Modulation of extracellular glutamate and pressor response to muscle contraction during NMDA-receptor blockade in the rostral ventrolateral medulla

Sex differences in NMDA GluN1 plasticity in rostral ventrolateral medulla neurons containing corticotropin-releasing factor type 1 receptor following slow-pressor angiotensin II hypertension

There are profound, yet incompletely understood, sex differences in the neurogenic regulation of blood pressure. Both corticotropin signaling and glutamate receptor plasticity, which differ between males and females, are known to play important roles in the neural regulation of blood pressure. However, the relationship between hypertension and glutamate plasticity in corticotropin-releasing factor (CRF)-receptive neurons in brain cardiovascular regulatory areas, including the rostral ventrolateral medulla (RVLM) and paraventricular nucleus of the hypothalamus (PVN), is not understood. In the present study, we used dual-label immuno-electron microscopy to analyze sex differences in slow-pressor angiotensin II (AngII) hypertension with respect to the subcellular distribution of the obligatory NMDA glutamate receptor subunit 1 (GluN1) subunit of the N-methyl-D-aspartate receptor (NMDAR) in the RVLM and PVN. Studies were conducted in mice expressing the enhanced green fluorescence protein (EGFP) under the control of the CRF type 1 receptor (CRF1) promoter (i.e., CRF1-EGFP reporter mice). By light microscopy, GluN1-immunoreactivity (ir) was found in CRF1-EGFP neurons of the RVLM and PVN. Moreover, in both regions tyrosine hydroxylase (TH) was found in CRF1-EGFP neurons. In response to AngII, male mice showed an elevation in blood pressure that was associated with an increase in the proportion of GluN1 on presumably functional areas of the plasma membrane (PM) in CRF1-EGFP dendritic profiles in the RVLM. In female mice, AngII was neither associated with an increase in blood pressure nor an increase in PM GluN1 in the RVLM. Unlike the RVLM, AngII-mediated hypertension had no effect on GluN1 localization in CRF1-EGFP dendrites in the PVN of either male or female mice. These studies provide an anatomical mechanism for sex-differences in the convergent modulation of RVLM catecholaminergic neurons by CRF and glutamate. Moreover, these results suggest that sexual dimorphism in AngII-induced hypertension is reflected by NMDA receptor trafficking in presumptive sympathoexcitatory neurons in the RVLM.

Physical (in)activity-dependent alterations at the rostral ventrolateral medulla: influence on sympathetic nervous system regulation

A sedentary lifestyle is a major risk factor for cardiovascular disease, and rates of inactivity and cardiovascular disease are highly prevalent in our society. Cardiovascular disease is often associated with overactivity of the sympathetic nervous system, which has both direct and indirect effects on multiple organ systems. Although it has been known for some time that exercise positively affects the brain in terms of memory and cognition, only recently have changes in how the brain regulates the cardiovascular system been examined in terms of physical activity and inactivity. This brief review will discuss the evidence for physical activity-dependent neuroplasticity related to control of sympathetic outflow. It will focus particularly on recent studies from our laboratory and others that have examined changes that occur in the rostral ventrolateral medulla (RVLM), considered one of the primary brain regions involved in the regulation and generation of sympathetic nervous system activity.

Angiotensin II type-2 (AT2) receptor antagonism alters cardiovascular responses to static exercise and simultaneously changes glutamate/GABA levels within the ventrolateral medulla

Angiotensin II receptors (Ang II), classified into AT1 and AT2 subtypes, are located in different regions of the central nervous system, including the cardiovascular control centers in the medulla oblongata. We previously reported the role of Ang II AT1 receptors within the medulla on cardiovascular responses and glutamate/GABA neurotransmission during the exercise pressor reflex [Patel, D., Böhlke, M., Phattanarudee, S., Kabadi, S., Maher, T.J., Ally, A., 2008. Cardiovascular responses and neurotransmitter changes during blockade of angiotensin II receptors within the ventrolateral medulla. Neurosci. Res. 60 (3), 340-348]. In this study, we investigated the role of the AT2 receptor subtype within the ventrolateral medullary region (VLM) in modulating increases in mean arterial pressure (MAP) and heart rate (HR) in response to static skeletal muscle contraction.

