Abstract
5-Hydroxytryptamine (5-HT) (5 x 10-8 to 5 x 10-6 M) perfused through the isolated heart of Limulus polyphemus, mimics stimulation of the cardio-inhibitory nerves by decreasing rate and strength of beating of the heart. 5-HT, like activity in the cardio-inhibitory nerves, also decreases the number of units discharging and the total duration of each burst of electrical activity in the cardiac ganglion. Bromlysergic acid diethylamide (1.6 x 10-5 M) decreases the ability of the cardio-inhibitory nerves to influence heart rate and also prevents the rate and strength changes engendered by 5-HT applied to the heart. Picrotoxin also prevents the rate changes produced by 5-HT, but synergizes with 5-HT with respect to the strength-decreasing abilities of 5-HT. No interaction between bromlysergic acid diethylamide and gamma aminobutyric acid could be demonstrated.