Septo-hippocampal pathway necessary for dentate theta production.

In acute experiments in urethane-anaesthetized rabbits a small lesion at the septo-hippocampal border, from 0.8 to 1.5 mm from the midline, abolished theta activity ipsilaterally. This lesion severed fibres from the lateral part of the medial septal nucleus. Lesions of the main body of the fimbria or the dorsal fornix, alvear bundle and perforant path or the hippocampal commissures failed to change the dentate theta activity. Theta activity also survived the establishment of a 2 mm wide 'gate' near the rostral pole of the hippocampus; the gate was produced by cutting the main body of the fimbria and medially adjoining tissue and the dorsal fornix and laterally adjoining tissue. Cuts behind the 'gate' showed successive reduction of the dentate theta when the lesion passed beyond a threshold depth of about 1.5 mm. The degree of theta reduction appeared dependent upon the amount of the hippocampal tissue destroyed below this level. Thus, the fibres of importance for theta production seem to run through a bottleneck just behind the medial septum. Their further course is found in the basal part of the fimbria on its medial aspect and the adjoining parts of CA3 (lower blade) and hilus of the dentate fascia. Because the whole of this area had to be destroyed to block all theta activity, it is suggested that the fibres of importance for theta production are relatively dispersed within this region.