- Email: [email protected]
Mediation of the cardiovascular response of adenosine A1 receptor through a GABAB receptor in the spinal cord of the rats
Neuroscience Letters 243 (1998) 81–84
Mediation of the cardiovascular response of adenosine A1 receptor through a GABAB receptor in the spinal cord of the rat Hyun Chul Koh a,*, In Chul Shin a, Se Jin Hwang b, Ju Seop Kang a, Chang Ho Lee a, Ji Hee Ha a, Doo Jin Paik b a
Department of Pharmacology, College of Medicine, Hanyang University, 17 Haengdang-Dong, Sungdong-Ku, Seoul 133–791, South Korea b Department of Anatomy, College of Medicine, Hanyang University, 17 Haengdang-Dong, Sungdong-Ku, Seoul 133–791, South Korea Received 3 October 1997; received in revised form 16 January 1998; accepted 16 January 1998
Abstract Cardiovascular inhibitory effects induced by intrathecal (i.t.) administration of adenosine A1 receptor agonist and its modulation by cyclic AMP was suggested by our previous report. In this experiment, we examined the mediation of cardiovascular effects of adenosine A1 receptor by g-aminobutyric acid receptors A and B (GABAA and GABAB) in the spinal cord. I.t. administration of 10 nmol of N6-cyclohexyladenosine (CHA), an adenosine A1 receptor agonist, and pretreatment with bicuculline (10 nmol, i.t), a GABAA receptor antagonist, and 5-aminovaleric acid (50 nmol, i.t.), a GABAB receptor antagonist, prior to injection of CHA were performed in anesthetized, artificially ventilated Sprague–Dawley rats. I.t. injection of 50 nmol of 5-aminovaleric acid significantly attenuated the inhibitory cardiovascular effects of CHA but 10 nmol of bicuculline did not alter CHA-induced cardiovascular actions. It is suggested that cardiovascular responses of adenosine A1 receptor is meditated by GABAB receptor in the spinal cord. 1998 Published by Elsevier Science Ireland Ltd.
Keywords: Adenosine A1 receptor; g-Aminobutyric acid A receptor; g-Aminobutyric acid B receptor; Spinal cord; Blood pressure; Heart rate
Adenosine A1 receptor, which is found in both central and peripheral nervous system [4], mediates various neuromodulatory actions of adenosine, including cardiovascular regulatory effects. Adenosine A1 receptors in the central nervous system mediates mainly inhibitory effects on synaptic transmission of glutamate, acetylcholine in hippocampus, striatum, hypothalamus and cerebral cortex [3,5,18]. Several lines of evidences suggest that adenosine A1 receptor is responsible for cardiovascular inhibitory effects in central and peripheral nervous system. The administration of adenosine or its agonist into the 4th ventricle [2] and nucleus tractus solitarius [17] resulted in depression of blood pressure (BP) and decrease of heart rate (HR). Previously, we have reported that intrathecal (i.t.) injection of cyclohexyladenosine (CHA), leads the cardiovascular inhibitory effects which is mediated by cyclic AMP but, not by cyclic GMP [14]. GABA is one of the major inhibitory neurotransmitters in the central nervous system and it is suggested to have a significant role in modulating the activity of sympathetic * Corresponding author. Tel.: +82 2 2900651; fax: +82 2 2926686.
