Glutamate Transmission in the Rostral Ventrolateral Medullary Sympathetic Premotor Pathway

1. The aim of these studies was to test the hypothesis that glutamate is the principal excitatory neurotransmitter in the sympathetic premotor pathway from the rostral ventrolateral medulla (RVLM) to the sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord.2. Iontophoretic and pressure ejection of glutamate receptor agonists and antagonists was made onto antidromically identified splanchnic and adrenal SPNs before and during electrical stimulation of the RVLM in urethane/chloralose-anesthetized, artificially ventilated rats.3. SPNs were excited by both NMDA and non-NMDA glutamate receptor agonists. Blockade of glutamate receptors in the IML interrupted the ability of electrical activation of sympathetic premotor neurons in the RVLM to excite SPNs. Within the IML, antergradely labeled terminals of RVLM neurons were found to contain glutamate immunoreactivity and to make asymmetric synapses on local dendrites.4. These data support a significant role for glutamate neurotransmission in mediating the tonic and phasic excitation of SPNs by the sympathetic premotor pathway from the RVLM. It seems likely that stimulation of the RVLM produces glutamate release from both C1 and non-PNMT-containing axon terminals in the IML.     

延髓头端腹外侧区血管紧张素Ⅱ对脊髓投射神经元氨基酸递质释放的调节

用脊髓(T8)中间外侧柱(IML)微透析方法结合高效液相色谱(HPLC)技术,研究延髓头端腹外侧区(RVLM)微量注射血管紧张素Ⅱ(ANGⅡ,100pmol,n=11)后脊髓IML氨基酸递质释放的变化.在RVLM区微量注射ANGⅡ(100pmol,n=11),能显著增加(P<0.01)脊髓(T8)内天门冬氨酸(ASP,从4.75±1.01升至8.90±2.28pmol/20μl)和谷氨酸(GLU,从18.99±8.64升至73.88±29.26pmol/20μl)的释放.在同一RVLM部位注射losartan(10nmol,n=8)可以显著抑制注射ANGⅡ引起的GLU释放升高反应(P<0.05).免疫荧光双标记结合共聚焦显微镜观察到RVLM内62%～91%的谷氨酸能神经元呈AT1受体免疫阳性.此结果提示ANGⅡ诱发的脊髓内谷氨酸释放可能来源于RVLM内AT1受体免疫阳性的谷氨酸能脊髓投射神经元.     

Coexistence of neurons integrating urinary bladder activity and pelvic nerve activity in the same cardiovascular areas of the pontomedulla in cats

The present study examines the coexistence of neurons in the same cardiovascular point of the pontomedulla that integrates urinary bladder (UB) motility, and pelvic nerve activity (PNA). Microinjection of monosodium L-glutamate (Glu) into the locus coeruleus (LC), the gigantocellular tegmental field (FTG), the rostral ventrolateral medulla (RVLM), and the dorsomedial medulla (DM) produced pressor responses, whereas injection into the lateral tegmental field (FTL), the nucleus of tractus solitarii (NTS), and the caudal ventrolateral medulla (CVLM) produced depressor responses. However, microinjection of Glu into the dorsomotor nucleus of the vagus (DMV) and the ambiguus nucleus (AN), where the vagus nerve originates, produced marked bradycardia. Many of these cardiovascular responses were accompanied by increased, or decreased parasympathetic PNA. In six animals, sympathetic renal nerve activity (RNA) and PNA also increased simultaneously during the pressor response. The present study also examines the connection between the DMV-AN and the sacral intermediolateral column (IML), where parasympathetic preganglionic neurons (PGNs) of the pelvic nerve located. Biotinylated dextran amine (BDA), an anterograde tracer, was iontophoretically injected into the DMV or AN. No labelled terminal or neuron was detected in the sacral IML, but labelled terminals were observed in the bilateral LC, and also in the bilateral sides of the FTG, FTL, RVLM, DM, and CVLM. These results suggest that neurons of the DMV and/or AN may indirectly regulate the sacral parasympathetic PGNs through the LC for supraspinal control of the pelvic nerve. Furthermore, these results also suggest the coexistence of multiple autonomic integrating mechanisms of different kinds within various cardiovascular areas of the pontomedulla.     