METHODS

Using microdialysis methods in anesthetized rats, we administered AR-AT2 antagonists into the rostral (RVLM) and caudal (CVLM) VLM and determined its effects on cardiovascular responses and glutamate/GABA neurotransmission following muscle contraction. Bilateral microdialysis of a selective AT2 antagonist, PD 123319 (10 microM), for 30 min into the RVLM augmented MAP and HR responses during a static muscle contraction. Simultaneously, the drug increased glutamate and decreased GABA levels within the RVLM. After 60 min of discontinuation of the drug, only MAP and HR values but not the neurotransmitter levels in response to a muscle contraction returned to baseline. In contrast, bilateral microdialysis of the drug into the CVLM attenuated cardiovascular responses during a static muscle contraction, decreased glutamate and increased GABA. However, only the cardiovascular responses recovered after 60 min of discontinuation of the drug. These results demonstrate that AT2 within both RVLM and CVLM plays important differential roles in modulating neurotransmission and cardiovascular function during the exercise pressor reflex.

Cardiovascular responses and neurotransmitter changes during blockade of angiotensin II receptors within the ventrolateral medulla

Angiotensin II (Ang II) receptors are located in different regions of the brain, particularly within the cardiovascular control centers in the brainstem. These Ang II receptors are divided into AT1 and AT2 subtypes. We investigated the role of AT1 receptor subtype within the rostral (RVLM) and caudal (CVLM) ventrolateral medulla on cardiovascular responses and glutamate/GABA neurotransmission during static exercise using microdialysis in anesthetized rats. Bilateral microdialysis of a selective AT1 receptor antagonist, ZD7155 (10 microM), for 30 min into the RVLM attenuated increases in mean arterial pressure (MAP) and heart rate (HR) during a static muscle contraction. Glutamate concentrations within the RVLM decreased while GABA levels increased simultaneously during the contraction period when compared to those before ZD7155. After 60 min of discontinuation of ZD7155, MAP, HR, glutamate, and GABA levels in response to another muscle contraction returned to baseline levels. Conversely, bilateral microdialysis of ZD7155 into the CVLM potentiated cardiovascular responses during a static muscle contraction; glutamate concentrations increased while GABA levels within the CVLM decreased. All responses recovered after 60 min of discontinuation of ZD7155. These results demonstrate that medullary AT1 receptors play an important role in modulating both neurotransmission and cardiovascular function during static exercise.

Neuronal Nitric Oxide Synthase (nNOS) blockade within the ventrolateral medulla differentially modulates cardiovascular responses and nNOS expression during static skeletal muscle contraction

Nitric oxide (NO) is synthesized from L-arginine through the activity of the enzyme, NO synthase (NOS). Previous studies have demonstrated the role of the 3 isoforms of NOS, namely endothelial NOS (eNOS), neuronal NOS (nNOS), and inducible NOS (iNOS) in cardiovascular regulation. Local blockade of nNOS in RVLM vs. CVLM differentially alters local glutamate and GABA release, and thereby results in opposite cardiovascular responses to static muscle contraction (Brain Res. 2003, 977, 80-89). In this study, we examined whether nNOS antagonism within the RVLM and CVLM affected cardiovascular responses during the exercise pressor reflex and simultaneously modulated medullary nNOS protein expression using anesthetized rats. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM, 1.0 microM) for 120 min into the RVLM, potentiated cardiovascular responses during a static muscle contraction. Western blot analysis of nNOS expression within the RVLM showed significant attenuation of the protein when compared to the data obtained from control animals microdialyzed with vehicle. In contrast, bilateral application of TRIM into the CVLM attenuated cardiovascular responses during muscle contractions and increased nNOS protein expression within the CVLM. These results demonstrated that nNOS protein expression within the brainstem was pharmacologically altered by nNOS blockade within the RVLM or CVLM, which in turn might have contributed to the augmentation or attenuation of cardiovascular responses, respectively, during static exercise.

