Morphine inhibits TRH-induced intestinal transit increases

The current study examined the effects of intraperitoneal (IP) and intracisternal (IC) administration of the opiate agonist, morphine, and an opioid, central beta-endorphin, on thyrotropin releasing hormone (TRH)-induced small intestinal transit increases. Anesthetized rats, 14-day and older, were studied to determine age-related differences. Results showed that in all age groups IP morphine (2 mg/kg) blocked TRH (15 μg)-induced increases in transit of a charcoal bolus. Morphine 1 μg and beta-endorphin 1 μg administered IC in 0.6 μl failed to block TRH (10 μg)-induced increases in intestinal transit in 14-day-old rats. However, both morphine and beta-endorphin 1 μg IC blocked TRH-induced increases in adult rats. Dose-response studies demonstrated that higher doses (> 1 μg) of morphine IC were required to block TRH-induced increases in preweaning rats.     

Stereospecific opiate receptors in the actions of thyrotropin releasing hormone and morphine on gastrointestinal transit

Studies in these laboratories have shown that morphine and thyrotropin releasing hormone (TRH) inhibit gastrointestinal transit in the mouse. Administration of morphine sulfate (5 mg/kg, s.c.) or TRH (10 microgram i.c.v.) to mice inhibited gastrointestinal transit as measured by the charcoal meal test. In order to determine whether the effects of TRH and morphine were mediated via stereospecific opiate receptors, the effects of two stereoisomers of an antagonist, (-) alpha -5,9-diethyl-2'-hydroxy-2-(3-furylmethyl)6,7-benzomorphan (MR2266), the active isomer and (+) alpha-5,9-diethyl-2'-hydroxy-2-(3-furylmethyl)6,7-benzomorphan (MR 2267), the inactive isomer, on morphine and TRH induced changes in gastrointestinal transit were determined. Morphine and THR induced inhibition of gastrointestinal transit was antagonized by MR 2266 (1 and 3 mg/kg, s.c.) but was unaffected by MR 2267. These studies provide evidence for the involvement of opiate receptors in the actions of morphine and TRH on gastrointestinal transit, and further suggest that the receptors are stereospecific in nature.     

Studies on the mechanism of thyrotropin releasing hormone induced inhibition of gastrointestinal transit

Intracerebral administration of thyrotropin releasing hormone (TRH) inhibited gastrointestinal transit in the mouse as determined by the charcoal meal test. A similar inhibitory effect was produced by morphine administered subcutaneously. TRH enhanced morphine-induced inhibition of gastrointestinal transit. Intracerebral injections of cyclo (His-Pro), a postulated metabolite, did not affect gastrointestinal transit either by itself or that produced by morphine. It is suggested that gastrointestinal transit effects of TRH are not mediated via its conversion to cyclo (His-Pro).     

Effects of intracerebroventricular injections of thyrotropin releasing hormone on gastrointestinal propulsive motility in rats

The effect of intracerebroventricular (ICV) injections of thyrotropin releasing hormone (TRH) on the propulsive motility of the gastrointestinal tract was examined in rats. The distance travelled by charcoal meal through the small intestine, measured in terms of percentage of its total length, was recorded as the index of propulsive motility. The results were as follows: (1) The propulsive distance of charcoal meal was significantly reduced in a dose-dependent manner after ICV injections of TRH (1 microgram/10 microliters, 5 micrograms/10 microliters or 10 micrograms/10 microliters) (P less than 0.01-0.001) The effects were abolished by injection of atropine (5 micrograms/10 microliters ICV). (2) The gastrointestinal propulsive motility decreased markedly (P less than 0.01) after injection of a larger dose of TRH (50 micrograms/100 g) into the hypodermis. The effects were not completely blocked by subcutaneous injections of propranolol (5 mg/kg). (3) No effects (P greater than 0.05) were found on the inhibition of gastrointestinal propulsive motility after ICV injections of regitine (2.5 mg/kg im, 50 micrograms/50 microliters ICV) or propranolol (5 mg/kg im, 50 micrograms/50 microliters ICV). The results indicate that TRH has an inhibitory effect on the propulsive motility of gastrointestinal tract, which may be mediated via the non-adrenergic inhibitory nerve of the vagal nerves.     

