Activity of aortic chemoreceptors in the anaesthetized rat

Intra-carotid body inter-cellular communication

The classic peripheral chemoreflex response is a critical homeostatic mechanism. In healthy individuals, appropriate chemoreflex responses are triggered by acute activation of the carotid body - the principal chemosensory organ in mammals. However, the aberrant chronic activation of the carotid body can drive the elevated sympathetic activity underlying cardio-respiratory diseases such as hypertension, diabetes and heart failure. Carotid body resection induces intolerable side effects and so understanding how to modulate carotid body output without removing it, and whilst maintaining the physiological chemoreflex response, represents the next logical next step in the development of effective clinical interventions. By definition, excessive carotid body output must result from altered intra-carotid body inter-cellular communication. Alongside the canonical synaptic transmission from glomus cells to petrosal afferents, many other modes of information exchange in the carotid body have been identified, for example bidirectional signalling between type I and type II cells via ATP-induced ATP release, as well as electrical communication via gap junctions. Thus, herein we review the carotid body as an integrated circuit, discussing a variety of different inter-cellular signalling mechanisms and highlighting those that are potentially relevant to its pathological hyperactivity in disease with the aim of identifying novel therapeutic targets.

A Novel Organ-Specific Approach to Selectively Target Sensory Afferents Innervating the Aortic Arch

The brain maintains cardiovascular homeostasis, in part, via the arterial baroreflex which senses changes in blood pressure (BP) at the level of the aortic arch. Sensory afferents innervating the aortic arch employ baroreceptors to convert stretch exerted on the arterial wall into action potentials carried by the vagus nerve to second order neurons residing within the nucleus of the solitary tract (NTS). Although the baroreflex was described more than 80 years ago, the specific molecular, structural, and functional phenotype of the baroreceptors remain uncharacterized. This is due to the lack of tools that provide the genetic and target organ specificity that is required to selectively characterize baroreceptor afferents. Here, we use a novel approach to selectively target baroreceptors. Male mice on a C57BL/6J background were anesthetized with isoflurane, intubated, and artificially ventilated. Following sternotomy, the aortic arch was exposed, and a retrograde adeno-associated virus was applied to the aortic arch to direct the expression of channelrhoropsin-2 (ChR2) and/or tdTomato (tdTom) to sensory afferents presumably functioning as baroreceptors. Consistent with the structural characteristics of arterial baroreceptors, robust tdTom expression was observed in nerve endings surrounding the aortic arch, within the fibers of the aortic depressor and vagus nerves, cell bodies of the nodose ganglia (NDG), and neural projections to the caudal NTS (cNTS). Additionally, the tdTom labeled cell bodies within the NDG also expressed mRNAs coding for the mechanically gated ion channels, PIEZO-1 and PIEZO-2. In vitro electrophysiology revealed that pulses of blue light evoked excitatory post-synaptic currents in a subset of neurons within the cNTS, suggesting a functional connection between the labeled aortic arch sensory afferents and second order neurons. Finally, the in vivo optogenetic stimulation of the cell bodies of the baroreceptor expressing afferents in the NDG produced robust depressor responses. Together, these results establish a novel approach for selectively targeting sensory neurons innervating the aortic arch. This approach may be used to investigate arterial baroreceptors structurally and functionally, and to assess their role in the etiology or reversal of cardiovascular disease.

Laterality Influences Central Integration of Baroreceptor Afferent Input in Male and Female Sprague Dawley Rats

We explored the effects of baroreceptor afferents laterality and sexual dimorphism on the expression of cardiovascular reflex responses to baroreflex activation in Sprague Dawley (SD) rats. Under urethane anesthesia, rats of either sex (total n = 18) were instrumented for left, right and bilateral aortic depressor nerve (ADN) stimulation (1–40 Hz, 0.2 ms, 0.4 mA for 20 s) and measurement of mean arterial pressure (MAP), heart rate (HR) and mesenteric (MVR) and femoral (FVR) vascular resistance. Female rats were matched for the diestrus phase of the estrus cycle. Left, right and bilateral ADN stimulation evoked frequency-dependent drops in MAP, HR, and MVR, and increases in FVR. Irrespective of sex, left and bilateral ADN stimulation as compared to right-sided stimulation mediated greater reflex reductions in MAP, HR, and MVR but not in FVR. In males, reflex bradycardic responses were greater in response to bilateral stimulation relative to both left- and right-sided stimulation. In females, left ADN stimulation evoked the largest increase in FVR. Left and bilateral ADN stimulations evoked greater reductions in MAP and MVR while left-sided stimulation produced larger increases in FVR in females compared with males. All other reflex responses to ADN stimulation were relatively comparable between males and females. These results show a differential baroreflex processing of afferent neurotransmission promoted by left versus right baroreceptor afferent inputs and sexual dimorphism in the expression of baroreflex responses in rats of either sex. Collectively, these data add to our understanding of physiological mechanisms pertaining to baroreflex control in both males and females.

Ventilatory and Autonomic Regulation in Sleep Apnea Syndrome: A Potential Protective Role for Erythropoietin?

Obstructive sleep apnea (OSA) is the most common form of sleep disordered breathing and is associated with wide array of cardiovascular morbidities. It has been proposed that during OSA, the respiratory control center (RCC) is affected by exaggerated afferent signals coming from peripheral/central chemoreceptors which leads to ventilatory instability and may perpetuate apnea generation. Treatments focused on decreasing hyperactivity of peripheral/central chemoreceptors may be useful to improving ventilatory instability in OSA patients. Previous studies indicate that oxidative stress and inflammation are key players in the increased peripheral/central chemoreflex drive associated with OSA. Recent data suggest that erythropoietin (Epo) could also be involved in modulating chemoreflex activity as functional Epo receptors are constitutively expressed in peripheral and central chemoreceptors cells. Additionally, there is some evidence that Epo has anti-oxidant/anti-inflammatory effects. Accordingly, we propose that Epo treatment during OSA may reduce enhanced peripheral/central chemoreflex drive and normalize the activity of the RCC which in turn may help to abrogate ventilatory instability. In this perspective article we discuss the potential beneficial effects of Epo administration on ventilatory regulation in the setting of OSA.

Lessons from single-cell transcriptome analysis of oxygen-sensing cells

The advent of single-cell RNA-sequencing (RNA-Seq) technology has enabled transcriptome profiling of individual cells. Comprehensive gene expression analysis at the single-cell level has proven to be effective in characterizing the most fundamental aspects of cellular function and identity. This unbiased approach is revolutionary for small and/or heterogeneous tissues like oxygen-sensing cells in identifying key molecules. Here, we review the major methods of current single-cell RNA-Seq technology. We discuss how this technology has advanced the understanding of oxygen-sensing glomus cells in the carotid body and helped uncover novel oxygen-sensing cells and mechanisms in the mice olfactory system. We conclude by providing our perspective on future single-cell RNA-Seq research directed at oxygen-sensing cells.