Sympathoinhibitory pathway from caudal midline medulla to RVLM is independent of baroreceptor reflex pathway

Glutamate stimulation of the caudal midline medulla (CMM) causes profound sympathoinhibition due to GABAergic inhibition of presympathetic neurons in the rostral ventrolateral medulla (RVLM). We investigated whether the sympathoinhibitory pathway from CMM to RVLM, like the central baroreceptor reflex pathway, includes a glutamatergic synapse in the caudal ventrolateral medulla (CVLM). In pentobarbital sodium-anesthetized rats, the RVLM on one side was inhibited by a muscimol microinjection. Then the response evoked by glutamate microinjections into the CMM or by baroreceptor stimulation was determined before and after 1) microinjection of the GABA receptor antagonist bicuculline into the RVLM on the other side or 2) microinjections of the glutamate receptor antagonist kynurenate bilaterally into the CVLM. Bicuculline in the RVLM greatly reduced both CMM- and baroreceptor-evoked sympathoinhibition. Compared with the effect of vehicle solution, kynurenate in the CVLM greatly reduced baroreceptor-evoked sympathoinhibition, whereas its effect on CMM-evoked sympathoinhibition was not different from that of the vehicle solution. These findings indicate that the output pathway from CMM sympathoinhibitory neurons, unlike the baroreceptor and other reflex sympathoinhibitory pathways, does not include a glutamatergic synapse in the CVLM.     

Sympathoexcitatory CVLM neurons mediate responses to caudal pressor area stimulation

Neurons in the caudal pressor area (CPA) are a source of tonic sympathoexcitation that is dependent on activation of cardiovascular sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM). In the present study, we sought to clarify the mechanism through which CPA neurons elicit increases in RVLM neuronal discharge, vasoconstrictor sympathetic tone, and arterial pressure. In urethan-chloralose-anesthetized, paralyzed, and artificially ventilated rats, bilateral disinhibition of CPA with bicuculline (Bic) after bilateral disinhibition of caudal ventrolateral medulla (CVLM) caused increases in splanchnic sympathetic nerve activity (+277% control) and arterial pressure (+54 mmHg). Inhibition of CVLM neurons with muscimol abolished the pressor response to activation of CPA neurons, suggesting that neurons within CVLM mediate the excitatory responses from CPA. Disinhibition of CVLM and CPA with Bic enhanced the sympathoexcitatory responses to stimulation of CPA with DL-homocysteic acid, which were blocked by microinjections of kynurenic acid into CVLM. We conclude that the pathway from CPA to RVLM involves an obligatory glutamatergic activation of sympathoexcitatory neurons in the vicinity of CVLM.     

Serotonin modulates glutamate action in the medulla to regulate cardiovascular functions in cats

We examined the effects of serotonin (5-HT) on cardiovascular responses and blood flows in the right common carotid artery (RCCA), superior mesenteric artery (SMA) and right femoral artery (RFA), stimulated by glutamate (Glu) in the dorsomedial medulla (DM), rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM). Microinjection of Glu into the DM produced increases in systemic arterial pressure (SAP) and flows in the RCCA and RFA, and decrease in flow in the SMA. Microinjection of Glu into the RVLM produced increases in SAP and decreases in flows in the RCCA, SMA and RFA. Prior microinjections of 5-HT into the same sites attenuated all the Glu-induced responses. Microinjection of Glu into the CVLM produced decreases in SAP and flows in the RCCA, SMA and RFA. These decreases were potentiated by prior injection of 5-HT. These findings suggest that 5-HT modulates the cardiovascular and blood flow responses induced by Glu in the medulla.     

