Hyperpolarizing after-potentials regulate generation of long-duration plateau depolarizations in rat paraventricular nucleus neurons

Central SDF-1/CXCL12 expression and its cardiovascular and sympathetic effects: the role of angiotensin II, TNF-α, and MAP kinase signaling

The chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) and its receptors are expressed by neurons and glial cells in cardiovascular autonomic regions of the brain, including the hypothalamic paraventricular nucleus (PVN), and contribute to neurohumoral excitation in rats with ischemia-induced heart failure. The present study examined factors regulating the expression of SDF-1 in the PVN and mechanisms mediating its sympatho-excitatory effects. In urethane anesthetized rats, a 4-h intracerebroventricular (ICV) infusion of angiotensin II (ANG II) or tumor necrosis factor-α (TNF-α) in doses that increase mean blood pressure (MBP) and sympathetic drive increased the expression of SDF-1 in PVN. ICV administration of SDF-1 increased the phosphorylation of p44/42 mitogen-activated protein kinase (MAPK), JNK, and p38 MAPK in PVN, along with MBP, heart rate (HR), and renal sympathetic nerve activity (RSNA), but did not affect total p44/42 MAPK, JNK, and p38 MAPK levels. ICV pretreatment with the selective p44/42 MAPK inhibitor PD98059 prevented the SDF-1-induced increases in MBP, HR, and RSNA; ICV pretreatment with the selective JNK and p38 MAPK inhibitors attenuated but did not block these SDF-1-induced excitatory responses. ICV PD98059 also prevented the sympatho-excitatory response to bilateral PVN microinjections of SDF-1. ICV pretreatment with SDF-1 short-hairpin RNA significantly reduced ANG II- and TNF-α-induced phosphorylation of p44/42 MAPK in PVN. These findings identify TNF-α and ANG II as drivers of SDF-1 expression in PVN and suggest that the full expression of their cardiovascular and sympathetic effects depends upon SDF-1-mediated activation of p44/42 MAPK signaling.

Gene transfer of neuronal nitric oxide synthase to the paraventricular nucleus reduces the enhanced glutamatergic tone in rats with chronic heart failure

Our previous studies have shown that the decreased NO and increased glutamatergic mechanisms on sympathetic regulation within the paraventricular nucleus (PVN) may contribute to the elevated sympathoexcitation during chronic heart failure (CHF). In the present study, we investigated the effects of neuronal NO synthase (nNOS) gene transfer on N-methyl-D-aspartic acid receptor subunit NR(1) in the rats with a coronary ligation model of CHF. Adenovirus vectors encoding nNOS (AdnNOS) or adenovirus vectors encoding β-galactosidase were transfected into the PVN in vivo. Five days after application of AdnNOS, the increased expression of nNOS within the PVN was confirmed by NADPH-diaphorase staining, real-time PCR, and Western blot. In anesthetized rats, AdnNOS treatment significantly enhanced the blunted renal sympathetic nerve activity, blood pressure, and heart rate responses to NO synthase inhibitor N(G)-monomethyl-L-arginine in the rats with CHF compared with CHF-adenovirus vectors encoding β-galactosidase group. AdnNOS significantly decreased the enhanced renal sympathetic nerve activity, blood pressure, and heart rate responses to N-methyl-D-aspartic acid in the rats with CHF (renal sympathetic nerve activity: 44±2% versus 79±6%; P<0.05) compared with CHF-adenovirus vectors encoding the β-galactosidase group. AdnNOS transfection significantly reduced the increased NR(1) receptor mRNA expression (Δ35±5%) and protein levels (Δ24±4%) within the PVN in CHF rats. Furthermore, in neuronal NG-108 cells, NR(1) receptor protein expression decreased in a dose-dependent manner after AdnNOS transfection. According to our results, nNOS downregulation enhances glutamate transmission in the PVN by increasing NR(1) subunit expression. This mechanism may enhance renal sympathetic nerve activity in CHF rats.

Exercise-induced neuronal plasticity in central autonomic networks: role in cardiovascular control

It is now well established that brain plasticity is an inherent property not only of the developing but also of the adult brain. Numerous beneficial effects of exercise, including improved memory, cognitive function and neuroprotection, have been shown to involve an important neuroplastic component. However, whether major adaptive cardiovascular adjustments during exercise, needed to ensure proper blood perfusion of peripheral tissues, also require brain neuroplasticity, is presently unknown. This review will critically evaluate current knowledge on proposed mechanisms that are likely to underlie the continuous resetting of baroreflex control of heart rate during/after exercise and following exercise training. Accumulating evidence indicates that not only somatosensory afferents (conveyed by skeletal muscle receptors, baroreceptors and/or cardiopulmonary receptors) but also projections arising from central command neurons (in particular, peptidergic hypothalamic pre-autonomic neurons) converge into the nucleus tractus solitarii (NTS) in the dorsal brainstem, to co-ordinate complex cardiovascular adaptations during dynamic exercise. This review focuses in particular on a reciprocally interconnected network between the NTS and the hypothalamic paraventricular nucleus (PVN), which is proposed to act as a pivotal anatomical and functional substrate underlying integrative feedforward and feedback cardiovascular adjustments during exercise. Recent findings supporting neuroplastic adaptive changes within the NTS-PVN reciprocal network (e.g. remodelling of afferent inputs, structural and functional neuronal plasticity and changes in neurotransmitter content) will be discussed within the context of their role as important underlying cellular mechanisms supporting the tonic activation and improved efficacy of these central pathways in response to circulatory demand at rest and during exercise, both in sedentary and in trained individuals. We hope this review will stimulate more comprehensive studies aimed at understanding cellular and molecular mechanisms within CNS neuronal networks that contribute to exercise-induced neuroplasticity and cardiovascular adjustments.

