Ipsilateral Rostral Ventrolateral Medulla Research Articles

Activation of the Rostral Ventrolateral Medulla Reveals Specialized Control of Sympathetic Nerves Involved in Regulation of Blood Glucose and Blood Pressure

BACKGROUND The sympathetic nervous system mediates the “fight-or-flight” response, including stimulation of the adrenal glands to secrete epinephrine. Epinephrine increases heart rate and contractility and increases blood glucose. Sympathetic nerve activity to functionally distinct targets is regulated differentially by the rostral ventrolateral medulla (RVLM). Therefore, it is important to recognize the numerous roles of the RVLM in control of both blood pressure and blood glucose. Anatomical evidence suggests that innervation of the adrenal gland is from ipsilateral spinal sympathetic neurons and, in turn by RVLM neurons primarily from the ipsilateral side; however, the functional implications of lateralized innervation are lacking. Functional evidence also suggests that withdrawal of tonic GABAergic inhibition in the RVLM during decreases in blood pressure is an important regulatory mechanism in the excitation of sympathetic vasoconstrictor nerves (e.g., splanchnic SNA). We were interested in whether similar neurotransmitter mechanisms mediate hypotension-induced increases in adrenal SNA (AdSNA). Thus, the goals of the present study were to 1) compare effects of contralateral vs. ipsilateral RVLM activation on sSNA and AdSNA, 2) determine the role of RVLM GABAA receptors in hypotension-induced increases in sSNA and AdSNA and 3) investigate functional outcomes in a physiologically relevant model of altered glucose regulation. We hypothesized that activation of the RVLM produces differential increases in sSNA versus AdSNA, given the unique functional role of AdSNA in glucose regulation. METHODS We measured AdSNA and sSNA in male Sprague-Dawley rats during ipsilateral or contralateral glutamatergic activation of the RVLM. We produced decreases in blood pressure (sodium nitroprusside, i.v.) before and after bilateral GABAA receptor blockade (bicuculline, 5mM 90nL). In related experiments, blood glucose was also recorded before and after 2-deoxyglucose (2-DG, i.v.). Lastly, adrenal glands from active (12 wk running wheels) and sedentary animals were analyzed to measure adrenal medulla size. RESULTS Our results indicated that glutamate (100 mM, 30nL) injected into the left RVLM produced increases in ipsilateral AdSNA that were more than twice that of increases following right RVLM (contralateral) injections (p<0.05; n=6). In contrast, sSNA responses were equivalent to glutamate injections on either side of the RVLM. In response to hypotension, both sSNA and AdSNA increased; however, increases in sSNA but not AdSNA were eliminated by bicuculline (p<0.05; n=4). In preliminary experiments, sedentary animals demonstrated potentially greater increases in glucose in response to 2-DG stimulation (p=0.21, n=4) and possibly smaller adrenal medulla size (p=0.142, n=3). CONCLUSION The results confirm previous anatomical findings that the RVLM has predominantly ipsilateral innervation of adrenal nerves. Baroreflex-mediated increases in sSNA but not AdSNA are mediated by GABAA receptors in the RVLM. A deeper understanding of neural control of adrenal function provides insight into comorbidities between cardiovascular and metabolic disease pathologies.

On the presence and functional significance of sympathetic premotor neurons with collateralized spinal axons in the rat.

