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Centrally administered ß;-endorphin produces prolonged changes in δ-opioid ligand activity in vivo
European Journal of Pharmacology, 120 (1986) 359-362
359
Elsevier
Short communication CENTRALLY A D M I N I S T E R E D [3-ENDORPH1N P R O D U C E S P R O L O N G E D C H A N G E S IN 8 - O P I O I D L I G A N D ACTIVITY IN VIVO A N D R E DRAY, LINDA N U N A N and WILLIAM WIRE
Universi(v of Arizona, Department of Pharmacology, Health Sciences Center. Tucson. A Z 85724, U.S.A.
Received 28 November 1985, accepted 3 December 1985
A. DRAY, L. NUNAN and W. WIRE, Centrally administered fl-endorphin produces prohmged changes in ~-opioid ligand activity' in vivo, European J. Pharmacol. 120 (1986) 359-362. Spontaneous reflex bladder contractions were recorded isometrically in urethane anesthetized rats. Bladder contractions were depressed by intracerebroventricular injections of the ~-opioid receptor agonist [DP¢la2,MePhe4,Gly(ol)S]enkephalin (DAGO) and the 8-agonist [2D-penicillamine,5D-penicillamine]enkephalin (DPDPE) respectively. The effect of DPDPE was selectively antagonized by ICI 174,864 (N,N-diallyl-Tyr-Aib-AibPhe-Leu-OH: Aib = a-aminoisobutyric acid). However following the administration of fl-endorphin the antagonistic action of ICI 174,864 could no longer be observed. In addition ICI 174,864 exhibited agonistic activity following fl-endorphin and the effects of DPDPE were prolonged in a dose related manner by fl-endorphin. These observations suggest that fl-endorphin may produce complex changes in central 6-opioid receptor activity. /3-Endorphins
8-Receptor changes
ICI 174,864
1. Introduction Opioids produce their effects by an interaction with one or other of the p,-, 6- or K-opioid receptor subtypes (Paterson et al., 1983). In addition recent reports have suggested that these opioid receptor subtypes may themselves interact or interconvert (see Rothman and Westfall, 1982; D ' A m a t o and Holaday, 1984) and that several opioid peptides including dynorphin and enkephalin may be involved in the physiological regulation of an opioid receptor macromolecular complex (Lee and Smith, 1984). The present experiments support this latter hypothesis by the demonstration that exogenously administered /3-endorphin produces prolonged changes in central ~-opioid ligand activity.
2. Materials and methods Experiments were performed in vivo using inhibition of reflex urinary bladder contractions as an index of central opioid activity (Dray and 0014-2999/86/$03.50 © 1986 Elsevier Science Publishers B.V.
Bladder reflexes
Nunan, 1984). Female Sprague-Dawley rats (200220 g) were anesthetized with urethane (1.2 ~ g / k g i.p.) and the urinary bladder was catheterized via the urethra. Intravesicular pressure was monitored via a pressure transducer and recorded continuously. The bladder was filled with saline until spontaneous contractions were initiated as a result of central reflex activity. Rhythmic contractions were recorded isometrically for prolonged periods and could be influenced by intracerebroventricularly (i.c.v.: lateral ventricle) administered substances. Bladder activity was inhibited by alternate injections of submaximal but equieffective doses of [D-Ala 2,Me-Phe4,Gly(ol) s]enkephalin ( D A G O ) and [2D-penicillamine,5 D-penicillamine]enkephalin (DPDPE) highly selective bt- and ~-opioid receptor agonists respectively (Handa et al., 1981: Mosberg et al., 1983). In approximately half of the experiments the order of agonist administrations was reversed. The selective 6-receptor antagonistic effect of ICI 174,864 (N,N-diallyl-Tyr-Aib-AibPhe-Leu-OH; Aib = a-aminoisobutyric acid) was then demonstrated. Following recovery from this
360
effect jS-endorphin was a d m i n i s t e r e d aud the test with ICI 174,864 was repeated. In other experim e n t s D A G O , D P D P E and ICI 174,864 were also tested s e p a r a t e l y before, and at various p e r i o d s after the a d m i n i s t r a t i o n of fl-endorphin. A naive a n i m a l was used for each test with each c o m pound. Drugs were o b t a i n e d from the following sources: D A G O , f i - e n d o r p h i n (Peninsula L a b o r a t o r i e s Inc., California), D P D P E (gift from Dr. H. Mosberg, D e p a r t m e n t of Chemistry, University of Arizona), ICI 174,864 (gift from Dr. R. Cotton, ICI Pharmaceuticals Division, Macclesfield, England) and were m a d e up to the required c o n c e n t r a t i o n with sterile physiological saline.