Cardiovascular responses and neurotransmitter changes during static muscle contraction following blockade of inducible nitric oxide synthase (iNOS) within the ventrolateral medulla

The enzyme nitric oxide synthase (NOS) which is necessary for the production of nitric oxide from L-arginine exists in three isoforms: neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). Our previous studies have demonstrated the roles of nNOS and eNOS within the rostral (RVLM) and caudal ventrolateral medulla (CVLM) in modulating cardiovascular responses during static skeletal muscle contraction via altering localized glutamate and GABA levels (Brain Res. 977 (2003) 80-89; Neuroscience Res. 52 (2005) 21-30). In this study, we investigated the role of iNOS within the RVLM and CVLM on cardiovascular responses and glutamatergic/GABAergic neurotransmission during the exercise pressor reflex. Bilateral microdialysis of a selective iNOS antagonist, aminoguanidine (AGN; 1.0 microM), for 60 min into the RVLM attenuated increases in mean arterial pressure (MAP), heart rate (HR), and extracellular glutamate levels during a static muscle contraction. Levels of GABA within the RVLM were increased. After 120 min of discontinuation of the drug, MAP and HR responses and glutamate/GABA concentrations recovered to baseline values during a subsequent muscle contraction. In contrast, bilateral application of AGN (1.0 microM) into CVLM potentiated cardiovascular responses and glutamate concentration while attenuating levels of GABA during a static muscle contraction. All values recovered after 120 min of discontinuation of the drug. These results demonstrate that iNOS within the ventrolateral medulla plays an important role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission that regulates the exercise pressor reflex.

Role of glutamate in a visceral sympathoexcitatory reflex in rostral ventrolateral medulla of cats

The rostral ventrolateral medulla (rVLM) is involved in processing visceral sympathetic reflexes. However, there is little information on specific neurotransmitters in this brain stem region involved in this reflex. The present study investigated the importance of glutamate and glutamatergic receptors in the rVLM during gallbladder stimulation with bradykinin (BK), because glutamate is thought to function as an excitatory neurotransmitter in this region. Stimulation of visceral afferents activated glutamatergic neurons in the rVLM, as noted by double-labeling with c-Fos and the cellular vesicular glutamate transporter 3 (VGLUT3). Visceral reflex activation significantly increased arterial blood pressure as well as extracellular glutamate concentrations in the rVLM as determined by microdialysis. Barodenervation did not alter the release of glutamate in the rVLM evoked by visceral reflex stimulation. Iontophoresis of glutamate into the rVLM enhanced the activity of sympathetic premotor cardiovascular rVLM neurons. Also, the responses of these neurons to visceral afferent stimulation with BK were attenuated significantly (70%) by blockade of glutamatergic receptors with kynurenic acid. Microinjection of either an N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanate (25 mM, 30 nl) or an dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (2 mM, 30 nl) into the rVLM significantly attenuated the visceral sympathoexcitatory reflex responses. These results suggest that glutamate in the rVLM serves as an excitatory neurotransmitter through a baroreflex-independent mechanism and that both NMDA and AMPA receptors mediate the visceral sympathoexcitatory reflex responses.

Attenuated pressor responses to amino acids in the rostral ventrolateral medulla after swimming training in conscious rats

