Brain monoamine levels and behaviour of young and adult chickens genetically selected on feather pecking.

Severe feather pecking (SFP) in chickens is a detrimental behaviour with possibly neurochemical deficits at its base. Recent neurological studies depicted conflicting results on the role of serotonin (5-hydroxytryptamine, 5-HT) and dopamine (DA) in the development and display of feather pecking. We studied brain monoamine levels and behaviour in domestic chickens divergently genetically selected on feather pecking behaviour, the Low Feather Pecking (LFP) and High Feather Pecking (HFP) lines, both at a young age and when adult, to elucidate the role of 5-HT and DA in feather pecking. Also pecking behaviour and the behavioural response to challenging test situations was determined. At 8 weeks of age, HFP had lower 5-HT and DA turnover in several brain areas than LFP, whereas these differences had disappeared or were even reversed at 25 weeks of age. Line differences in central monoamine activity were found both in emotion-regulating and motor-regulating areas. As expected from previous generations, HFP exceeded LFP in most types of pecking at other birds, including severe feather pecking. Furthermore, HFP responded more actively in most behavioural tests conducted, and seem more impulsive or (hyper)active in their way of coping with challenges. This paper shows different developmental trajectories of the neurochemical systems (5-HT and DA) for chickens divergently selected on feather pecking behaviour, and a remarkable reversion of differences in monoamine activity at a later stage of life. Whether this is a cause or consequence of SFP needs further investigation.

The role of oxytocin in male and female reproductive behavior.

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Stress revisited: a critical evaluation of the stress concept.

With the steadily increasing number of publications in the field of stress research it has become evident that the conventional usage of the stress concept bears considerable problems. The use of the term 'stress' to conditions ranging from even the mildest challenging stimulation to severely aversive conditions, is in our view inappropriate. Review of the literature reveals that the physiological 'stress' response to appetitive, rewarding stimuli that are often not considered to be stressors can be as large as the response to negative stimuli. Analysis of the physiological response during exercise supports the view that the magnitude of the neuroendocrine response reflects the metabolic and physiological demands required for behavioural activity. We propose that the term 'stress' should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism, in particular situations that include unpredictability and uncontrollability. Physiologically, stress seems to be characterized by either the absence of an anticipatory response (unpredictable) or a reduced recovery (uncontrollable) of the neuroendocrine reaction. The consequences of this restricted definition for stress research and the interpretation of results in terms of the adaptive and/or maladaptive nature of the response are discussed.

The potential and limitations of DOV 216,303 as a triple reuptake inhibitor for the treatment of major depression: a microdialysis study in olfactory bulbectomized rats.

DOV 216,303 belongs to a new class of antidepressants, the triple reuptake inhibitors (TRIs), that blocks serotonin, norepinephrine and dopamine transporters and thereby increases extracellular brain monoamine concentrations. The aim of the present study was to measure extracellular monoamine concentrations both in the prefrontal cortex (PFC) and dorsal hippocampus (DH) after chronic administration of DOV 216,303 in the OBX animal model of depression and to compare the effects with acute drug treatment. OBX animals showed lower dopamine levels in PFC upon acute administration of DOV 216,303 than sham animals for up to five weeks after surgery. No such changes were observed in the DH. Unexpectedly, a DOV 216,303 challenge in chronic DOV 216,303 treated sham animals resulted in a blunted dopamine response in the PFC compared to the same challenge in vehicle treated animals. This blunted response probably reflects pharmacokinetic adaptations and/or pharmacodynamic changes, since brain and plasma concentrations of DOV 216,303 were significantly lower after chronic administration compared to acute administration. Surprisingly, and in contrast what we have reported earlier, chronic DOV 216,303 treatment was unable to normalize the hyperactivity of the OBX animals. Interestingly, by measuring the drug plasma and brain levels, it was demonstrated that at the time of behavioral testing (24 h after last drug treatment) DOV 216,303 was not present anymore in either plasma or brain. This seems to indicate that this putative antidepressant drug has no lasting antidepressant-like behavioral effects in the absence of the drug in the brain.

The effects of acute tryptophan depletion on affective behaviour and cognition in Brown Norway and Sprague Dawley rats.