0
60 [
~LIj"~kf
£
0
~ rain
10 min
r"~
,~l
• DPDPE 4pg i.c.v.
• DAGO O. lpg i.c.v.
ICI 174,864 3lag i.c.v.
•DPDPE 10 rain
/
/
/
• DAGO Recovery 120 min
• DPDPE
3. Results 20 min after [3 Endorphin lpg i.c.v.
S u b m a x i m a l doses of D P D P E (4 /~g i.c.v.) a n d D A G O (0.1 # g i.c.v.) p r o d u c e d a p p r o x i m a t e l y e q u i p o t e n t inhibition of s p o n t a n e o u s b l a d d e r contractions. The s u b s e q u e n t a d m i n i s t r a t i o n of ICI 174,864 (3/~g i.c.v.) at a dose previously found to p r o d u c e selective & r e c e p t o r a n t a g o n i s m ( D r a y and N u n a n , 1984) p r o d u c e d no d e t e c t a b l e effect on b l a d d e r c o n t r a c t i o n frequency but a t t e n u a t e d or a b o l i s h e d the effect of D P D P E (16 of 16 experiments, fig. 1). Recovery from the a n t a g o n i s t i c effect of ICI 174,864 was o b s e r v e d some 92-240 rain later (fig. 1). A t this stage f l - e n d o r p h i n was a d m i n istered (1 /~g i.c.v.) and it p r o d u c e d an inhibition of reflex b l a d d e r c o n t r a c t i o n s (table 1). Some 20-60 rain after c o m p l e t e recovery from the i n h i b i t o r y effects of ~ - e n d o r p h i n , IC1 174,864 was retested. This c o m p o u n d now a p p e a r e d to have little or no a n t a g o n i s t i c activity t o w a r d s D P D P E (fig. 1). On the c o n t r a r y ICI 174,864 itself showed agonistic activity (3 ~g i.c.v. = 16_+ 4 rain, n = 14), while the inhibitory effect of D P D P E was significantly p r o l o n g e d (9 of 9 experiments; control 13 _+ 2 rain: post f l - e n d o r p h i n 25 _+ 6 min). In further e x p e r i m e n t s each s u b s t a n c e was tested s e p a r a t e l y 60 min before and at 60 a n d 120 rain after the i.c.v, a d m i n i s t r a t i o n of f l - e n d o r p h i n or saline. Since /Y-endorphin p r o d u c e d d o s e - r e l a t e d inhibition of b l a d d e r activity the effects of o t h e r substances were only tested after c o m p l e t e re-
• ICI 174,864
• DPDPE j % ~ f t e r
[~ Endorphin
5 min
/,,,a,,~ j 4
• ICI 174,864
Fig. 1. The effect of fl-endorphin on the antagonistic activity of ICI 174,864. The top two traces show the inhibition of reflex urinary bladder contractions by submaximal intracerebroventricular (i.c.v.) doses of the &receptor agonist [2D-penicillamine,5D-penicillamine]enkephalin (DPDPE, 4 /~g) and the /~-agonist [D-Ala2,Me-Phe4,Gly(ol)S]enkephalin (DAGO, 0.1 ~g i.c.v.). A subsequent administration of ICI 174,864 (3 /,tg i.c.v.) did not change bladder contractions but completely abolished the effect of DPDPE and not that of DAGO (middle traces). Some 120 min after the ICI 174,864 was administered the effect of DPDPE recovered. The bottom three traces show that when ICI 174,864 was tested 20 rain after the recovery from the inhibitory effect of fl-endorphin (1 p,g i.c.v.) it no longer abolished the action of DPDPE. In addition IC1 174,864 (3 p,g i.c.v.) administered 80 rain after recovery from /~-endorphin now exhibited agonistic activity (bottom trace). All traces are taken from the same experiment.
