| At present there are two lines of studies carried out on origin of the spontaneous oscillation in brain and mechanism of the synchrony activity:(1) a little pacemaker cells, which can generate spontaneous oscillation and drive other pyramidal cells; (2) group cells in the network doing same thing at the same time and then drive other pyramidal cells.Research on the oscillations in brain mainly concentrates on two points:(1) the cellular or neurochemical basis of oscillations and (2) the neural assembly by which oscillations propagate and synchronize over a large scale. Studies about these two possibilities are major on hippocampus in vitro and intracellular recording in vivo. The hippocampus is usually cut into slice with 400μm thickness for in vitro studies. During the process of the cutting, many fibers are damaged, leading to blocks of many synaptic connections. And then damage the internal network influencing the input and output of the pyramidal cells. Multichannel recording in vivo can record signal integration of much input, which is similar with physiological level. In our experiment, we choose the whole hippocampus in vitro, and record spontaneous slow frequency oscillations. This whole hippocampus model could maintain the intact internal network and eliminate the interference of the inputs from other brain structure, so this model was favorable to study the generation of the spontaneous and synchrony activity.1. GABAA Receptor-Activity Mediates Self-Generated Synchrony in Hippocampal CA3-CA1 RegionsAt present research about the oscillations in brain electrical activities mainly concentrates on two points:(1) the mechanism of generation; (2) the neural assemblies by which oscillation propagates and synchronizes over a large scale. We found that the whole hippocampus can generate slow frequency oscillation (0.8-2Hz), and simultaneous whole-cell recordings further revealed that the oscillations were synchronized among CA1-CA1, CA3-CA3, and CA3-CA1 neurons. By applying bicuculline—a GABAA receptor blocker—in the bath solution, we found that the frequency of the spontaneous oscillation was significantly reduced to 0.1—0.2 Hz. This was observed in both CA1 and CA3 regions. But the synchrony of the activity over CA3-CA1 regions remained at the low frequency oscillations. However, extracellular blockade of GABAB receptors by SCH50911 didn't alter the frequency of these oscillations. Extracellular treatment of ZD7288, which blocks hyperpolarization-activated (Ih) channels, also slowed down the hippocampal oscillations to～0.4 Hz, while did not influence the synchrony. Applying the DNQX, which is antagonist of the AMPA receptors, can block the generation of the slow oscillation in whole hippocampus. So the generation of the slow oscillation in whole hippocampus was dependent on AMPA receptors and GABAA receptors and Ih channels played an important role in this process.1. Synaptic connection between CA3 and CA1 pyramidal cellsAt present research about the synaptic connection of the different cells in hippocampus is concentrated on the slice. Normally, analyze the postsynaptic activity after presynaptic stimulus. Most fibers are damaged when the hippocampal slice is prepared. While in the whole hippocampus, the network and the synaptic connection of the hippocampus are intact. So it is benefit to analyze the ratio of the synaptic connection between CA3 and CA1 pyramidal cells. We record 10 pairs of pyramidal cells,3 of them had obvious postsynaptic excitable activity and the amplitude was about 2pA. This ratio of the synaptic connection was higher than that in the hippocampus slice. In the model of the whole hippocampal formation in vitro, we could analyze the ratio of the connection between CA3 and CA1 accurately, providing the basis for the research on the function of the hippocampus. |
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