Biomedical subjects

B Sakmann

Publications and source records attributed to B Sakmann.

At least 37 records · Page 2Linked to original sources

PubMed1999

Reliable synaptic connections between pairs of excitatory layer 4 neurones within a single 'barrel' of developing rat somatosensory cortex.

1. Dual whole-cell recordings were made from pairs of synaptically coupled excitatory neurones in the 'barrel field' in layer (L) 4 in slices of young (postnatal day 12-15) rat somatosensory cortex. The majority of interconnected excitatory neurones were spiny stellate cells with an asymmetrical dendritic arborisation largely confined to a single barrel. The remainder were star pyramidal cells with a prominent apical dendrite terminating in L2/3 without forming a tuft. 2. Excitatory synaptic connections were examined between 131 pairs of spiny L4 neurones. Single presynaptic action potentials evoked unitary EPSPs with a peak amplitude of 1.59 +/- 1.51 mV (mean +/- s. d.), a latency of 0.92 +/- 0.35 ms, a rise time of 1.53 +/- 0.46 ms and a decay time constant of 17.8 +/- 6.3 ms. 3. At 34-36 C, the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0. 37 +/- 0.16 and the percentage of failures to evoke an EPSP was 5.3 +/- 7.8 %. The c.v. and failure rate decreased with increasing amplitude of the unitary EPSP. 4. Postsynaptic glutamate receptors in spiny L4 neurones were of the AMPA and NMDA type. At -60 mV in the presence of 1 mM Mg2+, NMDA receptors contributed 39.3 +/- 12.5 % to the EPSP integral. In Mg2+-free solution, the NMDA receptor/AMPA receptor ratio of the EPSC was 0.86 +/- 0.64. 5. The number of putative synaptic contacts established by the projection neurone with the target neurone varied between two and five with a mean of 3.4 +/- 1.0 (n = 11). Synaptic contacts were exclusively found in the barrel in which the cell pair was located and were preferentially located on secondary to quarternary dendritic branches. Their mean geometric distance from the soma was 68.8 +/- 37.4 microm (range, 33.4-168.0 microm). The number of synaptic contacts and mean EPSP amplitude showed no significant correlation. 6. The results suggest that in L4 of the barrel cortex synaptic transmission between spiny neurones is largely restricted to a single barrel. The connections are very reliable, probably due to a high release probability, and have a high efficacy because of the compact structure of the dendrites and axons of spiny neurones. Intrabarrel connections thus function to amplify and distribute the afferent thalamic activity in the vertical directions of a cortical column.

Animals↗

PubMed1999

Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning.

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.

Action Potentials↗

PubMed1999

Developmental switch in the short-term modification of unitary EPSPs evoked in layer 2/3 and layer 5 pyramidal neurons of rat neocortex.

Amplitudes of EPSPs evoked by repetitive presynaptic action potentials can either decrease (synaptic depression) or increase (synaptic facilitation). To determine whether facilitation and depression in the connections between neocortical pyramidal cells varied with the identity of the pre- or the postsynaptic cell and whether they changed during postnatal development, whole-cell voltage recordings were made simultaneously from two or three pyramidal cells in layers 2/3 and 5 of the rat sensorimotor cortex. Unitary EPSPs were evoked when pre- and postsynaptic neurons were in the same and in different layers. In young [postnatal day 14 (P14)] cortex, EPSPs evoked in all connected neurons depressed. The degree of depression was layer specific and was determined by the identity of the presynaptic cell. EPSPs evoked by stimulation of presynaptic layer 5 neurons depressed significantly more than did those evoked by stimulation of layer 2/3 neurons. In mature cortex (P28), however, the EPSPs evoked in these connected neurons facilitated to a comparable degree regardless of the layer in which pre- and postsynaptic neurons were located. The results suggest that in young cortex the degree of synaptic depression in connected pyramidal cells is determined primarily by whether the presynaptic cell was in layer 2/3 or 5 and that maturation of the cortex involves a developmental switch from depression to facilitation between P14 and P28 that eliminates the layer-specific differences. A functional consequence of this switch is that in mature cortex the spread of excitation between neocortical pyramidal neurons is enhanced when action potentials occur in bursts.

Animals↗

PubMed1999

A new cellular mechanism for coupling inputs arriving at different cortical layers.

