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Effects of morphine and naloxone on feline colonic transit
Pergamon Press
vol. 44, PP. 873-879 Sciences, Printed in the U.S.A.
Life
EFFECTS OF MORPHINE
AND NALOXONE ON FELINE COLONIC TRANSIT
Benjamin Krevsky, Boris Libster, Alan H. Maurer, and Robert S. Fisher
Billee J. Chase,
Departments of Medicine and Diagnostic Imaging, Temple University School of Medicine, Philadelphia, PA 19140 (Receivedin final form January 31, 1989) Summary The effects of endogenous and exogenous opioid substances on feline colonic transit were evaluated using colonic transit scintigraphy. Naloxone (0.3 mg/kg, i.m.) accelerated emptying of the cecum and ascending colon, and filling of the transverse colon. Endogenous opioid peptides thus appear to play a significant role in the regulation of colonic transit. At a moderate dose of morphine (0.1 mg/kg, i.m.), cecum and ascending colon transit was accelerated, while at a larger dose (1.0 Since naloxone, a mg/kg, i.m.) morphine had no effect. relatively nonspecific opioid antagonist, and morphine, a principally mu opioid receptor agonist, both accelerate proximal colonic transit, a decelerating role for at least one of the other opioid receptors is inferred. The remedial effects of opioid substances on diarrhea1 illnesses have long been recognized (1). Moreover, constipation is a well described concomitant of narcotic analgesic use. Our understanding of the mechanism(s) by which these substances alter gastrointestinal function, however, remains enigmatic (2,3). Endogenous opioid peptides, exogenous opioid compounds, and opioid antagonists are all known to affect gastrointestinal transit and gastrointestinal motor activity (4,5). Morphine-like substances delay gastric emptying (6), increase segmental contractions of the large and small intestine, decrease propulsive contractions of the small and large intestine (7), and lengthen the mucosal contact time of luminal contents leading to enhanced absorption of fluids and electrolytes from the colon (8). They induce ileocecal sphincter contraction (g-11), and they stimulate propagated phase III migrating motor complex-like activity (12). In a qualitative study using a radiopaque marker, met-enkephalin was shown to delay feline colon transit (13). Finally, it has recently been shown in humans that morphine delays colonic transit and naloxone accelerates it (14). Although the efEects of opioid agonists and antagonists on feline smooth muscle have been studied (10,11,13,15), little is
0024-3205189$3.00 + .oo (c) 1989 Pergamon Press plc
Copyright
874
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
known about the importance of individual opioid receptor types on in vivo colonic transit. The cat has been found to be an accurate and reproducible animal model for the study of regional fecal transit using colonic transit scintigraphy (16), and was therefore chosen for this study. The aim of this study was two-fold: to determine the effects of a principally mu receptor agonist, morphine sulfate, on regional colonic transit in the cat; and to determine the importance of endogenous opioid peptides on colonic transit using the opioid antagonist, naloxone. Methods This study was performed using adult female cats. The animals were housed individually under controlled temperature and lighting conditions. They were permitted free access to food (Purina Cat Chow) and water. Feline colonic transit scintigraphy was performed as described earlier (16). After induction of anesthesia with ketamine (4.0 mg/kg, i.m.) and acepromazine (0.4 mg/kg, i.m.) a midline laparotomy was performed. A silicone catheter 1.65 mm O.D. and 0.75 mm I.D. was inserted into the cecum on the antimesenteric border and secured with a purse string suture. The catheter was tunnelled subcutaneously to the interscapular region where it terminated in a Luer stub adapter (Intermedic, Clay Adams, Parsippany, NJ). The cats received postoperative antibiotics and were allowed at least a two week recovery before colonic transit scintigraphy was performed. There were 6 animals, each studied twice, in the control group (saline 0.5 ml, i.m.), and 6 animals in each of 3 study 1) morphine sulfate (0.1 mg/kg, i.m.), 2) morphine groups: sulfate (1.0 mg/kg, i.m.). and 3) naloxone 0.3 mg/kg, i.m.). On the day of study a fasted animal was lightly sedated with ketamine. The animal was awake and mobile at the dose used. Ketamine was chosen for a sedative for several reasons: (a) there is no cross-tolerance to its analgesic effects with morphine (17), (b) gastrointestinal transit in mice treated with ketamine was not altered by naloxone (18), (c) its relaxation effect on muscle is minimal (19), and (d) it permitted sequential gamma camera images to be obtained over several hours, rather than sacrificing animals for each determination. Thirty mins after receiving an i.m. fplection of the test agent, 25 microcuries of Indium-diethylene triamine pentacetic acid (lllIn-DTPA) in a volume of 0.3 ml of normal saline were instilled into the cecum. The distribution of the cecal instillate was followed at 30 min intervals for 6 hrs using a Nuclear Chicago Pho-Gamma gamma camera (Searle) interfaced to a Med IV microcomputer with Modumen nuclear scintigraphy analysis software (Medical Data Systems, Ann Arbor, The colon was divided into 4 regions of interest (ROI): 1 = MI). cecum and ascending colon (CAC), 2 = transverse colon (TC), 3 = descending colon (DC), and 4 = excreted feces. The regions were defined using rectangular computer generated ROI. The transverse colon ROI was the area of activity which was oriented transversely, and contained the splenic and hepatic flexures. The CAC accounted for all the activity proximal to the contained the activity distal to the TC. After the TEi,Iflh;,;;
Opioids and Feline Colonic Trnasit
vol. 44, No. 13, 1989
875
counts were corrected for background and decay, counts in each ROI were quantitated using computer generated rectangular non-overlapping regions. The activity in each ROI at each acquisition time was expressed as a percent of the total instilled activity. A geometric center was calculated at each acquisition time as the sum of the fraction of activity in each region multiplied by the number assigned to that region (16). The half-emptyiny time for the cecum and ascending colon was determined for each study. This was calculated as the time until 50% of the instilled radionuclide remained in the CAC. The time elapsed until the peak activity occurred in the transverse colon was determined. Group values are expressed as the mean + S.E.M. Student's t-test for the difference between sample means was used to determine whether significant differences were present. Results were considered significant if p < 0.05. Evaluation of repeated measures over time was in accordance with O'Brien and Shampo (20).
CECVM
AND ASCENDING
DESCENDING
TRANSVERSE
COLON
COLON
COLON
Fig. 1. Effect of naloxone (0.3 mg/kg) on colonic transit in the cat. The effect of naloxone is compared to control (saline 0.5 ml fter cecal instillation of 50 microCurie of La In-DTPA. * p < 0.05 vs control.
876
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
Results Naloxone accelerated emptying of the CAC at 0.5, 1.0 and 1.5 hours compared to controls (p < 0.05) (Fig. 1). The halfemptying time for this region was accelerated by naloxone from 1.3720.25 to 0.48+0.20 hrs (p < 0.01). Naloxone increased radioisotope activity in the transverse colon (Fig. 1) at 0.5 and 1.0 hours (p < 0.05). Descending colon transit was not significantly altered by naloxone (Fig. 1). The geometric center analysis demonstrated an initial acceleration at 0.5 hr with naloxone (Fig. 2). There was no other difference between these curves.
Fig. 2. Effect of naloxone (0.3 mg/kg, i.m.) and morphine (0.1 or 1.0 mg/kg, i.m.) on feline colonic transit expressed as the progression of the geometric centerl~8~~n~~p~i~~l~~~er* p < lnstlllation of the 0.05 vs. control. Some error bars not shown for clarity. Morphine at 0.1 mg/kg, i.m. accelerated emptying of radioisotope activity in the CAC at 1.0, 1.5, 2.0, 3.5, and 4 hours (p < 0.05). The CAC T l/2 was reduced 50% by morphine 0.1 mg/kg (1.37tO.25 h to 0.69+0.14 h; pcO.025). Morphine at 1.0 mg/kg, had no significant effect on the CAC compared to control (Fig. 3). The CAC T l/2 after morphine 1.0 mg/kg (3.53t2.10 h) was not significantly different from control (1.37+0.25-h). At 0.1 mg/kg morphine caused a significantly increased accumulation of activity in the transverse colon at 0.5, 2.0, 2.5, 3.0, and 3.5 hours. At 1.0 mg/kg morphine had no significant effect on transit in the transverse colon (Fig. 3). Descending colon transit was not significantly altered by either dose of morphine (Fig. 3). The geometric center analysis demonstrated only minimal effects on transit from morphine at either dose (Fig. 2). There was no significant difference in the time from instillation until peak activity in the transverse colon after morphine 0.1 mg/kg (2.6 + .5 h), morphine 1.0 mg/kg (2.9 + .4 h), or naloxone (2.2 + .8 hT versus control (3.5 + .5 h).