Enhanced Firing in NTS Induced by Short-Term Sustained Hypoxia Is Modulated by Glia-Neuron Interaction

Humans ascending to high altitudes are submitted to sustained hypoxia (SH), activating peripheral chemoreflex with several autonomic and respiratory responses. Here we analyzed the effect of short-term SH (24 h, FIO210%) on the processing of cardiovascular and respiratory reflexes using an in situ preparation of rats. SH increased both the sympatho-inhibitory and bradycardiac components of baroreflex and the sympathetic and respiratory responses of peripheral chemoreflex. Electrophysiological properties and synaptic transmission in the nucleus tractus solitarius (NTS) neurons, the first synaptic station of afferents of baroreflexes and chemoreflexes, were evaluated using brainstem slices and whole-cell patch-clamp. The second-order NTS neurons were identified by previous application of fluorescent tracer onto carotid body for chemoreceptor afferents or onto aortic depressor nerve for baroreceptor afferents. SH increased the intrinsic excitability of NTS neurons. Delayed excitation, caused by A-type potassium current (IKA), was observed in most of NTS neurons from control rats. The IKA amplitude was higher in identified second-order NTS neurons from control than in SH rats. SH also blunted the astrocytic inhibition of IKA in NTS neurons and increased the synaptic transmission in response to afferent fibers stimulation. The frequency of spontaneous excitatory currents was also increased in neurons from SH rats, indicating that SH increased the neurotransmission by presynaptic mechanisms. Therefore, short-term SH changed the glia-neuron interaction, increasing the excitability and excitatory transmission of NTS neurons, which may contribute to the observed increase in the reflex sensitivity of baroreflex and chemoreflex in in situ preparation.

Open-loop characteristics of the arterial baroreflex after blockade of unmyelinated baroreceptors with resiniferatoxin

Arterial baroreceptors can be divided into two categories dependent on whether their axons are myelinated (A-fiber) or unmyelinated (C-fiber). We investigated the effect of periaxonal resiniferatoxin (RTX), a blocker of C-fiber baroreceptor activity, on the open-loop static characteristics of the arterial baroreflex. The baroreceptor region of the right aortic depressor nerve was isolated, and intra-baroreceptor region pressure (BRP) was changed from 60 to 180 mm Hg in 10 anesthetized Sprague-Dawley rats. Open-loop static characteristics of the neural arc from BRP to efferent sympathetic nerve activity (SNA), peripheral arc from SNA to arterial pressure (AP), and total reflex arc from BRP to AP were estimated. Although blocking C-fiber activity with RTX resulted in a lower response range (33.7±4.6% and 49.4±4.8%, P<0.01) and higher minimum SNA (78.0±4.7% and 53.6±5.0%, P<0.001) of the steady-state neural arc, the peak SNA response to BRP was greater at a BRP of 160 mm Hg (-37.87±5.83% and -26.28±4.90%, P=0.01). RTX also resulted in a lower response range (27.8±5.0 mm Hg and 40.9±5.2 mm Hg, P<0.01) and higher minimum AP (92.4±4.7 mm Hg and 79.1±4.9 mm Hg, P<0.001) of the total reflex arc. Despite these changes, the maximum slope of the neural arc and the maximum gain of the total reflex arc did not differ significantly after RTX. These data suggest that A-fiber baroreceptors can regulate AP and maintain the maximum gain when systemic AP is around the normal operating range. In contrast, C-fiber baroreceptors are critically important for reductions in SNA and AP when systemic AP is raised above the normal operating range.

Autocrine/paracrine modulation of baroreceptor activity after antidromic stimulation of aortic depressor nerve in vivo

Activation of the sensory nerve endings of non-myelinated C-fiber afferents evokes release of autocrine/paracrine factors that cause localized vasodilation, neurogenic inflammation, and modulation of sensory nerve activity. The aims of this study were to determine the effect of antidromic electrical stimulation on afferent baroreceptor activity in vivo, and investigate the role of endogenous prostanoids and hydrogen peroxide (H2O2) in mediating changes in nerve activity. Baroreceptor activity was recorded from the left aortic depressor nerve (ADN) in anesthetized rats before and after stimulating the ADN for brief (5–20 s) periods. The rostral end of the ADN was crushed or sectioned beforehand to prevent reflex changes in blood pressure. Antidromic stimulation of ADN using parameters that activate both myelinated A-fibers and non-myelinated C-fibers caused pronounced and long-lasting (> 1 min) inhibition of baroreceptor activity (n = 9, P < 0.05), with the magnitude and duration of inhibition dependent on the duration of the stimulation period (n = 5). Baroreceptor activity was only transiently inhibited after selective stimulation of A-fibers. The inhibition of activity after antidromic stimulation of A and C fibers was prolonged after administration of the cyclooxygenase inhibitor indomethacin (5 mg/kg, IV, n = 7) and abolished after administration of PEG-catalase (104 units/kg, IV, n = 7), an enzyme that catalyzes the decomposition of H2O2 to water and oxygen. The results demonstrate a long-lasting inhibition of baroreceptor activity after antidromic stimulation of ADN and suggest that endogenous prostanoids and H2O2 oppose and mediate the inhibition, respectively. These mechanisms may contribute to rapid baroreceptor resetting during acute hypertension and be engaged during chronic baroreceptor activation therapy in patients with hypertension.

Nitric oxide stimulates glutamatergic synaptic inputs to baroreceptor neurons through potentiation of Cav2.2-mediated Ca(2+) currents

Nitric oxide (NO) increases glutamate release to the second-order neurons in the nucleus tractus solitarius (NTS). N-type Ca(2+) channel is essential for triggering glutamate release at synaptic terminals. In this study, we determined the role of Cav2.2 subunit in NO-induced increase in glutamate synaptic inputs to NTS neurons. The second-order NTS neurons and nodose ganglionic (NG) neurons were identified by applying DiA, a fluorescent lipophilic tracer, on aortic depressor nerve in rats. NO donor DEA/NO significantly increased tractus solitarius (TS)-evoked excitatory postsynaptic currents (EPSCs) in second-order NTS neurons, an effect was abolished by pretreatment of slice with ODQ, an inhibitor for soluble isoform of guanylyl cyclase. DEA/NO decreased the paired-pulse ratio of TS-evoked EPSCs, while increased the frequency, but not the amplitude, of miniature EPSCs in second-order NTS neurons. Furthermore, DEA/NO significantly increased Ba(2+) currents in identified baroreceptor NG neurons. However, DEA/NO had little effect on the Ba(2+) currents in the presence of specific N-type Ca(2+) blocker ω-conotoxin GVIA. In addition, immunocytochemistry staining revealed that Cav2.2 subunit immunoreactivates were colocalized with DiA-labeled baroreceptor nerve terminals in the NTS. Collectively, these findings suggest that NO stimulates glutamatergic synaptic inputs to second-order NTS neurons through augmentation of Cav2.2-mediated N-type Ca(2+) currents.

