Brain Research, 568 (1991) 319-322 ~) 1991 Elsevier Science Publishers B.V. All fights reserved 0006-8993/91/$03.50
319
BRES 24991
Non-NMDA receptors in the nucleus of the tractus solitarius play the predominant role in mediating aortic baroreceptor reflexes Frank J. Gordon and Christina Leone Department of Pharmacology, Emory University School of Medicine, Atlanta, GA 30322 (U.S.A.) (Accepted 24 September 1991) Key words: Blood pressure; Central nervous system; 6,7-Dinitroqninoxahne-2,3-dione; Excitatory amino acid
The purpose of these studies was to determine the relatwe role of N-methyl-D-asparticacid (NMDA) receptors and non-NMDA receptors in the nucleus of the tractus solitafius (NTS) in mediating arterial baroreceptor reflexes evoked by electrical stimulation of the aortic nerve. Selective blockade of NMDA receptors in the NTS had little effect on aortic baroreflexes except at high frequencies of aortic nerve stimulation. In contrast, blockade of non-NMDA receptors in the NTS abolished aortic baroreceptor reflexes. These results suggest that although NMDA receptors may modulate baroreflex responses, synaptic activation of non-NMDA receptors in the NTS plays the predominant role in mediating aortic baroreceptor reflexes.
The baroreceptor reflex is the principal mechanism by which the central nervous system regulates cardiovascular function. The nucleus of the tractus solitarius (NTS) is the central termination site of peripheral baroreceptor afferents 17. Since the initial observation of Talman et al. 18 that microinjections into the NTS of the excitatory amino acid (EAA) receptor antagonist glutamate diethyl ester could abolish reflex-evoked bradycardia, it has become generally accepted that synaptic activation of E A A receptors in the NTS plays a critical role in the mediation of baroreceptor reflexes. With the elucidation that E A A receptors could be categorized into distinct subclasses 2 it became important to determine which of these receptor types might participate in mediating neural transmission of baroreceptor-derived information in the NTS. The purpose of the present study was to determine the relative role of N-methyl-D-aspartic acid (NMDA) receptors and non-NMDA receptors in the NTS in mediating aortic baroreceptor reflexes. Female Sprague-Dawley rats (Sasco) weighing 250300 g were anesthetized with urethane (1.0-1.5 g/kg, i.p.) and catheters were inserted into the femoral artery and vein for recording of arterial pressure and injection of drugs, respectively. The aortic nerve was prepared for electrical stimulation as described in detail previously 5' 13. Rats were then mounted in a stereotaxic frame with the animal's nose deflected ventrally to an angle of approximately 45 °. The rats were paralyzed with decamethonium (2.0 mg/kg, i.v.) and artificially respirated with
oxygen-enriched room air. The dorsal surface of the medulla was exposed and multibarrel glass pipettes (25-70/tin o.d.) were placed bilaterally into the NTS at stereotaxic coordinates 0.5 mm lateral and anterior to calamus scriptorius and 0.30.6 mm ventral to the medullary surface. Individual barrels of each pipette were filled with vehicle or drug solutions, or a 10% solution of Pontamine sky blue. All drugs were dissolved in artificial cerebrospinal fluid (ACSF) vehicle except for 6,7-dinitro-quinoxaline-2,3dione (DNQX) for which the ACSF vehicle also contained 1.5% dimethylsulfoxide (DMSO). Accurate placement of the pipettes and functional identification of the NTS was determined by noting depressor responses of 25-35 m m H g following microinjection into the NTS of 425-500 pmol of L-glutamate delivered from one of the pipette barrels. All microinjections were made in a volume of 50 nl delivered over a period of 5-15 s by applying pulses of pressurized nitrogen to individual pipette barrels. After placement of the pipettes, the aortic nerve was stimulated electrically for 20 s at supramaximal current and pulse duration of 80/zA and 2 ms, respectively6'13. The frequency of nerve stimulation then was adjusted for each rat to establish parameters that would elicit reproducible reflex-mediated depressor responses of approximately -10, -20, and -40 mmHg. Stimulation frequencies used in these experiments averaged 2.8 -+ 0.4, 6.5 -+ 1.1 and 16.1 - 2.4 Hz for low, mid and high fre-
Correspondence: EJ. Gordon, Department of Pharmacology, Rm. 5011, Rollins Research Center, Emory University School of Medicine, Atlanta. GA 30322, U.S.A. Fax: (1) (404) 727-0365.
