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Segal M, Bloom FE. 
“The Projection Of Midline Raphe Nuclei To The Hippocampus Of The Rat”. 
Fed Proc. 1974;33(3):299, Abs.No.538.
Activity of hippocampal pyramidal cells was monitored in anesthetized rats. Bipolar stimutation of dorsal and medial raphe nuclei caused cessation of spontaneous firing of hippocampal cells. This inhibition had a long latency 40-80 msec) and long duration [200 - 400 msec] and was pronounced when a train of stimuli was used. A similar inhibition was observed as a response to the microiontophoresis of serotonin. These inhibitions were qualitatively different from the responses to stimulation of the fornix or the nucleus locus coeruleus as well as to the iontophoresis of GABA or norepirephrine, all of which are inhibitory with respect to pyramidal spontaneous activity. Iontophoresis of LSD, BOL, methysergide and cyproheptadine as well es picrotoxir. antagonized the effects of serotonin. Some of these compounds also antagonized the response to raphe stimulation. A monosynaptic pathway was traced between the raphe and the hippocampus with anterograde and retrograde anatomical techniques. It is suggested that this pathway uses serotonin as a transmitter and that it is inhibitory with respect to hippocampal pyramidal cells.
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