Activation of NTS A(1) adenosine receptors inhibits regional sympathetic responses evoked by activation of cardiopulmonary chemoreflex

Previously we have shown that adenosine operating via the A(1) receptor subtype may inhibit glutamatergic transmission in the baroreflex arc within the nucleus of the solitary tract (NTS) and differentially increase renal (RSNA), preganglionic adrenal (pre-ASNA), and lumbar (LSNA) sympathetic nerve activity (ASNA>RSNA≥LSNA). Since the cardiopulmonary chemoreflex and the arterial baroreflex are mediated via similar medullary pathways, and glutamate is a primary transmitter in both pathways, it is likely that adenosine operating via A(1) receptors in the NTS may differentially inhibit regional sympathetic responses evoked by activation of cardiopulmonary chemoreceptors. Therefore, in urethane-chloralose-anesthetized rats (n = 37) we compared regional sympathoinhibition evoked by the cardiopulmonary chemoreflex (activated with right atrial injections of serotonin 5HT(3) receptor agonist phenylbiguanide, PBG, 1-8 μg/kg) before and after selective stimulation of NTS A(1) adenosine receptors [microinjections of N(6)-cyclopentyl adenosine (CPA), 0.033-330 pmol/50 nl]. Activation of cardiopulmonary chemoreceptors evoked differential, dose-dependent sympathoinhibition (RSNA>ASNA>LSNA), and decreases in arterial pressure and heart rate. These differential sympathetic responses were uniformly attenuated in dose-dependent manner by microinjections of CPA into the NTS. Volume control (n = 11) and blockade of adenosine receptor subtypes in the NTS via 8-(p-sulfophenyl)theophylline (8-SPT, 1 nmol in 100 nl) (n = 9) did not affect the reflex responses. We conclude that activation of NTS A(1) adenosine receptors uniformly inhibits neural and cardiovascular cardiopulmonary chemoreflex responses. A(1) adenosine receptors have no tonic modulatory effect on this reflex under normal conditions. However, when adenosine is released into the NTS (i.e., during stress or severe hypotension/ischemia), it may serve as negative feedback regulator for depressor and sympathoinhibitory reflexes integrated in the NTS.

Hypertension is critically dependent on the carotid body input in the spontaneously hypertensive rat

The peripheral chemoreflex is known to be enhanced in individuals with hypertension. In pre-hypertensive (PH) and adult spontaneously hypertensive rats (SHRs) carotid body type I (glomus) cells exhibit hypersensitivity to chemosensory stimuli and elevated sympathoexcitatory responses to peripheral chemoreceptor stimulation. Herein, we eliminated carotid body inputs in both PH-SHRs and SHRs to test the hypothesis that heightened peripheral chemoreceptor activity contributes to both the development and maintenance of hypertension. The carotid sinus nerves were surgically denervated under general anaesthesia in 4- and 12-week-old SHRs. Control groups comprised sham-operated SHRs and aged-matched sham-operated and carotid sinus nerve denervated Wistar rats. Arterial blood pressure was recorded chronically in conscious, freely moving animals. Successful carotid sinus nerve denervation (CSD) was confirmed by testing respiratory responses to hypoxia (10% O(2)) or cardiovascular responses to i.v. injection of sodium cyanide. In the SHR, CSD reduced both the development of hypertension and its maintenance (P<0.05) and was associated with a reduction in sympathetic vasomotor tone (as revealed by frequency domain analysis and reduced arterial pressure responses to administration of hexamethonium; P<0.05 vs. sham-operated SHR) and an improvement in baroreflex sensitivity. No effect on blood pressure was observed in sham-operated SHRs or Wistar rats. In conclusion, carotid sinus nerve inputs from the carotid body are, in part, responsible for elevated sympathetic tone and critical for the genesis of hypertension in the developing SHR and its maintenance in later life.

Effects of chemostimuli on [Ca2+]i responses of rat aortic body type I cells and endogenous local neurons: comparison with carotid body cells

Mammalian aortic bodies (ABs) are putative peripheral arterial chemoreceptors whose function remains controversial, partly because information on their cellular physiology is lacking. In this study, we used ratiometric Ca2+ imaging to investigate for the first time chemosensitivity in short-term cultures of dissociated cells of juvenile rat ABs, located near the junction of the left vagus and recurrent laryngeal nerves. Among the surviving cell population were glomus or type I cell clusters, endogenous local neurons and glia-like cells. A variety of chemostimuli, including hypoxia, isohydric or acidic hypercapnia, and isocapnic acidosis, caused a rise in intracellular [Ca2+] in AB type I cells. The [Ca2+]i responses were indistinguishable from those in carotid body (CB) type I cells grown in parallel cultures from the same animals, and responses to acidic hypercapnia were prevented by the non-specific voltage-gated Ca2+ channel antagonist, 2mM Ni2+. Furthermore, we identified a subpopulation (∼40%) of glia-like cells in AB cultures that resembled CB type II cells based on their approximately equal sensitivity to ATP and UTP, consistent with the expression of purinergic P2Y2 receptors. Finally, we showed that some local neurons, known to be uniquely associated with these AB paraganglia in situ, generated robust [Ca2+]i responses to these chemostimuli. Thus, these AB type I cells and associated putative type II cells resemble those from the well-studied CB. Unlike the CB, however, they also associate with a special group of endogenous neurons which we propose may subserve a sensory function in local cardiovascular reflexes.

Vasopressor and heart rate responses to systemic administration of bombesin in anesthetized rats

The aim of the present study was to investigate the effects of aortic depressor nerve (ADN) transection, supranodosal vagi denervation (NG vagi cut) and adrenergic receptor blocker treatment on the cardiovascular responses evoked by systemic injection of bombesin. The cardiovascular effects were studied in spontaneously breathing rats that were (i) bilaterally, midcervically vagotomized (MC vagi cut) and subjected to section of the aortic depressor nerves, (ii) midcervically vagotomized and subsequently vagotomized at the supranodosal level or (iii) midcervically vagotomized before and after pharmacological blockade of α- or β-adrenergic receptors with phentolamine and propranolol, respectively. An intravenous bolus of bombesin (10 μg/kg) in midcervically vagotomized and ADN denervated animals increased mean arterial blood pressure (MAP) and heart rate (HR). An approximate 20% increase in blood pressure occurred immediately following bombesin injection and lasted for 2-3 min. Augmentation of the heart rate occurred 30-60 s after the bombesin challenge and persisted for more than 10 min. After section of the supranodosal vagi, bombesin failed to induce an increase in heart rate. Blockade of α-adrenergic receptors with an intravenous dose of phentolamine significantly reduced post-bombesin hypertension. These results indicate that bombesin-evoked increases in blood pressure do not require aortic depressor nerves and supranodosal vagi and are presumably mediated by the activation of peripheral α-adrenergic receptors. Bombesin-induced tachycardia was dependent on an intact supranodose pathway and was amplified by activation of β-adrenoceptors.

