Brain-derived neurotrophic factor (BDNF) expression increases in the paraventricular nucleus of the hypothalamus (PVN) during stress, and our recent studies indicate that BDNF induces sympathoexcitatory and hypertensive responses when injected acutely or overexpressed chronically in the PVN. However, it remained to be investigated whether BDNF is involved in the mediation of stress-induced cardiovascular responses. Here we tested the hypothesis that inhibition of the high-affinity BDNF receptor TrkB in the PVN diminishes acute stress-induced cardiovascular responses. Male Sprague-Dawley rats were equipped with radiotelemetric transmitters for blood pressure measurement. BDNF-TrkB signaling was selectively inhibited by viral vector-mediated bilateral PVN overexpression of a dominant-negative truncated TrkB receptor (TrkB.T1, n = 7), while control animals ( n = 7) received green fluorescent protein (GFP)-expressing vector injections. Rats were subjected to acute water and restraint stress 3-4 wk after vector injections. We found that body weight, food intake, baseline mean arterial pressure (MAP), and heart rate were unaffected by TrkB.T1 overexpression. However, peak MAP increases were significantly reduced in the TrkB.T1 group compared with GFP both during water stress (GFP: 39 ± 2 mmHg, TrkB.T1: 27 ± 4 mmHg; P < 0.05) and restraint stress (GFP: 41 ± 3 mmHg, TrkB.T1: 34 ± 2 mmHg; P < 0.05). Average MAP elevations during the poststress period were also significantly reduced after both water and restraint stress in the TrkB.T1 group compared with GFP. In contrast, heart rate elevations to both stressors remained unaffected by TrkB.T1 overexpression. Our results demonstrate that activation of BDNF high-affinity TrkB receptors within the PVN is a major contributor to acute stress-induced blood pressure elevations. NEW & NOTEWORTHY We have shown that inhibition of the high-affinity brain-derived neurotrophic factor receptor TrkB in the paraventricular nucleus of the hypothalamus significantly reduces blood pressure elevations to acute stress without having a significant impact on resting blood pressure, body weight, and food intake.

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Brain derived neurotrophic factor (BDNF) is an important modulator of neuronal function, capable of mediating long‐term changes in neuronal structure and signaling in the central nervous system (CNS). Increased expression of BDNF in the PVN has been associated with elevated blood pressure (BP) and sympathetic nervous system activity. However, the mechanism mediating this effect of BDNF is unclear. BDNF is a modulator of catecholaminergic (CA‐ergic) neuronal function in the CNS, and could potentially influence CA‐ergic input to the PVN. The majority of CA‐ergic projections to the PVN come from the nucleus of the solitary tract (NTS), and these projections have been shown to exert a hypotensive effect. Here, we tested the hypothesis that increased BDNF expression in the PVN elevates BP in part by diminishing the inhibitory input from NTS CA‐ergic neurons projecting to the PVN by downregulating β‐receptors in the PVN. Sprague‐Dawley (SD) rats received bilateral PVN injections of AAV2 viral vectors expressing either green fluorescent protein (GFP) or BDNF and bilateral NTS injections of phosphate‐buffered saline (PBS) or anti‐dopamine‐β‐hydroxylase‐conjugated saporin (DSAP), a neurotoxin selective to noradrenaline‐ and adrenaline‐synthesizing neurons. BDNF overexpression in the PVN without lesioning NTS CAergic neurons significantly increased mean arterial pressure (MAP) (BDNF+PBS: 115±3 mmHg, p<0.01 vs GFP+PBS: 96±2 mmHg). DSAP treatment increased MAP in the GFP group by ~13 mmHg, whereas DSAP treatment in the BDNF group did not significantly alter MAP. This suggests that BDNF overexpression in the PVN may interfere with CA‐ergic communication between the NTS and PVN. Since previous reports suggested that adrenergic β‐receptors exert an inhibitory effect on PVN neurons and lower BP, we tested whether BDNF overexpression in the PVN diminishes hypotensive effects of β‐receptor activation in the PVN. SD rats received bilateral PVN injections of AAV2 viral vectors expressing either GFP or BDNF. Three weeks later, BP responses to an injection of Isoprenaline (125 μM or 250 μM), a non‐selective β‐adrenergic agonist, into the PVN were recorded under alpha chloralosed‐urethane anesthesia. Our results showed that BDNF treatment significantly attenuated MAP responses to Isoprenaline compared to the GFP group in a dose dependent manner. In the GFP group, peak decrease in MAP in response to 125 μM and 250 μM Iso was −21±4 mmHg and −29±4 mmHg, compared with −4±1 mmHg (p<0.01) and −8±1 mmHg (p<0.001) in the BDNF group. To test if the reduced effect of Isoprenaline is due to a BDNF‐induced change in adrenergic receptor expression in the PVN, we assessed adrenergic receptor expression using quantitative RT‐PCR in PVN brain punches from SD rats previously injected with an AAV2 viral vectors expressing either GFP or BDNF. BDNF treatment significantly reduced the expression of β1 adrenergic receptor mRNA expression, whereas α1a, α1b, α2a, and β2 adrenergic receptors were unaffected by BDNF treatment. In summary, our findings indicate ...