Previous studies in rats and humans have shown that the essential amino acid tryptophan (TRP) is depleted after consumption of a gelatin-based protein-carbohydrate mixture, which is lacking L-tryptophan (TRP-). In rats, TRP depletion caused impaired object recognition but only had a modest effect on affective behaviour. Because these studies were preformed with Wistar rats, the aim of the present experiment was to evaluate strain differences in behavioural responses to acute TRP depletion between Brown Norway (BN) and Sprague Dawley (SD) rats. The rats were repeatedly treated with TRP- or a balanced control (TRP+) and were tested in tests of anxiety- and depression-related behaviour (open-field test, home cage emergence test, social interaction test, forced swim test) and memory. SD rats, but not BNs, showed more anxiety- and depression-related behaviour and impaired object recognition after TRP- treatment. There was a dissociation between plasma TRP levels, central 5-HT concentrations and 5-HIAA/5-HT turnover. Both strains showed about 60% decrease in plasma TRP/SigmaLNAA levels, whereas hippocampal 5-HT levels were lower after TRP- in BN but not SD rats. Conversely, 5-HIAA/5-HT turnover was lower after TRP- in SD but not BN rats, suggesting a dissociation between 5-HT storage and release in SDs. The present study suggests that acute tryptophan depletion effects are strain dependent on the behavioural and the neurochemical level.

Olfactory bulbectomy induces rapid and stable changes in basal and stress-induced locomotor activity, heart rate and body temperature responses in the home cage.

BACKGROUND: Olfactory bulbectomy (OBX) in rats causes several behavioral and neurochemical changes. However, the extent and onset of physiological and behavioral changes induced after bulbectomy have been little examined. METHODS: Male Sprague-Dawley rats received telemetric implants. Before and immediately after OBX surgery, basal and stress-induced heart rate, body temperature, and locomotor activity were measured in the home cage in sham (n=9) and OBX animals (n=11). Stress was induced using novel cage stress or witness stress. RESULTS: Bulbectomized animals differed physiologically and behaviorally from shams. Nocturnally, OBX animals were significantly more active compared with shams, had a higher core body temperature and displayed a decreased heart rate variability. During the light period, OBX animals had a significantly lower basal heart rate and a reduced heart rate variability. These effects became apparent 2-3 days after OBX surgery, and were stable over time. After witness stress, OBX animals showed smaller autonomic (body temperature and heart rate) responses compared with shams, but showed no difference in locomotor responses. In contrast, novel cage stress led to increased locomotor responses in OBX rats compared with sham rats, while no differences were found in autonomic responses. CONCLUSION: Removal of the olfactory bulbs results in rapid, stable and persistent changes in basal locomotor activity, body temperature, heart rate and heart rate variability. Although the sleep-wake cycle of these parameters is not altered, increases in circadian amplitude are apparent within 3 days after surgery. This indicates that physiological changes in the OBX rat are the immediate result of olfactory bulb removal. Further, stress responsivity in OBX rats depends on stressor intensity. Bulbectomized rats display smaller temperature and heart rate responses to less intense witness stress compared with sham rats. Increased locomotor responses to more intense novel cage stress are present in the home cage as well as the open field. The present study shows that olfactory bulbectomy has rapid and persistent influence on basal and stress-induced physiological parameters.

The benzodiazepine brotizolam reduces fear in calves exposed to a novel object test.

The present study examined the effects of the intravenous administration of the anxiolytic drug brotizolam on the behavioral and physiological responsiveness of calves to novelty in a dose response fashion. Holstein Friesian heifer calves (39-41 weeks of age; body weight 200-300 kg) received an intravenous injection of either a vehicle control (12 calves) or one of four doses of brotizolam (8 calves per dose): 0.0125, 0.05, 0.2 and 0.8 mg/100 kg body weight. They were then individually subjected to a 'combined' test involving exposure to a novel environment (open field, OF) for 5 min followed by the sudden introduction of a novel object (NO) that remained in place for a further 10 min. Behavioral, heart rate and plasma cortisol responses were recorded in all animals. Compared to vehicle treatment, the highest dose of brotizolam dose-dependently and significantly increased the time spent in locomotion and the distance travelled near the NO, as well as the time spent in contact with the NO. In addition, post-test plasma cortisol concentrations changed in a dose-dependent manner over time: they decreased between 0 and 10 min after the test in calves that had received the two highest doses of brotizolam, whereas they increased in vehicle-treated and low-dosage calves. There were no effects of brotizolam on vocalization or locomotion during the OF phase of the test or on vocalization following introduction of the NO. These findings strongly support the notion that interaction with a novel object in a novel arena represents a behavioral index of fear and fearfulness in calves, and that vocalization and locomotion in an OF reflect other independent characteristics.