Carotid chemoafferent activity is not necessary for all phrenic long-term facilitation following acute intermittent hypoxia

Phrenic long-term facilitation (pLTF) is a form of respiratory plasticity induced by acute intermittent hypoxia (AIH) or episodic carotid chemoafferent neuron activation. Surprisingly, residual pLTF is expressed in carotid denervated rats. However, since carotid denervation eliminates baroreceptor feedback and causes profound hypotension during hypoxia in anesthetized rats, potential contributions of these uncontrolled factors or residual chemoafferent neuron activity to residual pLTF cannot be ruled out. Since ATP is necessary for hypoxic carotid chemotransduction, we tested the hypothesis that functional peripheral chemoreceptor denervation (with intact baroreceptors) via systemic P2X receptor antagonism blocks hypoxic phrenic responses and AIH-induced pLTF in anesthetized rats. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 100 mg/kg i.v.), a non-selective P2X receptor antagonist, was administered to anesthetized, vagotomized, paralyzed and ventilated male Sprague-Dawley rats prior to AIH (3, 5 min episodes of 10% O(2); 5 min intervals). Although PPADS strongly attenuated the short-term hypoxic phrenic response (20 ± 4% vs. 113 ± 15% baseline; P < 0.001), pLTF was reduced but not eliminated 60 min post-AIH (25 ± 4% vs. 51 ± 11% baseline; n = 8 and 7, respectively; P < 0.002). Thus, AIH initiates residual pLTF out of proportion to the diminished hypoxic phrenic response and chemoafferent neuron activation. Although the mechanism of residual pLTF following functional chemo-denervation remains unclear, possible mechanisms involving direct effects of hypoxia on the CNS are discussed.

The effect of pro-inflammatory cytokines on the discharge rate of vagal nerve paraganglia in the rat

Vagal paraganglia resemble the carotid body and are chemosensitive to reduction in the partial pressure of oxygen (PO2) (O’Leary et al., 2004). We hypothesised that they may also mediate communication between the immune system and the central nervous system and more specifically respond to the pro-inflammatory cytokines: interleukin-1 beta (IL-1β) and tumour necrosis factor-α (TNF-α). We recorded axonal firing rate of isolated superfused rat glomus cells – located at the bifurcation of the superior laryngeal nerve – to IL-1β or TNF-α at concentrations of 0.5 ng/ml, 5 ng/ml and 50 ng/ml. Twenty-three successful single fibre recordings were obtained from 10 animals. IL-1β and TNF-α had no statistically significant effect on the frequency of action potentials observed (p = 0.39 and 0.42, respectively, repeated measures ANOVA). The activity of both cytokines was tested by observing translocation of P65-NFκB from cytoplasm to nucleus in cultured HELA cells. In conclusion, an immune role for SLN paraganglia has not been established.

Effects of baroreceptor activation on respiratory variability in rat

Controversy surrounds the respiratory responses to baroreceptor activation. Although many reflexes that effect respiration (e.g. chemoreflexes and nociceptive reflexes) frequently affect cardiovascular parameters, the effect of baroreflex stimulation within normal physiological limits is generally considered to affect only blood pressure and heart rate. Even though previous authors have reported that baroreceptor activation can affect respiratory activity, the effects on respiratory frequency and amplitude are highly variable, and changes in perfusion evoked by blood pressure manipulation could account for the observed effects. Here, we determined the respiratory effects of activating arterial baroreceptors by intravenous injection of phenylephrine or angiotensin II, or by electrical stimulation of the aortic depressor nerve (ADN). In urethane-anesthetized vagotomized rats, 1, 2 and 4s trains of tetanic ADN stimulation evoked 3.1+/-1.1%, 11.2+/-13.6% and 21.9+/-8.9% increases in inspiratory (TI) time and 26.5+/-18%, 23.4+/-15.7% and 34.6+/-20.9% increases in expiratory (TE) time, respectively (P<0.05 in both cases), but no effect on the amplitude of bursts recorded in the phrenic nerve. Similar effects were observed following pressor trials evoked by intravenous PE (TE: +26.1+/-9.1%, P<0.01), but not Ang II. Intermittent ADN stimulation (single pulse, 1 Hz) significantly increased the variability of TI during periods of low respiratory drive (P<0.05) without significantly affecting any other parameters. We propose that a specific baroreceptor-respiratory response exists that is independent of changes in blood flow. In contrast to the effects of baroreceptor stimulation on sympathetic nerve activity, the baro-respiratory response is subtle and highly dependent on respiratory drive.

ACTIVATION OF PERIPHERAL CHEMORECEPTORS CAUSES POSITIVE INOTROPIC EFFECTS IN A WORKING HEART?BRAINSTEM PREPARATION OF THE RAT

1. The aim of the present study was to evaluate the effects of peripheral chemoreceptor activation on myocardial contractility in an anaesthetic-free decerebrated rat preparation. 2. In the decerebrated and retrogradely perfused working heart-brainstem preparation, we recorded phrenic nerve activity, left ventricular (LV) pressure (microtip Millar catheter), LV dP/dT, heart rate and aortic perfusion pressure before and after activating peripheral chemoreceptors with bolus intra-arterial injections of KCN. 3. Without cardiac pacing, chemoreflex activation caused falls in heart rate (-108 +/- 21 b.p.m.) and complex polyphasic changes in LV pressure and LV dP/dT. If the heart was paced, chemoreflex activation caused significant rises in LP pressure (+16 +/- 3 mmHg) and LV dP/dt (+778 +/- 93 mmHg/s). These positive inotropic effects were significantly and substantially attenuated by beta-adrenoceptor blockade with atenolol. In all instances, chemoreflex activation elicited potent tachypnoeic responses. 4. In conclusion, activation of peripheral chemoreceptors in non-anaesthetized rats evokes a positive inotropic response that is sympathetically mediated. This observation may be relevant for the evaluation of neurally induced effects of acute hypoxia on the ventricular myocardium.

Vesicular glutamate transporters and neuronal nitric oxide synthase colocalize in aortic depressor afferent neurons

The aortic depressor nerve (ADN) primarily transmits baroreceptor signals from the aortic arch to the nucleus tractus solitarii. Cell bodies of neurons that send peripheral fibers to form the ADN are located in the nodose ganglion (NG). Studies have implicated glutamate and nitric oxide in transmission of baroreflex signals; therefore, we tested the hypothesis that ADN neurons contain either vesicular glutamate transporters (VGLUTs) or neuronal nitric oxide synthase (nNOS) or both. We applied a fluorescent tracer, tetramethyl rhodamine dextran (TRD), to rat ADN to identify ADN neurons and then performed immunofluorescent labeling for nNOS and VGLUTs 1, 2, and 3 in NG sections. We found that VGLUT2-immunoreactivity (IR) and VGLUT3-IR was present in a significantly higher proportion of TRD positive neurons than in TRD negative neurons. In contrast, the percentage of TRD positive neurons containing VGLUT1-IR or nNOS-IR did not differ from that of TRD negative neurons. We also observed that the percentage of TRD positive neurons containing both VGLUT2-IR and nNOS-IR and the percentage of TRD positive neurons containing both VGLUT3-IR and nNOS-IR were significantly higher than that of TRD negative neurons. On the other hand, colocalization of VGLUT1-IR and nNOS-IR in TRD positive neurons did not differ from that of TRD negative neurons. These results support our hypothesis and suggest prominent roles of VGLUT2-IR containing neurons and VGLUT3-IR containing neurons in transmitting cardiovascular signals via the ADN to the brain stem.