320 quency stimulation, respectively. To assess the effect of pharmacologic blockade of E A A receptors on aortic baroreflexes, 100 pmol of D N Q X or 2.0 nmol of D-2amino-5-phosphonovaleric acid (D-AP5) was microinjected bilaterally into the NTS from one of the pipette barrels. D N Q X was employed to selectively block nonNMDA, i.e A M P A (a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainic acid receptors in the NTS. D-AP5 was used to block N M D A receptors. Ten minutes after drug or vehicle administration, the aortic nerve stimulation series was repeated and resultant depressor responses were recorded. In separate groups of rats, the selectivity of E A A receptor blockade by D N Q X and D-AP5 was empirically determined. This was accomplished by microinjecting into the NTS, 50 nl vols. of N M D A (7.8 pmol), A M P A (0.49 pmol), kainic acid (1.2 pmol) and L-glutamate (425 pmol) before and 10 min after administration of E A A receptor antagonists from an adjacent pipette barrel. The submaximal agonist doses used in these studies were derived from dose-effect curves previously published by this laboratory 13 and were selected so as to produce depressor responses approximately equivalent to those evoked by the highest frequencies of aortic nerve stimulation used in these studies. In most experiments, 50 nl of 10% Pontamine sky blue dye was ejected from one of the pipette barrels to mark the injection sites. Rats were then perfused transcardially with saline followed by 10% phosphate-buffered formalin. Brains were removed and stored in formalin for subsequent localization of the injection sites by microscopic examination of 60 /~m serial sections of the medulla stained with Neutral red. As previously reported 5'13, injection sites were centered in the dorsomedial portion of the NTS between obex and the midpoint of the area postrema. Data were analyzed by Student's t-test when only two groups were to be compared, or by analysis of variance for factorial experiments. Dunn's procedure was used for post-hoc multiple comparisons among means 8. The criterion for statistical significance was set at the 0.05 level. Numerical values cited in the text refer to the mean --S.E.M. Blockade of non-NMDA receptors in the NTS. Bilateral microinjections into the NTS of vehicle solution (ACSF containing 1.5% DMSO) had no effect on MAP (mean arterial pressure) (95 ± 6 vs 95 -+ 5 mmHg, n = 6). Graded electrical stimulation of the aortic nerve elicited frequency related decreases in MAP (Figs. 1 and 2). Reductions in MAP evoked by aortic nerve stimulation were not different before and after vehicle injection into the NTS. Control responses to low, mid and high frequencies of aortic nerve stimulation averaged -9 --- 1,
-23 - 1, and -39 --- 3 m m H g respectively before microinjection of vehicle and -9 ~ 1, -22 +- 5 and -38 + 3 m m H g after vehicle injection into the NTS (n = 4). Bilateral injections into the NTS of 100 pmol of D N Q X produced a sustained (/>30 min) increase in arterial pressure averaging 27 -+ 3 m m H g (P < 0.001; n = 22) from a baseline MAP of 97 +- 3 mmHg. Ten minutes after D N Q X administration MAP averaged 122 - 5 m m H g and baroreflex mediated decreases in arterial pressure were virtually abolished (Fig. 1). The selectivity of D N Q X as an antagonist of non-NMDA receptors is shown in Fig. 1. D N Q X markedly reduced depressor responses evoked by microinjections into the NTS of AMPA and kainic acid without significantly affecting those produced by N M D A or L-glutamate. Blockade of NMDA receptors in the NTS. Bilateral microinjections into the NTS of 2 nmol of the N M D A receptor antagonist D-AP5 had no effect on baseline MAP. MAP averaged 95 -x-_2 m m H g before administration of D-AP5 and 94 _ 3 m m H g 10 rain after D-AP5 was injected into the NTS (n = 18). Micromjections into the NTS of ACSF vehicle also had no effect on MAP
