Microinjection Of AP5 Research Articles

Calcineurin inhibition causes persistent hypertension through hypothalamic NMDA receptor‐dependent sympathetic outflow

Calcineurin is a calcium/calmodulin‐dependent serine/threonine protein phosphatase, and hypertension is a major side effect of clinically used calcineurin inhibitors. Although increased sympathetic nerve discharges are associated with calcineurin inhibitor‐induced hypertension, the sources of sympathetic outflow are unknown. In this study, we determined the role of the paraventricular nuclear (PVN) of the hypothalamus in calcineurin inhibitor‐induced hypertension. Immunocytochemistry labeling showed that calcineurin was expressed on spinally projecting PVN neurons. Radiotelemetry recordings showed that systemic administration of tacrolimus (FK506), a specific calcineurin inhibitor, for 14 days caused a gradual and significant increase in arterial blood pressure in rats, which lasted at least another 7 days after discontinuing FK506 treatment. FK506 significantly decreased calcineurin activity in the PVN and increased the protein level of GluN1, the NMDA receptor obligatory subunit, and serine/threonine phosphorylation levels of GluN1 in the PVN synaptosomes. Electrophysiological recordings revealed that the spontaneous firing rate, frequency of mEPSCs, and amplitude of puff NMDA‐elicited currents in spinally projecting PVN neurons were significant greater in FK506‐treated rats than in vehicle‐treated rats. Remarkably, treatment with the NMDA receptor antagonist AP5 normalized the stimulatory effects of FK506 on PVN neurons of FK506‐treated rats. Furthermore, microinjection of AP5 into the PVN decreased renal sympathetic nerve discharges and blood pressure increased in FK506‐treated rats. Systemic administration of memantine, a clinically used NMDA receptor antagonist, also significantly lowered the high blood pressure in FK506‐treated rats. Collectively, our findings indicate that calcineurin inhibitor‐induced persistent hypertension results from augmented sympathetic output through increased NMDA receptor phosphorylation and activity in the PVN.

α2δ-1-Dependent NMDA Receptor Activity in the Hypothalamus Is an Effector of Genetic-Environment Interactions That Drive Persistent Hypertension.

The interplay between genetic and environmental factors is critically involved in hypertension development. The paraventricular nucleus (PVN) of the hypothalamus regulates sympathetic output during stress responses and chronic hypertension. In this study, we determined mechanisms of synaptic plasticity in the PVN in chronic stress-induced persistent hypertension in male borderline hypertensive rats (BHR), the first offspring of spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. In Wistar-Kyoto rats, chronic unpredictable mild stress (CUMS) increased arterial blood pressure (ABP) and heart rate, which quickly returned to baseline after CUMS ended. In contrast, in BHR, CUMS caused persistent elevation in ABP, which lasted at least 2 weeks after CUMS ended. CUMS also increased the mRNA level of α2δ-1 and synaptic protein levels of GluN1, α2δ-1, and α2δ-1-GluN1 complexes in the PVN in BHR. Furthermore, CUMS significantly increased the frequency of miniature EPSCs and the amplitude of NMDAR currents in spinally projecting PVN neurons in BHR; these increases were normalized by blocking NMDARs with AP5, inhibiting α2δ-1 with gabapentin, or disrupting the α2δ-1-NMDAR interaction with α2δ-1Tat peptide. Microinjection of AP5 or α2δ-1Tat peptide into the PVN normalized elevated ABP and renal sympathetic nerve activity in stressed BHR. In addition, systemically administered gabapentin or memantine attenuated higher ABP induced by CUMS in BHR. Our findings indicate that chronic stress-induced persistent hypertension is mediated by augmented sympathetic outflow via α2δ-1-bound NMDARs in the PVN. This new information provides a cellular and molecular basis for how the genetic-environment interactions cause persistent hypertension.SIGNIFICANCE STATEMENT Chronic stress is a major risk factor for hypertension development, especially for individuals with a genetic predisposition to hypertension. Using a rat model of borderline hypertension, we showed that chronic stress induced long-lasting hypertension and sympathetic nerve hyperactivity, which were maintained by NMDAR activation in the hypothalamus. Chronic stress also increased the expression of α2δ-1, previously regarded as a Ca2+ channel subunit, promoting physical interaction with and synaptic trafficking of NMDARs in the hypothalamus. Inhibiting α2δ-1, blocking NMDARs, or disrupting α2δ-1-bound NMDARs reversed chronic stress-induced sympathetic outflow and persistent hypertension. Thus, α2δ-1-dependent NMDAR activity in the hypothalamus is an effector of genetic-environment interactions and may be targeted for treating stress-induced neurogenic hypertension.