Endomorphin-2 in the medial NTS attenuates the responses to baroreflex activation

We have previously reported that microinjections of endomorphin-2 (E-2; an endogenous mu-receptor agonist) into the medial subnucleus of the NTS (mNTS) elicit depressor and bradycardic responses via activation of ionotropic glutamate receptors located on secondary mNTS-neurons. Based on this report, it was hypothesized that activation of secondary mNTS neurons by E-2 may result in an exaggeration of baroreflex responses. In order to test this hypothesis, baroreflex responses were studied in adult, urethane-anesthetized, artificially ventilated, male Wistar rats before and after the microinjections of E-2 into the mNTS. Baroreceptors were stimulated by applying pressure increments (80-100 mm Hg) in the carotid sinus and by electrical stimulation (stimulus intensity: 0.5 V, frequencies 5, 10, and 25 pulses/s, pulse duration: 1 ms) of the aortic nerve for 30-s periods. Baroreceptor stimulation elicited depressor and bradycardic responses. Microinjections (100 nl) of E-2 (0.4 mmol/l) into the mNTS attenuated the baroreflex responses. Microinjections of naloxone (an opioid receptor antagonist) into the mNTS (0.5 mmol/l) did not alter baroreflex responses. Based on these results, it was concluded that activation of mu-opioid receptors in the mNTS attenuates baroreflex responses. Possible mechanisms for excitatory effects of E-2 in the mNTS resulting in depressor and bradycardic responses, on one hand, and inhibitory effects resulting in attenuation of baroreflex responses, on the other, are discussed.

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.

Immunohistochemical evidence for species-specific coexistence of catecholamines, serotonin, acetylcholine and nitric oxide in glomus cells of rat and guinea pig aortic bodies

The aortic bodies are small paraganglia distributed along the vagus nerve and its branches in the vicinity of the aortic arch which, like the carotid bodies, act as arterial chemoreceptors. In the rat carotid body, corelease of ATP and acetylcholine (ACh) from glomus cells is considered to be the main mechanism mediating fast hypoxic chemotransmission while dopamine, serotonin, and nitric oxide (NO) exert modulating effects. The present study was aimed at determination of the endogenous sources of serotonin, ACh and NO within rat and guinea pig aortic bodies by immunohistochemical double- and triple-labeling approaches, utilizing antibodies to serotonin, the NO and ACh synthesizing enzymes neuronal NO synthase (nNOS) and choline acetyltransferase (ChAT), respectively, as well as to the vesicular acetylcholine transporter (VAChT). Additional marker antibodies were directed against the rate-limiting enzyme of catecholamine synthesis, i.e. tyrosine hydroxylase (TH), and the vesicular protein, synaptophysin (SYN). In both species, all aortic body glomus cells were immunoreactive to serotonin and cholinergic markers. In the rat, all glomus cells were additionally catecholaminergic, as indicated by TH-immunoreactivity, whereas this applied only to a subgroup of guinea pig glomus cells. On the other hand, all guinea pig glomus cells were nNOS-immunoreactive, whereas only nerve fibers but not glomus cells exhibited nNOS-immunoreactivity in the rat. These data support the concept that the chemoexcitatory transmitters ACh and serotonin are involved in hypoxic excitation of aortic chemoreceptor terminals in both species. The production of the inhibitory modulators, dopamine and NO, however, appears to be species-specifically regulated.

Relation between myelin area and axon diameter in the aortic depressor nerve of spontaneously hypertensive rats

The hypothesis that the aortic depressor nerve (ADN) from spontaneously hypertensive rats (SHR) does not show the expected correlation between myelin sheath area and the axonal diameter of myelinated fibers detected in normotensive rat myelinated fibers was tested by means of regression analysis. Proximal and distal segments of ADN from 13 normotensive Wistar-Kyoto rats (WKY) and nine SHR were prepared for light microscopy study. With an image analysis system, the area of the myelin sheath and the axonal diameter of all myelinated fibers in each nerve were automatically measured. Regression lines were calculated for all nerve segments from each group. Differences between the regression lines were tested for slope and intercept and differences between the correlation coefficients were also tested. Regression lines for WKY data showed no differences between the proximal and distal segments either for slope or intercept. Proximal and distal SHR regression lines were not coincident between segments or when compared to WKY data. These results agree with previous observations that there are morphological differences between WKY and SHR myelinated fibers of the ADN suggesting that the SHR depressor nerve fibers present characteristics of axonal atrophy and/or remyelination.

Attenuation of aortic baroreflex responses by microinjections of endomorphin-2 into the rostral ventrolateral medullary pressor area of the rat

The presence of μ-opioid receptors and endomorphins has been demonstrated in the general area encompassing the rostral ventrolateral medullary pressor area (RVLM). This investigation was carried out to test the hypothesis that endomorphins in the RVLM may have a modulatory role in regulating cardiovascular function. Blood pressure and heart rate (HR) were recorded in urethane-anesthetized male Wistar rats. Unilateral microinjections of endomorphin-2 (0.0125–0.5 mmol/l) into the RVLM elicited decreases in mean arterial pressure (16–30 mmHg) and HR (12–36 beats/min), which lasted for 2–4 min. Bradycardia was not vagally mediated. The effects of endomorphin-2 were mediated via μ-opioid receptors because prior microinjections of naloxonazine (1 mmol/l) abolished these responses; the blocking effect of naloxonazine lasted for 15–20 min. Unilateral stimulations of aortic nerve for 30 s (at frequencies of 5, 10, and 25 pulses/s; each pulse 0.5 V and 1-ms duration) elicited depressor and bradycardic responses. These responses were significantly attenuated by microinjections of endomorphin-2 (0.2 and 0.4 mmol/l). The inhibitory effect of endomorphin-2 on baroreflex responses was prevented by prior microinjections of naloxonazine. Microinjections of naloxonazine alone did not affect either baseline blood pressure and HR or baroreflex responses. These results indicate that endomorphin-2 elicits depressor and bradycardic responses and inhibits baroreflex function when injected into the RVLM. These effects are consistent with the known hyperpolarizing effect of opioid peptides on RVLM neurons.

Autonomic cardiovascular responses to hypercapnia in conscious rats: the roles of the chemo- and baroreceptors

The role of the autonomic nervous system, the central and peripheral chemoreceptors, and the arterial baroreceptors was examined in the cardiovascular response to hypercapnia in conscious rats chronically instrumented for the measurement of arterial blood pressure (ABP), heart rate (HR), and renal sympathetic nerve activity (RSNA). Rats were exposed to hypercapnia (6% CO2), and the cardiovascular and autonomic nervous responses in intact and carotid chemo- and/or aortic denervated rats were compared. In intact and carotid chemo-denervated rats, hypercapnia induced significant increases in mean ABP (MABP) and RSNA, and a significant decrease in HR. The HR decrease was reversed by atropine and eliminated by bilateral aortic denervation, which procedure, however, did not affect the MABP or RSNA response. Bilateral carotid chemo-denervation did not affect the baroreflex control of HR, although this control was attenuated by aortic denervation. Hypercapnia did not affect baroreflex sensitivity in intact rats. These results suggest that hypercapnia induces an increase in MABP due to an activation of sympathetic nervous system via central chemoreceptors and a decrease in HR due to a secondary reflex activation of the parasympathetic nervous system via arterial baroreceptors in response to the rise in ABP. In addition, carotid chemoreceptors do not play a major role in the overall cardiovascular response to hypercapnia in conscious rats. The mechanism responsible for the parasympatho-excitation may also involve CO2 induced aortic chemoreceptor simulation.