Spinally-projecting neurons of the rostral ventrolateral medulla (RVLM) determine sympathetic outflow to different territories of the body. Previous studies suggest the existence of RVLM neurons with distinct functional classes, such as neurons that target sympathetic nerves bound for functionally-similar tissue types (e.g. muscle vasculature). The existence of RVLM neurons with more general actions had not been critically tested. Using viral tracing, we show that a significant minority of RVLM neurons send axon collaterals to disparate spinal segments (T2 and T10 ). Furthermore, optogenetic activation of sympathetic premotor neurons projecting to lumbar spinal segments also produced activation of sympathetic nerves from rostral spinal segments that innervate functionally diverse tissues (heart and forelimb muscle). These findings suggest the existence of individual RVLM neurons for which the axons branch to drive sympathetic preganglionic neurons of more than one functional class and may be able to produce global changes in sympathetic activity. We investigate the extent of spinal axon collateralization of rat rostral ventrolateral medulla (RVLM) sympathetic premotor neurons and its functional consequences. In anatomical tracing experiments, two recombinant herpes viral vectors with retrograde tropism and expressing different fluorophores were injected into the intermediolateral column at upper thoracic and lower thoracic levels. Histological analysis revealed that ∼21% of RVLM bulbospinal neurons were retrogradely labelled by both vectors, indicating substantial axonal collateralization to disparate spinal segments. In functional experiments, another virus with retrograde tropism, a canine adenovirus expressing Cre recombinase, was injected into the left intermediolateral horn around the thoracolumbar junction, whereas a Cre-dependent viral vector encoding Channelrhodopsin2 under LoxP control was injected into the ipsilateral RVLM. In subsequent terminal experiments, blue laser light (473nm×20ms pulses at 10mW) was used to activate RVLM neurons that had been transduced by both vectors. Stimulus-locked activation, at appropriate latencies, was recorded in the following pairs of sympathetic nerves: forelimb and hindlimb muscle sympathetic fibres, as well as cardiac and either hindlimb muscle or lumbar sympathetic nerves. The latter result demonstrates that axon collaterals of lumbar-projecting RVLM neurons project to, and excite, both functionally similar (forelimb and hindlimb muscle) and functionally dissimilar (lumbar and cardiac) preganglionic neurons. Taken together, these findings show that the axons of a significant proportion of RVLM neurons collateralise widely within the spinal cord, and that they may excite preganglionic neurons of more than one functional class.

An in vitro experimental model for analysis of central control of sympathetic nerve activity

Newborn rat brainstem-spinal cord preparations are useful for in vitro analysis of various brainstem functions including respiratory activity. When studying the central control of sympathetic nerve activity (SNA), it is important to record peripheral outputs of the SNA. We developed an in vitro preparation in which neuronal connections between the cardiovascular center in the medulla and SNA peripheral outputs are preserved. Zero- to 1-day-old rats were deeply anesthetized with isoflurane, and the brainstem and spinal cord were isolated with a partial right thoracic cage to record sympathetic nerve discharge from the right thoracic sympathetic nerve trunk (T9-T11). SNA in this preparation was strongly modulated by inspiratory activity. Single-shot electrical stimulation of the ipsilateral rostral ventrolateral medulla (RVLM) induced a transient increase of SNA. Bath application of angiotensin II induced an increase of SNA, and local ipsilateral microinjection of angiotensin II to the RVLM induced a transient increase of SNA. This preparation allows analysis of the central control of the SNA in vitro.

Sedentary conditions and enhanced responses to GABA in the RVLM: role of the contralateral RVLM.

A sedentary lifestyle is a major risk factor for cardiovascular disease, and both conditions are associated with overactivity of the sympathetic nervous system. Ongoing discharge of sympathetic nerves is regulated by the rostral ventrolateral medulla (RVLM), which in turn is modulated by the primary excitatory and inhibitory neurotransmitters glutamate and γ-amino-butyric acid (GABA), respectively. We reported previously that sedentary conditions enhance GABAergic modulation of sympathoexcitation in the RVLM, despite overall increased sympathoexcitation. Thus the purpose of this study was to test the hypothesis that sedentary conditions increase responsiveness to GABA in RVLM. Male Sprague-Dawley rats performed either chronic wheeling running or remained sedentary for 12-15 wk. Animals were instrumented to perform RVLM microinjections under Inactin anesthesia while mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) were recorded. Unilateral microinjections of GABA (30 nl, 0.3-600 mM) into the RVLM produced dose-dependent decreases in MAP and SSNA; however, no group differences were observed. Inhibition of the contralateral RVLM (muscimol, 2 mM, 90 nl) caused decreases in MAP and SSNA that were not different between groups but enhanced decreases in SSNA to GABA in sedentary rats only. In sinoaortic denervated rats, GABA microinjections before or after inhibition of the contralateral RVLM caused decreases in MAP and SSNA that were not different between groups. Our results suggest that the contralateral RVLM plays an important role in buffering responses to inhibition of the ipsilateral RVLM under sedentary but not physically active conditions. Based on these studies and others, sedentary conditions appear to enhance both sympathoinhibitory and sympathoexcitatory mechanisms in the RVLM. Enhanced sympathoinhibition may act to reduce already elevated sympathetic nervous system activity following sedentary conditions.

