Brain Research, 305 (1984) 361-364 361 Elsevier BRE20261 Hypothalamic paraventricular nucleus lesions decrease pressor responses to subfornical organ stimulation A. V. FERGUSON and L. P. RENAUD Neurosciences Unit, The Montreal General Hospital and McGiU University, 1650 Cedar A venue, Montreal, P.Q. H3G 1A 4 (Canada) (Accepted March 6th, 1984) Key words: subfornical organ - - paraventricular nucleus - - pressor responses Electrical stimulation of the subfornical organ (SFO) in urethane anesthetized male Sprague-Dawley rats was associated with in- tensity and frequency dependent increases in arterial blood pressure. Stimulation in the hypothalamic paraventricular nucleus (PVN) also evoked increases in arterial blood pressure. Electrolytic lesions of the PVN significantly reduced SFO induced pressor responses, suggesting that the PVN constitutes part of the efferent pathways through which SFO stimulation elicits increases in blood pressure. The subfornical organ (SFO), a circumventricular structure located at the ventral surface of the hippo- campal commissure, participates in the regulation of body fluid balance through its efferent projections. Their destruction has been shown to abolish circulat- ing angiotensin II-induced drinking behavior 2A1,14, and an associated release of vasopressin in the pos- terior pituitary 5. The SFO is also a site where both electrical 4.7 and chemical 8 stimulation can increase arterial blood pressure. The hypothalamic paraven- tricular nucleus (PVN), another site in the central nervous system where electrical stimulation can cause an elevation in blood pressure 1°, has been shown to receive afferent projections from the SFO 6,9. The present study was designed first to more clearly characterize the pressor responses to electri- cal stimulation of the SFO and secondly, to examine whether the PVN might play a role in these cardio- vascular effects. All experiments utilized male Sprague-Dawley rats (150-300 g) anesthetized with a single intraperi- toneal injection of urethane (1.4 g/kg). Arterial blood pressure was monitored continuously with a pressure transducer connected to a femoral arterial catheter (PE 50 Intramedic). Animals were placed in a stereotaxic frame and a burr hole was placed in the dorsal surface of the skull to permit introduction of electrodes in the SFO. Stimulation was delivered through monopolar glass coated tungsten electrodes (tip exposure 50-100/~m), and consisted of trains of cathodal stimuli at 1-75 Hz (25-400/~A; pulse width 100/~s) lasting 10 s applied from an isolated stimula- tion unit controlled by a pulse generator. Stimulating electrodes were advanced in 0.1 mm increments through the SFO area, and the effects of electrical stimulation on systolic blood pressure were assessed at each site. SFO stimulation sites were later verified histologically in cresyl violet stained 100/am sections by the reconstruction of electrode penetrations which were marked by small electrolytic lesions placed at the top and bottom of each penetration. Effects of SFO stimulation on blood pressure. Ini- tial experiments on 7 animals examined the topo- graphy and optimal parameters for electrical stimula- tion in SFO to induce a rise in blood pressure. Re- sponses were site specific; as illustrated in Fig. 1, stimulation in SFO, but not in adjacent regions (e.g. the hippocampal commissure), produced a short la- tency intensity-dependent increase in arterial pres- sure (maximum change within 3 s), followed by a ra- pid return to normal (return to baseline within 5 s) at the end of the stimulation period. The observed pres- Correspondence: A. V. Ferguson, Neurosciences Unit, Montreal General Hospital Research Institute, 1650 Cedar Avenue, Montre- al, Quebec H3G 1A4 Canada. 0006-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.