Arterial chemoreceptors in the superior laryngeal nerve of the rat

Paraganglia resembling the carotid body have been described in the superior laryngeal nerve (SLN) of the rat and the aim of the present study was to determine if this tissue is chemosensitive. We developed a novel isolated SLN preparation superfused with HEPES-buffered Tyrode solution at 35 degrees C in vitro. A glass suction microelectrode was used to record the electrical activity of single SLN units and a micropipette was used to pressure-eject small volumes of sodium cyanide (NaCN; 250-500 ng in 5 microl) near glomus tissue located at the main bifurcation of the SLN. The duration of the NaCN response and the number of spikes evoked after application of NaCN were compared in normoxia and hyperoxia (PO2 > 300 mmHg). Hyperoxia significantly reduced the duration and spike number of the NaCN response and a negative linear correlation existed between PO2 and response duration. In addition, hypoxia (PO2 < 60 mmHg) triggered SLN firing. Therefore, we can conclude that the paraganglia of the SLN are not only morphologically similar to the carotid body but are also excited by similar stimuli.

Modulation of Dural Nociceptor Mechanosensitivity by the Nitric Oxide-Cyclic GMP Signaling Cascade

The role of the nitric oxide (NO)-cGMP signaling cascade in modulation of peripheral nociception is controversial. Although behavioral studies have suggested both pro- and anti-nociceptive effects, little is known about the direct action of this signaling cascade on primary afferent nociceptive neurons that mediate these behaviors. Here, using single-unit recordings, we examined the direct effect of NO-cGMP signaling on spontaneous activity and mechanical responses of nociceptive afferents that innervate the dura mater. We found that the NO donor sodium nitroprusside (SNP), when applied topically to the neuronal receptive field, induced both sensitization and inhibition of the mechanical responses, albeit in different populations of neurons, which could be distinguished based on their baseline mechanical thresholds. SNP, however, did not change the level of spontaneous activity. Administration of the cGMP analogue 8-pCPT-cGMP mimicked only the inhibitory effect. When SNP was co-applied with either an inhibitor of guanylyl cyclase or a cGMP blocker, sensitization never occurred, and the inhibitory effect of SNP could also be blocked. Our findings suggest that NO can either increase or decrease the mechanical responsiveness of nociceptors and that its action might depend, in part, on the baseline level of neuronal excitability. Our results also implicate cGMP in mediating the inhibitory effect of NO.

Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats

Episodic hypoxia elicits a long-lasting augmentation of phrenic inspiratory activity known as long-term facilitation (LTF). We investigated the respective contributions of carotid chemoafferent neuron activation and hypoxia to the expression of LTF in urethane-anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats. One hour after three 5-min isocapnic hypoxic episodes [arterial Po(2) (Pa(O(2))) = 40 +/- 5 Torr], integrated phrenic burst amplitude was greater than baseline in both carotid-denervated (n = 8) and sham-operated (n = 7) rats (P < 0.05), indicating LTF. LTF was reduced in carotid-denervated rats relative to sham (P < 0.05). In this and previous studies, rats were ventilated with hyperoxic gas mixtures (inspired oxygen fraction = 0.5) under baseline conditions. To determine whether episodic hyperoxia induces LTF, phrenic activity was recorded under normoxic (Pa(O(2)) = 90-100 Torr) conditions before and after three 5-min episodes of isocapnic hypoxia (Pa(O(2)) = 40 +/- 5 Torr; n = 6) or hyperoxia (Pa(O(2)) > 470 Torr; n = 6). Phrenic burst amplitude was greater than baseline 1 h after episodic hypoxia (P < 0.05), but episodic hyperoxia had no detectable effect. These data suggest that hypoxia per se initiates LTF independently from carotid chemoafferent neuron activation, perhaps through direct central nervous system effects.

Glutamate circuits in selected medullo-spinal areas regulating cardiovascular function

1. The importance of the medullo-spinal neuronal pools in the regulation of cardiovascular function has been known for a long time. However, important groups of these neurons, interconnections between them and the neurotransmitters released at their projections have been identified with certainty only during the past two decades. 2. Some of the medullo-spinal neuronal pools mediating cardiovascular function include the nucleus tractus solitarius, caudal ventrolateral medullary depressor area, rostral ventrolateral medullary pressor area, nucleus ambiguus and intermediolateral cell column of the thoracolumbar spinal cord. Interactions between these selected neuronal groups and neurotransmitters in the pathways connecting them are discussed in the present short review.

Critical developmental period for hyperoxia-induced blunting of hypoxic phrenic responses in rats

Hypoxic ventilatory and phrenic responses are reduced in adult rats reared in hyperoxia (60% O(2)) for the first month of life but not after hyperoxia as adults. In this study, we identified the developmental window for susceptibility to hyperoxia. Phrenic nerve responses to hypoxia were recorded in anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats (aged 3-4 mo) exposed to 60% O(2) for the first, second, third, or fourth postnatal week. Responses were compared with control rats and with rats exposed to 60% O(2) for the first month of life. Phrenic minute activity (burst amplitude x frequency) increased less during isocapnic hypoxia (arterial PO(2) = 60, 50, and 40 Torr) in rats exposed to hyperoxia for the first or second week, or the first month, of life (P < 0.01 vs. control). Functional impairment caused by 1 wk of hyperoxia diminished with increasing age of exposure (P = 0.005). Adult hypoxic phrenic responses are impaired by 1 wk of hyperoxia during the first and second postnatal weeks in rats, indicating a developmental window coincident with carotid chemoreceptor maturation.

Effects of carotid and aortic chemoreceptor denervation in newborn piglets

The objective of the present study was to test the hypothesis that in neonatal piglets there would be no hypoventilation after sham denervation or aortic denervation (AOD) alone, but there would be transient hypoventilation after carotid body denervation (CBD) and the hypoventilation would be greatest after combined carotid and aortic denervation (CBD+AOD). There was a significant (P < 0.05) hypoventilation in CBD and CBD+AOD piglets denervated at 5, 15, and 25 days of age. The hypoventilation in CBD+AOD piglets denervated at 5 days of age was greater (P < 0.05) than that of all other groups. Conversely, sham-denervated and AOD piglets did not hypoventilate after denervation. Injections of sodium cyanide showed that aortic chemoreceptors were a site of recovery of peripheral chemosensitivity after CBD. This aortic sodium cyanide response was abolished by prior injection of a serotonin 5a receptor blocker. Residual peripheral chemosensitivity after CBD+AOD was localized to the left ventricle. We conclude that 1) aortic chemoreceptors contribute to eupneic breathing in piglets that were carotid denervated at 5 days of age and 2) there are multiple sites of residual peripheral chemosensitivity after CBD.

