Evidence for the Association of a Deleted Variant in the 5'-Flanking Region of the Chicken serotonin transporter (5-HTT) Gene with a Temporary Increase in Feed Intake and Growth Rate

Molecular biology of serotonergic systems in avian brains

Serotonin (5-hydroxytryptamine, 5-HT) is a phylogenetically conserved neurotransmitter and modulator. Neurons utilizing serotonin have been identified in the central nervous systems of all vertebrates. In the central serotonergic system of vertebrate species examined so far, serotonergic neurons have been confirmed to exist in clusters in the brainstem. Although many serotonin-regulated cognitive, behavioral, and emotional functions have been elucidated in mammals, equivalents remain poorly understood in non-mammalian vertebrates. The purpose of this review is to summarize current knowledge of the anatomical organization and molecular features of the avian central serotonergic system. In addition, selected key functions of serotonin are briefly reviewed. Gene association studies between serotonergic system related genes and behaviors in birds have elucidated that the serotonergic system is involved in the regulation of behavior in birds similar to that observed in mammals. The widespread distribution of serotonergic modulation in the central nervous system and the evolutionary conservation of the serotonergic system provide a strong foundation for understanding and comparing the evolutionary continuity of neural circuits controlling corresponding brain functions within vertebrates. The main focus of this review is the chicken brain, with this type of poultry used as a model bird. The chicken is widely used not only as a model for answering questions in developmental biology and as a model for agriculturally useful breeding, but also in research relating to cognitive, behavioral, and emotional processes. In addition to a wealth of prior research on the projection relationships of avian brain regions, detailed subdivision similarities between avian and mammalian brains have recently been identified. Therefore, identifying the neural circuits modulated by the serotonergic system in avian brains may provide an interesting opportunity for detailed comparative studies of the function of serotonergic systems in mammals.

Brain gain-Is the cognitive performance of domestic hens affected by a functional polymorphism in the serotonin transporter gene?

The serotonin transporter (5-HTT) plays an important role in regulating serotonergic transmission via removal of serotonin (5-HT) from synaptic clefts. Alterations in 5-HTT expression and subsequent 5-HT transmission have been found to be associated with changes in behaviour, such as fearfulness or activity, in humans and other vertebrates. In humans, alterations in 5-HTT expression have been suggested to be able to lead to better learning performance, with more fearful persons being better at learning. Similar effects of the variation in the 5-HTT on fearfulness have been found in chickens, and in this study, we investigated effects on learning. Therefore, we tested 52 adult laying hens, differing in their functional 5-HTT genotype (W/W, W/D and D/D) in an operant learning paradigm in three different phases (initial learning, reversal learning and extinction) and in a tonic immobility test for fearfulness. We found that the 5-HTT polymorphism affects the initial learning performance of laying hens, with homogeneous wild-type (W/W) hens being the slowest learners, and the most fearful birds. W/W hens, showed significantly more choices to solve the initial learning task (LME, p = 0.031) and had the highest latencies in a tonic immobility test (p = 0.039), indicating the highest fearfulness. Our results provide interesting first insights into the role of 5-HTT in chickens and its sensitive interaction with the environment. We further suggest that the 5-HTT gene can be an interesting target gene for future breeding strategies as well as for further experimental studies.

Behavioural consequences of divergent selection on general locomotor activity in chickens

General locomotor activity is a highly variable phenotypic trait of animals. In domestic chickens it is different within and between breeds. The general locomotor activity is a substantially heritable trait and has been shown to be correlated with several other behavioural traits, such as for example feather pecking and anxiety in chickens. However, whether there is a relation between different levels of general locomotor activity and behavioural changes remained unclear. Therefore, a selection line model system has been established, where hens from the same founder population were selected over eight generations for either high or low general locomotor activity. The selection led to significant increases, respectively decreases in general locomotor activity and differences in growth. We here tested 128 hens of the 8^th generation in three behavioural tests. We assumed fearfulness to be affected from selection on general locomotor activity, which we tested in a tonic immobility test. Socio-positive and socio-negative behaviours were tested in respective test paradigms. Fearfulness was higher in hens selected for high general locomotor activity. Social behavioural traits and feather pecking were not affected by selection for general locomotor activity. Evolutionary mechanisms that link fear and general locomotor activity are discussed and also why social behaviours and feather pecking seems not to be affected from selection on general locomotor activity. Our results provide interesting new insights on how selection on one trait, general locomotor activity, affects the behavioural phenotype in other dimensions too.

