Electrophysiological and pharmacological evidence in favor of amino acid neurotransmission in fimbria-fornix fibers innervating the lateral septal complex of rats.

Electrical stimulation of fimbria-fornix (fi-fx) fibers monosynaptically activated many of the neurons tested in the lateral septal complex (LSC) of the rat. The orthodromically activated LSC neurons were classified as "strongly" orthodromically activated (SOA) or "weakly" orthodromically activated ( WOA ) cells according to their threshold for eliciting a response, stability of the response latency, frequency following and the stimulus-response ratio. Microiontophoretically applied glutamate (GLU) could excite both SOA and WOA neurons. However, the expelling currents needed to activate the SOA cells were often considerably lower than those necessary to excite the WOA cells suggesting higher sensitivity to GLU of those cells which receive a strong fi-fx innervation. Iontophoretically administered glutamic acid diethylester (GDEE) in general reversibly attenuated excitatory responses of LSC cells to GLU but not to acetylcholine. GDEE was also effective in blocking the synaptic responses of SOA septal cells to fi-fx stimuli. In addition, GDEE administered topically reversibly suppressed the field potential induced in the LSC by fi-fx stimulation. These electrophysiological and pharmacological results support recent biochemical observations suggesting that the excitatory innervation of LSC neurons by fi-fx fibers is mediated by GLU or a closely related excitatory amino acid.