Frequency response of renal sympathetic nervous activity to aortic depressor nerve stimulation in the anaesthetized rat

1. The contribution of central baroreceptor reflex pathways to the dynamic regulation of sympathetic nervous activity (SNA) has not been properly examined thus far. The aim of this study was to characterize the transfer function of the central arc of the baroreceptor reflex (from baroreceptor afferent activity to SNA) over a wide range of frequencies. 2. In nine baroreceptor-intact and six sino-aortic baroreceptor-denervated rats anaesthetized with urethane, the renal SNA was recorded while applying sinusoidal stimulation to the aortic depressor nerve at 26 discrete frequencies ranging from 0.03 to 20 Hz. At each modulation frequency, cross-power spectrum analysis using a fast Fourier transform algorithm was performed between the stimulation and renal SNA, which provided the transfer function of the central arc. 3. In both baroreceptor intact and denervated rats, the transfer gain increased by a factor of about three between 0.03 and 1 Hz. At higher frequencies, the gain decreased but remained above the static gain of the system up to 12 Hz. There was a slight phase lead up to 0.4 Hz, then a continuously increasing phase lag. A three-element linear model satisfactorily described the experimental transfer function. The model combined a derivative gain (corner frequency approximately 0.15 Hz), an overdamped second-order low-pass filter (natural frequency approximately 1 Hz) and a fixed time delay (approximately 100 ms). 4. These results indicate that the central arc of the baroreceptor reflex shows derivative properties that are essential for compensating the filtering of fast oscillations of baroreceptor afferent activity and thus for the generation of fast oscillations of renal SNA (e.g. those related to the cardiac cycle).

Properties of solitary tract neurones responding to peripheral arterial chemoreceptors

Despite the highly integrated pattern of response evoked by peripheral chemoreceptor stimulation, limited information exists regarding the neurones within the nucleus of the solitary tract that mediate this reflex. Using a working heart-brainstem preparation, we describe evoked synaptic response patterns, some intrinsic membrane properties, location, morphology and axonal projections of physiologically characterised 'chemoreceptive' neurones located in the solitary tract nucleus in the rat. From 172 whole cell recordings, 56 neurones were identified as chemoreceptive since they responded to aortic injections of low doses of sodium cyanide (2-5 microg). Chemoreceptive neurones had a mean resting membrane potential of -52+/-1 mV and input resistance was 297+/-15 M(Omega) (n=56). Synaptic responses evoked included excitatory synaptic potentials alone, excitatory-inhibitory post-synaptic potential complexes, inhibitory synaptic potentials alone and central respiratory modulated synaptic potentials. Synaptic response latency data were obtained by stimulating electrically the solitary tract: the mean excitatory synaptic latency was 5.2+/-0.4 ms (range 2.5-8.0 ms; n=17). Chemoreceptive neurones showed a heterogeneity in their intrinsic membrane properties: neurones displayed either steady state, augmenting or adapting firing responses to depolarising current injection and, in some neurones, either delayed excitation or rebound activity following hyperpolarising pulses. Eleven chemoreceptive neurones were labelled and provided the first morphological data of these cells. Labelled somata were detected dorsomedial or medial to the solitary tract spanning the obex. Neurones typically had three to eight primary dendrites which often entered the solitary tract as well as extending across the ipsilateral region of the nucleus of the solitary tract. Axons were mostly unmyelinated with boutons of the en passant variety and often ramified within the solitary tract nucleus as well as coursed towards the ipsilateral ventral medulla. In summary, this study provides new data on the neurophysiological, anatomical and morphological properties of nucleus of the solitary tract neurones responding to arterial chemoreceptors in the rat.

Mortality after carotid body denervation in rats

Carotid body denervation (CBD) in neonatal goats and piglets results in minimal irregular breathing and no fatalities. Redundancy and/or plasticity of peripheral chemosensitivity and a relatively mature ventilatory control system at birth may contribute to the paucity of CBD effects in these species. In the present study, we tested the hypothesis that CBD mortality would be greater in neonates of a less mature species such as the rat. We found that the mortality in rats denervated at 2-3 and 7-8 days of age was significantly higher (P < 0.05) than in sham-CBD rats. In all surviving rats, pulmonary ventilation during hypoxia was lower in CBD than in sham operated rats 2 days after denervation. In surviving rats denervated during the 7th and 8th postnatal days, there was also reduced weight gain and pulmonary ventilation during eupnea, including apneas up to 20 s in duration. However, the effects of CBD were compensated within 3 wk after denervation. Local injections of NaCN indicated that aortic chemoreceptors might have been one of the sites of recovery of peripheral chemosensitivity. We concluded that CBD has higher mortality in newborn rats than in other mammals, possibly because of the relative immaturity of these animals at birth. Nonetheless, in survivors there was enough redundancy and plasticity in the control of breathing to eventually compensate for the consequences of CBD.

Impaired ventilatory responses to hypoxia in mice deficient in endothelin-converting-enzyme-1

Endothelin-converting-enzyme (ECE-1) catalyzes the proteolytic activation of big endothelin-1 to mature endothelin-1. Most homozygous ECE-1-/- embryos die in utero and show severe craniofacial, enteric, and cardiac malformations precluding ventilatory function assessment. In contrast, heterozygous ECE-1+/- embryos develop normally. Their respiratory function at birth has not been studied. Taking into account previous respiratory investigations in mice with endothelin-1 gene disruption, we hypothesized that ECE-1-deficient mice may have impaired ventilatory control. We analyzed ventilatory responses to hypercapnia (8% CO(2)) and hypoxia (10% O(2)) in newborn and adult mice heterozygous for ECE-1 deficiency (ECE-1+/-) and in their wild-type littermates (ECE-1+/+). Ventilation, breath duration, and tidal volume were measured using whole-body plethysmography. Ventilatory responses to hypoxia were significantly weaker in ECE-1+/- than in ECE-1+/+ newborn mice (percentage ventilation increase: 1 +/- 25% versus 33 +/- 29%, p = 0.010). Baseline breathing variables and ventilatory responses to hypercapnia were normal in the ECE-1+/- newborn mice. No differences were observed between adult ECE-1+/- and ECE-1+/+ mice. We conclude that ECE-1 is required for normal ventilatory response to hypoxia at birth.