Effects of morphine-3-glucuronide on the antinociceptive activity of peptide and nonpeptide opioid receptor agonists in mice

The effects of morphine-3-glucuronide (M3G), a metabolite of morphine, were determined on the antinociceptive actions, as measured by the tail flick test, of morphine, a μ-opioid receptor agonist, of U-50,488H, a κ-opioid receptor agonist, of [ -Pen², -Pen⁵]enkephalin (DPDPE), a δ₁-opioid receptor agonist, and of [ -Ala²,Glu⁴]deltorphin II (deltorphin II), a δ₂-opioid receptor agonist in mice. Morphine administered ICV (2.5 μg/mouse) or SC (10 mg/kg), U-50,488H (25 mg/kg, IP), DPDPE (15 μg/mouse; ICV), and deltorphin II (15 μg/mouse, ICV) produced antinociception in mice. Intraperitoneal or ICV injections of M3G did not produce any effect on the tail flick latency nor did it affect the antinociception-induced by morphine, U-50,488H, DPDPE, or deltorphin II. Previously M3G has been shown to antagonize the antinociceptive effects of morphine in the rat. It is concluded that in the mouse, M3G neither produces hyperalgesia nor modifies the actions of μ-, κ-, δ₁-, or δ₂-opioid receptor agonists.     

Cardiovascular effects of dynorphin A (1–13) in conscious rats and its modulation of morphine bradycardia over time

The short-term cardiovascular effects of dynorphin A (1–13), as well as its effects upon morphine bradycardia were investigated. In unanesthetized, unrestrained rats, intracerebroventricular (ICV) dynorphin A (1–13) injections (10–20 μg) produced a dose-related pressor effect, whereas intravenous (IV) dynorphin A (1–13) (1.0 mg/kg) produced a depressor effect; these responses persisted less than five min. Heart rate was not significantly altered by these doses or routes of administration. Dynorphin A (1–13) also produced behavioral effects in the unanesthetized animals, such as wet dog shakes in response to IV administration and wet dog shakes accompanied by barrel rolling in response to ICV administration. To evaluate the effects of dynorphin A (1–13) pretreatment on the bradycardic response to IV morphine, rats were pretreated with 10 μg dynorphin A (1–13) ICV four, six or eight hours prior to challenge with morphine sulfate (0.1 mg/kg IV). Four hour pretreatment with dynorphin A (1–13) (tested at 14:00 hr) resulted in a potention of morphine bradycardia, with six hours pretreatment (tested at 16:00 hr) no effect was observed, and eight hours following dynorphin A (1–13) pretreatment (tested at 18:00 hr) morphine bradycardia was attenuated. Additionally, the bradycardic response to IV morphine alone became more exaggerated as rats approached their nocturnal activity cycle. These data further establish that dynorphin A (1–13) exerts a potent, long lasting modulatory effect on morphine bradycardia and emphasize the importance of circadian variables in altering the magnitude of cardiovascular responses to opioid agonists.     

Actions of intracerebroventricular administration of kyotorphin and an analog on thermoregulation in the mouse

Intracerebroventricular (ICV) administration of kyotorphin (L-Tyr-L-Arg) and cyclo (N-methyl-L-Tyr-L-Arg), its analog, produced significant dose-dependent hypothermic responses in mice at an ambient temperature of 24°C. The hypothermic action of kyotorphin was much greater than that of Met-enkephalin (Met-ENK) but less than that of cyclo NMTA. This action was slightly but not significantly reversed by intraperitoneally administered naloxone (8 mg/kg), an opioid receptor antagonist. Met-ENK utilized as a control peptide in this study also produced a dose-dependent hypothermia which was slightly antagonized by naloxone (8 mg/kg, IP). Thyrotropin releasing hormone (TRH) injected ICV produced hyperthermia dose-dependently. The hypothermia induced by kyotorphin, its cyclic analog and Met-ENK was prevented by a small dose of TRH (0.18 μg=0.5 nmol/animal) which by itself had little effect on body temperature. A TRH neuronal system in the brain may explain the mechanism of kyotorphin-induced hypothermia. However, there was little evidence of involvement of opioid receptors. The present study demonstrates a potent action of kyotorphin and its analog on thermoregulation.     