o "~
-10
E E
-2o
L ,~
-30 -e--
-40
After
DNQX
-50 LOW
MID
HIGH
ADN Stimulation (Hz)
m
Control
[~
After DNQX
-15 -30 -45
-60 NMDA AMPA
KA
GLU
Fig. 1 Top: changes m mean arterial pressure (MAP) produced by graded electrical stimulation of the aortic nerve before (Control), and 10 mm after bilateral blockade of non-NMDA receptors m the NTS with 100 pmol of DNQX (n = 6) Bottom changes m MAP produced by unilateral mlcromlecUons into the NTS of exotatory amino aod agomsts before (Control) and 10 rain after rejection of DNQX from an adjacent pipette barrel (n = 6-16 per group). Doses of agomst were NMDA, 7.8 pmol; AMPA, 0 49 pmol; KA, 1 2 pmol and GLU, 425 pmol Standard error bars, where not shown, are smaller than the data point markers *P < 0.05 compared to Control
321 (97 - 5 vs 97 - 7 mmHg, n = 5). ACSF did not affect depressor responses produced by graded electrical stimulation of the aortic nerve (control = -10 - 1, -21 -+ 2, -44 _ 1 mmHg; ACSF = -10 - 2, -21 - 2, -40 - 4 mmHg; n = 5). Blockade of NMDA receptors in the NTS with o-AP5 attenuated baroreflex-mediated reductions in arterial pressure produced by electrical stimulation of the aortic nerve (Fig. 2). However, this effect was statistically significant only at the highest frequency of aortic nerve stimulation employed in these experiments. o-AP5 virtually eliminated depressor responses produced by microinjections into the NTS of NMDA without affecting reductions in arterial pressure elicited by AMPA, kainic acid or L-glutamate (Fig. 2). The non-NMDA receptor antagonist, DNQX, abolished aortic baroreflexes when it was microinjected into the NTS. Blockade of non-NMDA receptors in the NTS also produced a significant increase in arterial blood pressure, an observation consistent with a role for nonNMDA receptors in the mediation of baroreceptor reflexes17. DNQX selectively antagonized cardiovascular responses evoked by NTS injections of AMPA and kainic acid without significantly affecting those produced by
0
,r
= 5.0 Hz) afferent stimulation and consequent activation of N M D A receptors is required to establish long-term potentiation of fast synaptic responses in hippocampus 2. If similar mechanisms were operative in the NTS, we might speculate that baroreceptor-initiated fast synaptic transmission is m e d i a t e d via activation of n o n - N M D A receptors, whereas under conditions of high frequency stimulation, activation of N M D A receptors would potentiate neural transmission and thus aug-
1 Andresen, M.C. and Yang, M., Non-NMDA receptors mediate sensory afferent synaptie transmission in medial nucleus tractus sohtarius, Am. J Physiol., 259 (1990) H1307-H1311. 2 Collingridge, G.L. and Lester, R.A.J., Excitatory amino acid receptors in the vertebrate central nervous system, Pharmacol. Rev., 40 (1989) 143-210, 3 Drewe, J.A., Miles, R. and Kunze, D.L., Excitatory amino acid receptors of gmnea pig medial nucleus tractus solitarius neurons, Am. J Physiol., 259 (1990) H1389-H1395. 4 Florentino, A., Varga, K. and Kunos, G., Mechanism of the cardiovascular effects of GABAn receptor activation m the nucleus tractus solitarii of the rat, Brain Research, 535 (1990) 264270. 