Unilateral blockade of the dorsal ascending noradrenergic bundle and septal elicitation of hippocampal theta rhythm.

Unilateral blockade of the dorsal ascending noradrenergic bundle was produced by injections of procaine or of 6-hydroxydopamine. The threshold current of septal stimulation required to drive the hippocampal theta rhythm was then investigated at frequencies between 5.9 and 10.0 Hz. With both types of blockade the threshold-frequency function showed a normal 7.7 Hz minimum ipsilaterally and an abolition of the minimum contralaterally. Ventral noradrenergic bundle lesions had no effect. Septal elicitation of hippocampal theta rhythm may, therefore, be dependent on a noradrenergic mechanism controlling aspects of the theta rhythm, e.g. interhemispheric balance, which would not be obvious in ordinary spontaneous records.