Pyramidal neurons in layer 5 of the neocortex of the brain extend their axons and dendrites into all layers. They are also unusual in having both an axonal and a dendritic zone for the initiation of action potentials. Distal dendritic inputs, which normally appear greatly attenuated at the axon, must cross a high threshold at the dendritic initiation zone to evoke calcium action potentials but can then generate bursts of axonal action potentials. Here we show that a single back-propagating sodium action potential generated in the axon facilitates the initiation of these calcium action potentials when it coincides with distal dendritic input within a time window of several milliseconds. Inhibitory dendritic input can selectively block the initiation of dendritic calcium action potentials, preventing bursts of axonal action potentials. Thus, excitatory and inhibitory postsynaptic potentials arising in the distal dendrites can exert significantly greater control over action potential initiation in the axon than would be expected from their electrotonically isolated locations. The coincidence of a single back-propagating action potential with a subthreshold distal excitatory postsynaptic potential to evoke a burst of axonal action potentials represents a new mechanism by which the main cortical output neurons can associate inputs arriving at different cortical layers.

Action Potentials↗

PubMed1999

Effect of changes in action potential shape on calcium currents and transmitter release in a calyx-type synapse of the rat auditory brainstem.

We studied the relation between the size of presynaptic calcium influx and transmitter release by making simultaneous voltage clamp recordings from presynaptic terminals, the calyces of Held and postsynaptic cells, the principal cells of the medical nucleus of the trapezoid body, in slices of the rat brainstem. Calyces were voltage clamped with different action potential waveforms. The amplitude of the excitatory postsynaptic currents depended supralinearly on the size of the calcium influx, in the absence of changes in the time-course of the calcium influx. This result is in agreement with the view that at this synapse most vesicles are released by the combined action of multiple calcium channels.

Action Potentials↗

PubMed1999

Calcium channel types with distinct presynaptic localization couple differentially to transmitter release in single calyx-type synapses.

We studied how Ca2+ influx through different subtypes of Ca2+ channels couples to release at a calyx-type terminal in the rat medial nucleus of the trapezoid body by simultaneously measuring the presynaptic Ca2+ influx evoked by a single action potential and the EPSC. Application of subtype-specific toxins showed that Ca2+ channels of the P/Q-, N-, and R-type controlled glutamate release at a single terminal. The Ca2+ influx through the P/Q-type channels triggered release more effectively than Ca2+ influx through N- or R-type channels. We investigated mechanisms that contributed to these differences in effectiveness. Electrophysiological experiments suggested that individual release sites were controlled by all three subtypes of Ca2+ channels. Immunocytochemical staining indicated, however, that a substantial fraction of N- and R-type channels was located distant from release sites. Although these distant channels contributed to the Ca2+ influx into the terminal, they may not contribute to release. Taken together, the results suggest that the Ca2+ influx into the calyx via N- and R-type channels triggers release less effectively than that via P/Q-type because a substantial fraction of the N- and R-type channels in the calyx is localized distant from release sites.

Action Potentials↗

PubMed1999

NMDAR channel segments forming the extracellular vestibule inferred from the accessibility of substituted cysteines.

In NMDA receptor channels, the M2 loop forms the narrow constriction and the cytoplasmic vestibule. The identity of an extracellular vestibule leading toward the constriction remained unresolved. Using the substituted cysteine accessibility method (SCAM), we identified channel-lining residues of the NR1 subunit in the region preceding M1 (preM1), the C-terminal part of M3 (M3C), and the N-terminal part of M4 (M4N). These residues are located on the extracellular side of the constriction and, with one exception, are exposed to the pore independently of channel activation, suggesting that the gate is at the constriction or further cytoplasmic to it. Permeation of Ca2+ ions was decreased by mutations in M3C and M4N, but not by mutations in preM1, suggesting a functionally distinct contribution of the segments to the extracellular vestibule of the NMDA receptor channel.

Amino Acid Sequence↗

PubMed1999

Imaging synaptic activity in intact brain and slices with FM1-43 in C. elegans, lamprey, and rat.

