Longitudinal hair cortisol in participants with BD is higher than in controls
We compared hair cortisol from participants with BD (n = 26) to control participants (n = 59). We measured hair cortisol in 12 cm of hair, corresponding to about a year of growth. Cortisol was evaluated in 6 segments of 2 cm each. Cortisol levels declined along the length of hair as previously described 21. In all six hair segments, measurements in participants with BD had about 2-fold higher median cortisol than control participants (Fig. 1).
BD participants had enhanced year-scale fluctuations in cortisol levels
We next studied the variation of cortisol over time in each individual. We accounted for the natural decline along the hair and evaluated the cortisol fluctuations in both control and BD participants using the methods developed in 21. This resulted in normalized cortisol readings for six 2cm segments (representing 2 months of growth) points from each 12 cm hair sample (Fig. 2A).
To evaluate the contribution of different timescales to the cortisol dynamics we used Fourier analysis. Fourier analysis decomposes the signal into a sum of oscillatory components at different frequencies, and quantifies the contribution of each frequency component as a Fourier amplitude. Six segments, spanning one year, allow three frequencies to be detected: 1 \(yea{r}^{-1}\), 2 \(yea{r}^{-1}\), and 3 \(yea{r}^{-1}\), representing periods of a year, 6 months, and 4 months, respectively.
In both groups the Fourier amplitudes declined with frequency (Fig. 2B-C), in agreement with 21. In controls, median amplitude at the lowest frequency (1 \(yea{r}^{-1}\)) was about 1.3 (p = 0.02, Mann-Whitney U test) times higher than the median amplitude at the highest frequency (3 \(yea{r}^{-1}\)). In the BD group, the median amplitude at the lowest frequency was higher by a factor of about 2.5 (p = 0.006, Mann-Whitney U test) than that of the highest frequency.
The Fourier amplitudes of the BD group were significantly higher than controls (p = 0.001, 0.0003, 0.02 for the three frequencies, Mann-Whitney U test). At the lowest frequency, the median amplitude is 4 times larger in BD than in controls. This shows that BD participants exhibit year-scale peaks and troughs in cortisol levels which are much larger than those of the control group.
Hair cortisol of participants with BD correlated with depression and anxiety mood scores
We also tested whether hair cortisol was associated with mood scales. At the time the BD participants contributed the 12cm hair sample, we evaluated them with four mood scales, the YMRS for mania, BDI and HDRS for depression and HAM-A for anxiety (see Methods). Thirteen participants returned and donated a second hair sample more than two months after the first sample, and were evaluated using these mood scales a second time. One participant did not fill out the scales. In total we had 38 hair samples with 38 corresponding sets of mood scores.
Since the mood scales are designed to estimate mood over the recent weeks, we compared the mood scale scores to the 2 cm segment of hair most proximal to the scalp, which corresponds to cortisol in the 2 months prior to hair collection.
Low YMRS mania scale scores were recorded for all participants (mean score = 2, std = 3.3), indicating that none of the participants showed significant manic symptoms during the study. YMRS scores did not correlate with log cortisol (\(r=0.23, p=0.26\), adjusted partial Pearson correlation).
A wide range of scores were recorded in the depression and anxiety scales, indicating a range of depression and anxiety symptoms in the BD group. The two depression scales BDI and HDRS both correlated positively with log hair cortisol (\(r=0.47\), \(p=0.007\) BDI, \(r=0.55\), \(p=0.001\) HDRS, adjusted partial Pearson correlation). The anxiety scale HAM-A also correlated positively with log cortisol (\(r=0.45, p=0.01\), adjusted partial Pearson correlation).
We also repeated the analysis excluding the 13 repeat measurements. The HDRS correlation remained significant (\(r=0.53, p=0.025\)). The other two scales show insignificant positive correlation trends (BDI \(r=0.38, p=0.09\) and HAM-A \(r=0.3, p=0.15\)).
The BDI, HAM-A and HDRS scale scores also correlated well with each other (\(r=0.88, p=6\cdot 1{0}^{-13}\) BDI vs. HDRS, \(r=0.72, p=5\cdot 1{0}^{-7}\) BDI vs. HAM-A, \(r=0.66, p=7\cdot 1{0}^{-6}\) HDRS vs. HAM-A) indicating satisfactory reliability of the questionnaires.
We conclude that hair cortisol in the 2cms most proximal to the scalp correlates with depression and anxiety mood scales.