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
Discussion Several animals models have been used to study the effects of Transit opioid compounds on gastrointestinal transit (4,21-23). has generally been determined by measuring the progression of a labeled meal from ingestion until the animal was sacrificed (24,25). This permits the assesment of overall transit only after a single fixed time interval. In contrast, colonic transit scintigraphy evaluates regional colonic transit by labeling the cecal contents with a radioisotope marker (26). The effects on each region may be assessed over a period of several hours. Naloxone was used to study the effects of endogenous opioids on colonic transit and was found to accelerate transit in the cecum and ascending colon. This finding suggests that endogenous opioid compounds play a role in the regulation of colonic transit in the cat, even under fasting conditions. An unexpected finding was that both morphine sulfate and naloxone accelerated transit in the cecum and ascending colon. f;ii effect occurred at a moderate dose of morphine (0.1 mg/kg). , in the cat, the mu receptor appears to stimulate transit.
100.
100
CECUM AND ASCENDING COLON
DESCENDING
100 r
TRANSVERSE COLON
COLON
Effect of morphine (0.1 or 1.0 mg/kg, i.m.) on colonic transit in the cat. The effect of morphine is compared to control (saline 0.5 ml) er cecal instillation of 50 microcuries of af! In-DTPA. * p < 0.05 vs. control.
877
878
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
This is unlike the human situation, where morphine, even at the same dose, had a delaying effect on transit in the CAC (14). In the cat, acceleration of CAC emptying by low dose morphine may have been due to an effect of mu receptor stimulation. At a higher dose (1.0 mg/kg), morphine may also stimulate delta and kappa receptors. If they are inhibitory in their effect (13), then large dose morphine may cause a balanced effect on transit, with no net change compared to controls. Alternative hypotheses for the effects of morphine include a) a difference in central and peripheral effects which are dose dependent, b) a direct effect on colonic smooth muscle which may be the opposite of receptor effects, and c) a prejunctional agonist effect on the inhibitory opioid receptors which modulate adrenergic or nonadrenergicnoncholinergic nerves (15) and which may be reversed by a direct myogenic or muscle opioid receptor effects at higher doses. There are several possible explanations for the effect of naloxone. First, naloxone may have had an agonist effect (27). This is unlikely since the dose used in this study was well below those used in studies demonstrating an agonist effect. Second, and more likely, the acceleration caused by naloxone may have been due to non-mu receptor inhibition. Since naloxone is a non-specific opioid antagonist, there may be tonic inhibition of transit by a non-mu receptor type in the CAC. By inhibiting these receptors, naloxone permitted more rapid transit to occur. The effects of other opioid receptors (e.g. kappa, delta, sigma) on feline colonic transit are largely unknown, but there is some evidence that the delta receptor may be inhibitory (13). Increased activity levels in the TC after morphine at 0.1 mg/kg, i-m. were probably an effect of accelerated emptying of the CAC. Naloxone produced an effect on transit which was earlier than that seen with morphine (30 min. vs. 60 min., respectively). Possible explanations for the different time sequence include varying bioavailability, different drug metabolism and distribution, altered receptor binding kinetics, different end-organ response to agonists and antagonists, and different receptor specificities (i.e. nonspecific opioid antagonist vs. mu specific morphine). The results of this study suggest that endogenous opioid peptides play a role in the regulation of feline colonic transit. Acceleration of cecum and ascending colon transit by both a primarily mu receptor agonist (morphine) and a nonspecific antagonist (naloxone) suggests that different opioid receptor types have a heterogeneity of function in the feline colon. Since the methodology of this study could not distinguish between individual receptor types, or even central vs. peripheral effects, further studies with selective agents (e.g. kappa and delta agonists) and different routes of administration (i.c.v. vs. peripheral), will help to clarify the mechanism of action of exogenous and endogenous opioids. Acknowledgements This research was supported by grant R23 DK36402 from the National Institutes of Health. The authors appreciate the assistance of Lynne Rivers in the preparation of the manuscript.
Vol. 44, No. 13, 1989
Opioids and Feline Colonic Transit
879
References 1.