Roles of glutamate and GABA of the Kölliker-Fuse nucleus in generating the cardiovascular chemoreflex.

The Kölliker-Fuse (KF) nucleus is a part of the parabrachial complex, located in the dorsolateral pons. It is involved in the chemoreflex-evoked cardiovascular and respiratory changes, but the role of GABA and glutamate in cardiovascular chemoreflex has not been shown yet. This study was performed to determine the role of GABA, glutamate, and their interaction in the KF, in cardiovascular chemoreflex in anesthetized rat. The antagonists were microinjected into the KF, and arterial pressure, heart rate, and single-unit responses were recorded simultaneously. The chemoreflex was evoked by i.v. injection of KCN, consisted of a short pressor followed by long bradycardia responses. Both responses were significantly attenuated by injection of a synaptic blocker (CoCl2) into the KF, confirming involvement of the KF in generating the reflex. Microinjection of AP5, an NMDA receptor antagonist, into the KF significantly attenuated the pressor and bradycardia responses, while blocking the AMPA receptors by CNQX had no significant effect. Blockade of GABAA receptors by bicuculline methiodide (BMI) potentiated both responses. Co-injection of BMI and CNQX potentiated the responses too. Co-injection of BMI and AP5 had no significant effect on the pressor response but significantly attenuated the bradycardia response. In conclusion, the KF plays a role in generating cardiovascular chemoreflex via its glutamate NMDA but not AMPA receptors. GABA inhibits both components of this reflex through GABAA receptors. There is an interaction between GABAA and NMDA receptors in regulating the bradycardia response of the reflex. Single-unit results were also presented which were correlated with and supported the homodynamic findings.

Blockade of Glutamate Receptors within the Prelimbic Cortex Attenuate Concentration of Excitatory Amino Acids in the Morphine Self-administration in Rats.

Background:The attitude of research on addiction has been done on the key role of glutamate. As a regard, the prelimbic cortex (PrL) has an important role in addiction, learning, and memory. We tried to investigate the level of glutamate and aspartate concentration after glutamate receptors blockade in this region in the morphine-addicted rats.Materials and Methods:In this study, we examined the effects of local infusion of the N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonists, 2-amino-5-phosphonovaleric acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), into the PrL cortex on the level of excitatory amino acids (EAAs) and glycine. After 11 days of self-administration, the prelimbic area of the brain was taken out, and the EAAs and glycine concentration was measured by high-performance liquid chromatography.Results:Morphine resulted in the significant increase in the EAAs concentration within this area (P ≤ 0.001). Microinjection of AP5 into this region before using of morphine significantly decreased the morphine-induced glutamate and aspartate concentration (P ≤ 0.001). CNQX had the same effect and significantly reduced the EAAs concentration compared to the morphine group (P ≤ 0.001). In addition, microinjection of AP5 and CNQX simultaneously increased glycine concentration (P ≤ 0.001).Conclusions:These results show that morphine stimulates the EAAs release in the prelimbic area. It seems that microinjection of AP5 or CNQX in this region is effective in reducing morphine-induced EAA. It is suggested that EAA transmission in the PrL cortex may be a possible target for treatment of morphine addiction.

