Effects of cyanide and hypoxia on membrane currents in neurones acutely dissociated from the rostral ventrolateral medulla of the rat

Modeling the effects of extracellular potassium on bursting properties in pre-Bötzinger complex neurons

There are many types of neurons that intrinsically generate rhythmic bursting activity, even when isolated, and these neurons underlie several specific motor behaviors. Rhythmic neurons that drive the inspiratory phase of respiration are located in the medullary pre-Bötzinger Complex (pre-BötC). However, it is not known if their rhythmic bursting is the result of intrinsic mechanisms or synaptic interactions. In many cases, for bursting to occur, the excitability of these neurons needs to be elevated. This excitation is provided in vitro (e.g. in slices), by increasing extracellular potassium concentration (K out) well beyond physiologic levels. Elevated K out shifts the reversal potentials for all potassium currents including the potassium component of leakage to higher values. However, how an increase in K out , and the resultant changes in potassium currents, induce bursting activity, have yet to be established. Moreover, it is not known if the endogenous bursting induced in vitro is representative of neural behavior in vivo. Our modeling study examines the interplay between K out, excitability, and selected currents, as they relate to endogenous rhythmic bursting. Starting with a Hodgkin-Huxley formalization of a pre-BötC neuron, a potassium ion component was incorporated into the leakage current, and model behaviors were investigated at varying concentrations of K out. Our simulations show that endogenous bursting activity, evoked in vitro by elevation of K out , is the result of a specific relationship between the leakage and voltage-dependent, delayed rectifier potassium currents, which may not be observed at physiological levels of extracellular potassium.

Somatostatin in the rat rostral ventrolateral medulla: Origins and mechanism of action

Somatostatin (SST) or agonists of the SST-2 receptor (sst2 ) in the rostral ventrolateral medulla (RVLM) lower sympathetic nerve activity, arterial pressure, and heart rate, or when administered within the Bötzinger region, evoke apneusis. Our aims were to describe the mechanisms responsible for the sympathoinhibitory effects of SST on bulbospinal neurons and to identify likely sources of RVLM SST release. Patch clamp recordings were made from bulbospinal RVLM neurons (n = 31) in brainstem slices prepared from juvenile rat pups. Overall, 58% of neurons responded to SST, displaying an increase in conductance that reversed at -93 mV, indicative of an inwardly rectifying potassium channel (GIRK) mechanism. Blockade of sst2 abolished this effect, but application of tetrodotoxin did not, indicating that the SST effect is independent of presynaptic activity. Fourteen bulbospinal RVLM neurons were recovered for immunohistochemistry; nine were SST-insensitive and did not express sst2a . Three out of five responsive neurons were sst2a -immunoreactive. Neurons that contained preprosomatostatin mRNA and cholera-toxin-B retrogradely transported from the RVLM were detected in: paratrigeminal nucleus, lateral parabrachial nucleus, Kölliker-Fuse nucleus, ventrolateral periaqueductal gray area, central nucleus of the amygdala, sublenticular extended amygdala, interstitial nucleus of the posterior limb of the anterior commissure nucleus, and bed nucleus of the stria terminalis. Thus, those brain regions are putative sources of endogenous SST release that, when activated, may evoke sympathoinhibitory effects via interactions with subsets of sympathetic premotor neurons that express sst2 .

Heme oxygenase-1 and chronic hypoxia

A myriad of changes are necessary to adapt to chronic hypoxemia. Key among these changes increases in arterial oxygen carrying capacity, ventilation and sympathetic activity. This requires the induction of several gene products many of which are regulated by the activity of HIF-1α, including HO-1. Induction of HO-1 during chronic hypoxia is necessary for the continued breakdown of heme for the enhanced production of hemoglobin and the increased respiratory and sympathetic responses. Several human HO-1 polymorphisms have been identified that can affect the expression or activity of HO-1. Associations between these polymorphisms and the prevalence of hypertension have recently been assessed in specific populations. There are major gaps in our understanding of the mechanisms of how HO-1 mediates changes in the activity of the hypoxia-sensitive chemosensors and whether HO-1 polymorphisms are an important factor in the integrated response to chronic hypoxia. Understanding how HO-1 mediates cardiorespiratory responses could provide important insights into clinical syndromes such as obstructive sleep apnea.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Heme oxygenase is necessary for the excitatory response of cultured neonatal rat rostral ventrolateral medulla neurons to hypoxia

