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Release probability increases towards distal dendrites boosting high-frequency signal transfer in the rodent hippocampus

  1. Thomas P Jensen  Is a corresponding author
  2. Olga Kopach
  3. James P Reynolds
  4. Leonid P Savtchenko
  5. Dmitri A Rusakov  Is a corresponding author
  1. Queen Square UCL Institute of Neurology, University College London, United Kingdom
Research Article
Cite this article as: eLife 2021;10:e62588 doi: 10.7554/eLife.62588
6 figures, 1 table and 1 additional file

Figures

Figure 1 with 1 supplement
Optical quantal analysis at individual CA3-CA1 synapses reports higher release probability towards distant dendrites.

(A) CA1 pyramidal cell held in whole-cell mode (acute hippocampal slice), dialysed with 50 µM AF 594 and 300 µM Fluo-8 (75 µm z-stack average, λx2p = 800 nm; AF 594 channel). Dotted rectangles (S1 and S2), two ROIs to record from two dendritic spines; stimulating electrode positions are illustrated by dotted cones. (B) Image panels, ROIs as in (A), shown at higher magnification; arrows, linescan positioning at the dendritic spines of interest. Traces, Ca2+ signal (ΔG/R, Fluo-8 green-channel signal ΔG related to red-channel AF 594 signal R) recorded as the width-integrated linescan intensity, at two spines as indicated, in response to paired-pulse afferent stimuli (arrows). Release failures and responses to the first pulse can be clearly separated (Figure 1—figure supplement 1D). P1 and P2 show average release probability in response to the 1st and 2nd stimulus, respectively, calculated as P1 = {(1,0) + (1,1)} / N and P2 = {(0,1) + (1,1)} / N, and P2* is the adjusted 2nd-stimulus release probability calculated as P2* = (0,1) / {(0,0) + (0,1)} where brackets indicate the counts of paired-pulse successes (1) or failures (0), and N is the number of trials. (C) Average release probability (Pr; shown as P1 in B) at individual synapses plotted against distance to the soma. Solid line, linear regression (p value to reject H0 = zero slope and Pearson's r shown; n = 67). (D) Paired-pulse ratio P2/P1 plotted against distance to the soma. Other notations as in (C) (n = 41; average PPR mean ± SEM: 2.14 ± 0.17; spines with no reliable detection of 2nd responses, and no detectable (0,1) responses, were excluded). (E) Adjusted paired-pulse ratio PPR*=P2*/P1 plotted against distance to the soma. Other notations as in (C) (n = 41; average PPR*=2.05 ± 0.18).

Figure 1—figure supplement 1
Optical quantal analysis at individual CA3-CA1 synapses in acute hippocampal slices: second-order dendrite.

(A) CA1 pyramidal cell held in whole-cell mode dialysed with 50 µM AF 594 and 400 µM OGB-1 (75 µm z-stack average, λx2p = 800 nm; DIC + AF 594 channel combined). Patch pipette (patch) and stimulating electrode (stim) can be seen; dotted rectangles (S1 and S2), ROIs to record from individual dendritic spines. (B) ROIs as shown in (A) by dotted, at higher magnification (AF 594 channel only); dotted circles, two dendritic spines of interest. (C) Examples of Ca2+ linescan signal traces (ΔG/R, green-channel OGB-1 increment signal ΔG related to red-channel AF 594 signal R) recorded in two dendritic spines as shown in (B), in response to two afferent stimuli (arrows) applied by a stimulating electrode (as in A). Release failures and successes can be clearly separated; notations P1, P2, and P2* as in Figure 1B. (D) Average amplitudes (10–45 ms interval post-pulse, mean ± SD) of the first-response successes (red) and failures (blue) for 67 dendritic spines as shown in Figure 1C.

Figure 2 with 1 supplement
Selected features of excitatory synapses with respect to their dendritic location.

(A) Average release probability (Pr) at individual synapses plotted against distance to the first dendrite branching point. Solid line, linear regression (p value to reject H0 = zero slope and Pearson's r shown; n = 67). (B) Percentage difference in Pr between two synapses on one dendritic branch (as in Figure 1A, Figure 1—figure supplement 1A), plotted against the distance between them along the branch. Other notations as in (A) (n = 15). (C) Average release probability (Pr) plotted against relative synapse position at the dendritic branch: synapse co-ordinate was scaled to the 0–1 range representing the branch origin and the end, as indicated (n = 63; several spines with unidentifiable distal branch ends were excluded). (D) Probability of release success upon both afferent stimuli, plotted against distance from the soma. Other notations as in (A) (n = 40). (E) Apparent spine density along the dendrite (smallest/thinnest spines could be undetectable, see Discussion), plotted against distance from the soma. Other notations as in (A) (n = 65; several spines with unidentifiable local spine density were excluded). (F) Release probability (Pr), plotted against distance from the soma. Other notations as in (A) (n = 67).