The cardiovascular effects of microinjection of the amino acids glutamate and glycine within the rostral ventrolateral medulla (RVLM) after swimming training (ST) in unrestrained awake rats were investigated. Unilateral microinjection of l-glutamate (5, 20 and 50 mM, in 100 nl) produced a dose dependent increase in mean arterial pressure (MAP) in control (C) (16+/-5 mm Hg; 29+/-6 mm Hg; 43+/-6 mm Hg) and swim (SW) (1+/-1 mm Hg; 16+/-2 mm Hg; 25+/-3 mm Hg) groups. However, the magnitude of this response was lower in the swim group. Prazosin injection produced hypotension and tachycardia in both groups (C=-43+/-3 mm Hg/98+/-16 bpm; SW=-61+/-5 mm Hg/115+/-32 bpm). In the SW group the hypotension caused by prazosin was greater compared to C group, but the tachycardia was not different between them. After prazosin, glutamate response in RVLM was blocked in both groups as well. When glycine (10 mM or 1 M, in 100 nl) were microinjected into the RVLM of C group we observed two different effects: decrease in MAP with the lower dose and an increase in MAP with the higher dose (10 mM=-13+/-2 mm Hg; 1 M=47+/-6 mm Hg). However, after ST the hypertensive response to glycine was blunted with no alterations in the hypotensive response (10 mM=-14+/-1 mm Hg; 1 M=18+/-4 mm Hg). These findings suggest that RVLM is involved in the modulation of the sympathetic outflow to the cardiovascular system during exercise training.

Neurochemistry within ventrolateral medulla and cardiovascular effects during static exercise following eNOS antagonism

Nitric oxide synthase (NOS), necessary for the production of nitric oxide from l-arginine, exists in three isoforms: neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). We have previously demonstrated that blockade of nNOS within the rostral (RVLM) and caudal ventrolateral medulla (CVLM) differentially modulated cardiovascular responses to static exercise [Ishide, T., Nauli, S.M., Maher, T.J., Ally, A., 2003. Cardiovascular responses and neurotransmitter changes following blockade of nNOS within the ventrolateral medulla during static muscle contraction. Brain Res. 977, 80-89]. In this study, we have examined the effects of bilaterally microdialyzing a specific eNOS antagonist into the RVLM and CVLM on cardiovascular responses and glutamatergic/GABAergic neurotransmission during the exercise pressor reflex in anesthetized rats. Bilateral microdialysis of a selective eNOS antagonist, l-N(5)-(1-iminoethyl)ornithine (l-NIO; 10.0 microM) into the RVLM potentiated cardiovascular responses and increased extracellular fluid glutamate levels during a static muscle contraction. At the same time, levels of GABA within the RVLM were decreased. The cardiovascular responses and neurochemical changes to muscle contraction recovered after discontinuation of the drug. In contrast, bilateral application of the eNOS antagonist into the CVLM attenuated cardiovascular responses and glutamate concentrations during a static muscle contraction, but augmented levels of GABA. These results demonstrate that eNOS within the ventrolateral medulla plays an important role in modulating glutamate/GABAergic neurotransmission, that in turn regulates the exercise pressor reflex. The present study provides further evidence of simultaneous sympathoexcitatory and sympathoinhibitory effects of nitric oxide within the RVLM and CVLM involved in the neural control of circulation during static exercise.

Cardiovascular responses and neurotransmission in the ventrolateral medulla during skeletal muscle contraction following transient middle cerebral artery occlusion and reperfusion

We hypothesized that static skeletal muscle contraction-induced systemic cardiovascular responses, and central glutamate/GABA release in rostral (RVLM) and caudal ventrolateral medulla (CVLM), would be modulated by cerebral ischemia. In sham-operated rats, a 2-min tibial nerve stimulation induced static contraction of the triceps surae, evoked pressor responses, increased glutamate in both the RVLM and CVLM, decreased GABA in the CVLM, and increased GABA in the RVLM. In rats with a temporary 90-min left middle cerebral artery occlusion (MCAO) followed by 24 h reperfusion, pressor responses during muscle contractions were attenuated, as were glutamate within the left RVLM and left CVLM. Glutamate within the right RVLM and right CVLM were unaltered and similar to those in sham rats. In contrast, GABA increases during muscle contractions were enhanced in the left RVLM and CVLM but changes within the right CVLM and RVLM were similar to those in sham rats. These results indicate that unilateral ischemia increases ipsilateral GABA/glutamate ratios during muscle contraction in the RVLM. In contrast, opposite changes in ipsilateral glutamate and GABA release within the RVLM and CVLM were observed following a 90-min right-sided MCAO followed by 24 h reperfusion. However, cardiovascular responses during muscle contraction were depressed following such an ischemic brain injury. These data suggest that transient ischemic brain injury attenuates cardiovascular responses to static exercise via modulating neurotransmission within the ventrolateral medulla.