Heme oxygenase has been linked to the oxygen-sensing function of the carotid body, pulmonary vasculature, cerebral vasculature, and airway smooth muscle. We have shown previously that the cardiorespiratory regions of the rostral ventrolateral medulla are excited by local hypoxia and that heme oxygenase-2 (HO-2) is expressed in the hypoxia-chemosensitive regions of the rostral ventrolateral medulla (RVLM), the respiratory pre-Bötzinger complex, and C1 sympathoexcitatory region. To determine whether heme oxygenase is necessary for the hypoxic-excitation of dissociated RVLM neurons (P1) cultured on confluent medullary astrocytes (P5), we examined their electrophysiological responses to hypoxia (NaCN and low Po(2)) using the whole-cell perforated patch clamp technique before and after blocking heme oxygenase with tin protoporphyrin-IX (SnPP-IX). Following the electrophysiological recording, immunocytochemistry was performed on the recorded neuron to correlate the electrophysiological response to hypoxia with the expression of HO-2. We found that the responses to NaCN and hypoxia were similar. RVLM neurons responded to NaCN and low Po(2) with either depolarization or hyperpolarization and SnPP-IX blocked the depolarization response of hypoxia-excited neurons to both NaCN and low Po(2) but had no effect on the hyperpolarization response of hypoxia-depressed neurons. Consistent with this observation, HO-2 expression was present only in the hypoxia-excited neurons. We conclude that RVLM neurons are excited by hypoxia via a heme oxygenase-dependent mechanism.

Evidence that estrogen directly and indirectly modulates C1 adrenergic bulbospinal neurons in the rostral ventrolateral medulla

Blood pressure in women increases after menopause, and sympathetic tone in female rats decreases with estrogen injections in the rostral ventrolateral medulla (RVLM) region that contains bulbospinal C1 adrenergic neurons and is involved in blood pressure control. We investigated the anatomical and physiological basis for estrogen effects in the RVLM. Neurons with alpha- or beta-subtypes of estrogen receptor (ER) immunoreactivity (-ir) overlapped in distribution with tyrosine hydroxylase (TH)-containing C1 neurons. Immunoelectron microscopy revealed that ERalpha- and ERbeta-ir had distinct cellular and subcellular distributions. ERalpha-ir was most commonly in TH-lacking profiles, many of which were axons and peptide-containing afferents that contacted TH-containing dendrites. ERalpha-ir was also in some TH-containing dendrites. ERbeta-ir was most frequently in TH-containing somata and dendrites, particularly on endoplasmic reticula, mitochondria, and plasma membranes. In whole-cell patch clamp recordings from isolated bulbospinal RVLM neurons, 17beta-estradiol dose-dependently reduced voltage-gated Ca(++) currents, especially the long-lasting (L-type) component. This inhibition was reversed by washing or prevented by adding the non-subtype-selective ER antagonist ICI182780. An ERbeta-selective agonist, but not an ERalpha-selective agonist, reproduced the Ca(++) current inhibition. The data indicate that estrogens can modulate the function of RVLM C1 bulbospinal neurons either directly, through extranuclear ERbeta, or indirectly through extranuclear ERalpha in selected afferents. Moreover, Ca(++) current inhibition may underlie the decrease in sympathetic tone evoked by local 17beta-estradiol application. These findings provide a structural and functional basis for the effects of estrogens on blood pressure control and suggest a mechanism for the modulation of cardiovascular function by estrogen in women.

Hyperglycemic response to hemorrhage is modulated by baroreceptors unloading but not by peripheral chemoreceptors activation

The aim of this study was to assess the relative participation of carotid baro- and chemoreceptors on plasma glucose and lactate level in response to hemorrhagic hypotension. We also evaluated the effects of selective activation of carotid chemoreceptors. One week before the experiments, male Wistar rats (250-300 g) were submitted to bilateral total carotid denervation (BCD-group), or to bilateral ligature of the carotid body artery (ChD-group). During the same surgical procedure, a chronic jugular catheter for blood sampling and hemorrhage (1.2 mL/100 g/2 min) and polyethylene cannula was inserted into the left femoral artery for cardiovascular monitoring. One group submitted to fictitious surgery was used as a surgical control (Sham-group). Carotid chemoreceptors were selectively activated by sodium cyanide (NaCN, 40 microg/0.1 mL i.v.) in the Sham and ChD group. The results showed that hyperglycemic response to hemorrhage in the BCD-group was reduced whereas in the ChD-group there was no significant change in this parameter compared to the Sham group (8.6 +/- 0.5 mM, Sham-hemorrhaged, n = 8; 7.2 +/- 0.3 mM, BCD-hemorrhaged, n = 8 and 9.4 +/- 0.6 mM, ChD, n = 8, p < 0.05). Increased plasma lactate levels following hemorrhage were observed in all the three experimental groups throughout the experimental period and there were no differences between the groups. Chemoreceptor stimulation by NaCN also produced hyperglycemia, as well as an increase in blood pressure and bradycardia but did not affect plasma lactate concentration. Ligature of the carotid body artery annulled the cardiovascular responses induced by NaCN, but did not change the hyperglycemic response to hypoxia. In conclusion, our data indicate that carotid chemoreceptors do not play any major role in overall metabolic response to hypoxia or hemorrhagic hypotension. Furthermore, the results suggest that carotid baroreceptors unloading play a predominant role as main source of afferent impulses leading to the hyperglycemic response to hemorrhage. In addition our data shows that the metabolic response and cardiovascular adjustment to hypoxia can be dissociated by ligature of the carotid body artery.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

