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

Abstract

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.

The effects of brucine and alcuronium on the inhibition of [3H]acetylcholine release from rat striatum by muscarinic receptor agonists

“…Data from a recent quantitative immunocytochemical study indicate that, of the total population of striatal muscarinic receptors in adult rats, 66% belong to the M 4 , 23% to the M 1′ 9% to the M 2 and 3% to the M 3 subtype (Tice et al , 1996). Biochemically, the prevalence of the M 4 (compared to the M 2 ) receptor subtype in the striatum is supported by data indicating that it is the M 4 subtype which is responsible for the inhibition of adenylyl cyclase in striatal membranes by muscarinic receptor agonists (Onali et al , 1994; Olianas et al , 1996). Experiments with subtype‐selective antimuscarinic toxins indicate that more than 90% of muscarinic binding sites in rat striatum belong to the M 4 and M 1 receptor subtypes (Purkerson & Potter, 1998).…”

Identification of Muscarinic Receptor Subtypes in RINm5F Cells by Means of Polymerase Chain Reaction, Subcloning, and DNA Sequencing

“…Pharmacological studies strongly suggest the involvement of the M 3 receptor subtype, rather than the M 4 subtype, in the muscarinic stimulation of insulin release in the cells. It should be noted, however, that we cannot rule out an additional minor role for the M 4 receptor if it too had a novel signal transduction coupling, an example of such a coupling being the recent report of an M 4 receptor positively coupled with adenylyl cyclase (35,36). We conclude from these results and our previous findings (11) that the M 3 receptor is most likely responsible for the unusual carbachol response in the RINm5F cell and that a novel signaling pathway, other than elevated [Ca 2+ ] ; and DAG, is involved.…”

Rat striatal muscarinic receptors coupled to the inhibition of adenylyl cyclase activity: potent block by the selective m4 ligand muscarinic toxin 3 (MT3)

“…Acetylcholine (ACh) inhibition of forskolin-stimulated adenylyl cyclase activity was determined as previously described (Onali et al, 1994). The reaction mixture (final volume: 100,ul) contained: HEPES/NaOH 50 mM (pH 7.4), MgCl2 2.3 mM, EGTA 0.3 mM, [a 32P] adenosine-5'-triphosphate (ATP) 0.05 mM (50-70 c.p.m.…”

“…Lazareno et al 1990). The possibility that the M4 subtype mediates the muscarinic inhibition of striatal adenylyl cyclase was also supported by the studies of Olianas & Onali (1991) and Onali et al (1994). However, the lack of selective M4 'Author for correspondence.…”