Meredith Hay

1. Neurons of the nodose ganglia provide the sole connection between many types of visceral sensory inputs and the central nervous system. Electrophysiological studies of isolated nodose neurons provide a practical means of measuring individual cell membrane currents and assessing their putative contributions to the overall response properties of the neuron and its terminations. Here, we present a comprehensive mathematical model of an isolated nodose sensory neuron that is based upon numerical fits to quantitative voltage- and current-clamp data recorded in our laboratory. Model development was accomplished using an iterative process of electrophysiological recordings, nonlinear parameter estimation, and computer simulation. This work is part of an integrative effort aimed at identifying and characterizing the fundamental ionic mechanisms participating in the afferent neuronal limb of the baroreceptor reflex. 2. The neuronal model consists of two parts: a Hodgkin-Huxley-type membrane model coupled to a lumped fluid compartment model that describes Ca2+ ion concentration dynamics within the intracellular and external perineuronal media. Calcium buffering via a calmodulin-type buffer is provided within the intracellular compartment. 3. The complete model accurately reproduces whole-cell voltage-clamp recordings of the major ion channel currents observed in enzymatically dispersed nodose sensory neurons. Specifically, two Na+ currents exhibiting fast (INaf) and slow tetrodotoxin (TTX)-insensitive (INas) kinetics; low- and high-threshold Ca2+ currents exhibiting transient (ICa,t) and long-lasting (ICa,n) dynamics, respectively; and outward K+ currents consisting of a delayed-rectifier current (IK), a transient outward current (I(t)) and a Ca(2+)-activated K+ current (IK,Ca). 4. Whole-cell current-clamp recordings of somatic action-potential dynamics were performed on enzymatically dispersed nodose neurons using the perforated patch-clamp technique. Stimulus protocols consisted of both short ( or = 2.0 ms) and long (> or = 200 ms) duration current pulses over a wide range of membrane holding potentials. These studies clearly revealed two populations of nodose neurons, often termed A- and C-type cells, which exhibit markedly different action-potential signatures and stimulus response properties. 5. Using a single set of equations, the model accurately reproduces the electrical behavior of both A- and C-type nodose neurons in response to a wide variety of stimulus conditions and membrane holding potentials. The structure of the model, as well as the majority of its parameters are the same for both A- and C-type implementations.(ABSTRACT TRUNCATED AT 400 WORDS)

This study reports the effects of angiotensin II (ANG II), arginine vasopression (AVP), phenylephrine (PE), and sodium nitroprusside (SNP) on baroreflex control of heart rate in the presence and absence of the area postrema (AP) in conscious mice. In intact, sham-lesioned mice, baroreflex-induced decreases in heart rate due to increases in arterial pressure with intravenous infusions of ANG II were significantly less than those observed with similar increases in arterial pressure with PE (slope: -3.0 +/- 0.9 vs. -8.1 +/- 1.5 beats x min(-1) x mmHg(-1)). Baroreflex-induced decreases in heart rate due to increases in arterial pressure with intravenous infusions of AVP were the same as those observed with PE in sham animals (slope: -5.8 +/- 0.7 vs. -8.1 +/- 1.5 beats x min(-1) x mmHg(-1)). After the AP was lesioned, the slope of baroreflex inhibition of heart rate was the same whether pressure was increased with ANG II, AVP, or PE. The slope of the baroreflex-induced increases in heart rate due to decreases in arterial blood pressure with SNP were the same in sham- and AP-lesioned animals. These results indicate that, similar to other species, in mice the ability of ANG II to acutely reset baroreflex control of heart rate is dependent on an intact AP.

