Spread of synaptic potentials through electrical synapses in Retzius neurones of the leech
Francisco F. De-Miguel*,
Mariana Vargas-Caballero and
Elizabeth García-Pérez
Departamento de Biofísica, Instituto de Fisiología Celular, UNAM, Apartado Postal 70-253, 04510, D.F., México
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Fig. 1. Morphology and potential contact sites of Retzius neurones. (A) Three-dimensional reconstruction from serial confocal images of one Retzius neurone filled with Lucifer Yellow showing multiple fine neurites emerging from the primary process. The visualization of the somata of the contralateral neurone and of two small neurones in the lower right-hand corner resulted from autofluorescence. Anterior is to the left. (B) Three-dimensional confocal reconstruction of the dendritic arborization of a pair of Retzius neurones. The neurone coloured yellow was stained with Lucifer Yellow, while the blue neurone was injected with Texas Red. The somata were removed to avoid optical interference. Note the extensive dendritic overlap. The white dots are potential contact sites between the neurones. Autofluorescent somata of multiple neurones served to outline the ganglion. The arrow points to the anterior. Scale bar for both images, 30 µm.
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Fig. 2. Spontaneous synaptic activity in Retzius neurones. (A) Simultaneous somatic recordings of Retzius neurones in leech Ringer’s solution showing spontaneous and synchronous excitatory postsynaptic potentials. V1 is the recording from one of the neurones and V2 is the recording from the other. Six consecutive series are shown. (B) Partial substitution of Ca2+ by Mg2+ in the Ringer’s solution abolished the spontaneous activity in both neurones. (C) Synaptic activity was restored in normal Ringer’s solution.
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Fig. 3. Synaptic events in Retzius neurones. (A) Selected simultaneous recordings from a pair of Retzius neurones. V1 is the recording from one of the neurones and V2 is the recording from the other. The amplitudes of excitatory synaptic potentials (EPSPs) varied from one neurone to the other and from one event to the next in the same neurone (examples are marked by the asterisks). The arrows point to events that are slower and smaller than those in their contralateral partners, suggesting their spread from the other neurone. An example of a large EPSP can be seen at the bottom of the figure. (B) Amplitude distribution of EPSPs showing populations of small and large events (N=686). (C) The small events had monotonic rise times. (D) The large EPSPs had complex rising phases, suggesting the summation of several EPSPs. Note the scale differences between C and D.
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Fig. 4. Rise times of synaptic potentials. (A) Four excitatory postsynaptic potentials (EPSPs) with different rise times recorded from the same neurone. The dotted line passes through the peak of the fastest EPSP (a). The EPSPs shown in b and c had similar rise times but different amplitudes, suggesting either that they had been produced in the same site with different quantal contents or that the two EPSPs had been produced on different sides of the gap junction. The EPSP shown in d was smaller and slower, suggesting that it was produced electrotonically at a point more more distant from the soma being recorded. (B) Characteristic rise time distribution of one Retzius neurone recorded in normal saline solution. The distribution had two Gaussian components marked by the arrows. (C) The EPSP rise time distribution of a Retzius neurone recorded in the presence of 1 mmol l-1 Mg2+ in the external fluid. Again, two Gaussian peaks, which are marked by the arrows, were present. In this case, the peak at the long rise time was clearer. (D) Theoretical model showing two Retzius neurones with dendrites coupled by a resistance (r) and sharing a common presynaptic input. The lettering of the inputs corresponds to those in B and C and has as reference the recording electrode.
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Fig. 5. Different relationships between pairs of synchronous excitatory postsynaptic potentials (EPSPs). V1 is the recording from one of the neurones and V2 is the recording from the other. (A) Simultaneous recordings from paired neurones showing pairs of synchronous EPSPs. (B) Amplification of two pairs of EPSPs from the first traces boxed in A, with different amplitudes but similar rise times. (C) In the second two pairs of synchronous EPSPs boxed in A, the amplitudes of EPSPs in V2 were fractions of those in V1 and the EPSPs had slower rise times, suggesting their propagation from V1. (D) The relationship between the rise times of synchronous pairs of EPSPs in normal saline solution (N=60). (E) The relationship between the rise times of synchronous pairs of EPSPs in the presence of 1 mmol l-1 Mg2+ in the external solution in a different neurone (N=52). Fast times in V1 clearly correlated with slow times in V2 and vice versa.
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Fig. 6. Spread of artificial excitatory postsynaptic potentials (EPSPs) from one neurone to the other. (A) Current protocol used to produce artificial EPSPs in the soma of one of the neurones. (B) Artificial EPSP with a similar shape to natural EPSPs in the same neurone. (C) Artificial EPSPs arriving at the contralateral soma were smaller and had a slower rise time. The coupling ratio V2/V1, defined as the follower voltage (V2) divided by the driving voltage (V1), in these neurones was 0.31 (N=10). (D) The coupling ratio of natural EPSPs was 0.42 (N=32), greater than that of artificial EPSPs, as expected because of the different distances that each EPSP had to spread. Values given in the figure are means ± S.E.M.
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Fig. 7. The excitatory synaptic potential (EPSP) coupling ratios were smaller than the steady-state coupling ratios in each neuronal pair. (A) The steady-state coupling ratio measured by the end of the voltage deflections in this pair of neurones was 0.31. (B) Three synchronous pairs of EPSPs from the same pair of neurones as in A with coupling ratios of 0.2–0.26. (C) The EPSP coupling ratios plotted versus the steady-state coupling ratios of 12 pairs of cells. Each symbol is the average coupling ratio of 35 EPSPs. The line indicates a slope of 1 and highlights the fact that the steady-state coupling ratios are always larger than their corresponding EPSP coupling ratios. V1 is the recording from one of the neurones and V2 is the recording from the other.
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© The Company of Biologists Ltd 2001
