TEMPORAL DYNAMICS OF GRADED SYNAPTIC TRANSMISSION IN THE LOBSTER STOMATOGASTRIC GANGLION

Synaptic transmission between neurons in the stomatogastric ganglion o f the lobster Panulirus interruptus is a graded function of membrane p otential, with a threshold for transmitter release in the range of -50 to -60 mV. We studied the dynamics of graded transmission between the lateral pyloric (LP) neuron and the pyloric dilator (PD) neurons afte r blocking action potential-mediated transmission with 0.1 mu M tetrod otoxin. We compared the graded IPSPs (gIPSPs) from LP to PD neurons ev oked by square pulse presynaptic depolarizations with those potentials evoked by realistic presynaptic waveforms of variable frequency, ampl itude, and duty cycle. The gIPSP shows frequency-dependent synaptic de pression. The recovery from depression is slow, and as a result, the g IPSP is depressed at normal pyloric network frequencies. Changes in th e duration of the presynaptic depolarization produce nonintuitive chan ges in the amplitude and time course of the postsynaptic responses, wh ich are again frequency-dependent. Taken together, these data demonstr ate that the measurements of synaptic efficacy that are used to unders tand neural network function are best made using presynaptic waveforms and patterns of activity that mimic those in the functional network.