Spatial distribution of hippocampal-dentate theta rhythm following colchicine injection into the hippocampal formation of the rat.

This investigation confronts the basic question of dentate granule cell participation in the generation of superficial and/or deep theta recorded from the dorsal hippocampal formation in the ether-anesthetized rat. Granular cells were destroyed unilaterally by local injection of 2 micrograms of colchicine (COL) dissolved in 0.2 microliter of distilled water. Hippocampal EEG was recorded 8 days after the neurotoxin injection. The COL injection which destroyed most dentate granular cells but left nearby pyramidal cells of the hippocampal CA1 area virtually unaffected, failed to disrupt significantly either superficial or deep theta. In contrast, the COL injection which destroyed most dentate granular cells and, also, caused some damage to the nearby pyramidal cells of the CA1 area, resulted in a sharp alteration of both theta. These data strongly suggest that granular cells of the dentate gyrus may not be critically involved in theta production, at least in the ether-anesthetized rat. They also confirm a previous conclusion that the neuroanatomical integrity of the CA1 area of the dorsal hippocampus is necessary for the normal appearance and maintenance of both superficial and deep theta. These data are discussed in the context of the generally accepted notion that superficial and deep theta are produced by the rhythmic activity of CA1 pyramidal cells and dentate granular neurons respectively.