5 Gordon, F.J., Opioids and central baroreflex control: a site of action in the nucleus tractus sohtarius, Pepudes, 11 (1990) 305309. 6 Gordon, F.J. and Mark, A.L., Mechanism of impaired baroreflex control in prehypertensive Dahl salt-sensitive rats, C~rc Res., 54 (1984) 378-387. 7 Honore, T., Davies, S.N., Drejer, J., Fletcher, E.J., Jacobsen, P., Lodge, D. and Nielsen, F.E., Quinoxahnediones: potent competitive non-NMDA glutamate receptor antagomsts, Science, 241 (1988) 701-703. 8 Kirk, R.E., Experimental Design: Procedures for the Behavioral Sciences, Brooks/Cole, Belmont, CA, 1968. 9 Kogo, N., Graft, J., Grieve, P.A. and Talman, W.T., NMDA receptors and the baroreceptor reflex, Soc. Neurosci. Abstr., 14 (1988) 503. 10 Kubo, T. and Kihara, M., Evidence of N-methyl-o-aspartate receptor-methated modulation of the aortic baroreceptor reflex
ment b a r o r e c e p t o r reflexes. Blockade of n o n - N M D A receptors would then be expected to eliminate baroreflexes by blocking fast synaptic transmission of b a r o r e c e p t o r derived information. In contrast, b l o c k a d e of N M D A receptors would not eliminate baroreflexes, but instead would reduce their magnitude by removing the potentiating effect of N M D A r e c e p t o r activation in p r o p o r t i o n to the frequency of afferent input. This interpretation would be consistent with the results of the present study and may be helpful in reconciling those of o t h e r investigators as well. In summary, the results of these experiments indicate that n o n - N M D A receptors in the NTS play the p r e d o m inant role in mediating aortic b a r o r e c e p t o r reflexes in the rat. In addition, they also suggest an important m o d ulatory role for N M D A receptors in the NTS in central baroreflex control. The excellent technical assistance of Ms. Louise A. McCann is gratefully acknowledged This work was supported by NIH Grant HL36907 F.J.G. 1s an Estabhshed Investigator of the American Heart Association and Sandoz Pharmaceutical Corp. These experiments were conducted during C.L.'s tenure as a predoctoral fellow of the Pharmaceuucal Manufacturer's Associatmn Foundation.
11
12
13 14 15 16 17 18
in the rat nucleus tractus sohtarii, Neurosct. Len., 87 (1988) 69-74 Kubo, T. and Klhara, M., Undateral blockade of excRatory amino acid receptors in the nucleus tractus solitaru produces an inhibition of baroreflexes m rats, Naunyn-Schmtedeberg's Arch Pharmacol., 343 (1991) 317-322. Le Galloudec, E., Merahi, N. and Laguzzi, R., Cardiovascular changes induced by the local application of glutamate-related drugs m the rat nucleus tractus solitaru, Bram Research, 503 (1989) 322-325. Leone, C and Gordon, FJ., Is L-glutamate a neurotransrmtter of baroreceptor mformat~on in the nucleus of the tractus solitarius?, J. Pharmacol Exp. Ther , 250 (1989) 953-962 Mayer, M.L. and Westbrook, G.L., The physiology of excitatory amino acids m the vertebrate central nervous system, Prog. Neurobiol., 28 (1987) 197-276. Miller, B.D. and Felder, R.B., Excitatory amino acid receptors intnnsic to synapt~c transmission m nucleus tractus sohtaril, Brain Research, 456 (1988) 333-343. Sah, P., Hestrin, S. and Nicoll, R.A., Tome activation of NMDA receptors by ambient glutamate enhances excitability of neurons, Science, 246 (1989) 815-818. Spyer, K.M., Neural organisation and control of the baroreceptor reflex, Rev. Physiol. Biochem Pharmacol., 88 (1981) 23124. Talman, W.T, Perrone, M.H., Scher, P., Kwo, S. and Reis, D J., Antagonism of the baroreceptor reflex by glutamate diethyl ester, an antagonist to L-glutamate, Brain Research, 217 (1981) 186-191.
