Figure 8.

Functional effects of excitatory hilar synapses. A₁, Presumptive mossy cell-to-mossy cell excitatory connection with a mean onset latency of 2.5 ms. Average postsynaptic response shown in bold trace superimposed on example unitary responses (gray traces). A₂, The postsynaptic hilar neuron (distance metric, 1.57; MC2) in this paired recording responded to a depolarizing step with an initial burst followed by irregular spiking and small AHPs, characteristic of mossy cells. B, Averaged postsynaptic response to paired-pulse activation of a mossy cell. Hilar EPSPs showed modest paired-pulse facilitation. C, Summary of short-term plasticity in hilar synapses. Inhibitory responses segregated by phenotype of postsynaptic target; excitatory connections pooled. *p < 0.05. D₁, Spike transmission through mossy cell synapses. The presynaptic mossy cell (bottom traces) fired repetitively at ∼50 Hz when activated by an α-function current waveform, evoking a series of summating EPSPs and a single AP (truncated) in the postsynaptic hilar interneuron (top trace). D₂, Summary plots of the probability of firing in the presynaptic and postsynaptic cells, versus time, when the presynaptic neuron was driven by trains of α-function current waveforms as in D₁. Repetitive mossy cell firing at 50 Hz reproducibly evoked firing in postsynaptic neurons. E₁, Average spike-aligned responses to the first 4 APs in the α-function-triggered presynaptic bursts. Example presynaptic mossy cell burst shown in inset. The EPSP evoked by the second presynaptic AP was larger than the response to the first AP; subsequent EPSPs were smaller than the initial EPSP response. E₂, Summary plots of EPSP amplitude and postsynaptic spike probability versus presynaptic spike number. Same stimulus protocol as D₁ and E₁.