. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 320 NEUROBIOLOGY AND BIOPHYSICS Inhibitory Effects of Nonhydrolyzable Guanine Nucleotides on Neurotransmitter Release at the Squid Giant Synapse Peter Doroshenko, Stephen Hess, and George Augustine (Duke University Medical Center) We are examining the role of GTP-binding regulatory proteins in the control of neurotransmitter release. To this end, we in- jected two nonhydrolyzable guanine nucleotides into the giant presynaptic terminal of the most distal synapse of the stellate ganglion of the squid, Loligo pealei, and studie
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 320 NEUROBIOLOGY AND BIOPHYSICS Inhibitory Effects of Nonhydrolyzable Guanine Nucleotides on Neurotransmitter Release at the Squid Giant Synapse Peter Doroshenko, Stephen Hess, and George Augustine (Duke University Medical Center) We are examining the role of GTP-binding regulatory proteins in the control of neurotransmitter release. To this end, we in- jected two nonhydrolyzable guanine nucleotides into the giant presynaptic terminal of the most distal synapse of the stellate ganglion of the squid, Loligo pealei, and studied their effects on transmitter release evoked by action potentials. As an assay of release, the maximum rate of rise of postsynaptic potentials (PSPs) produced by intracellular stimulation of the presynaptic neuron was measured. The terminal was also pre-loaded with the Ca-sensitive dye Fura-2 to perform dual wavelength mea- surements of presynaptic free Ca24 ion concentration, ([Ca:+],). lontophoretic injection of GTP-yS into the giant presynaptic terminal caused a marked depression of synaptic transmission. The rate of rise of PSPs began to decrease within a few minutes after the onset of injection and continued progressively for several tens of minutes after injection has been stopped. One hundred microCoulomb injections inhibited transmission by 60-80%. An upper estimate of the intraterminal concentration of injected nucleotide is in the high micromolar range. Additional injections did not seem to add to the inhibition. Inhibition of synaptic transmission was not accompanied by any significant changes in the electrical properties of the presynaptic terminal: neither presynaptic resting nor action potentials were affected by GT?7S injections. Further, the changes in the presynaptic [Ca:t], induced by a train of 250 action potentials (5s. 50 Hz) were not affected by injection of GT?7S. There was also no steady depolarization of the postsynaptic membrane potential, suggesting
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