Donghai Huangfu

Ventrolateral medulla and sympathetic chemoreflex in the rat

Splanchnic sympathetic nerve discharge (SND), phrenic nerve activity (PND) and putative sympathetic premotor neurons of the rostral ventrolateral medulla (RVL) were recorded in urethane-anesthetized vagotomized rats without aortic baroreceptor afferents. Carotid chemoreceptor stimulation with brief N2 inhalation increased SND by 101 +/- 7%, raised mean arterial pressure (MAP) and increased the discharge rate of RVL premotor neurons by 46 +/- 12% (N = 32). During chemoreceptor activation. SND and most RVL neurons displayed pronounced central respiratory rhythmicity with maximal firing probability immediately after cessation of the PND (postinspiratory phase) and lowest probability during PND (inspiratory phase). Bilateral microinjection of the breed spectrum glutamate receptor antagonist kynurenic acid (Kyn, 5 nmol in 100 nl) into RVL blocked the sympathetic chemoreflex but left the sympathetic baroreflex intact. In contrast, bilateral microinjection of the same dose of Kyn into the caudal ventrolateral medulla (at obex level CVL) blocked the baroreflex but left the sympathetic chemoreflex intact. Bilateral microinjection of the GABAA agonist muscimol (87.5 pmol in 50 nl) into CVL produced effects identical to those of Kyn. These results confirm that the caudal ventrolateral medulla contains an essential relay of the sympathetic baroreflex and demonstrate that the same area plays no role in the sympathetic chemoreflex. The data suggests that these two reflexes could have a largely independent course through the medulla oblongata and that integration between the baroreceptor and chemoreceptor information used for sympathetic vasomotor control may occur as late as the premotor neuronal stage in RVL.

Chapter 8 Role of medulla oblongata in generation of sympathetic and vagal outflows

Publisher Summary This chapter reviews the present understanding of the medullary circuits that regulate the vasomotor sympathetic outflow and cardiovagal tone. It illustrates some of the recent progress in this area of integrative neurophysiology. Most concepts are based on the results of experimentation conducted under anesthesia but several components of the circuits have also been validated by neurophysiological work in unanesthetized preparations and by studies in behaving animals based on c-fos protooncogene expression. These medullary networks may be viewed from a reductionist perspective as embedded within a hierarchy of control systems that include higher centers involved in emotional behavior. Vast areas of uncertainty remain and competing theories are mentioned. A yet unknown fraction of spinal preganglionic neurons involved in vasomotor control may receive direct inputs from structures rostra1 to the medulla; however, individual components of the medullary circuits is described in this chapter are almost certainly recruited by supramedullary structures to generate the hemodynamic patterns characteristic of specific emotions.

Rostral ventrolateral medullary neurons projecting to locus coeruleus have cardiorespiratory inputs.

This study was designed to characterize some of the properties of the rostral ventrolateral medullary (RVLM) cells with axonal projection to the locus coeruleus (LC) in urethane anesthetized, vagotomized, paralyzed and artificially respirated rats. The vast majority of RVLM units antidromically (AD) activated from LC (RVLM-LC units) were silent and unresponsive to peripheral chemoreceptor stimulation or nociceptive stimulation. Twenty seven spontaneously active RVLM-LC neurons, AD activated from LC with currents below 30 microA (17 +/- 2 microA) were analyzed. AD mapping (n = 18) indicated that the lowest threshold for AD activation occurred within the LC itself. Axonal branching within or close to LC was suggested by the presence of sudden jumps in AD latency. Maximal AD latencies ranged from 7 to 37 ms. Most spontaneously active RVLM-LC neurons displayed marked central respiratory modulation characterized by either a post-inspiratory or an inspiratory pattern. The majority of the tested neurons were affected (excited or inhibited) by brief peripheral chemoreceptor stimulation (N2 inhalation). Most cells were inhibited by raising arterial pressure but none exhibited any detectable pulse synchrony. Reticulospinal sympathetic premotor neurons of RVLM were not found to project to LC (sample of 9) and very few RVLM cells with on-off respiratory discharges appeared to project to LC (2 out of 110). This study suggests that much of the information conveyed by the RVLM to LC could be of a mixed cardiorespiratory nature.

α2-Adrenergic autoreceptors in A5 and A6 neurons of neonate rats

A5 noradrenergic neurons control sympathetic outflow, nociception, and respiration. The presence of α2-adrenergic receptors (α2-ARs) in A5 cells has been suggested by immunohistochemistry. In the present experiments, we analyze the response of spinally projecting A5 cells to α2-AR agonists, and we compare it with that of locus ceruleus (A6) neurons. Whole cell recordings were obtained from 52 spinally projecting neurons in the ventrolateral pons of neonate rats. Immunohistochemistry showed that 60% of the recorded cells were A5 cells. In A5 cells clamped at -55 mV, norepinephrine (NE) in the presence of the α1-AR antagonist prazosin produced a Ba2+-sensitive outward current (20.4 ± 2.6 pA; n = 28). The α2-AR-induced current reversed at the K+ equilibrium potential ( E K) at three different extracellular K+ concentrations. Replacement of 82% of the extracellular Na concentration with N-methyl-d-glucamine did not change the reversal potential. The 19 presumably noncatecholaminergic neurons responded weakly or not at all to NE (2.5 ± 0.6 pA outward current). Pontospinal A6 neurons ( n = 11) were also recorded. Six A6 cells displayed large tetrodotoxin (TTX)-resistant membrane oscillations. In these cells, the current induced by α2-AR stimulation did not reverse over the voltages tested (-50 to -130 mV) or reversed at potentials more negative than E K (less than -114 mV). In A6 neurons that did not display large oscillations ( n = 5), the α2-AR-induced current reversed at or close to the E K (-90 ± 1.6 mV). In conclusion, A5 cells, like locus ceruleus neurons, have α2-ARs that may function as autoreceptors. In both cases, α2-AR activation increases an inwardly rectifying K+conductance. In A5 cells, we found no evidence that α2-AR activation decreases a resting Na+ conductance. The inhibition of A5 cells by clonidine and other agents with α2-AR agonist activity is likely to contribute to the ability of these drugs to decrease sympathetic tone and arterial pressure.

