Meredith Hay

This study investigated sex differences in the sensitization of angiotensin (ANG) II-induced hypertension and the role of central estrogen and ANG-(1-7) in this process. Male and female rats were implanted for telemetered blood pressure (BP) recording. A subcutaneous subpressor dose of ANG II was given alone or concurrently with intracerebroventricular estrogen, ANG-(1-7), an ANG-(1-7) receptor antagonist A-779 or vehicle for 1 wk (induction). After a 1-wk rest (delay), a pressor dose of ANG II was given for 2 wk (expression). In males and ovariectomized females, subpressor ANG II had no sustained effect on BP during induction, but produced an enhanced hypertensive response to the subsequent pressor dose of ANG II during expression. Central administration of estrogen or ANG-(1-7) during induction blocked ANG II-induced sensitization. In intact females, subpressor ANG II treatment produced a decrease in BP during induction and delay, and subsequent pressor ANG II treatment given during expression produced only a slight but significant increase in BP. However, central blockade of ANG-(1-7) by intracerebroventricular infusion of A-779 during induction restored the decreased BP observed in females during induction and enhanced the pressor response to the ANG II treatment during expression. RT-PCR analyses indicated that estrogen given during induction upregulated mRNA expression of the renin-angiotensin system (RAS) antihypertensive components, whereas both central estrogen and ANG-(1-7) downregulated mRNA expression of RAS hypertensive components in the lamina terminalis. The results indicate that females are protected from ANG II-induced sensitization through central estrogen and its regulation of brain RAS.

Synaptic vesicle exocytosis in primary cultures of baroreceptor neurons is reduced during high-frequency stimulation. Calcium influx through voltage-gated calcium channels (VGCC) is a key step in neurotransmitter release. With the help of FM2-10, a marker of synaptic vesicle recycling, the present study investigates the differential contribution of several VGCC subtypes to exocytosis in neuronal processes and how this contribution is altered at high frequencies. In control experiments, field stimulation at 0.5 Hz evoked about 66 +/- 5% destaining. Combined blockade of N- and P/Q-subtypes with Ctx-MVIIC was far more effective in reducing exocytosis (11 +/- 8%) than blocking N-type (49 +/- 5%, Ctx-GVIA) or P-type (46 +/- 1%, Agatoxin) alone. The effectiveness of the blockers also varied with the duration of stimulation: Ctx-GVIA attenuating exocytosis significantly in the first 60 s and Agatoxin affecting exocytosis only towards the end of 180 s stimulation period. Field stimulation at 10 Hz evoked exocytosis (36 +/- 18%) comparable to that evoked by 0.5 Hz in the presence of Ctx-GVIA. While blockade with Agatoxin had no effects, Ctx-GVIA, Ctx-MVIIC and L-type blocker Nifedepine had small but similar inhibitory effects on exocytosis at 10 Hz. The data suggest that N-type is the major contributor to exocytosis at 0.5 Hz, and this contribution is reduced during prolonged stimulation periods and at high frequencies.

Mineralocorticoid excess increases superoxide production by activating NADPH oxidase (NOX), and intracerebroventricular infusions of NADPH oxidase inhibitors attenuate aldosterone (Aldo)/salt-induced hypertension. It has been hypothesized that increased reactive oxygen species (ROS) in the brain may be a key mechanism in the development of hypertension. The present study investigated the brain regional specificity of NADPH oxidase and the role of NOX2 and NOX4 NADPH oxidase subunits in the hypothalamic paraventricular nucleus (PVN) in Aldo/salt-induced hypertension. PVN injections of adenoviral vectors expressing small interfering (si)RNA targeting NOX2 (AdsiRNA-NOX2) or NOX4 (AdsiRNA-NOX4) mRNAs were used to knock down NOX2 and NOX4 proteins. Three days later, delivery of Aldo (0.2 mg·kg(-1)·day(-1) sc) via osmotic pump commenced and 1% NaCl was provided in place of water. PVN injections of either AdsiRNA-NOX2 or AdsiRNA-NOX4 significantly attenuated the development of Aldo/NaCl-induced hypertension. In an additional study, Aldo/salt-induced hypertension was also significantly attenuated in NOX2 (genomic) knockout mice compared with wild-type controls. When animals from both functional studies underwent ganglionic blockade, there was a reduced fall in blood pressure in the NOX2 and NOX4 knockdown/knockout mice. Western blot analyses of the PVN of siRNA-NOX2- or siRNA-NOX4-injected mice confirmed a marked reduction in the expression of NOX2 or NOX4 protein. In cultured PVN neurons, silencing either NOX2 or NOX4 protein production by culturing PVN cells with siRNA-NOX2 or siRNA-NOX4 attenuated Aldo-induced ROS. These data indicate that both NOX2 and NOX4 in the PVN contribute to elevated sympathetic activity and the hypertensivogenic actions induced by mineralocorticoid excess.

Serotonergic fibers have been identified within the granule cell layer of the cerebellar cortex; however, their functional significance has not been identified. In this study the effect of serotonin on granule cell spontaneous activity was determined in the rat cerebellum. Of the 136 granule cells tested, 44.8% displayed a decrease in firing rate, 21.3% increased firing rate and 33.8% were not affected. The serotonin-induced changes in activity were not blocked by bicuculline or methysergide. The serotonin agonist 1,3 (trifluoromethylphenyl) piperazine mimicked the serotonin-induced suppressive response. Iontophoretically applied serotonin was also found to modulate GABA-induced suppression of granule cell activity. The variable effects of serotonin on spontaneous activity suggests the presence of more than one type of serotonergic receptor in the cerebellar granule cell layer.

The nucleus tractus solitarius (NTS) receives information from both area postrema (AP) and peripheral afferents. It is, therefore, one likely site of interaction between AP and peripheral afferent fibers. The present study's purpose was to determine the influence of AP stimulation on solitary tract-induced modulation of NTS neuronal activity. With the use of an in vitro rabbit brain slice preparation, extracellular recordings were made from 58 NTS neurons in which action potentials were evoked by both solitary tract and AP stimulation. In the majority of the cells tested, simultaneous stimulation of solitary tract and AP, at voltage levels that evoked no action potentials when stimulated separately, resulted in production of either single or multiple action potentials. In 27 units, stimulation levels to the solitary tract and to the AP were adjusted such that their respective separate stimulations produced an NTS action potential less than 30% of the time. When the two inputs were stimulated together, simultaneous stimulations produced an NTS action potential 100% of the time, suggesting a facilitatory interaction between the AP and the solitary tract on NTS neuronal activity. In nine cells, perfusion of the slice with clonidine induced a facilitation of solitary tract-evoked NTS response to a level similar to that seen during simultaneous stimulation of the solitary tract with the AP. Application of the alpha 2-adrenergic receptor antagonist yohimbine blocked the ability of both clonidine and AP to facilitate the solitary tract-evoked response. These results support a possible interaction between AP and peripheral afferents and suggest that AP stimulation facilitates effects of solitary tract activation at the level of the NTS.