Abstract
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.
Highlights
Neuronal serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in the central nervous system and has been shown to be a major mediator of several physiological and neurobehavioral processes, including locomotion (Holmes et al, 2003; Uçeyler et al, 2010), feeding (Lam et al, 2010), emotion (Jonassen and Landrø, 2014), impulsivity (Landrø et al, 2015; Nomura et al, 2015) and fear-related behavior (Gross and Hen, 2004; Berger et al, 2009)
Because all the altered behavioral behaviors have been found in adult animals, we further examined the role of the 5-HTT in fear-like behavior in newly hatched birds and in young birds in the present study
Our results indicate that chicks with the 5-HTT polymorphism at early post-hatch ages and at 5 weeks of age showed significantly different levels of fear and arousal
Summary
Neuronal serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in the central nervous system and has been shown to be a major mediator of several physiological and neurobehavioral processes, including locomotion (Holmes et al, 2003; Uçeyler et al, 2010), feeding (Lam et al, 2010), emotion (Jonassen and Landrø, 2014), impulsivity (Landrø et al, 2015; Nomura et al, 2015) and fear-related behavior (Gross and Hen, 2004; Berger et al, 2009). It was shown that human lymphoblast cells carrying two copies of the s allele had reduced 5-HTT gene expression, leading to a decrease in 5-HT uptake when compared to cells with one or two copies of the l allele (Lesch et al, 1996). Individuals carrying two copies of the s allele showed significantly lower 5-HTT expression in the brain than individuals with l/l and s/l genotypes, which are clinically linked with depression and varying response to alcohol abuse (Little et al, 1998; Heinz et al, 2000; Pezawas et al, 2005). Behavioral studies following lesions damaging the arcopallium indicated that this brain region is involved in the control of fear-related behaviors (Phillips, 1964; Cohen, 1975; Dafters, 1975; Phillips and Youngren, 1986; Saint-Dizier et al, 2009)
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