Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

Abstract

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

Fig. 1.

Coronal brain sections illustrating the neuroantatomical connection between the subfornical organ (SFO), median preoptic nucleus (MnPO), and paraventricular nucleus (PVN). A: bilateral injection of the retrograde marker latex green was made into the PVN. Retrograde marker was recovered in neuron cell bodies within the boundaries of the dorsal and ventral MnPO (B) and the SFO (C). AC, anterior commissure.

Fig. 2.

A: representative raw tracings of the responses to N-methyl-d-aspartic acid (NMDA) in the MnPO alone and with prior AP5 in the PVN. Activation of MnPO neurons with NMDA (200 pmol; n = 7) (A and B) or Bic (200 pmol, n = 5) (C) increased renal sympathetic activity (RSNA), mean arterial pressure (MAP), and heart rate (HR). The responses to NMDA and Bic were also recorded after prior microinjection of AP5 (4 nmol) into the PVN. iRSNA, integrated RSNA. *P < 0.05 vs. before AP5.

Fig. 3.

The firing response of PVN neurons to NMDA in MnPO. A: segments of original recordings show firing activity of one PVN neuron increases after NMDA microinjection (200 pmol) into the MnPO (arrow). B: mean data of changes in discharge rate after NMDA microinjection into the MnPO. *P < 0.05 vs. before NMDA.

Fig. 4.

Activation of SFO neurons with ANG II (1 nmol) increased RSNA, MAP, and HR. A: this response was attenuated with prior blocking of the PVN with AP5 (4 nmol, n = 5). B: response to ANG II microinjection into the SFO with prior microinjection of losartan (Los) (4 nmol) into the PVN (n = 5). *P < 0.05 vs. before AP5.

Fig. 5.

Activation of SFO neurons with Bic (200 pmol) increased RSNA, MAP, and HR. A: this response was attenuated with prior blocking of the PVN with AP5 (4 nmol, n = 5). B: the response to bicuculline (Bic) into the SFO after prior microinjection of Los (4 nmol) into the PVN (n = 5). *P < 0.05 vs. before AP5.

Fig. 6.

Representative injection sites in the brain. Solid circles denote hits; open circles denote misses. A: confirmed injection sites from the experiments shown in Fig. 2, NMDA microinjection into the MnPO. B: confirmed injection sites from the experiments shown in Fig. 4, ANG II microinjection into the SFO. C: confirmed injection sites from the experiments shown in Fig. 2: AP5 bilateral microinjection into the PVN. dMnPO, dorsal median preoptic nucleus; AC, anterior commissure; vMnPO, ventral medial preoptic nucleus; PVN, paraventricular nucleus; AH, anterior hypothalamus; 3V, third ventricle.