Red LED light promotes hair follicle development to improve fibre quality in Su line Angora rabbit

Background: Light has crucial roles in animal physiological activities. This study aimed to investigate the effects of different colours of light-emitting diodes (LEDs) on rabbit fibre quality and hair follicle development. 50 three-month-old Su line Angora rabbits were randomly assigned to five groups. Treatment groups were exposed to same intensities of red, green and blue LED light under 16 h light:8 h dark photoperiod regimes. Control groups were exposed to white light and black. The trial spanned 73 days. Results: Results showed that LED colours exerted different effects on wool yield, fibre quality, hormones and hair follicle development. The wool yield of red group was higher than that of white, green and black groups (P<0.05). The shoulder fibre length of red group was higher than that of white and green groups (P<0.05). The coarse fibre diameter of white group was lower than that of green and black groups (P<0.05). The fibre diameter of red group was the lowest and was lower by 13.9% than that of control group (P>0.05). The coarse fibre ratio of green group was higher (13.31%) than that of red group (3.81%, P<0.05). The follicle groups of white, green and black groups consisted of 1 primary follicle associated with 3 or 4 secondary follicle groups and those of blue group consisted of 1 primary follicle associated with 5–10 secondary follicle groups. The follicle of red group consisted of numerous secondary follicles and a few primary follicles. In same magnification, the numbers of follicle groups of white, red, green, blue and black groups were 14.0, 16.5, 10.0, 11.67 and 11.0, respectively. The numbers of follicle groups of red and green groups significantly differed (P<0.05). Serum melatonin (MT) of red group was highest than that of white and green groups (P<0.01), higher than that of black group (P<0.05),

serumTriiodothyronine (T 3 ) of red group was higher than that of white and black groups (P<0.05). Conclusions: Thus, the data reveal that red LED light can improve fibre quality, this may be due to red LED light which can enhance the secretion of melatonin to promote hair follicle development .

Background
Light, an environmental factor, markedly affects animal biological processes, such as seasonal reproductive cycles [1], pelage growth [2], spring moulting [3], appetite and weight changes [4] and horn growth [5]. Light plays an important role in animal production, and artificial lighting is often applied in livestock farms. Numerous studies on the effect of light on rabbits have been performed. Virag et al. [6] observed that milk production by rabbit does and kit litter weight under 1. The rabbit is a light-sensitive species. The feed intake of rabbit does is reduced under 12 h of daily intermittent lighting with 40 lux light intensity [6]. The European Food Safety Authority [10] stated that the light intensity of 50 lux is necessary for the visual illumination of conspecifics, investigation of surroundings and stimulation of physical activity of rabbits. Light also regulates pelage growth [2]. However, few works have focused on the regulation of pelage growth by light, and most related works have focused on the use of low-level laser therapy (LLLT) to treat hair loss in humans. LLLT stimulates hair growth in individuals with male-pattern hair loss, female-pattern hair loss and alopecia areata with good outcomes and minimal side effects [11][12][13]. Fushimi et al. [14] reported that red LED light stimulates hair growth in mice and induces several potential mediators to stimulate hair growth from human dermal papilla cells. Fibre growth is also influenced by the characteristics of light (colour, length and intensity).
The present study aimed to determine the effects of different LED colours applied with the same photoperiod and intensity on the fibre quality and hair follicle development of Su line Angora rabbits. The results of this work provide a theoretical foundation for the use of LED light to improve wool production.

Animals, Diets and Feeding Procedures
Su line Angora rabbit is a new breed of rabbit from Jiangsu Academy of Agricultural Sciences and is raised as species resource in Liuhe animal science base. The owners of the Su line Angora rabbits permitted experimentation on their animals. Threemonth-old Su line Angora rabbit wools were simultaneously sheared before the trial.
A total of 50 rabbits (BW 2.245 ± 0.296 kg, similar wool yield) were housed in individual cages (66 cm × 44 cm × 52 cm) and provided with pellet feed ad libitum.
The ingredients and chemical composition of the pellet feed are listed in Table 1.
The basal diet was formulated in accordance with the recommended nutritional requirements for rabbits [15]. The feed was provided twice daily (08:30 and 16:00) in two equal portions. Animals were given free access to tap water throughout the experimental period. The body weight and feed intake of the rabbits were recorded before morning feeding every 14 days. After the trial, all rabbits continued to be raised as genetic resource in Liuhe animal science base.

Experimental design
The rabbits were randomly divided into five groups. Each group comprised 10 rabbits (5 males and 5 females), one cage of a rabbit is a replication. Rabbits in the experimental groups were exposed to red, green or blue LED lights (red, green, blue treatment groups) under a 16L: 8D photoperiod regime. Rabbits in the control groups were exposed to white light and black (control) under 16L: 8D and 0L:24D photoperiod regime. The trial spanned 73 days. An LED lamp band was obtained from the NVC Lighting Holding Limited and suspended at a distance of 50 cm from the rabbits. The light intensity was 40-50 lux.

Fibre sample and quality analysis
At the end of the trial, fibre samples were taken from the shoulders, backs and abdomens of the rabbits. The sampling areas had dimensions of approximately 2 cm × 2 cm. Fibre length was estimated using a scaled ruler. A visual subjective test was conducted to identify fine and coarse fibres, and the dry weights of the fibres were determined to calculate coarse fibre ratio. Fibre diameter was measured with a random sample of 200 fibres and a projection microscope [16]. Moisture content was measured using an oven method [17].