2. 3. 4. 2: 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
J.H. JAFFE and W.R. MARTIN. Opioid analgesics and antagonists In: A.G. Gilman, L.S. Goodman, A. Gilman eds. The Pharmacological Basis of Therapeutics. 6th ed. p. -494-534 Macmillan, New York (1980). M. WEINBECK and J. CHRISTENSEN. Gastroenterology 61, 470-478 (1971). T.J. BORODY, E.M.M. QUIGLEY, S.F. PHILLIPS, M. WEINBECK, R.L. TUCKER, A. HADDAD, AND A.R. ZINSMEISTER. Gastroenterology e, 652-570 (1985). L. MANARA AND A. BIANCHETTI. Ann. Rev. Pharmacol. Toxicol. 25, 249-273 (1985). S.J. KONTUREK. Am. J. Gastroenterol. 74, 285-291 (1980). W.S. NIMMO. R.C. HEADING. J. WILSON. P. TOTHILL. and L.F. PRESCOTT. -Br. J. Clin. Pharmacol. i, 509-513 (1975). E.M. VAUGHAN WILLIAMS. Pharmacol. Rev 6, 159-90 (1954). L.R. SCHILLER, G.R. DAVIS, C.A. SANTA ANA, S.G. MORAWSKI, and J.S. FORDTRAN. J. Clin. Invest. 70, 999-1008 (1982). A. OUYANG, C.J. CLAIN, W.J. SNAPE and S. COHEN. J. Clin. Invest. 69, 507-515 (1982). G. BERTIGER, A. OUYANG, J.C. REYNOLDS, S. COHEN. Life Sci. 42, 1697-1703 (1988). A. OUYANG, P. VOS, S. COHEN. Am. J. Physiol. 245, G224-231 (1988). S.J. KONTUREK, P. THOR, R. KROL, A. DEMBINSKI, and A.V. ORSCHALLY. Am. J. Physiol. 238, G384-389 (1980). M. WEINBECK, M. BLASBERG. 2. Gastroenterol. 24, 179-187 (1986). P.N. KAUFMAN, 8. KREVSKY, L.S. MALMUD, A.H. MAURER, M.B. SOMERS, J.A. SIEGEL and R.S. FISHER. Gastroenterology 94, 1351-1356 (1986). C. KENNEDY, J. KRIER. Br. J. Pharmacol. 92, 291-298 (1987). B. KREVSKY, M.B. SOMERS, A.H. MAURER, L.S. MALMUD, L.C. KNIGHT, and R.S. FISHER. 255, G529-G534 Am. J. Phvsiol _ (1988): M.G. DESIMONI, R. GIGLIO, G. DAL-TOSO , I. CONFORTI, and S. ALIGERI. Neuropharmacol. 2, 541-545 (1985). R.N. TAKAHASHI, G.S. MORATO, and G.A. RAE. Gen. Pharmacol. 2, 201-302 (1987) A.G. GILMAN, L.S. GOODMAN, T.W. RALL, and F. MURAD. The pharmacological basis of therapeutics , 7th ed., pp. 298-299, MacMillan Publ. Co., N.Y. 1985). P.C. O'BRIEN, M.A. SHAMPO. Mayo Clin. Proc. 63, 918-920 (1988). J.J. GALLIGAN, H-1. MOSBERG, R. HUSRT, V.J. HRUBY and T.F. BURKS. J. Pharmacol. Exp. Ther. 229, 641-648 (1984). F. PORRECA, H.I. MOSBERG, R. HURST, V.J. HRUBY, and T.F. BURKS. J. Pharmacol. Exp. Ther. 230, 341-348 (1984). D.E. GMEREK, A. COWAN, and J.H. WOODS. J. Pharmacol. Exp. Ther. 236, 8-13 (1986). J.J. STEWART, N.W. WEISBRODT, and T.F. BURKS. J. Pharmcol. Exp. Ther. 205, 547-555 (1978). A.F. GREEN. Br. J. Pharmacol. 14, 26-34 (1959). B. KREVSKY, L.S. MALMUD, F. D'ERCOLE, A.H. MAURER, and R.S. FISHER. Gastroenterology 91, 1102-1112 (1986). J. SAWYNOK, C. PINSKY, F.S. LABELLA. Life Sci. 25, 1620-1632 (1979).
vol. 44, PP. 873-879 Sciences, Printed in the U.S.A.
