Reciprocal hindlimb scratching and putative subtype-selective muscarinic agents

Area of Science:

• Neuropharmacology
• Behavioral Neuroscience

Background:

• The reciprocal hindlimb scratching (RHS) response in mice, induced by intrathecal administration of muscarinic agents, has been previously proposed to be mediated by M1 muscarinic receptors.
• Understanding the specific receptor subtypes involved in mediating behavioral responses is crucial for developing targeted pharmacological interventions.

Purpose of the Study:

• To further investigate the muscarinic receptor subtype(s) responsible for mediating the RHS response in mice.
• To differentiate the pharmacological profiles of benzhexol, methoctramine, and RS 86 using the RHS assay as an in vivo endpoint.
• To provide further validation for the use of the RHS assay in the study of muscarinic agents.

Main Methods:

• Intrathecal injections of pilocarpine to elicit RHS in mice.
• Administration of varying doses of benzhexol, methoctramine, and RS 86 to assess their effects on pilocarpine-induced RHS.
• Determination of inhibitory dose 50 (ID50) values for compounds exhibiting inhibitory effects.

Main Results:

• Benzhexol demonstrated potent inhibition of pilocarpine-induced RHS with an ID50 of 0.5 ng.
• Methoctramine did not fully block the RHS response.
• RS 86 neither produced nor blocked the RHS response.

Conclusions:

• The data differentiate benzhexol, methoctramine, and RS 86 in the context of the in vivo RHS assay.
• The results substantiate previous binding studies, characterizing benzhexol and methoctramine as potential M2 and M2 antagonists, respectively.
• The study further supports the utility of the reciprocal hindlimb scratching assay for investigating the pharmacology of muscarinic agents.