Optogenetic stimulation of PVN‐RVLM neurons increases sympathetic nerve activity in anesthetized rats

The hypothalamic paraventricular nucleus (PVN) sends direct projections to the rostral ventrolateral medulla (RVLM). The present study was conducted to examine the effect of optogenetic stimulation of PVN‐RVLM neurons on sympathetic nerve activity in anesthetized rats. Male rats anesthetized with pentobarbital received unilateral microinjection into the PVN with an adeno‐associated virus (AAV) vector that encodes the reporter protein palGFP (control rats, N=4) or the channel rhodopsin variant, ChIEF‐tdTomato (ChIEF rats, N=8). Two‐to‐five weeks later, in the rats anesthetized with a mixture of urethane and α‐chloralose, photostimulation (473 nm wavelength, 8–10 mW) was provided to the RVLM ipsilateral to the PVN into which AAV had been injected. In ChIEF rats, photostimulation of the RVLM at 40 Hz (5‐ms pulse duration) for 2 min significantly (P &lt; 0.05) evoked renal sympathoexcitatory (+126 ± 20% from baseline, means ± SE) and pressor (+13 ± 1 mmHg) responses while this stimulation had no effect in control rats. Moreover, immunofluorescence staining revealed abundant distribution of PVN‐derived, tdTomato‐labeled axons in the ipsilateral RVLM as compared with that in the contralateral RVLM. Confocal imaging further showed close association of tdTomato‐labeled axons with tyrosine hydroxylase‐positive neurons in the RVLM. These observations suggest that excitation of the PVN‐RVLM monosynaptic pathway elicits sympathoexcitation via, at least in part, stimulation of RVLM catecholaminergic neurons, potentially playing a role in sympathetic adjustments to stress such as exercise.Support or Funding InformationJSPS KAKENHI 15H05367 &amp; 16K15190 (SK); 16H05128 &amp; 15H05932 (KN).

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 24h 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.

GABAergic receptors in rostral ventrolateral medulla mediates the cardiovascular responses to activation of bed nucleus of the stria terminalis in the female rat

The bed nucleus of the stria terminalis (BST) is known to contain estrogen (E)- concentrating neurons. In addition, injections of E into BST have been reported to potentiate the sympathoinhibitory arterial pressure (AP) and heart rate (HR) responses elicited by glutamate (Glu) stimulation. In this study, the effect of GABA-A antagonist receptors, bicuculline methiodide (BMI), in the rostral ventrolateral medulla (RVLM) on the cardiovascular responses to Glu stimulation of BST (0.25 M; 20 nl) was investigated in the chloralose anaesthetized, ovariectomized (OVX; n = 6) or OVX+E treated (30 pg/ml plasma E, n = 7) female Wistar rats. Glu stimulation of the BST elicited decreases in AP (-28.1 ± 3.7 mmHg) and HR (-14.2 ± 3.5 bpm) in the OVX+E animals and in OVX only animals (AP, -22.48 ± 2.68 mmHg; HR, -12.2 ± 2.1 bpm). Microinjection of GABA-A receptor antagonist, BMI (1.0mM, 100nl) into the ipsilateral RVLM reversibly attenuates glutamate induced bradycardia and depressor responses to BST stimulation in both group of animals (OVX; n=6 AP,-8.03 ± 3.15 and HR,-5.8 ± 4.2) but in OVX+E animals only attenuates glutamate induced bradycardia but not depressor response (OVX+E;n=7 AP,-16.0 ± 2.4 and HR, -5.84 ± 2.0). These data suggest RVLM sympathetic premotor neurons contain GABA receptors, which mediate the sympathoinhibitory responses to stimulation of BST in OVX animals.

