Calcium in motor nerve terminals associated with posttetanic potentiation

We have used fura-2 fluorescence to study the effects of repetitive stimulation producing posttetanic potentiation (PTP) at crayfish neuromuscular junctions on presynaptic calcium concentration. Fura-2 was injected into the preterminal axon of the excitor motor neuron to the claw opener muscle of a walking leg. Pictures of presynaptic terminals on the muscle surface were obtained with a charge-coupled device camera, ratioed, and converted to spatial images of intracellular calcium concentration. Stimulation of the motor nerve for 7–10 min at 20–33 Hz produced potentiation during the tetanus and PTP following the tetanus. Presynaptic calcium levels in terminal boutons and varicosities rose to about 2 microM during the tetanus and decayed at first rapidly and then slowly back to levels near the initial concentration of about 200 nM. The decay rate of potentiated synaptic transmission was the same as the decay rate of the elevated calcium concentration during the posttetanic period dominated by PTP, when facilitation and augmentation had dissipated. A 13-fold potentiation corresponded to a 500 nM elevation of calcium to about 700 nM. The linear dependence we observed is not consistent with the power law formulation of a residual calcium hypothesis for PTP. During the tetanus, the enhancement of synaptic transmission due to facilitation, augmentation, and potentiation exceeded that expected from the correspondence between PTP and posttetanic calcium levels. This may occur because during the tetanus there is insufficient time for calcium to equilibrate spatially between action potentials, and the submembrane calcium will be higher than the volume-average calcium levels that we detect. Following low-frequency trains (typically 8 Hz for about 35 sec), enhanced synaptic transmission and elevated presynaptic calcium decayed rapidly, within a few seconds. Short high-frequency trains (50– 100 Hz for 1–2 min) elicited an additional hours-long elevation of presynaptic calcium, corresponding to, and perhaps responsible for, part of the long-term potentiation of transmission that such stimulation produces at this synapse.

[1]  J. Desmedt,et al.  Inhibition of the intracellular release of calcium by Dantrolene in barnacle giant muscle fibres. , 1977, The Journal of physiology.

[2]  E. Alnaes,et al.  On the role of mitochondria in transmitter release from motor nerve terminals. , 1975, The Journal of physiology.

[3]  K L Magleby,et al.  The effect of repetitive stimulation on facilitation of transmitter release at the frog neuromuscular junction , 1973, The Journal of physiology.

[4]  R. Zucker,et al.  Role of presynaptic calcium ions and channels in synaptic facilitation and depression at the squid giant synapse. , 1982, The Journal of physiology.

[5]  H. Atwood,et al.  Short-term and long-term plasticity and physiological differentiation of crustacean motor synapses. , 1986, International review of neurobiology.

[6]  I. Cohen,et al.  The calcium dependence of spontaneous and evoked quantal release at the frog neuromuscular junction , 1983, The Journal of physiology.

[7]  R. Zucker,et al.  Post-tetanic decay of evoked and spontaneous transmitter release and a residual-calcium model of synaptic facilitation at crayfish neuromuscular junctions , 1983, The Journal of general physiology.

[8]  E. Neher,et al.  The Ca signal from fura‐2 loaded mast cells depends strongly on the method of dye‐loading , 1985, FEBS letters.

[9]  R. Tsien,et al.  A new generation of Ca2+ indicators with greatly improved fluorescence properties. , 1985, The Journal of biological chemistry.

[10]  R. Zucker,et al.  Aequorin response facilitation and intracellular calcium accumulation in molluscan neurones , 1980, The Journal of physiology.

[11]  A Mallart,et al.  An analysis of facilitation of transmitter release at the neuromuscular junction of the frog , 1967, The Journal of physiology.

[12]  S. Erulkar,et al.  Changes in transmitter release induced by ion-containing liposomes. , 1978, Proceedings of the National Academy of Sciences of the United States of America.

[13]  R S Zucker,et al.  Presynaptic calcium diffusion from various arrays of single channels. Implications for transmitter release and synaptic facilitation. , 1985, Biophysical journal.

[14]  R. Eisenberg,et al.  Action Potentials without Contraction in Frog Skeletal Muscle Fibers with Disrupted Transverse Tubules , 1967, Science.

[15]  B. Katz,et al.  The role of calcium in neuromuscular facilitation , 1968, The Journal of physiology.

[16]  R. Tsien,et al.  Calcium rises abruptly and briefly throughout the cell at the onset of anaphase. , 1986, Science.

[17]  J. Rosenthal.  Post‐tetanic potentiation at the neuromuscular junction of the frog , 1969, The Journal of physiology.

[18]  R Llinás,et al.  Intraterminal injection of synapsin I or calcium/calmodulin-dependent protein kinase II alters neurotransmitter release at the squid giant synapse. , 1985, Proceedings of the National Academy of Sciences of the United States of America.

[19]  K. Magleby.  The effect of tetanic and post‐tetanic potentiation on facilitation of transmitter release at the frog neuromuscular junction , 1973, The Journal of physiology.

[20]  H. Atwood,et al.  Organization and synaptic physiology of crustacean neuromuscular systems , 1976, Progress in Neurobiology.

[21]  D. Tank,et al.  Spatially resolved calcium dynamics of mammalian Purkinje cells in cerebellar slice. , 1988, Science.

[22]  J. Connor,et al.  Calcium levels measured in a presynaptic neurone of Aplysia under conditions that modulate transmitter release. , 1986, The Journal of physiology.

[23]  K. Magleby,et al.  Augmentation: A process that acts to increase transmitter release at the frog neuromuscular junction. , 1976, The Journal of physiology.

[24]  D. Weinreich.  Ionic mechanism of post‐tetanic potentiation at the neuromuscular junction of the frog , 1971, The Journal of physiology.

[25]  R. Llinás,et al.  Compartmentalization of the submembrane calcium activity during calcium influx and its significance in transmitter release. , 1985, Biophysical journal.

[26]  D. A. Baxter,et al.  Quantal mechanism of long-term synaptic potentiation. , 1985, Proceedings of the National Academy of Sciences of the United States of America.

[27]  B. Wallin,et al.  Intracellular ion concentrations in single crayfish axons. , 1967, Acta physiologica Scandinavica.

[28]  S. Erulkar,et al.  The role of calcium ions in tetanic and post‐tetanic increase of miniature end‐plate potential frequency. , 1978, The Journal of physiology.