Identification of pathways mediating cardiovascular responses elicited by stimulation of the septum in the rat.

1. Experiments were done in rats anaesthetized with sodium pentobarbitone to localize pathways mediating the cardiovascular responses elicited by electrical stimulation of the septum. The major efferent projections from the septum were first identified anatomically by the Fink-Heimer II technique and these pathways were subsequently lesioned in acute experiments to establish their role in the mediation of the cardiovascular responses elicited by septal stimulation. 2. Electrical stimulation of histologically localized sites in the lateral septum elicited hypotension and bradycardia whereas stimulation of sites in the medial septum elicited hypertension and bradycardia. 3. Selective lesions of cardiovascular responsive sites in either the lateral or medial septum produced a pattern of degeneration essentially similar to that of previous anatomical studies, i.e. the main efferent projections were localized to the fornix, stria medullaris and the medial forebrain bundle. 4. Stimulation of the fornix did not elicit cardiovascular changes and lesions of this pathway did not alter cardiovascular responses to septal stimulation. 5. Stimulation of the stria medullaris elicited hypotension and bradycardia whereas stimulation of the medial forebrain bundle elicited hypertension and bradycardia. Ipsilateral lesions of the stria medullaris, signicicantly attenuated the hypotension and bradycardia elicited by stimulation of the lateral septum but did not affect the responses to stimulation of the medial septum. Bilateral lesions of the medial forebrain bundle in the region of the lateral hypothalamus abolished the hypertension but did not affect the bradycardia elicited by medial septum stimulation. Cardiovascular, reponses elicited by stimulation of the lateral septum were not affected by medial forebrain bundle lesions. 6. It is suggested that the cardiovascular responses elicited by stimulation of the lateral septum are mediated via the stria medullaris and that the hypertension elicited by stimulation of the medial septum is mediated via the medial forebrain bundle. On the other hand, the bradycardia elicited by stimulation of the medial septum is probably mediated by a pathway presently unknown.