Synaptic modulation by a neuropeptide depends on temperature and extracellular calcium.

The crayfish neuropeptide DRNFLRFamide increases transmitter release from synaptic terminals onto muscle cells. As temperature decreases from 20 to 8 degrees C, the size of excitatory junctional potentials (EJPs) decreases, and the peptide becomes more effective at increasing EJP amplitude. The goal of the present study was to determine whether the enhanced effectiveness of the peptide is strictly a temperature-related effect, or whether it is related to the fact that the EJPs are smaller at low temperature, allowing a greater range for EJP amplitude to increase. Decreasing temperature reduced the number of quanta of transmitter released per nerve impulse (assessed by recording synaptic currents) and increased input resistance in muscle fibers. As in earlier work, the ability of the peptide to increase EJP amplitude was enhanced by decreasing temperature. However, the peptide was also more effective at increasing EJP amplitude when transmitter output was lowered by reducing the ratio of calcium to magnesium ions in the bath. Thus the effectiveness of the peptide may be related to the level of output from the synaptic terminals.