Transmitter release from nerve terminals undergoing suppression

Abstract

Experiments aimed at investigating the mechanism behind the decline in quantal content of foreign nerve transmission in the axolotl supracoracoideus muscle were performed. The effect of K + on spontaneous transmitter release from suppressed foreign terminals was investigated after the degeneration of native terminals. All terminals studied responded with an increase in spontaneous miniature endplate potential frequency. The possibility that transmitter release from suppressed nerve terminals is less sensitive to Ca 2+ than in non-suppressed terminals was investigated directly. Quantal content of transmission from foreign terminals undergoing suppression and from foreign terminals in muscles not reinnervated by the native nerve was measured at 3 different Ca 2+ concentrations. The relationship between the log of quantal content and log Ca 2+ concentration was similar for suppressed foreign and non-suppressed foreign terminals suggesting that there is no difference in the sensitivity to Ca 2+ of the transmitter release mechanism in the two classes of terminals. Facilitation was compared for the two classes of terminals using a pair of stimuli separated by 20 msec. No significant difference was found. No difference was also found in the degree of depression of quantal content during 1 Hz stimulation for the two classes of nerve terminal. Some of these results differ considerably from those reported previously. The possibility that the lower level of spontaneous and evoked transmitter release from suppressed terminals may be due to a reduced overall number of Ca 2+ channels in the terminal membrane or a reduced availability of transmitter release sites is compatible with reduction in the size of the nerve terminal during suppression.