Myrna Herrera-Rosales

Ang II–Salt Hypertension Depends on Neuronal Activity in the Hypothalamic Paraventricular Nucleus but Not on Local Actions of Tumor Necrosis Factor-α

Development of angiotensin II (Ang II)–dependent hypertension involves microglial activation and proinflammatory cytokine actions in the hypothalamic paraventricular nucleus (PVN). Cytokines activate receptor signaling pathways that can both acutely grade neuronal discharge and trigger long-term adaptive changes that modulate neuronal excitability through gene transcription. Here, we investigated contributions of PVN cytokines to maintenance of hypertension induced by subcutaneous infusion of Ang II (150 ng/kg per min) for 14 days in rats consuming a 2% NaCl diet. Results indicate that bilateral PVN inhibition with the GABA-A receptor agonist muscimol (100 pmol/50 nL) caused significantly greater reductions of renal and splanchnic sympathetic nerve activity (SNA) and mean arterial pressure in hypertensive than in normotensive rats (P<0.01). Thus, ongoing PVN neuronal activity seems required for support of hypertension. Next, the role of the prototypical cytokine tumor necrosis factor-&agr; was investigated. Whereas PVN injection of tumor necrosis factor-&agr; (0.3 pmol/50 nL) acutely increased lumbar and splanchnic SNA and mean arterial pressure, interfering with endogenous tumor necrosis factor-&agr; by injection of etanercept (10 &mgr;g/50 nL) was without effect in hypertensive and normotensive rats. Next, we determined that although microglial activation in PVN was increased in hypertensive rats, bilateral injections of minocycline (0.5 &mgr;g/50 nL), an inhibitor of microglial activation, failed to reduce lumbar or splanchnic SNA or mean arterial pressure in hypertensive or in normotensive rats. Collectively, these findings indicate that established Ang II–salt hypertension is supported by PVN neuronal activity, but short term maintenance of SNA and arterial blood pressure does not depend on ongoing local actions of tumor necrosis factor-&agr;.

Chronic Hypertension Enhances the Postsynaptic Effect of Baclofen in the Nucleus Tractus Solitarius

Microinjection of the inhibitory neurotransmitter &ggr;-aminobutyric acid B-subtype receptor agonist baclofen into the nucleus tractus solitarius increases arterial blood pressure and sympathetic nerve discharge. The baclofen-induced pressor response is enhanced in chronic hypertension. We hypothesized that a postsynaptic mechanism contributes to the enhanced responses to baclofen in hypertension. We investigated the postsynaptic effect of baclofen on second-order baroreceptor neurons, identified by 1,1′-dilinoleyl-3,3,3′,3′-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate labeling of the aortic nerve, in nucleus tractus solitarius slices from sham-operated normotensive and unilateral nephrectomized, renal-wrap hypertensive rats. After 4 weeks, arterial blood pressure was 153±7 mm Hg in hypertensive rats (n=9) and 93±3 mm Hg in normotensive rats (n=8; P<0.05). There was no difference in resting membrane potential (54.5±0.7 versus 53.3±0.6 mV) or input resistance (1.07±0.11 versus 1.03±0.11 G&OHgr;) between hypertensive and normotensive neurons (both n=18). Baclofen induced a net outward current in nucleus tractus solitarius neurons in the presence of 1 &mgr;mol/L tetrodotoxin. The EC50 of the baclofen effect was greater in normotensive cells (9.1±3.2 &mgr;mol/L; n=5) than hypertensive cells (3.0±0.5 &mgr;mol/L; n=7; P<0.05), and baclofen (10 &mgr;mol/L) induced a greater decrease in input resistance in hypertensive cells (61±2%; n=6) than in normotensive cells (45±4%; n=9; P<0.05). Both potassium and calcium channels were involved in the baclofen-evoked whole-cell current. The results suggest an enhanced postsynaptic response to activation of inhibitory neurotransmitter &ggr;-aminobutyric acid B-subtype receptors in second-order baroreceptor neurons in the nucleus tractus solitarius in renal-wrap hypertensive rats. This enhanced inhibition could alter baroreflex function in chronic hypertension.

