Plasma growth hormone and prolactin response to FK 33-824, a synthetic opioid agonist, in broiler chickens.

A synthetic opioid agonist, FK 33-824 ([D-Ala2,N-Me-Phe4,Met-(O)5-ol]- enkephalin), was administered intramuscularly at levels of 0, 1, 25, or 625 micrograms/kg of body weight to 4-wk-old Arbor Acres x Arbor Acres broiler cockerels. All levels of FK 33-824 caused the birds to be sedated, with the highest dose causing deep sedation over the 240-min test period. A significant time by treatment interaction was seen for plasma growth hormone (GH). At 30 min after treatment, a significant increase in GH occurred, but this transitory increase returned to control levels at 60 min. A secondary GH peak was observed in the 240-min samples from cockerels given the 625 micrograms/kg dose. Significant treatment and time effects and a time by treatment interaction were seen in the plasma prolactin (PRL) response to FK 33-824. Prolactin was increased at 30 and 60 min after treatment with 1 microgram/kg, whereas higher doses seemed to suppress PRL concentrations. Over the 240-min experimental period, PRL concentrations tended to increase in all treatments except in the treatment with the lowest FK 33-824 dose, but the largest transitory increase was observed at 240 min in the birds given the 625 micrograms/kg dose. These results suggest that FK 33-824 affected GH and PRL secretion in chickens in a manner different from other opioid agonists. This difference was probably due to the ability of FK 33-824 to bind to multiple opiate receptors, with the highest affinity for mu receptors and lesser affinity for delta receptors, whereas other opioids have high affinity for delta receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding in vivo of selective mu and delta opioid agonists: localization by autoradiography.

The in vivo binding properties of cerebral opioid receptors were investigated in mice after intracerebroventricular (i.c.v.) injections of iodinated FK33-824 and [D.Ala2]deltorphin-I which behave in vitro as highly selective ligands possessing high affinity for mu and delta receptors, respectively. [125I]FK33-824 and [125I][D.Ala2] deltorphin-I exhibited similar diffusion kinetics after i.c.v. injection and bound specifically to sites characterized pharmacologically as mu and delta receptors respectively. Autoradiographic analysis revealed that after i.c.v. administration, concentrations of [125I]FK33-824 and [125I][D.Ala2]deltorphin-I remained higher in the circumventricular than in the deep structure of the brain and that specific sites labelled in vivo were differently distributed from those observed after in vitro labelling. FK33-824 was 250 times more analgesic than [D.Ala2]deltorphin-I in the tail-flick test and at doses producing a similar analgesia, [D.Ala2]deltorphin-I occupied a high proportion of mu receptors. Furthermore, analgesic effect of [D.Ala2]deltorphin-I was antagonized by pretreatment with naltrexone but not by naltrindole, a selective antagonist of delta-opioid receptors. These experiments reveal the localization of mu and delta opioid receptors reached after i.c.v. injection and provide evidence to support the suggestion that delta-opioid receptors contribute little or none to the supraspinal antinociception.

A synthetic opioid peptide increases plasma growth hormone and prolactin in Holstein calves.

The effect of the synthetic opioid agonist D-Ala2,N-Me-Phe4,Met(O)5-ol enkephalin (DAMME) on plasma growth hormone (GH) and prolactin (PRL) concentrations in Holstein heifer calves was investigated in this study. The possible site of action of DAMME was determined by pretreating calves with an opioid antagonist that crosses the blood-brain barrier poorly if at all (N-methyl levallorphan-methane sulphonate [MLM]) or one that crosses readily (naloxone [NAL]). All calves were assigned to one of three treatment groups: 1) pretreatment with saline, 2) pretreatment with NAL, or 3) pretreatment with MLM. All groups were injected with DAMME 30 min after pretreatments. Plasma PRL increased after injection of DAMME in calves pretreated with saline. Prolactin concentrations were not different before and after injection of DAMME in calves pretreated with either NAL or MLM. Plasma GH increased after injection of DAMME in saline- and MLM-pretreated calves but was unchanged in NAL-pretreated calves. These data show that peripherally administered DAMME increases plasma GH and PRL in Holstein heifer calves and suggest that DAMME mediates GH release through receptors located somewhere inside the blood-brain barrier, but it can induce PRL secretion at a site located outside the barrier.

Control of growth hormone secretion during the postpartum period.

The postpartum period is characterized by high levels of circulating steroids which condition hypothalamo-pituitary activities. In pregnancy growth hormone (GH) levels are greatly increased and lack pulsatility. In order to investigate the behavior of GH during the postpartum period, the GH response to GH-releasing hormone stimulation (50 and 100 micrograms), to amphetamine, a dopamine receptor agonist, and to FK 33-824, an opiate receptor agonist, was investigated in women during the first 5 days after delivery. In all groups GH responses were significantly lower (p less than 0.01) than in normal women studied during the early follicular phase. FK-33-824-induced GH release was similar in postpartum and control women. These results demonstrate reduced pituitary GH response to GH-releasing hormone and to amphetamine in women during the postpartum period, confirming the peculiarity of the hypothalamopituitary component.