Ketazocines and morphine: effects on gastrointestinal transit after central and peripheral administration

The mu agonist, morphine, and the prototype kappa agonists, ketocyclazocine and ethylketocyclazocine (EK), were studied for their effects on gastrointestinal transit. Following s.c. administration, both morphine (0.3-3 mg/kg) and ketocyclazocine (0.3-10 mg/kg) antagonized transit of an opaque marker through the small intestines of mice. Morphine (0.1-1 microgram) was also effective after intracerebroventricular (icv) administration in mice whereas ketocyclazocine (0.3-30 micrograms) was not. Similarly, while both morphine (0.3-5 mg/kg) and EK (0.6-10 mg/kg) slowed transit after s.c. injection to rats, only morphine (1-10 micrograms), but not EK (0.3-300 micrograms), was active following icv administration. Icv infusion of the mu benzomorphan, phenazocine (10-100 micrograms), slowed transit in a dose-related manner. These results indicate that there may be an anatomically distinct distribution of receptors for benzomorphan kappa agonists in both the mouse and rat, with these opiate receptors not being located near the lateral cerebral ventricles. The difference in efficacy between morphine and ketazocines in slowing gastrointestinal transit after icv administration to rodents suggests that (a) inactivity in this endpoint is a characteristic of benzomorphan kappa compounds and (b) the model may serve as a useful screen when establishing in vivo profiles of kappa agonists in mice and rats.     

Central and peripheral components of dermorphin's effect on rat intestinal propulsion in comparison to morphine

Dermorphin, injected intracerebroventricularly (ICV) to rats, provokes, like to morphine, an inhibition of intestinal propulsion linearly related to the log of the administered doses (in the range from 0.06 to 0.56 μg/rat), but it is 143 times more active than morphine. Naloxone, ICV or IP, antagonizes dermorphin less effectively than morphine. Quaternary naloxone ICV administered antagonizes the intestinal effect of ICV dermorphin, while IP administered it is not effective until 8 mg/kg. The dose of dermorphin maximally active by the ICV route (0.56 μg/rat) is completely inactive when injected IP. Increasing doses of dermorphin IP (from 12 to 6400 μg/kg) inhibit intestinal propulsion to the same extent irrespectively of the doses employed, but never by more than 50%. Only a high dose of naloxone (30 mg/kg/IP) antagonizes this IP effect. The central and peripheral components of this intestinal effect of dermorphin are discussed.     

Antinociceptive, prolactin releasing and intestinal motility inhibiting activities of dermorphin and analogues after subcutaneous administration in the rat

A series of analogues and shorter homologues of dermorphin (DM), a frog skin heptapeptide with potent morphine-like activity, have been assayed in the rat after subcutaneous (SC) administration at the screening dose of 4 mg/kg. The effects taken into account are: analgesia (tail-pinch test), stimulation of prolactin (PRL) secretion, and inhibition of gastro-intestinal (GI) motility (charcoal meal transit). Effective doses were calculated for the most active compounds. The potency of DM (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) in the different tests was: tail-pinch: ED50 = 0.83 mg/kg; PRL release: ED100 = 0.3 mg/kg; inhibition of GI motility: ED30 = 1.8 mg/kg.     

Pupillary effects of leucine and methionine enkephalin in rats after intraperitoneal administration

Changes in pupil size after peripheral administration of met-enkephalin, leu-enkephalin, or morphine were studied in the rat. With a simple pupillographic technique, the pupil diameter of male, S.D. rats (250–300 g) was measured by a series of photographs taken every 60 sec for at least 45 min after the last drug injection. Morphine (8 mg/kg, SC) caused mydriasis characterized by rapid and marked fluctuations of pupil size. Mydriasis also occurred after leu-enkephalin (5 and 10 mg/kg, IP) and met-enkephalin (20 mg/kg, IP). Both peptides induced morphine-like fluctuations. When given 15 min after morphine, leu-enkephalin (5 and 10 mg/kg) increased the mydriatic effect of morphine from 172 percent of control to 224 and 272 percent, respectively. Met-enkephalin (20 mg/kg, but not 10 mg/kg) also enhanced the mydriatic response of morphine, to 244 percent of control. These interactions appear to represent simple addition rather than potentiation. The effects of both peptides were reversed by naloxone (1 mg/kg, SC), suggesting an opiate receptor interaction for the pupillary effects of the enkephalins. The rat pupil thus provides one of the few in vivo models permitting quantification of enkephalin action after parenteral administration.     