The fluorescent probe FM1-43 has been used extensively for imaging vesicle recycling; however, high nonspecific adsorption resulting in elevated background levels has precluded its use in certain tissues, notably brain slices. We have found that a sulfobutylated derivative of beta-cyclodextrin (ADVASEP-7) has a higher affinity for FM1-43 than the plasma membrane. ADVASEP-7 was used as a carrier to remove FM1-43 nonspecifically bound to the outer leaflet of the plasma membrane or extracellular molecules, significantly reducing background staining. This has enabled us to visualize synaptic vesicle recycling in the nematode C. elegans, intact lamprey spinal cord, and rat brain slices.

Animals↗

PubMed1999

Dendritic GABA release depresses excitatory transmission between layer 2/3 pyramidal and bitufted neurons in rat neocortex.

GABAergic, somatostatin-containing bitufted interneurons in layer 2/3 of rat neocortex are excited via glutamatergic excitatory postsynaptic potentials (EPSPs) by pyramidal neurons located in the same cortical layer. Pair recordings showed that short bursts of backpropagating dendritic action potentials (APs) reduced the amplitude of unitary EPSPs. EPSP depression was dependent on a rise in dendritic [Ca2+]. The effect was blocked by the GABA(B) receptor (GABA(B)-R) antagonist CGP55845A and was mimicked by the GABA(B)-R agonist baclofen. As presynaptic GABA(B)-Rs were activated neither by somatostatin nor by GABA released from axon collaterals of the bitufted cell, we conclude that GABA(B)-Rs were activated by a retrograde messenger, most likely GABA, released from the dendrite. Because synaptic depression was prevented by loading bitufted neurons with GDP-beta-S, it is likely to be caused by exocytotic GABA release from dendrites.

Action Potentials↗

PubMed1999

Coincidence detection and changes of synaptic efficacy in spiny stellate neurons in rat barrel cortex.

Paired whole-cell voltage recordings were made from synaptically connected spiny stellate neurons in layer 4 of the barrel field in young (P14) rat somatosensory cortex. When postsynaptic action potentials (APs) followed each of 5 presynaptic APs in a 10- or 20-Hz train by less than 25 ms, subsequent unitary EPSP amplitudes were persistently reduced. Induction of long-term depression (LTD) depended on activation of group II metabotropic glutamate receptors, but not on NMDA or AMPA receptors. Reducing postsynaptic increases in intracellular calcium ([Ca2+]i) by intracellular loading with a fast- (BAPTA) or a slow- (EGTA) acting Ca2+ buffer blocked synaptic depression. Analysis of EPSP failures suggested mediation of LTD by a reduction in release probability. We propose a mechanism by which coincident activity results in long-lasting reduction of synaptic efficacy between synaptically connected neurons.

Animals↗

PubMed1998

Postsynaptic Ca2+ influx mediated by three different pathways during synaptic transmission at a calyx-type synapse.

Whole-cell recordings and Ca2+ flux measurements were made at a giant calyx-type synapse in rat brainstem slices to determine the contribution of glutamate receptor (GluR) channels and voltage-dependent Ca2+ channels (VDCCs) to postsynaptic Ca2+ influx during synaptic transmission. A single presynaptic action potential (AP) evoked an EPSP, followed by a single AP. The EPSP-AP sequence caused a postsynaptic Ca2+ influx of approximately 3.0 pC, primarily through VDCCs ( approximately 70%) and NMDA-type (up to 30%) channels but also through AMPA-type (<5%) GluR channels. At -80 mV, the fractional Ca2+ current (Pf) mediated by AMPA receptor (AMPAR) and NMDA receptor (NMDAR) channels was 1.3 and 11-12%, respectively. Simulations of the time course of Ca2+ influx through GluR channels suggested that the small contribution of AMPAR channels occurred only during the first few milliseconds of an EPSP, whereas influx through NMDAR channels dominated later. The NMDAR-mediated Ca2+ influx was localized in regions covered by the presynaptic terminal, whereas the Ca2+ influx mediated by VDCCs was more homogeneously distributed. Because of the temporal and spatial differences, calcium ions entering through the three different pathways are likely to activate different intracellular targets in the postsynaptic cell.

Animals↗

PubMed1998

Transmitter release modulation in nerve terminals of rat neocortical pyramidal cells by intracellular calcium buffers.