Diencephalic and mesencephalic influences on ponto-medullary respiratory control in normoxic and hypoxic conditions: an in vitro study on central nervous system preparations from newborn rat

We investigated the effects of the diencephalon and mesencephalon on the central respiratory drive originating from ponto-medullary regions in normoxic and hypoxic conditions, using central nervous system preparations from newborn rats. We used two approaches: 1) electrophysiological analysis of respiratory frequency and the amplitude of inspiratory C4 activity and 2) immunohistochemical detection of Fos protein, an activity-dependent neuronal marker. We found that, in normoxic conditions, the mesencephalon moderated respiratory frequency, probably by means of an inhibitory effect on ventral medullary respiratory neurons. Diencephalic inputs restored respiratory frequency. Moreover, O(2)-sensing areas in the diencephalon (caudal lateral and posterior hypothalamic areas) and mesencephalon (ventrolateral and dorsolateral periaqueductal gray) seem to increase the amplitude of respiratory bursts during adaptation of the central respiratory drive to hypoxia. In contrast, decrease in respiratory frequency during hypoxia is thought to be mediated by a cluster of ventral hypothalamic neurons.

Sodium-mediated axonal degeneration in inflammatory demyelinating disease

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS). The mechanisms responsible for the degeneration remain unclear, but evidence suggests that a failure to maintain axonal sodium ion homeostasis may be a key step that underlies at least some of the degeneration. Sodium ions can accumulate within axons due to a series of events, including impulse activity and exposure to inflammatory factors such as nitric oxide. Recent findings have demonstrated that partial blockade of sodium channels can protect axons from nitric oxide-mediated degeneration in vitro, and from the effects of neuroinflammatory disease in vivo. This review describes some of the reasons why sodium ions might be expected to accumulate within axons in MS, and recent observations suggesting that it is possible to protect axons from degeneration in neuroinflammatory disease by partial sodium channel blockade.

Oxygen-sensing neurons in the central nervous system

This mini-review summarizes the present knowledge regarding central oxygen-chemosensitive sites with special emphasis on their function in regulating changes in cardiovascular and respiratory responses. These oxygen-chemosensitive sites are distributed throughout the brain stem from the thalamus to the medulla and may form an oxygen-chemosensitive network. The ultimate effect on respiratory or sympathetic activity presumably depends on the specific neural projections from each of these brain stem oxygen-sensitive regions as well as on the developmental age of the animal. Little is known regarding the cellular mechanisms involved in the chemotransduction process of the central oxygen sensors. The limited information available suggests some conservation of mechanisms used by other oxygen-sensing systems, e.g., carotid body glomus cells and pulmonary vascular smooth muscle cells. However, major gaps exist in our understanding of the specific ion channels and oxygen sensors required for transducing central hypoxia by these central oxygen-sensitive neurons. Adaptation of these central oxygen-sensitive neurons during chronic or intermittent hypoxia likely contributes to responses in both physiological conditions (ascent to high altitude, hypoxic conditioning) and clinical conditions (heart failure, chronic obstructive pulmonary disease, obstructive sleep apnea syndrome, hypoventilation syndromes). This review underscores the lack of knowledge about central oxygen chemosensors and highlights real opportunities for future research.

Effect of estradiol sulfamate (ES-J995) on affinity of hemoglobin for oxygen, cardiovascular function and acid-base balance in ovariectomized rats

Oral administration of estradiol sulfamate (ES, prodrug of estradiol) leads to increased systemic and reduced hepatic effects than estradiol because ES is accumulated in erythrocytes. However, possible alterations of erythrocytic oxygen transport by intraerythrocytic ES accumulation has not been studied. Therefore, ovariectomized adult female rats (n = 58; body wt.) were randomly treated orally either with single doses (day 1) or multiple dose (days 1-4) with vehicle, with estradiol sulfamate (ES-J995, 1 mg x kg(-1) b.w.) or with estradiol (30 mg x kg(-1) b.w.). Under general anesthesia arterial blood pressure, heart rate, blood gases, and acid-base balance were measured. Hypoxia was performed by lowering the inspired fraction of oxygen from 0.35 to 0.12. In addition, individual oxygen dissociation curves and ES-J995 distribution in blood and plasma were estimated. ES-J995 was accumulated in erythrocytes by approximately 98% (P < 0.01), but oxygen transport capacity was not altered (P50: 35.6 +/- 1.0 mm Hg to 37.1 +/- 1.1 mm Hg). Blood gases and acid-base balance parameters were not altered after ES-J995 treatment under normoxic and hypoxic conditions. In conclusion, ES-J995 accumulation in erythrocytes does not alter the affinity of hemoglobin for oxygen nor any function which would indicate an impaired oxygen delivery to the body.

