Opioid modulation of feeding and drinking in fowls

Anticipatory Behavior for a Mealworm Reward in Laying Hens Is Reduced by Opioid Receptor Antagonism but Not Standard Feed Intake

It is widely accepted that the absence of suffering no longer defines animal welfare and that positive affective experiences are imperative. For example, laying hens may be housed in environments that do not cause chronic stress but may lack particular resources that promote positive affective experiences, such as conspecifics or effective enrichment. Despite a consensus of how important positive affect is for animal welfare, they are difficult to identify objectively. There is a need for valid and reliable indicators of positive affect. Pharmacological interventions can be an effective method to provide insight into affective states and can assist with the investigation of novel indicators such as associated biomarkers. We aimed to validate a pharmacological intervention that blocks the subjective hedonistic phase associated with reward in laying hens via the administration of the non-selective (μ, δ, and κ) opioid receptor antagonist, nalmafene. We hypothesized that nonfood deprived, hens that did not experience a positive affective state when presented with a mealworm food reward due to the administration of nalmefene, would show minimal anticipatory and consummatory behavior when the same food reward was later presented. Hens (n = 80) were allocated to treatment groups, receiving either nalmefene or vehicle (0.9% saline) once or twice daily, for four consecutive days. An anticipatory test (AT) was performed on all days 30 min post-drug administration. Behavioral responses during the appetitive and consummatory phase were assessed on days 1, 3 and 4. Anticipatory behavior did not differ between treatment groups the first time hens were provided with mealworm food rewards. However, antagonism of opioid receptors reduced anticipatory and consummatory behavior on days 3 and 4. Feed intake of standard layer mash was not impacted by treatment, thus nalmefene reduced non-homeostatic food consumption but not homeostatic consumption. Behavioral observations during the AT provided no evidence that nalmefene treated hens were fearful, sedated or nauseous. The results suggest that we successfully blocked the hedonistic subjective component of reward in laying hens and provide evidence that this method could be used to investigate how hens perceive their environment and identify associated novel indicators to assess hen welfare.

Modulation of opioid-induced feeding behavior by endogenous nitric oxide in neonatal layer-type chicks

The current study was designed to evaluate the effects of central administration of L-arginine (The precursor of nitric oxide), N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, selective opioid receptor agonists and involvement of central nitrergic/opioidergic systems on feeding behavior in neonatal layer-type chicks. The results of this study showed that the intracerebroventricular (ICV) injection of L-arginine (400 and 800 nmol) significantly decreased food intake (P < 0.001) but the injection of 200 nmol L-arginine had no effect on cumulative food intake in FD3 chickens (P > 0.05). The ICV injection of L-NAME (200 and 400 nmol) increased food intake (P < 0.001) but 100 nmol of L-NAME had no significant effect (P > 0.05). On the other hand, the co-injection of 100 nmol L-NAME significantly attenuated the anorexigenic effect of 800 nmol L-arginine (P < 0.001). Moreover, the food intake of chicks was significantly decreased by ICV injection of DAMGO (μ-opioid receptor agonist, 125 pmol) (P < 0.001) while both DPDPE (δ-opioid receptor agonist, 40 pmol) and U-50488H (κ-opioid receptor agonist, 30 nmol) significantly stimulated food intake (P < 0.001). In addition, the hypophagic effect of DAMGO was significantly amplified by administration of L-arginine (P < 0.001) while the administration of L-NAME attenuated the hypophagic effect of DAMGO (P < 0.001). In contrast, co-injection of L-arginine or L-NAME with DPDPE had no effect on the hyperphagia induced by DPDPE as well as the hyperphagic effect of U-50488H on food intake was not affected by concurrent injection of L-arginine or L-NAME (P > 0.05). These results suggest that nitrergic and opioidergic systems have an important role on feeding behavior in the CNS of neonatal layer-type chicks and it seems that interaction between them is mediated by μ-opioid receptor.

Effects of N-terminal fragments of beta-endorphin on feeding in chicks

It is known that N-terminal fragments of beta-endorphin have biological activities, such as an antagonism effect of beta-endorphin (1-31) on the secretion of hormones or thermoregulation in mammals. We studied the effects of the N-terminal fragments on feeding behavior in male broiler chicks. Intracerebroventricular administration of beta-endorphin (1-27) (0.4nmol) stimulated feeding behavior compared with saline control during the 60-min experimental period. beta-Endorphin (1-17) (2.0nmol) also increased food intake at 30min postinjection. Co-injection of either beta-endorphin (1-27) or (1-17) was effective in reducing full-length beta-endorphin-induced feeding in chicks. These data suggest that the N-terminal fragments of beta-endorphin act as a partial agonist, and may regulate the activity of the central opioidergic system in chicks.