Simultaneous glutamate and gamma-aminobutyric acid release within ventrolateral medulla during skeletal muscle contraction in intact and barodenervated rats

The purpose of this study was to determine if baroreflex modulates cardiovascular responses and neurotransmitter release within rostral (RVLM) and caudal (CVLM) ventrolateral medulla during static contraction of skeletal muscle using anesthetized rats. We evoked cardiovascular responses by a static muscle contraction and measured simultaneous release of glutamate and gamma-aminobutyric acid (GABA) in both the RVLM and CVLM using microdialysis probes, two inserted bilaterally into the RVLM and two into the CVLM. In intact anesthetized rats, a muscle contraction increased release of glutamate concomitantly in both the RVLM and CVLM along with significant increases in heart rate and arterial blood pressure. In contrast, concentrations of GABA increased within the RVLM, but decreased significantly within the CVLM during the pressor response. These changes were due to contraction-evoked activation of muscle afferents since tibial nerve stimulation following muscle paralysis failed to evoke glutamate, GABA, or any cardiovascular changes. On the other hand, static muscle contractions in baroreceptor denervated rats augmented the increases in heart rate and blood pressure. Furthermore, muscle contraction significantly enhanced the release of glutamate in the RVLM but attenuated its release in the CVLM. In addition, concentrations of GABA within the RVLM were attenuated following a muscle contraction in denervated rats without any changes in GABA within the CVLM. These results demonstrate that the baroreceptors influence cardiovascular responses to static muscle contraction associated with dynamic changes in glutamate and GABA release within the RVLM and CVLM.

Effects of nitric oxide and GABA interaction within ventrolateral medulla on cardiovascular responses during static muscle contraction

We hypothesized that nitric oxide (NO) has opposing roles in regulating cardiovascular responses within the rostral (RVLM) and caudal (CVLM) ventrolateral medulla by modulating release of gamma-aminobutyric acid (GABA). We have measured GABA concentrations within the RVLM and CVLM during increases in mean arterial pressure (MAP) and heart rate (HR) following a 2-min tibial nerve stimulation-evoked static muscle contraction before and after microdialysis of the NO precursor, L-arginine (1.0 microM), for 30 min, and after the NO inhibitor, L-NMMA (1.0 microM), for 30 min. In eight anesthetized rats, muscle contraction significantly increased MAP, HR and GABA levels within the RVLM area (from 0.53+/-0.09 to 1.22+/-0.10 ng/10 microl). Following microdialysis of L-arginine, muscle contraction augmented GABA levels (from 0.45+/-0.07 to 2.18+/-0.09 ng/10 microl) and attenuated changes in MAP and HR. Subsequent application of L-NMMA significantly decreased GABA levels (from 0.47+/-0.08 to 0.22+/-0.07 ng/10 microl) but potentiated MAP and HR responses to a muscle contraction. In contrast, muscle contraction significantly increased MAP and HR but decreased GABA concentrations within the CVLM (from 1.20+/-0.20 to 0.78+/-0.17 ng/10 microl). Following microdialysis of L-arginine, muscle contraction significantly attenuated GABA levels (from 1.34+/-0.19 to 0.33+/-0.10 ng/10 microl) and augmented changes in MAP and HR in response to muscle contraction. A subsequent microdialysis of L-NMMA into the CVLM reversed the effects of L-arginine. These results demonstrate that NO within the RVLM and CVLM differentially modulates cardiovascular responses during static muscle contraction and that NO influences exercise-induced cardiovascular responses by modulating GABA release within the ventrolateral medulla.