Impact of mitochondrial inhibition on excitability and cytosolic Ca2+ levels in brainstem motoneurones from mouse

Motoneurones (MNs) are particularly affected by the inhibition of mitochondrial metabolism, which has been linked to their selective vulnerability during pathophysiological states like hypoxia and amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder. To elucidate underlying events, we used sodium cyanide (CN) as a pharmacological inhibitor of complex IV of the mitochondrial respiratory chain ('chemical hypoxia') and investigated the cellular response in vulnerable and resistant neurone types. Bath application of 2 mm CN activated TTX-insensitive Na+ conductances in vulnerable hypoglossal MNs, which depolarized these MNs by 10.2 +/- 1.1 mV and increased their action potential activity. This response was mimicked by sodium azide (2 mm) and largely prevented by preincubation with the antioxidants ascorbic acid (1 mm) and Trolox (750 microm), indicating an involvement of reactive oxygen species (ROS) in the activation mechanism. CN also elevated cytosolic [Ca2+] levels through (i) Ca2+ release from mitochondria-controlled stores, (ii) significant retardation of cytosolic Ca2+ clearance rates, even when cytosolic ATP levels were held constant during whole-cell recording, and (iii) secondary Ca2+ influx during elevated firing rates. Blocking mitochondrial ATP production additionally raised cytosolic Ca2+ levels and prolonged recovery of Ca2+ transients with a delay of 5-6 min. Comparative studies on hypoglossal MNs, facial MNs and dorsal vagal neurones suggested that CN responses were dominated by the activation of K+ conductances in resistant neurones, thus reducing excitability during mitochondrial inhibition. In summary, our observations therefore support a model where selective MN vulnerability results from a synergistic accumulation of risk factors, including low cytosolic Ca2+ buffering, strong mitochondrial impact on [Ca2+]i, and a mitochondria-controlled increase in electrical excitability during metabolic disturbances.

Endogenous rhythm generation in the pre-Bötzinger complex and ionic currents: modelling and in vitro studies

The pre-Bötzinger complex is a small region in the mammalian brainstem involved in generation of the respiratory rhythm. As shown in vitro, this region, under certain conditions, can generate endogenous rhythmic bursting activity. Our investigation focused on the conditions that may induce this bursting behaviour. A computational model of a population of pacemaker neurons in the pre-Bötzinger complex was developed and analysed. Each neuron was modelled in the Hodgkin-Huxley style and included persistent sodium and delayed-rectifier potassium currents. We found that the firing behaviour of the model strongly depended on the expression of these currents. Specifically, bursting in the model could be induced by a suppression of delayed-rectifier potassium current (either directly or via an increase in extracellular potassium concentration, [K+]o) or by an augmentation of persistent sodium current. To test our modelling predictions, we recorded endogenous population activity of the pre-Bötzinger complex and activity of the hypoglossal (XII) nerve from in vitro transverse brainstem slices (700 micro m) of neonatal rats (P0-P4). Rhythmic activity was absent at 3 mm[K+]o but could be triggered by either the elevation of [K+]o to 5-7 mm or application of potassium current blockers (4-AP, 50-200 micro m, or TEA, 2 or 4 mm), or by blocking aerobic metabolism with NaCN (2 mm). This rhythmic activity could be abolished by the persistent sodium current blocker riluzole (25 or 50 micro m). These findings are discussed in the context of the role of endogenous bursting activity in the respiratory rhythm generation in vivo vs. in vitro and during normal breathing in vivo vs. gasping.

Consequences of in utero caffeine exposure on respiratory output in normoxic and hypoxic conditions and related changes of Fos expression: a study on brainstem-spinal cord preparations isolated from newborn rats

Several aspects of the central regulation of respiratory control have been investigated on brainstem-spinal cord preparations isolated from newborn rats whose dam was given 0.02% caffeine in water as drinking fluid during the whole period of pregnancy. Analysis of the central respiratory drive estimated by the recording of C4 ventral root activity was correlated to Fos ponto-medullary expression. Under normoxic conditions, preparations obtained from the caffeine-treated group of animals displayed a higher respiratory frequency than observed in the control group (9.2 +/- 0.5 versus 7.2 +/- 0.6 burst/min). A parallel Fos detection tends to indicate that the changes of the respiratory rhythm may be due to a decrease in neuronal activity of medullary structures such as the ventrolateral subdivision of the solitary tract, the area postrema, and the nucleus raphe obscurus. Under hypoxic conditions, the preparations displayed a typical hypoxic respiratory depression associated with changes in the medullary Fos expression pattern. In addition, the hypoxic respiratory depression is clearly emphasized after in utero exposure to caffeine and coincides with an increased Fos expression in the area postrema and nucleus raphe obscurus, two structures in which it is not increased in the absence of caffeine. Taken together, these results support the idea that in utero caffeine exposure could affect central respiratory control.

