Synaptic control of pyramidal cell activation in the hippocampal slice preparation in the rat.

Recordings were made from CA1 pyramidal neurons in a rat hippocampal slice preparation to compare the effectiveness of orthodromic stimuli when delivered at different distances from the cells under study. A stimulating electrode placed in stratum radiatum was less effective in driving nearby pyramidal cells (within 200 micron) than those farther away (greater than 800 micron). Thus for a given field excitatory postsynaptic potential both the intracellular excitatory postsynaptic potential and the evoked population spike were smaller when evoked from a local stimulating electrode than from one more distant. Laminar mapping experiments indicated that the spatial distribution of activated excitatory synapses over the pyramidal cell dendrites was similar for local and distant stimuli. The firing threshold, and the amplitude of hyperpolarizing inhibitory postsynaptic potentials, were also similar for the two stimuli. Responses evoked by the local stimulating electrode were more sensitive to morphine, penicillin and pentobarbital than responses elicited by the distant stimulus, suggesting that some form of GABAergic inhibition limited the efficacy of the local stimulus. The data suggest that in the CA1 region a vertically oriented synaptic inhibitory system exists that powerfully regulates the ability of an orthodromic stimulus to activate pyramidal cells. These results also illustrate the practical importance of controlling the distance between stimulating and recording electrodes, when performing quantitative pharmacological studies of synaptic transmission in the hippocampus.