We tested the hypothesis that the probability of vesicular exocytosis at synapses is positively correlated with the pools of readily releasable synaptic vesicles, as shown for mammalian neurons grown in tissue culture. We compared synapses of two identified glutamatergic neurons: phasic (high-output, depressing) and tonic (low-output, facilitating) crustacean motor neurons, which differ 100- to 1000-fold in quantal content. Estimates of vesicles available for exocytosis were made from depletion during forced release and from electron microscopic observation of vesicles docked at synaptic membranes near active zones. Both measurements showed a significantly larger pool of readily releasable vesicles in facilitating synapses, despite their much lower quantal output during stimulation. Thus, the probability for release of docked vesicles is very much lower at facilitating synapses, and the presence of more docked vesicles does not predict higher synaptic release probability in these paired excitatory neurons.
Neuropeptides are found in both mammals and invertebrates and can modulate neural function through activation of G-protein-coupled receptors (GPCRS). The precise mechanisms by which many of these GPCRs modulate specific signaling cascades to regulate neural function are not well defined. We used Drosophila melanogaster as a model to examine both the cellular and behavioral effects of DPKQDFMRFamide, the most abundant peptide encoded by the dFMRF gene. We show that DPKQDFMRFamide enhanced synaptic transmission through activation of two G-protein-coupled receptors, Fmrf Receptor ( FR ) and Dromyosupressin Receptor-2 (DmsR-2 ). The peptide increased both the presynaptic Ca 2+ response and the quantal content of released transmitter. Peptide-induced modulation of synaptic function could be abrogated by depleting intracellular Ca 2+ stores or by interfering with Ca 2+ release from the endoplasmic reticulum through disruption of either the ryanodine receptor or the inositol 1,4,5-trisphosphate receptor. The peptide also altered behavior. Exogenous DPKQDFMRFamide enhanced fictive locomotion; this required both the FR and DmsR-2 . Likewise, both receptors were required for an escape response to intense light exposure. Thus, coincident detection of a peptide by two GPCRs modulates synaptic function through effects of Ca 2+ -induced Ca 2+ release, and we hypothesize that these mechanisms are involved in behavioral responses to environmental stress.
ABSTRACT Long-term adaptation (LTA), a phenomenon previously studied in the crayfish claw, was examined in one of the motoneurones innervating the phasic abdominal extensor muscles. The motoneurone was conditioned by electrically stimulating the second root of the third abdominal ganglion in situ for 4 h per day, using trains of stimuli with an average impulse frequency of 2·5 Hz. In juvenile crayfish, 3 days of conditioning produced a marked (81%) reduction in EPSP amplitude, which recovered only slightly during the succeeding 7 days. The quantal content of synaptic currents also decreased (by an average of 65 %). Estimated values of the binomial parameter p were lower for conditioned neurones than for controls, suggesting that the observed decrease in transmitter release involves a decrease in release probability. Conditioned neurones also displayed less synaptic depression than controls during repetitive stimulation at 5 Hz. In adult crayfish, conditioning for 7 days also produced a marked (74%) reduction in EPSP amplitude and resistance to synaptic depression. These results differ from those of previous work with the phasic axon of the claw closer muscle, which shows virtually no synaptic changes in adults after conditioning for 2 weeks. The ability to exhibit LTA, therefore, is not lost with age in all neurones.
Motor neurons innervate muscles through structures containing calcium channels and vesicles poised to release transmitter. A newly described protein organizes these structures.