Potential switch from eupnea to fictive gasping after blockade of glycine transmission and potassium channels

This study evaluated possible neuronal mechanisms responsible for the transition from normal breathing (eupnea) to gasping. We hypothesized that a blockade of both inhibitory glycinergic synaptic transmission and potassium channels, combined with an increase in extracellular concentration of potassium, would induce a switch from an eupneic respiratory pattern to gasping. Efferent activities of the phrenic, vagal, and hypoglossal nerves were recorded during eupnea and ischemia-induced gasping in a perfused in situ preparation of the juvenile rat (4-6 wk of age). To block potassium channels, 4-aminopyridine (4-AP, 1-10 microM) was administered. Strychnine (0.2-0.6 microM) was used to block glycinergic neurotransmission. After administrations of 4-AP, excess extracellular potassium (10.25-17.25 mM), and strychnine, the incrementing pattern of eupneic phrenic activity was altered to a decrementing discharge. Hypoglossal and vagal activities became concentrated to the period of the phrenic burst with expiratory activity being reduced or eliminated. These changes in neural activities were similar to those in ischemia-induced gasping. Results are consistent with the concept that the elicitation of gasping represents a switch from a network-based rhythmogenesis for eupnea to a pacemaker-driven mechanism.

The network vs. pacemaker theory of the activity of RVL presympathetic neurons--a comparison with another putative pacemaker system

Intracellular studies previously conducted in our laboratory on adult rats indicate that the activity of spinally projecting RVL neurons (neurons located in the Rostral Ventrolateral Medulla) results from synaptic inputs. The data obtained by others in medullary slices suggest that the firing of these neurons (RVL C1 and/or non-C1 type, depending on experimental conditions) is mainly determined by their 'beating' pacemaker properties. Interestingly, there is an analogy between the contrasting views on the role of the network vs. pacemakers in the generation of sympathetic tone, and a debate regarding the relative role of such mechanisms in other types of 'spontaneously' active neurons, including dopaminergic neurons of the Substantia Nigra/Ventral Tegmental Area (in ventral mesencephalon). This short review discusses our previous in vivo studies and more recent data obtained in vitro after acute cell isolation, showing that under both experimental conditions, the RVL neurons display no clear pacemaker-like properties. Interestingly, pacemaker activity of dopaminergic mesencephalic neurons can be easily demonstrated in brain slices and after acute isolation, but not in vivo. These findings strongly suggest that under normal in vivo conditions, individual neurons belonging to these two neural systems function as elements of networks.

Domoic acid lesions in nucleus of the solitary tract: time-dependent recovery of hypoxic ventilatory response and peripheral afferent axonal plasticity

The nucleus of the solitary tract (NTS) plays a pivotal role in the ventilatory response to hypoxia (HVR). However, the effects of excitotoxic lesions and the potential for functional recovery and plasticity remain unknown. Domoic acid (DA) or vehicle were bilaterally injected within the NTS of adult male Sprague Dawley rats. HVR (10% O(2)) and anatomical changes were assessed at 5-90 d after surgery. DA induced dose-dependent HVR attenuations ( approximately 70% at peak effect) that exhibited saturation at concentrations of 0.75-1.0 mm. However, although sodium cyanide-induced ventilatory responses were virtually abolished, DA did not modify baroreceptor gain. Consistent with ventilatory reductions, NTS neurons showed a significant degeneration 3 d after DA injection. In addition, the projection fields and the density of vagal afferent terminals to the NTS, and the motor neurons in the dorsal motor nucleus of the vagus were substantially reduced at 15 d. At 30 d, no functional or neural recovery were apparent. However, at day 60, the reduction in HVR was only approximately 40% of control, and at 90 d, HVR returned to control levels, paralleling regeneration of vagal afferent terminals within the NTS. The regeneration was particularly prominent in the commissural and dorsomedial subnuclei in the absence of cellular recovery. Thus, the integrity of the NTS is critical for HVR, spontaneous HVR recovery occurs after excitotoxic lesions in the NTS, and vagal-glossopharyngeal terminal sprouting in the NTS may underlie the anatomical substrate for such spontaneous functional recovery. The adult brainstem/NTS has self-repairing capabilities and will compensate for functional losses after structural damage by rewiring of its neural circuitry.

Influence of levels of carbon dioxide and oxygen upon gasping in perfused rat preparation

In vivo, the augmenting pattern of integrated phrenic nerve discharge of eupnea is altered to the decrementing pattern of gasping in severe hypoxia or ischaemia. Identical alterations in phrenic discharge are found in perfused in situ preparations of the juvenile rat. In this preparation, gasping was produced by equilibration of the perfusate with various levels of carbon dioxide and oxygen. The duration of the phrenic burst, the interval between bursts and the burst amplitude were not significantly different following equilibration with 21-6%O(2) at 5% CO(2) or with 0-9% CO(2) at 6% O(2), with the exception that the burst amplitude was significantly greater in hypercapnic-hypoxia (9% CO(2) at 6% O(2)). It is proposed that hypoxia-induced gasping results from the release of an endogenous pacemaker activity of rostral medullary neurons. This release is caused by cellular mechanisms that change the balance between membrane ionic currents. Moreover, these cellular mechanisms may be explicitly induced by alterations in the ionic and metabolic homeostasis.

