A LONG-LASTING INCREASE AND DECREASE IN SYNAPTIC EXCITABILITY IN THE RAT LATERAL SEPTUM ARE ASSOCIATED WITH HIGH AND LOW SHUTTLE BOX PERFORMANCE, RESPECTIVELY

In a series of experiments with rats, using evoked held potentials, th e influence of massed trial training in 2-way shuttle box avoidance an d step-through passive avoidance tasks was studied on the synaptic exc itability of the lateral septum (LS) neurons and on the induction of l ong-term potentiation in the lateral septum in vivo. The majority of r ats that attained a high performance level in the shuttle box task exh ibited, after the shuttle box training, a long-lasting enhancement of synaptic excitability of lateral septum neurons, whereas most of the r ats with low performance in the shuttle box showed a long-lasting depr ession in the LS synaptic excitability. Both types of excitability cha nges disappeared within 24 h. Neither the first habituation session in the passive avoidance apparatus nor the subsequent one-trial learning in passive avoidance task had a marked influence on lateral septum sy naptic excitability. Both highperformance and low-performance rats exh ibited a long-term potentiation (LTP)-like potentiation of synaptic ex citability of the lateral septum neurons after high frequency stimulat ion of the fimbria fibers although the amount of LTP in high performan ce rats was slightly higher than that in low performance animals.