Jpn. J. Pharmacol. 81 (1), 34 - 37 (1999)

Antinociceptive Effect of R-(+)-Hyoscyamine on the Conjunctival Reflex Test in Rabbits

Carla Ghelardini¹, Nicoletta Galeotti¹, Lia Fantetti¹, Fulvio Gualtieri²,
Serena Scapecchi² and Alessandro Bartolini<sup>1


1</sup>Department of Pharmacology, University of Florence, Viale G. Pieraccini, 6, I-50139 Florence, Italy
²Department of Pharmaceutical Sciences, University of Florence, Via G. Capponi, 9, I-50121 Florence, Italy

Abstract: R-(+)-Hyoscyamine (1 - 10 μg/kg, s.c.) dose-dependently increased the local anesthetic effect of procaine (50 μg/ml) and lidocaine (50 μg/ml) in the conjunctival reflex test in the rabbit. This potentiating effect is completely prevented by the M₁ antagonist dicyclomine (10 mg/kg, s.c.). The intensity of R-(+)-hyoscyamine antinociception was comparable to that induced by morphine (2 mg/kg, s.c.) and minaprine (15 mg/kg, s.c.), used as analgesic reference drugs. In the same experimental conditions, the S-(-)-enantiomer of atropine (0.1 - 10 μg/kg, s.c.), was completely ineffective. The present results confirm the ability of R-(+)-hyoscyamine to produce a paradoxical antinociceptive effect mediated by a cholinergic mechanism not only in rodents but also in the rabbit.

Keywords: Atropine, R-(+)-Hyoscyamine, Antinociception, Local anesthetics, Cholinergic system