1. Dual whole-cell voltage recordings were made from synaptically connected layer 5 (L5) pyramidal neurones in slices of the young (P14-P16) rat neocortex. The Ca2+ buffers BAPTA or EGTA were loaded into the presynaptic neurone via the pipette recording from the presynaptic neurone to examine their effect on the mean and the coefficient of variation (c.v.) of single fibre EPSP amplitudes, referred to as unitary EPSPs. 2. The fast Ca2+ buffer BAPTA reduced unitary EPSP amplitudes in a concentration dependent way. With 0.1 mM BAPTA in the pipette, the mean EPSP amplitude was reduced by 14 +/- 2.8% (mean +/- s.e.m., n = 7) compared with control pipette solution, whereas with 1.5 mM BAPTA, the mean EPSP amplitude was reduced by 72 +/- 1.5% (n = 5). The concentration of BAPTA that reduced mean EPSP amplitudes to one-half of control was close to 0.7 mM. 3. Saturation of BAPTA during evoked release was tested by comparing the effect of loading the presynaptic neurone with 0.1 mM BAPTA at 2 and 1 mM [Ca2+]o. Reducing [Ca2+]o from 2 to 1 mM, thereby reducing Ca2+ influx into the terminals, decreased the mean EPSP amplitude by 60 +/- 2.2% with control pipette solution and by 62 +/- 1.9% after loading with 0.1 mM BAPTA (n = 7). 4. The slow Ca2+ buffer EGTA at 1 mM reduced mean EPSP amplitudes by 15 +/- 2.5% (n = 5). With 10 mM EGTA mean EPSP amplitudes were reduced by 56 +/- 2.3 % (n = 4). 5. With both Ca2+ buffers, the reduction in mean EPSP amplitudes was associated with an increase in the c.v. of peak EPSP amplitudes, consistent with a reduction of the transmitter release probability as the major mechanism underlying the reduction of the EPSP amplitude. 6. The results suggest that in nerve terminals of thick tufted L5 pyramidal cells the endogenous mobile Ca2+ buffer is equivalent to less than 0.1 mM BAPTA and that at many release sites of pyramidal cell terminals the Ca2+ channel domains overlap, a situation comparable with that at large calyx-type terminals in the brainstem.

Algorithms↗

PubMed1998

Facilitation of presynaptic calcium currents in the rat brainstem.

1. To study use-dependent changes in the presynaptic Ca2+ influx and their contribution to transmitter release, we made simultaneous voltage clamp recordings from presynaptic terminals (the calyces of Held) and postsynaptic cells (the principal cells of the medial nucleus of the trapezoid body) in slices of the rat auditory brainstem. 2. Following a short (2 ms) prepulse to 0 mV, calcium channels opened faster during steps to negative test potentials. During trains of action potential waveforms the Ca2+ influx per action potential increased. At the same time, however, the amplitude of the EPSCs decreased. 3. The facilitation of the calcium currents appeared to depend on a build-up of intracellular Ca2+, since its magnitude was proportional to the Ca2+ influx and it was reduced in the presence of 10 mM BAPTA. 4. Facilitation of the presynaptic calcium currents may contribute to short-term facilitation of transmitter release, observed when quantal output is low. Alternatively, it may counteract processes that contribute to synaptic depression.

Action Potentials↗

PubMed1998

Calcium dynamics in single spines during coincident pre- and postsynaptic activity depend on relative timing of back-propagating action potentials and subthreshold excitatory postsynaptic potentials.

We compared the transient increase of Ca2+ in single spines on basal dendrites of rat neocortical layer 5 pyramidal neurons evoked by subthreshold excitatory postsynaptic potentials (EPSPs) and back-propagating action potentials (APs) by using calcium fluorescence imaging. AP-evoked Ca2+ transients were detected in both the spines and in the adjacent dendritic shaft, whereas Ca2+ transients evoked by single EPSPs were largely restricted to a single active spine head. Calcium transients elicited in the active spines by a single AP or EPSP, in spines up to 80 micro(m) for the soma, were of comparable amplitude. The Ca2+ transient in an active spine evoked by pairing an EPSP and a back-propagating AP separated by a time interval of 50 ms was larger if the AP followed the EPSP than if it preceded it. This difference reflected supra- and sublinear summation of Ca2+ transients, respectively. A comparable dependence of spinous Ca2+ transients on relative timing was observed also when short bursts of APs and EPSPs were paired. These results indicate that the amplitude of the spinous Ca2+ transients during coincident pre- and postsynaptic activity depended critically on the relative order of subthreshold EPSPs and back-propagating APs. Thus, in neocortical neurons the amplitude of spinous Ca2+ transients could encode small time differences between pre- and postsynaptic activity.

