The action of calcium on neuronal synapses in the squid

Publisher Website
Google Scholar
Abstract
The isolated stellate ganglion of the squid (L. pealii) was studied with intracellular and extracellular micro-electrodes. Three or four nerve fibers in the preganglionic nerve establish synaptic relations with the giant axon in the last stellar nerve. Accordingly, 1-3 small presynaptic spikes (< 1 mV) could be recorded from within the post-synaptic axon. A micro-electrode was inserted in the presynaptic fiber and used to polarize and record simultaneously. In the distal (giant) synapse, hyperpolarization of the ending produced an increase in the size of the presynaptic action potential and post-synaptic potential (PSP). Depolarization had the opposite effect. These effects of polarization took more than 10 sec to develop fully, and declined with a similar time course at the end of polarization. Analogous results were obtained with two other preganglionic fibers, which make contacts in the proximal synaptic region. The 2nd of a pair of preganglionic impulses evoked a PSP larger than the first. This facilitation of PSP was sometimes accompanied by a small increase in the size of the second action potential in the presynaptic axon. At some shorter intervals, the 2nd presynaptic action potential was reduced in amplitude, but the PSP was still increased. Hyperpolarization of the presynaptic terminal increased the size of both PSPs in a pair and abolished the facilitation. With stronger hyperpolarization the 2nd PSP was even smaller than the 1st. Removing or reducing the Ca in the bathing fluid reversibly abolished the post-synaptic response. The small presynaptic spikes remained practically unaffected. In these conditions a nerve impulse still invaded the ending and normal action potentials could be recorded from the presynaptic terminal. This shows that electrical coupling between pre- and post-synaptic axons is insufficient to account for synaptic transmission. In low-Ca solution synaptic transmission could be restored locally by extracellular ionophoretic application of Ca to a small portion of the synapse. At sensitive spots a post-synaptic current (recorded with the Ca pipete) and PSP could be detected earlier than 1 sec after commencing the application of Ca. Ca was ineffective when injected intracellularly into the presynaptic fiber at a spot where extracellular ionophoresis of Ca restored the PSP. The results indicate that synaptic transmission in the squid stellate ganglion is not electrical but due to the release of an unidentified transmitter. Release of this transmitter by the presynaptic nerve impulse requires the presence of Ca in the external medium. During the impulse Ca would combine with a ''Ca-receptor'' in the membrane and initiate the reactions which lead to transmitter release. It appears that the "Ca-receptor" is only accessible from the outside of the membrane.