Neuropeptides and thermoregulation: the interactions of bombesin, neurotensin, TRH, somatostatin, naloxone and prostaglandins

In this study we have examined the interactions of bombesin (1 microgram ICV), neurotensin (1 microgram ICV), TRH (10 micrograms ICV), somatostatin (10 micrograms ICV), PGE2 (10 micrograms ICV) and naloxone (10 mg/kg SC) on thermoregulation in the rat at room temperature (20 +/- 1 degree C). Given alone, bombesin, neurotensin, somatostatin and naloxone all produced hypothermia (bombesin greater than neurotensin greater than somatostatin congruent to naloxone). PGE2 was hyperthermic, and TRH had no effect. Bombesin and PGE2 neutralized one another's effects. Neurotensin had no effect on PGE2-induced hyperthermia. Naloxone enhanced the hypothermic effect of bombesin and somatostatin enhanced the rate of onset of hypothermia after bombesin. TRH had no effect on bombesin-induced hypothermia. TRH, somatostatin and naloxone had no effect on neurotensin-induced hypothermia. TRH antagonized the hypothermia due to naloxone and somatostatin.     

Comparison of the effects of chronic central administration and chronic peripheral administration of islet amyloid polypeptide on food intake and meal pattern in the rat

Islet amyloid polypeptide (IAPP) is postulated to act as a hormonal signal from the pancreas to the brain to inhibit food intake and reduce adipose energy reserves. The present study compared the effects of chronic peripheral and chronic central administration of IAPP on food intake and meal pattern in rats. IAPP was administered subcutaneously (SC) for 7 days at doses of 0, 0.25, 2.5 and 25 pmol kg(-1) min(-1) using an osmotic minipump or administered centrally at doses of 0, 0.025, 0.25 and 2.5 pmol kg(-1) min(-1) using an osmotic minipump connected to an intracerebroventricular (ICV) catheter inserted into the third ventricle. Both SC and ICV infusion decreased total food intake dose-dependently. The minimal effective dose was 2.5 pmol IAPP kg(-1) min(-1) for SC administration and 0.25 pmol kg(-1) min(-1) for ICV infusion. The decrease in food intake produced by infusion of IAPP was mainly due to decreased meal size, although a significant decrease in meal number also occurred at the highest SC and ICV doses. SC administration produced a larger, more persistent decrease in food intake during the light period than in the dark period, while ICV infusion caused a larger, more persistent decrease during the dark period. The 10-fold difference in minimal effective doses indicates that ICV-administered IAPP acted primarily in the brain to inhibit food intake. The difference between the effects of IAPP on meal pattern with the two methods of administration suggests that IAPP does not act on the same target(s) when administered centrally as it does when it is administered peripherally.     

Inhibition of apomorphine-induced yawning and penile erection by neurotensin.

The yawns and penile erection elicited in rats by apomorphine (100 micrograms/kg SC) are dose-dependently suppressed by the enkephalinase-resistant analog of NT, [D-Trp11]NT, intracerebroventricularly (ICV) injected (10-120 ng per rat). This antagonistic effect was shared by NT (0.75-3 micrograms per rat) administered ICV. The yawns induced by pilocarpine (2 mg/kg IP) were similarly antagonized by [D-Trp11]NT (30-120 ng per rat). The enkephalinase inhibitor acetorphan (5 mg/kg IV) reduced in a naloxone (2 mg/kg, SC)-resistant manner the apomorphine-induced penile erection or yawning.     