preganglionic neurons in the spinal cord [1,8]. Both GABAA and GABAB receptors in these neurons mediate cardiovascular regulatory effects of GABA in the spinal cord [10]. Colocalization and functional relations between adenosine receptors and GABA receptors, were suggested by some authors [7,15]. Gerber and Ga¨hwiler [6] suggested that actions of adenosine and GABAB receptors in hippocampal pyramidal cells are mediated by a common signal transduction pathway. This study was performed to examine the role of GABA receptors on cardiovascular depressive action induced by stimulation of adenosine A1 receptor in lower thoracic cord level. The experimental animals were categorized into three groups. One of these groups was treated only with CHA (10 nmol) and the other two groups were treated with GABAA receptor antagonist, bicuculline and the GABAB receptor antagonist, 5-aminovaleric acid [19], respectively, prior to the administration of the CHA. All drugs were purchased from RBI Chemical Company (MA, USA). CHA and 5-aminovaleric acid were dissolved in 0.9% NaCl solution and bicuculline was dissolved in
0304-3940/98/$19.00 1998 Published by Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940(98) 00089- 5
82
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
2.0% dimethylsulfoxide (DMSO) solution. Male Sprague– Dawley rats (300–350 g) were anesthetized with urethane (1.15 g/kg, i.p.), paralyzed with D-tubocurarine (0.5 mg/kg, i.m.) and artificially ventilated (Ugo Basile, Varese, Italy). BP and HR were continuously monitored via a femoral arterial catheter (PE-50) connected to a pressure transducer (Spectramed, MA, USA) and a polygraph (Grass, MA, USA). Mean arterial pressure was calculated as diastolic pressure +1/3 (systolic pressure − diastolic pressure). Rectal temperature was maintained at 37 ± 0.5°C with a heating pad. The rats were placed in stereotaxic instrument (Stoelting, IL, USA) in prone position. The posterior atlantooccipital membrane was exposed by an occipital incision. The
atlantooccipital membrane was cut and a guide cannula (PE10) was inserted intrathecally; its tip was positioned at the lower thoracic vertebral level (6.5 cm from the lower margin of the occipital bone). I.t. administration of drugs were made using the injector cannula (33-gauge stainless steel) through the guide cannula. CHA (10 nmol) was delivered in a volume of 5 ml in 1 min with Hamilton syringe mounted on a micrometer. Bicuculline (10 nmol) and 5-aminovaleric acid (50 nmol) were injected 10 min before administration of CHA. Data were expressed as the mean ± SE of the maximal response following drug administration. Student’s t-test for paired or unpaired data was used for statistical evaluation of the results.
Fig. 1. Representative tracings depicting the changes of blood pressure (BP) and heart rate (HR) following intrathecal injections of the N6cyclohexyladenosine (CHA). (A) CHA (10 nmol, i.t.) injection only, (B) Bicuculline (10 nmol, i.t.) and CHA (10 nmol, i.t.) injection, (C) 5aminovaleric acid (50 nmol, i.t.) and CHA (10 nmol, i.t.) injection. In (B) and (C), bicuculline and 5-aminovaleric acid were injected 10 min prior to CHA injection.
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
83
Fig. 2. Changes in mean arterial pressure (MAP) and heart rate (HR) following treatment with N6-cyclohexyladenosine (CHA; 10 nmol, i.t.) only and CHA after pretreatment with bicuculline (BC; 10 nmol, i.t.). Data are the mean ± SE.
A representative experiment showing effects of i.t. administration of CHA (10 nmol) on BP and HR is illustrated in Fig. 1. I.t. injection of CHA caused a decrease in MAP that reached maximum in 17.3 ± 4.1 min after injection. The decrease of MAP evoked by CHA was 29.7 ± 6.2 mmHg (n = 5). Decrease of HR was also in– duced by CHA administration, maximum 18.2 ± 5.2 min after injection. HR was decreased by 120 ± 25.4 beats/ min (Figs 2 and 3). Baseline MAP and HR for these rats were 86.7 ± 7.7 mmHg and 345.5 ± 10.3 beats/ min, respectively. Administration (i.t.) of an equivalent volume of normal saline did not affect the basal MAP and HR. Pretreatment with 5-aminovaleric acid (50 nmol, i.t.) sig-
nificantly attenuated the CHA-induced cardiovascular responses; decrease in MAP and HR were 18.1 ± 4.7 mmHg and 64.0 ± 12.0 beats/min (n = 5; Figs. 1 and 3), respectively. However, i.t. administration of bicuculline (10 nmol, i.t.) prior to injection of CHA did not alter the depressor and bradycardiac response elicited by CHA; decrease in MAP and HR were 25.1 ± 5.0 mmHg and 112 ± 19.2 beats/min (n = 5; Figs. 1 and 2), respectively. I.t. injection of 5-aminovaleric acid (50 nmol) and bicuculline (10 nmol) had no effects on basal MAP and HR. In the previous report, we demonstrated that adenosine A1 receptor-mediated cardiovascular response was modulated by cAMP in the spinal cord of rats. In the present experiment, we examined the modulation of cardiovascular effects
Fig. 3. Changes in mean arterial pressure (MAP) and heart rate (HR) following treatment with N6-cyclohexyladenosine (CHA; 10 nmol, i.t.) only and CHA after pretreatment with 5-aminovaleric acid (AV; 50 nmol, i.t). Data are the mean ± SE. *P , 0.01, compared to CHA only group.
