DIRECT ACCESS OF IONS TO THE SQUID STELLATE GANGLION GIANT SYNAPSE BY AORTIC PERFUSION: EFFECTS OF CALCIUM-FREE...

The giant synapse in the squid stellate ganglion has served as a model in the understanding of normal synaptic transmission, but has not been used extensively in the study of changes in external ion concentrations or pharmacological agents. This anomaly is due primarily to the substantial diffusion barrier that exists between the synapse and the bathing medium. The present study describes a technique for the rapid introduction of substances into the synapse by perfusion through the arterial blood supply that improves access by at least 50-fold. This is demonstrated by treatments known to block the Ca²⁺ activated release of transmitter from the nerve terminal: Ca²⁺-free medium, La³⁺, and Cd²⁺. Whereas such treatments take 20 minutes to 3 hours to block transmitter release with bath application, with perfusion they act within a few seconds. The excitatory postsynaptic potential (EPSP) is reduced below action potential threshold in 5 to 22 seconds, and disappears completely in less than a minute. In addition, the use of pressurized O₂ to drive the perfusate through the preparation eliminates the need for superfusion with O₂ and aids in the long term maintenance of the ganglion. This study confirms the important role of Ca²⁺ in the release of transmitter at the giant synapse, and opens up this neurobiologically important preparation for ionic and pharmacologic evaluation.