Recovery from disrupted ultradian glucocorticoid rhythmicity reveals a dissociation between hormonal and behavioural stress responsiveness

Abstract

Ultradian release of glucocorticoids is thought to be essential for homeostasis and health. Furthermore, deviation from this pulsatile release pattern is considered to compromise resilience to stress-related disease, even after hormone levels have normalised. In the present study, we investigate how constant exposure to different concentrations of corticosterone affects diurnal and ultradian pulsatility. The rate of recovery in pulsatile hypothalamic-pituitary-adrenal (HPA) activity after withdrawal of exogenous corticosterone is also examined. Finally, the behavioural and neuroendocrine responsiveness to an audiogenic stressor is studied. Adrenally intact male rats were subcutaneously implanted with vehicle, 40% or 100% corticosterone pellets for 7 days. The continuous release of corticosterone from these implants abolished diurnal and ultradian corticosterone variation, as measured with high-frequency automated blood sampling. Pellet removal on post-surgery day 8 allowed rapid recovery of endogenous rhythms in animals previously exposed to daily average concentrations (40%) but not after exposure to high concentrations (100%) of corticosterone. Behavioural and neuroendocrine responsiveness to stress was distinctly different between the treatment groups. Audiogenic stimulation 1 day after pellet removal resulted in a similar corticosterone response in animals previously exposed to 40% corticosterone or vehicle. The 40% pellet group, however, showed less and shorter behavioural activity (i.e. locomotion, risk assessment) to noise stress compared to 100% corticosterone and vehicle-treated animals. In conclusion, unlike the animals impanted with 100% corticosterone, we find that basal HPA axis activity in the 40% group, which had mean daily levels of circulating corticosterone in the physiological range, rapidly reverts to the characteristic pulsatile pattern of corticosterone secretion. Upon reinstatement of the ultradian rhythm, and despite the fact that these animals did not differ from controls in their response to noise stress, they did show substantial changes in their behavioural response to stress.

Fig. 1

Effect of subcutaneous corticosterone (Cort) pellet implantation on diurnal and ultradian plasma corticosterone rhythms. Data represent (a) group averages (mean ± SEM) and representative individual plasma corticosterone profiles of rats implanted with (b) vehicle (n = 4), (c) 40% (n = 7) and (d) 100% corticosterone pellets (n = 7). Blood samples were collected at 10-min intervals from 07.00–10.00 h and 18.00–21.00 h on day 7 post-surgery. Significant ultradian fluctuations in corticosterone concentrations were only detected in the evening in vehicle-treated animals (repeated measures anova), indicating successful flattening of ultradian pulses in 40% and 100% corticosterone pellet animals. ***Effect of time with Bonferroni’s post-hoc test: F_1.9,5.8 = 9.1; P = 0.02. Grey bar indicates the dark phase.

Fig. 2

Effect of corticosterone pellet removal (washout) on diurnal and ultradian plasma corticosterone (Cort) rhythms. Data represent (a) group averages (mean ± SEM) and individual plasma corticosterone profiles measured after pellet removal in rats previously implanted with (b) vehicle (n = 4), (c) 40% (n = 6) or (d) 100% corticosterone pellets (n = 6). Pellets were removed at 08.00 of day 8 post-surgery; rats were reconnected to the ABS at 09.30 h and samples were collected every 20 min (10.00–12.40 h) or 10 min (13.00–05.00 h). Grey bar indicates the dark phase.

Fig. 3

Effect of previous constant corticosterone (Cort) exposure on stress-induced plasma corticosterone release. Values are represented as mean ± SEM of plasma corticosterone concentration measured in rats 1 day after removal of vehicle (n = 4), 40% (n = 6) or 100% corticosterone pellets (n = 5). Blood samples were automatically collected every 20 min. Rats were exposed to 10 min of noise stress (99 dB; hatched bar) at 06.00 h, which increased corticosterone levels in vehicle and 40% corticosterone animals [effect of time: F(7,96) = 15.7; P < 0.001]. No difference in stress-induced corticosterone levels was observed between vehicle and 40% corticosterone animals. Animals previously treated with 100% corticosterone showed no significant endocrine response to the stressor as basal concentrations were elevated (*P < 0.01; two-way repeated measures anova and Bonferroni’s post-hoc test).

Fig. 4

Behavioural responses to 10 min of noise stress (99 dB). Behavioural changes to noise stress are presented in 1-min bins over time in (a) locomotion (F_10,60 = 13.4; P < 0.001) (b) sitting (F_10,60 = 12.3; P < 0.001) and (c) risk assessment (F_3.7,22.0 = 17.9; P < 0.001). Analysis of interaction effect showed that in the 40% animals, the stressor induced significantly less locomotion (F_20,120 = 2.8; P < 0.001) and risk assessment (F_20,120 = 1.7; P = 0.04) compared to vehicle, indicating less total activity. Data are presented as the mean ± SEM duration in seconds per 1-min bin for vehicle (n = 6), 40% (n = 7) and 100% corticosterone (Cort) pellet animals (n = 7). *P < 0.05 compared to vehicle as tested by two-way repeated measures anova and Bonferroni’s post-hoc test.