p-Fluoro-hexahydro-sila-difenidol exhibits poor selectivity between M3 and M1 muscarinic receptors

Muscarinic receptor subtypes in rat dorsal cochlear nucleus

We previously reported that responses of spontaneously active rat dorsal cochlear nucleus (DCN) neurons to cholinergic agonists are mediated predominantly by muscarinic receptors. We have now tested the effects of 7 antagonists with differing affinities for the muscarinic receptor subtypes M1-M4 on the responses to constant, submaximal doses of carbachol in rat brainstem slices. Each slice was exposed to one or more concentrations of one antagonist applied during extracellular recording of a DCN neuron. The concentrations yielding 50% reduction of test responses (IC50) of regular and bursting neurons were estimated for each antagonist. Correlation coefficients were calculated between log(IC50) values and log(Ki) values of the drugs for the receptor subtypes. Correlation coefficients for both regular and bursting neurons were not significant (P > 0.05) for M1 and M3, but were significant (P < 0.02) for M4. Bursting but not regular neurons also showed a significant correlation for M2 (P < 0.05). Our results suggest that (1) M4 contributes to the cholinergic responses in DCN and M2 may also contribute to the responses of bursting neurons, but the contribution of other subtypes cannot be completely excluded; (2) muscarinic subtypes in DCN probably differ from those reported for cochlea and some brain regions.

Using microphysiometry to study the pharmacology of exogenously expressed m1 and m3 muscarinic receptors

The microphysiometer, an instrument that uses a semiconductor-based sensor to monitor cellular metabolic activity, has been shown to detect the activation of a variety of receptors in living cells, largely irrespective of the signal-transduction mechanism. Using the Cytosensor Microphysiometer, we have studied agonist concentration responses for the activation of CHO-K1 cell lines exogenously expressing rat m1 or m3 receptors. Three levels of receptor expression were investigated for each subtype. Carbachol is more potent for m3 than m1 receptors (0.5 to 1.0 log unit lower EC50); for both, potency correlates positively with receptor density. The results agree well with those obtained by measuring phosphoinositide hydrolysis and intracellular [CA++] in the same cells. We also determined that two subtype-selective antagonists, pirenzepine (for m1) and p-fluoro-hexahydrosila-difenidol (for m3) displayed appropriate differential ability to shift carbachol concentration-response curves in the microphysiometer. This study provides additional evidence that pharmacological results obtained by microphysiometry are consistent with those obtained by more conventional functional assays.