Life
EFFECTS OF MORPHINE
AND NALOXONE ON FELINE COLONIC TRANSIT
Benjamin Krevsky, Boris Libster, Alan H. Maurer, and Robert S. Fisher
Billee J. Chase,
Departments of Medicine and Diagnostic Imaging, Temple University School of Medicine, Philadelphia, PA 19140 (Receivedin final form January 31, 1989) Summary The effects of endogenous and exogenous opioid substances on feline colonic transit were evaluated using colonic transit scintigraphy. Naloxone (0.3 mg/kg, i.m.) accelerated emptying of the cecum and ascending colon, and filling of the transverse colon. Endogenous opioid peptides thus appear to play a significant role in the regulation of colonic transit. At a moderate dose of morphine (0.1 mg/kg, i.m.), cecum and ascending colon transit was accelerated, while at a larger dose (1.0 Since naloxone, a mg/kg, i.m.) morphine had no effect. relatively nonspecific opioid antagonist, and morphine, a principally mu opioid receptor agonist, both accelerate proximal colonic transit, a decelerating role for at least one of the other opioid receptors is inferred. The remedial effects of opioid substances on diarrhea1 illnesses have long been recognized (1). Moreover, constipation is a well described concomitant of narcotic analgesic use. Our understanding of the mechanism(s) by which these substances alter gastrointestinal function, however, remains enigmatic (2,3). Endogenous opioid peptides, exogenous opioid compounds, and opioid antagonists are all known to affect gastrointestinal transit and gastrointestinal motor activity (4,5). Morphine-like substances delay gastric emptying (6), increase segmental contractions of the large and small intestine, decrease propulsive contractions of the small and large intestine (7), and lengthen the mucosal contact time of luminal contents leading to enhanced absorption of fluids and electrolytes from the colon (8). They induce ileocecal sphincter contraction (g-11), and they stimulate propagated phase III migrating motor complex-like activity (12). In a qualitative study using a radiopaque marker, met-enkephalin was shown to delay feline colon transit (13). Finally, it has recently been shown in humans that morphine delays colonic transit and naloxone accelerates it (14). Although the efEects of opioid agonists and antagonists on feline smooth muscle have been studied (10,11,13,15), little is
0024-3205189$3.00 + .oo (c) 1989 Pergamon Press plc
Copyright
874
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
known about the importance of individual opioid receptor types on in vivo colonic transit. The cat has been found to be an accurate and reproducible animal model for the study of regional fecal transit using colonic transit scintigraphy (16), and was therefore chosen for this study. The aim of this study was two-fold: to determine the effects of a principally mu receptor agonist, morphine sulfate, on regional colonic transit in the cat; and to determine the importance of endogenous opioid peptides on colonic transit using the opioid antagonist, naloxone. Methods This study was performed using adult female cats. The animals were housed individually under controlled temperature and lighting conditions. They were permitted free access to food (Purina Cat Chow) and water. Feline colonic transit scintigraphy was performed as described earlier (16). After induction of anesthesia with ketamine (4.0 mg/kg, i.m.) and acepromazine (0.4 mg/kg, i.m.) a midline laparotomy was performed. A silicone catheter 1.65 mm O.D. and 0.75 mm I.D. was inserted into the cecum on the antimesenteric border and secured with a purse string suture. The catheter was tunnelled subcutaneously to the interscapular region where it terminated in a Luer stub adapter (Intermedic, Clay Adams, Parsippany, NJ). The cats received postoperative antibiotics and were allowed at least a two week recovery before colonic transit scintigraphy was performed. There were 6 animals, each studied twice, in the control group (saline 0.5 ml, i.m.), and 6 animals in each of 3 study 1) morphine sulfate (0.1 mg/kg, i.m.), 2) morphine groups: sulfate (1.0 mg/kg, i.m.). and 3) naloxone 0.3 mg/kg, i.m.). On the day of study a fasted animal was lightly sedated with ketamine. The animal was awake and mobile at the dose used. Ketamine was chosen for a sedative for several reasons: (a) there is no cross-tolerance to its analgesic effects with morphine (17), (b) gastrointestinal transit in mice treated with ketamine was not altered by naloxone (18), (c) its relaxation effect on muscle is minimal (19), and (d) it permitted sequential gamma camera images to be obtained over several hours, rather than sacrificing animals for each determination. Thirty mins after receiving an i.m. fplection of the test agent, 25 microcuries of Indium-diethylene triamine pentacetic acid (lllIn-DTPA) in a volume of 0.3 ml of normal saline were instilled into the cecum. The distribution of the cecal instillate was followed at 30 min intervals for 6 hrs using a Nuclear Chicago Pho-Gamma gamma camera (Searle) interfaced to a Med IV microcomputer with Modumen nuclear scintigraphy analysis software (Medical Data Systems, Ann Arbor, The colon was divided into 4 regions of interest (ROI): 1 = MI). cecum and ascending colon (CAC), 2 = transverse colon (TC), 3 = descending colon (DC), and 4 = excreted feces. The regions were defined using rectangular computer generated ROI. The transverse colon ROI was the area of activity which was oriented transversely, and contained the splenic and hepatic flexures. The CAC accounted for all the activity proximal to the contained the activity distal to the TC. After the TEi,Iflh;,;;
Opioids and Feline Colonic Trnasit
vol. 44, No. 13, 1989
875
counts were corrected for background and decay, counts in each ROI were quantitated using computer generated rectangular non-overlapping regions. The activity in each ROI at each acquisition time was expressed as a percent of the total instilled activity. A geometric center was calculated at each acquisition time as the sum of the fraction of activity in each region multiplied by the number assigned to that region (16). The half-emptyiny time for the cecum and ascending colon was determined for each study. This was calculated as the time until 50% of the instilled radionuclide remained in the CAC. The time elapsed until the peak activity occurred in the transverse colon was determined. Group values are expressed as the mean + S.E.M. Student's t-test for the difference between sample means was used to determine whether significant differences were present. Results were considered significant if p < 0.05. Evaluation of repeated measures over time was in accordance with O'Brien and Shampo (20).
CECVM
AND ASCENDING
DESCENDING
TRANSVERSE
COLON
COLON
COLON
Fig. 1. Effect of naloxone (0.3 mg/kg) on colonic transit in the cat. The effect of naloxone is compared to control (saline 0.5 ml fter cecal instillation of 50 microCurie of La In-DTPA. * p < 0.05 vs control.
876
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
Results Naloxone accelerated emptying of the CAC at 0.5, 1.0 and 1.5 hours compared to controls (p < 0.05) (Fig. 1). The halfemptying time for this region was accelerated by naloxone from 1.3720.25 to 0.48+0.20 hrs (p < 0.01). Naloxone increased radioisotope activity in the transverse colon (Fig. 1) at 0.5 and 1.0 hours (p < 0.05). Descending colon transit was not significantly altered by naloxone (Fig. 1). The geometric center analysis demonstrated an initial acceleration at 0.5 hr with naloxone (Fig. 2). There was no other difference between these curves.
Fig. 2. Effect of naloxone (0.3 mg/kg, i.m.) and morphine (0.1 or 1.0 mg/kg, i.m.) on feline colonic transit expressed as the progression of the geometric centerl~8~~n~~p~i~~l~~~er* p < lnstlllation of the 0.05 vs. control. Some error bars not shown for clarity. Morphine at 0.1 mg/kg, i.m. accelerated emptying of radioisotope activity in the CAC at 1.0, 1.5, 2.0, 3.5, and 4 hours (p < 0.05). The CAC T l/2 was reduced 50% by morphine 0.1 mg/kg (1.37tO.25 h to 0.69+0.14 h; pcO.025). Morphine at 1.0 mg/kg, had no significant effect on the CAC compared to control (Fig. 3). The CAC T l/2 after morphine 1.0 mg/kg (3.53t2.10 h) was not significantly different from control (1.37+0.25-h). At 0.1 mg/kg morphine caused a significantly increased accumulation of activity in the transverse colon at 0.5, 2.0, 2.5, 3.0, and 3.5 hours. At 1.0 mg/kg morphine had no significant effect on transit in the transverse colon (Fig. 3). Descending colon transit was not significantly altered by either dose of morphine (Fig. 3). The geometric center analysis demonstrated only minimal effects on transit from morphine at either dose (Fig. 2). There was no significant difference in the time from instillation until peak activity in the transverse colon after morphine 0.1 mg/kg (2.6 + .5 h), morphine 1.0 mg/kg (2.9 + .4 h), or naloxone (2.2 + .8 hT versus control (3.5 + .5 h).