Glutaminergic receptors in rostral ventrolateral medulla mediate the cardiovascular responses to activation of bed nucleus of the stria terminalis in female rats

The bed nucleus of the stria terminalis (BST) has been known to contain estrogen (E)-concentrating neurons. In addition, injections of E into BST have been reported to potentiate the sympathoinhibitory arterial pressure (AP) and heart rate (HR) responses elicited by glutamate (Glu) stimulation. In this study, the effect of glutamate antagonist receptors in the rostral ventrolateral medulla (RVLM) on the cardiovascular responses to Glu stimulation of BST (0.25 M; 20 nl) was investigated in the chloralose anaesthetized, ovariectomized (OVX; n=6) or OVX+E-treated (30 pg/ml plasma E; n=10) female Wistar rats. Glu stimulation of the BST decreased AP (-15 ± 5 mmHg) and HR (-25.23± 2 bpm) in the OVX+E animals and in OVX only animals (AP, -29.78 ± 4.17 mmHg; HR, -17.3 ± 3.1 bpm). Microinjection of glutamate antagonist receptors, kynurenic acid (5.0 mM, 100nl) into the ipsilateral RVLM reversibly attenuated glutamate-induced bradycardi and depressor responses to BST stimulation in OVX only animals (OVX; n=8 AP,-13.7±2.6 and HR,-8.1±4.3) but in OVX+E animals only attenuated glutamate induced bradycardia, but not depressor response (OVX+E;n=7 AP,-11.7±3.8 and HR, -16.7±2.3). These data suggest that RVLM sympathetic premotor neurons contain glutaminergic receptors mediate the sympathoinhibitory responses to stimulation of BST in OVX animals.

RVLM glycine receptors mediate GABA A and GABA B independent sympathoinhibition from CVLM in rats

The caudal ventrolateral medulla (CVLM) provides tonic inhibitory and also excitatory inputs to the rostral ventrolateral medulla (RVLM). These experiments evaluated the role of RVLM γ-amino butyric acid (GABA) receptor subtypes and glycine receptors in mediating CVLM sympathoinhibition. In Inactin anesthetized female rats, the CVLM and RVLM were functionally defined by pressor and depressor responses to microinjected GABA (500 pmol, 50 nl). Although reduced, pressor and sympathoexcitatory responses due to inhibition of the CVLM with GABA persisted following ipsilateral RVLM GABA A receptor blockade (bicuculline, BIC, 400 pmol, 100 nl; n = 12) in rats with contralateral nucleus tractus solitarius (NTS) lesion. In the presence of either ipsilateral (+ contralateral NTS lesion; n = 8) or bilateral ( n = 6) GABA A and GABA B receptor blockade of the RVLM (400 pmol BIC + 400 pmol CGP35348, 100 nl), inhibition of the CVLM still increased MAP and renal sympathetic nerve activity (RSNA). Thus neither GABA B receptors nor a contralateral CVLM to RVLM GABAergic pathway explains residual responses to CVLM blockade. The addition of strychnine (300 pmol, 100 nl) to the RVLM eliminated responses to CVLM inhibition, suggesting that a GABA A and GABA B independent sympathoinhibitory influence from CVLM to RVLM is mediated by glycine receptors. Decreases in MAP and RSNA due to activation of the CVLM with glutamate (500 pmol, 50 nl) were reversed to increases in the presence of RVLM GABA A receptor blockade ( n = 7). Thus, a sympathoexcitatory pathway from the CVLM can be activated in the presence of RVLM GABA receptor blockade, but sympathoinhibitory influences from the CVLM predominate.

Angiotensin II Excites Paraventricular Nucleus Neurons That Innervate the Rostral Ventrolateral Medulla: An In Vitro Patch-Clamp Study in Brain Slices

