Variable properties in a single class of excitatory spinal synapse

Abstract

Although synaptic properties are specific to the type of synapse examined, there is evidence to suggest that properties can vary in individual synaptic populations. Here, a large sample of monosynaptic connections made by excitatory interneurons (EINs) onto motor neurons in the lamprey spinal cord locomotor network has been used to examine the properties of a single class of spinal synapse in detail. The properties and activity-dependent plasticity of EIN-evoked EPSPs varied considerably. This variability occurred at convergent inputs made by several EINs onto single motor neurons. This suggests that it was an intrinsic network property and not simply related to differences between animals or experiments. The activity-dependent plasticity of EIN-evoked EPSPs could be negatively or positively related to the initial EPSP amplitude (P1 and P2 connections, respectively). This reflected the development of facilitation and depression from either small or large initial EPSPs. To identify differences in presynaptic properties that could contribute to the synaptic variability, the quantal amplitude, release probability, number of release sites, and size of the available vesicle pool were examined. This analysis suggested that the variable amplitude and plasticity of EPSPs at P1 and P2 connections reflected an interaction between the release probability and the size of the available transmitter store. There is thus significant functional variability in EIN synaptic properties. Synapses ranged from strong (evoked postsynaptic spikes) to weak (small depressing EPSPs). The selection of interneurons with different synaptic properties could provide an intrinsic mechanism for modifying excitatory network interactions and the locomotor network output.

Fig. 1.

The properties of EIN-evoked EPSPs. Histograms of EPSP amplitudes (A), rise times (B), and half-widths (C) are shown. Di,Dii, An example of an electrical connection between an EIN and a motor neuron. Notice that the chemical component varies, but the electrical component is constant. The chemical but not electrical component was blocked by cadmium (200 μm).E, An example of a connection in which EIN stimulation at 20 Hz consistently evoked spikes in the postsynaptic motor neuron.F, The relationship of the coefficient of variation to the initial EPSP amplitude. Each symbol represents a single connection.

Fig. 2.

The plasticity of EIN inputs over trains of 20 spikes at 5–20 Hz. Graphs show summed data at depressing (A), facilitating (B), biphasic (C), and unchanged connections (D). Insets on the graphs show examples of each type of plasticity over the first five spikes during a 20 Hz spike train.

Fig. 3.

The properties of convergent EIN inputs to single motor neurons. The graphs and traces show inputs made by two different EINs onto a single motor neuron. Ai, Aii, Two convergent inputs to a single motor neuron in which the initial EPSP amplitude and plasticity differed. Bi,Bii, Convergent inputs in which the same type of plasticity developed from different initial EPSP amplitudes.Ci, Cii, Convergent inputs that evoked different plasticity from two EPSPs with similar initial amplitudes.Di, Dii, Convergent inputs where EPSPs with different initial amplitudes and plasticity resulted in the input equalizing during the spike train.

Fig. 4.

Examination of the locus of plasticity using the CV⁻². Graph showing the relationship of the CV⁻²(CV⁻²_PP/CV⁻²_Init) to paired pulse plasticity (EPSP₂/EPSP₁) at 20 Hz (A), 10 Hz (B), and 5 Hz (C). Each symbol represents a single connection. With a presynaptic mechanism the CV⁻²_PP/CV⁻²_Initshould increase with facilitation but reduce with depression. As can be seen from the graphs, facilitation tended to follow a presynaptic mechanism. However, with depression the response was more variable, and at higher frequencies there was an increase in the number of responses in which the CV⁻²_PP/CV⁻²_Initincreased with depression. The increase in the CV⁻² with depression was not removed by correcting for the effect of driving force resulting from the initial EPSP (D) but was reduced in low-calcium Ringer's solution (E).F, Graph showing the CV⁻²_PP/CV⁻²_Initat depressing connections in control, after correcting for driving force, and in low-calcium Ringer's solution. G, Example trace showing a significant polysynaptic inhibitory input evoked in a motor neuron by EIN stimulation.

Fig. 5.

The relationship of PP plasticity (EPSP₂/EPSP₁) to the initial EPSP amplitude. A, Graph showing PP plasticity at different frequencies. For clarity only PP plasticity with values of <2.5 is shown. B, The relationship of PP plasticity to the initial EPSP amplitude is shown at 20 Hz, when connections that show an inverse relationship between initial EPSP amplitude and plasticity (P1 connections, ▪) are plotted separately to connections that have PP plasticity positively related to EPSP amplitude (P2 connections, ■). Notice that above 2 mV the EPSP was not related to PP plasticity in either type of connection. The inset shows sample traces of paired-pulse plasticity from P1 and P2 connections at 20 Hz.

Fig. 6.

Variance–mean analysis of EIN synaptic transmission. A, Traces showing 100 EIN (top traces) or reticulospinal (bottom traces)-evoked EPSPs (evoked at 0.2 Hz) in control and in high- and low-calcium Ringer's solution.B, Graph showing EPSP amplitudes over the stimulation train in different calcium levels (●, control; ■, high calcium; ▿, low calcium). C, Plot of the EPSP variance against the EPSP mean in control and in high- and low-calcium Ringer's solution. Di–Diii, Histograms of theq_w,N_min, andp_w of EIN and reticulospinal (RS) inputs to motor neurons. Histograms showing the quantal amplitude (q_w) (Ei), minimum number of release sites (N_min) (Eii), and release probability (p_w,) (Eiii) at facilitating and depressing EIN connections.

Fig. 7.

The releasable vesicle pool size in EINs.A, Graph showing the depression of an EIN–evoked EPSP in a single connection, illustrating the parameters used to estimate the available vesicle numbers. B, Histogram showing vesicle numbers in all depressing connections and in P1 and P2 connections. The estimated releasable vesicle pool was significantly larger at P1 than at P2 connections.