84
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
of CHA by GABAA and GABAB receptor antagonist in the spinal cord. Administration (i.t.) of 5-aminovaleric acid significantly attenuated the cardiovascular depressor actions elicited by CHA. Both adenosine A1 and GABAB receptors mediate mainly inhibitory effects in central nervous system. Lohse et al. [16] have suggested that adenosine A1 receptor may interact with GABA receptor in a competitive manner in receptor binding study, but little is known about the relation of two receptors in cardiovascular response. Our result is the first evidence for interaction of two receptors in terms of mechanism of cardiovascular regulation. Koh et al. [14] suggested the modulation of cardiovascular response of adenosine A1 receptors by cAMP in the spinal cord. Actions of GABAB receptors are also mediated by cAMP in cardiovascular modulatory action in the spinal cord [13]. We suggest that these mechanism, in the spinal cord, may contribute to mediation of cardiovascular regulation of CHA by 5-aminovaleric acid, and that adenosine A1 receptor might interact with GABAB receptor in the spinal cardiovascular regulation because 50 nmol of 5-aminovaleric acid alone did not altered basal BP and HR. This can be supported by the following studies. Kamachi and Ticku [11,12] suggested that the action of GABAB receptors involved the G proteins and adenylate cyclase, and that this is under modulatory control by protein kinase A and C. Gerber and Ga¨hwiler [6] also suggested that adenosine receptors and GABA receptors are seemed to be present in the cell membrane of same hippocampal neuron, and contact with same Gi protein pool or separate G proteins which are converged to same adenylate cyclase. Additionally, cardiovascular action of GABAB receptor was blocked by adenosine A1 receptor antagonist, but not by A2 receptor in spinal cord in our experiment (unpublished data). In fact, the GABAA receptor in the spinal cord revealed inhibitory cardiovascular actions [9] as adenosine A1 receptor. But in our experiment, administration (i.t.) of bicuculline (at the dose of no change in basal BP and HR) did not alter the cardiovascular depressive action elicited by CHA. This may account for the different modes of the action of GABAA receptor from that of adenosine A1 receptor in cardiovascular regulation in the spinal cord. GABAA receptors leads to the opening of specific Cl− channels [20] but adenosine A1 receptors leads to decrease of cAMP by inhibition of adenylate cyclase in the spinal cord [14]. In conclusion, we suggest that cardiovascular response of adenosine A1 receptor is mediated by GABAB receptor, not by GABAA receptor in the spinal cord. [1] Bacon, S.J. and Smith, A.D., Preganglionic sympathetic neurons innervating the rat adrenal medulla: immunocytochemical evidences of synaptic input from nerve terminals containing substance-P, GABA or 5-hydroxytryptamine, J. Auton. Nerv. Syst., 24 (1988) 97–122. [2] Barraco, R.A., Campbell, W.R., Parizon, M., Shoener, E.P. and Shein, S.E., Cardiovascular effects of microinjections of adenosine analogs into the fourth ventricle of rats, Brain Res., 424 (1987) 17–25.
