Brain Research, 382 (1986) 119-128 119 Elsevier BRE 11990 Electrophysiological Identification of Forebrain Connections of the Subfornical Organ MICHAEL B. GUTMAN, JOHN CIRIELLO and GORDON J. MOGENSON Department of Physiology, Health Sciences Centre, Universityof Western Ontario, London, Ont. (Canada) (Accepted February llth, 1986) Key words: angiotensin II - - vasopressin - - circumventricular organ - - hypothalamus - - cardiovascular regulation - - drinking Experiments were performed in 17 urethane-anesthetized rats to investigate electrophysiologically neurons in the subfornical organ (SFO), which send efferent axons directly to the region of the paraventricular nucleus of the hypothalamus (PVH), the supraoptic nu- cleus (SON) and the nucleus medianus (NM). Extraceilular single unit recordings were made from spontaneously active and silent neurons in the region of SFO (n = 130) and the nucleus triangularis (NT; n = 20). Sixty-five units in SFO were antidromically activated by stimulation of either PVH, SON or NM with latencies corresponding to conduction velocities of 0.54 + 0.07 (n = 24), 0.44 + 0.05 (n = 17) and 0.23 + 0.02 (n = 24) m/s, respectively. Axons of SFO units projecting to NM conducted at significantly slower velocities than those to PVH and SON. An additional 11 units were antidromically activated in NT by stimulation of these forebrain structures. Sixty-seven units were found to respond orthodromically to stimulation of PVH, SON and NM: 58 in SFO and 9 in NT. Orthodromic responses were primarily excitation or inhibition. These data have demonstrated bidirectional pathways between SFO and forebrain structures which are likely involved in the dipsogenic and arterial pressure responses to activation of SFO by blood-borne angiotensin II. INTRODUCTION There is experimental evidence suggesting that blood-borne angiotensin II exerts an effect on the subfornical organ (SFO) to increase arterial pressure 16, to elicit water intake 3°, and to induce the release of arginine vasopressin 18. In addition, electri- cal and chemical stimulation of the SFO has been shown to elicit water intake 25 and increased arterial pressure 7'1°. Taken together, these observations sug- gest that SFO contributes to homeostatic mecha- nisms which maintain body fluid balance and arterial pressure. The neuronal circuits, which are likely involved in mediating these responses to stimulation of SFO, have been extensively studied using neuroanatomical tract tracing techniques 14,19,2°,24,27,2s. These studies have shown that SFO projects directly to 3 major fo- rebrain regions: the regions of the parvocellular and magnocellular components of the paraventricular nu- cleus of the hypothalamus (PVH), the supraoptic nu- cleus (SON) and the nucleus medianus (NM). These forebrain structures have been implicated in mediat- ing the release of arginine vasopressin 17-2°, the in- crease in arterial pressure 7,1° and the drinking re- sponse 13'17 observed during stimulation of SFO. On the other hand, few studies have investigated electro- physiologically the connections of SFO. Electrical stimulation of SFO has been shown to alter the firing frequency of neurohypophyseal and tuberoinfundi- bular PVH neurons 5 and of paraventriculomedullary neurons 6. In addition, stimulation of SON 29 and PVH 32 have been shown to antidromically and ortho- dromically activate a small number of SFO neurons. However, in these latter studies, there were not enough neurons to adequately investigate their elec- trophysiological characteristics. In the present study, experiments were done to ex- amine some of the electrophysiological characteris- tics of SFO neurons which project directly to the re- gion of PVH, SON and NM. Extracellular single unit recordings were made in SFO during electrical stimu- Correspondence: J. Ciriello, Department of Physiology, Health Sciences Centre, University of Western Ontario, London, Ont., Canada N6A 5C1. 0006-8993/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)