Spatiotemporal properties of neural activity propagation from the subicular complex to the posterior cingulate cortex in rat brain slices detected bythe optical recording technique

Using the optical technique, we investigated the functional neural circuits from the subicular complex to the posterior cingulate cortex in rat brain slices. In 11 out of 98 slices, an electrical stimulation to the subicular complex induced an excitation wave that spread into the posterior cingulate cortex (PCC) and propagated along its superficial layer. The process of th is propagation was clearly divided into three steps. The first step was a f ast conduction process in the superficial layers of the subiculum, which mi ght arise from propagation of action potentials directly evoked by the stim ulation. The second one was a slow process around the boundary between the subiculum and PCC, during which significant signal enhancement was observed via a pathway in the middle to deep layers. The third step was a slow prop agating process along the superficial layers of PCC. Application of the non -NMDA receptor antagonist, CNQX, restricted propagation in the first step, suggesting that a synaptic relay exists between the first and second steps. In the rest of the slices (87 out of 98), signal propagation showed only t he first step in response to electrical stimulation. However, when bicucull ine, a GABAA receptor antagonist, was applied to these slices, the signal p ropagation spread into PCC in a manner indistinguishable from the one chara cterized above. It is therefore plausible that, under the conditions we ado pted for the silces, the propagation pathway to PCC usually remains suppres sed by GABAergic synaptic mechanisms.