Diversity of postsynaptic amplitude and failure probability of unitary excitatory synapses between CA3 and CA1 cells in the rat hippocampus

A gradient of frequency-dependent synaptic properties along the longitudinal hippocampal axis

Background

The hippocampus is a functionally heterogeneous brain structure and specializations of the intrinsic neuronal network may crucially support the functional segregation along the longitudinal axis of the hippocampus. Short-term synaptic plasticity plays fundamental roles in information processing and may be importantly involved in diversifying the properties of local neuronal network along the hippocampus long axis. Therefore, we aimed to examine the properties of the cornu ammonis 1 (CA1) synapses along the entire dorsoventral axis of the rat hippocampus using field excitatory postsynaptic potentials from transverse rat hippocampal slices and a frequency stimulation paradigm.

Results

Applying a ten-pulse stimulus train at frequencies from 0.1 to 100 Hz to the Schaffer collaterals we found a gradually diversified pattern of frequency-dependent synaptic effects along the dorsoventral hippocampus axis. The first conditioned response was facilitated along the whole hippocampus for stimulus frequencies 10–40 Hz. However, steady-state responses or averaged responses generally ranged from maximum synaptic facilitation in the most dorsal segment of the hippocampus to maximum synaptic depression in the most ventral segment of the hippocampus. In particular, dorsal synapses facilitated for stimulus frequency up to 50 Hz while they depressed at higher frequencies (75–100 Hz). Facilitation at dorsal synapses was maximal at stimulus frequency of 20 Hz. On the contrary, the most ventral synapses showed depression regardless of the stimulus frequency, only displaying a transient facilitation at the beginning of 10–50 Hz stimulation. Importantly, the synapses in the medial hippocampus displayed a transitory behavior. Finally, as a whole the hippocampal synapses maximally facilitated at 20 Hz and increasingly depressed at 50–100 Hz.

Conclusion

The short-term synaptic dynamics change gradually along the hippocampal long axis in a frequency-dependent fashion conveying distinct properties of information processing to successive segments of the structure, thereby crucially supporting functional segregation along the dorsoventral axis of the hippocampus.

Differences in uniquantal amplitude between sites reduce uniquantal variance when few release sites are active

In many types of central neurons, the coefficient of variation (CV) of stimulus-evoked uniquantal events inferred from quantal analysis is small, frequently less than 20%. In contrast, spontaneous putative uniquantal events (minis) from the same neurons are much more variable in amplitude, having a CV of roughly 50% or more. One explanation for this discrepancy is that, if the variance in mini amplitude were generated by differences between release sites, the small number of sites activated during stimulation would sometimes fortuitously have similar quantal amplitudes. Only in these fortuitous cases where uniquantal variance is small could quantal peaks be resolved, and therefore the uniquantal CV seen in the subset of cells where quantal analysis can be performed would systematically be much smaller than predicted by the mini distribution. We have explored this possibility by Monte Carlo simulation assuming all variance in mini amplitude to be due to intersite differences in uniquantal amplitude. We find that when a small number of release sites are activated under these conditions, there is a reduction in the expected uniquantal variance. However, the expected uniquantal CV is highly variable from one experiment to the next, and low uniquantal CVs are not expected to be seen often enough to account for the high frequency with which quantal peaks with a uniquantal CV < 20% are observed experimentally. We conclude that variance in mini amplitude between release sites cannot account for the small uniquantal CV seen in quantal analysis of many central synapses.

Testing the fit of a quantal model of neurotransmission

Many studies of synaptic transmission have assumed a parametric model to estimate the mean quantal content and size or the effect upon them of manipulations such as the induction of long-term potentiation. Classical tests of fit usually assume that model parameters have been selected independently of the data. Therefore, their use is problematic after parameters have been estimated. We hypothesized that Monte Carlo (MC) simulations of a quantal model could provide a table of parameter-independent critical values with which to test the fit after parameter estimation, emulating Lilliefors's tests. However, when we tested this hypothesis within a conventional quantal model, the empirical distributions of two conventional goodness-of-fit statistics were affected by the values of the quantal parameters, falsifying the hypothesis. Notably, the tests' critical values increased when the combined variances of the noise and quantal-size distributions were reduced, increasing the distinctness of quantal peaks. Our results support two conclusions. First, tests that use a predetermined critical value to assess the fit of a quantal model after parameter estimation may operate at a differing unknown level of significance for each experiment. Second, a MC test enables a valid assessment of the fit of a quantal model after parameter estimation.