covery of s p o n t a n e o u s reflex contractions. The effect of D A G O was u n c h a n g e d b y / ~ - e n d o r p h i n whereas the p r o l o n g a t i o n of the D P D P E effect a n d the I C I 174,864 agonistic effect were clearly
361 TABLE 1 Summary of data concerning the interaction of intracerebroventricular fl-endorphin with DAGO, D P D P E and IC! 174,864. The first column shows the test substances and subsequent columns show their inhibitory effect on reflex bladder contractions determined 60 min before (control) and at periods of 60 and 120 min following the administration of fl-endorphin (0.1 p,g, 0.5 /tg and 1.0 /,tg) of saline (2 /~l) respectively. The dose-related inhibitory effect of fl-endorphin is indicated in the fl-endorphin columns. Responses are shown as the period of bladder quiescence (min _+S.D.). n = 6-10 for each determination. /LEndorphin at 1.0 ~tg produced no change in the response to DAGO. The interactions of DAGO following smaller doses of fl-endorphin were not tested. No significant changes in the response to DPDPE or ICI 174,864 were observed with 0.1 /tg of fl-endorphin. However the effect of DPDPE was significantly prolonged after 0.5 p~g and 1.0 ,ug of fl-endorphin. ICI 174,864 also exhibited agonistic activity following these doses of fl-endorphin. Saline administrations did not change the response to DPDPE or 1CI 174,864. fl-Endorphin 0.1 p,g (4_+2 min) -
fl-Endorphin 0.5 p,g (14_+4 rain) -
fl-Endorphin 1.0 p,g (20+_9 min)
DAGO (0.1 p~g)
Control 60 120
DPDPE (4/tg)
Control 60 120
13 _+3 14±3 15_+2
12 ± 2 27±7 * 15±2
19 ± 5 3 9 + 9 ** 29±3 *
ICI 174,564 (3/tg)
Control 60 120
0 3+_4 0
0 14±4"* 4±2
0 1 7 ± 5 ** 7_+4 *
Saline (2 /11)
16 + 4 14+_4 17+5 16 ± 4 15+3 15±2 0 0 0
** P < 0.01, * P < 0.05: Wilcoxon's signed-ranks test.
related magnitude and duration to the dose of fl-endorphin administered (table 1). Saline administrations (2 ~1) did not alter reflex bladder contractions or the effects of each compound tested (table 1).
4. Discussion Our present observations suggest that the prior administration of fl-endorphin produced a prolonged change in central &opioid receptor activity. Thus ICI 174,864 no longer exhibited 6-receptor antagonism but showed agonistic activity, the basis of which is presently unknown. In addition fl-endorphin prolonged the activity of the highly selective 8-receptor agonist DPDPE. In each respect the effects of fl-endorphin were dose related. It is unclear how these effects were mediated but they could have resulted from fl-endorphin itself or a fragment derived from in vivo metabolism. The prolonged time course of these phenomenon may relate to the fact that this peptide is
relatively slowly processed (Davis et al., 1982) and being highly lipophilic may remain bound in a biologically active form for a considerable period. At a molecular level /3-endorphin has been postulated to interact in a complex manner with a number of opioid receptor subtypes (for references see Houghten et al., 1984). But these interactions may or may not be relevant to the present observations because the changes in 8-1igand activity occurred for a prolonged period after the direct agonistic effect of fl-endorphin had been observed. It is attractive to speculate that the alterations in a 8-opioid receptor complex were preceded by an activation of a closely associated macromolecule which affected allosterically, both the 'antagonistic' and 'agonistic' conformation of the 8-receptor. An interaction with a common receptor complex has been suggested for a number of opioids with differing receptor activities, including dynorphin, enkephalin and fl-endorphin (Lee and Smith, 1984). Complex opioid actions involving /.t- and 6-receptor interconversion and interactions have also been reported (see Rothman and Westfall, 1982; Lee and Smith, 1984; D'Amato and Hola-
362
