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Cruickshank, Nick

Pharmacology

2017

M.S.

Considerable evidence supports the claim that a hyperactive sympathetic nervous system (SNS) is involved in most cases of human hypertension, and therefore a more thorough understanding of the central regulation of the SNS may help elucidate novel therapeutic options. The PVN is a key region in SNS regulation of blood pressure (BP) and heart rate (HR). Stimulation of the parvocellular PVN neurons has been shown to enhance sympathetic outflow and thereby increase BP. Brain-derived neurotrophic factor (BDNF), a modulator of neuronal activity is upregulated in the paraventricular nucleus of the hypothalamus (PVN) in response to several hypertensive stimuli such as stress and hyperosmolarity, and previous studies from our lab demonstrated that both acute injections or chronic overexpression of BDNF in the PVN elevate SNS activity and BP. However, the BDNF-mediated hypertensive mechanisms are not completely understood. PVN neurons are under tonic inhibition from NTS catecholaminergic projections under baseline condition as indicated by significant BP increase after selective lesioning of NTS NE-ergic neurons. In addition, BDNF has been shown to alter NE-ergic signaling in multiple brain regions raising the possibility that BDNF may increase SNS activity and BP by interfering with NE-ergic inhibition of PVN sympathoregulatory neurons. Therefore, we tested the hypothesis that BDNF increases SNS activity and BP in part by disabling inhibitory actions of NTS catecholaminergic projections to the PVN by altering the expression of adrenergic receptors and NET in the PVN. First, blood pressure was recorded using radiotelemetry in male Sprague-Dawley rats following bilateral microinjections of adeno-associated viral vectors expressing green fluorescent protein (GFP) or myc-tagged BDNF in the PVN and microinjections of phosphate saline buffer (PBS) or Anti-Dopaine Beta Hydroxylase (DBH)-conjugated saporin (DSAP), a catecholaminergic neuron-specific neurotoxin, into the NTS. Blood pressure was monitored both during resting conditions and during acute stress tests. A second group of rats received bilateral microinjections of adeno-associated viral vectors expressing GFP or myc-tagged BDNF in the PVN, and were sacrificed after 5 weeks. PVN and NTS samples were then selectively isolated using a brain punch tool, and expression of TH, DBH, 1a, 1b, 2a, 1, 2 receptors, and norepinephrine transporter (NET) was analyzed using quantitative RT-PCR. Our results show that BDNF overexpression in the PVN leads to increased expression of catecholamine synthesizing enzymes in the NTS. In addition, both BDNF overexpression in the PVN, and DSAP lesioning in the NTS increased MAP compared to control rats. However, combined treatment with BDNF and DSAP failed to have any additional hypertensive effects suggesting that BDNF treatment may abolish the inhibitory effect of NTS catecholaminergic projections. Lesioning the NTS catecholaminergic neurons didn't appear to have a significant effect on mean arterial pressure response to the stress tests, although DSAP treatment appeared to decrease the initial heart rate response to acute stress, and this effect was most pronounced in GFP rats. These results indicate that BDNF overexpression in the PVN desensitizes sympathoregulatory neurons to inhibitory NTS catecholaminergic projections during baseline conditions.