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
Discussion Several animals models have been used to study the effects of Transit opioid compounds on gastrointestinal transit (4,21-23). has generally been determined by measuring the progression of a labeled meal from ingestion until the animal was sacrificed (24,25). This permits the assesment of overall transit only after a single fixed time interval. In contrast, colonic transit scintigraphy evaluates regional colonic transit by labeling the cecal contents with a radioisotope marker (26). The effects on each region may be assessed over a period of several hours. Naloxone was used to study the effects of endogenous opioids on colonic transit and was found to accelerate transit in the cecum and ascending colon. This finding suggests that endogenous opioid compounds play a role in the regulation of colonic transit in the cat, even under fasting conditions. An unexpected finding was that both morphine sulfate and naloxone accelerated transit in the cecum and ascending colon. f;ii effect occurred at a moderate dose of morphine (0.1 mg/kg). , in the cat, the mu receptor appears to stimulate transit.
100.
100
CECUM AND ASCENDING COLON
DESCENDING
100 r
TRANSVERSE COLON
COLON
Effect of morphine (0.1 or 1.0 mg/kg, i.m.) on colonic transit in the cat. The effect of morphine is compared to control (saline 0.5 ml) er cecal instillation of 50 microcuries of af! In-DTPA. * p < 0.05 vs. control.
877
878
Opioids and Feline Colonic Transit
Vol. 44, No. 13, 1989
This is unlike the human situation, where morphine, even at the same dose, had a delaying effect on transit in the CAC (14). In the cat, acceleration of CAC emptying by low dose morphine may have been due to an effect of mu receptor stimulation. At a higher dose (1.0 mg/kg), morphine may also stimulate delta and kappa receptors. If they are inhibitory in their effect (13), then large dose morphine may cause a balanced effect on transit, with no net change compared to controls. Alternative hypotheses for the effects of morphine include a) a difference in central and peripheral effects which are dose dependent, b) a direct effect on colonic smooth muscle which may be the opposite of receptor effects, and c) a prejunctional agonist effect on the inhibitory opioid receptors which modulate adrenergic or nonadrenergicnoncholinergic nerves (15) and which may be reversed by a direct myogenic or muscle opioid receptor effects at higher doses. There are several possible explanations for the effect of naloxone. First, naloxone may have had an agonist effect (27). This is unlikely since the dose used in this study was well below those used in studies demonstrating an agonist effect. Second, and more likely, the acceleration caused by naloxone may have been due to non-mu receptor inhibition. Since naloxone is a non-specific opioid antagonist, there may be tonic inhibition of transit by a non-mu receptor type in the CAC. By inhibiting these receptors, naloxone permitted more rapid transit to occur. The effects of other opioid receptors (e.g. kappa, delta, sigma) on feline colonic transit are largely unknown, but there is some evidence that the delta receptor may be inhibitory (13). Increased activity levels in the TC after morphine at 0.1 mg/kg, i-m. were probably an effect of accelerated emptying of the CAC. Naloxone produced an effect on transit which was earlier than that seen with morphine (30 min. vs. 60 min., respectively). Possible explanations for the different time sequence include varying bioavailability, different drug metabolism and distribution, altered receptor binding kinetics, different end-organ response to agonists and antagonists, and different receptor specificities (i.e. nonspecific opioid antagonist vs. mu specific morphine). The results of this study suggest that endogenous opioid peptides play a role in the regulation of feline colonic transit. Acceleration of cecum and ascending colon transit by both a primarily mu receptor agonist (morphine) and a nonspecific antagonist (naloxone) suggests that different opioid receptor types have a heterogeneity of function in the feline colon. Since the methodology of this study could not distinguish between individual receptor types, or even central vs. peripheral effects, further studies with selective agents (e.g. kappa and delta agonists) and different routes of administration (i.c.v. vs. peripheral), will help to clarify the mechanism of action of exogenous and endogenous opioids. Acknowledgements This research was supported by grant R23 DK36402 from the National Institutes of Health. The authors appreciate the assistance of Lynne Rivers in the preparation of the manuscript.
Vol. 44, No. 13, 1989
Opioids and Feline Colonic Transit
879
References 1.
2. 3. 4. 2: 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
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