Fos study of ponto-medullary areas involved in the in vitro hypoxic respiratory depression

In this study, the brainstem-spinal cord preparation isolated from newborn rats, an established model for the study of the hypoxic respiratory depression (HRD), has been used. The comparison of Fos expression in ponto-medullary areas in these preparations placed either in normoxic or hypoxic conditions suggests that only the retrotrapezoid nucleus (RTN) and the ventrolateral medulla (VLM) are involved in the in vitro HRD. Hypoxic preparations exhibit a Fos expression enhanced in the RTN, suggesting that the RTN might play a crucial role in the HRD. As well as this, VLM neurons presented a decrease in Fos expression that could be related to the decline of the respiratory output induced by hypoxia.

Measurement of single-cell gene expression using capillary electrophoresis

Capillary electrophoresis with laser-induced fluorescence detection was used to monitor gene expression in individual mammalian cells using the reverse transcriptasepolymerase chain reaction. Specifically, beta-actin expression in single LNCaP (prostate cancer) cells was measured. A sieving matrix containing hydroxypropyl methyl cellulose was used to effect size-based separation. Ethidium bromide fluorescence of the product DNA was used as the detection scheme and yielded excellent sensitivity. The beta-actin product, resulting from an individual cell lysed by a freeze-thaw method, gave an average signal-to-noise ratio (S/N) of 77+/-27 (n = 2). Chemical lysis of a single cell, using a dilute solution of SDS, gave a S/N of 26+/-2 (n = 2), roughly 3-fold lower than for freeze-thaw lysis. An initial detection limit (not considering fully optimized conditions) was calculated from an amplified cDNA standard to correspond to a concentration of approximately 133 starting molecules/nL (of beta-actin mRNA).

Specific actions of cyanide on membrane potential and voltage-gated ion currents in rostral ventrolateral medulla neurons in rat brainstem slices

The present study examined specific effects of sodium cyanide (CN) on the membrane potential (MP), spontaneous discharge (SD) and voltage-gated ion current of the identified bulbospinal rostral ventrolateral medulla (RVLM) neuron in the rat pup brainstem slice. 125 microM CN rapidly depolarized MP in the RVLM neuron by 11.6 mV as well as enhanced the SD rate by 300%. In contrast, the same dose of CN immediately hyperpolarized unlabeled, non-RVLM neurons by 4.8 mV. 50 microM CN did not significantly affect voltage-gated Ca(++) or A-type K(+) currents. The same concentration of CN, however, rapidly and reversibly suppressed voltage-gated Na(+) currents and sustained outward K(+) currents in the RVLM neuron by 22.5% and 23%, respectively. Tetraethylammonium could mimic the effect of CN on MP, SD and sustained K(+) current in the RVLM neuron. It is concluded that: (1) like that from the adult rat, the rat pup bulbospinal RVLM neuron can be selectively and rapidly excited by CN; (2) the hypoxia-sensitive, sustained outward K(+) channel may play an important role in the acute hypoxia-induced excitation of the RVLM neurons.

Neurons of a limited subthalamic area mediate elevations in cortical cerebral blood flow evoked by hypoxia and excitation of neurons of the rostral ventrolateral medulla

Sympathoexcitatory reticulospinal neurons of the rostral ventrolateral medulla (RVLM) are oxygen detectors excited by hypoxia to globally elevate regional cerebral blood flow (rCBF). The projection, which accounts for >50% of hypoxic cerebral vasodilation, relays through the medullary vasodilator area (MCVA). However, there are no direct cortical projections from the RVLM/MCVA, suggesting a relay that diffusely innervates cortex and possibly originates in thalamic nuclei. Systematic mapping by electrical microstimulation of the thalamus and subthalamus revealed that elevations in rCBF were elicited only from a limited area, which encompassed medial pole of zona incerta, Forel's field, and prerubral zone. Stimulation (10 sec train) at an active site increased rCBF by 25 +/- 6%. Excitation of local neurons with kainic acid mimicked effects of electrical stimulation by increasing rCBF. Stimulation of the subthalamic cerebrovasodilator area (SVA) with single pulses (0.5 msec; 80 microA) triggered cortical EEG burst-CBF wave complexes with latency 24 +/- 5 msec, which were similar in shape to complexes evoked from the MCVA. Selective bilateral lesioning of the SVA neurons (ibotenic acid, 2 microg, 200 nl) blocked the vasodilation elicited from the MCVA and attenuated hypoxic cerebrovasodilation by 52 +/- 12% (p < 0.05), whereas hypercarbic vasodilation remained preserved. Lesioning of the vasodilator site in the basal forebrain failed to modify SVA-evoked rCBF increase. We conclude that (1) excitation of intrinsic neurons of functionally restricted region of subthalamus elevates rCBF, (2) these neurons relay signals from the MCVA, which elevate rCBF in response to hypoxia, and (3) the SVA is a functionally important site conveying vasodilator signal from the medulla to the telencephalon.