Central respiratory modulation of facial motoneurons in rats

Facial motoneurons (FMN) were recorded intracellularly in Sprague-Dawley rats anesthetized with halothane. The animals were vagotomized, paralyzed, and artificially ventilated. The average membrane potential of the cells was 62.6 +/- 1.9 mV and their input impedance ranged from 5 to 30 M omega (9.8 +/- 1.1 M omega, n = 38). The membrane potential of most FMNs varied throughout the central respiratory cycle and four distinct patterns were detected. Type I (post-inspiratory) cells (21/44) showed a two-phase Cl(-)-mediated hyperpolarization during the respiratory cycle, one during central inspiration and the second during late expiration. Type II cells (early inspiratory, n = 10) showed early inspiratory depolarization. Type III (n = 2, stage-2 expiratory) cells displayed late expiratory depolarization and one cell (type IV or throughout inspiratory) exhibited expiratory Cl(-)-mediated hyperpolarization. The remaining 10 cells showed no detectable respiratory modulation. The results reflect the heterogeneity of the central respiratory modulation of FMNs and suggest that these cells receive both excitatory and inhibitory inputs from elements of the central respiratory pattern generating network.

Autoactivity of A5 neurons: role of subthreshold oscillations and persistent Na+current

A5 noradrenergic neurons play a key role in autonomic regulation, nociception, and respiration. The purpose of the present experiments was to characterize some of the intrinsic properties of A5 cells in vitro. Whole cell recordings were obtained from 85 spinally projecting neurons of the ventrolateral pons of neonate rats. Immunohistochemistry showed that 60% of the ventrolateral pontine cells were noradrenergic. Eighty percent of A5 neurons were spontaneously active (0.1-5.5 spikes/s). Their discharge rate was unchanged by a mixture of synaptic blockers that eliminated postsynaptic potentials (PSPs). The nonnoradrenergic cells could not be distinguished from A5 cells on the basis of discharge rate, action potential duration, inward rectification, input resistance, or accommodation. A5 cells displayed subthreshold irregular oscillations of the membrane potential (main frequency component 0.5-2 Hz). These oscillations were unchanged in the presence of low external Ca2+-high Mg2+ and were very reduced by hyperpolarizing the cells below -65 mV. The oscillations were partially attenuated by 1 μM tetrodotoxin (TTX) and were eliminated by reducing external Na+ (27 mM). Stepping the membrane potential from -65 to -50 mV for 200 ms revealed the presence of a transient and a persistent inward current that were both blocked by 0.1 μM TTX or by extracellular Na+ reduction. In conclusion, most A5 neurons are spontaneously active in vitro. They display irregular subthreshold membrane potential oscillations generated by voltage-activated conductances that include a persistent TTX-sensitive Na+ current. Most of the activity of A5 cells appears due to intrinsic properties rather than to synaptic inputs.

Bulbospinal C1-adrenergic neurons: electrophysiological properties in the neonate rat.

Publisher Summary This chapter briefly summarizes data from whole cell recordings of more than 250 rostral ventrolateral medulla (RVLM) bulbospinal C1 neurons using the thin slice approach in neonate rats (3–12 days after birth). All recordings were done at room temperature. Such studies in the neonate rat indicate that C1 adrenergic neurons have two basic characteristics that are reminiscent of other central nervous system (CNS) catecholaminergic neurons, namely a slow nonbursting pattern of discharge that is because of intrinsic properties of the neurons rather than synaptic drives and somatodendritic catecholaminergic inhibitory receptors (autoreceptors). The autoactivity of C1 cells in vitro appears because, in large part, of a persistent sodium current, which leads to slow, irregular fluctuations of the membrane potential from which action potentials are triggered. Bulbospinal C1 cells have somatodendritic α 2 -adrenergic receptors. These α 2 receptors probably contribute to the inhibition of C1 cells in vivo by low doses of clonidine and related antihypertensive agents and secondarily to the sympatholytic and hypotensive action of these agents. The presence of inhibitory α 2 -adrenergic receptors on C1 cells probably accounts for their sensitivity to low doses of clonidine in vivo . The somatodendritic receptors so far characterized on C1 cells are in well agreement with the type of synaptic inputs previously described by anatomical means (GABA, acetylcholine, substance P, angiotensin) and are reminiscent of those present on the locus ceruleus.