Role of the solitary tract nucleus and caudal ventrolateral medulla in temperature responses in endotoxemic rats

In experiments on conscious rats it was found that preliminary microinjection of 100 nl 100 microM glutamic acid to the rostral commissural part of the solitary tract nucleus or to the caudal ventrolateral medulla increased a rise in colonic temperature induced by systemically applied endotoxin (3 microg/kg Escherichia coli lipopolysaccharide, i.p.) as compared to animals with intrabulbar injection of vehicle (control group). Preliminary microinjection of glutamate to the caudal commissural part of the solitary tract nucleus levelled the endotoxin-induced temperature response. After glutamate treatment of the caudal ventrolateral medulla there was a significant decrease in the noradrenaline content and decrease in the adrenaline level in the caudal (not significant) and rostral ventrolateral medulla (significant), as well as a small rise in noradrenergic activity at the solitary tract nucleus as compared to control animals. The post-mortem measurement of the optical density of brainstem tissues revealed its significant attenuation at the solitary tract nucleus and caudal ventrolateral medulla after glutamate as compared with these structures after vehicle. The involvement of monoaminergic systems of both structures under study in the initiation and control of temperature responses during endotoxemia is suggested.     

Modulatory Inputs on Sympathetic Neurons in the Rostral Ventrolateral Medulla in the Rat

1. The first part of this study looks at spontaneously active neurons located in the rostral ventrolateral medulla (RVLM) with projections to the thoracic spinal cord. Sixteen neurons were intracellularly recorded in vivo. Four out of 16 neurons were antidromically activated from the thoracic spinal cord (axonal conduction velocities varied from 1.8 m/s to 9.5 m/s).2. The simultaneous averages of the neuronal membrane potential and arterial blood pressure triggered by the pulsatile arterial wave or the EKG-R wave demonstrated changes in membrane potential (hyperpolarization or depolarization) locked to the cardiac cycle in four neurons in this group. These neurons (three of them bulbospinal) were further tested for barosensitivity by characterizing the responses to electrical stimulation of the aortic depressor nerve. Four neurons responded with inhibitory hyperpolarizing responses characterized as inhibitory postsynaptic potentials (IPSP) to aortic nerve stimulation (onset latency: 32.3 ± 5.0 ms; mean ± SEM).3. In two neurons in the RVLM, one of them characterized as barosensitive, electrical stimulation of the opposite RVLM (0.5 Hz, 1.0 ms pulse duration, 25–100 A) elicited excitatory postsynaptic potentials (EPSPs) with latencies of 9.07 and 10.5 ms. At resting membrane potential, the onset latency of the evoked EPSPs did not change with increasing stimulus intensities. Some of the recorded neurons were intracellularly labelled with biocytin for visualization. They were found in the RVLM.4. These experiments in vivo would support the idea of a functional commissural pathway between the RVLM of both sides.5. Anatomical data have shown that some of those commissural bundle fibers originate in the C1 adrenergic neuronal group in the RVLM. In the second part of this study, we used an intracellular recording technique in vitro to investigate the effects of the indirect adrenergic agonist tyramine on neurons in the RVLM with electrophysiological properties similar to premotor sympathetic neurons in vivo.6. Tyramine (0.5–1 mM) produced a pronounced inhibitory effect with hyperpolarization and increase in membrane input resistance on two neurons characterized as regularly firing (R), and on one neuron characterized as irregularly firing (I). This effect was preceded by a transient depolarization with increases in firing rate.7. These results would indicate that neurons in the RVLM recorded in vitro and with properties similar to premotor sympathetic neurons can be modulated by catecholamines released from terminals probably making synaptic contacts.     