[3] Brown, S.J., James, S., Reddington, M. and Richardson, P.J., Both A1 and A2a purine receptors regulate striatal acetylcholine release, J. Neurochem., 55 (1990) 31–38. [4] Choca, J.I., Proudfit, H.K. and Green, R.D., Identification of A1 and A2 adenosine receptors in the rat spinal cord, J. Pharmacol. Exp. Ther., 242 (1987) 905–910. [5] Coardetti, R., Lo Conte, G., Moroni, F., Passani, M.B. and Peteu, G., Adenosine decrease aspartate and glutamate release from rat hippocampal slices, Eur. J. Pharmacol., 104 (1984) 19–26. [6] Gerber, U. and Ga¨hwiler, B.H., GABAB and adenosine receptors mediate enhancement of the K + current, IAHP, by reducing adenylyl cyclase activity in rat CA3 hippocampal neurons, J. Neurophysiol., 72 (1994) 2360–2367. [7] Goodman, R.R. and Snyder, S.H., Autoradiographic localization of adenosine receptor in rat brain using [3H] cyclohexyladenosine, J. Neurosci., 2 (1982) 1230–1241. [8] Gordon, F.J., Spinal GABA receptors and central cardiovascular control, Brain Res., 328 (1985) 165–169. [9] Hassessian, H., Prat, A., Champlain, J.D. and Couture, R., Regulation of cardiovascular sympathetic neurons by substance P and gamma aminobutyric acid in rat spinal cord, Eur. J. Pharmacol., 202 (1991) 51–60. [10] Hong, Y. and Henry, J.L., Phaclofen-reversible effects of GABA in the spinal cord of the rat, Eur. J. Pharmacol., 201 (1991) 171–177. [11] Kamachi, G.L. and Ticku, M.K., Functional coupling of presynaptic GABAB receptors with voltage-gated Ca2 + channel: regulation by protein kinase A and C in cultured spinal cord neurons, Mol. Pharmacol., 38 (1990) 342–347. [12] Kamachi, G.L. and Ticku, M.K., A functional assay to measure postsynaptic g-aminobutyric acidB response in cultured spinal cord neurons: heterologous regulation of the same K + channel, J. Pharmacol. Exp. Ther., 256 (1991) 426–431. [13] Karbon, E.W. and Enna, S.J., Characterization of the relationship between g-aminobutyric acid B agonist and transmittercoupled cyclic nucleotide-generating systems in rat brain, Mol. Pharmacol., 27 (1985) 53–59. [14] Koh, H.C., Shin, I.C., Hwang, S.J. and Paik, D.J., Modification of cardiovascular response of adenosine A1 receptor agonist by cyclic AMP in the spinal cord of the rats, Neurosci. Lett., 219 (1996) 195–198. [15] Lee, K.S., Reddington, M., Schubert, R. and Kreutzberg, G., Regulation of the strength of adenosine modulation in the hippocampus by a differential distribution of the density of A1 receptor, Brain Res., 260 (1983) 156–159. [16] Lohse, M.J., Boser, S., Klotz, K.N. and Schwabe, U., Affinities of barbiturates for the GABA-receptor complex and A1 adenosine receptors: a possible explanation of their excitatory effects, Naunyn Schmiedeberg’s Arch Pharmacol., 336 (1987) 211– 217. [17] Mosqueda, G.R., Tseng, C.J., Appalsamy, M., Beck, C. and Robertson, D., Cardiovascular excitatory effects of adenosine in the nucleus of the solitary tract, Hypertension, 18 (1991) 494–502. [18] Phillis, J.W., Edstrom, J.P., Kostopoulos, G.K. and Kirkpatrick, J.R., Effects of adenosine and adenine nucleotides on synaptic transmission in the cerebral cortex, Can. J. Pharmacol., 57 (1979) 1289–1312. [19] Schwarz, M., Klockgether, T., Wullner, U., Turkski, L. and Sontag, K.H., Delta-aminovaleric acid antagonizes the pharmacological actions of baclofen in the central nervous system, Exp. Brain Res., 70 (1988) 618–626. [20] Sighart, W., GABAA receptors: ligand-gated Cl − ion channels modulated by multiple drug binding sites, Trends Pharmacol. Sci., 13 (1992) 446–450.
