Reciprocal hindlimb scratching and putative subtype-selective muscarinic agents.

We have previously proposed that the reciprocal hindlimb scratching (RHS) of mice elicited by intrathecal injection of muscarinic agents is mediated by M1 muscarinic receptors. In this study, benzhexol potently inhibited pilocarpine-induced RHS (ID50 = 0.5 ng), methoctramine did not fully block RHS and RS 86 neither produced nor blocked RHS. These data (1) differentiate the three muscarinic agents using this in vivo endpoint, (2) substantiate binding studies characterizing benzhexol and methoctramine as putative M2 and M2 antagonists, respectively, and (3) further support using RHS for studying muscarinic agents.