Single-cell RT-PCR as a tool to study gene expression in central and peripheral autonomic neurones

In studies of the central and peripheral autonomic nervous system, it has become increasingly important to be able to investigate mRNA expression patterns within specific neuronal populations. Traditionally, the identification of mRNA species in discrete populations of cells has relied upon in situ hybridization. An alternative, relatively simple procedure is 'multiplex' reverse transcription-polymerase chain reaction (RT-PCR), conducted on single neurons after their in vitro isolation. Multiplex single-cell RT-PCR can be used to examine the expression of multiple genes within individual cells, and can be combined with electrophysiological, pharmacological and anatomical (retrograde labelling) studies. This review focuses on a number of key aspects of this approach, methodology, and both the advantages and the limitations of the technique. We also provide specific examples of work performed in our laboratory, examining the expression of alpha 2-adrenergic receptors in catecholaminergic cells of the rat brainstem and adrenal medulla. The application of single-cell RT-PCR to future studies of the autonomic nervous system will hopefully provide information on how physiological and pathological conditions affect gene expression in autonomic neurones.

Biophysical characterization of rat caudal hypothalamic neurons: calcium channel contribution to excitability

Neurons in the caudal hypothalamus (CH) are responsible for the modulation of various processes including respiratory and cardiovascular output. Previous results from this and other laboratories have demonstrated in vivo that these neurons have firing rhythms matched to the respiratory and cardiovascular cycles. The goal of the present study was to characterize the biophysical properties of neurons in the CH with particular emphasis in those properties responsible for rhythmic firing behavior. Whole cell, patch-clamped CH neurons displayed a resting membrane potential of -58.0 +/- 1.1 mV and an input resistance of 319.3 +/- 16.6 MOmega when recorded in current-clamp mode in an in vitro brain slice preparation. A large proportion of these neurons displayed postinhibitory rebound (PIR) that was dependent on the duration and magnitude of hyperpolarizing current as well as the resting membrane potential of the cell. Furthermore these neurons discharged tonically in response to a depolarizing current pulse at a depolarized resting membrane potential (more positive than -65 mV) but switched to a rapid burst of firing to the same stimulus when the resting membrane potential was lowered. The PIR observed in these neurons was calcium dependent as demonstrated by the ability to block its amplitude by perfusion of Ca(2+)-free bath solution or by application of Ni(2+) (0.3-0.5 mM) or nifedipine (10 microM). These properties suggest that low-voltage-activated (LVA) calcium current is involved in the PIR and bursting firing of these CH neurons. In addition, high-voltage-activated calcium responses were detected after blockade of outward potassium current or in Ba(2+)-replacement solution. In addition, almost all of the CH neurons studied showed spike frequency adaptation that was decreased following Ca(2+) removal, indicating the involvement of Ca(2+)-dependent K(+) current (I(K,Ca)) in these cells. In conclusion, CH neurons have at least two different types of calcium currents that contribute to their excitability; the dominant current is the LVA or T-type. This LVA current appears to play a significant role in the bursting characteristics that may underlie the rhythmic firing of CH neurons.

Hypoxic augmentation of fast-inactivating and persistent sodium currents in rat caudal hypothalamic neurons

Previous work from this laboratory has indicated that TTX-sensitive sodium channels are involved in the hypoxia-induced inward current response of caudal hypothalamic neurons. Since this inward current underlies the depolarization and increased firing frequency observed in these cells during hypoxia, the present study utilized more detailed biophysical methods to specifically determine which sodium currents are responsible for this hypoxic activation. Caudal hypothalamic neurons from approximately 3-wk-old Sprague-Dawley rats were acutely dissociated and patch-clamped in the voltage-clamp mode to obtain recordings from fast-inactivating and persistent (noninactivating) whole cell sodium currents. Using computer-generated activation and inactivation voltage protocols, rapidly inactivating sodium currents were analyzed during normal conditions and during a brief (3-6 min) period of severe hypoxia. In addition, voltage-ramp and extended-voltage-activation protocols were used to analyze persistent sodium currents during normal conditions and during hypoxia. A polarographic oxygen electrode determined that the level of oxygen in this preparation quickly dropped to 10 Torr within 2 min of initiation of hypoxia and stabilized at <0.5 Torr within 4 min. During hypoxia, the peak fast-inactivating sodium current was significantly increased throughout the entire activation range, and both the activation and inactivation values (V(1/2)) were negatively shifted. Furthermore both the voltage-ramp and extended-activation protocols demonstrated a significant increase in the persistent sodium current during hypoxia when compared with normoxia. These results demonstrate that both rapidly inactivating and persistent sodium currents are significantly enhanced by a brief hypoxic stimulus. Furthermore the hypoxic-induced increase in these currents most likely is the primary mechanism for the depolarization and increased firing frequency observed in caudal hypothalamic neurons during hypoxia. Since these neurons are important in modulating cardiorespiratory activity, the oxygen responsiveness of these sodium currents may play a significant role in the centrally mediated cardiorespiratory response to hypoxia.

