EFFECTS OF REVERSIBLE INACTIVATION OF THE MEDIAL SEPTAL AREA ON LONG-TERM POTENTIATION AND RECURRENT INHIBITION OF HIPPOCAMPAL POPULATION SPIKES IN RATS

We assessed the effects of reversible inactivation of the medial septa l area (MSA) on long-term potentiation (LTP) and recurrent inhibition in the dentate gyrus of urethane-anesthetized rats, in vivo. The septa l input to the hippocampus was temporarily eliminated by injection of tetrodotoxin (TTX, 10 ng/l mu l) into the MSA. In Experiment 1, LTP in ducibility was examined in the perforant-dentate gyrus synapses in the MSA inactivated and control rats by 2 high-frequency stimulation (HFS ), 5 min apart, applied to the perforant pathway (PP). The magnitude o f potentiation was evaluated as the percentage change in the populatio n spike (PS) amplitude al 5, 30, 60 or 120 min after the second HFS. T he PS amplitude in the MSA inactivated rats was significantly lower th an those of control group at 120 but not 5, 30 or 60 min after the sec ond HFS, The MSA inactivation itself had no effect on the basal respon ses evoked by test stimuli. In Experiment 2, the MSA inactivation did not affect the efficacy of recurrent inhibition in the perforant-denta te gyrus synapses produced by paired pulses applied to the PP at 10- a nd 20-ms interpulse intervals, These results indicate that: (1) althou gh hippocampal synapses can be still potentiated after the HFS in the MSA inactivated animals, a faster decay of LTP may be due to eliminati on of the MSA output amplification on synaptic responses mediated by e xcitatory amino acids; and (2) the recurrent inhibition mechanism in t he dentate gyrus of the hippocampus is not probably affected by the MS A inactivation.