Changes in growth hormone and luteinizing hormone following acute or chronic administration of an opioid agonist, FK33-824, in wethers.

Two experiments were conducted to determine the effect of acute or chronic administration of an opioid agonist, (D-Ala2,MePhe4,Met(O)5-ol)-enkephalin (FK33-824), on concentrations of growth hormone (GH) and LH in castrate sheep (wethers). In Exp. 1, we evaluated the effect of acute administration (two injections or infusion for 2 h) of FK33-824 on GH and LH. Wethers (31 +/- 1 kg; n = 12) received FK33-824 or saline via one of the three following routes of deliver: 1) i.v. (8 micrograms/kg) at 0 and 60 min (i.v., n = 4), 2) s.c. (16 micrograms/kg) at 0 and 60 min (s.c., n = 4), or 3) i.v. infusion (8 micrograms.kg-1.h-1) for 120 min (inf., n = 4) preceded by a single injection (8 micrograms/kg). Wethers received FK33-824 and saline on d 1 and 2 or d 2 and 1, respectively. Concentrations of GH were consistently elevated (P less than .05) by FK33-824 regardless of route of delivery. Infusion of FK33-824 for 120 min maintained GH concentrations greater (P less than .05) than pretreatment or those during infusion of saline. Average LH was decreased consistently by FK33-824, independent of its route of administration; episodic release of LH was suppressed. In Exp. 2, crossbred wethers (36 +/- 2 kg; n = 12) were used to determine the effect of chronic administration (s.c.) of FK33-824 on the concentrations of GH and LH and retention of N and energy.(ABSTRACT TRUNCATED AT 250 WORDS)

D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin in the nucleus tractus solitarius of the rat produces cardiorespiratory depression.

1. The synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin (FK 33-824). 1 or 2 micrograms, after its injection into the nucleus tractus solitarius (NTS) of Wistar rats, anesthetized with pentobarbital and breathing spontaneously, produced a transient increase in blood pressure followed by sustained and significant (P less than 0.05) hypotension and bradycardia. This occurred in a dose dependent manner. 2. FK 33-824 in the NTS, 1 or 2 micrograms, also produced a marked respiratory depression. 3. In anesthetized rats, in which hypoventilation was prevented by mechanical ventilation, there was a definite reduction in blood pressure and heart rate that was considerably and significantly (P less than 0.05) less than that observed in spontaneously breathing rats. 4. Blood pressure fluctuations occurred after NTS injection that were more marked in spontaneously breathing animals but still occurred in animals that were ventilated mechanically. 5. FK 33-824, 1 and 2 micrograms in the NTS was fatal within 100 min for all animals but was prevented by mechanical ventilation. Higher doses of FK 33-824, 10 micrograms in the NTS, however, induced fatal ventricular arrhythmias even in the mechanically ventilated rat. 6. Thus, FK 33-824 in the NTS decreases blood pressure and heat rate in spontaneously breathing as well as mechanically ventilated rats, but much of the effect on blood pressure and heart rate is due to the profound respiratory depression in the spontaneously breathing rat.

Effect of indomethacin on opioid-induced gastric protection in cold-restrained stress.

Morphine and the synthetic opioid met-enkephalin analog [D-Ala2, MePhe4, Met(0)5ol] enkephalin (FK 33-824) injected intraperitoneally to rats at doses of 5-20 and 0.5-2 mg/kg respectively showed a protective effect on gastric lesion induced by cold-restraint stress. This protective effect was abolished by pretreatment with indomethacin. This suggests a role for prostaglandins in the protection, induced by opioids of the gastric mucosa against the development of stress-induced ulcers.

Cardiorespiratory responses to D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin after administration into the fourth cerebral ventricle of the rat: interaction with cholinergic mechanisms.

The purpose of this study was to investigate the effect of the synthetic Met-enkephalin, D-Ala-2-Me-Phe-4-Met-(O)-ol-enkephalin (FK 33-824), on blood pressure, heart rate, respiratory rate and survival, after its injection into the 4th cerebral ventricle of Wistar rats. The animals were either anesthetized with pentobarbital and breathing spontaneously or unanesthetized. The unanesthetized rats were previously instrumented with cannulas in the 4th cerebral ventrical and a systemic artery. In anesthetized rats, intracerebroventricular administration of FK 33-824 produced a transient increase in blood pressure followed by sustained hypotension, bradycardia and respiratory depression in a dose-dependent manner. Fatalities were observed over a 150-min observation period and were a function of dose. Pretreatment with atropine sulfate (1 mg/kg i.v.) produced an accentuated response with greater hypotension, bradycardia and shorter survival. In another group of anesthetized rats, in which hypoventilation was prevented by mechanical ventilation, blood pressure and heart rate were not as reduced as in spontaneously breathing rats. Hypotension, bradycardia and hypoventilation were less marked in unanesthetized rats, compared to anesthetized rats. Thus, FK 33-824 in the 4th cerebral ventricle of the rat produces marked changes in blood pressure in anesthetized as well as unanesthetized animals, but these changes were less in the unanesthetized or mechanically ventilated animal and greater after atropine, suggesting that these effects are mediated by respiratory depression and are antagonized by the cholinergic nervous system.