Mediation of the cardiovascular response of adenosine A1 receptor through a GABAB receptor in the spinal cord of the rat Hyun Chul Koh a,*, In Chul Shin a, Se Jin Hwang b, Ju Seop Kang a, Chang Ho Lee a, Ji Hee Ha a, Doo Jin Paik b a
Department of Pharmacology, College of Medicine, Hanyang University, 17 Haengdang-Dong, Sungdong-Ku, Seoul 133–791, South Korea b Department of Anatomy, College of Medicine, Hanyang University, 17 Haengdang-Dong, Sungdong-Ku, Seoul 133–791, South Korea Received 3 October 1997; received in revised form 16 January 1998; accepted 16 January 1998
Abstract Cardiovascular inhibitory effects induced by intrathecal (i.t.) administration of adenosine A1 receptor agonist and its modulation by cyclic AMP was suggested by our previous report. In this experiment, we examined the mediation of cardiovascular effects of adenosine A1 receptor by g-aminobutyric acid receptors A and B (GABAA and GABAB) in the spinal cord. I.t. administration of 10 nmol of N6-cyclohexyladenosine (CHA), an adenosine A1 receptor agonist, and pretreatment with bicuculline (10 nmol, i.t), a GABAA receptor antagonist, and 5-aminovaleric acid (50 nmol, i.t.), a GABAB receptor antagonist, prior to injection of CHA were performed in anesthetized, artificially ventilated Sprague–Dawley rats. I.t. injection of 50 nmol of 5-aminovaleric acid significantly attenuated the inhibitory cardiovascular effects of CHA but 10 nmol of bicuculline did not alter CHA-induced cardiovascular actions. It is suggested that cardiovascular responses of adenosine A1 receptor is meditated by GABAB receptor in the spinal cord. 1998 Published by Elsevier Science Ireland Ltd.
Keywords: Adenosine A1 receptor; g-Aminobutyric acid A receptor; g-Aminobutyric acid B receptor; Spinal cord; Blood pressure; Heart rate
Adenosine A1 receptor, which is found in both central and peripheral nervous system [4], mediates various neuromodulatory actions of adenosine, including cardiovascular regulatory effects. Adenosine A1 receptors in the central nervous system mediates mainly inhibitory effects on synaptic transmission of glutamate, acetylcholine in hippocampus, striatum, hypothalamus and cerebral cortex [3,5,18]. Several lines of evidences suggest that adenosine A1 receptor is responsible for cardiovascular inhibitory effects in central and peripheral nervous system. The administration of adenosine or its agonist into the 4th ventricle [2] and nucleus tractus solitarius [17] resulted in depression of blood pressure (BP) and decrease of heart rate (HR). Previously, we have reported that intrathecal (i.t.) injection of cyclohexyladenosine (CHA), leads the cardiovascular inhibitory effects which is mediated by cyclic AMP but, not by cyclic GMP [14]. GABA is one of the major inhibitory neurotransmitters in the central nervous system and it is suggested to have a significant role in modulating the activity of sympathetic * Corresponding author. Tel.: +82 2 2900651; fax: +82 2 2926686.