Neurons of the hypothalamic paraventricular nucleus (PVN) are key controllers of sympathetic nerve activity and receive input from angiotensin II (ANG II)-containing neurons in the forebrain. This study determined the effect of ANG II on PVN neurons that innervate in the rostral ventrolateral medulla (RVLM)-a brain stem site critical for maintaining sympathetic outflow and arterial pressure. Using an in vitro brain slice preparation, whole cell patch-clamp recordings were made from PVN neurons retrogradely labeled from the ipsilateral RVLM of rats. Of 71 neurons tested, 62 (87%) responded to ANG II. In current-clamp mode, bath-applied ANG II (2 muM) significantly (P < 0.05) depolarized membrane potential from -58.5 +/- 2.5 to -54.5 +/- 2.0 mV and increased the frequency of action potential discharge from 0.7 +/- 0.3 to 2.8 +/- 0.8 Hz (n = 4). Local application of ANG II by low-pressure ejection from a glass pipette (2 pmol, 0.4 nl, 5 s) also elicited rapid and reproducible excitation in 17 of 20 cells. In this group, membrane potential depolarization averaged 21.5 +/- 4.1 mV, and spike activity increased from 0.7 +/- 0.4 to 21.3 +/- 3.3 Hz. In voltage-clamp mode, 41 of 47 neurons responded to pressure-ejected ANG II with a dose-dependent inward current that averaged -54.7 +/- 3.9 pA at a maximally effective dose of 2.0 pmol. Blockade of ANG II AT1 receptors significantly reduced discharge (P < 0.001, n = 5), depolarization (P < 0.05, n = 3), and inward current (P < 0.01, n = 11) responses to locally applied ANG II. In six of six cells tested, membrane input conductance increased (P < 0.001) during local application of ANG II (2 pmol), suggesting influx of cations. The ANG II current reversed polarity at +2.2 +/- 2.2 mV (n = 9) and was blocked (P < 0.01) by bath perfusion with gadolinium (Gd(3+), 100 muM, n = 8), suggesting that ANG II activates membrane channels that are nonselectively permeable to cations. These findings indicate that ANG II excites PVN neurons that innervate the ipsilateral RVLM by a mechanism that depends on activation of AT1 receptors and gating of one or more classes of ion channels that result in a mixed cation current.

Involvement of I 1-imidazoline receptors in baroreceptor reflex in the caudal ventrolateral medulla of rats

There is ample evidence to show the existence of center I 1-imidazoline receptors that are involved in the regulation of cardiovascular activities. The purpose of this study was to examine the possible role of I 1-imidazoline receptors and α 2-adrenoceptors within the caudal ventrolateral medulla (CVLM) in mediating the baroreceptor reflex in anesthetized rats. Unilateral microinjection of idazoxan (2 nmol in 50 nl), a mixed antagonist of I 1-imidazoline receptors and α 2-adrenoceptors, into the CVLM significantly ( P<0.01) decreased blood pressure (BP), heart rate (HR), and the firing rate of presympathetic neurons in the rostral ventrolateral medulla (RVLM) by 21±6 mmHg, 25±5 beats per min and 3.5±0.9 spikes/s, respectively. Moreover, unilateral injection of idazoxan into the CVLM significantly ( P<0.01) reduced the inhibitory responses of the ipsilateral RVLM presympathetic neurons evoked by stimulation of aortic nerve and elevation of BP, and partially inhibited the neuronal cardiac cycle-related rhythm. Depressor responses evoked by aortic nerve stimulation were significantly ( P<0.01) attenuated 10 and 20 min after bilateral microinjection of idazoxan (2 nmol in 50 nl for each side) into the CVLM (−20±4 and −30±4 vs. −40±1 mmHg). However, injection of yohimbine (500 pmol in 50 nl), a selective α 2-adrenoceptor antagonist, into the CVLM did not affect the resting cardiovascular activities and baroreceptor reflex. It is concluded that the CVLM I 1-imidazoline receptors are involved in maintenance of tonic cardiovascular activities and transmission of the baroreceptor reflex.

Pressor response to unilateral carotid chemoreceptor activation is not affected by ipsilateral antagonism of excitatory amino acid receptors in the rostral ventrolateral medulla of awake rats

The importance of the integrity of the ipsilateral rostral ventrolateral medulla (RVLM) in the pressor response to activation of unilateral arterial chemoreceptors was evaluated. To achieve this goal, the left carotid body artery was ligated prior to the experiment and a guide cannula was implanted in the direction of the right RLVM, i.e. the side where the carotid body artery was kept intact. On the day of the experiment, the chemoreflex was activated with potassium cyanide (KCN, i.v.) before and after unilateral microinjection of kynurenic acid into the rostral or caudal aspect of the RVLM. The data indicated that microinjection of kynurenic into the rostral or caudal aspect of the RVLM produced no effect on the pressor response of chemoreflex activation. These data suggest that chemoreflex activation excites a neuronal network in which the processing of the sympatho-excitatory component of the chemoreflex is not restricted to an excitatory projection from the nucleus tractus solitarii to the ipsilateral RVLM.

