Serotonin release in the caudal nidopallium of adult laying hens genetically selected for high and low feather pecking behavior: an in vivo microdialysis study.

Severe feather pecking (FP) is a detrimental behavior causing welfare problems in laying hens. Divergent genetic selection for FP in White Leghorns resulted in strong differences in FP incidences between lines. More recently, it was shown that the high FP (HFP) birds have increased locomotor activity as compared to hens of the low FP (LFP) line, but whether these lines differ in central serotonin (5-hydroxytryptamine, 5-HT) release is unknown. We compared baseline release levels of central 5-HT, and the metabolite 5-HIAA in the limbic and prefrontal subcomponents of the caudal nidopallium by in vivo microdialysis in adult HFP and LFP laying hens from the ninth generation of selection. A single subcutaneous d-fenfluramine injection (0.5 mg/kg) was given to release neuronal serotonin in order to investigate presynaptic storage capacity. The present study shows that HFP hens had higher baseline levels of 5-HT in the caudal nidopallium as compared to LFP laying hens. Remarkably, no differences in plasma tryptophan levels (precursor of 5-HT) between the lines were observed. d-fenfluramine increased 5-HT levels in both lines similarly indirectly suggesting that presynaptic storage capacity was the same. The present study shows that HFP hens release more 5-HT under baseline conditions in the caudal nidopallium as compared to the LFP birds. This suggests that HFP hens are characterized by a higher tonic 5-HT release.

Selection for low mortality in laying hens affects catecholamine levels in the arcopallium, a brain area involved in fear and motor regulation.

Feather pecking (FP) in laying hens may cause mortality due to cannibalism. Novel breeding methods using survival days of group-housed siblings allow for the genetic selection of laying hens with low mortality (LML: low mortality line) due to cannibalism. Previous studies have demonstrated less fear-related behavior and also less FP in LML hens compared to CL. Selection also caused changes in locomotor behavior in an open field. It is unknown, however, whether selection for low mortality affects central neurotransmitter levels. In this study, brain monoamine levels were measured in the dorsal thalamus, medial striatum, hippocampus and arcopallium of adult laying hens of both LML and CL using HPLC. Brain samples were collected after 5-min of manual restraint. The most prominent line differences were found in the arcopallium. Compared to CL, LML had lower levels of noradrenaline (NA) and 3,4-dihydroxyphenylacetic acid (DOPAC) and tended to have lower levels of dopamine (DA), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). Levels of serotonin (5-HT), 5-HT- and DA-turnover in this brain area were not affected by line. LML showed less fear-related behavior during the restraint than CL. These findings show that selection for low mortality in hens leads to changes of predominantly the dopaminergic system in the chicken's arcopallium, a forebrain somatomotor area also related to fear. This suggests a relationship between catecholamine functioning in this brain area and FP and cannibalistic behavior in chickens and underpins previously found relationships between FP, fear and high activity.

Effects of feather pecking phenotype (severe feather peckers, victims and non-peckers) on serotonergic and dopaminergic activity in four brain areas of laying hens (Gallus gallus domesticus).

Severe feather pecking (SFP) in laying hens is a detrimental behavior causing loss of feathers, skin damage and cannibalism. Previously, we have associated changes in frontal brain serotonin (5-HT) turnover and dopamine (DA) turnover with alterations in feather pecking behavior in young pullets (28-60 days). Here, brain monoamine levels were measured in adult laying hens; focusing on four brain areas that are involved in emotional behavior or are part of the basal ganglia-thalamopallial circuit, which is involved in obsessive compulsive disorders. Three behavioral phenotypes were studied: Severe Feather Peckers (SFPs), Victims of SFP, and Non-Peckers (NPs). Hens (33 weeks old) were sacrificed after a 5-min manual restraint test. SFPs had higher 5-HIAA levels and a higher serotonin turnover (5-HIAA/5-HT) in the dorsal thalamus than NPs, with intermediate levels in victims. NPs had higher 5-HT levels in the medial striatum than victims, with levels of SFPs in between. 5-HT turnover levels did not differ between phenotypes in medial striatum, arcopallium and hippocampus. DA turnover levels were not affected by feather pecking phenotype. These findings indicate that serotonergic neurotransmission in the dorsal thalamus and striatum of adult laying hens depends on differences in behavioral feather pecking phenotype, with, compared to non-pecking hens, changes in both SFP and their victims. Further identification of different SFP phenotypes is needed to elucidate the role of brain monoamines in SFP.

The relation between fearfulness in young and stress-response in adult laying hens, on individual and group level.

Fearfulness of an individual can affect its sensitivity to stress, while at the same time the social situation in which an animal lives can affect its fear level. It is however unknown what the long-term effects of high fearfulness on sensitivity to stress are, on individual or group level in laying hens. We hypothesize that increased fearfulness at a young age results in increased sensitivity to stress at an adult age, and that this relation can differ between groups, due to differences in group composition. Therefore, we studied the relation between fearfulness in an Open Field (OF) test at six weeks of age and plasma-corticosterone (CORT) levels after a 5-min Manual Restraint test (MR) at 33 weeks of age, and assessed behavior in the home pen. We used birds from a low mortality line, selected for four generations on low mortality due to feather pecking and cannibalism and a control line (n=153 in total, eight pens/line). These lines are known to differ in fearfulness and stress physiology. Chicks from the low mortality line were more active in the OF compared to chicks from the control line. Chicks that showed a fearful response (no walking, no vocalizing) in the OF test had higher CORT at 33 weeks of age than chicks that walked and/or vocalized in the OF test and had higher activity in the home pen as adults. On group level, a passive response in the OF was related to high CORT levels after MR. Presence of at least one fearful bird in a group led to higher CORT in the other group mates compared to birds from groups with no fearful birds present. Birds from groups in which more than 50% of birds had severe comb lesions had higher CORT levels compared to birds from groups with less than 50% of birds affected. High fearfulness of laying hen chicks can on individual level have a long-term effect on stress sensitivity. The presence of fearful birds in a group as well as signs of social instability in a group, indicated by comb lesions, can affect sensitivity to stress of birds from the same group. The mechanism by which this occurs can lie in social transmission of (fear related) behavior, but this suggestion needs further investigation.