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Antagonism by (R)‐ and (S)‐trihexyphenidyl of muscarinic stimulation of adenylyl cyclase in rat olfactory bulb and inhibition in striatum and heart

1 Activation of muscarinic receptors in rat olfactory bulb stimulates adenylyl cyclase activity. This response was competitively antagonized by the (R)‐ and (S)‐enantiomers of trihexyphenidyl with pA2 values of 8.84 and 6.09, respectively. 2 Similarly, in rat striatal homogenates, muscarinic inhibition of adenylyl cyclase activity was antagonized by the (R)‐ and (S)‐enantiomers with pA2 values of 8.75 and 6.12, respectively. 3 In contrast, in rat myocardium the muscarinic inhibition of the adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) formation was more weakly antagonized by trihexyphenidyl, with a particularly marked loss (15 fold) in activity of the (R)‐enantiomer. The (R)‐ and (S)‐enantiomers had pA2 values of 7.64 and 5.72, respectively. 4 Each muscarinic response was completely antagonized by increasing concentrations of (R)‐trihexyphenidyl with a Hill coefficient not significantly different from unity. 5 The present study shows that the muscarinic receptors coupled to stimulation of adenylyl cyclase in the olfactory bulb display high stereoselectivity for the enantiomers of trihexyphenidyl. The affinities of these receptors for the antagonists are similar to those shown by the striatal receptors. This finding supports the hypothesis that both the muscarinic stimulation of adenylyl cyclase in the olfactory bulb and the muscarinic inhibition of the enzyme in striatum are mediated by activation of a receptor subtype pharmacologically equivalent to the m4 gene product. On the other hand, the weaker affinities and the lower stereoselectivity for the trihexyphenidyl enantiomers exhibited by the muscarinic inhibition of adenylyl cyclase in the heart are consistent with the involvement of M2 receptors in this response.