day, 1984) while in addition ~c-ligands (dynorphin(1-13)) produced changes in both /, (morphine, naloxone) and &receptor (ICI 174,864) (Lee and Smith, 1984: Long et al., 1984) activity. In this latter regard there are direct similarities with the present observations: thus pretreatment with dynorphin-(1-13) prevented the effect of ICI 174,864 on endotoxin-induced hypotension (Long et al., 1984). In conclusion a variety of complex regulatory activities may be performed by opioid agonists or antagonists acting on a common macromolecule. The present experiments suggest that ,8-endorphin may be one of a number of neuropeptides that can function in this way. References D'Amato, R. and J.W. Holaday, 1984, Multiple opioid receptors in endotoxic shock: evidence for 8 involvement and /Z-8 interactions in vivo, Proc. Natl. Acad. Sci. U.S.A. 81, 2898. Davis, T.P.. H. Schoemaker, A. Chen, A. Culling and H.I. Yamamura, 1982, Isolation and characterization of in vitro
endorphm metabolism in the brain, Proc. West. Pharmacol. Soc 25, 101. Dray, A. and L Nunan, 1984, Selective 8-opioid receptor antagonism by ICl 174,864 in the central nervous system, Peptides 5, 1015. Handa, B.K., A.C. Lane, J.A.H. Lord, B.A. Morgan, M.J. Rance and C F . C . Smith, 1981, Analogs of bcta-l, PH~, I ~,4 possessing selective agonist activity of /*-opiate receptors, European J. Pharmacol. 70, 531. Houghten, R.A., N. Johnson and G.W. Pasternak, 1984. [~H],8-Endorphin binding in rat brain, J. Neurosci. 4, 24611. Lee, N.M. and A.P. Smith, 1984, Possible regulatory function of dynorphin and its clinical implications. Trends Pharmacol. Sci. 5, 108. Long, ,I.B.. B.A. Ruvio, C.E. Glatt and J.W. Holaday, 1984, l('l 174,864, a putative delta opioid antagonist, reverses cndotoxemic hypotension: pretreatment with dynorphin 1-13, a K-agonist, blocks this action, Neuropeptides 5, 29I. Mosberg, H.I., R. Hurst, V.J. Hruby, K. Gee, H.I. Yamamura, ,1.,I. Galligan and T.F. Burks, 1983, Bis-penicillaminc cnkephalins possess highly improved specificity towards delta opioid receptors, Proc. Natl. Acad. Sci. U.S.A. 80, 5871. Paterson, S.J., L.E. Robson and H.W. Kosterlitz, 1983, Classification of opioid receptors, Br. Med. Bull. 39, 31. Rothman, R.B. and T.C. Westfall, 1982, Allostcric coupling between morphine and enkephalin receptors in vitro. Mol. Pharmacol. 2I, 548.
359
Elsevier
Short communication CENTRALLY A D M I N I S T E R E D [3-ENDORPH1N P R O D U C E S P R O L O N G E D C H A N G E S IN 8 - O P I O I D L I G A N D ACTIVITY IN VIVO A N D R E DRAY, LINDA N U N A N and WILLIAM WIRE
Universi(v of Arizona, Department of Pharmacology, Health Sciences Center. Tucson. A Z 85724, U.S.A.
Received 28 November 1985, accepted 3 December 1985
A. DRAY, L. NUNAN and W. WIRE, Centrally administered fl-endorphin produces prohmged changes in ~-opioid ligand activity' in vivo, European J. Pharmacol. 120 (1986) 359-362. Spontaneous reflex bladder contractions were recorded isometrically in urethane anesthetized rats. Bladder contractions were depressed by intracerebroventricular injections of the ~-opioid receptor agonist [DP¢la2,MePhe4,Gly(ol)S]enkephalin (DAGO) and the 8-agonist [2D-penicillamine,5D-penicillamine]enkephalin (DPDPE) respectively. The effect of DPDPE was selectively antagonized by ICI 174,864 (N,N-diallyl-Tyr-Aib-AibPhe-Leu-OH: Aib = a-aminoisobutyric acid). However following the administration of fl-endorphin the antagonistic action of ICI 174,864 could no longer be observed. In addition ICI 174,864 exhibited agonistic activity following fl-endorphin and the effects of DPDPE were prolonged in a dose related manner by fl-endorphin. These observations suggest that fl-endorphin may produce complex changes in central 6-opioid receptor activity. /3-Endorphins
8-Receptor changes
ICI 174,864
1. Introduction Opioids produce their effects by an interaction with one or other of the p,-, 6- or K-opioid receptor subtypes (Paterson et al., 1983). In addition recent reports have suggested that these opioid receptor subtypes may themselves interact or interconvert (see Rothman and Westfall, 1982; D ' A m a t o and Holaday, 1984) and that several opioid peptides including dynorphin and enkephalin may be involved in the physiological regulation of an opioid receptor macromolecular complex (Lee and Smith, 1984). The present experiments support this latter hypothesis by the demonstration that exogenously administered /3-endorphin produces prolonged changes in central ~-opioid ligand activity.