Modulation of pressor response to muscle contraction via monoamines following AMPA-receptor blockade in the ventrolateral medulla

We hypothesized that cardiovascular responses to static muscle contraction are mediated via changes in extracellular concentrations of monoamines (norepinephrine, dopamine and serotonin) following the administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an AMPA-receptor antagonist) into the rostral (RVLM) or caudal (CVLM) ventrolateral medulla. For the RVLM experiments (n= 8), a 2-min static muscle contraction increased the mean arterial pressure (MAP) and heart rate (HR) by 23 +/- 2 mmHg and 28 +/- 8 bpm, respectively. During this contraction, the concentrations of norepinephrine, dopamine, and serotonin within the RVLM increased by 278 +/- 52%, 213 +/- 23%, and 232 +/- 24%, respectively. Microdialysis of CNQX (1.0 microM) for 30 min into the RVLM attenuated the increases in MAP and HR ( 11 +/- 2 mmHg and 14 +/- 5 bpm) without a change in developed muscle tension. The levels of norepinephrine, dopamine, and serotonin within the RVLM were also attenuated. In contrast, microdialysis of CNQX into the CVLM (n= 8) potentiated the contraction-evoked responses in MAP ( 21 +/- 2 vs 33 +/- 5 mmHg) and HR ( 25 +/- 5 vs 46 +/- 8 bpm) without any effect on the monoamine levels within the CVLM region. These results suggest that AMPA-receptor blockade within the RVLM and CVLM has opposing effects on cardiovascular responses during static muscle contraction. In addition, such receptor blockade modulates extracellular concentrations of monoamines within the RVLM but not in the CVLM. These results provide evidence that AMPA receptors within the ventrolateral medulla play a role in exercise pressor reflex.

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Cardiovascular changes induced by central hypertonic saline are accompanied by glutamate release in awake rats

To elucidate neurochemical mechanisms responsible for cardiovascular responses induced by central salt loading, we directly perfused the paraventricular nucleus (PVN) of the hypothalamus region with hypertonic saline (0.3 or 0.45 M) by using an in vivo brain microdialysis technique. We then measured the extracellular concentrations of glutamate in the PVN region in conscious rats along with the blood pressure and heart rate. Blood pressure, heart rate, and glutamate levels were increased by perfusion of 0.45 M saline; however, they did not change by perfusion of 0.3 M saline. Next, we examined the possible involvement of glutamate in the cardiovascular responses induced by hypertonic saline. Dizocilpine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuated the increases of blood pressure and heart rate, although 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the non-NMDA receptor, did not affect the blood pressure and heart rate. Our results show that local perfusion of the hypothalamic PVN region with hypertonic saline elicits a local release of glutamate, which may act via NMDA-type glutamate receptors to produce cardiovascular responses.

Further evidence that extracellular serotonin in the rostral ventrolateral medulla modulates 5-HT(1A) receptor-mediated attenuation of exercise pressor reflex

We determined changes in extracellular levels of glutamate, serotonin (5-HT), norepinephrine (NE), and dopamine (DA) within rostral ventrolateral medulla (RVLM) during 5-HT(1A)-receptor stimulation-mediated inhibition of cardiovascular responses to static muscle contraction using anesthetized rats. In ten rats, muscle contraction significantly increased (P<0.01) mean arterial pressure (MAP) by 29+/-4 mm Hg, heart rate (HR) by 25+/-3 bpm, and glutamate levels by 4.5+/-0.8 ng/5 microl. Microdialysis of a 5-HT(1A) receptor agonist, 8-OH-DPAT (10 mM), into the RVLM for 30 min attenuated cardiovascular responses to muscle contraction and had no effect on glutamate levels. A subsequent administration of 10 mM WAY100635, a 5-HT(1A) antagonist, into the RVLM antagonized the attenuating effects of 8-OH-DPAT. In another ten rats, muscle contraction significantly increased (P<0.01) MAP and HR by 20+/-2 mmHg and 25+/-8 bpm, respectively. In addition, levels of 5-HT, NE, and DA in the RVLM significantly increased (P<0.01) by 3.6+/-0.3, 3.2+/-0.3, and 3.3+/-0.4 pg/10 microl, respectively. Administration of 8-OH-DPAT (10 mM) into the RVLM for 30 min attenuated cardiovascular responses to muscle contraction and had no effects on NE and DA levels. However, the drug significantly attenuated 5-HT levels following a muscle contraction. Microdialysis of 10 mM WAY100635 into the RVLM reversed both cardiovascular and 5-HT changes. These results suggest that stimulation of 5-HT(1A)-receptors within the RVLM attenuates cardiovascular responses to static exercise via a reduction of extracellular 5-HT concentration and most likely not through changes in glutamate, NE or DA levels.

