Endogenous opioid peptides in the control of food intake in cats

A comprehensive review of the pharmacodynamics, pharmacokinetics, and clinical effects of the neutral endopeptidase inhibitor racecadotril

Racecadotril, via its active metabolite thiorphan, is an inhibitor of the enzyme neutral endopeptidase (NEP, EC 3.4.24.11), thereby increasing exposure to NEP substrates including enkephalins and atrial natriuretic peptide (ANP). Upon oral administration racecadotril is rapidly and effectively converted into the active metabolite thiorphan, which does not cross the blood–brain-barrier. Racecadotril has mainly been tested in animal models and patients of three therapeutic areas. As an analgesic the effects of racecadotril across animal models were inconsistent. In cardiovascular diseases such as hypertension or congestive heart failure results from animal studies were promising, probably related to increased exposure to ANP, but clinical results have not shown substantial therapeutic benefit over existing treatment options in cardiovascular disease. In contrast, racecadotril was consistently effective in animal models and patients with various forms of acute diarrhea by inhibiting pathologic (but not basal) secretion from the gut without changing gastro-intestinal transit time or motility. This included studies in both adults and children. In direct comparative studies with loperamide in adults and children, racecadotril was at least as effective but exhibited fewer adverse events in most studies, particularly less rebound constipation. Several guidelines recommend the use of racecadotril as addition to oral rehydration treatment in children with acute diarrhea.

Up-regulation of methionine-enkephalin-like immunoreactivity by 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment in the forebrain of the Long-Evans rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is considered to be one of the most toxic environmental contaminants, named dioxin. Exposure to TCDD induces a plethora of intoxication symptoms, including anorexia and hypothermia, in several mammals and human. Enkephalin, an endogenous pentapeptide, is an important neuroregulator of autonomic functions, such as food intake and body temperature. In this study, we investigated the effects of TCDD gastric administration on methionine-enkephalin (MEK) immunoreactivity in the brain of the Long-Evans rat, the species strain considered to be the most TCDD-susceptible, using immunohistochemical staining. A single dose of TCDD (dissolved in olive oil, 50 microg/kg) or olive oil alone was administrated to the rats by gavage. Compared with the vehicle-treated rat, a marked increase in the density of MEK immunoreactive cell bodies, fibers and terminals was found 2 weeks after TCDD treatment in the forebrain of the TCDD-treated rat, i.e. the central amygdaloid nucleus, field CA3 of the hippocampus, paraventricular hypothalamic nucleus, medial preoptic nucleus, interstitial nucleus of the posterior limb of the anterior commissure, lateral globus pallidus, ventral pallidum and lateral division of the bed nucleus of the stria terminalis. These results demonstrated for the first time a site-specific increased enkephalinergic activity in certain brain regions of the Long-Evans rat. It is suggested that the increased MEK immunoreactivity may act as a compensatory adaptation for the pathophysiological alterations caused by TCDD exposure.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

Microinjection of opioid antagonists into the substantia nigra reduces stress-induced eating in rats

Stress produced by pinching the tail has been shown to cause satiated animals to eat and to display oral stereotypies. Endogenous opioids and central dopamine systems have been implicated in the mediation of these effects. In order to test the possibility that the substantia nigra (SN) might be involved, the amount of food intake and gnawing produced by mild tail pinch were assessed following bilateral microinjections of opioid antagonists into the SN. Evaluations of nociceptive thresholds were also conducted using tail flick and hot plate tests. Eating induced by tail pinch was reduced by microinjections of the non-selective opioid antagonist naloxone (3, 10, 20 and 30 nmol) and by the mu-selective antagonist Cys2, Tyr3, Orn5, Pen7 Amide (CTOP) (1, 3 and 10 nmol). These effects on eating occurred in the absence of effects on gnawing. kappa- and delta-antagonists (10 nmol) had no effect on eating or gnawing. Naloxone did not alter either tail flick or hot-plate response latencies. The highest dose of CTOP increased response latency on the hot-plate test only. The results are interpreted as suggesting that the SN may be an important central site of action for opioid antagonists in reducing stress-induced eating. The possibility that the SN may be a central site mediating the effects of dopamine on this phenomenon is also discussed.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.

Central versus peripheral opioid regulation of ingestive behavior in the domestic fowl

1. Three experiments were conducted to determine whether opioid regulation of ingestive behavior in the domestic fowl is mediated at sites within the central nervous system (CNS) or peripheral tissues. 2. Food and water intake were significantly decreased by the intramuscular (im) injection of naloxone hydrochloride (NHCl) and naloxone methobromide, which have a high and low ability, respectively, to cross the blood-brain barrier. 3. Water, but not food, intake was significantly decreased by the intracerebroventricular (ICV) injection of NHCl. However, water intake was not affected by the im injection of doses which were effective when given ICV. 4. These results suggest that in the domestic fowl there is a peripheral component to opioid regulation of food intake, while opioid regulation of water intake seems to be mediated at peripheral sites and within the CNS.