Action Potentials↗

PubMed1998

R-type Ca2+ currents evoke transmitter release at a rat central synapse.

Voltage-dependent Ca2+ currents evoke synaptic transmitter release. Of six types of Ca2+ channels, L-, N-, P-, Q-, R-, and T-type, only N- and P/Q-type channels have been pharmacologically identified to mediate action-potential-evoked transmitter release in the mammalian central nervous system. We tested whether Ca2+ channels other than N- and P/Q-type control transmitter release in a calyx-type synapse of the rat medial nucleus of the trapezoid body. Simultaneous recordings of presynaptic Ca2+ influx and the excitatory postsynaptic current evoked by a single action potential were made at single synapses. The R-type channel, a high-voltage-activated Ca2+ channel resistant to L-, N-, and P/Q-type channel blockers, contributed 26% of the total Ca2+ influx during a presynaptic action potential. This Ca2+ current evoked transmitter release sufficiently large to initiate an action potential in the postsynaptic neuron. The R-type current controlled release with a lower efficacy than other types of Ca2+ currents. Activation of metabotropic glutamate receptors and gamma-aminobutyric acid type B receptors inhibited the R-type current. Because a significant fraction of presynaptic Ca2+ channels remains unidentified in many other central synapses, the R-type current also could contribute to evoked transmitter release in these synapses.

Action Potentials↗

PubMed1998

Intracellular Mg2+ interacts with structural determinants of the narrow constriction contributed by the NR1-subunit in the NMDA receptor channel.

1. N-methyl-D-aspartate (NMDA) receptor channels are blocked by intracellular Mg2+ in a voltage-dependent manner. Amino acid residues positioned at or near the narrow constriction that interact with intracellular Mg2+ were identified in recombinant NR1-NR2A channels expressed in Xenopus oocytes or human embryonic kidney (HEK) 293 cells. 2. In the absence of extracellular Ca2+, the block of wild-type channel by intracellular Mg2+ measured using macroscopic currents showed a voltage dependence (delta) of around 0.38 and a voltage-independent affinity for the channel of 4 mM. These parameters were independent of the Mg2+ concentration (0.05-10mM), and were indistinguishable from those found for the reduction of single channel amplitudes under the same ionic conditions. Under bionic conditions with high intracellular Mg2+ and K+ extracellularly, Mg2+ was weakly permeant. Mg2+ efflux, however, attenuated the block only at positive potentials (> +80 mV). 3. Substitutions of the N-site asparagine in the NR1-subunit altered intracellular Mg2+ block over physiological membrane potentials (+10 to +50 mV). Substitution of glycine, glutamine or serine attenuated the extent of block whereas the negatively charged aspartate enhanced it, consistent with the side chain of the native asparagine at this position contributing to a blocking site for intracellular Mg2+. 4. Substitutions of the N-site or N + 1 site asparagine in the NR2A-subunit, which form a blocking site for extracellular Mg2+, also altered the block by intracellular Mg2+. However, for the NR2A-subunit N-site asparagine, the block was reduced but only at non-physiological high potentials (> +70 mV). 5. The NR2A-subunit N + 1 site asparagine, which together with NR1-subunit N-site asparagine forms the narrow constriction of the channel, also contributed to a blocking site for intracellular Mg2+. However, it did so to lesser extent than the NR1-subunit N-site and in a manner different from its contribution to a blocking site for extracellular Mg2+. 6. It is concluded that intracellular Mg2+ interacts with residues that form the narrow constriction in the NMDA receptor channel with the N-site asparagine of the NR1-subunit representing the dominant blocking site. Thus, intracellular Mg2+ interacts with different asparagine residues at the narrow constriction than extracellular Mg2+, although the two blocking sites are positioned very close to each other.

Animals↗

PubMed1998

Adjacent asparagines in the NR2-subunit of the NMDA receptor channel control the voltage-dependent block by extracellular Mg2+.

