Upregulation of brain-derived neurotrophic factor expression in nodose ganglia and the lower brainstem of hypertensive rats

Abstract

Hypertension leads to structural and functional changes at baroreceptor synapses in the medial nucleus tractus solitarius (NTS), but the underlying molecular mechanisms remain unknown. Our previous studies show that brain-derived neurotrophic factor (BDNF) is abundantly expressed by rat nodose ganglion (NG) neurons, including baroreceptor afferents and their central terminals in the medial NTS. We hypothesized that hypertension leads to upregulation of BDNF expression in NG neurons. To test this hypothesis, we used two mechanistically distinct models of hypertension, the spontaneously hypertensive rat (SHR) and the deoxycorticosterone acetate (DOCA)-salt rat. Young adult SHRs, whose blood pressure was significantly elevated compared with age-matched Wistar-Kyoto (WKY) control rats, exhibited dramatic upregulation of BDNF mRNA and protein in the NG. BDNF transcripts from exon 4, known to be regulated by activity, and exon 9 (protein-coding region) showed the largest increases. Electrical stimulation of dispersed NG neurons with patterns that mimic baroreceptor activity during blood pressure elevations led to increases in BDNF mRNA that were also mediated through promoter 4. The increase in BDNF content of the NG in vivo was associated with a significant increase in the percentage of BDNF-immunoreactive NG neurons. Moreover, upregulation of BDNF in cell bodies of NG neurons was accompanied by a significant increase in BDNF in the NTS region, the primary central target of NG afferents. A dramatic increase in BDNF in the NG was also detected in DOCA-salt hypertensive rats. Together, our study identifies BDNF as a candidate molecular mediator of activity-dependent changes at baroafferent synapses during hypertension.

Figure 1. BDNF transcripts are upregulated in nodose ganglia of SHR compared to WKY rats

(A) Mean arterial pressure of WKY (n=6) and SHR (n=4) rats measured in the week preceding tissue collection; (B) Levels of BDNF transcripts 1, 4, 6 and the protein-coding region (pcr) relative to the reference gene glyceraldehyde 3-phosphate dehydrogenase (GAPDH), measured by quantitative RT-PCR in nodose ganglia of WKY rats (n=6) and SHRs (n=4); * P<0.05, *** P<0.001.

Figure 2. Electrical stimulation of NG neurons in vitro at the frequency that mimics in vivo activity of baroreceptor afferents leads to BDNF transcription

Levels of BDNF transcript 4 (which showed significant upregulation in hypertensive rats in vivo, Fig. 1B) in newborn NG neurons in vitro in control (Unstimulated, n=4) and following a 4-hour electrical field stimulation at 6 Hz with 0.2 ms/100 mA pulses (6 Hz, n=4). The levels of the BDNF transcript were measured by quantitative RT-PCR, with glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as the reference gene, and normalized (the transcript levels in unstimulated cultures are considered 1); * P<0.05.

Figure 3. BDNF protein is upregulated in nodose ganglia of SHR compared to WKY rats

(A) Mean arterial pressure of WKY (n=14) and SHR (n=13) rats measured in the week preceding tissue collection; (B) Levels of BDNF protein measured by sandwich ELISA in nodose ganglia of WKY rats (n=14) and SHRs (n=13); *** P<0.001.

Figure 4. The proportion of BDNF-immunoreactive neurons is increased in nodose ganglia of SHR compared to WKY rats

(A) Mean arterial pressure of WKY (n=3) and SHR (n=3) rats measured in the week preceding tissue collection; (B) Mean percentage of BDNF-immunoreactive (BDNF-IR) cells among all NG somata with a clearly delineated plasma membrane and a nuclear profile of WKY and SHR rats; WKY (n=3 rats; 703 neurons), SHR (n=3 rats; 591 neurons); ** P<0.01, *** P<0.001; (C) A representative section through the right NG of a WKY (left panel) and SHR (right panel) rat, immunostained for BDNF; scale bar, 100 μm.

Figure 5. BDNF protein is upregulated in the dorsal medulla, the primary central target of NG afferents, in SHR compared to WKY rats

(A) Mean arterial pressure of WKY (n=9) and SHR (n=8) rats measured in the week preceding tissue collection; (B) Levels of BDNF protein measured by sandwich ELISA in the dorsal medulla of WKY rats (n=9) and SHRs (n=8); *** P<0.001; (C) Levels of BDNF protein in the dorsal medulla (circles) and the NG (triangles) of each individual WKY rat and SHR analyzed.

Figure 6. BDNF expression is increased in nodose ganglia of the deoxycorticosterone acetate (DOCA)-salt rats, a mechanistically distinct model of hypertension

(A) Mean arterial pressure of sham-salt (Sham, n=3) and DOCA-salt (DOCA, n=4) rats measured one day before tissue collection; (B) Levels of BDNF protein measured by sandwich ELISA in nodose ganglia of sham-salt (Sham, n=7) and DOCA-salt (DOCA, n=9) rats; * P<0.05, ** P<0.01.