2. Materials and methods Experiments were performed in vivo using inhibition of reflex urinary bladder contractions as an index of central opioid activity (Dray and 0014-2999/86/$03.50 © 1986 Elsevier Science Publishers B.V.
Bladder reflexes
Nunan, 1984). Female Sprague-Dawley rats (200220 g) were anesthetized with urethane (1.2 ~ g / k g i.p.) and the urinary bladder was catheterized via the urethra. Intravesicular pressure was monitored via a pressure transducer and recorded continuously. The bladder was filled with saline until spontaneous contractions were initiated as a result of central reflex activity. Rhythmic contractions were recorded isometrically for prolonged periods and could be influenced by intracerebroventricularly (i.c.v.: lateral ventricle) administered substances. Bladder activity was inhibited by alternate injections of submaximal but equieffective doses of [D-Ala 2,Me-Phe4,Gly(ol) s]enkephalin ( D A G O ) and [2D-penicillamine,5 D-penicillamine]enkephalin (DPDPE) highly selective bt- and ~-opioid receptor agonists respectively (Handa et al., 1981: Mosberg et al., 1983). In approximately half of the experiments the order of agonist administrations was reversed. The selective 6-receptor antagonistic effect of ICI 174,864 (N,N-diallyl-Tyr-Aib-AibPhe-Leu-OH; Aib = a-aminoisobutyric acid) was then demonstrated. Following recovery from this
360
effect jS-endorphin was a d m i n i s t e r e d aud the test with ICI 174,864 was repeated. In other experim e n t s D A G O , D P D P E and ICI 174,864 were also tested s e p a r a t e l y before, and at various p e r i o d s after the a d m i n i s t r a t i o n of fl-endorphin. A naive a n i m a l was used for each test with each c o m pound. Drugs were o b t a i n e d from the following sources: D A G O , f i - e n d o r p h i n (Peninsula L a b o r a t o r i e s Inc., California), D P D P E (gift from Dr. H. Mosberg, D e p a r t m e n t of Chemistry, University of Arizona), ICI 174,864 (gift from Dr. R. Cotton, ICI Pharmaceuticals Division, Macclesfield, England) and were m a d e up to the required c o n c e n t r a t i o n with sterile physiological saline.
0
60 [
~LIj"~kf
£
0
~ rain
10 min
r"~
,~l
• DPDPE 4pg i.c.v.
• DAGO O. lpg i.c.v.
ICI 174,864 3lag i.c.v.
•DPDPE 10 rain
/
/
/
• DAGO Recovery 120 min
• DPDPE
3. Results 20 min after [3 Endorphin lpg i.c.v.
S u b m a x i m a l doses of D P D P E (4 /~g i.c.v.) a n d D A G O (0.1 # g i.c.v.) p r o d u c e d a p p r o x i m a t e l y e q u i p o t e n t inhibition of s p o n t a n e o u s b l a d d e r contractions. The s u b s e q u e n t a d m i n i s t r a t i o n of ICI 174,864 (3/~g i.c.v.) at a dose previously found to p r o d u c e selective & r e c e p t o r a n t a g o n i s m ( D r a y and N u n a n , 1984) p r o d u c e d no d e t e c t a b l e effect on b l a d d e r c o n t r a c t i o n frequency but a t t e n u a t e d or a b o l i s h e d the effect of D P D P E (16 of 16 experiments, fig. 1). Recovery from the a n t a g o n i s t i c effect of ICI 174,864 was o b s e r v e d some 92-240 rain later (fig. 1). A t this stage f l - e n d o r p h i n was a d m i n istered (1 /~g i.c.v.) and it p r o d u c e d an inhibition of reflex b l a d d e r c o n t r a c t i o n s (table 1). Some 20-60 rain after c o m p l e t e recovery from the i n h i b i t o r y effects of ~ - e n d o r p h i n , IC1 174,864 was retested. This c o m p o u n d now a p p e a r e d to have little or no a n t a g o n i s t i c activity t o w a r d s D P D P E (fig. 1). On the c o n t r a r y ICI 174,864 itself showed agonistic activity (3 ~g i.c.v. = 16_+ 4 rain, n = 14), while the inhibitory effect of D P D P E was significantly p r o l o n g e d (9 of 9 experiments; control 13 _+ 2 rain: post f l - e n d o r p h i n 25 _+ 6 min). In further e x p e r i m e n t s each s u b s t a n c e was tested s e p a r a t e l y 60 min before and at 60 a n d 120 rain after the i.c.v, a d m i n i s t r a t i o n of f l - e n d o r p h i n or saline. Since /Y-endorphin p r o d u c e d d o s e - r e l a t e d inhibition of b l a d d e r activity the effects of o t h e r substances were only tested after c o m p l e t e re-
