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antagonist analgesics and nigrostriatal dopamine metabolism in the rat: Evidence for receptor dualism
Life Sciences, Vol. 33, Sup. I, 1983, pp. 759-762 Printed in the U.S.A.
Pergamon Press
AGONIST/ANTAGONIST ANALGESICS AND NIGROSTRIATAL DOPAMINE METABOLISM IN THE RAT: EVIDENCE FOR RECEPTOR DUALISM
Paul L. Wood*, P. McQuade, J.W. Richard and M. Thakur Douglas Hospital Research Centre, 6875 Boul. LaSalle, Verdun, Qu6. H4H IR3 (Received in final form June 26, 1983) Summary Agonist/antagonist (Ag/Ant) analgesics possess bell-shaped dose-response curves with regard to nigrostriatal dopamine (DA) metabolism in the rat. Using local drug injections as well as parenteral drug, after acute hemisection, we have determined that this phenomenon is not the result of autoinhibition. Our data clearly indicate that the receptor population antagonizing the agonist actions of Ag/Ant is not localized within the striatum or substantia nigra. These results therefore present evidence for receptor dualism with Ag/Ant analgesics. By the use of defined opioid receptor agonists and antagonists, we have been able to demonstrate that the rat nigrostriatal dopaminergic pathway receives independent P2 and 6 regulatory inputs (I-5). In contrast to p and 6 agonists, the Ag/Ant possess bell-shaped dose-response curves with respect to their actions on nigrostriatal DA metabolism (1,4). Similar bell-shaped curves have been reported for Ag/Ant-dependent analgesia (4 for review). The hypotheses concerning these curves invoke either autoinhibition (6) or receptor dualism (7). In this report, we support the hypothesis of receptor dualism. Methods Striatal DA, homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) were measured by gas chromatography-mass fragmentography as described previously (2). Stereotaxic surgery and local drug injections were performed as reported previously (3). For hemisection experiments, rats were hemisected under ether anaesthesia and administered butorphanol l hour later (3). Male Sprague Dawley rats (180-200g) in groups of 7 to lO animals were used. Results Drug combinations Butorphanol alone produced a classical bell-shaped dose-response curve while morphine administration elicited a dose-dependent agonist action on striatal DOPAC levels (Fig. I). However, with the combination of these drugs, no greater effect than that observed with butorphanol was observed except in the case of morphine (64 mg/kg) and butorphanol (2 mg/kg).
*Correspondence:
Douglas Hospital Research Centre, 6875 Boul. LaSalle, Verdun, Quebec H4H IR3 0024-3205/83 $3.00 + .00 Copyright (c) 1983 Pergamon Press L t d
760
Opioid Receptor Dualism
Vol. 33, Sup. I, 1983
140 •
Butorphanol
C) Morphine (2) 120
[]
Morphine (16)
/~ Morphine ( 6 4 ) 100 c0 UJ G: O Z
O < a.
80
60
o
40
20
O 0
i
,
L
,
,
2
4
8
16
32
L
64
BUTORPHANOL (mg/Kg)
FIG. I Actions of morphine, butorphanol and morphine/butorphanol combinations on rat striatal DOPAC levels. Butorphanol was administered at -6 min, morphine at time 0 and the rats sacrificed at 60 min. Points are means of 7-I0 animals with SEM less 6%. Local drug injections The local administration of butorphanol into the striatum or substantia nigra (SN) resulted in only agonist dose-response curves (Table I). TABLE I
Actions of intrastriatal and intranigral butorphanol striatal DA metabolites (60 min) Treatment
(~g)
Intrastrlatal
on
DOPAC HVA (% of vehicle treated rats) Butorphanol
Intranlgral Butorphanol * p <0.O5 (N = 7-10)
(20)
I18"
117
(50)
129"
121*
(lO0)
149"
136"
(200)
181"
160"
(I0)
202*
169"
(I00)
241"
185"
Vol. 33, Sup. I, 1983
Opioid Receptor Dualism
761
Acute hem±section After acute isolation of striatal DA nerve endings from their cell bodies in the SN,'parenteral butorphanol acted as an agonist producing increases in striatal DA metabolites equivalent to those produced by morphine (1). TABLE II Butorphanol actions on striatal DA metabolism after acute hem±section (60 min) Treatment (m~/k~)
DOPAC
HVA (pmol/m 9 protein)
DA
Control side Saline Butorphanol
I08 ± (8) (64)
8
72 ±
4
668 ±
40
140 ± lO* ll2 ± 3
93 ± 82 ±
7* 3
682 ± 596 ±
44 17
173 ± 20
82 ±
4
Lesion side Saline Butorphanol
(8) (64)
166 ± 18 374 ± 67*
93 ± II 284 ± 48*
1420 ± 181 lOl2 ± 59 1265 ± 228
