Plasma corticosterone concentrations were <2 ng/ml in 21 of 24 birds at -1.5 min (see Fig. 6.4 for individual corticosterone responses). There were no changes in plasma corticosterone in control birds treated with saline, and corticosterone remained constant between -1.5 and 60 min in birds treated with 2 mg/kg of LPS. Corticosterone
increased between 60 and 120 min in four of the six birds treated with 0.5 mg/kg LPS, and in five of the six birds treated with either 1.0 or 2.0 mg/kg LPS. Corticosterone then decreased or remained relatively constant from 120 to 240 min in 16 of 18 birds treated with LPS.
Figure 6.4. Individual plasma corticosterone responses in control quail and in quail treated with 0.5, 1.0 or 2.0 mg/kg of lipopolysaccharide (LPS). Initial blood samples were collected 1.5 min before injections were given at 0 min. N = 6 per group.
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 60 120 240 0 5 10 15 20 Time (min) C o rt ic os te ro ne ( n g /m l) Control 0.5 mg 1.0 mg 2.0 mg
Mean corticosterone concentrations did not differ between groups immediately before injection with saline or LPS (Kruskal-Wallis one way ANOVA K3 = 3.259, p = 0.353; Fig. 6.5, see Appendix Table 6.3 for further statistics), but differed between groups at all other times (K3 = 9.465, p = 0.024; K3 = 14.940, p = 0.002; K3 = 13.200, p = 0.004). Mean corticosterone was greater than in the control group at 120 and 240 min in birds treated with 0.5 mg/kg of LPS (U = 2.000, p = 0.010; U = 0.000, p = 0.004), and at all times in birds treated with either 1.0 (U = 3.000, p = 0.016; U = 0.000, p = 0.004; U = 0.000, p = 0.004) or 2.0 mg/kg of LPS (U = 3.500, p = 0.019; U = 1.000, p = 0.006; U = 0.000, p = 0.004).
Mean corticosterone concentrations did not change with time in the control group (Friedman’s one way repeated measures ANOVA F3 = 0.650, p = 0.885, Fig. 6.5, see Appendix Table 6.4 for further statistics), but differed significantly between times in groups treated with LPS (F3 = 12.350, p = 0.006; F3 = 11.800, p = 0.008; F3 = 11.000, p = 0.012). Corticosterone in birds treated with 0.5 mg/kg of LPS did not change between -1.5 and 60 min, increased to a maximum of 4.46 ± 1.01 ng/ml at 120 min, and
remained relatively constant between 120 and 240 min (-1.5 vs 60 min, F3 = -0.405, p = 0.686; 60 vs 120 min, F3 = 2.201, p = 0.028; 120 vs 240 min, F3 = -0.524, p = 0.600). Corticosterone in birds treated with 1.0 mg/kg of LPS increased from -1.5 to 60 min, reached a maximum of 9.01 ± 1.34 ng/ml at 120 min, then decreased from 120 to 240 min (-1.5 vs 60 min, F3 = 2.201, p = 0.028; 60 vs 120 min, F3 = 0.943, p = 0.345; 120 vs 240 min, F3 = -1.992, p = 0.046). Corticosterone in birds treated with 2.0 mg/kg of LPS remained relatively constant from -1.5 to 60 min, increased to a maximum of 6.66 ± 1.65 ng/ml at 120 min, then decreased slightly but not significantly from 120 to 240 min (-1.5 vs 60 min, F3 = 1.782, p = 0.075; 60 vs 120 min, F3 = 1.992, p = 0.046; 120 vs 240 min, F3 = -1.572, p = 0.116).
Figure 6.5. Plasma corticosterone responses in control quail and in quail treated with lipopolysaccharide (LPS). Control, —□—; 0.5 mg/kg LPS, ۔۔۔■۔۔۔; 1.0 mg/kg LPS, ···○···; 2.0 mg/kg LPS, —∆—. Initial blood samples were collected 1.5 min before injections were given at 0 min. Data are presented as mean ± S.E.; N = 6 per group.
0 60 120 240 0 2 4 6 8 10
Time (min)
C
o
rt
ic
os
te
rone
(
n
g/
m
l)
6.4 Discussion
Treatment of Japanese quail with 4 IU/kg of insulin increased plasma corticosterone concentrations in three of the six birds, and treatment with 0.5, 1.0 or 2.0 mg/kg of LPS elevated corticosterone in at least five of the six birds receiving each dose. These findings show that treatment of quail with insulin or LPS can activate the HPA axis, and hence can be classified as physical stressors in quail.
