Spontaneous CA1 Oscillations Generated In Whole Hippocampal Formation In Vitro And Their Dependence On GABA_A Receptor-and Gap Junction-mediated Activity

Hippocampus plays a crucial role in the formation and maintenance of specific types of learning and memory,in which function hippocampal oscillation is deeply involved.In the past half century,systematic in vivo and in vitro studies have established a series of molecular,cellular and network mechanisms contributing to hippocampal oscillation.Both in vivo and in vitro methods have its own merits and defects:pharmacological studies can be easily implemented on slices,but extra drug administration is required to induce hippocampal oscillation due to the destruction of hippocampal networks;hippocampal oscillation can be easily recorded in animals alive but pharmacological methods are inevitably constrained.In the start of our study,we established a new technique to isolate complete hippocampus,so that the original hippocampal oscillation can be easily recorded.All the following work was conducted on complete hippocampus from young rats of 14—18 postnatal days.First,we recorded the activity in hippocampal CA1 region.Local field potential recording shows that oscillation in region CA1 of complete isolated hippocampus is in the frequency range of 0.5—1.5 Hz,and the oscillation is mainly composed of depolarized membrane potential,over 90%of which causes action potential firing,according to perforated patch clamp recording.What's more,strong activity synchronization was observed among CA1 neurons.Further investigations showed that the completeness of CA1 networks is essential for the CA1 oscillation.When‘CA3-DG'region was removed,CA1 oscillation was not significantly affected,but further removal of part of CA1 region caused the frequency of the remaining CA1dropped markedly.Next,we investigated how synaptic and ion channel activities contribute to the oscillation of complete isolated hippocampus.When GABA_A antagonist was administered,CA1 oscillation frequency was found to decrease significantly,while GABA_A agonist administering increased CA1 oscillation frequency significantly.And the activity of CA1 interneuron and pyramidal cell was recorded simultaneously revealing that their membrane potential was highly synchronized,and that there existed an action potential timing precedency of interneuron over pyramidal cell.These results suggest that CA1 oscillation depends on the activity of GABAergic interneurons.Furthermore,we administered CBX,a broad-spectrum antagonist of gap junction,discovering that CA1 oscillation was completely blocked.And the same result was observed when Mefloquine,a specific antagonist of connexin 36(electric synapse between neurons),was administered.These results show that CA1 oscillation depends on broad-spectrum connexin and electric synapse.During experiments,it was observed that even though when chemical synapse(e.g.AMPA receptor,Ca2~+channels,etc.)or gap junction was blocked,CA1oscillation was totally destructed,after washing out the drugs,CA1 oscillation re-originated“from nothing”.we further administered,in addition to previous drugs,TTX to block the activity of Na~+channel,CA1 oscillation didn't reappear after washing out.This finding suggested that after the blockade of chemical and electric synapse,there still exists Na~+channel dependent activity,which is indispensable to the re-origination of CA1 oscillation.In conclusion,with the technique to isolate complete hippocampus,we observed the basic characteristics of CA1 oscillation,and proved its dependency on CABA_A receptor and connexin.These findings will provide us with new clues for a better understanding of hippocampal oscillation.