O2-sensing after carotid chemodenervation: hypoxic ventilatory responsiveness and upregulation of tyrosine hydroxylase mRNA in brainstem catecholaminergic cells

Ventilatory responses to acute and long-term hypoxia are classically triggered by carotid chemoreceptors. The chemosensory inputs are carried within the carotid sinus nerve to the nucleus tractus solitarius and the brainstem respiratory centres. To investigate whether hypoxia acts directly on brainstem neurons or secondarily via carotid body inputs, we tested the ventilatory responses to acute and long-term hypoxia in rats with bilaterally transected carotid sinus nerves and in sham-operated rats. Because brainstem catecholaminergic neurons are part of the chemoreflex pathway, the ventilatory response to hypoxia was studied in association with the expression of tyrosine hydroxylase (TH). TH mRNA levels were assessed in the brainstem by in situ hybridization and hypoxic ventilatory responses were measured in vivo by plethysmography. After long-term hypoxia, TH mRNA levels in the nucleus tractus solitarius and ventrolateral medulla increased similarly in chemodenervated and sham-operated rats. Ventilatory acclimatization to hypoxia developed in chemodenervated rats, but to a lesser extent than in sham-operated rats. Ventilatory response to acute hypoxia, which was initially low in chemodenervated rats, was fully restored within 21 days in long-term hypoxic rats, as well as in normoxic animals which do not overexpress TH. Therefore, activation of brainstem catecholaminergic neurons and ventilatory adjustments to hypoxia occurred independently of carotid chemosensory inputs. O2-sensing mechanisms unmasked by carotid chemodenervation triggered two ventilatory adjustments: (i) a partial acclimatization to long-term hypoxia associated with TH upregulation; (ii) a complete restoration of acute hypoxic responsivity independent of TH upregulation.

Neural structures that mediate sympathoexcitation during hypoxia

The sympathetic adjustments triggered by acute mild hypoxia (sympathetic chemoreflex) are initiated by activation of peripheral chemoreceptors whereas more severe hypoxia activates the sympathetic outflow via direct effects on the brainstem. In both cases the rostral ventrolateral medulla (RVLM) plays a critical role in these responses. The first part of this review briefly describes the general input-output properties of the presympathetic neurons of RVLM before focusing on the neural pathways leading to their excitation in response to peripheral chemoreceptor stimulation. The extent to which the central respiratory network contributes to the sympathetic chemoreflex is then discussed before briefly alluding to its role in obstructive sleep apnea and other pathologies. The second half of the review examines the direct effects of hypoxia on RVLM neurons and whether this region and the presympathetic neurons in particular qualify as a physiological central oxygen sensor. The literature is also examined in the context of cerebral ischemia, the Cushing response and the genesis of certain forms of hypertension.

Detection of mRNA species in bulbospinal neurons isolated from the rostral ventrolateral medulla using single-cell RT-PCR

The rostral ventrolateral medulla (RVL) contains neurons which are critically involved in the tonic and reflex control of blood pressure. Some of these neurons project to the intermediolateral cell column of the thoracolumbar spinal cord and excite preganglionic sympathetic neurons. In order to gain a better understanding of the properties of the RVL neurons at the cellular and molecular level, a protocol was developed utilizing acute dissociation and the reverse transcription-polymerase chain reaction (RT-PCR) to study the expression of several genes in single RVL neurons. Neurons were dissociated from the RVL region of young rats, and classified as spinally projecting or non-spinal by the presence or absence of retrogradely transported fluorescent beads injected into the upper thoracic segments of the spinal cord. Individual neurons were collected by aspiration into a glass micropipette and analysed by RT-PCR. The presence of either glyceraldehyde 3-phosphate dehydrogenase (GAPDH) or neuron-specific enolase (NSE) mRNA was used as the criterion for selecting cells for further analysis. A subpopulation (50%) of spinally projecting, GAPDH- or NSE-positive neurons expressed mRNA for tyrosine hydroxylase (TH) or phenylethanolamine N-methyltransferase (PNMT), indicative of catecholaminergic or C1 adrenergic neurons, respectively. Some bulbospinal RVL neurons, including those that were TH- or PNMT-positive, were also found to express mRNA for the mineralocorticoid receptor (MR), the glucocorticoid receptor (GR), noradrenaline transporter (NET), and neuronal glutamate transporter (EAAC1). The glial glutamate transporter (GLT), glycine transporter (GLYT2), glutamic acid decarboxylase (GAD67) and gamma-amino butyric acid (GABA) transporter (GAT-1) were not expressed. The single-cell RT-PCR protocol is a powerful, yet simple and relatively rapid method for analysis of mRNA expression in a defined neuronal population. It can be combined with whole-cell patch-clamp recording prior to RT-PCR analysis, allowing linkage of the molecular analysis of mRNA expression to the electrophysiological and pharmacological properties of single neurons. The method is very sensitive, enabling mRNA transcripts in low abundance to be detected, and its application in our recent studies provided novel information about neurons involved in blood-pressure regulation at the molecular and cellular level.