Pain behavior and the expression of Fos in the related brain regions of rats with chronic construction injury

Objective To observe pain behavior and the expression of Fos in the related brain regions, including Anterior Cingulate Cortex (ACC), Periaqueductal Gray (PAG), Rostral ventromedial nucleus (RVM) in chronic constrictive injury (CCI) rats and to explore whether ACC modulate spinal nociceptive transmission through endogenous descending facilitatory system. Methods A total of 48 male SD rats were randomly divided into six groups: Naive group, Sham group (just separated the left sciatic nerves without ligation), CCI group (the left sciatic nerve was ligated), CCI+ P group (on the 14 th day after surgery, intraperitoneal injection of 10 mg/kg paroxetine 45 min before behavior test), ACC-Sham group (bilateral microinjection of 0.9% NaCl in ACC, 1μl/each side) and ACC-AP5 group (bilateral microinjection of AP5 25 mM in ACC, 1 μl/each side on the 13th day after surgery). On the 14th day after light-dark transition test, forced swimming test, paw-withdrawal mechanical threshold(PWMT) and paw-withdrawal thermal patency(PWTL) were performed, the rats were terminally anesthetized and ACC, PAG, RVM and the spinal cord was rapidly removed, and then the expression of c-fos was measured by immunohistochemistry. Results (1)Rats in CCI group demonstrated nociceptive hypersensitivity and depressive-like behaviors compared with Naive rats, while rats in CCI+ P group and in ACC-AP5 group showed less nociceptive hypersensitivity and less depressive-like behaviors compared with CCI group. (2)Compared with Naive group, the number of c-fos positive neurons in the bilateral ACC (left, surgery side, 4920.6±1053.7; right, 2059.3±409.1), VIPAG (9074.8±2320.3), RVM (6195.4±895.0) and bilateral spinal cord (left, 15148.8±3080.2; right, 6400.2±1558.4) was significantly enhanced in CCI group, especially in the left side. In contrast, the amount of fos labeled neurons declined in the bilateral ACC (left, 2776.4±820.1; right, 1120.5±141.4), VIPAG (4002.2±1171.8), RVM (2938.9±910.3) and bilateral spinal cord (left, 8742.0±1131.0; right, 3933.1±858.9) in CCI+ P group and also declined in the bilateral ACC (left, 3623.1±667.4; right, 696.5±164.8), VIPAG (5668.8±1403.3), RVM (3972.3±851.7) and bilateral spinal cord (left, 10675.4±1725.3; right, 3818.3±1085.1) in ACC-AP5 group. Conclusion Endogenous descending facilitatory system may contribute to ACC modulating spinal nociceptive transmission. Key words: Anterior cingulate cortex; Spinal; Periaqueductal grey; Rostroventral medulla; Fos protein

Exercise-Induced Pain Requires NMDA Receptor Activation in the Medullary Raphe Nuclei

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP. Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task. All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP. Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.

Effect of glutamate stimulation of the cuneiform nucleus on cardiovascular regulation in anesthetized rats: Role of the pontine Kolliker–Fuse nucleus

Cuneiform nucleus (CnF) is a reticular nucleus of the midbrain involved in cardiovascular function and stress. There is no report on the cardiovascular effects of the glutamatergic system in the CnF. In the present study, we investigated the cardiovascular effects of glutamate and its NMDA and AMPA/kainate receptors in the CnF. In addition, the possible mediation of Kolliker–Fuse (KF) nucleus in the cardiovascular effects of the CnF was explored. l-glutamate, AP5 (an NMDA receptor antagonist), and CNQX (an AMPA/kainate receptor antagonist) (50–100nl) were microinjected into the CnF of anesthetized rats. Also, the KF was blocked by cobalt chloride (CoCl2) then l-glutamate was microinjected into the CnF. The maximum changes of blood pressure and heart rate were compared with the pre-injection (paired t-test) and control (independent t-test) values. Microinjection of glutamate (25nmol/100nl) into the CnF produced either a short pressor and bradycardic or a long pressor and tachycardic responses. Microinjection of AP5 or CNQX alone did not affect the basal arterial pressure and heart rate. However, co-injection of glutamate with AP5 strongly attenuated the short and moderately attenuated the long cardiovascular responses elicited by glutamate. Co-injection of glutamate with CNQX did not attenuate the short and weakly attenuated the long cardiovascular responses elicited by glutamate. These data suggest that the responses are mediated mainly through NMDA receptors. Blockade of the KF nucleus strongly attenuated the short response and weakly attenuated the long response to glutamate microinjection, suggesting that the cardiovascular effects of glutamate in the CnF, especially the short responses, were mediated by the KF nucleus.