SSR149415, a non-peptide vasopressin V1b receptor antagonist, has long-lasting antidepressant effects in the olfactory bulbectomy-induced hyperactivity depression model.

Olfactory bulbectomy (OBX) in rats causes several behavioral and neurochemical CNS changes, reminiscent of symptoms of human depression. Such depression-like behavior after OBX can be reversed with antidepressants. Recently, a connection between the vasopressin 1b (V1b) receptor and the development of depression has been suggested; therefore, a vasopressin V1b receptor antagonist (SSR149415) was investigated in the OBX model. Male rats received olfactory bulbectomy or sham surgery. After recovery, animals received 14 consecutive daily doses of SSR149415 (10 or 30 mg/kg), imipramine (20 mg/kg), or vehicle (5% hydroxy-propyl methylcellulose). Animals were tested in an open field after acute treatment, on days 7 and 14 of treatment and 1 week after cessation of treatment. Similar to imipramine, repeated, but not acute, administration of SSR149415 completely reversed OBX-induced hyperactivity, leaving activity in shams unaffected. This reversal of OBX-induced hyperactivity in the SSR149415 treated rats was still present 7 days after cessation of treatment. Although the behavioral effects of treatment with SSR149415 were specific for the OBX animals, adrenal gland weights were reduced in both sham and OBX animals treated with 30 mg/kg SSR149415. Chronic but not acute administration of SSR149415 normalizes OBX-induced hyperactivity up to 1 week after cessation of treatment, suggesting that a V1b receptor antagonist may have long-lasting antidepressant activity.

The triple monoaminergic reuptake inhibitor DOV 216,303 has antidepressant effects in the rat olfactory bulbectomy model and lacks sexual side effects.

Current antidepressants have a delayed onset of action and disturbing side effects, including inhibition of sexual behavior. It is hypothesized that novel drugs, hitting multiple disease-relevant targets, may yield a new generation of superior antidepressants. One such approach is simultaneous inhibition of serotonin, norepinephrine and dopamine transporters. We tested the triple uptake inhibitor (TUI), DOV 216,303 (5, 10 and 20 mg/kg) after 1, 7 and 14 days administration in the olfactory bulbectomized (OBX) rat depression model, and in a model of rat sexual behavior to detect putative sexual side effects. Chronic, but not acute treatment of DOV 216,303 (20 mg/kg) normalized OBX-induced hyperactivity in the open field, similar to the effect of imipramine (20 mg/kg). None of the doses of DOV 216,303 had any effect on sexual behavior at any time point. The results indicate that DOV 216,303 displays antidepressant efficacy and is devoid of sexual side effects.

Acute tryptophan depletion dose dependently impairs object memory in serotonin transporter knockout rats.

RATIONALE: Acute tryptophan depletion (ATD) transiently lowers central serotonin levels and can induce depressive mood states and cognitive defects. Previous studies have shown that ATD impairs object recognition in rats. OBJECTIVES: As individual differences exist in central serotonin neurotransmission, the impact of ATD may vary accordingly. In this experiment, we investigated the hypothesis that male serotonin transporter knockout (SERT(-/-)), rats marked by a lower SERT function, are more vulnerable to the effects of ATD in an object recognition task than male wildtype (SERT(+/+)) and heterozygous (SERT(+/-)) rats. MATERIALS AND METHODS: Twelve male SERT(+/+), SERT(+/-), and SERT(-/-) rats were treated with standard dose and low-dose ATD using a gelatine-based protein-carbohydrate mixture lacking tryptophan. In the control treatment, L: -tryptophan was added to the mixture. Four hours after treatment, the rats were subjected to the object recognition task. In addition, the effects of ATD on plasma amino acid concentrations were measured, and concentrations of 5-HT and 5-HIAA were measured in the frontal cortex and hippocampus of these rats. RESULTS: Plasma TRP levels and central 5-HT and 5-HIAA levels were decreased in all genotypes after ATD, but effects were stronger in SERT(-/-) rats. The standard dose of ATD impaired object recognition in all genotypes. SERT(-/-) and SERT(+/-) rats were more vulnerable to low dose of ATD in the object recognition task compared to SERT(+/+) rats. CONCLUSIONS: These results indicate a greater sensitivity to ATD in SERT(-/-) and SERT(+/-) rats, which may be related to stronger central depletion effects in these rats.