Ether-a-go-go-related gene potassium channels: what's all the buzz about?

Antipsychotic drugs are thought to exert their therapeutic action by antagonizing dopamine receptors but are also known to produce side effects in the heart by inhibiting cardiac ether-a-go-go-related gene (ERG) K(+) channels. Recently, it has been discovered that the same channels are present in the brain, including midbrain dopamine neurons. ERG channels are most active after the cessation of intense electrical activity, and blockade of these channels prolongs plateau potentials in bursting dopamine neurons. This change in excitability would be expected to alter dopamine release. Therefore, the therapeutic action of antipsychotic drugs may depend on inhibition of both postsynaptic dopamine receptors and presynaptic ERG K(+) channels.

Transient potassium conductances protect nucleus tractus solitarius neurons from NMDA induced excitotoxic plateau depolarizations

Ischemic insults, followed by excessive accumulation of extracellular glutamate, destroy most, but not all, neurons in affected area(s) of the central nervous system (CNS). Characterization of the unique properties of cells resistant to such excitotoxic challenge may identify novel preventive/therapeutic strategies to reduce cell death. We have previously reported that transient potassium conductances expressed in magnocellular neurons of the paraventricular nucleus protect these cells from excitotoxic cell death. In the present study, in vitro patch-clamp recording techniques were used to assess the roles of similar potassium conductances in protecting delayed excitation (DE) neurons of the nucleus tractus solitarius (NTS) from over-excitation after N-methyl-d-aspartate (NMDA) receptor activation. DE neurons show a reduced sensitivity (compared to NTS neurons which lack these potassium conductances) to NMDA receptor activation which protects against long duration plateau depolarizations (LDPDs). We identify two types of transient K(+) conductances (I(A) and I(D)), which contribute to the rapid repolarization of the membrane after a strong depolarization, and show that inhibition of these currents with 4-aminopyridine increases neuronal excitability after NMDA receptor activation such that DE cells now respond with LDPDs. In contrast, lower concentrations of 4-AP (100 mM) which inhibit only the I(D) have no effect on NMDA induced depolarization. These results suggest that the reduced sensitivity of DE neurons in NTS to NMDA receptor activation is the result of the large transient potassium conductance I(A) expressed in these neurons, and identify this as a common mechanism protecting against NMDA receptor mediated excitotoxicity in both PVN and NTS neurons.

Dendritic action potentials in magnocellular neurons

In the central nervous system, information is traditionally thought to flow from synapses to dendrites to soma. Recent evidence, however, suggests that dendrites play more of an active role in signal processing than previously thought. This review will examine the evidence in support of dendritic spikes in magnocellular neurons. Additionally, it will shed light on a number of important roles these spikes may play in regulating the excitability of magnocellular neurons.

NMDA-mediated increase in renal sympathetic nerve discharge within the PVN: role of nitric oxide

The paraventricular nucleus (PVN) of the hypothalamus is an important site of integration in the central nervous system for sympathetic outflow. Both glutamate and nitric oxide (NO) play an important role in the regulation of sympathetic nerve activity. The purpose of the present study was to examine the interaction of NO and glutamate within the PVN in the regulation of renal sympathetic nerve activity in rats. Renal sympathetic nerve discharge (RSND), arterial blood pressure (BP), and heart rate (HR) were measured in response to administration of N-methyl-D-aspartic acid (NMDA) and N(G)-monomethyl-L-arginine (L-NMMA) into the PVN. We found that microinjection of NMDA (25, 50, and 100 pmol) into the PVN increased RSND, BP, and HR in a dose-dependent manner, reaching 53 +/- 9%, 19 +/- 3 mmHg, and 32 +/- 12 beats/min, respectively, at the highest dose. These responses were significantly enhanced by prior microinjection of L-NMMA. On the other hand, inhibition of NO within the PVN by microinjection of L-NMMA also induced increases in RSND, BP, and HR in a dose-dependent manner, reaching 48 +/- 6.5%, 11 +/- 4 mmHg, and 55 +/- 16 beats/min, respectively, at the highest dose. This sympathoexcitatory response was eliminated by prior microinjection of DL-2-amino-5-phosphonovaleric acid, an antagonist of the NMDA receptor. Furthermore, with the use of the push-pull technique, perfusion of glutamate (0.5 micromol) or NMDA (0.1 nmol) into the PVN induced an increase in NO release. In conclusion, our data indicate that NMDA receptors within the PVN mediate an excitatory effect on renal sympathetic nerve activity, arterial BP, and HR. NO in the PVN, which is released by activation of the NMDA receptor, also inhibits NMDA-mediated increases in sympathetic nerve activity. This negative feedback of NO on the glutamate system within the PVN may play an important role in maintaining the overall balance and tone of sympathetic outflow in normal and pathophysiological conditions known to have increased sympathetic tone.