preganglionic neurons in the spinal cord [1,8]. Both GABAA and GABAB receptors in these neurons mediate cardiovascular regulatory effects of GABA in the spinal cord [10]. Colocalization and functional relations between adenosine receptors and GABA receptors, were suggested by some authors [7,15]. Gerber and Ga¨hwiler [6] suggested that actions of adenosine and GABAB receptors in hippocampal pyramidal cells are mediated by a common signal transduction pathway. This study was performed to examine the role of GABA receptors on cardiovascular depressive action induced by stimulation of adenosine A1 receptor in lower thoracic cord level. The experimental animals were categorized into three groups. One of these groups was treated only with CHA (10 nmol) and the other two groups were treated with GABAA receptor antagonist, bicuculline and the GABAB receptor antagonist, 5-aminovaleric acid [19], respectively, prior to the administration of the CHA. All drugs were purchased from RBI Chemical Company (MA, USA). CHA and 5-aminovaleric acid were dissolved in 0.9% NaCl solution and bicuculline was dissolved in
0304-3940/98/$19.00 1998 Published by Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940(98) 00089- 5
82
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
2.0% dimethylsulfoxide (DMSO) solution. Male Sprague– Dawley rats (300–350 g) were anesthetized with urethane (1.15 g/kg, i.p.), paralyzed with D-tubocurarine (0.5 mg/kg, i.m.) and artificially ventilated (Ugo Basile, Varese, Italy). BP and HR were continuously monitored via a femoral arterial catheter (PE-50) connected to a pressure transducer (Spectramed, MA, USA) and a polygraph (Grass, MA, USA). Mean arterial pressure was calculated as diastolic pressure +1/3 (systolic pressure − diastolic pressure). Rectal temperature was maintained at 37 ± 0.5°C with a heating pad. The rats were placed in stereotaxic instrument (Stoelting, IL, USA) in prone position. The posterior atlantooccipital membrane was exposed by an occipital incision. The
atlantooccipital membrane was cut and a guide cannula (PE10) was inserted intrathecally; its tip was positioned at the lower thoracic vertebral level (6.5 cm from the lower margin of the occipital bone). I.t. administration of drugs were made using the injector cannula (33-gauge stainless steel) through the guide cannula. CHA (10 nmol) was delivered in a volume of 5 ml in 1 min with Hamilton syringe mounted on a micrometer. Bicuculline (10 nmol) and 5-aminovaleric acid (50 nmol) were injected 10 min before administration of CHA. Data were expressed as the mean ± SE of the maximal response following drug administration. Student’s t-test for paired or unpaired data was used for statistical evaluation of the results.
Fig. 1. Representative tracings depicting the changes of blood pressure (BP) and heart rate (HR) following intrathecal injections of the N6cyclohexyladenosine (CHA). (A) CHA (10 nmol, i.t.) injection only, (B) Bicuculline (10 nmol, i.t.) and CHA (10 nmol, i.t.) injection, (C) 5aminovaleric acid (50 nmol, i.t.) and CHA (10 nmol, i.t.) injection. In (B) and (C), bicuculline and 5-aminovaleric acid were injected 10 min prior to CHA injection.
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
83
Fig. 2. Changes in mean arterial pressure (MAP) and heart rate (HR) following treatment with N6-cyclohexyladenosine (CHA; 10 nmol, i.t.) only and CHA after pretreatment with bicuculline (BC; 10 nmol, i.t.). Data are the mean ± SE.
A representative experiment showing effects of i.t. administration of CHA (10 nmol) on BP and HR is illustrated in Fig. 1. I.t. injection of CHA caused a decrease in MAP that reached maximum in 17.3 ± 4.1 min after injection. The decrease of MAP evoked by CHA was 29.7 ± 6.2 mmHg (n = 5). Decrease of HR was also in– duced by CHA administration, maximum 18.2 ± 5.2 min after injection. HR was decreased by 120 ± 25.4 beats/ min (Figs 2 and 3). Baseline MAP and HR for these rats were 86.7 ± 7.7 mmHg and 345.5 ± 10.3 beats/ min, respectively. Administration (i.t.) of an equivalent volume of normal saline did not affect the basal MAP and HR. Pretreatment with 5-aminovaleric acid (50 nmol, i.t.) sig-
nificantly attenuated the CHA-induced cardiovascular responses; decrease in MAP and HR were 18.1 ± 4.7 mmHg and 64.0 ± 12.0 beats/min (n = 5; Figs. 1 and 3), respectively. However, i.t. administration of bicuculline (10 nmol, i.t.) prior to injection of CHA did not alter the depressor and bradycardiac response elicited by CHA; decrease in MAP and HR were 25.1 ± 5.0 mmHg and 112 ± 19.2 beats/min (n = 5; Figs. 1 and 2), respectively. I.t. injection of 5-aminovaleric acid (50 nmol) and bicuculline (10 nmol) had no effects on basal MAP and HR. In the previous report, we demonstrated that adenosine A1 receptor-mediated cardiovascular response was modulated by cAMP in the spinal cord of rats. In the present experiment, we examined the modulation of cardiovascular effects
Fig. 3. Changes in mean arterial pressure (MAP) and heart rate (HR) following treatment with N6-cyclohexyladenosine (CHA; 10 nmol, i.t.) only and CHA after pretreatment with 5-aminovaleric acid (AV; 50 nmol, i.t). Data are the mean ± SE. *P , 0.01, compared to CHA only group.