Repeated social defeat in female pigs does not induce neuroendocrine symptoms of depression, but behavioral adaptation.

The aim of this study was to develop an animal model of major depression. Since two thirds of depressive patients are women, it is important to develop specific female animal models of depression. We therefore determined the consequences of chronic social defeat in individually housed prepubertal female pigs confronted with a dominant, older pig. Repeated defeat increased the salivary cortisol level, measured immediately after the confrontations, but this effect diminished after repeated confrontations. Neither organ weights nor the number of glucocorticoid (GR) and mineralocorticoid (MR) receptors in the ventral hippocampus were affected by repeated defeat. Serotonin turnover in the dorsal hippocampus was also unaffected. Behavioral analysis revealed that across confrontations, the pigs reduced the time spent actively attacking the dominant pigs, whereas the time increased in which the pigs passively underwent aggression and/or actively avoided aggression. Therefore, we conclude that the repeated social defeat paradigm does not induce long-lasting depression-like neuroendocrine effects as a consequence of behavioral adaptations (changes in the fighting strategy) in the young female pigs.

Mapping quantitative trait loci affecting feather pecking behavior and stress response in laying hens.

In the European Union, legislation concerning animal housing is becoming stricter because of animal welfare concerns. Feather pecking (FP) in large group housing systems is a major problem. It has been suggested that corticosterone (CORT) response to manual restraint as a measure for stress is associated with FP behavior. The aim of the current study was to identify QTL involved in FP behavior and stress response in laying hens. An F2 population of 630 hens was established from a cross between two commercial lines of laying hens differing in their propensity to feather peck. The behavioral traits, measured at 6 and 30 wk of age, were gentle FP, severe FP, and aggressive pecking. Toe pecking was measured at 30 wk of age and CORT response to manual restraint was measured at 32 wk. All animals were genotyped for 180 microsatellite markers. A QTL analysis was performed using a regression interval mapping method. At 6 wk of age, a suggestive QTL on GGA10 was detected for gentle FP. At 30 wk of age, suggestive QTL were detected on GGA1 and GGA2 for gentle FP. A significant QTL was detected on GGA2 for severe FP. At 32 wk of age, a suggestive QTL was detected on GGA18 for CORT response to manual restraint. In addition, a suggestive QTL was detected on GGA5 with possible maternal parent-of-origin effect for CORT response.

Stress responses during milking; comparing conventional and automatic milking in primiparous dairy cows.

A comparative study was performed to evaluate the differences in behavioral and physiological stress responses during milking between cows that were milked by an automated milking system (AM-cows) and cows that were milked in a conventional tandem parlor (TM-cows). In a randomized design, 36 primiparous Holstein-Friesian dairy cows were observed and blood sampled (1-min intervals) individually during milking. AM-cows spent less time standing with their heads outside the feeding trough than TM-cows and had a lower heart rate. In addition, AM-cows had lower maximum plasma adrenaline and noradrenaline concentrations during milking. No differences were found in the number of steps. After tactile stimulation of the teats either by hand or by the cleaning brush, mean oxytocin concentrations did not differ. In AM-cows, however, elevated oxytocin levels were prolonged at the end of milking. Averaged over the first five blood samples, AM-cows tended to have higher plasma cortisol concentrations than TM-cows, but median fecal concentrations of the cortisol metabolite dioxoandrostane were comparable. Maximum quarter milk flow, maximum udder milk flow and residual milk as a percentage of the total milk volume was comparable. From this study it is concluded that behavioral and physiological responses, both in automatically and in conventionally milked cows, were relatively low and were typical for cows being milked. We therefore conclude that, as far as the welfare of the dairy cow during milking is concerned, automatic milking and conventional milking are equally acceptable.