Figure 2—figure supplement 1
Selected features of excitatory synapses with respect to their dendritic location.

(A) Average probability of the second release (paired-pulse stimuli 50 ms apart) plotted against distance to the soma; sample mean ± SEM: 0.633 ± 0.20 (n = 41). (B) Paired-pulse ratio (as P2/P1 in C) plotted against spine density along dendrites; sample mean ± SEM: 2.14 ± 0.17 (n = 41).

Figure 3 with 1 supplement
Evoked glutamate release from Schaffer collaterals shows lower paired-pulse ratios at greater distances from pyramidal cell bodies.

(A) Experimental design: area of the hippocampal slice with iGluSnFR expressed in neuronal membranes (green channel); arrow, measured distance d between CA1 pyramidal cell body layer and the axonal bouton of interest; stim, stimulating electrode. (B) Examples of recorded axonal boutons (image panels, dotted circles; position of spiral 'Tornado' linescans is illustrated) showing characteristic glutamate signals in response to two afferent stimuli 50 ms apart (green traces, individual trials; black, average), at two distances from the s. pyramidale, as indicated. The lack of release failures reflects detection of glutamate escaping from multiple neighbouring synapses. (C) Paired-pulse ratio for optical glutamate signals: ΔF/F0(1) / ΔF/F0(2) averaged over 18–36 trials at individual boutons, plotted against distance to the soma. Other notations as in Figure 1C (n = 33). (D) Amplitude of the first glutamate response, ΔF/F0(1) averaged over 18–36 trials at individual boutons, plotted against distance to the soma. The amplitude values reflect the average amount of glutamate released from the bouton of interest, and glutamate escaping from its neighbours; other notations as in (C) (n = 33).

Figure 3—figure supplement 1
Optical (multi-synaptic) glutamate signal evoked by short bursts of Schaffer collateral stimulation at varied distances from s.pyramidale.

(A) Examples of recorded ROIs (iGluSnFR green fluorescence channel, dotted ovals) in s. radiatum, with groups of stimulation-responding tentative axonal boutons expressing iGluSnFR, at different distances to the s. pyramidale border (as illustrated in Figure 3A; distance reference shown is measured to the horizontal midline of the area shown; distance to each ROI was measured to the ROI centroid). (B) ROI-average iGluSnFR responses (green line, temporal resolution ~25 ms) to five stimuli 50 ms apart. The underlying sensor signal kinetics (light grey line) was reconstructed using the fitting algorithm ΔF/F0=Aiexp((tΔt(i1))τ1) (i = 1,..., 5) where Ai is the ith signal amplitude (fitted directly to the recoded amplitude), Δt = 50 ms, and the decay constant τ obtained directly from fitting the signal tail after the fifth pulse; dash-dotted lines, the linear regression line for the five ΔF/F0 response peaks (at the five pulse onsets). (C) Average slope of linear regression for the five ΔF/F0 response peaks as shown in (B), plotted against the distance from the s. pyramidale. (n = 61 boutons from N = 7 animals / slices).

Figure 4 with 1 supplement
Computer simulations of CA1 pyramidal cell with stochastic excitatory synapses.

(A) Diagram, NEURON model of a reconstructed CA1 pyramidal cell (Migliore et al., 1999) (ModelDB 2796; variable time step dt, t = 34°C). 50 excitatory inputs (blue dots) generate bi-exponential conductance change (rise and decay time, 1 ms and 20 ms, respectively) stochastically, in accord with Pr. Traces, simulated somatic response to paired-pulse stimuli (50 ms apart), with Pr distributed either uniformly randomly (black), or in accord with the distance-dependent trend (as in Figure 1C; red), and both Pr and synaptic density trends (as in Figure 2E, blue; same average Pr = 0.36 ). (B) Summary of simulation tests in (A); dots, individual runs (n = 100); bars, mean EPSP amplitude (left, mean ± SEM: 2.90 ± 0.060, 3.27 ± 0.061, 3.20 ± 0.068, for the three conditions, respectively, as indicated) and paired-pulse ratios (right, mean ± SEM: 1.96 ± 0.047, 1.57 ± 0.036, 1.52 ± 0.0437, notation as above) are shown; ***p<0.005. (C) Input-output spiking rate relationship over the physiological range of input firing frequencies (per axon, bottom axis; total, top axis); hollow circles, uniform distribution of Pr; solid symbols, Pr follows the distance-dependent trend (as in Figure 1C); mean ± SEM are shown (n = 100 simulation runs). (D) Trace: A characteristic cell spiking burst (model as in A) in response to a Poisson-process afferent spiking input (~50 Hz per synapse) incorporating the experimental kinetics of short-term plasticity (STP) at CA3-CA1 synapses (Mukunda and Narayanan, 2017) (see Figure 4—figure supplement 1C–F for detail). Graph: Input-output spiking rate relationship across the physiological range of average input firing frequencies, with experimental STP incorporated; other notations as in (C); the top abscissa scale is nonlinear because STP affects average Pr in a biphasic, non-monotonous manner (see Figure 4—figure supplement 1C).