Synaptic refractory period provides a measure of probability of release in the hippocampus

Despite extensive research, much controversy remains regarding the locus of expression of long-term potentiation (LTP) in area CA1 of the hippocampus, specifically, whether LTP is accompanied by an increase in the probability of release (p(r)) of synaptic vesicles. We have developed a novel method for assaying p(r), which utilizes the synaptic refractory period--a brief 5-6 ms period following release during which the synapse is incapable of transmission (Stevens and Wang, 1995). We show that this assay is sensitive to a battery of manipulations that affect p(r) but find no change following either NMDA receptor-dependent LTP or long-term depression (LTD).

Physiology and anatomy of synaptic connections between thick tufted pyramidal neurones in the developing rat neocortex

1. Dual voltage recordings were made from pairs of adjacent, synaptically connected thick tufted layer 5 pyramidal neurones in brain slices of young rat (14-16 days) somatosensory cortex to examine the physiological properties of unitary EPSPs. Pre- and postsynaptic neurones were filled with biocytin and examined in the light and electron microscope to quantify the morphology of axonal and dendritic arbors and the number and location of synaptic contacts on the target neurone. 2. In 138 synaptic connections between pairs of pyramidal neurones 96 (70%) were unidirectional and 42 (30%) were bidirectional. The probability of finding a synaptic connection in dual recordings was 0.1. Unitary EPSPs evoked by a single presynaptic action potential (AP) had a mean peak amplitude ranging from 0.15 to 5.5 mV in different connections with a mean of 1.3 +/- 1.1 mV, a latency of 1.7 +/- 0.9 ms, a 20-80% rise time of 2.9 +/- 2.3 ms and a decay time constant of 40 +/- 18 ms at 32-24 degrees C and -60 +/- 2 mV membrane potential. 3. Peak amplitudes of unitary EPSPs fluctuated randomly from trial to trial. The coefficient of variation (c.v.) of the unitary EPSP amplitudes ranged from 0.13 to 2.8 in different synaptic connections (mean, 0.52; median, 0.41). The percentage of failures of single APs to evoke a unitary EPSP ranged from 0 to 73% (mean, 14%; median, 7%). Both c.v. and percentage of failures decreased with increasing mean EPSP amplitude. 4. Postsynaptic glutamate receptors which mediate unitary EPSPs at -60 mV were predominantly of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor type. Receptors of the N-methyl-D-aspartate (NMDA) type contributed only a small fraction (< 20%) to the voltage-time integral of the unitary EPSP at -60 mV, but their contribution increased at more positive membrane potentials. 5. Branching patterns of dendrites and axon collaterals of forty-five synaptically connected neurones, when examined in the light microscope, indicated that the axonal and dendritic anatomy of both projecting and target neurones and of uni- and bidirectionally connected neurones was uniform. 6. The number of potential synaptic contacts formed by a presynaptic neurone on a target neurone varied between four and eight (mean, 5.5 +/- 1.1 contacts; n = 19 connections). Synaptic contacts were preferentially located on basal dendrites (63%, 82 +/- 35 microns from the soma, n = 67) and apical oblique dendrites (27%, 145 +/- 59 microns, n = 29), and 35% of all contacts were located on tertiary basal dendritic branches. The mean geometric distances (from the soma) of the contacts of a connection varied between 80 and 585 microns (mean, 147 microns; median, 105 microns). The correlation between EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0.2 and 0.26, respectively). 7. Compartmental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electrotonic distances between 0.07 and 0.33 from the soma (mean, 0.13). Simulations of unitary EPSPs, assuming quantal conductance changes with fast rise time and short duration, indicated that amplitudes of quantal EPSPs at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10-fold depending on the dendritic location of synaptic contacts. The inferred quantal peak conductance increase varied between 1.5 and 5.5 nS (mean, 3 nS). 8. The combined physiological and morphological measurements in conjunction with EPSP simulations indicated that the 20-fold range in efficacy of the synaptic connections between thick tufted pyramidal neurones, which have their synaptic contacts preferentially located on basal and apical oblique dendrites, was due to differences in transmitter release probability of the projecting neurones and, to a lesser extent, to differenc

The number of postsynaptic currents necessary to produce locomotor-related cyclic information in neurons in the neonatal rat spinal cord

To understand better how synaptic signaling contributes to network activity, we analyzed the potential contribution of putative unitary postsynaptic currents (PSCs) to locomotor-related information received by spinal interneurons in neonatal rats. The average cyclic modulation of the whole-cell current in 13 neurons was quantified as the difference between the current integral (charge) during the first and second halves of the cyclic locomotor network output. Between 7.6 and 303 average unitary PSCs per second were needed to produce the cyclic modulation. This number is so low that very few (1-5) of the synapses contributing to the cyclic information need to be active simultaneously. This suggests that individual presynaptic cells in a central locomotor network can have a powerful influence on other neurons.