EVIDENCE THAT CENTRAL 5-HT1A-RECEPTORS PLAY A ROLE IN THE VONBEZOLD-JARISCH REFLEX IN THE RAT

The effect of intracisternal (i.c.) application of putative 5-hydroxytryptamine (5-HT)(1A) antagonists on the reflex bradycardia evoked by injection of phenylbiguanide (i.v.) were investigated in anaesthetized, atenolol-pretreated rats. Intracisternal application of spiperone (100 μg kg-1) reversibly attenuated the reflex bradycardia whilst the same dose given i.v. had no effect. The bradycardia was also attenuated by i.c. methiothepin (200 μg kg-1), (±)-pindolol (100 μg kg-1) and buspirone (200 μg kg-1) but was not attenuated by antagonists selective for α1-adrenoceptors (alfuzosin; 100 μg kg-1), 5-HT2-receptors (BW 501C67; 100 μg kg-1) or dopamine D2-receptors ((-)-sulpiride; 100 μg kg-1) given i.c. It is concluded that the 5-HT(1A)-receptor antagonist action of intracisternally applied spiperone, methiothepin, (±)-pindolol and buspirone is responsible for the ability of these drugs to attenuate reversibly the excitation of cardiac vagal motoneurones caused by activation of the von Bezold-Jarisch reflex.