This is the first report of physical stressors in Japanese quail. Corticotropin-releasing factor (CRF), arginine vasotocin (AVT), and adrenocorticotropic hormone (ACTH) are all components of the HPA axis, and treatment of quail with these hormones is not considered to represent treatment with a physical stressor. Also, although food and water deprivation can activate the HPA axis in quail (Scott et al., 1983), this treatment is not solely a physical stressor. Visual and metabolic stimuli associated with food and water deprivation can influence plasma corticosterone concentrations (Harvey et al., 1983; Harvey et al., 1985), so this stressor has both emotional and physical components. Corticosterone and glucose concentrations have for the first time been measured
together in plasma samples collected from Japanese quail following treatment with insulin. Insulin-induced hypoglycaemia can elevate plasma corticosterone
concentrations in mammals (Robinson et al., 1992; Romero et al., 1993; Koenig and Cho, 2005) and in chickens. For example, chickens with a mean body weight of approximately 1 200 g were given intravenous injections of insulin at a dose of 4 IU/animal (Niezgoda et al., 2005). Plasma glucose decreased significantly from
approximately 11 mmol/l at the time of injection to 7 mmol/l at 2 h, and was still lower than in controls at 6 h, whereas plasma corticosterone was significantly elevated at 2 h, reached a maximum concentration of around 9 ng/ml at 4 h, and was still higher than in controls at 6 h (Niezgoda et al., 2005). The results of the present study support the notion that elevated plasma corticosterone concentrations in quail treated with insulin resulted from hypoglycaemia. Whilst just 53 of the 96 samples collected during insulin dose-response tests had sufficient plasma for measurement of glucose concentrations, in many cases plasma glucose in quail treated with insulin was markedly lower than in untreated control quail. Plasma glucose concentrations were also noticeably reduced in treated quail that had markedly elevated plasma corticosterone concentrations.
Furthermore, mean glucose concentrations were markedly lower in all insulin treatment groups than in controls at 60 min, and lower in the two highest insulin treatment groups than in controls at 150 min. In general, however, plasma glucose concentrations in untreated Japanese quail appeared to be slightly higher than in untreated chickens (see Niezgoda et al., 2005).
The current findings show that treatment of quail with LPS can activate the HPA axis and increase the secretion of corticosterone, as has been shown in chickens.
Intravenous treatment of male broilers with 1 mg/kg of LPS elevated plasma
corticosterone to approximately 9 ng/ml after 2 h (Baert et al., 2005). Corticosterone concentrations in chickens treated with higher doses of LPS differed between studies. In eight week old male white Leghorns treated with 8 mg/kg of LPS, mean
corticosterone was >100 ng/ml at 1 h, decreased to around 50 ng/ml at 6 h, and returned to similar concentrations to controls after 24 h (Gehad et al., 2002). Conversely, in brown Hyline chicks treated with 8 mg/kg of LPS, mean corticosterone concentrations were <8 ng/ml at 1 h, and around 6 ng/ml at 3 h, before returning to initial
concentrations at 24 h (Shini et al., 2008). There is substantial evidence that activation of the HPA axis following treatment with the endotoxin LPS is mediated by cytokines (Turnbull and Rivier, 1999; Arkins et al., 2001; Beishuizen and Thijs, 2003). Studies in mammals show that different cytokines can act at more than one level of the HPA axis to increase glucocorticoid secretion. Indeed, the secretion of the cytokines IL-1, IL-6 and TNF-α occurred simultaneously with increased plasma corticosterone
concentrations in chickens treated with LPS (Nakamura et al., 1998; Gehad et al., 2002; Ferdous et al., 2008).
This is the first report to demonstrate marked individual differences in the plasma corticosterone responses of birds to physical stressors. For example, three of the six quail treated with 4 IU/kg insulin had markedly elevated plasma corticosterone concentrations at 150 min, whilst plasma corticosterone in the other three quail in this group remained relatively constant after treatment with insulin. There was also considerable variation in the responses of individual birds to treatment with LPS, with quail showing relatively low or high corticosterone responses to treatment with all three doses. Whilst this is the first report of individual variation in corticosterone responses of birds to physical stressors, there is already considerable interest in the variation
between birds in their responses to emotional stressors. Studies that investigate
individual variation in corticosterone responses typically measure responses of birds to standardised emotional stressors, such as capture followed by restraint in captive (Cockrem and Silverin, 2002b) or wild (Cockrem et al., 2009) free-living species, or handling followed by confinement in domesticated species (Littin and Cockrem, 2001). The functional significance of variation in corticosterone responses to emotional
stressors has been examined (Blas et al., 2007; Cockrem, 2007; Williams, 2008;
Breuner et al., 2008), but there are no data on how individual variation in corticosterone responses to physical stressors might relate to birds in an evolutionary context.
Variation in corticosterone responses to emotional stressors may result from differences in the way a stressor is perceived, or from differences in the functioning of the HPA axis, and both genetics and previous experience may influence the response (Satterlee and Johnson, 1988; Cockrem, 2007). It is likely that these factors also account for some of the variation in corticosterone responses to physical stressors, although the perception of stimuli as threats, and hence as emotional stressors, will be absent when birds
respond to physical stressors. Instead, factors such as the distribution and number of receptors in insulin target cells, and the concentrations of circulating cytokines may account for some of the variation in corticosterone responses of birds to the physical stressors used in the present study. To further examine the functional significance of individual variation in plasma corticosterone responses the first objective would clearly be to determine if responses of birds to emotional and physical stressors are related. It would then be apparent whether or not birds show consistent individual differences in plasma corticosterone response to stressors in general.
In conclusion, these findings in quail are consistent with reports of elevated plasma corticosterone concentrations in chickens following treatment with insulin or LPS. It is probable that decreased plasma glucose concentrations and increased secretion of cytokines were responsible for activating the HPA axis following treatment with insulin or LPS respectively. The corticosterone responses to different doses of insulin or LPS provide a basis for the choice of doses (4 IU/kg of insulin or 1.2 mg/kg of LPS) and sampling times (insulin, 60, 150 and 240 min; LPS, 60, 120 and 240 min) to be used in a study of plasma corticosterone responses to emotional and physical stressors in quail.