Glutamate neurotransmission and nitric oxide interaction within the ventrolateral medulla during cardiovascular responses to muscle contraction

We previously reported that nitric oxide, within the RVLM and CVLM, plays an opposing role in modulating cardiovascular responses during static muscle contraction [B.J. Freda, R.S. Gaitonde, R. Lillaney, A. Ally, Cardiovascular responses to muscle contraction following microdialysis of nitric oxide precursor into ventrolateral medulla, Brain Res. 828 (1999) 60-67]. In this study, we determined whether the effects of administering L-arginine, a precursor for the synthesis of nitric oxide, and N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase inhibitor, into the rostral (RVLM) and caudal (CVLM) ventrolateral medulla on cardiovascular responses elicited during static muscle contraction were mediated via an alteration of localized glutamate concentrations using microdialysis techniques. In experiments within the RVLM (n=8), muscle contraction increased MAP and HR by 21+/-2 mmHg and 22+/-3 bpm, respectively. Glutamate increased from 1.1+/-0.4 to 4.4+/- 0.6 ng/5 microl measured from bilateral RVLM areas. Microdialysis of L-arginine (1.0 microM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels. After subsequent microdialysis of L-NMMA (1.0 microM) into the RVLM, contraction augmented the pressor and tachycardic responses and glutamate release. In experiments within CVLM (n=8), muscle contraction increased MAP and HR by 22+/-3 mmHg and 20+/-2 bpm, respectively. Glutamate increased from 0.8+/-0. 4 to 3.6+/-0.6 ng/5 microl measured from the CVLM. L-Arginine augmented the cardiovascular responses and glutamate release and L-NMMA attenuated all the effects. Results suggest that nitric oxide within the RVLM and CVLM plays opposing roles in modulating cardiovascular responses during static exercise via decreasing and increasing, respectively, extracellular glutamate levels.

Rostral ventrolateral medulla opioid receptor activation modulates glutamate release and attenuates the exercise pressor reflex

We previously reported that the administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the rostral (RVLM) but not into the caudal ventrolateral medulla (CVLM), attenuated increases in mean arterial pressure (MAP) and heart rate (HR) during static muscle contraction that had been blocked by prior microdialysis of the opioid receptor antagonist, naloxone [Am. J. Physiol. 274 (1998) H139-H146]. In this study, we determine whether this RVLM-mediated opioidergic-modulation of cardiovascular responses is associated with localized changes in extracellular concentrations of glutamate, an excitatory amino acid, using microdialysis techniques in anesthetized rats. Muscle contraction increased MAP and HR by 37+/-5 mmHg and 23+/-3 bpm, respectively. Extracellular glutamate concentrations, determined using HPLC-ECD, increased from 0.8+/-0.2 to 6.6+/-1.2 ng/5 microliter in the bilateral RVLM areas. Microdialysis of DAME (100 microM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels (20+/-4 mmHg, 10+/-2 bpm, and 1.8+/-0.2 ng/5 microliter, respectively). After microdialysis of naloxone (100 microM) for 30 min into the RVLM, muscle contraction blocked the attenuations (35+/-5 mmHg, 26+/-4 bpm, and 5.8+/-1.0 ng/5 microliter, respectively). Developed muscle tensions were similar throughout the protocol (676+/-38, 678+/-37 and 687+/-37 g, respectively). These results suggest that an opioidergic receptor-mediated mechanism within the RVLM attenuates cardiovascular responses during static exercise via modulating extracellular concentrations of glutamate in the RVLM.