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AN ELECTROPHYSIOLOGICAL ANALYSIS OF THE EFFECTS OF MORPHINE ON THE CALCIUM DEPENDENCE OF NEUROMUSCULAR TRANSMISSION IN THE MOUSE VAS DEFERENS

1 The effects of morphine on the Ca‐dependence of the synaptic potential amplitude in the mouse vas deferens have been determined. 2 The synaptic potential increased with a power factor of 2.4 for [Ca]o between 0.7 mm and 1.8 mm. Morphine (40 nm) decreased the synaptic potential, without altering the second power relationship between the synaptic potential and [Ca]o. 3 Morphine reversed the depression in the synaptic potential which develops during a short high‐frequency (10 Hz) train of impulses to facilitation. Consequently the synaptic potential beyond the tenth impulse was unaffected by morphine. 4 Morphine did not alter the facilitation of the synaptic potential which develops during a short low‐frequency (≤2 Hz) train of impulses in normal [Ca]o. Consequently morphine decreased the synaptic potential for each impulse by about the same percentage amount. 5 Morphine increased the small facilitation in the synaptic potential which occurs during a short low‐frequency (≤2 Hz) train of impulses in high [Ca]o. This facilitation approximated the predictions based on the assumption that each impulse leaves residual Ca ions bound to receptors involved in transmitter release from the nerve terminal.