Contribution to sympathetic vasomotor tone of tonic glutamatergic inputs to neurons in the RVLM

The role of excitatory amino acid (EAA) receptors in the rostral ventrolateral medulla (RVLM) in maintaining resting sympathetic vasomotor tone remains unclear. It has been proposed that EAA receptors in the RVLM mediate excitatory inputs both to presympathetic neurons and to interneurons in the caudal ventrolateral medulla (CVLM), which then provide a counterbalancing inhibition of RVLM presympathetic neurons. In this study, we tested this hypothesis by determining the effect of blockade of EAA receptors in the RVLM on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), after inhibition of CVLM neurons. In anesthetized rats, bilateral injections of muscimol in the CVLM increased MAP, HR, and RSNA. Subsequent bilateral injections of kynurenic acid (Kyn, 2.7 nmol) in the RVLM caused a modest reduction of approximately 20 mmHg in the MAP but had no effect, when compared with the effect of vehicle injection alone, on HR or RSNA. By approximately 50 min after the injections of Kyn or vehicle in the RVLM, the MAP had stabilized at a level close to its original baseline level, but the HR and RSNA stabilized at levels above baseline. The results indicate that removal of tonic EAA drive to RVLM neurons has little effect on the tonic activity of RVLM presympathetic neurons, even when inputs from the CVLM are blocked. Thus the tonic activity of RVLM presympathetic neurons under these conditions is dependent on excitatory synaptic inputs mediated by non-EAA receptors and/or the autoactivity of these neurons.     

Fos,RVLM-projecting neurons,and spinally projecting neurons in the PVN following hypertonic saline infusion

Hypertonic saline (HTS; 1.7 M) infused intravenously into conscious rats increases the production of Fos, a marker of cell activation, in the hypothalamic paraventricular nucleus (PVN). The parvocellular PVN contains subpopulations of neurons. However, which subpopulations are activated by HTS is unknown. We determined whether PVN neurons that innervate the rostral ventrolateral medulla (RVLM) or the spinal cord (important autonomic sites) expressed Fos following HTS. Experiments were performed 24-96 h after chronic implantation of an intravenous cannula. HTS significantly increased the number of Fos-positive cells. In the parvocellular PVN, the maximum number of Fos-positive cells occurred rostral of the anterior-posterior level at which the number of neurons that projected to the medulla or spinal cord peaked. Compared with controls, HTS did not significantly increase the number of double-labeled neurons. These findings demonstrate that an elevation in plasma osmolality activates PVN neurons but not the subgroups of PVN neurons with projections to the RVLM or to the spinal cord.     

Expression of angiotensin II type 1 (AT(1)) receptor in the rostral ventrolateral medulla in rats.

In the present study, the changes of amino acids release in the spinal cord after the application of angiotensin II (ANG II) in the rostral ventrolateral medulla (RVLM) and the distribution of ANG receptors on neurons of the RVLM were investigated. A microdialysis experiment showed that microinjection of angiotensin II into the RVLM significantly (P < 0.01) increased the release of aspartate and glutamate in the intermediolateral column of the spinal cord. Immunofluorescence technique combined with confocal microscopy demonstrated that most of the glutamatergic and GABAergic neurons in the RVLM of both Wistar and spontaneously hypertensive rats (SHR) were double labeled with ANG type 1 (AT1) receptor. Immunocytochemical studies demonstrated that the mean optic density of AT1 receptor of the cell surface as well as the whole cell was higher (P < 0.05) in SHR than that in Wistar rats, indicating that the higher expression of AT1 receptors in the RVLM may contribute to the higher responsiveness of SHR to ANG II stimulation. Immunogold staining and electronmicroscopic study demonstrated that AT1 receptor in the RVLM was distributed on the rough endoplasmic reticulum, cell membrane, and nerve processes. The results suggest that effects evoked by ANG II in the RVLM are closely related to glutamatergic and GABAergic pathways. These results indirectly support the hypothesis that ANG II in the RVLM may activate vasomotor sympathetic glutamatergic neurons, leading to an increase in sympathetic nerve activity and arterial blood pressure.     

