In vivo modulation of gastrointestinal motor activity by Met-enkephalin, morphine and enkephalin analogs in chickens

Perspectives on Endogenous Opioids in Birds

The present review summarizes the state of knowledge of endogenous opioids in birds. Endogenous opioid peptides acts in a neuromodulatory, hormonal and paracrine manner to mediate analgesic and other physiological functions. These peptides act through specific G-protein coupled receptors. Opioid receptors consist of a family of four closely-related proteins. The three types of opioid receptors are the mu (MOR or μ), delta (DOR or δ), and kappa (KOR or κ) opioid receptor proteins. The role of the fourth member of the opioid receptor family, the nociceptin or orphanin FQ receptor (ORL), is not clear. The ligands for opioid receptors are: β –endorphin (MOR), Met- enkephalin, Leu-enkephalin (DOR) and dynorphin (KOR), together with probably endomorphins 1 and 2. In spite of long history of research on endogenous opioid peptides, there are no studies of endogenous opioids per se in wild birds and few in poultry species. β-endorphin is present in all birds investigated and there is close agreement between the structures of β-endorphin in different birds. Plasma concentrations of β-endorphin are increased by ether stress in geese. There is evidence that β-endorphin plays a role in the control of luteinizing hormone release in chickens. Met-enkephalin is present in tissues such as the retina, hypothalamus, pituitary gland, and adrenals together with circulation of birds. Stresses such as crowding and withholding water increase circulating concentrations of Met-enkephalin in chickens. The structures of chicken dynorphin A and B have been deduced from cDNA. What is missing are comprehensive studies of plasma concentrations and expression of the full array of endogenous opioids in multiple avian species under different situations. Also, what is not known is the extent to which circulating or locally released or intra-cellular Met-enkephalin influence physiological process in birds. Thus, there is considerable scope for investigation of the physiology of endogenous opioids in birds.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

Effect of endomorphin on somatostatin secretion in the isolated perfused rat stomach

Recently the two peptides endomorphin-1 and -2 were isolated from bovine brain, which are postulated to be endogenous agonists for mu-opiate receptors in the CNS. Since exogenous and endogenous opioids have been shown to influence gastric functions, it was of interest to examine the effects of endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2), EM-1) and -2 (Tyr-Pro-Phe-Phe-NH(2), EM-2) in the isolated perfused rat stomach.

RESULTS

EM-1 10(-8) M and 10(-6) M inhibited somatostatin (SLI) levels from a mean of 79 +/- 2.7 pg/min and 73 +/- 2.7 pg/min to 52 +/- 4.0 pg/min (n = 5, n.s.) and 27 +/- 3.0 pg/min (n = 5, P < 0.05), respectively. To characterize the effect on stimulated SLI-secretion, it was prestimulated for 30 min with gastric inhibitory polypeptide (GIP, 10(-9) M). EM-1 decreased prestimulated SLI-secretion in a concentration-dependent manner from a mean of 469 +/- 64.9 pg/min during the immediately preceding 15 minutes to 184 +/- 12.1 pg/min (67 +/- 4.0 %) at 10(-7) M and from a mean of 1146 +/- 269.6 pg/min to 111 +/- 14.1 pg/min (94 +/- 2.2 %) at 10(-6) M (each n = 6, each P < 0.05). In addition EM-2 was also examined at a concentration of 10(-6) M, which inhibited prestimulated SLI-secretion from a mean of 514 +/- 14.9 pg/min to a nadir of 204 +/- 44.7 pg/min (42 +/- 5 %, n = 6, P < 0.05). Application of the specific mu-opiate receptor antagonist CTOP in doses of 10(-7) to 10(-5) M significantly attenuated the inhibitory effect of EM-1 10(-7) M from 67 +/- 4.0 % to 34 +/- 4.7 % (10(-7) M), 33 +/- 3.0 % (10(-6) M) or 30 +/- 8.6 % (10(-5) M), respectively. This residual inhibition, however, was still significantly different from the preceding perfusion period. On the other hand, naloxone 10(-6) M completely abolished the inhibitory effect of EM-1 10(-7) M. Similarly, the inhibitory effect of 10(-6) M EM-1 was also significantly reduced by CTOP from 94 +/- 2.2 % to 60 +/- 10.9 % (10(-7) M), 61 +/- 5.5 % (10(-6) M) or 51 +/- 12.5 % (10(-5) M), respectively, and the residual effect was significantly different from the preceding perfusion period as well. At this higher dose of EM-1 (10(-6) M) naloxone 10(-6) M reduced the effect to 35 +/- 8.2 %, but there was still a significant difference of SLI levels compared to the preceding stimulation period (P < 0.05). Naloxone 10(-6) M reduced the inhibitory effect of EM-2 10(-6) M from 42 +/- 5.0 % to 20 +/- 5.0 % (P < 0.05), which was still significantly different compared to the preceding stimulation period. EM-1 at the doses of 10(-12) M, 10(-10) M, 10(-8) M and 10(-6) M had no significant effect neither on basal gastrin, bombesin (BLI) and vasoactive intestinal polypeptide (VIP) release nor during concomitant infusion of GIP.

CONCLUSIONS

EM-1 and -2 inhibit basal and prestimulated SLI secretion in the isolated perfused rat stomach, which is in part attenuated by the mu-receptor antagonist CTOP. The greater inhibitory effect of naloxone, which can be demonstrated at least during the lower dose of EM-1, indicates that other opiate receptors contribute as well. The failure of naloxone to completely antagonize the effect of the higher concentration of EM-1 or EM-2 could be due to insufficient dosage or might indicate the involvement of non-opiate receptor mechanisms.

Initiation of phase III contractions in the jejunum by atropine, hexamethonium and xylocaine in conscious dogs

Mechanisms of initiation of phase III contractions in the jejunum during the digestive state are not well understood. To test whether phase III can be induced by a local injection of various agents in a jejunal segment, a polyethylene tube was chronically placed in a branch of the jejunal artery, and force transducers were chronically placed in the upper jejunum. Local injection of atropine, hexamethonium and xylocaine induced caudal-migrating phase III in the injected segment only in the digestive state, and simultaneous intra-arterial infusions of L-arginine, an NK-1 antagonist, or 5-hydroxytryptamine (5-HT) 1P and 3 antagonists inhibited the induced phase III. Intravenous atropine and hexamethonium also inhibited xylocaine-induced phase III contractions. Atropine and hexamethonium-induced phase III were brought about by inhibition of neural transmission at nicotinic receptors in the inhibitory pathway to NO neurones. NK-1, 5-HT1P and 5-HT3 receptors are present in the excitatory but not the inhibitory pathway to NO neurones. Xylocaine appears to stop neuronal transmission from mechanoreceptors to NO neurones. Thus, the initiation of spontaneous occurrence of phase III in the digestive jejunum is likely to be brought about by transient cessation of postprandial contractions in a segment of the jejunum.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.