An arcuate-ventrolateral periaqueductal gray reciprocal circuit participates in electroacupuncture cardiovascular inhibition

Electroacupuncture (EA) suppresses elevated blood pressure (BP) by activating the arcuate nucleus, ventrolateral periaqueductal gray (vlPAG), and inhibiting cardiovascular sympathetic neurons in the rostral ventrolateral medulla. This study investigated the reciprocal neural circuit between arcuate and vlPAG during EA inhibition of reflex increases in blood pressure. In α-chloralose anesthetized cats the gallbladder or splanchnic nerve was stimulated to induce cardiovascular sympathoexcitatory reflexes. Electrophysiological recordings showed that EA facilitates the arcuate neural response to splanchnic nerve stimulation. Bilateral vlPAG microinjection of d, l-homocysteic acid (DLH) facilitated the arcuate response to splanchnic nerve stimulation, while microinjection of kainic acid blocked EA (P 5-6 acupoints on pericardial meridian, overlying the median nerves) excitation of arcuate neurons. Retrograde microsphere tracer labeling in the arcuate or vlPAG perikarya was found after respective microinjection of the tracer in the vlPAG or arcuate of rats, demonstrating reciprocal direct connections between the two nuclei. EA inhibition of reflex-induced BP elevation was blocked by injection of glutamate or cholinergic receptor antagonist, kynurenic acid or atropine, into the arcuate. Excitation of vlPAG neurons during EA was blocked by arcuate microinjection of glutamate NMDA and non-NMDA receptor antagonists, AP-5 and CNQX, or the cholinergic receptor antagonist, atropine. Microinjection of DLH or acetylcholine (ACh) into the arcuate facilitated EA excitation of vlPAG neurons. Microinjection of AP5 and CNQX, but not atropine, into the vlPAG blocked EA excitation of arcuate neurons. Thus, a reciprocal excitatory glutamatergic neural circuit between the arcuate and vlPAG contributes to long-lasting EA cardiovascular inhibition. ACh in the arcuate but not in the vlPAG participates in the reciprocal excitation.

Glutamatergic activity modulates the phase‐shifting effects of gastrin‐releasing peptide and light

Previous studies have established that microinjection of gastrin-releasing peptide (GRP) into the suprachiasmatic nucleus (SCN) region or third ventricle causes circadian phase shifts similar to those produced by light pulses. Activation of N-methyl-d-aspartate (NMDA) receptors in the SCN region also produces light-like phase shifts. This study was designed to test the effects of (+/-)-2-amino-5-phosphonopentanoic acid (AP5), an NMDA antagonist, and l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a glutamate reuptake inhibitor, on GRP-induced phase shifts. Adult male Syrian hamsters equipped with a surgically implanted guide cannula aimed at the third ventricle were housed in constant darkness until stable free-running rhythms of wheel-running activity were apparent. Microinjection of GRP into the third ventricle at circadian time (CT)13 induced large phase delays. These GRP-induced phase delays were completely blocked by co-administration of AP5, suggesting that GRP-induced phase delays require concurrent activation of NMDA receptors. Microinjection of AP5 alone did not induce significant phase shifts. A second set of experiments was designed to test whether GRP-induced phase shifts would be enhanced by PDC. Co-administration of PDC and GRP elicited significantly larger phase delays at CT13 than GRP alone. However, administration of PDC alone did not induce a significant phase shift. Finally, when administered just prior to a light pulse, PDC elicited significantly larger phase delays than light pulse plus vehicle controls. These data suggest that the effects of GRP on the circadian clock phase are highly dependent on the level of excitation provided by activated NMDA receptors.