Responses of aortic depressor nerve-evoked neurones in rat nucleus of the solitary tract to changes in blood pressure

Using electrophysiological techniques, the discharge of neurones in the nucleus of the solitary tract (NTS) receiving aortic depressor nerve (ADN) inputs was examined during blood pressure changes induced by I.V. phenylephrine or nitroprusside in anaesthetized, paralysed and artificially ventilated rats. Various changes in discharge rate were observed during phenylephrine-induced blood pressure elevations: an increase (n = 38), a decrease (n = 5), an increase followed by a decrease (n = 4) and no response (n = 11). In cells receiving a monosynaptic ADN input (MSNs), the peak discharge frequency response was correlated to the rate of increase in mean arterial pressure (P < 0.01) but was not correlated to the absolute increase in blood pressure. The peak discharge frequency response of cells receiving a polysynaptic ADN input (PSNs) was correlated to neither the absolute increase in blood pressure nor the rate of increase in mean arterial pressure. Diverse changes in discharge rate were observed during nitroprusside-induced reductions in blood pressure: an increase (n = 3), a decrease (n = 10), an increase followed by a decrease (n = 3) and no response (n = 6). Reductions in pressure of 64 +/- 2 mmHg produced weak reductions in spontaneous discharge of 1.3 +/- 0.9 Hz and only totally abolished spontaneous discharge in one neurone. These response patterns of NTS neurones during changes in arterial pressure suggest that baroreceptor inputs are integrated differently in MSNs compared to PSNs. The sensitivity of MSNs to the rate of change of pressure provides a mechanism for the rapid regulation of cardiovascular function. The lack of sensitivity to the mean level of a pressure increase in both MSNs and PSNs suggests that steady-state changes in pressure are encoded by the number of active neurones and not graded changes in the discharge of individual neurones. Both MSNs and PSNs receive tonic excitatory inputs from the arterial baroreceptors; however, these tonic inputs appear to be insufficient to totally account for their spontaneous discharge.

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.

Compensatory vasoconstrictor effects of sodium pentobarbital on the hindquarters of conscious normotensive control and lumbar-sympathectomized Wistar rats

The aim of this study was to compare the vasoconstrictor effect of sodium pentobarbital on the hindquarter resistance of intact control Wistar rats with the effect on lumbar-sympathectomized rats. For this purpose, mean arterial pressure (MAP) and hindquarter (supplied terminal aorta) flow (HQF) were simultaneously measured in these conscious rats with an arterial in dwelling cannula and electromagnetic flow probe implanted around the terminal aorta. Hindquarter resistance (HQR) was calculated as MAP divided by HQF. In the intact control conscious rats, subsequent pentobarbital anesthesia (30 mg/kg, i.v.) caused an increase in HQR (+43.5 +/- 7.4%, mean +/- S.E.M.) and a decrease in MAP (-17.0 +/- 3.2%). After pentobarbital anesthesia, subsequent ganglionic blockade with hexamethonium bromide (C6; 25 mg, i.v.) induced a significant decrease in HQR (-30.9 +/- 3.0%) with a further lowering of MAP (-20.9 +/- 1.6%). However, in rats not anesthetized with sodium pentobarbital, C6 alone induced almost no change in HQR (-3.4 +/- 5.3%), even when MAP was lowered (-24.2 +/- 2.5%). In the lumbar-sympathectomized rats, pentobarbital anesthesia produced almost no change in HQR (-11.7 +/- 4.4%), although MAP decreased significantly (-24.3 +/- 2.2%). These findings suggest that: (1) sodium pentobarbital anesthesia newly generates a compensatory vasoconstrictor tone in the hindquarters acting against the depressor effect, and (2) the vasocompensator tone is controlled by the efferent fibers, including those in the lumbar sympathetic nerves.

Carotid and cardiopulmonary chemoreceptor activity increases hippocampal theta rhythm in conscious rabbits

We have examined whether activation of carotid artery chemoreceptors causes alerting in conscious rabbits. Injection of phenylbiguanide (a 5-hydroxytryptamine(3)-receptor agonist) into the common carotid artery of conscious rabbits increased the proportion of theta rhythm in the hippocampal EEG, commencing in the first 5-s epoch after the injection. Intravenous injection of phenylbiguanide also increased the proportion of theta rhythm in the hippocampal electroencephalogram (EEG), but the onset of the change was not until the second 5-s epoch following injection. Injection of Ringer solution, either into the common carotid artery or into the marginal ear vein, did not affect the hippocampal EEG. Results suggest that phenylbiguanide-mediated activation of carotid and cardiopulmonary chemoreceptor afferents alerts the animal, as assessed by induction of theta rhythm in the hippocampal EEG. This alerting response presumably reflects the action of neural inputs that enter the brain with the carotid sinus nerve at the level of the medulla oblongata.

Ventilatory and central neurochemical reorganisation of O2 chemoreflex after carotid sinus nerve transection in rat

1. The first step of this study was to determine the early time course and pattern of hypoxic ventilatory response (HVR) recovery following irreversible bilateral carotid sinus nerve transection (CSNT). The second step was to find out if HVR recovery was associated with changes in the neurochemical activity of the medullary catecholaminergic cell groups involved in the O2 chemoreflex pathway. 2. The breathing response to acute hypoxia (10% O2) was measured in awake rats 2, 6, 10, 45 and 90 days after CSNT. In a control group of sham-operated rats, the ventilatory response to hypoxia was principally due to increased respiratory frequency. There was a large reduction in HVR in the CSNT compared to the sham-operated rats (-65%, 2 days after surgery). Within the weeks following denervation, the CSNT rats progressively recovered a HVR level similar to the sham-operated rats (-37% at 6 days, -27% at 10 days, and no difference at 45 or 90 days). After recovery, the CSNT rats exhibited a higher tidal volume (+38%) than the sham-operated rats in response to hypoxia, but not a complete recovery of respiratory frequency. 3. Fifteen days after CSNT, in vivo tyrosine hydroxylase (TH) activity had decreased in caudal A2C2 (-35%) and A6 cells (-35%). After 90 days, the CSNT rats displayed higher TH activity than the sham-operated animals in caudal A1C1 (+51%), caudal A2C2 (+129%), A5 (+216%) and A6 cells (+79%). 4. It is concluded that HVR following CSNT is associated with a profound functional reorganisation of the central O2 chemoreflex pathway, including changes in ventilatory pattern and medullary catecholaminergic activity.

Is the aortic depressor nerve involved in arterial chemoreflexes in rats?

Recent anatomical and physiological studies showed that chemoreceptor afferent fibers are present in the rat aortic depressor nerve (ADN), which has been considered to contain exclusively baroreceptor afferent fibers. However, it remains to be proven whether the chemoreceptor afferents of the ADN are practically involved in chemoreflexes. The present study was performed in chloralose/urethane-anesthetized rats of either Sprague-Dawley (SD) or Wistar strain to examine whether the ADN carries sufficient information regarding arterial hypoxia and hypercapnia, and whether the ADN indeed participates in chemoreflexes, the circulatory and respiratory components. It was found in either strain that afferent discharges of the ADN were not affected at all by hypoxia or hypercapnia, whereas those of the carotid sinus nerve (CSN) markedly increased due to these stimuli. Hypoxia produced hypertension, transient bradycardia followed by tachycardia, and respiratory facilitation, which characterize the chemoreflexes. Any of these responses was not affected at all by the ADN section, but all were abolished by the CSN section. Intraaortic injection of cyanide also induced transient bradycardia and respiratory facilitation, but any of them was not affected by the ADN section while all were abolished by the CSN section. Furthermore, electrical stimulation of the ADN produced solely baroreflex responses, i.e. hypotension and respiratory suppression, whereas that of the CSN provoked chemoreflex responses, i.e. early, transient hypertension and respiratory facilitation. In conclusion, the rat ADN does not contain a functionally significant number of chemoreceptor afferent fibers, if at all, and does not appreciably contribute to generation of chemoreflexes.