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Fig. 3. Serotonin (5-HT) increases the amplitude of EJPs and neurally evoked
contractions. (A) Representative recordings of EJPs (upper traces) and the
corresponding twitch contractions (lower traces) at 2°C are shown under
control conditions and 22 min after the addition of 50 nmol
l–1 serotonin. Each trace is the average of ten consecutive
recordings in response to a 200 ms train of 10 Hz stimulation. Serotonin
increased EJP size in this muscle fiber by approximately 30% and increased
twitch tension recorded from the entire muscle by 800%. (B) Serotonin elicits
a dose-dependent increase in EJP amplitude, with a threshold of approximately
1 nmol l–1 and maximal effects at 1 µmol
l–1. Results are expressed as the logarithm of the ratio of
EJP amplitude after application of serotonin with respect to its control
value. Symbols represent 13 measurements from 10 different preparations at a
temperature of 2°C. (C) Serotonin is most effective at potentiating small
EJPs. Symbols illustrate the magnitude of potentiation of EJPs by 50 nmol
l–1 serotonin recorded at bath temperatures of 16°C
(squares) and 2°C (triangles) expressed as the logarithm of the ratio of
EJP amplitude after application of serotonin with respect to its control
value. Each symbol represents a measurement from a different preparation.
Slopes of linear fits to data (not shown) are negative and significantly
different from zero (at 2°C slope=–0.03, P=0.002; at
16°C slope=–0.13, P=0.03). (D) Serotonin triggers muscle
action potentials. At 3°C in the presence of 50 nmol l–1
serotonin (broken trace) an action potential appeared superimposed on the
fourth EJP evoked by a 30 Hz, 600 ms stimulus train. The control recording at
the same temperature is shown as a solid trace.
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