Stereoselective increase in cholinergic transmission by R-(+)-hyoscyamine

R-(+)-hyoscyamine, the dextro enantiomer of atropine, has been shown to amplify cholinergic transmission. R-(+)-hyoscyamine, unlike S-(-)-hyoscyamine, was able to increase acetylcholine release both in vitro and in vivo at a range of concentrations (10(-14) to 10(-12) M) and doses (5 microg/kg i.p.) which were inadequate for blocking muscarinic receptors. The increase over control values in ACh release was 15.9 +/- 2.1% in in vitro experiments performed in rat phrenic nerve-hemidiaphragm preparations (n = 6), and 63.3 + 16.3% in cortical microdialysis performed in free-moving rats (n = 5). The maximum ACh release was reached 60 min after R-(+)-hyoscyamine administration in in vivo experiments. At the same doses and concentrations, R-(+)-hyoscyamine was also able to elicit: antinociception of a cholinergic type (55.6-112.7% depending on the test used); complete prevention of scopolamine- and dicyclomine-induced amnesia; potentiation of muscular contractions electrically evoked in isolated guinea-pig ileum (16.7 +/- 3.6%) and in rat phrenic nerve-hemidiaphragm (19.9 +/- 3.2%) preparations. Antinociception was performed using the hot-plate and acetic acid abdominal constriction tests in mice, and the paw pressure test in rats, while prevention of induced amnesia was evaluated in mice using the passive-avoidance test. The respective affinities (pA2) for R-(+)- and S-(-)-hyoscyamine vs M1 (rabbit vas deferens), M2 (rat atrium) and M3 (rat ileum) receptor subtypes were as follows: 7.05 +/- 0.05/9.33 +/- 0.03 for M1; 7.25 +/- 0.04/8.95 +/- 0.01 for M2; 6.88 +/- 0.05/9.04 +/- 0.03 for M3. The respective pKi values for R-(+)- and S-(-)-hyoscyamine vs the five human muscarinic receptor subtypes expressed in Chinese hamster oocytes (CHO-K1) were as follows: 8.21 +/- 0.07/9.48 +/- 0.18 for m1; 7.89 +/- 0.06/9.45 +/- 0.31 for m2; 8.06 +/- 0.18/9.30 +/- 0.19 for m3; 8.35 +/- 0.11/9.55 +/- 0.13 for m4; 8.17 +/- 0.08/9.24 +/- 0.30 for m5.