* p <0.05; mean ± SEM (N = 7-I0) Discussion Our experiments with local drug injections of butorphanol as well as parenteral drug in acutely hemisected animals clearly indicate that the receptor population antagonizing the agonist actions of butorphanol are outside the sphere of the nigrostriatal pathway. Furthermore the hemisection experiments indicate this antagonist action must be caudal to the striatum. A further complexity in this experimental paradigm is defining the receptor population mediating the agonist actions of butorphanol on the nigrostriatal system. Clearly it is an opiate receptor, since the agonist actions are reversed by naloxone (1). However, cross-tolerance studies (4) indicate that it is unlikely to involve 9, 6 or < receptors. This conclusion is further supported by the observations reported in Fig. I. Irrespective of the dose ratio of the agonist to Ag/Ant (8), butorphanol acts as an antagonist of morphine. These data indicate that throughout the bell-shaped dose-response curve, butorphanol is acting as a ~ antagonist and cannot be eliciting its agonist actions on the nigrostriatal pathway via a ~ receptor. Similar experiments with D-Ala2-D-Leu5 enkephalin have negated a ~ receptor agonism (4). In summary, our data indicate that butorphanol is a potent ~2 and 6 antagonist with regard to the rat nigrostriatal pathway. Therefore, the receptor population responsible for the agonist actions of this drug on DA metabolism remains to be defined. Similarly, the receptor system, which is activated at high drug doses and which opposes the agonist actions, remains to be defined both in character and exact location. Present data indicates that this receptor is caudal to the striatum. Acknowledgements We wish to thank Dr. W.F. Minor ( B r i s t o l ) f o r the generous supply o f b u t o r phanol t a r t r a t e . This work was supported by the Douglas Hospital Research Centre and the Canadian Medical Research C o u n c i l .
762
Opioid Receptor Dualism
Vol. 33, Sup. I, 1983
References I.
5. 6.
P.L. WOOD, M. STOTLAND, J.W. RICHARD and A. RACKHAM, J. Pharmacol. Exp. Ther. 215 697-703 (]980). P.L. WOOD, J.W. RICHARD and M. THAKUR, Life Sci. 31 2313-2317 (1982). P.L. WOOD and J.W. RICHARD, Neuropharmacology 21 1305-1310 (1982). P.L. WOOD, D. SANSCHAGRIN, J.W. RICHARD and M. THAKUR, J. Pharmacol. Exp. Ther. (in press). P.L. WOOD and G. PASTERNAK, Neurosci. Lett. (in press)a E.J. ARIENS, J.M. VAN ROSSUM and A.M. SIMONIS, Pharmacol. Rev. 9 218-236
7.
(1957). W,R. MARTIN, Pharmacol. Rev. 19 463-521 (1967).
8.
M.J. RANCE, Br. J. Clin. Pharma---col. ~ 2815-2865 (1979).
2. 3. 4.
Pergamon Press
AGONIST/ANTAGONIST ANALGESICS AND NIGROSTRIATAL DOPAMINE METABOLISM IN THE RAT: EVIDENCE FOR RECEPTOR DUALISM
Paul L. Wood*, P. McQuade, J.W. Richard and M. Thakur Douglas Hospital Research Centre, 6875 Boul. LaSalle, Verdun, Qu6. H4H IR3 (Received in final form June 26, 1983) Summary Agonist/antagonist (Ag/Ant) analgesics possess bell-shaped dose-response curves with regard to nigrostriatal dopamine (DA) metabolism in the rat. Using local drug injections as well as parenteral drug, after acute hemisection, we have determined that this phenomenon is not the result of autoinhibition. Our data clearly indicate that the receptor population antagonizing the agonist actions of Ag/Ant is not localized within the striatum or substantia nigra. These results therefore present evidence for receptor dualism with Ag/Ant analgesics. By the use of defined opioid receptor agonists and antagonists, we have been able to demonstrate that the rat nigrostriatal dopaminergic pathway receives independent P2 and 6 regulatory inputs (I-5). In contrast to p and 6 agonists, the Ag/Ant possess bell-shaped dose-response curves with respect to their actions on nigrostriatal DA metabolism (1,4). Similar bell-shaped curves have been reported for Ag/Ant-dependent analgesia (4 for review). The hypotheses concerning these curves invoke either autoinhibition (6) or receptor dualism (7). In this report, we support the hypothesis of receptor dualism. Methods Striatal DA, homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) were measured by gas chromatography-mass fragmentography as described previously (2). Stereotaxic surgery and local drug injections were performed as reported previously (3). For hemisection experiments, rats were hemisected under ether anaesthesia and administered butorphanol l hour later (3). Male Sprague Dawley rats (180-200g) in groups of 7 to lO animals were used. Results Drug combinations Butorphanol alone produced a classical bell-shaped dose-response curve while morphine administration elicited a dose-dependent agonist action on striatal DOPAC levels (Fig. I). However, with the combination of these drugs, no greater effect than that observed with butorphanol was observed except in the case of morphine (64 mg/kg) and butorphanol (2 mg/kg).