The purpose of this study was to identify the complement of metabotropic glutamate receptors (mGluRs) expressed in nodose ganglia and the nucleus tractus solitarius (NTS). mRNA from these tissues was isolated and amplified with standard RT-PCR with primers specific for each mGluR subtype. The results of this analysis showed that the NTS expresses all eight mGluR subtypes, whereas nodose ganglia express only group III mGluRs: mGluR4, mGluR6, mGluR7, and mGluR8. Application of the group III-specific mGluR agonist L-(+)-2-amino-4-phosphonobutyric acid (100 microM) reversibly inhibited voltage-gated calcium currents isolated from DiI-labeled aortic baroreceptor neurons and unlabeled nodose neurons. The results of this study suggest that group III mGluRs are the primary mGluR subtype expressed in visceral afferent neurons and that these receptors may be involved in afferent central transmission.

The identification of the specific estrogen receptor (ER) subtypes that are involved in estrogen protection from hypertension and their specific locations in the central nervous system is critical to our understanding and design of effective estrogen replacement therapies in women. Using selective ER agonists and recombinant adeno-associated virus (AAV) carrying small interference (si) RNA to silence either ERα (AAV-siRNA-ERα) or ERβ (AAV-siRNA-ERβ), the present study investigated regional specificity of different ER subtypes in the protective actions of estrogen in aldosterone (Aldo)-induced hypertension. Intracerebroventricular infusions of either diarylpropionitrile, a selective ERβ agonist, or propyl-pyrazole-triol, a selective ERα agonist, attenuated Aldo/NaCl-induced hypertension in ovariectomized rats. In contrast, intracerebroventricular injections of siRNA-ERα or siRNA-ERβ augmented Aldo-induced hypertension in intact females. Site-specific paraventricular nucleus (PVN) or rostroventrolateral medulla (RVLM) injections of siRNA-ERβ augmented Aldo-induced hypertension. However, rats with PVN or RVLM injections of siRNA-ERα did not significantly increase blood pressure induced by Aldo. Real-time polymerase chain reaction analyses of the PVN and RVLM of siRNA-injected rat confirmed a marked reduction in the expression of ERα and ERβ. In cultured PVN neurons, silencing either ERα or ERβ by culturing PVN neurons with siRNA-ERα or siRNA-ERβ enhanced Aldo-induced reactive oxygen species production. Ganglionic blockade after Aldo infusion showed an increase in sympathetic activity in ERβ knockdown rats. These results indicate that both PVN and RVLM ERβ, but not ERα in these nuclei, contribute to the protective effects of estrogen against Aldo-induced hypertension. The brain regions responsible for the protective effects of estrogen interaction with ERα in Aldo-induced hypertension still need to be determined.

This study was designed to determine if arginine vasopressin (AVP) facilitates the response of nucleus of the solitary tract (NTS) neurons to baroreceptor input. In anesthetized sinoaortic-denervated vagotomized rabbits, AVP was intravenously infused (15 microg x kg(-1) x min(-1), 1 min) or microinjected into the area postrema (AP; 1 ng/nl, 10 nl). Extracellular recordings of evoked NTS neuronal responses to electrical stimulation of the aortic depressor nerve (ADN) or vagus nerve (1 Hz, 2-20 V, 0.05-0.6 ms) were evaluated before and after AVP administration. In neurons receiving input from the ADN (n = 19), 58% of them increased their responses after AVP (40.3 +/- 5.0 to 71.5 +/- 4,8%, P 0.001). Similarly, in neurons activated by vagal stimulation (n = 22), 55% of them were facilitated during AVP administration (59.7 +/- 12.8 to 90.8 +/- 10.7%, P 0.01). This action of AVP was independent of the mode of AVP administration, since either microinjection or venous infusion was effective in augmenting responses of NTS neurons to aortic/vagal stimulation. In an additional 37 spontaneous NTS neurons, AVP showed no effect on the mean baseline firing rate (8.9 +/- 1.3 vs. 9.6 +/- 1.3 spikes/s, P > 0.05), but increased neuronal activity in 54% of neurons (6.9 +/- 1.3 vs. 13.1 +/- 1.7 spikes/s, P 0.01). In two rabbits pretreated with vasopressin antagonist (15 microg/kg iv), AVP failed to produce facilitatory effects (n = 8). The results of this study provide evidence in support of the hypothesis that circulating peptides modulate the arterial baroreflex via activation of neurons in the AP.