Theta-related gating cells in hippocampal formation: in vivo and in vitro study

In this study we extended our earlier in vitro findings concerning the discovery of a novel type of theta-related cells, which we have termed gating cells. There were two main objectives of our present investigations. The first was to determine the distribution of theta gating cells in the separated CA1 and CA3 generators in three different pharmacological conditions: (i) the presence of a cholinergic agonist-carbachol, (ii) the presence of carbachol and GABA(A) ergic antagonist-bicuculline, (iii) the presence of carbachol and GABA(B) ergic antagonist-2-hydroxysaclofen. The second objective of our studies was to verify our earlier in vitro findings and to demonstrate, for the first time, gating cells in intact hippocampus during the generation of Type II theta in urethane anaesthetized rats. Two hundred ninety-nine theta-related cells were isolated and recorded from in vivo and in vitro hippocampal formation. Twenty out of all 299 neurons (6.6%) were classified as gating cells. The neuron was classified as a gating cell if it met one of the following criteria: (i) the cell discharges occurred precisely in the beginning and at the end of each theta epoch (gating cell A); (ii) the cell began to discharge just before the transition from non-theta interval/LIA into the theta epoch (gating cell B); (iii) the cell began to discharge just after the transition from the theta epoch into non-theta interval/LIA (gating cell C). Our data demonstrates that the appearance of theta epochs and their length, as well as the appearance of non-theta states (in vivo recorded LIA or in vitro recorded intervals between theta epochs) and their length, may require the existence of a specific population of hippocampal neurons which we termed gating cells.