Two components of transmitter release in retinal bipolar cells: exocytosis and mobilization of synaptic vesicles.
ABSTRACT Ca2+-transmitter release coupling was examined using bipolar cells with large presynaptic terminals dissociated from the goldfish retina. Presynaptic Ca2+ current (I(Ca)) was recorded under the whole-cell voltage clamp. Release of excitatory amino acid transmitter was simultaneously monitored as the current through N-methyl-D-asperate (NMDA) receptors of reporter cells or as the membrane capacitance (C(m)) change associated with exocytosis. When I(Ca) was activated by a long depolarizing pulse, a double-peaked transmitter-induced current (I(tr)) was elicited in reporter cells. The rapid component of I(tr) was evoked immediately after the onset of depolarization, and was affected only slightly by intracellularly applied Ca2+ chelators. The delayed slow component of I(tr) was elicited during depolarization once a fixed amount of Ca2+ was accumulated in presynaptic terminals, and its appearance was suppressed or retarded by Ca2+ chelators. Two components of transmitter release were also recognized by monitoring C(m) changes elicited by the activation of I(Ca). These results suggest that bipolar cells have at least two pools of synaptic vesicles; a small, immediately releasable pool and a large releasable pool. The rapid and the delayed slow components of transmitter release may reflect exocytosis and mobilization of synaptic vesicles, respectively.
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ABSTRACT: • We examined whether transmitter release could be modified by the activation of protein kinase C (PKC) of retinal bipolar cells. A bipolar cell with a large axon terminal was isolated from the goldfish retina. The presynaptic Ca2+ current was measured under whole-cell voltage clamp, and the released transmitter (probably glutamate) was detected electrophysiologically by using the response of NMDA receptors of catfish horizontal cells as a reporter. • Transmitter release was potentiated by a PKC activator, phorbol 12-myristate 13-acetate (PMA), but not by an ineffective phorbol ester, 4α-phorbol 12,13-didecanoate. A PKC inhibitor, bisindolylmaleimide I, did not affect the transmitter release by itself but blocked the PMA-induced potentiation of transmitter release. These results suggest that the actions of PMA were mediated via the activation of PKC. • Introduction of 5 mmEGTA into the presynaptic terminals of bipolar cells revealed two separate components of transmitter release. A rapid component was triggered immediately after depolarization while a slow component appeared with a delay. Application of PMA selectively potentiated the slow component without affecting the Ca2+ dependence of exocytosis. • We suggest that the activation of PKC may modify the recruitment process of synaptic vesicles in retinal bipolar cells.The Journal of Physiology 09/2004; 512(1):219 - 225. · 4.38 Impact Factor
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ABSTRACT: Neural activity-induced long-term potentiation (LTP) of synaptic transmission is believed to be one of the cellular mechanisms underlying experience-dependent developmental refinement of neural circuits. Although it is well established that visual experience and neural activity are critical for the refinement of retinal circuits, whether and how LTP occurs in the retina remain unknown. Using in vivo perforated whole-cell recording and two-photon calcium imaging, we find that both repeated electrical and visual stimulations can induce LTP at excitatory synapses formed by bipolar cells on retinal ganglion cells in larval but not juvenile zebrafish. LTP induction requires the activation of postsynaptic N-methyl-D-aspartate receptors, and its expression involves arachidonic acid-dependent presynaptic changes in calcium dynamics and neurotransmitter release. Physiologically, both electrical and visual stimulation-induced LTP can enhance visual responses of retinal ganglion cells. Thus, LTP exists in developing retinae with a presynaptic locus and may serve for visual experience-dependent refinement of retinal circuits.Neuron 08/2012; 75(3):479-89. · 15.77 Impact Factor
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ABSTRACT: The neurotransmitter glutamate is used by most neurons in the brain to activate a multitude of different types of glutamate receptors and transporters involved in fast and relatively slower signaling. Synaptic ribbons are large presynaptic structures found in neurons involved in vision, balance, and hearing, which use a large number of glutamate-filled synaptic vesicles to meet their signaling demands. To directly measure synaptic vesicle release events, the ribbon-type presynaptic terminals of goldfish retinal bipolar cells were coaxed to release a false transmitter that could be monitored with amperometry by placing the carbon fiber directly on the larger synaptic terminal. Spontaneous secretion events formed a unimodal charge distribution, but single spike properties were heterogeneous. Larger events rose exponentially without interruption (τ ∼ 30 μs), and smaller events exhibited a stammer in their rising phase that is interpreted as a brief pause in pore dilation, a characteristic commonly associated with large dense core granule fusion pores. These events were entirely Ca(2+)-dependent. Holding the cells at -60 mV halted spontaneous release; and when the voltage was stepped to >-40 mV, secretion ensued. When stepping the voltage to 0 mV, novel kinetic phases of vesicle recruitment were revealed. Approximately 14 vesicles were released per ribbon in two kinetic phases with time constants of 1.5 and 16 ms, which are proposed to represent different primed states within the population of docked vesicles.Journal of Neuroscience 05/2013; 33(19):8144-58. · 6.91 Impact Factor