Central neuropeptide FF reduces feed consumption and affects hypothalamic chemistry in chicks

Information on the physiological functions of neuropeptide FF; NPFF, a morphine modulating octapeptide in avians is lacking. Thus, we designed a study to investigate the effects of central NPFF with particular emphasis on appetite-related processes. Cobb-500 chicks were intracerebroventricularly (ICV) injected with 0, 4.16, 8.32 or 16.6nmol NPFF, and feed and water intake were quantified. Feed intake was linearly decreased as NPFF dose increased, and this effect decayed over time and was not significant by 120min post-injection. Water intake was not affected by ICV NPFF. In a second exp, we observed that naloxone completely reversed the NPFF-induced decrease in feed intake. The amount of time a visible marker took to travel through the total length of the alimentary canal linearly increased as NPFF dose increased. We measured neuronal activation in the lateral hypothalamus (LH), paraventricular nucleus (PVN) dorsomedial nucleus (DMN) and ventromedial hypothalamus (VMN) of the hypothalamus, and nucleus dorsomedialis posterior thalami (DMP) of the thalamus. The DMN, DMP, PVN and VMH were all activated by ICV NPFF while the LH was not affected. Finally, we determined that the anorexigenic effect of ICV NPFF is primarily behavior specific, since behaviors unrelated to ingestion were not increased the same duration of time as was consumatory pecking. We conclude that NPFF causes anorexigenic effects in chicks that are primarily behavior specific.

Effects of various mu- and delta-opioid ligands on food intake in the meat-type chick

The present study was designed to examine the effects of mu- and delta-opioid receptor ligands on feeding behavior in meat-type chicks. Intracerebroventricular (ICV) injection of naltrexone (mu- and delta-antagonist), beta-funaltrexamine (beta-FNA; mu-antagonist), ICI-174,864 (ICI; delta-antagonist), or naloxonazine (NAL; mu1-antagonist) significantly decreased deprivation-induced feeding at 30 min postinjection. Co-injection of beta-FNA, but not NAL, significantly blocked the depressive effect of [D-Ala2, N-MePhe4, Gly5-ol]-enkephalin (mu-opioid agonist) under ad libitum conditions. Central injection of ICI attenuated significant effects of [D-Pen(2,5)]-enkephalin (delta-opioid agonist) on feeding behavior in ad libitum fed chicks. Co-injection of beta-FNA, but not ICI, significantly attenuated the orexigenic effect of [D-Ala2, D-Leu3]-enkephalin (mu- and delta-opioid agonist). These results suggest that the endogenous opioid peptides, which act on the mu- and/or delta-opioid receptor, have an important role in feeding behavior in the central nervous system of meat-type chicks.

Feeding responses to mu-, delta- and kappa-opioid receptor agonists in the meat-type chick

The present study was designed to examine the effect of specific opioid agonists on feeding behavior in neonatal chicks. The food intake of broiler chicks was significantly decreased by intracerebroventricular injection of DAMGO and beta-casomorphin, micro-opioid receptor agonists, at 30-min postinjection. In contrast, both delta-opioid receptor agonists (DADLE and DPDPE) stimulated the food intake of the chick. Similar to the delta-opioid receptor agonists, food intake was elevated by the kappa-opioid receptor agonist (U-50488H and U-62066) in a dose-dependent manner. These results suggest that the endogenous opioid peptides have an important role for feeding behavior in the central nervous system of chicks.

Response of neuropeptide Y-induced feeding to μ-, δ- and κ-opioid receptor antagonists in the neonatal chick

It is known that opioid antagonists reduce the orexigenic effect of neropeptide Y (NPY) in mammals. We studied the effect of three opioid antagonists on NPY-induced feeding in male broiler chicks. Beta-funaltrexamine (beta-FNA), naloxonazine (NAL), ICI-174,864 (ICI) or nor-binaltorphimine (nor-BNI), antagonists of mu-, mu1-, delta- or kappa-receptors, and NPY were co-injected in chicks. Food intake was measured 30 min after treatment. Co-injection of beta-FNA or NAL was effective in reducing NPY-induced feeding, whereas ICI and nor-BNI had little effect on NPY-induced feeding. These data suggest that the mu-opioid receptor, especially the mu1-opioid has some relation to NPY-induced feeding, and implies that an endogenous ligand, such as beta-endorphin, participates in the orexigenic effect of NPY in neonatal chicks.