Transient middle cerebral artery occlusion and reperfusion alters inducible NOS expression within the ventrolateral medulla and modulates cardiovascular function during static exercise

A major cause of stroke is cerebral ischemia in regions supplied by the middle cerebral artery (MCA). In this study, we hypothesized that compromised cardiovascular function during static exercise may involve altered expression of inducible NOS (iNOS) protein within the rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM). We compared cardiovascular responses and iNOS protein expression within the left and right sides of both RVLM and CVLM in sham-operated rats and in rats with a 90 min left-sided MCA occlusion (MCAO) followed by 24?h of reperfusion. Increases in blood pressure during a static muscle contraction were attenuated in MCAO rats compared with sham-operated rats. Also, iNOS expression within the left RVLM was augmented compared with the right RVLM in MCAO rats and compared with both RVLM quadrants in sham-operated rats. In contrast, compared with sham-operated rats and the right CVLM of MCAO rats, iNOS expression was attenuated in the left CVLM in left-sided MCAO rats. These data suggest that the attenuation of pressor responses during static exercise in MCAO rats involves overexpression of iNOS within the ipsilateral RVLM and attenuation in iNOS within the ipsilateral CVLM. Differential expression of iNOS within the medulla plays a role in mediating cardiovascular responses during static exercise following stroke.     

Increased GABA(A) inhibition of the RVLM after hindlimb unloading in rats

Attenuated baroreflex-mediated increases in renal sympathetic nerve activity (RSNA) in hindlimb unloaded (HU) rats apparently are due to changes within the central nervous system. We hypothesized that GABA(A) receptor-mediated inhibition of the rostral ventrolateral medulla (RVLM) is increased after hindlimb unloading. Responses to bilateral microinjection of the GABA(A) antagonist (-)-bicuculline methiodide (BIC) into the RVLM were examined before and during caudal ventrolateral medulla (CVLM) inhibition in Inactin-anesthetized control and HU rats. Increases in mean arterial pressure (MAP), heart rate (HR), and RSNA in response to BIC in the RVLM were significantly enhanced in HU rats. Responses to bilateral CVLM blockade were not different. When remaining GABA(A) inhibition in the RVLM was blocked by BIC during CVLM inhibition, the additional increases in MAP and RSNA were significantly greater in HU rats. These data indicate that GABA(A) receptor-mediated inhibition of RVLM neurons is augmented after hindlimb unloading. Effects of input from the CVLM were unaltered. Thus, after cardiovascular deconditioning in rodents, the attenuated increase in sympathetic nerve activity in response to hypotension is associated with greater GABA(A) receptor-mediated inhibition of RVLM neurons originating at least in part from sources other than the CVLM.     

Modulation of respiratory responses to chemoreflex activation by L-glutamate and ATP in the rostral ventrolateral medulla of awake rats

Presympathetic neurons in the different anteroposterior aspects of rostral ventrolateral medulla (RVLM) are colocalized with expiratory [B?tzinger complex (B?tC)] and inspiratory [pre-B?tzinger complex (pre-B?tC)] neurons of ventral respiratory column (VRC), suggesting that this region integrates the cardiovascular and respiratory chemoreflex responses. In the present study, we evaluated in different anteroposterior aspects of RVLM of awake rats the role of ionotropic glutamate and purinergic receptors on cardiorespiratory responses to chemoreflex activation. The bilateral ionotropic glutamate receptors antagonism with kynurenic acid (KYN) (8 nmol/50 nl) in the rostral aspect of RVLM (RVLM/B?tC) enhanced the tachypneic (120 ± 9 vs. 180 ± 9 cpm; P < 0.01) and attenuated the pressor response (55 ± 2 vs. 15 ± 1 mmHg; P < 0.001) to chemoreflex activation (n = 7). On the other hand, bilateral microinjection of KYN into the caudal aspect of RVLM (RVLM/pre-B?tC) caused a respiratory arrest in four awake rats used in the present study. Bilateral P2X receptors antagonism with PPADS (0.25 nmol/50 nl) in the RVLM/B?tC reduced chemoreflex tachypneic response (127 ± 6 vs. 70 ± 5 cpm; P < 0.001; n = 6), but did not change the chemoreflex pressor response. In addition, PPADS into the RVLM/B?tC attenuated the enhancement of the tachypneic response to chemoreflex activation elicited by previous microinjections of KYN into the same subregion (188 ± 2 vs. 157 ± 3 cpm; P < 0.05; n = 5). Our findings indicate that: 1) L-glutamate, but not ATP, in the RVLM/B?tC is required for pressor response to peripheral chemoreflex and 2) both transmitters in the RVLM/B?tC are required for the processing of the ventilatory response to peripheral chemoreflex activation in awake rats.     

