Potent action of leumorphin on consummatory behaviors in rats: comparison with other opioid peptides

Translational science: Newly emerging science in biology and medicine - Lessons from translational research on the natriuretic peptide family and leptin

Translation is the process of turning observations in the laboratory, clinic, and community into interventions that improve the health of individuals and the public, ranging from diagnostics and therapeutics to medical procedures and behavioral changes. Translational research is defined as the effort to traverse a particular step of the translation process for a particular target or disease. Translational science is a newly emerging science, distinct from basic and clinical sciences in biology and medicine, and is a field of investigation focused on understanding the scientific and operational principles underlying each step of the translational process. Advances in translational science will increase the efficacy and safety of translational research in all diagnostic and therapeutic areas. This report examines translational research on novel hormones, the natriuretic peptide family and leptin, which have achieved clinical applications or for which studies are still ongoing, and also emphasizes the lessons that translational science has learned from more than 30 years' experience in translational research.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

The kappa-opioid U-50,488H suppresses the initiation of nocturnal spontaneous drinking in normally hydrated rats

The effect of a systemic (IP) treatment with 1.0, 3.0 and 9.0 mg/kg U-50,488H (U50), a highly selective kappa-agonist, on spontaneous, nocturnal ingestive behavior of the rat was studied using a microcomputer controlled data acquisition system. The latency to initiate drinking was increased and drinking behavior was suppressed in the first hour after injection in a dose-dependent manner. The consummatory indices of drinking were not affected. After this period of adipsia, a phase of polydipsia, that was probably due to the diuretic effect of U50, was evident. This prophagic effect of U50 was evident only at the dose of 3 mg/kg and was accompanied by an increased duration of feeding episodes but not by a reduced latency to feed. These results suggest that kappa-receptors play a pivotal role in modulating spontaneous drinking in the normally hydrated rat and that this control is mainly exerted on the motivational aspect of drinking.

Effects of intracerebroventricular injection of dynorphin, leumorphin and alpha neo-endorphin on operant feeding in pigs

Young pigs, which are useful experimental animals for biomedical research, were prepared with lateral intracerebroventricular (ICV) cannulae and housed individually in cages fitted with operant panels, with food and water ad lib. ICV injection of 200 micrograms of dynorphin A 1-17 or 1-13 resulted in a significant meal commencing within 2-5 min. Shorter fragments of dynorphin (1-10, 1-9, 1-8) were ineffective at inducing feeding as was dynorphin B (rimorphin). In the same situation, leumorphin and alpha neo-endorphin (200 micrograms) elicited significant feeding but beta neo-endorphin did not. Dynorphin 1-17 or 1-13, administered 5 min before feeding started, increased meal size when pigs were fed after 4-h deprivation. Naloxone ICV (0.4 mg) significantly reduced food intake in pigs feeding after 4-h deprivation and its main effect was in the second half of the meal. Naloxone also abolished the effect of ICV dynorphin. It is concluded that dynorphin and related endogenous opioids are involved in the regulation of food intake in pigs.

Kappa-selective opioid receptor agonists leumorphin and dynorphin inhibit supraoptic neurons in rat hypothalamic slice preparations

Abstract To clarify the effects of opioid peptides, and in particular the effects of kappa-receptor agonists on the activity of supraoptic neurons, extracellular recordings were made from 71 spontaneously firing neurons in the rat hypothalamic slice preparation. Of 71 neurons, 28 showed a phasic firing pattern (phasic neurons: putative vasopressin neurons). The mean firing rate of phasic neurons was 2.6 spikes/s (intraburst firing rate 5.4 +/- 2.2 spikes/s). The mean firing rate of neurons classified as non-phasic neurons (putative oxytocin neurons) was 4.5 spikes/s. Following bath application of leumorphin (LM) at 10(-7) M, which has potent opioid activity at kappa-receptors, 17 (61%) of 28 phasic neurons were inhibited and 22 (51%) of 43 non-phasic neurons were inhibited. Excitation was observed in only one non-phasic neuron. The dose-dependence of the response to LM was tested in five supraoptic neurons. There was an inverse relationship between LM concentration and percent change in firing rate. The threshold concentration of LM was approximately 10(-8) M. The relatively selective kappa-receptor antagonist, MR-2266, completely blocked the LM-induced responses. Its effects were long-lasting and only partial recovery was observed 2 h after the application of MR-2266. Dynorphin had similar inhibitory effects on supraoptic neurons to those obtained with LM when tested on the same neurons. In another series of experiments the mu-receptor agonist morphine and the delta-receptor agonist [D-Ala, D-leu]-enkephalin (DADLE) were applied to 28 supraoptic neurons (12 phasic and 16 non-phasic neurons) at 10(-7) M and their actions compared directly with that of LM. Only two of 12 phasic neurons tested were inhibited by DADLE and none of five phasic neurons tested was inhibited by morphine, while eight of the 12 neurons tested were inhibited by LM. By contrast the non-phasic neurons tested were inhibited by application of each of the peptides; seven of 16 neurons tested were not only inhibited by LM, but also five of 11 neurons by DADLE and seven of 15 by morphine. The magnitude of the responses varied from cell to cell. These results suggest that LM acts at the same receptors as dynorphin, and that opioids acting preferentially at kappa-receptors inhibit both vasopressin and oxytocin neurons while delta- and mu-receptor agonists inhibit primarily oxytocin neurons.