*Correspondence:
Douglas Hospital Research Centre, 6875 Boul. LaSalle, Verdun, Quebec H4H IR3 0024-3205/83 $3.00 + .00 Copyright (c) 1983 Pergamon Press L t d
760
Opioid Receptor Dualism
Vol. 33, Sup. I, 1983
140 •
Butorphanol
C) Morphine (2) 120
[]
Morphine (16)
/~ Morphine ( 6 4 ) 100 c0 UJ G: O Z
O < a.
80
60
o
40
20
O 0
i
,
L
,
,
2
4
8
16
32
L
64
BUTORPHANOL (mg/Kg)
FIG. I Actions of morphine, butorphanol and morphine/butorphanol combinations on rat striatal DOPAC levels. Butorphanol was administered at -6 min, morphine at time 0 and the rats sacrificed at 60 min. Points are means of 7-I0 animals with SEM less 6%. Local drug injections The local administration of butorphanol into the striatum or substantia nigra (SN) resulted in only agonist dose-response curves (Table I). TABLE I
Actions of intrastriatal and intranigral butorphanol striatal DA metabolites (60 min) Treatment
(~g)
Intrastrlatal
on
DOPAC HVA (% of vehicle treated rats) Butorphanol
Intranlgral Butorphanol * p <0.O5 (N = 7-10)
(20)
I18"
117
(50)
129"
121*
(lO0)
149"
136"
(200)
181"
160"
(I0)
202*
169"
(I00)
241"
185"
Vol. 33, Sup. I, 1983
Opioid Receptor Dualism
761
Acute hem±section After acute isolation of striatal DA nerve endings from their cell bodies in the SN,'parenteral butorphanol acted as an agonist producing increases in striatal DA metabolites equivalent to those produced by morphine (1). TABLE II Butorphanol actions on striatal DA metabolism after acute hem±section (60 min) Treatment (m~/k~)
DOPAC
HVA (pmol/m 9 protein)
DA
Control side Saline Butorphanol
I08 ± (8) (64)
8
72 ±
4
668 ±
40
140 ± lO* ll2 ± 3
93 ± 82 ±
7* 3
682 ± 596 ±
44 17
173 ± 20
82 ±
4
Lesion side Saline Butorphanol
(8) (64)
166 ± 18 374 ± 67*
93 ± II 284 ± 48*
1420 ± 181 lOl2 ± 59 1265 ± 228
* p <0.05; mean ± SEM (N = 7-I0) Discussion Our experiments with local drug injections of butorphanol as well as parenteral drug in acutely hemisected animals clearly indicate that the receptor population antagonizing the agonist actions of butorphanol are outside the sphere of the nigrostriatal pathway. Furthermore the hemisection experiments indicate this antagonist action must be caudal to the striatum. A further complexity in this experimental paradigm is defining the receptor population mediating the agonist actions of butorphanol on the nigrostriatal system. Clearly it is an opiate receptor, since the agonist actions are reversed by naloxone (1). However, cross-tolerance studies (4) indicate that it is unlikely to involve 9, 6 or < receptors. This conclusion is further supported by the observations reported in Fig. I. Irrespective of the dose ratio of the agonist to Ag/Ant (8), butorphanol acts as an antagonist of morphine. These data indicate that throughout the bell-shaped dose-response curve, butorphanol is acting as a ~ antagonist and cannot be eliciting its agonist actions on the nigrostriatal pathway via a ~ receptor. Similar experiments with D-Ala2-D-Leu5 enkephalin have negated a ~ receptor agonism (4). In summary, our data indicate that butorphanol is a potent ~2 and 6 antagonist with regard to the rat nigrostriatal pathway. Therefore, the receptor population responsible for the agonist actions of this drug on DA metabolism remains to be defined. Similarly, the receptor system, which is activated at high drug doses and which opposes the agonist actions, remains to be defined both in character and exact location. Present data indicates that this receptor is caudal to the striatum. Acknowledgements We wish to thank Dr. W.F. Minor ( B r i s t o l ) f o r the generous supply o f b u t o r phanol t a r t r a t e . This work was supported by the Douglas Hospital Research Centre and the Canadian Medical Research C o u n c i l .
762
Opioid Receptor Dualism
Vol. 33, Sup. I, 1983
References I.
5. 6.
P.L. WOOD, M. STOTLAND, J.W. RICHARD and A. RACKHAM, J. Pharmacol. Exp. Ther. 215 697-703 (]980). P.L. WOOD, J.W. RICHARD and M. THAKUR, Life Sci. 31 2313-2317 (1982). P.L. WOOD and J.W. RICHARD, Neuropharmacology 21 1305-1310 (1982). P.L. WOOD, D. SANSCHAGRIN, J.W. RICHARD and M. THAKUR, J. Pharmacol. Exp. Ther. (in press). P.L. WOOD and G. PASTERNAK, Neurosci. Lett. (in press)a E.J. ARIENS, J.M. VAN ROSSUM and A.M. SIMONIS, Pharmacol. Rev. 9 218-236
7.
(1957). W,R. MARTIN, Pharmacol. Rev. 19 463-521 (1967).
8.
M.J. RANCE, Br. J. Clin. Pharma---col. ~ 2815-2865 (1979).
2. 3. 4.