Blood-borne interleukin-1β acts on the subfornical organ to upregulate the sympathoexcitatory milieu of the hypothalamic paraventricular nucleus

Abstract

We previously reported that microinjection of the proinflammatory cytokine interleukin-1β (IL-1β) into the subfornical organ (SFO) elicits a pressor response accompanied by increases in inflammation and renin-angiotensin system (RAS) activity in the SFO and hypothalamic paraventricular nucleus (PVN). The present study sought to determine whether blood-borne IL-1β induces similar neurochemical changes in the SFO and PVN and, if so, whether increased inflammation and RAS activity at the SFO level orchestrate the sympathoexcitatory response to circulating IL-1β. In urethane-anesthetized male Sprague-Dawley rats, intravenous injection of IL-1β (500 ng) increased blood pressure, heart rate, renal sympathetic nerve activity, and mRNA for angiotensin-converting enzyme, angiotensin II type 1a receptor, cyclooxygenase-2, tumor necrosis factor-α, and IL-1β, as well as the tumor necrosis factor-α p55 receptor and the IL-1 receptor, in the SFO and PVN. Pretreatment with SFO microinjections of the angiotensin II type 1a receptor blocker losartan (1 µg), the angiotensin-converting enzyme inhibitor captopril (1 µg), or the cyclooxygenase-2 inhibitor NS-398 (2 µg) attenuated expression of these excitatory mediators in the SFO and downstream in the PVN and the IL-1β-induced pressor responses. An SFO lesion minimized the IL-1β-induced expression of inflammatory and RAS components as well as c-Fos, an indicator of neuronal excitation, in the PVN. These studies demonstrate that circulating IL-1β, which increases in cardiovascular disorders such as hypertension and heart failure, acts on the SFO to increase inflammation and RAS activity in the SFO and PVN and that intervening in these neurochemical processes in the SFO can significantly reduce the sympathetic response.

Keywords: brain; cyclooxygenase-2; proinflammatory cytokines; renin-angiotensin system; sympathetic nervous system.

Fig. 1.

Representative traces (A–D) and grouped data (E) showing effects of intravenous (IV) injection of interleukin (IL)-1β (500 ng) on blood pressure (BP), heart rate (HR, beats/min), and renal sympathetic nerve activity (RSNA), windowed (spikes/s) and integrated (mV), in rats pretreated with subfornical organ (SFO) microinjections of vehicle (Veh), losartan, captopril, and NS-398. Arrows indicate timing of injections. MBP, mean blood pressure; ΔRSNA, change from baseline in integrated RSNA. Values are means ± SE. *P < 0.05 vs. baseline; †P < 0.05 vs. SFO Veh + IV IL-1β.

Fig. 2.

Quantitative analysis by real-time PCR showing effects of intravenous (IV) injection of interleukin (IL)-1β or 0.9% saline on mRNA expression of angiotensin-converting enzyme (ACE), angiotensin II type 1a receptor (AT_1aR), cyclooxygenase (COX)-2, and COX-1 in the cortex, subfornical organ (SFO), and hypothalamic paraventricular nucleus (PVN) of rats pretreated with SFO microinjection of vehicle (Veh), losartan, captopril, and NS-398. Values are means ± SE (n = 6 for each group) and expressed as fold change relative to Veh control. *P < 0.05 vs. SFO Veh + IV saline; †P < 0.05 vs. SFO Veh + IV IL-1β.

Fig. 3.

Quantitative analysis by real-time PCR showing effects of intravenous (IV) injection of interleukin (IL)-1β or 0.9% saline on mRNA expression of tumor necrosis factor (TNF)-α, IL-1β, TNF-α receptor 1 (TNFR1), and IL-1 receptor (IL-1R) in the cortex, subfornical organ (SFO), and hypothalamic paraventricular nucleus (PVN) of rats pretreated with SFO microinjection of vehicle (Veh), losartan, captopril, and NS-398. Values are means ± SE (n = 6 for each group) and expressed as fold change relative to Veh control. *P < 0.05 vs. SFO Veh + IV saline; †P < 0.05 vs. SFO Veh + IV IL-1β.

Fig. 4.

Quantitative analysis by real-time PCR showing effects of intravenous (IV) injection of interleukin (IL)-1β or 0.9% saline on mRNA expression of angiotensin-converting enzyme (ACE), angiotensin II type 1a receptor (AT_1aR), cyclooxygenase (COX)-2, COX-1, tumor necrosis factor (TNF)-α, IL-1β, TNF-α receptor 1 (TNFR1), and IL-1 receptor (IL-1R) in the hypothalamic paraventricular nucleus (PVN) of rats with a subfornical organ (SFO) lesion (SFO-x) or a sham SFO lesion (SFO-s). Values are means ± SE (n = 6 for each group) and expressed as fold change relative to Veh control. *P < 0.05 vs. SFO-s + IV saline; †P < 0.05 vs. SFO-s + IV IL-1β.

Fig. 5.

Western blot analysis showing effects of intravenous (IV) injection of interleukin (IL)-1β or 0.9% saline on expression of the transcription factor nuclear factor NF-κB (NF-κB) p65 subunit (A), inhibitor of κB (IκB-α, B), and c-Fos (C) in the hypothalamic paraventricular nucleus (PVN) of rats with a subfornical organ lesion (SFO-x) or a sham SFO lesion (SFO-s). Values are means ± SE and normalized to β-actin (n = 6 in each group). *P < 0.05 vs. SFO-s + IV saline; †P < 0.05 vs. SFO-s + IV IL-1β. Representative Western blots are shown above each bar.

Fig. 6.

Histological images of the subfornical organ (SFO) from rats subjected to a sham lesion (SFO-s, A), an electrolytic lesion (SFO-x, B), and SFO microinjection (C). Arrow in C indicates a microinjection site labeled with pontamine sky blue.