Occurrence of muscarinic acetylcholine receptors in wild type and cholinergic mutants of Caenorhabditis elegans

Crude homogenates of the nematode worm Caenorhabditis elegans were shown to bind the cholinergic antagonists [3H]N-methylscopolamine and quinuclidinyl benzilate ([3H]QNB) with high affinity. The dissociation constant for [3H]N-methylscopolamine binding determined from equilibrium saturation experiments (KD = 3.7 x 10-l' M) was in good agreement with that determined from forward and reverse rate constants (KD = k&k,, = 5 x 10-l' M). These binding sites were blocked stereospecifically by the (+) enantiomer of the muscarinic antagonist benzetimide, as would be expected of true muscarinic receptors. Furthermore, competition experiments with unlabeled cholinergic agonists and antagonists indicate that C3H]QNB and [3H]N-methylscopolamine bind to the same sites in nematode homogenates and that these sites are similar but not identical to muscarinic receptors in vertebrates. The concentration of [3H]Z'V-methylscopolamine and [3H]QNB binding sites in adult populations of wild-type nematodes was approximately 10 fmol/mg of protein. This was approximately 4-fold lower than the concentration of binding sites in young (Ll and L2 stage) juveniles. These stage-specific differences in binding site concentrations parallel differences in acetylcholinesterase activity in larval and adult nematodes (Culotti, J., G. von Ehrenstein, M. R. Culotti, and R. L. Russell (1981