5-Hydroxytryptamine causes depression of twitch height in the partially curarized rat phrenic nerve hemidiaphragm preparation.

5-Hydroxytryptamine (5-HT) has significant effects on the skeletal neuromuscular junction. We have evaluated the potential neuromodulatory effects of 5-HT on the rat hemidiaphragm using a non-fatiguing pattern of stimulation (0.3 Hz) in a partially curarized preparation using tubocurarine (EC50 = 560 (range 400-650) nmol litre-1). 5-HT was added to the partially curarized preparation in the following concentrations: 0, 12.5, 25, 50, 100, 200 and 400 nmol litre-1 and the twitch response was recorded at each concentration. These measurements were expressed as a ratio of control values, plotted against time and the area under the curve (AUC) calculated. The AUC values for test and control hemidiaphragms were compared by paired t test analysis accepting a 5% significance level. The mean AUC for test (3327 mm2) and control (4102 mm2) preparations were significantly different (P = 0.003). We have shown that the addition of low concentrations of 5-HT to the indirectly stimulated partially curarized rat diaphragm preparation caused depression of twitch height.