Corticosteroids in relation to fear, anxiety and psychopathology.

Corticosteroids play extremely important roles in fear and anxiety. The mechanisms by which corticosteroids exert their effects on behavior are often indirect, because, although corticosteroids do not regulate behavior, they induce chemical changes in particular sets of neurons making certain behavioral outcomes more likely in certain contexts as a result of the strengthening or weakening of particular neural pathways. The timing of corticosteroid increase (before, during or after exposure to a stressor) determines whether and how behavior is affected. The present review shows that different aspects of fear and anxiety are affected differentially by the occupation of the mineralocorticoid receptor (MR) or glucocorticoid receptor (GR) at different phases of the stress response. Corticosteroids, at low circulating levels, exert a permissive action via brain MRs on the mediation of acute freezing behavior and acute fear-related plus-maze behavior. Corticosteroids, at high circulating levels, enhance acquisition, conditioning and consolidation of an inescapable stressful experience via GR-mechanisms. Brain GR-occupation also promotes processes underlying fear potentiation. Fear potentiation can be seen as an adjustment in anticipation of changing demands. However, such feed-forward regulation may be particularly vulnerable to dysfunction. MR and/or GR mechanisms are involved in fear extinction. Brain MRs may be involved in the extinction of passive avoidance, and GRs may be involved in mediating the extinction of active avoidance. In the developing brain, corticosteroids play a facilitatory role in the ontogeny of freezing behavior, probably via GRs in the dorsal hippocampus, and their influence on the development of the septo-hippocampal cholinergic system. Corticosteroids can exert maladaptive rather than adaptive effects when their actions via MRs and GRs are chronically unbalanced due to chronic stress. Both mental health of humans and animal welfare is likely to be seriously threatened after psychosocial stress, prolonged stress, prenatal stress or postnatal stress, especially when maternal care or social support is absent, because these can chronically dysregulate the central MR/GR balance. In such circumstances the normally adaptive corticosteroid responses can become maladaptive.

Farm animal welfare research in interaction with society.

Over the last 30 years concern about farm animal welfare has increased and has become a public issue in the Netherlands. Public discussion has stimulated research in this field, financed by both government and industry. Dutch society in general and consumers of animal products in particular, want to see high standards of welfare for production animals. Good animal welfare has gradually gained more impact in the total quality concept of the product. This will encourage scientists to continue to analyse the welfare status of animals and to come up with innovative solutions for the remaining problems. At ID-Lelystad much effort is put into farm animal welfare research. This research includes for example, the development of behavioural tests for quantifying and interpreting fear in cattle, investigations into the effects of dietary iron supply and a lack of roughage on behaviour, immunology, stress physiology, and pathology in veal calves, studies of the ontogeny of tail biting in finishing pigs and feather pecking in laying hens as well as evaluation of the welfare effects of automatic milking in dairy cows. The results of these projects contribute to concrete improvements in animal husbandry and expertise and support policy making and legislation. The animal industry as well as retailers should aim at the further implementation of this knowledge and to specify welfare standards to guarantee consumer acceptance of animal production.

The amount of free corticosterone is increased during lipopolysaccharide-induced fever.

The relation between lipopolysaccharide (LPS)-induced fever and bioavailability of corticosterone (B) was examined in male Wistar rats. Animals were injected with LPS (2.5 mg/kg i.p.) or saline and core temperature and heart rate were monitored continuously using a biotelemetry system. Blood samples were withdrawn from freely moving rats via jugular catheters for estimation of total and free plasma B. LPS induced a long-lasting increase (24-48 h) in core temperature and B secretion and a short-lasting increase (90 min) in heart rate. LPS-induced fever was accompanied by a significant increase in the free/total B ratio. In contrast, an acute injection of B, which resulted in circulating B levels similar to those found after LPS, did not affect the free/total B ratio. The important role of LPS-induced fever in the hormone secretion pattern and the equilibrium between free and total B was further demonstrated in an in vitro study showing that an increase in the temperature by 3 degrees C elevated the free B fraction and the free/total B ratio of plasma samples with concentrations of B in the physiological range (5-40 microg/dl). Taken together, these findings indicate that during LPS-induced fever there is an increase in the amount of biologically available B. Exposure of glucocorticoid-sensitive targets to elevated levels of free B could contribute to the restoration of homeostasis that is disturbed during inflammation.

