Muscarinic and Nicotinic Cholinergic Binding Sites in the Terminal Abdominal Ganglion of the Cricket (Acheta domesticus)

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Abstract
Cricket (Acheta domesticus) terminal abdominal ganglia (TG) contain high concentrations (∼2 pmol/ mg protein) of muscarinic and nicotinic cholinergic binding sites, based on the capacity of TG to bind specifically the labelled ligands L-[3H]quinuclidinyl benzilate ([3H]QNB) and [125I]α-bungarotoxin ([125I]α-BGT) with high affinity. For both ligands, binding is saturable and reversible. Competitive displacement experiments indicate that the [3H]QNB and [125I]α-BGT binding sites probably represent pharmacologically distinct classes of putative TG acetylcholine receptors (AChRs). Results from physiological recording and autoradiographic localization experiments demonstrate that a portion of the putative nicotinic AChRs is localized in synaptic regions of the well-characterized cercal sensory-giant interneuron pathway in the TG, where they are likely to serve as functional synaptic AChRs. Unlike nicotinic ligands, muscarinic agents do not appear to be pharmacologically active in this pathway. Therefore, in the insect CNS, putative muscarinic and nicotinic AChRs coexist at high density, but can be pharmacologically distinguished from one another on the basis of criteria derived from both ligand binding and physiological methods.