Presympathetic neurons in the paraventricular nucleus of the hypothalamus (PVN) play a key role in cardiovascular regulation. We have previously shown that brain-derived neurotrophic factor (BDNF), acting in the PVN, increases sympathetic activity and blood pressure and serves as a key regulator of stress-induced hypertensive responses. BDNF is known to alter glutamatergic and GABA-ergic signaling broadly in the central nervous system, but whether BDNF has similar actions in the PVN remains to be investigated. Here, we tested the hypothesis that increased BDNF expression in the PVN elevates blood pressure by enhancing NMDA receptor (NMDAR)- and inhibiting GABAA receptor (GABAAR)-mediated signaling. Sprague Dawley rats received bilateral PVN injections of AAV2 viral vectors expressing GFP or BDNF. Three weeks later, cardiovascular responses to PVN injections of NMDAR and GABAAR agonists and antagonists were recorded under α-chloralose-urethane anesthesia. Additionally, expressions of excitatory and inhibitory signaling components in the PVN were assessed using immunofluorescence. Our results showed that NMDAR inhibition led to a greater decrease in blood pressure in the BDNF vs GFP group, while GABAAR inhibition led to greater increases in blood pressure in the GFP group compared to BDNF. Conversely, GABAAR activation decreased blood pressure significantly more in GFP vs BDNF rats. In addition, immunoreactivity of NMDAR1 was upregulated, while GABAAR-a1 and K+/Cl- cotransporter 2 were downregulated by BDNF overexpression in the PVN. In summary, our findings indicate that hypertensive actions of BDNF within the PVN are mediated, at least in part, by augmented NMDAR and reduced GABAAR signaling.

The PVN is an important cardiovascular and autonomic center involved in sympathetic nerve activity and blood pressure (BP) regulation. Stimulation of PVN presympathetic neurons elevate sympathetic activity and BP via excitatory glutamatergic projections to the rostral ventrolateral medulla and the intermediolateral cell column of the spinal cord. Presympathetic neuronal activity is regulated by the balance of excitatory glutamatergic and inhibitory GABA‐ and noradrenergic signaling. BDNF is a neurotrophic factor capable of modulating glutamatergic, GABAergic and catecholaminergic (CA‐ergic) signaling in the CNS by changing expression and membrane trafficking of neurotransmitter receptors and transporters and by increasing the soma size of neurons. Within the PVN, BDNF expression is upregulated in response to physical and psychological stressors; therefore, BDNF may play a significant role in mediating stress‐induced changes in autonomic regulation of BP. Thus, we set out to examine the long‐term effects of BDNF overexpression within the PVN on NMDA‐, GABAA‐ and adrenergic receptor‐mediated BP mechanisms and its effect on PVN neuronal soma size. To test this, Sprague Dawley (SD) rats received bilateral PVN injections of AAV2 viral vectors expressing either GFP or BDNF. Three weeks later, BP and heart rate (HR) responses to PVN injections of AP5 (10 mM), an NMDA antagonist, gabazine (2 mM), a GABAA receptor antagonist, and isoprenaline (250 μM), a non‐selective β‐adrenergic agonist, were recorded under alpha chloralosed‐urethane anesthesia. To test if BDNF alters the soma size of PVN neurons, fixed PVN brain sections were analyzed in ImageJ to quantify the soma size of BDNF or GFP expressing neurons. Our results showed that in the BDNF group, peak decreases in BP and HR in response to AP5 were −21±4 mmHg and − 38±13 BPM, compared with −2±1 mmHg (p<0.0001) and −3±1 BPM (p<0.0001) in the GFP group. In response to gabazine, peak increases in BP and HR in the BDNF group were 52±3 mmHg and 85±19 BPM, compared with 91±6 mmHg (p<0.01) and 20±3 BPM (p=0.06, n.s.) in the GFP group. In response to isoprenaline, peak decreases in BP and HR in the BDNF group were −8±1 mmHg and −19±12 BPM, compared with −35±2 mmHg (p<0.001) and −34±9 BPM (p=0.39, n.s.) in the GFP group. Analysis of PVN neuronal dimensions showed that the BDNF treated group had a significantly larger average soma size of 220±11 μm2 compared to the GFP group of 122±6 μm2 (p<0.0001). In summary, our findings indicate BDNF may alter the excitatory‐inhibitory balance in the PVN by enhancing glutamatergic and diminishing GABAergic and CA‐ergic signaling mechanisms within the PVN. In addition, BDNF‐induced increases in the volume and membrane surface area of PVN neurons could also contribute to long‐term changes in BP regulatory mechanisms. Support or Funding Information Supported by R01 HL133211‐01A1 and UVM start‐up funds. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB J...