延髓腹外侧区微量注射L－NNA和SNP对大鼠血压、心率和肾交感神经活动的影响

在麻醉大鼠观察了向延髓腹外侧区微量注射ＮＯ合成酶抑制剂Ｎ－硝基左旋精氨酸（ＬＮＮＡ）和硝普钢（ＳＮＰ）对血压、心率和肾交感神经活动的影响,旨在探讨中枢左旋精氨酸－ＮＯ通路在动脉血压调节中的作用及其机制。实验结果如下：（１）向延髓腹外侧头端区（ＲＶＬＭ）注射Ｌ－ＮＮＡ后,平均动脉压（ＭＡＰ）升高,肾交感神经活动（ＲＳＮＡ）增强;心率（ＨＲ）减慢,但无统计学意义。ＭＡＰ和ＲＳＮＡ的变化持续３０ｍｉｎ以上;此效应可被预先静注左旋精氨酸所逆转。（２）向ＲＶＬＭ微量注射ＳＮＰ,ＭＡＰ降低,ＲＳＮＡ减弱;但ＨＲ的变化无统计学意义。（３）向延髓腹外侧尾端区（ＣＶＬＭ）注射Ｌ－ＮＮＡ,ＭＡＰ降低,ＨＲ减慢,ＲＳＮＡ减弱。（４）向ＣＶＬＭ微量注射ＳＮＰ,ＭＡＰ升高,ＲＳＮＡ增强,而心率无明显变化。以上结果表明,中枢左旋精氨酸－ＮＯ通路对延髓腹外侧部的神经元活动有调变作用。     

Substance P and enkephalin in transected axons of medulla and spinal cord

The accumulation of transported materials in cut axons is demonstrated by the light and electron microscopic immunocytochemical localization of substance P and enkephalin in the caudal medulla and cervical spinal cord of adult rat. Two days following unilateral knife-cuts in the caudal medulla or spinal (C2-C3) levels, substance P and enkephalin-like immunoreactivity (SPLI and ELI) are detected in lesioned axons located rostral and caudal to the transection. Rostrally, SPLI and ELI are detected in the lateral reticular region and ventrolateral fasciculus corresponding to the location of previously identified bulbospinal pathways. Caudally, previously unidentified, propriospinal pathways showing SPLI are detected in the dorsal columns and in the dorsolateral fasciculus. In contrast, ELI is found caudal to the transection only in the reticular region of the medulla. For both peptides, immunoreactivity is present throughout axons containing numerous large, dense core, and small clear vesicles. These results support the concept of both particulate and soluble modes of transport for substance P and enkephalin within axons of the central nervous system.     

Descending pathways mediating cardiovascular response from dorsomedial hypothalamic nucleus

Physiological and anatomic methods were used to determine whether neurons in the rostral ventrolateral medulla (RVLM), nucleus tractus solitarius (NTS), or hypothalamic paraventricular nucleus (PVN) mediate the cardiovascular response evoked from the dorsomedial hypothalamic nucleus (DMH), which is believed to play a key role in mediating responses to stress. In urethane-anesthetized rats, activation of neurons in the DMH by microinjection of bicuculline resulted in a large increase in arterial pressure, heart rate, and renal sympathetic nerve activity. The pressor and sympathoexcitatory responses, but not the tachycardic response, were greatly reduced after bilateral muscimol injections into the RVLM even when baseline arterial pressure was maintained at a constant level. These responses were not reduced by muscimol injections into the PVN or NTS. Retrograde tracing experiments identified many neurons in the DMH that projected directly to the RVLM. The results indicate that the vasomotor and cardiac components of the response evoked from the DMH are mediated by pathways that are dependent and independent, respectively, of neurons in the RVLM.     

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-B?tzinger 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.