1. The voltage-dependent block of N-methyl-D-aspartate (NMDA) receptor channels by extracellular Mg2+ is a critical determinant of its contribution to CNS synaptic physiology. The function of the narrow constriction of the channel in determining the block was investigated by analysing the effects of a set different amino acid substitutions at exposed residues positioned at or near this region. NMDA receptor channels, composed of wild-type and mutant NR1- and NR2A-subunits, were expressed in Xenopus oocytes or human embryonic kidney (HEK) 293 cells. 2. In wild-type channels, the voltage dependence (delta) of the block Mg2+ was concentration dependent with values of delta of integral of 0.82 in 0.07 mM and higher concentrations. Under bionic conditions with high extracellular Mg2+ and K+ as the reference ion, Mg2+ weakly permeated the channel. Over intermediate potentials (approximately -60 to -10 mV), this weak permeability had no apparent effect on the block but at potentials negative to approximately -60mV, it attenuated the extent and voltage dependence of the block. 3. Substitutions of glycine, serine, glutamine or aspartate for the N-site asparagine in the NR1-subunit enhanced the extent of block over intermediate potentials but left the voltage dependence of the block unchanged indicating that structural determinants of the block remained. These same substitutions either attenuated or left unchanged the apparent Mg2+ permeability. 4. In channels containing substitutions of glycine, serine or glutamine for the N-site asparagine in the NR2A-subunit, the block Mg2+ was reduced at negative potentials. Over intermediate potentials, the block was not strongly attenuated except for the glutamine substitution which reduced the voltage dependence of the block to integral of 0.57 in 0.7 mM Mg2+. 5. Equivalent substitutions for the N + 1 site asparagine in the NR2A-subunit strongly attenuated the block over the entire voltage range. In 0.7 mM Mg2+, the voltage dependence of the block was reduced to 0.50 (glycine), 0.53 (serine) and 0.46 (glutamine). 6. Channels containing substitutions of the N-site or N + 1 site asparagines in the NR2A-subunit showed an increased Mg2+ permeability suggesting that these adjacent asparagines form a barrier for inward Mg2+ flux. Changes in this barrier contribute, at least in part, to the mechanism underlying disruption of the block following substitution of these residues. 7. The adjacent NR2A-subunit asparagines are positioned at or near the narrow constriction of the channel. Pore size, however, did not determine how effectively Mg2+ blocks mutant channels. 8. It is concluded that, at the narrow constriction in the NMDA receptor channel, the adjacent NR2A-subunit asparagines, the N-site and N + 1 site, but not the N-site asparagine of the NR1-subunit, form a critical blocking site for extracellular Mg2+. The contribution to the blocking site, in contrast to the prevailing view, is stronger for the N + 1 site than for the N-site asparagine. The block may involve binding of Mg2+ to these residues.

Amino Acid Sequence↗

PubMed1998

Calcium current during a single action potential in a large presynaptic terminal of the rat brainstem.

1. The calcium current of a 'giant' synaptic terminal (the calyx of Held) was studied using two-electrode voltage clamp in slices of the rat brainstem. 2. In terminals with a long axon (length > 100 microns), the passive current transient decayed biexponentially following voltage steps. In terminals with a short axon (length < 30 microns), the slow component was reduced or absent. These terminals also had small slow calcium tail currents following long depolarizing voltage steps, suggesting that these are largely due to axonal calcium channels. 3. Terminals were voltage clamped with action potential waveform commands. At both 24 and 36 degrees C the calcium current began shortly after the peak of the action potential and ended before the terminal was fully repolarized. 4. The calcium current during the repolarization phase was 69 +/- 1% (n = 3) of maximal, judged from the increase in this current when a plateau phase was added to the action potential waveform. 5. A Hodgkin-Huxley m2 model, based on the measured activation and deactivation of the calcium current, reproduced both the time course and the amplitude increase of the calcium currents during the different action potential waveforms well. 6. The fast gating of the calcium channels in the terminal ensures that they are effectively opened during the repolarization phase of an action potential. This implies that the distance between open calcium channels is minimized, which is in agreement with the view that multiple calcium channels are needed to release a vesicle in this synapse.

Action Potentials↗