Behavior and behavioral needs

An understanding of behavior is important in any consideration of poultry welfare. Behavior is a good indicator of states of suffering such as fear, frustration, and pain. It might also be possible to use social interactions as indicators of welfare. The possibility of using "luxury" behavior, such as play and exploratory behavior, as an indicator of positive emotional states, requires investigation. Important welfare consequences arise from the fact that some behavior may be so strongly motivated as to constitute a "need". A behavioral need will arise in the case of behavior, such as nesting, that is controlled largely by internal factors, because these factors will be present no matter what type of environment is provided. Behavior triggered largely by external stimuli, such as responses to predators, will not give rise to a need if the external factors can be removed from the environment. Dustbathing is an example of behavior controlled by complex interactions between internal and external factors; the extent to which this constitutes a need is still being debated. If a behavioral need arises, then it is important that the environment provided allows it to be performed without damage to the performer or other birds. It should also be remembered that birds may need to perform behavior, including appetitive as well as consummatory elements, although the functional consequences of these are no longer required for survival. Finally, the performance of certain behavior leads to an increase in health or physical condition that improves welfare later in life.

Feeding-induced tonic pain suppression in the chicken: reversal by naloxone

Endogenous analgesia induced by changes in motivation has been identified in the chicken in previous studies but either the motivational changes were difficult to interpret or the motivation was unpredictable. Experimental sodium urate (SU) arthritis of the ankle joint resulted in pain-coping behaviour (one-legged standing or sitting) for a 2-h period in non-food-deprived birds without access to food. Complete analgesia or marked hypoalgesia was observed in birds which had been food deprived overnight and given access to food immediately after SU injection. This analgesia seen during feeding behaviour in the food-deprived bird could be completely reversed by intravenous injection of naloxone. These results demonstrate that feeding motivation can totally suppress, in some animals, the severe tonic pain of SU arthritis and that this analgesia may be opioid mediated.

Intestine fullness influences feeding behaviour and crop filling in the domestic turkey

Image-intensification radiology showed that artificial introduction of food slurry into the intestines of 6-12 wk-old turkey hens significantly increased the proportion of boli entering the crop during an evening meal, hence decreasing the proportion of boli travelling directly to the gizzard. Since bolus movement is directed by esophageal motility, esophageal motility may be partially mediated by stretch or chemoreceptors in the distal duodenum. Increased crop filling during afternoon meals lends support to the widely held belief that the crop increases gut storage capacity and helps "tide birds over" the nightly fast. Artificial filling of the distal duodenum paradoxically increased food intake in birds observed by radiology while eating a single meal (morning and evening), and in birds observed by eye over a 3 h period in the morning. This phenomenon is similar to that previously reported for rabbits and domestic chicken. Conversely, duodenal filing reduced food intake over a full day (11 h), more than compensating for the introduced food. Rapid filling of the small intestine (within 25 min. of the start of the meal) suggests a modification of the function of the domestic turkey duodenum to serve as a "mixing chamber," possibly enhancing digestive efficiency. The ways in which this modification affects digestion and absorption in the duodenum warrant investigation.

Assessment of the welfare of food restricted male broiler breeder poultry with musculoskeletal disease

The general and sexual activity of food restricted male broiler breeder poultry was assessed for evidence of behavioural changes associated with musculoskeletal lesions. The activity and fertility of male birds given betamethasone (an anti-inflammatory steroid) or saline were compared in a two-period crossover experiment. Behavioural changes occurred and the birds' mating activity and fertility were decreased when they were given the steroid, but these effects were not associated with the presence of lesions. In a second experiment, there were no differences in sexual motivation between birds either with or without leg disorders. The birds were trained to walk down an alley for their food and the speed of walking was compared in a two-period crossover experiment. Betamethasone decreased their walking speed in period one and the carryover effect was significant in period two. Naloxone decreased the walking speed of birds with lesions more than of those without lesions. This effect was taken as evidence for analgesia by endogenous opioids and may help to explain the lack of response of the birds to the analgesic agent. The evidence that these food restricted male broiler breeder birds experienced pain was equivocal.

[The regulation of feed intake and selection with special reference to poultry]

Feed intake is regulated in a dialogue between the animal and the feed, which is influenced by numerous factors. The hypothalamus has a central integrative function. Furthermore, caudal brain areas (medulla oblongata, pons) are of importance because these areas are relays of peripheral signals and gustatory afferents. All peripheral informations are integrated by various neurotransmitters and neurohormones. The function of this neuronal system is not exactly known yet. Sensorial informations, mechano-, chemo- and osmoreceptors of the gastrointestinal tract and gastrointestinal hormones are discussed as influences of the periphery. The physiological satiety function of cholecystokinin is questionable in poultry. Hepatic chemoreceptors, which are activated by various metabolites, influence the amount of feed ingested. The feed choice appears to be regulated by the same mechanisms. Our knowledge about the translation of peripheral signals into choice behaviour by changes of neurotransmitter systems is limited.

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.