• ICI 174,864
• DPDPE j % ~ f t e r
[~ Endorphin
5 min
/,,,a,,~ j 4
• ICI 174,864
Fig. 1. The effect of fl-endorphin on the antagonistic activity of ICI 174,864. The top two traces show the inhibition of reflex urinary bladder contractions by submaximal intracerebroventricular (i.c.v.) doses of the &receptor agonist [2D-penicillamine,5D-penicillamine]enkephalin (DPDPE, 4 /~g) and the /~-agonist [D-Ala2,Me-Phe4,Gly(ol)S]enkephalin (DAGO, 0.1 ~g i.c.v.). A subsequent administration of ICI 174,864 (3 /,tg i.c.v.) did not change bladder contractions but completely abolished the effect of DPDPE and not that of DAGO (middle traces). Some 120 min after the ICI 174,864 was administered the effect of DPDPE recovered. The bottom three traces show that when ICI 174,864 was tested 20 rain after the recovery from the inhibitory effect of fl-endorphin (1 p,g i.c.v.) it no longer abolished the action of DPDPE. In addition IC1 174,864 (3 p,g i.c.v.) administered 80 rain after recovery from /~-endorphin now exhibited agonistic activity (bottom trace). All traces are taken from the same experiment.
covery of s p o n t a n e o u s reflex contractions. The effect of D A G O was u n c h a n g e d b y / ~ - e n d o r p h i n whereas the p r o l o n g a t i o n of the D P D P E effect a n d the I C I 174,864 agonistic effect were clearly
361 TABLE 1 Summary of data concerning the interaction of intracerebroventricular fl-endorphin with DAGO, D P D P E and IC! 174,864. The first column shows the test substances and subsequent columns show their inhibitory effect on reflex bladder contractions determined 60 min before (control) and at periods of 60 and 120 min following the administration of fl-endorphin (0.1 p,g, 0.5 /tg and 1.0 /,tg) of saline (2 /~l) respectively. The dose-related inhibitory effect of fl-endorphin is indicated in the fl-endorphin columns. Responses are shown as the period of bladder quiescence (min _+S.D.). n = 6-10 for each determination. /LEndorphin at 1.0 ~tg produced no change in the response to DAGO. The interactions of DAGO following smaller doses of fl-endorphin were not tested. No significant changes in the response to DPDPE or ICI 174,864 were observed with 0.1 /tg of fl-endorphin. However the effect of DPDPE was significantly prolonged after 0.5 p~g and 1.0 ,ug of fl-endorphin. ICI 174,864 also exhibited agonistic activity following these doses of fl-endorphin. Saline administrations did not change the response to DPDPE or 1CI 174,864. fl-Endorphin 0.1 p,g (4_+2 min) -
fl-Endorphin 0.5 p,g (14_+4 rain) -
fl-Endorphin 1.0 p,g (20+_9 min)
DAGO (0.1 p~g)
Control 60 120
DPDPE (4/tg)
Control 60 120
13 _+3 14±3 15_+2
12 ± 2 27±7 * 15±2
19 ± 5 3 9 + 9 ** 29±3 *
ICI 174,564 (3/tg)
Control 60 120
0 3+_4 0
0 14±4"* 4±2
0 1 7 ± 5 ** 7_+4 *
Saline (2 /11)
16 + 4 14+_4 17+5 16 ± 4 15+3 15±2 0 0 0
** P < 0.01, * P < 0.05: Wilcoxon's signed-ranks test.
related magnitude and duration to the dose of fl-endorphin administered (table 1). Saline administrations (2 ~1) did not alter reflex bladder contractions or the effects of each compound tested (table 1).