Involvement of the ipsilateral rostral ventrolateral medulla in the pressor response to l-glutamate microinjection into the nucleus tractus solitarii of awake rats

Microinjection of l-glutamate into the lateral commissural nucleus tractus solitarii (NTS) of unanesthetized rats evokes increases in mean arterial pressure (MAP) and a bradycardia. In a previous study we verified that this increase in MAP is mediated sympathetically because prazosin (iv) blocks this response. The aim of the present study was to evaluate the role of the rostral ventrolateral medulla (RVLM) in the pressor response produced by l-glutamate microinjected into the NTS of unanesthetized rats. l-Glutamate was microinjected into the NTS before and 15 and 90 min after microinjection of kynurenic acid into the ipsilateral RVLM. Pressor (+24±3 vs. +6±3 mm Hg) and bradycardic (−101±10 vs. −3±12 bpm) responses to l-glutamate microinjected into the NTS ( n=8) were almost abolished 15 min after microinjection of kynurenic acid into the RVLM when compared with control responses. Both pressor (+23±6 mm Hg) and bradycardic (−93±16 bpm) responses to l-glutamate into the NTS returned to control values 90 min after microinjection of kynurenic acid into the RVLM. These data indicate that the pressor response to l-glutamate into the NTS is essentially dependent on the ipsilateral RVLM and also that this sympatho-excitatory response is mediated by excitatory amino acid receptors in RVLM neurons.

Endogenous angiotensin within the rostral ventrolateral medulla facilitates the somatosympathetic reflex

To determine whether endogenous angiotensin modifies the synaptic excitation of sympatho-excitatory neurons in the rostral part of the ventrolateral medulla. Experiments were performed on anaesthetized rabbits with denervated arterial and cardiopulmonary baroreceptors. Arterial pressure, heart rate and renal sympathetic nerve activity were measured. The average sympatho-excitatory reflex response evoked by short-train stimulation of the sciatic nerve was measured before and at various times after micro-injection of the non-specific angiotensin receptor antagonist [Sar1,Thr8]-angiotensin II (80 pmol) into the contralateral rostral ventrolateral medulla. Because the central pathway mediating this somatosympathetic reflex includes a synapse within the contralateral (but not ipsilateral) rostral ventrolateral medulla, a change in the evoked response after blockade of angiotensin receptors in the contralateral rostral ventrolateral medulla indicates a role of these receptors in synaptic transmission within this region. As a control, the effects on this reflex of micro-injection of [Sar1,Thr8]-angiotensin II into the ipsilateral rostral ventrolateral medulla were also measured. After injection of [Sar1,Thr8]-angiotensin II into the contralateral rostral ventrolateral medulla, the evoked sympatho-excitatory reflex response was significantly reduced compared with the response before injection. In contrast, injection of [Sar1,Thr8]-angiotensin II into the ipsilateral rostral ventrolateral medulla had no significant effect on the evoked sympatho-excitatory reflex response, although contralateral and ipsilateral injections of [Sar1,Thr8]-angiotensin II had very similar effects on baseline arterial pressure and renal sympathetic nerve activity. The results indicate that, in the anaesthetized rabbit with denervated arterial and cardiopulmonary baroreceptors, endogenous angiotensin is tonically released within the rostral ventrolateral medulla and facilitates the synaptic excitation of sympatho-excitatory neurons reflexly evoked by stimulation of afferent fibres in the sciatic nerve.

