POSTSYNAPTIC ACTIVATION AT THE SQUID GIANT SYNAPSE BY PHOTOLYTIC RELEASE OF L-GLUTAMATE FROM A CAGED L-GLUTAMATE

1. Pharmacological evidence SuggeStS L-glutamate is a strong candidate as a transmitter at the giant synapse of the squid. Postsynaptic acti vation at the giant synapse cannot be effected by conventional applica tion of putative neurotransmitters by iontophoresis or perfusion, appa rently because the complex structure of the synapse prevents a suffici ently rapid change in concentration at the postsynaptic membrane. Flas h photolytic release Of L-glutamate from a pharmacologically inert 'ca ged' L-glutamate pre-equilibrated in the stellate ganglion of Alloteut his or Loligo was used to determine whether L-glutamate can produce po stsynaptic activation when released rapidly in the synaptic clefts. 2. The preparation, reaction mechanism and properties of the caged L-glu tamate, N-1-(2-nitrophenyl)ethoxycarbonyl-L-glutamate, are described. The product quantum yield on photolysis was 0.65 (+/- 0.05). On flash photolysis glutamate release followed a single exponential timecourse in the pH range 5.5-7.8. The rate constant was proportional to [H+] an d was 93 s-1 at pH 5.5 and 16-degrees-C in artificial sea water (ionic strength, I = 0.68 M). 3. At pH 7.8 flash photolysis of caged glutama te pre-equilibrated in the synapse caused only a slow depolarization. A second photolytic release Of L-glutamate or transynaptic activation produced no further depolarization, suggesting desensitization and ina ctivation of postsynaptic mechanisms by the initial pulse Of L-glutama te. 4. Synaptic transmission in the giant synapse was normal at pH 5.5 . Flash photolysis at pH 5.5 caused rapid production of L-glutamate wi thin the synaptic cleft and a fast postsynaptic depolarization which g enerated postsynaptic action potentials. 5. These results, together wi th appropriate controls, provide direct evidence that L-glutamate is a neurotransmitter in the squid giant synapse.