84
H.C. Koh et al. / Neuroscience Letters 243 (1998) 81–84
of CHA by GABAA and GABAB receptor antagonist in the spinal cord. Administration (i.t.) of 5-aminovaleric acid significantly attenuated the cardiovascular depressor actions elicited by CHA. Both adenosine A1 and GABAB receptors mediate mainly inhibitory effects in central nervous system. Lohse et al. [16] have suggested that adenosine A1 receptor may interact with GABA receptor in a competitive manner in receptor binding study, but little is known about the relation of two receptors in cardiovascular response. Our result is the first evidence for interaction of two receptors in terms of mechanism of cardiovascular regulation. Koh et al. [14] suggested the modulation of cardiovascular response of adenosine A1 receptors by cAMP in the spinal cord. Actions of GABAB receptors are also mediated by cAMP in cardiovascular modulatory action in the spinal cord [13]. We suggest that these mechanism, in the spinal cord, may contribute to mediation of cardiovascular regulation of CHA by 5-aminovaleric acid, and that adenosine A1 receptor might interact with GABAB receptor in the spinal cardiovascular regulation because 50 nmol of 5-aminovaleric acid alone did not altered basal BP and HR. This can be supported by the following studies. Kamachi and Ticku [11,12] suggested that the action of GABAB receptors involved the G proteins and adenylate cyclase, and that this is under modulatory control by protein kinase A and C. Gerber and Ga¨hwiler [6] also suggested that adenosine receptors and GABA receptors are seemed to be present in the cell membrane of same hippocampal neuron, and contact with same Gi protein pool or separate G proteins which are converged to same adenylate cyclase. Additionally, cardiovascular action of GABAB receptor was blocked by adenosine A1 receptor antagonist, but not by A2 receptor in spinal cord in our experiment (unpublished data). In fact, the GABAA receptor in the spinal cord revealed inhibitory cardiovascular actions [9] as adenosine A1 receptor. But in our experiment, administration (i.t.) of bicuculline (at the dose of no change in basal BP and HR) did not alter the cardiovascular depressive action elicited by CHA. This may account for the different modes of the action of GABAA receptor from that of adenosine A1 receptor in cardiovascular regulation in the spinal cord. GABAA receptors leads to the opening of specific Cl− channels [20] but adenosine A1 receptors leads to decrease of cAMP by inhibition of adenylate cyclase in the spinal cord [14]. In conclusion, we suggest that cardiovascular response of adenosine A1 receptor is mediated by GABAB receptor, not by GABAA receptor in the spinal cord. [1] Bacon, S.J. and Smith, A.D., Preganglionic sympathetic neurons innervating the rat adrenal medulla: immunocytochemical evidences of synaptic input from nerve terminals containing substance-P, GABA or 5-hydroxytryptamine, J. Auton. Nerv. Syst., 24 (1988) 97–122. [2] Barraco, R.A., Campbell, W.R., Parizon, M., Shoener, E.P. and Shein, S.E., Cardiovascular effects of microinjections of adenosine analogs into the fourth ventricle of rats, Brain Res., 424 (1987) 17–25.
