Nematode cholinergic pharmacology

Nematode acetylcholine (ACh) receptors were characterized using both biochemical and electrophysiological techniques, including: (1) receptor binding studies in crude homogenates of the free-living nematode Caenorhabditis elegans and the parasitic nematode Ascaris lumbricoides with the high-affinity probe ({dollar}sp3{dollar}H) N-methylscopolamine ( ({dollar}sp3{dollar}H) NMS) which binds to muscarinic receptors in many vertebrate and invertebrate tissues; (2) measurement of depolarization and contraction induced by a variety of cholinergic agents, including N-methylscopolamine (NMS), in an innervated dorsal muscle strip preparation of Ascaris; (3) examination of the antagonistic actions of d-tubocurarine (dTC) and NMS at dorsal neuromuscular junction; (4) measurement of input resistance changes in Ascaris commissural motorneurons induced by ACh, dTC, NMS, pilocarpine and other cholinergic drugs.; The binding assay revealed a population of high affinity ({dollar}sp3{dollar}H) NMS binding sites (K{dollar}sb{lcub}rm D{rcub}{dollar} = 0.5nM) in both C.elegans and larval Ascaris with characteristics similar but not identical to muscarinic receptors in other systems. Ascaris muscle depolarized and contracted in a dose-dependent manner in response to ACh, nicotine (a nicotinic agonist) and pilocarpine (a muscarinic agonist); however, dTC was a better antagonist for these drugs than NMS. 100{dollar}mu{dollar}M dTC antagonized all evoked output from dorsal excitatory motorneurons to muscle. 100{dollar}mu{dollar}M NMS apparently antagonized only DE1 and DE3 motorneuron classes, but had no effect on DE2 output to muscle. Motorneuron input resistance measurements taken in cobalt-containing saline demonstrated that 100{dollar}mu{dollar}M NMS reduced the input resistance of the DE1 class of motorneurons, but had no effect on DE2, suggesting that the apparent antagonism of NMS at the DE1 neuromuscular junction may be caused by conduction failure in the motorneuron rather than receptor blockade.; All motorneurons examined possess dTC-sensitive ACh receptors which mediate reductions in their input resistance. However, these cells can be distinguished by differences in response to other cholinergic agents. NMS does not have physiological activity in nanomolar concentrations, but high (100{dollar}mu{dollar}M) doses induce changes in muscle and in specific subsets of commissural motorneurons. These data suggest that the high affinity ({dollar}sp3{dollar}H) NMS binding sites are not present in the Ascaris neuromuscular preparation.