Modulation of Fear and Arousal Behavior by Serotonin Transporter (5-HTT) Genotypes in Newly Hatched Chickens

The serotonin transporter (5-HTT) plays a key role in regulating serotonergic transmission via removal of serotonin (5-hydroxytryptamine, 5-HT) from synaptic clefts. Alterations in 5-HTT expression and 5-HT transmission have been shown to cause changes to adult behavior including fear. The objective of the present study was to investigate the 5-HTT role in fear in birds at the very early stages of post-hatching life. Using an avoidance test with an elevated balance beam, which was based on depth perception and the respective fear of heights, we assessed fear-related avoidance behaviors of newly hatched chicks of the three functional 5-HTT genotypes W/W, W/D and D/D. Newly hatched chicks of the genotype D/D, which was linked to high 5-HTT expression, showed less intensive avoidance responses as measured by decreased latency to jump than W/W and W/D chicks. Further, significantly fewer D/D hens than W/W hens showed fear-like behavior that resembled a freezing response. Furthermore, in an arousal test the arousal reaction of the chicks in response to an acute short-term visual social deprivation in the home compartment was assessed 5 weeks after hatching, which also revealed that D/D chicks exhibited decreased arousal reaction, compared to W/W chicks. Thus, the results indicate that fear responses differ in D/D chicks in the early post-hatching periods, possibly due to the different expression of 5-HTT respectively 5-HT levels in this strain.

A polymorphism in the 5'-flanking region of the serotonin transporter (5-HTT) gene affects fear-related behaviors of adult domestic chickens

The neural serotonin (5-HT)/serotonin transporter (5-HTT) system is involved in the regulation of physiological processes and emotional states. In humans, the short (S) allele in the 5-HTT gene-linked polymorphic region, which decreases 5-HTT expression, has been shown to be associated with behavioral changes including an increased level of anxiety. Also in birds a polymorphism in the 5-HTT gene is described, a deletion (D) has been found to have functional consequences on growth and locomotion. Furthermore, the D-allele leads to an increased 5-HTT expression compared to the wild type (W), a feature which is linked to lower levels of fear in mammalian species. Thus, we aimed here to test whether the polymorphism in the chicken 5-HTT gene also leads to respective alternations of fear-related behaviors. We tested 268 hens of three genotypes (W/W, W/D, D/D) in two behavioral paradigms (open field, light-dark test) to assess fear-related behavior. Both tests revealed that hens possessing the D-allele showed lower levels of fear than those having the W-allele. These similar outcomes in fear-related behaviors in an avian and a mammalian species are associated with an increased 5-HTT expression. In the human 5-HTT gene, the long (L) allele is linked to such increased expression, whereas in chickens it is the D-allele. Thus, increased 5-HTT expression causing decreased fear may be a general mechanism in vertebrates.

Enhanced Expression of Serotonin Receptor 5-Hydroxytryptamine 2C is Associated with Increased Feather Damage in Dongxiang Blue-Shelled Layers

The gene encoding the serotonin receptor 5-hydroxytraptamine 2C (HTR2C) has been implicated in behavioral phenotypes in a number of species. In previous studies, a mutation in the chicken HTR2C gene was found to be associated with feather condition, thereby suggesting a relationship between the gene and receiving feather pecking activity. The present study analyzed the chicken HTR2C gene at both the genomic make-up and expression level in Dongxiang blue-shelled layer. A significant association between the single nucleotide polymorphism (SNP) rs13640917 (C/T) and feather condition was confirmed in the Chinese local layer. Enhanced HTR2C gene expression (151.1-fold) that was associated with high feather damage indicated that the right cerebrum might be the critical region for HTR2C to participate in the regulation of receiving feather pecking behavior.