319
BRES 24991
Non-NMDA receptors in the nucleus of the tractus solitarius play the predominant role in mediating aortic baroreceptor reflexes Frank J. Gordon and Christina Leone Department of Pharmacology, Emory University School of Medicine, Atlanta, GA 30322 (U.S.A.) (Accepted 24 September 1991) Key words: Blood pressure; Central nervous system; 6,7-Dinitroqninoxahne-2,3-dione; Excitatory amino acid
The purpose of these studies was to determine the relatwe role of N-methyl-D-asparticacid (NMDA) receptors and non-NMDA receptors in the nucleus of the tractus solitafius (NTS) in mediating arterial baroreceptor reflexes evoked by electrical stimulation of the aortic nerve. Selective blockade of NMDA receptors in the NTS had little effect on aortic baroreflexes except at high frequencies of aortic nerve stimulation. In contrast, blockade of non-NMDA receptors in the NTS abolished aortic baroreceptor reflexes. These results suggest that although NMDA receptors may modulate baroreflex responses, synaptic activation of non-NMDA receptors in the NTS plays the predominant role in mediating aortic baroreceptor reflexes.
The baroreceptor reflex is the principal mechanism by which the central nervous system regulates cardiovascular function. The nucleus of the tractus solitarius (NTS) is the central termination site of peripheral baroreceptor afferents 17. Since the initial observation of Talman et al. 18 that microinjections into the NTS of the excitatory amino acid (EAA) receptor antagonist glutamate diethyl ester could abolish reflex-evoked bradycardia, it has become generally accepted that synaptic activation of E A A receptors in the NTS plays a critical role in the mediation of baroreceptor reflexes. With the elucidation that E A A receptors could be categorized into distinct subclasses 2 it became important to determine which of these receptor types might participate in mediating neural transmission of baroreceptor-derived information in the NTS. The purpose of the present study was to determine the relative role of N-methyl-D-aspartic acid (NMDA) receptors and non-NMDA receptors in the NTS in mediating aortic baroreceptor reflexes. Female Sprague-Dawley rats (Sasco) weighing 250300 g were anesthetized with urethane (1.0-1.5 g/kg, i.p.) and catheters were inserted into the femoral artery and vein for recording of arterial pressure and injection of drugs, respectively. The aortic nerve was prepared for electrical stimulation as described in detail previously 5' 13. Rats were then mounted in a stereotaxic frame with the animal's nose deflected ventrally to an angle of approximately 45 °. The rats were paralyzed with decamethonium (2.0 mg/kg, i.v.) and artificially respirated with
oxygen-enriched room air. The dorsal surface of the medulla was exposed and multibarrel glass pipettes (25-70/tin o.d.) were placed bilaterally into the NTS at stereotaxic coordinates 0.5 mm lateral and anterior to calamus scriptorius and 0.30.6 mm ventral to the medullary surface. Individual barrels of each pipette were filled with vehicle or drug solutions, or a 10% solution of Pontamine sky blue. All drugs were dissolved in artificial cerebrospinal fluid (ACSF) vehicle except for 6,7-dinitro-quinoxaline-2,3dione (DNQX) for which the ACSF vehicle also contained 1.5% dimethylsulfoxide (DMSO). Accurate placement of the pipettes and functional identification of the NTS was determined by noting depressor responses of 25-35 m m H g following microinjection into the NTS of 425-500 pmol of L-glutamate delivered from one of the pipette barrels. All microinjections were made in a volume of 50 nl delivered over a period of 5-15 s by applying pulses of pressurized nitrogen to individual pipette barrels. After placement of the pipettes, the aortic nerve was stimulated electrically for 20 s at supramaximal current and pulse duration of 80/zA and 2 ms, respectively6'13. The frequency of nerve stimulation then was adjusted for each rat to establish parameters that would elicit reproducible reflex-mediated depressor responses of approximately -10, -20, and -40 mmHg. Stimulation frequencies used in these experiments averaged 2.8 -+ 0.4, 6.5 -+ 1.1 and 16.1 - 2.4 Hz for low, mid and high fre-
Correspondence: EJ. Gordon, Department of Pharmacology, Rm. 5011, Rollins Research Center, Emory University School of Medicine, Atlanta. GA 30322, U.S.A. Fax: (1) (404) 727-0365.