Figure 4—figure supplement 1
Simulating the experiment-based kinetics of short-term plasticity (STP) in the CA3-CA1 circuit.

(A) Traces show simulated average somatic response to five-burst 20 Hz stimuli (NEURON model as in Figure 4A), with synaptic Pr values distributed either uniformly randomly (black line), or in accord with the distance-dependent trend of Pr (as in Figure 1C–D) and the five-pulse ΔF/F0 slope (as in Figure 3—figure supplement 1C). (B) Summary of stochastic simulations shown in (A); ordinate, voltage transfer (area under the voltage curve over the five EPSPs); dots, individual runs (n = 100); bars, average values; ***p<0.001. (C) Linearised representation of the STP ratio profile for the 1–50 Hz range of presynaptic spiking frequencies replicating the experimental band-pass structure of the CA3–CA1 Schaffer collateral synapses (Mukunda and Narayanan, 2017). (D–F) Simulated EPSP response in a CA1 pyramidal cell (NEURON model as in Figure 4A) to a burst of afferent stimuli, with synaptic Pr values distributed either uniformly randomly (black), or in accord with the distance-dependent trend of Pr (as in Figure 1C; red); green dots, experimental data from Mukunda and Narayanan, 2017, normalised to the first EPSP amplitude, as indicated; postsynaptic response incorporates STP kinetics, as shown in (C).

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Tables

Key resources table
Reagent type (species) or resourceDesignationSource or referenceIdentifiersAdditional information
Chemical compound, drugPicrotoxinTocrisCat. # 1128
Chemical compound, drugD-SerineSigma AldrichCat. # S4250
Chemical compound, drugOregon Green 488 BAPTA-1, Hexapotassium SaltThermoFisherCat. #O6806
Chemical compound, drugAlexa Fluor-594 HydrazideThermoFisher ScientificCat.# A10438
Chemical compound, drugFluo-8, potassium saltStratech ScientificCat.# 21087-AATOriginal Source: (Teflabs)
No longer exists
Chemical compound, drugFluo-4, Pentapotassium SaltThermoFisher ScientificCat.# F14200
Chemical compound, drugAgaroseSigma-AldrichCat. #9539; CAS: 9012-36-6Freshly prepared (4%)
Chemical compound, drugD-glucoseSigma-AldrichCat. #G8270; CAS: 50-99-7Freshly prepared (10 mM)
Chemical compound, drugKClSigma-AldrichCat. #P9333; CAS: 7447-40-7
Recombinant DNA reagentAAV9.hSynap.iGluSnFr.WPRE.SV40Penn Vector CoreAddgene
Cat. # 98929-AAV9
Strain, strain background
(Sprague-Dawley rat)
Sprague-DawleyCharles River UKCrl: CD (SD)
Strain: 0204
Strain, strain background
(C57BL/6J mouse)
C57BL/6JCharles River UKC57BL/6NCrl
Strain: 0159
Software, algorithmImageJNIHRRID:SCR_003070
https://imagej.nih.gov/ij/
Software, algorithmpClamp10Molecular DevicesRRID:SCR_011323
https://www.moleculardevices.com/products/axon-patch-clamp-system/acquisition-and-analysis-software/pclamp-software-suite
Software, algorithmOriginProOriginLab IncRRID:SCR_014212
https://www.originlab.com/origin
Software, algorithmMES 4.x-5.xFemtonics Ltd.RRID:SCR_018309
https://uk.mathworks.com/products/connections/product_detail/femtonics-mes.html
Software, algorithmWinWCP Versions 4.x-5.xStrathclyde Electrophysiology SoftwareRRID:SCR_014270 http://spider.science.strath.ac.uk/sipbs/software_ses.htm
Software, algorithmMatlabMathworksRRID:SCR_001622 https://uk.mathworks.com/products/matlab.html
Software, algorithmNEURON 7.6 × 64https://www.neuron.yale.edu/neuron/RRID:SCR_005393
Software, algorithmKinetics of CA1 pyramidal neuronhttps://senselab.med.yale.edu/ModelDB/ShowModel?model=2796#tabs-2RRID:SCR_005393
model = 2796#tabs-2
Software, algorithmreconstructed CA1 pyramidal neuronshttps://senselab.med.yale.edu/ModelDB/ShowModel?model=7509#tabs-1RRID:SCR_005393
model = 7509#tabs-1
OtherMulticlamp 700BMolecular DevicesRRID:SCR_018455
OtherOlympus FluoView1000OlympusRRID:SCR_014215
OtherFemto3D RCFemtonicsFemto3D RC
OtherBioRad Radiance 2100BioRadRadiance 2100
OtherLeica VT1200S vibratomeLeica BiosystemsRRID:SCR_018453

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