Interaction of GABA and glutamate in the horizontal limb of diagonal band of Broca (hDB): role in cardiovascular responses

The diagonal band of Broca (DBB) is a part of the limbic system that consists of two parts: the vertical limb (vDB) and the horizontal limb (hDB). It has been shown that microinjection of glutamate into the hDB of the anesthetized rat elicited depressor responses. We have previously shown that microinjection of AP5 (an NMDA receptor antagonist, 2.5 mM, 50 nl) and CNQX (an AMPA receptor antagonist, 1 mM, 50 nl) caused no significant changes in the blood pressure and heart rate. Microinjection of bicuculline (BMI: a GABA A receptor antagonist, 1 mM, 50 nl) resulted in the increase of both the blood pressure and heart rate. In this study, we investigated the possible interaction of the GABAergic and glutaminergic systems of the hDB by coinjection of the antagonists of both systems. Experiments were performed on the 24 urethane-anesthetized rats. Repeated measures ANOVA was used for data analysis. Our results showed that coinjection of 50 nl of 1 mM of BMI and 2.5 mM of AP5 significantly (ANOVA, P < 0. 01) decreased the pressor effects of BMI. Also, coinjection of 50 nl of BMI (1 mM) and CNQX (1 mM) significantly (ANOVA, P < 0.01) decreased the pressor effects of BMI. Coinjection of the previous amounts of BMI and both of the glutamate receptor antagonists also produced the same results. These results showed that the cardiovascular effects of blockade of GABAergic inhibition in the hDB are dependent on the activation of local NMDA and non-NMDA receptors of glutamate. A possible interpretation of the results is that, the GABAergic neurons inhibit the glutaminergic neurons.

Involvement of NMDA receptors in the pressor response to microinjection of 5-HT 3 agonist into the NTS of awake rats

Previous studies have shown that the activation of 5-HT 3 receptors in the nucleus tractus solitarii (NTS) increases the baseline mean arterial pressure (MAP). In the present study, we evaluated the possible involvement of NMDA receptors in this pressor response. Four days before the experiments, under tribromoethanol anesthesia, rats received two guide cannulas in the direction of the NTS, and 1 day before the experiments, under tribromoethanol anesthesia, the femoral artery was cannulated for pulsatile arterial pressure (PAP), MAP, and heart rate (HR) measurements. On the day of the experiments, 2-methyl-serotonin, a 5-HT 3 agonist, was microinjected into the NTS after microinjection of saline or AP-5, a selective NMDA receptor antagonist. Microinjection of 2-methyl-serotonin (5 nmol/50 nl) into the NTS after the vehicle (saline) produced a significant increase in MAP (+20±5 mm Hg, n=8) while microinjection of 2-methyl-serotonin after microinjection of AP-5 (10 nmol/50 nl) produced no change in baseline MAP (−1±3 mm Hg, n=11). Microinjection of AP-5 into the NTS produced no significant changes in the baseline MAP and HR. The data show that the increase in MAP in response to microinjection of a 5-HT 3 agonist into the NTS is dependent on NMDA receptors.

Neurotransmission of the cardiovascular reflexes in the nucleus tractus solitarii of awake rats.