320 quency stimulation, respectively. To assess the effect of pharmacologic blockade of E A A receptors on aortic baroreflexes, 100 pmol of D N Q X or 2.0 nmol of D-2amino-5-phosphonovaleric acid (D-AP5) was microinjected bilaterally into the NTS from one of the pipette barrels. D N Q X was employed to selectively block nonNMDA, i.e A M P A (a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainic acid receptors in the NTS. D-AP5 was used to block N M D A receptors. Ten minutes after drug or vehicle administration, the aortic nerve stimulation series was repeated and resultant depressor responses were recorded. In separate groups of rats, the selectivity of E A A receptor blockade by D N Q X and D-AP5 was empirically determined. This was accomplished by microinjecting into the NTS, 50 nl vols. of N M D A (7.8 pmol), A M P A (0.49 pmol), kainic acid (1.2 pmol) and L-glutamate (425 pmol) before and 10 min after administration of E A A receptor antagonists from an adjacent pipette barrel. The submaximal agonist doses used in these studies were derived from dose-effect curves previously published by this laboratory 13 and were selected so as to produce depressor responses approximately equivalent to those evoked by the highest frequencies of aortic nerve stimulation used in these studies. In most experiments, 50 nl of 10% Pontamine sky blue dye was ejected from one of the pipette barrels to mark the injection sites. Rats were then perfused transcardially with saline followed by 10% phosphate-buffered formalin. Brains were removed and stored in formalin for subsequent localization of the injection sites by microscopic examination of 60 /~m serial sections of the medulla stained with Neutral red. As previously reported 5'13, injection sites were centered in the dorsomedial portion of the NTS between obex and the midpoint of the area postrema. Data were analyzed by Student's t-test when only two groups were to be compared, or by analysis of variance for factorial experiments. Dunn's procedure was used for post-hoc multiple comparisons among means 8. The criterion for statistical significance was set at the 0.05 level. Numerical values cited in the text refer to the mean --S.E.M. Blockade of non-NMDA receptors in the NTS. Bilateral microinjections into the NTS of vehicle solution (ACSF containing 1.5% DMSO) had no effect on MAP (mean arterial pressure) (95 ± 6 vs 95 -+ 5 mmHg, n = 6). Graded electrical stimulation of the aortic nerve elicited frequency related decreases in MAP (Figs. 1 and 2). Reductions in MAP evoked by aortic nerve stimulation were not different before and after vehicle injection into the NTS. Control responses to low, mid and high frequencies of aortic nerve stimulation averaged -9 --- 1,
-23 - 1, and -39 --- 3 m m H g respectively before microinjection of vehicle and -9 ~ 1, -22 +- 5 and -38 + 3 m m H g after vehicle injection into the NTS (n = 4). Bilateral injections into the NTS of 100 pmol of D N Q X produced a sustained (/>30 min) increase in arterial pressure averaging 27 -+ 3 m m H g (P < 0.001; n = 22) from a baseline MAP of 97 +- 3 mmHg. Ten minutes after D N Q X administration MAP averaged 122 - 5 m m H g and baroreflex mediated decreases in arterial pressure were virtually abolished (Fig. 1). The selectivity of D N Q X as an antagonist of non-NMDA receptors is shown in Fig. 1. D N Q X markedly reduced depressor responses evoked by microinjections into the NTS of AMPA and kainic acid without significantly affecting those produced by N M D A or L-glutamate. Blockade of NMDA receptors in the NTS. Bilateral microinjections into the NTS of 2 nmol of the N M D A receptor antagonist D-AP5 had no effect on baseline MAP. MAP averaged 95 -x-_2 m m H g before administration of D-AP5 and 94 _ 3 m m H g 10 rain after D-AP5 was injected into the NTS (n = 18). Micromjections into the NTS of ACSF vehicle also had no effect on MAP