Blood samples
At the end of the trial, blood samples were collected into test tubes through the ear vein at 9:00 in morning. Samples were collected before the morning feeding and watering. To isolate the serum, test tubes were first placed in a slanted position for 45 min at 4 °C and subsequently centrifuged for 15 min at 3,000×g. The serum was removed, and serum samples were frozen at −20 °C in 2 ml polyethylene tubes until analysis, which were used to test Melatonin (MT), Prolactin (PRL), Triiodothyronine (T 3 ), Thyroxine (T 4 ) and Growth hormone (GH).

Skin samples, staining and follicle analysis
At the end of the trial, skin samples were collected from the shoulders of the test rabbits. A pair of forceps was used to stretch a section of skin upwards. The

Statistical Analysis
Data were analysed as a completely randomised block design through one-way ANOVA with SPSS 17.0 program. All cages of the Su line Angora rabbits served as the experimental units. Differences among means were tested using Duncan's multiple range tests. Effects were considered significant at P<0.05.

Effects of different LED colours on growth performance and fibre quality
The effects of LED colours on the growth performance and fibre quality of the Su line Angora rabbits are illustrated in Tables 2-5. Performance was influenced by different LED colours. Specifically, the wool yield of the red group was higher than that of the white, green and black groups (P<0.05). Final weight, average daily gain and feed intake were not influenced by LED colour (P>0.05). The shoulder fibre length of the red group was longer than that of the control and green groups (P<0.05). Back fibre length exhibited the same tendency as shoulder fibre length (P>0.05). Abdominal fibre length was unaffected (P>0.05). Fine and coarse fibre diameters were measured. Coarse fibre diameter was significantly influenced by different LED colours and was lower in the white group than in the green and black groups (P<0.05). However, fine fibre diameter was unaffected, and the fine fibre diameter of the red group was small and was lower by 13.9% than that of the white group (P>0.05). Coarse fibre ratio and moisture content are shown in Table 4. The coarse fibre ratio of the green group (13.31%) was higher than that of the red group

Effects of different LED colours on serum hormones
The effects of different LED colours on hair follicle development are shown in Table   6. The serum MT, PRL and T 3 were influenced by LED light. The serum MT of the red group was highest than that of the white and green groups (P<0.01), higher than that of black group (P<0.05). The serum PRL of the black group was lower than that of the white and green groups (P<0.05). The serumT 3 of the red group was higher than that of the white and black groups (P<0.05)

Effects of different LED colours on hair follicle development
The effects of different LED colours on hair follicle development are presented in

Discussion
Wool yield and fibre quality are important indicators of animal fur production. Fibre quantity and quality are influenced by numerous factors, including age, heredity, environment and nutrition. Considerable research has been conducted on the effects of heredity and nutrition on animal fur production. Different protein levels or supplements can influence the mohair production performance and fibre characteristics of Angora goats [16,18]. Zhang et al. [19] reported that the addition of 20 mg Cu/kg DM to basal diets (containing 5.60 mg Cu/kg DM) enhances the growth performance of cashmere goats. However, minimal research has been conducted on the effect of environmental factors, particularly lighting, on fur production. The present study aimed to investigate the effects of different LED colours on fibre quality. Results showed that wool yield and fibre characteristics are influenced by different colours of LED lighting applied with 16L: 8D photoperiod regimes. Red LED light can increase wool yield and fibre length and decrease fine fibre diameter and coarse fibre ratio. Sheen et al. [20] found that in mice, anagen entry is faster under red light than under green and blue lights. Han et al. [21] reported that in an in vitro culture model, 655 nm red light + LED promotes human hair growth. Some reports have shown that fibre growth is regulated by melatonin.
In this trial, we also tested the serum hormones including melatonin and found Red LED light could enhance the concentration of serum melatonin and T 3 . In animals, light with a certain level of intensity sends a signal to the pineal gland to initiate or terminate melatonin synthesis and secretion, and melatonin transmits a signal with a circannual rhythm to regulate seasonal reproduction and other biological processes, such as hibernation, migration and pelage changes [22]. Numerous studies have been conducted on melatonin. Coelho et al. [23] reported that in the southern hemisphere, hair and wool ewe lambs exhibit the same annual pattern of plasma melatonin concentration under natural photoperiods at low latitudes.
Exogenous melatonin treatment during spring positively affects the medium-and long-term indices of the wool quality of Rasa Aragonesa ewes [24]. Moreover, exogenous melatonin can improve wool production and fibre quality [25]. Duan et al. [26] found that melatonin implantation (2 mg/kg BW) on two occasions (late April and June) increases cashmere yield by inducing cashmere fleece growth and decreases fibre diameter without changing dam growth rate or reproductive performance. Yang et al. [27] reveal that melatonin serves to promote secondary hair follicle development in early postnatal cashmere goats and expands our understanding of melatonin application in cashmere production. Cong et al. [28] stated that melatonin implantation during the winter solstice can effectively extend the cashmere growth phase of Liaoning cashmere goats. Melatonin treatment led to an increase in both the quantity and quality of cashmere fibre. Therefore, the improvement in fibre quality through light treatment may be related to melatonin.
In the present study, the effects of red LED light are better than those of natural light or other LED light colours. Therefore, red LED light plays a key role in the fibre growth of Angora rabbits.

Availability of data and material
All data generated or analysed during this study are included in this manuscript.

Consent for publication
Not applicable.

Competing interests
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted work. The authors alone are responsible for the content and writing of this article.

Funding
This work was supported by a grant from the National Rabbit Industry Technology System, Nanjing Comprehensive Experimental Station (CARS-43-G-2).