Chemoreflex activation with potassium cyanide (i.v.) produces pressor and bradycardic responses in awake rats. Microinjection of AP-5, a selective NMDA receptor antagonist, into the nucleus tractus solitarii (NTS) produced a dose-dependent blockade of the bradycardic response; while microinjection of DNQX, a selective non-NMDA receptor antagonist, or kynurenic acid, a nonselective ionotropic receptor antagonist, produced only a partial reduction in the pressor response, indicating that the bradycardic component of the chemoreflex is mediated by NMDA receptors and that the sympathoexcitatory component may involve neurotransmitters other than excitatory amino acids. With respect to the baroreflex, we verified that the gain of baroreflex bradycardia in response to phenyleprine (Phe) infusion was significantly reduced in a dose-dependent manner by microinjection of AP-5 into the NTS, indicating that the parasympathetic component of the baroreflex is mediated mainly by NMDA receptors. However, in a series of experiments involving the electrical stimulation of the aortic depressor nerve (ADN) we observed that the maximal bradycardic response was almost blocked by the combination of microinjection of NMDA and non-NMDA receptor antagonists into the NTS, while the depressor response was only partially reduced. These data indicate that the bradycardic response produced by the activation of the baroreflex with Phe is mediated by mechanisms differing from those in response to the electrical stimulation of the ADN because phenylephrine also activates carotid and aortic baroreceptors, while unilateral electrical stimulation of the ADN involves only one specific set of baroreceptor afferents. These data also indicate that the sympatho inhibitory component of this response may involve neurotransmitters other than L-glutamate. We discuss the possibility that two different afferent systems of arterial baroreceptors are involved in the modulation of parasympathoexcitation and sympathoinhibition: one activated within the normal range of pulsatile arterial pressure (on a pulse-to-pulse basis) and the other acting under circumstances of challenge to the pulsatile arterial pressure above the normal range.

Bradycardic and hypotensive responses to microinjection of l-glutamate into the lateral aspect of the commissural NTS are blocked by an NMDA receptor antagonist

Baroreflex activation by phenylephrine infusion produces a bradycardic response while microinjection of l-glutamate into the most lateral aspect of the commissural nucleus tractus solitarius (NTS, 0.8 mm lateral to the midline) produces bradycardic and hypotensive responses. In the present study we investigated the role of NMDA receptors in the lateral aspect of the commissural NTS (0.8 mm lateral to the midline) in the bradycardic and hypotensive responses to microinjection of l-glutamate as well as in the processing of the bradycardic response to the baroreflex activation. The hypotensive and bradycardic responses to l-glutamate microinjection into the NTS were blocked by methyl-atropine (intravenous, i.v.), indicating that the hypotensive response was secondary to the bradycardia. Microinjection of l-glutamate (1 nmol/50 nl) into the NTS was performed before and after microinjection of increasing doses of phosphonovaleric acid (AP-5, a selective NMDA antagonist) at the same site. The microinjection of AP-5 [0.5 ( n=9), 2.0 ( n=8) and 10.0 nmol/50 nl ( n=7)] into the NTS (0.8 mm lateral to the midline) produced a dose-dependent blockade of the bradycardic and hypotensive responses to l-glutamate. In a specific group of rats the microinjection of 10 nmol/50 nl of AP-5 produced a significant reduction in baroreflex sensitivity 2 min after microinjection into the lateral NTS [gain=−1.48±0.12 vs. −0.5±0.2 beats/mmHg, ( n=5)], which was reversible. The data show that the bradycardic responses produced by microinjection of l-glutamate into the most lateral aspect of the commissural NTS or by activation of the baroreflex were blocked by microinjection of AP-5, indicating that the neurotransmission of the parasympathetic component of the baroreflex in the neurons of the lateral aspect of the commissural NTS involves NMDA receptors.

NMDA receptor antagonist blocks the bradycardic but not the pressor response to l-glutamate microinjected into the nucleus tractus solitarius (NTS) of unanesthetized rats

In the present study we evaluated the role of NMDA receptors on the pressor and bradycardic responses to l-Glutamate ( l-G1u) microinjected into the nucleus tractus solitarius (NTS) of unanesthetized rats. l-Glu (1 nmol/100 nl) was microinjected into the NTS before and 10 min after microinjection of phosponovaleric acid (AP-5), a selective NMDA receptor antagonist, into the NTS of three different groups of rats (0.5, 2.0 and 10.0 nmol/100 nl). Microinjection of AP-5 into the NTS produced a dose-dependent reduction in the bradycardic response to l-Glu. However, no significant change in the pressor response to l-Glu was observed. These results indicate that the activation of the cardiovagal component (bradycardia) by l-Glu involves NMDA receptors and suggest that the activation of the sympatho-excitatory component (pressor response) by l-Glu in the commissural NTS is mediated by non-NMDA receptors.