o "~
-10
E E
-2o
L ,~
-30 -e--
-40
After
DNQX
-50 LOW
MID
HIGH
ADN Stimulation (Hz)
m
Control
[~
After DNQX
-15 -30 -45
-60 NMDA AMPA
KA
GLU
Fig. 1 Top: changes m mean arterial pressure (MAP) produced by graded electrical stimulation of the aortic nerve before (Control), and 10 mm after bilateral blockade of non-NMDA receptors m the NTS with 100 pmol of DNQX (n = 6) Bottom changes m MAP produced by unilateral mlcromlecUons into the NTS of exotatory amino aod agomsts before (Control) and 10 rain after rejection of DNQX from an adjacent pipette barrel (n = 6-16 per group). Doses of agomst were NMDA, 7.8 pmol; AMPA, 0 49 pmol; KA, 1 2 pmol and GLU, 425 pmol Standard error bars, where not shown, are smaller than the data point markers *P < 0.05 compared to Control
321 (97 - 5 vs 97 - 7 mmHg, n = 5). ACSF did not affect depressor responses produced by graded electrical stimulation of the aortic nerve (control = -10 - 1, -21 -+ 2, -44 _ 1 mmHg; ACSF = -10 - 2, -21 - 2, -40 - 4 mmHg; n = 5). Blockade of NMDA receptors in the NTS with o-AP5 attenuated baroreflex-mediated reductions in arterial pressure produced by electrical stimulation of the aortic nerve (Fig. 2). However, this effect was statistically significant only at the highest frequency of aortic nerve stimulation employed in these experiments. o-AP5 virtually eliminated depressor responses produced by microinjections into the NTS of NMDA without affecting reductions in arterial pressure elicited by AMPA, kainic acid or L-glutamate (Fig. 2). The non-NMDA receptor antagonist, DNQX, abolished aortic baroreflexes when it was microinjected into the NTS. Blockade of non-NMDA receptors in the NTS also produced a significant increase in arterial blood pressure, an observation consistent with a role for nonNMDA receptors in the mediation of baroreceptor reflexes17. DNQX selectively antagonized cardiovascular responses evoked by NTS injections of AMPA and kainic acid without significantly affecting those produced by
0
,r
= 5.0 Hz) afferent stimulation and consequent activation of N M D A receptors is required to establish long-term potentiation of fast synaptic responses in hippocampus 2. If similar mechanisms were operative in the NTS, we might speculate that baroreceptor-initiated fast synaptic transmission is m e d i a t e d via activation of n o n - N M D A receptors, whereas under conditions of high frequency stimulation, activation of N M D A receptors would potentiate neural transmission and thus aug-
1 Andresen, M.C. and Yang, M., Non-NMDA receptors mediate sensory afferent synaptie transmission in medial nucleus tractus sohtarius, Am. J Physiol., 259 (1990) H1307-H1311. 2 Collingridge, G.L. and Lester, R.A.J., Excitatory amino acid receptors in the vertebrate central nervous system, Pharmacol. Rev., 40 (1989) 143-210, 3 Drewe, J.A., Miles, R. and Kunze, D.L., Excitatory amino acid receptors of gmnea pig medial nucleus tractus solitarius neurons, Am. J Physiol., 259 (1990) H1389-H1395. 4 Florentino, A., Varga, K. and Kunos, G., Mechanism of the cardiovascular effects of GABAn receptor activation m the nucleus tractus solitarii of the rat, Brain Research, 535 (1990) 264270. 