The occurrence of cross-tolerance between morphine and ethyl-ketocyclazocine using the tail-flick test: Lack of effect of diazepam,phenobarbital, or amphetamine

Experiments were designed to test for short-term tolerance to morphine and ethyl-ketocyclazocine (EKC), mu and kappa agonists, respectively, and cross-tolerance between the two drugs. Mice were primed with one of the drugs, using doses that did not affect the tail-flick response when tested at a time 1 or 3 hours later, when the same or alternate test drug was administered. All animals were injected with the priming drug IP. In one series of experiments, the test drugs were given SC, and in the other, the test drugs were injected ICV under brief halothane anesthesia. Priming with morphine (30 or 100 mg/kg) significantly raised the ED₅₀ for ICV morphine. Priming with EKC (2 or 6 mg/kg) similarly elevated the ED₅₀'s for SC and ICV EKC. Symmetrical cross-tolerance was produced in experiments where the test drugs were administered SC when tested at 3 hrs. The effects of priming with EKC on morphine analgesia was evident when the interval between priming and test drugs was 1 hour. When the test drugs were given ICV, cross-tolerance was also symmetrical: priming with EKC significantly raised the ED₅₀ for morphine and priming with morphine raised the ED₅₀ for EKC when tested at 3 hrs. These data suggest that both agonists act on a common site to produce analgesia as similar pA₂ values for naloxone antagonism were determined. The occurrence of short-term tolerance and cross-tolerance to the opiates was unaltered by chronic pretreatment with diazepam, phenobarbital, or amphetamine.     

In vivo studies with ICI 154,129, a putative delta receptor antagonist

We studied the in vivo pharmacology of ICI 154,129, a new antagonist that is claimed to show selectivity for delta opiate receptors. At s.c. doses of 30 and 100 mg/kg, ICI 154,129 had no marked effect on the gastrointestinal transit of a charcoal meal in mice. In this test, ICI 154,129 reversed the inhibitory action of metkephamid (a proposed delta receptor agonist) but not that of levorphanol. ICI 154,129 was proconvulsant in the mouse picrotoxin potentiation test; the dose-response curve had a low ceiling and was biphasic. Naloxone (1 mg/kg, s.c.) enhanced the proconvulsant action of ICI 154,129 (40 mg/kg, s.c.) by an unknown mechanism.     

Inhibitory effect of intracerebroventricularly-administered [D-Arg(2), beta-Ala(4)]-dermorphin (1-4) on gastrointestinal transit

The inhibitory effect of intracerebroventricularly-administered [D-Arg(2), beta-Ala(4)]-dermorphin (1-4) (TAPA), a highly selective mu(1)-opioid receptor agonist, on mouse gastrointestinal transit was compared with that of morphine and [D-Ala(2), N-methyl-Phe(4), Gly(5)-ol]-enkephalin (DAMGO). When administered intracerebroventricularly 5 min before the oral injection of charcoal meal, TAPA (10-100 pmol), morphine (0.25-4 nmol), and DAMGO (20-80 pmol) dose-dependently inhibited gastrointestinal transit of charcoal. The inhibitory effect of each mu-opioid receptor agonist was completely antagonized by naloxone, a nonselective opioid receptor antagonist. The inhibitory effects of morphine and DAMGO were significantly antagonized by both beta-funaltrexamine, a selective mu-opioid receptor antagonist, and naloxonazine, a selective mu(1)-opioid receptor antagonist. In contrast, the inhibitory effect of TAPA was not affected at all by beta-funaltrexamine, naloxonazine, nor-binaltorphimine (a selective kappa-opioid receptor antagonist), or naltrindole (a selective delta-opioid receptor antagonist). These results suggest that the inhibitory effect of TAPA on gastrointestinal transit may be mediated through an opioid receptor mechanism different from that of morphine and DAMGO.     

