Unitary inhibitory synaptic potentials in the guinea‐pig hippocampus in vitro.

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Abstract
Mechanisms involved in the generation of synaptic inhibition were investigated by making simultaneous intracellular recordings from pairs of neurons in the CA3 pyramidal cell field of guinea pig hippocampal slices. Inhibitory post-synaptic potentials dependent on single presynaptic action potentials (i.p.s.p.) and mediated through monosynaptic and disynaptic connections were identified. The recurrent nature of some hippocampal inhibition was demonstrated by showing that activity in a single cell may initiate feedback i.p.s.p. onto itself. The observation of synchronous i.p.s.p. in recordings from 2 cells illustrates the divergence of synaptic contacts made by inhibitory neurons. The peak conductance change associated with an i.p.s.p. was in the range 5-9 nS and it reversed uniformly throughout its time course at membrane potentials between -73 and -80 mV. The shortest time-to-peak of synaptic potentials was .apprx. 3 ms and in this case the i.p.s.p. decayed with a time constant comparable to the passive membrane time constant of the post-synaptic neuron. The peak amplitude of i.p.s.p. fluctuated in a way consistent with the quantal release of inhibitory neurotransmitter. Inhibitory neurons could fire bursts of action potentials not unlike those generated by pyramidal cells in this area. A comparison of the conductance change associated with identified i.p.s.p. with that associated with the maximal inhibitory post-synaptic potential resulting from electrical stimulation of fiber pathways suggested that, in the slice, a pyramidal cell is innervated by up to 15 inhibitory neurons.