4. Discussion Our present observations suggest that the prior administration of fl-endorphin produced a prolonged change in central &opioid receptor activity. Thus ICI 174,864 no longer exhibited 6-receptor antagonism but showed agonistic activity, the basis of which is presently unknown. In addition fl-endorphin prolonged the activity of the highly selective 8-receptor agonist DPDPE. In each respect the effects of fl-endorphin were dose related. It is unclear how these effects were mediated but they could have resulted from fl-endorphin itself or a fragment derived from in vivo metabolism. The prolonged time course of these phenomenon may relate to the fact that this peptide is
relatively slowly processed (Davis et al., 1982) and being highly lipophilic may remain bound in a biologically active form for a considerable period. At a molecular level /3-endorphin has been postulated to interact in a complex manner with a number of opioid receptor subtypes (for references see Houghten et al., 1984). But these interactions may or may not be relevant to the present observations because the changes in 8-1igand activity occurred for a prolonged period after the direct agonistic effect of fl-endorphin had been observed. It is attractive to speculate that the alterations in a 8-opioid receptor complex were preceded by an activation of a closely associated macromolecule which affected allosterically, both the 'antagonistic' and 'agonistic' conformation of the 8-receptor. An interaction with a common receptor complex has been suggested for a number of opioids with differing receptor activities, including dynorphin, enkephalin and fl-endorphin (Lee and Smith, 1984). Complex opioid actions involving /.t- and 6-receptor interconversion and interactions have also been reported (see Rothman and Westfall, 1982; Lee and Smith, 1984; D'Amato and Hola-
362
day, 1984) while in addition ~c-ligands (dynorphin(1-13)) produced changes in both /, (morphine, naloxone) and &receptor (ICI 174,864) (Lee and Smith, 1984: Long et al., 1984) activity. In this latter regard there are direct similarities with the present observations: thus pretreatment with dynorphin-(1-13) prevented the effect of ICI 174,864 on endotoxin-induced hypotension (Long et al., 1984). In conclusion a variety of complex regulatory activities may be performed by opioid agonists or antagonists acting on a common macromolecule. The present experiments suggest that ,8-endorphin may be one of a number of neuropeptides that can function in this way. References D'Amato, R. and J.W. Holaday, 1984, Multiple opioid receptors in endotoxic shock: evidence for 8 involvement and /Z-8 interactions in vivo, Proc. Natl. Acad. Sci. U.S.A. 81, 2898. Davis, T.P.. H. Schoemaker, A. Chen, A. Culling and H.I. Yamamura, 1982, Isolation and characterization of in vitro
endorphm metabolism in the brain, Proc. West. Pharmacol. Soc 25, 101. Dray, A. and L Nunan, 1984, Selective 8-opioid receptor antagonism by ICl 174,864 in the central nervous system, Peptides 5, 1015. Handa, B.K., A.C. Lane, J.A.H. Lord, B.A. Morgan, M.J. Rance and C F . C . Smith, 1981, Analogs of bcta-l, PH~, I ~,4 possessing selective agonist activity of /*-opiate receptors, European J. Pharmacol. 70, 531. Houghten, R.A., N. Johnson and G.W. Pasternak, 1984. [~H],8-Endorphin binding in rat brain, J. Neurosci. 4, 24611. Lee, N.M. and A.P. Smith, 1984, Possible regulatory function of dynorphin and its clinical implications. Trends Pharmacol. Sci. 5, 108. Long, ,I.B.. B.A. Ruvio, C.E. Glatt and J.W. Holaday, 1984, l('l 174,864, a putative delta opioid antagonist, reverses cndotoxemic hypotension: pretreatment with dynorphin 1-13, a K-agonist, blocks this action, Neuropeptides 5, 29I. Mosberg, H.I., R. Hurst, V.J. Hruby, K. Gee, H.I. Yamamura, ,1.,I. Galligan and T.F. Burks, 1983, Bis-penicillaminc cnkephalins possess highly improved specificity towards delta opioid receptors, Proc. Natl. Acad. Sci. U.S.A. 80, 5871. Paterson, S.J., L.E. Robson and H.W. Kosterlitz, 1983, Classification of opioid receptors, Br. Med. Bull. 39, 31. Rothman, R.B. and T.C. Westfall, 1982, Allostcric coupling between morphine and enkephalin receptors in vitro. Mol. Pharmacol. 2I, 548.