Effects of environmental enrichment on behavioral responses to novelty, learning, and memory, and the circadian rhythm in cortisol in growing pigs.

Previously we showed that pigs reared in an enriched environment had higher baseline salivary cortisol concentrations during the light period than pigs reared under barren conditions. In the present experiment, it was investigated whether these higher baseline salivary cortisol concentrations were a real difference in cortisol concentration or merely represented a phase difference in circadian rhythm. The effects of different cortisol concentrations on the behavioral responses to novelty and learning and long-term memory in a maze test were also studied in enriched and barren housed pigs. At 9 weeks of age enriched and barren housed pigs did not differ in baseline salivary cortisol concentrations nor in circadian rhythm, but at 22 weeks of age barren housed pigs had a blunted circadian rhythm in salivary cortisol as compared to enriched housed pigs. The differences in baseline salivary cortisol concentrations between enriched- and barren-housed pigs are age-dependent, and become visible after 15 weeks of age. Enriched- and barren-housed piglets did not differ in time spent on exploration in the novel environment test. Barren-housed pigs had an impaired long-term memory in the maze test compared to enriched-housed pigs; however, no differences in learning abilities between enriched- and barren-housed pigs were found. Because blunted circadian cortisol rhythms are often recorded during states of chronic stress in pigs and rats or during depression in humans, it is suggested that the blunted circadian rhythm in cortisol in barren-housed pigs similarily may reflect decreased welfare.

Coping styles in animals: current status in behavior and stress-physiology.

This paper summarizes the current views on coping styles as a useful concept in understanding individual adaptive capacity and vulnerability to stress-related disease. Studies in feral populations indicate the existence of a proactive and a reactive coping style. These coping styles seem to play a role in the population ecology of the species. Despite domestication, genetic selection and inbreeding, the same coping styles can, to some extent, also be observed in laboratory and farm animals. Coping styles are characterized by consistent behavioral and neuroendocrine characteristics, some of which seem to be causally linked to each other. Evidence is accumulating that the two coping styles might explain a differential vulnerability to stress mediated disease due to the differential adaptive value of the two coping styles and the accompanying neuroendocrine differentiation.

Y chromosomal and sex effects on the behavioral stress response in the defensive burying test in wild house mice.

Genetically selected short attack latency (SAL) and long attack latency (LAL) male wild house mice behave differently in the defensive burying test. When challenged, SAL males respond actively with more time spent on defensive burying, whereas LAL males are more passive with more time remaining immobile. The first aim of this study was to find out whether the nonpairing part of the Y chromosome (Y(NPAR)) affects the behavioral stress response in this paradigm. Second, to determine if the differential behavioral profile found in males is also present in females, SAL and LAL females were tested. Third, nonattacking and attacking LAL males were compared. Five behavioral elements were recorded: defensive burying, immobility, rearing, grooming, and exploration. Males were first tested for attack latency. The results show that the Y(NPAR) influences defensive burying. However, the size of this effect is overshadowed by the background of the mice. Furthermore, although females differed from males, they tended to demonstrate the same behavioral profile as males. Nongenetic factors may also play a role, as attacking LAL males showed more defensive burying than nonattacking LAL males.

High carbon dioxide tension (PCO2) and the incidence of cardiac arrhythmias in rapidly growing broiler chickens.

The purpose of this investigation was to determine whether two-week-old rapidly growing broiler chickens with high metabolic activity have an increased risk of the development of heart failure three to five weeks later. The incidence of cardiac arrhythmias was assessed in broiler chickens with either a relatively high carbon dioxide tension (PCO2) or a low PCO2 in their venous blood. Their electrocardiograms (ECGS) were measured when the birds were between five and seven weeks old by means of a biotransplant which allowed them to move freely. Premature ventricular beats were observed in all the birds, but the largest numbers were observed in birds that had had a high PCO2 when they were two weeks old.