5 Gordon, F.J., Opioids and central baroreflex control: a site of action in the nucleus tractus sohtarius, Pepudes, 11 (1990) 305309. 6 Gordon, F.J. and Mark, A.L., Mechanism of impaired baroreflex control in prehypertensive Dahl salt-sensitive rats, C~rc Res., 54 (1984) 378-387. 7 Honore, T., Davies, S.N., Drejer, J., Fletcher, E.J., Jacobsen, P., Lodge, D. and Nielsen, F.E., Quinoxahnediones: potent competitive non-NMDA glutamate receptor antagomsts, Science, 241 (1988) 701-703. 8 Kirk, R.E., Experimental Design: Procedures for the Behavioral Sciences, Brooks/Cole, Belmont, CA, 1968. 9 Kogo, N., Graft, J., Grieve, P.A. and Talman, W.T., NMDA receptors and the baroreceptor reflex, Soc. Neurosci. Abstr., 14 (1988) 503. 10 Kubo, T. and Kihara, M., Evidence of N-methyl-o-aspartate receptor-methated modulation of the aortic baroreceptor reflex
ment b a r o r e c e p t o r reflexes. Blockade of n o n - N M D A receptors would then be expected to eliminate baroreflexes by blocking fast synaptic transmission of b a r o r e c e p t o r derived information. In contrast, b l o c k a d e of N M D A receptors would not eliminate baroreflexes, but instead would reduce their magnitude by removing the potentiating effect of N M D A r e c e p t o r activation in p r o p o r t i o n to the frequency of afferent input. This interpretation would be consistent with the results of the present study and may be helpful in reconciling those of o t h e r investigators as well. In summary, the results of these experiments indicate that n o n - N M D A receptors in the NTS play the p r e d o m inant role in mediating aortic b a r o r e c e p t o r reflexes in the rat. In addition, they also suggest an important m o d ulatory role for N M D A receptors in the NTS in central baroreflex control. The excellent technical assistance of Ms. Louise A. McCann is gratefully acknowledged This work was supported by NIH Grant HL36907 F.J.G. 1s an Estabhshed Investigator of the American Heart Association and Sandoz Pharmaceutical Corp. These experiments were conducted during C.L.'s tenure as a predoctoral fellow of the Pharmaceuucal Manufacturer's Associatmn Foundation.
11
12
13 14 15 16 17 18
in the rat nucleus tractus sohtarii, Neurosct. Len., 87 (1988) 69-74 Kubo, T. and Klhara, M., Undateral blockade of excRatory amino acid receptors in the nucleus tractus solitaru produces an inhibition of baroreflexes m rats, Naunyn-Schmtedeberg's Arch Pharmacol., 343 (1991) 317-322. Le Galloudec, E., Merahi, N. and Laguzzi, R., Cardiovascular changes induced by the local application of glutamate-related drugs m the rat nucleus tractus solitaru, Bram Research, 503 (1989) 322-325. Leone, C and Gordon, FJ., Is L-glutamate a neurotransrmtter of baroreceptor mformat~on in the nucleus of the tractus solitarius?, J. Pharmacol Exp. Ther , 250 (1989) 953-962 Mayer, M.L. and Westbrook, G.L., The physiology of excitatory amino acids m the vertebrate central nervous system, Prog. Neurobiol., 28 (1987) 197-276. Miller, B.D. and Felder, R.B., Excitatory amino acid receptors intnnsic to synapt~c transmission m nucleus tractus sohtaril, Brain Research, 456 (1988) 333-343. Sah, P., Hestrin, S. and Nicoll, R.A., Tome activation of NMDA receptors by ambient glutamate enhances excitability of neurons, Science, 246 (1989) 815-818. Spyer, K.M., Neural organisation and control of the baroreceptor reflex, Rev. Physiol. Biochem Pharmacol., 88 (1981) 23124. Talman, W.T, Perrone, M.H., Scher, P., Kwo, S. and Reis, D J., Antagonism of the baroreceptor reflex by glutamate diethyl ester, an antagonist to L-glutamate, Brain Research, 217 (1981) 186-191.