Central effect of the enkephalin analogue FK-33824 on vasopressin secretion in conscious sheep.

The role of opioids in the regulation of arginine vasopressin release from the posterior pituitary is a subject of controversy. In the present study, we examined the effects of central administration of met-enkephalin, leu-enkephalin, the enkephalin analogue FK-33824, and the opiate antagonist naloxone, and the effects of systemic administration of met-enkephalin and FK-33824 on AVP secretion in conscious normal sheep. Intracerebroventricular infusion of FK-33824 significantly increased the plasma concentration of immunoreactive AVP in a dose-dependent manner, but met-enkephalin, leu-enkephalin and naloxone failed to change plasma concentration of AVP. Intravenous infusion of met-enkephalin and FK-33824 also failed to change plasma concentration of AVP. The opiate antagonist naloxone given both centrally and systemically attenuated the increase in plasma concentration of AVP induced by FK-33824. We conclude that basal AVP release is stimulated by central administration of FK-33824.

Central histaminergic effect on basal and opioid-induced prolactin secretion.

1. The participation of the central histaminergic system in basal and opioid-induced prolactin secretion in male rats was investigated. 2. Central injections of cimetidine, an H2 antihistaminic, reduced basal plasma prolactin levels from 5.08 +/- 0.76 to 3.11 +/- 0.50 ng/ml (N = 10) while diphenhydramine, an H1 antihistaminic, had no effect. 3. Intracerebroventricular injections of FK 33824, a synthetic opioid peptide, produced a dose-dependent increase of prolactin secretion. This effect was partially blocked (-50%) by pretreatment with 20 ng diphenhydramine 15 min before administration of the opioid peptide. Cimetidine pretreatment failed to modify the prolactin rise in this case. 4. We conclude that brain histaminergic pathways exert a stimulatory tone on basal prolactin secretion which is mediated by H2 receptors and that the opioid-induced prolactin rise depends at least in part on activation of H1-histamine receptors.

Opiate receptor subtypes in the regulation of thyrotropin and prolactin secretion in the rat.

In the present study both MR 2034 (kappa-agonist) and DAMME (mu-agonist) decreased thyrotropin (TSH) secretion stimulated by cold in the rat when infused into the 3rd ventricle. After infusion into the posterior hypothalamus (PH), a small dose of MR 2034 increased the TSH response to cold whereas other doses did not. The stimulatory (at PH) but not the inhibitory (at 3rd ventricle) effect of MR 2034 was antagonized by naloxone. DAMME had no statistically significant effect at this location. Both the mu- and kappa-agonist stimulated prolactin secretion when infused into the 3rd ventricle, but DAMME was more effective than MR 2034. Furthermore, the stimulatory effect of DAMME, but not that of MR 2034, on prolactin secretion was antagonized by naloxone.

Opioid-induced linear running in obese (ob/ob) and lean mice.

Earlier research has shown that opioids stimulate behavioral activation in mice whereas opioid antagonists attenuate this activation. We conducted an experiment to determine the dose-response curve of FK33824, a potent Met-enkephalin analogue. FK33824 produced an unusual form of behavioral activation we called "linear running" in which the mice ran continuously in one direction and were nearly oblivious to environmental stimuli. This may be the kind of running that occurs naturally during migration. Wheel running activity of genetically obese (ob/ob) and lean (C57BL/6J ?/+) mice was measured following the intracerebroventricular infusion of 0.1, 1.0, 10.0 and 100.0 ng of FK33824. The lowest dose did not alter baseline running, whereas the 1.0 and 10.0 ng doses significantly increased running in both genotypes. We found a genotype difference with the highest dose tested, the lean mice ran at baseline levles and displayed ataxia whereas the obese mice continued to show increased running without ataxia. We hypothesize that genetic differences in the enkephalin mechanisms of C57 lean and obese mice are responsible for linear running.

Systemic and intracerebroventricular effects of opioid peptides in withdrawn morphine-dependent rhesus monkeys.

The effects of the degradation-resistant enkephalin analogs FK 33-824 and metkephamid were determined after systemic and intracerebroventricular (i.c.v.) administration in withdrawn morphine-dependent rhesus monkeys. Both peptides suppressed completely signs of 12-hr morphine deprivation, as does the prototype mu-receptor agonist morphine. The peptides were 100 and 2000 times more potent, respectively, after i.c.v. than s.c. injection. Thus, although peptidase-resistant, these compounds have restricted entrance into the central nervous system after systemic administration. The i.c.v. administration of compounds in rhesus monkeys should prove to be a valuable tool in the study of peptide ligands for opiate receptors.