A tetrapeptide isolated from hamster embryo with central opiate properties on gastrointestinal motility but not on pain perception

The effects of centrally administered kentsin (H-Thr-Pro-Arg-Lys-OH) on intestinal motility and on pain perception were investigated in rats chronically equipped with lateral ventricle catheters. Intestinal motility was recorded electromyographically from electrodes placed on the duodeno-jejunum; analgesia was evaluated by the hot-plate and tail-flick tests. Kentsin (4.0 ug/kg), injected intracerebroventricularly (ICV) 2 hours after the beginning of a meal, restores the "fasted" i.e. the migrating myoelectric complex of intestinal motility, while a 5 times higher dose administered subcutaneously was inactive. The ICV effect of kentsin was blocked by previous ICV administration of naloxone (400 ug/kg). In contrast, kentsin administered ICV (40 ug/kg) or SC (200 ug/kg) did not affect significantly (P greater than 0.05) the time latency in the two analgesic tests during 90 minutes after its administration and did not significantly modify the analgesic effects of (D5-Ala2, Met5) enkephalinamide. We conclude that kentsin when centrally administered acts on opiate receptors to alter gastrointestinal motility but without effects on pain perception.     

Effects of juice from Morinda citrifolia (Noni) on gastric emptying in male rats

The effects of juice from Morinda citrifolia (noni) on gastric emptying, gastrointestinal transit, and plasma level of cholecystokinin (CCK) in rats were studied. Male rats were given noni by gavage at levels of 0.25, 1, or 4 ml/kg once per day for one or 7 days. The rats in the control group were given water, while the rats in the experimental group were fasted overnight before measurement of gastrointestinal motility. Gastrointestinal motility was assessed in rats 15 min after intragastric instillation of a test meal containing charcoal (10%) and Na251CrO4 (0.5 microCi/ml). Gastric emptying was determined by measuring the amount of radiolabeled chromium contained in the small intestine as a percentage of the initial amount received. Then, gastrointestinal transit was evaluated by calculating the geometric center of distribution of the radiolabeled marker. Finally, blood samples were collected for measurement of CCK by radioimmunoassay. The administration of noni at 0.25 ml/kg, but not at 1 ml/kg and 4 ml/kg, for 1 day significantly inhibited gastric emptying. In contrast, gastric emptying was significantly inhibited by oral noni (0.25, 1, or 4 ml/kg) for 7 days. Intraperitoneal injection of lorglumide (5 or 10 mg/kg), a selective CCK1 receptor antagonist, effectively attenuated the noni-induced inhibition of gastric emptying. The intestinal transit and body weight, food intake, water intake, urine volume as well as feces weight were not altered by the administration of noni either acutely or chronically, but the administration of oral noni (1 ml/kg) for 7 days increased the level of plasma CCK in male rats. These results suggest that oral noni inhibits gastric emptying in male rats via a mechanism involving stimulation of CCK secretion and CCK1 receptor activation.     

Influence of enkephalinase inhibitors on gastric emptying in mice depends on the nature of the meal

The effects of two enkephalinase inhibitors (thiorphan and acétorphan) and DALAMIDE on gastric emptying of fat or non-fat meals were evaluated in mice. When administered intraperitonally at low doses (0.1 and 0.2 mg/kg) 30 min prior to a fatty (milk) meal, both thiorphan and acetorphan increased significantly (P less than 0.01) gastric emptying; these effects were maximal for 0.2 and 0.1 mg/kg respectively and decreased progressively to be not significant for doses higher than 5 mg/kg for thiorphan and 0.5 mg/kg for acetorphan. Similarly DALAMIDE given IP increased significantly (P less than 0.05) gastric emptying at doses of 0.5 and 1 mg/kg while a slowing of gastric emptying was obtained for 10 times higher doses. The effects of thiorphan (0.2 mg/kg) and DALAMIDE (0.5 mg/kg) were blocked by previous administration of naloxone (0.3 mg/kg) and methyl-naloxone (0.5 mg/kg) while only naloxone (0.3 mg/kg) blocked the slowing effect of high dose of DALAMIDE. Administered prior to a non-fat meal, thiorphan (1 mg/kg) stimulated gastric emptying and inhibited it at higher dosage (10 mg/kg). Neither acetorphan nor DALAMIDE at similar dosages affected the gastric emptying of a non-fat meal and the effects of thiorphan (1 and 0.1 mg/kg) were not blocked by naloxone (0.3 mg/kg). It is concluded that enkephalinase inhibitors (thiorphan and acetorphan) administered systemically stimulate the gastric emptying of a fat meal by increasing enkephalin levels in peripheral tissues, while thiorphan exhibits non-opiate effects on gastric emptying of a non-fat meal.