level. The experiments were performed under biophysically controlled conditions on freshly dissociated leg motoneurons to avoid secondary effects from the network. To allow for unequivocal identification, the leg motoneurons were backfilled with a fluorescent label through the main leg nerve prior to cell dissociation. In 87% of the motoneurons, IAChconsisted of a fast-desensitizing ( IACh1) and a slow-desensitizing component ( IACh2), both of which were concentration dependent, with EC50values of 3.7 × 10−5and 2.0 × 10−5M, respectively. Ca2+imaging revealed that a considerable portion of IACh(∼18%) is carried by Ca2+, suggesting that IACh, besides mediating fast synaptic transmission, could also induce Ca2+-dependent processes. Using specific nicotinic and muscarinic acetylcholine receptor ligands, we showed that IAChwas exclusively mediated by nicotinic acetylcholine receptors. Distinct concentration–response relations of IACh1and IACh2for these ligands indicated that they are mediated by different types of nicotinic acetylcholine receptors.