Pressor response from rostral dorsomedial medulla is mediated by excitatory amino acid receptors in rostral VLM

Excitatory amino acid (EAA) receptors in the rostral part of the ventrolateral medulla (VLM) have been shown to mediate pressor responses elicited by stimulation of various peripheral afferent fibers as well as other central nuclei. This study tested the hypothesis that these receptors are a critical component in the central pathway mediating the powerful pressor response that is produced by stimulation of a group of neurons within a circumscribed region in the rostral dorsomedial medulla (RDM). In anesthetized rabbits, the pressor response elicited by unilateral microinjection of glutamate into this RDM region was measured before and after injection of kynurenic acid (Kyn), a broad-spectrum EAA receptor antagonist, into the physiologically identified pressor region of either the ipsilateral or contralateral rostral VLM. The pressor response to RDM stimulation was greatly reduced (to 24 +/- 4% of control) 5-10 min after injection of Kyn (but not the vehicle solution) into the ipsilateral rostral VLM; this response returned completely to its control value within 30-60 min after Kyn injection. By contrast, after Kyn injection into the contralateral rostral VLM, the pressor response to RDM stimulation was not affected (106 +/- 15% of control). The results indicate that the descending pressor pathway from the RDM to the spinal cord is mediated by EAA receptors in the rostral VLM pressor region. Furthermore, the pathway from the RDM to the rostral VLM is predominantly, if not exclusively, ipsilateral.

Monosynaptic connection from caudal to rostral ventrolateral medulla in the baroreceptor reflex pathway

Experiments were done to test the hypothesis that caudal ventrolateral medulla (CVLM) neurons excited by activation of arterial baroreceptors and by stimulation of depressor sites in the nucleus tractus solitarii (NTS) project monosynaptically to the rostral ventrolateral medulla (RVLM). In urethan anaesthetized and artificially ventilated rats we recorded extracellular activity from 46 spontaneously firing units in the CVLM. Twenty of these units were excited by baroreceptor activation (1–3 μg phenylephrine i.v.) and of these 6 were excited (mean latency of9.8 ± 2.3ms) by single pulses (0.1 ms,30 ± 8.3 μA) delivered once per second to a depressor site in the ipsilateral NTS. These 6 units were also antidromically activated with a latency of4.1 ± 0.12ms by stimulation of a pressor region in the ipsilateral RVLM. These results provide evidence for the existence of an excitatory projection from the NTS to the CVLM which, in turn, projects monosynaptically to sympathoexcitatory neurons in the RVLM.

Cardiovascular responses to combined microinjection of substance P and acetylcholine in the intermediolateral nucleus of the rat

As microinjection of either substance P (SP) or acetylcholine (ACh) into the right intermediolateral cell nucleus (IML) at the T2 level elicits increases in heart rate (HR) in the anesthetized rat, we investigated the possibility of a synergistic effect on HR and arterial pressure (AP) of ACh and SP microinjected in this nucleus. Moreover, we studied the effect on HR and AP of microinjection of either ACh or SP into the IML combined with activation of cardiovascular neurons in the ipsilateral rostral ventrolateral medulla (RVLM) by microinjection of glutamate (Glu). Male Wistar rats (n = 16) were anesthetized with urethane (1.4 g/kg i.p.), artificially ventilated, and the dorsal medulla and spinal cord (T1-T3) were exposed. Micropipettes containing SP and ACh were positioned in the right IML at the T2 level. Microinjection of threshold amounts of ACh (5 x 10(-2) M, 2-10 nl) and SP (3 x 10(-6) M, 2-10 nl) that caused small or no changes in HR or AP (less than 10 bpm or mmHg) elicited statistically significant synergistic increases in HR (22.9 +/- 3.3 bpm) but no changes in AP. Threshold microinjections of Glu (0.18 M, 2-10 nl) into the right RVLM combined with microinjections of threshold amounts of SP or ACh into the ipsilateral IML elicited significant synergistic increases in HR of 13.1 +/- 1.9 bpm and 10.6 +/- 1.9 bpm and in AP of 9.7 +/- 1.9 mmHg and 10.8 +/- 1.7 mmHg, respectively. These results indicate that SP and ACh interact to influence cardioacceleratory spinal preganglionic neurons (SPN) and interact with the transmitter released in the IML by RVLM stimulation to elicit increases in HR.(ABSTRACT TRUNCATED AT 250 WORDS)