Involvement of GABAergic and cholinergic medial septal neurons in hippocampal theta rhythm

Hippocampal theta rhythm (HPC theta) may be important for various phenomena, including attention and acquisition of sensory information. Two types of HPC theta (types I and II) exist based on pharmacological, behavioral, and electrophysiological characteristics. Both types occur during locomotion, whereas only type II (atropine-sensitive) is present under urethane anesthesia. The circuit of HPC theta synchronization includes the medial septum-diagonal band of Broca (MSDB), with cholinergic and gamma-aminobutyric acid (GABA)ergic neurons comprising the two main projections from MSDB to HPC. The primary aim of the present study was to assess the effects of GABAergic MSDB lesions on urethane- and locomotion-related HPC theta, and compare these effects to those of cholinergic MSDB lesions. Saline, kainic acid (KA), or 192 IgG-saporin (SAP) was injected into MSDB before recording. KA preferentially destroys GABAergic MSDB neurons, whereas SAP selectively eliminates cholinergic MSDB neurons. A fixed recording electrode was placed in the clentate mid-molecular layer, and stimulating electrodes were placed in the posterior hypothalamus (PH), and medial perforant path (PP). Under urethane anesthesia, HPCO was induced by tail pinch, PH stimulation, and systemic physostigmine; none of the rats with KA or SAP showed HPC theta in any of these conditions. During locomotion, HPC theta was attenuated, but not eliminated, in rats with KA or SAP lesions. Intraseptal KA in combination with either intraseptal SAP or PP lesions reduced locomotion-related HPC theta beyond that observed with each lesion alone, virtually eliminating HPC theta. In contrast, intraseptal SAP combined with PP lesions did not reduce HPC theta beyond the effect of each lesion alone. We conclude that both GABAergic and cholinergic MSDB neurons are necessary for HPC theta under urethane, and that each of these septohippocampal projections contributes to HPC theta during locomotion. (c) 2005 Wiley-Liss, Inc.