Rats selectively bred for differences in aerobic capacity have similar hypertensive responses to chronic intermittent hypoxia

Exposure to chronic intermittent hypoxia (CIH) is an animal model that mimics the repetitive bouts of hypoxemia experienced by humans with sleep apnea. Rats exposed to CIH develop hypertension that depends on the activation of sympathetic nerve activity (SNA). Since obesity and metabolic syndrome have been linked to neurogenic hypertension and sleep apnea, and because sleep apnea can adversely affect aerobic exercise capacity, we tested the hypothesis that rats bred for selection of low aerobic capacity running (LCR) would have a greater hypertensive response to CIH than rats bred for high aerobic capacity running (HCR). Blockade of ganglionic transmission was performed to compare the contribution of SNA to the maintenance of resting mean arterial pressure (MAP). Next, hypertensive responses to 7 days of CIH were compared across LCR and HCR rats (14-16 mo old). Finally, the contribution of the hypothalamic paraventricular nucleus (PVN) to the maintenance of SNA and hypertension after CIH was determined and compared across groups. Although LCR rats were less active and had greater body weights than HCR rats, resting MAP, the contribution of ongoing SNA to the maintenance of MAP, and hypertensive responses to CIH were similar between groups. Contrary to our hypothesis, chemical inhibition of the PVN with muscimol (1 mmol/100 nl) caused a larger fall of MAP in HCR rats than in LCR rats. We conclude that LCR rats do not have resting hypertension or an exaggerated hypertensive response to CIH. Interestingly, the maintenance of CIH hypertension in LCR rats compared with HCR rats appears less reliant on ongoing PVN neuronal activity.

Identification of Active Central Nervous System Sites in Renal Wrap Hypertensive Rats

To identify central neurons participating in cardiovascular regulation in hypertension, we studied Fos staining, a marker for synaptically activated neurons, in adult male normotensive and hypertensive (HT) rats. At 1 and 4 weeks after induction of unilateral nephrectomy, renal wrap hypertension mean arterial pressure was 138±4 mm Hg (n=6) in 1-week HT rats and 159±6 mm Hg (n=6) in 4-week HT rats. Mean arterial pressure was 103±2 mm Hg (n=6) in sham-operated, normotensive rats. Mean arterial pressure was greater in both HT groups compared with normotensive rats, and the mean arterial pressure in 4-week HT rats was greater than that in 1-week HT rats. Rats were anesthetized and perfused, brains sectioned and processed using a Fos antibody, and the number of Fos immunoreactive neurons counted in sections through various brain regions. Hypertension of 1 or 4 weeks did not alter the number of Fos immunoreactive neurons in the area postrema, the supraoptic nucleus, and the median preoptic nucleus. The number of Fos immunoreactive neurons was increased after 1 and 4 weeks in the nucleus of the solitary tract, both the caudal and ventral lateral medulla, and the organum vasculosum of the lamina terminalis. In addition, after 4 weeks of HT, the number of Fos immunoreactive neurons was increased in the parabrachial nucleus and the paraventricular nucleus of the hypothalamus. The results indicate central regions active in acute and chronic HT rats and suggest certain areas that may be differentially activated depending on the duration of the hypertension.

Constitutive plasma membrane monoamine transporter (PMAT, Slc29a4) deficiency subtly affects anxiety-like and coping behaviours

Originally, uptake‐mediated termination of monoamine (e.g., serotonin and dopamine) signalling was believed to only occur via high‐affinity, low‐capacity transporters (“uptake1”) such as the serotonin or dopamine transporters, respectively. Now, the important contribution of a second low‐affinity, high‐capacity class of biogenic amine transporters has been recognised, particularly in circumstances when uptake1 transporter function is reduced (e.g., antidepressant treatment). Pharmacologic or genetic reductions in uptake1 function can change locomotor, anxiety‐like or stress‐coping behaviours. Comparable behavioural investigations into reduced low‐affinity, high‐capacity transporter function are lacking, in part, due to a current dearth of drugs that selectively target particular low‐affinity, high‐capacity transporters, such as the plasma membrane monoamine transporter. Therefore, the most direct approach involves constitutive genetic knockout of these transporters. Other groups have reported that knockout of the low‐affinity, high‐capacity organic cation transporters 2 or 3 alters anxiety‐like and stress‐coping behaviours, but none have assessed behaviours in plasma membrane monoamine transporter knockout mice. Here, we evaluated adult male and female plasma membrane monoamine transporter wild‐type, heterozygous and knockout mice in locomotor, anxiety‐like and stress‐coping behavioural tests. A mild enhancement of anxiety‐related behaviour was noted in heterozygous mice. Active‐coping behaviour was modestly and selectively increased in female knockout mice. These subtle behavioural changes support a supplemental role of plasma membrane monoamine transporter in serotonin and dopamine uptake, and suggest sex differences in transporter function should be examined more closely in future investigations.