Effects of the kappa opioid receptor antagonist MR-2266-BS on the acquisition of ethanol preference

Using a paradigm by which rats forced to drink a weak ethanol solution (2.5% w/v) (conditioning session) develop ethanol preference in consecutive retention testing days, the effects of the administration of the kappa opioid antagonist MR-2266-BS, prior to or after the forced ethanol session, were studied. Pre-conditioning subcutaneous (s.c.) administration of 1 mg/kg of MR-2266-BS induced a decrease in subsequent ethanol consumption without significantly modifying the acquisition of ethanol preference. Post-conditioning administration of MR-2266-BS (0.1, 1, 5 or 10 mg/kg) induced both a dose-dependent reduction in ethanol consumption and in preference throughout the three following days. The results of the present study provide further support of the involvement of kappa-type opioids on drinking behavior, and suggest that kappa receptors may be involved in the consumption and development of preference to ethanol.

Centrally administered opioid peptides stimulate saccharin intake in nondeprived rats

Endogenous opioid peptides are thought to play a role in mediating the pleasurable or rewarding aspects of the ingestion of certain foods and liquids. We therefore measured the effects of central administration of selective opioid agonists and naloxone on the intake of two concentrations of saccharin solution. All tests were performed on nondeprived rats, such that the taste of the solutions provided the primary incentive to consume. Intracerebroventricular (ICV) administration of the selective mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and the selective delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (3 nmol) increased intake of a 0.15% saccharin solution by approximately 10 ml over 3 hr. Water was available simultaneously, but intake was minimal. The selective kappa agonist U-50,488H did not increase intake of the saccharin solution. Naloxone (30 and 100 micrograms, ICV) caused a 44% reduction in saccharin solution intake in the first hour; two- and three-hour cumulative intakes were not different from control. DAGO and DTLET were also tested when rats were given a weaker saccharin solution (0.006%) along with water. Both agonists caused small increases in saccharin and water intake, but the increases above baseline were much smaller than those observed with the more palatable 0.15% saccharin solution. These results are consistent with reports by others which suggest that endogenous opioids influence taste preferences or palatability. Further, they indicate a role for central mu and delta opioid receptors in the mediation of this influence.

Implication of leumorphin in inhibitory control of vasopressin secretion in conscious rats

The effects of leumorphin, a kappa-agonist derived from proenkephalin B (neoendorphin and dynorphin precursor), on vasopressin secretion were studied under basal and stimulated conditions in conscious, unrestrained rats. Intracerebroventricular injection of leumorphin (60 or 600 pmol) significantly inhibited basal vasopressin secretion. The vasopressin response induced by intracerebroventricular injection of angiotensin II (100 pmol) was significantly suppressed, in a dose-dependent fashion, by the simultaneous intracerebroventricular injection of leumorphin (6, 60, or 600 pmol). Intravenous pretreatment with naloxone (0.5 mg/kg body weight) diminished the inhibitory action of leumorphin (60 pmol) on vasopressin secretion. Moreover, naloxone (0.5 mg/kg body weight) prolonged the vasopressin secretion induced by intracerebroventricular injection of angiotensin II (100 pmol). These results indicate that leumorphin possesses a potent inhibitory effect on vasopressin secretion and that, alone or in combination with other endogenous opioid peptides, it plays an important role in the control of vasopressin secretion.

Opioid modulation of vasopressin secretion in conscious rats

To clarify the role of endogenous opioid peptides in the control of vasopressin (AVP) secretion, the effects of an endogenous kappa-agonist, leumorphin, derived from proenkephalin B and an opioid antagonist, naloxone, on AVP secretion were examined in conscious and freely moving rats. Intraperitoneal injection of nicotine markedly increased AVP secretion in rats. The nicotine-induced AVP secretion was significantly suppressed by intracerebroventricular (i.c.v.) pretreatment with leumorphin. Intravenous injection of naloxone significantly increased the basal AVP level and carbachol-induced AVP secretion. These results indicate that endogenous opioid peptides have an inhibitory effect on AVP secretion in rats.

Involvement of kappa type opioids on ethanol drinking

The effects of the administration of the kappa agonist dynorphin1-17 and/or the kappa antagonist MR-2266-BS on ethanol preference was investigated using a paradigm by which rats develop alcohol preference. Administration of dynorphin shortly before or after the conditioning session (forced ethanol exposure) failed to affect later ethanol preference. However, dynorphin treatment prior to the first choice session reduced ethanol preference during the three consecutive testing days. This effect was reversed by the simultaneous administration of the kappa antagonist MR-2266-BS. The results of the present study provide further support for evidence of the involvement of dynorphinergic systems on drinking behavior and suggest that kappa-type opioid mechanisms may be involved in the consumption and development of preference to ethanol in rats.