[3] Brown, S.J., James, S., Reddington, M. and Richardson, P.J., Both A1 and A2a purine receptors regulate striatal acetylcholine release, J. Neurochem., 55 (1990) 31–38. [4] Choca, J.I., Proudfit, H.K. and Green, R.D., Identification of A1 and A2 adenosine receptors in the rat spinal cord, J. Pharmacol. Exp. Ther., 242 (1987) 905–910. [5] Coardetti, R., Lo Conte, G., Moroni, F., Passani, M.B. and Peteu, G., Adenosine decrease aspartate and glutamate release from rat hippocampal slices, Eur. J. Pharmacol., 104 (1984) 19–26. [6] Gerber, U. and Ga¨hwiler, B.H., GABAB and adenosine receptors mediate enhancement of the K + current, IAHP, by reducing adenylyl cyclase activity in rat CA3 hippocampal neurons, J. Neurophysiol., 72 (1994) 2360–2367. [7] Goodman, R.R. and Snyder, S.H., Autoradiographic localization of adenosine receptor in rat brain using [3H] cyclohexyladenosine, J. Neurosci., 2 (1982) 1230–1241. [8] Gordon, F.J., Spinal GABA receptors and central cardiovascular control, Brain Res., 328 (1985) 165–169. [9] Hassessian, H., Prat, A., Champlain, J.D. and Couture, R., Regulation of cardiovascular sympathetic neurons by substance P and gamma aminobutyric acid in rat spinal cord, Eur. J. Pharmacol., 202 (1991) 51–60. [10] Hong, Y. and Henry, J.L., Phaclofen-reversible effects of GABA in the spinal cord of the rat, Eur. J. Pharmacol., 201 (1991) 171–177. [11] Kamachi, G.L. and Ticku, M.K., Functional coupling of presynaptic GABAB receptors with voltage-gated Ca2 + channel: regulation by protein kinase A and C in cultured spinal cord neurons, Mol. Pharmacol., 38 (1990) 342–347. [12] Kamachi, G.L. and Ticku, M.K., A functional assay to measure postsynaptic g-aminobutyric acidB response in cultured spinal cord neurons: heterologous regulation of the same K + channel, J. Pharmacol. Exp. Ther., 256 (1991) 426–431. [13] Karbon, E.W. and Enna, S.J., Characterization of the relationship between g-aminobutyric acid B agonist and transmittercoupled cyclic nucleotide-generating systems in rat brain, Mol. Pharmacol., 27 (1985) 53–59. [14] Koh, H.C., Shin, I.C., Hwang, S.J. and Paik, D.J., Modification of cardiovascular response of adenosine A1 receptor agonist by cyclic AMP in the spinal cord of the rats, Neurosci. Lett., 219 (1996) 195–198. [15] Lee, K.S., Reddington, M., Schubert, R. and Kreutzberg, G., Regulation of the strength of adenosine modulation in the hippocampus by a differential distribution of the density of A1 receptor, Brain Res., 260 (1983) 156–159. [16] Lohse, M.J., Boser, S., Klotz, K.N. and Schwabe, U., Affinities of barbiturates for the GABA-receptor complex and A1 adenosine receptors: a possible explanation of their excitatory effects, Naunyn Schmiedeberg’s Arch Pharmacol., 336 (1987) 211– 217. [17] Mosqueda, G.R., Tseng, C.J., Appalsamy, M., Beck, C. and Robertson, D., Cardiovascular excitatory effects of adenosine in the nucleus of the solitary tract, Hypertension, 18 (1991) 494–502. [18] Phillis, J.W., Edstrom, J.P., Kostopoulos, G.K. and Kirkpatrick, J.R., Effects of adenosine and adenine nucleotides on synaptic transmission in the cerebral cortex, Can. J. Pharmacol., 57 (1979) 1289–1312. [19] Schwarz, M., Klockgether, T., Wullner, U., Turkski, L. and Sontag, K.H., Delta-aminovaleric acid antagonizes the pharmacological actions of baclofen in the central nervous system, Exp. Brain Res., 70 (1988) 618–626. [20] Sighart, W., GABAA receptors: ligand-gated Cl − ion channels modulated by multiple drug binding sites, Trends Pharmacol. Sci., 13 (1992) 446–450.