A longitudinal BD mood scale study show similar frequency spectra to hair cortisol
We next assessed whether the low frequency components observed in the hair cortisol dynamics of BD participants resemble the frequencies found in a much larger study of longitudinal mood measurements. For this purpose we analyzed longitudinal mood scale scores data from the Prechter BD cohort, where participants complete the PHQ-9 and the ASRM questionnaires every two months over multiple years 46 (see Methods).
The Prechter BD cohort includes 541 bipolar type-1 patients and 267 controls. We analyzed the BD participants with at least two years of consecutive mood measurements and took their longest consecutive time series for each mood scale. Thus, we included 266 BD and 179 control time series of depression scores (PHQ-9), and 273 BD and 178 control time series of mania scores (ASRM). In total we analyzed 24,627 mood measurements.
To estimate the Fourier frequency spectrum of the mood time series we used the Lomb-Scargle method 63–65 which is suitable for time series of differing lengths. The PHQ-9 exhibited a Fourier spectrum displaying the highest amplitudes at low frequencies (Fig. 3A). The 1 \(yea{r}^{-1}\) median amplitude was 1.2-fold (p = 0.01, Mann-Whitney U test) higher than the 3 \(yea{r}^{-1}\) median amplitude. The Fourier amplitudes were higher in BD participants than in controls. A similar feature was observed in the ASRM scores with declining frequency amplitudes along the Fourier spectrum, where the 1 \(yea{r}^{-1}\) median amplitude was 1.4-fold (p = 0.001, Mann-Whitney U test) higher than the 3 \(yea{r}^{-1}\) median amplitude (Fig. 3B).
We conclude that hair cortisol and mood scales in BD show similar frequency distributions, with dominant low frequencies on the scale of a year that have higher amplitude than those on the scale of months.
Mathematical model of the HPA axis in BD predicts enhanced year-scale fluctuations
In one of our prior studies 21, a Fourier spectrum constructed from hair cortisol levels measured from control participants was similar to that in Fig. 2B-C, with dominant low frequencies on the scale of a year. We showed that these frequencies cannot be explained by the classical mechanism of the HPA axis. The classical mechanism works on the timescale of the hormone lifetime, about an hour, and thus can not show timescales of months and years. We then showed that a recent mathematical model of the HPA axis 18,20 that considers the temporal changes in the size of the HPA glands over months (Fig. 4A) is able to reproduce the frequency spectrum of control participants. Stress inputs that vary from day to day, modeled as white noise, cause the glands to grow and shrink on the timescale of many months, providing the observed Fourier spectrum. The HPA system modulates the typical flat Fourier spectrum of the noisy input (Fig. 4C) and amplifies low frequencies (Fig. 4E) due to gland changes.
Here we extend this mathematical model to identify a possible reason why the BD group exhibits a 4-fold increase in low frequency amplitudes compared to controls. We propose that the daily stress inputs to the hypothalamus in people with BD are higher than in the control population. We modeled this as a white noise input to the HPA equations which has a larger amplitude than in controls (Fig. 4B-C).
We find, using simulations of the mathematical model, that the BD cortisol spectrum can be obtained by providing daily stress noise that is 4-fold larger than the noise needed to obtain the Fourier spectrum of control participants (Fig. 4D-E). We thus conclude that a possible explanation of the enhanced slow cortisol fluctuations in BD participants may be larger day-to-day fluctuations in their stress inputs to the HPA axis.
Hypothesis for a pathophysiological mechanism for the timescales of BD
Taking the present findings together, we propose a mechanism that could explain the timescales of the observed mood phenotypes in BD based on the HPA axis. This is built on decades of research showing the connection between the HPA axis and BD 27,66. The new aspect of the proposed mechanism is the ability of the HPA axis to generate fluctuations in cortisol on the time-scale of many months 21 due to changes in the effective mass of the glands 18 (Fig. 4A).
The basic premise of the proposed mechanism is that individuals susceptible to BD have two neurobiological traits (Fig. 5A): a susceptibility in which high cortisol levels over weeks can trigger mood episodes, and emotional reactivity in which individuals generate larger daily stress inputs to the HPA axis in response to life events than the typical population. The HPA axis amplifies these enhanced inputs to generate large cortisol fluctuations over months that can trigger mania and depression.
These two neurobiological traits are supported by multiple lines of evidence. Susceptibility to cortisol-induced mania is found in studies on long-term use of glucocorticoid steroids, which are cortisol analogues. According to a large-scale study, people treated with glucocorticoids have a 2-fold higher risk of developing depression and a 4-5-fold higher risk of developing mania compared with people unexposed to glucocorticoids 36. The effect is dose-dependent (Fig. 5B), with an apparent threshold of about 40 mg prednisone, equivalent to about 10 times the normal level of cortisol 67.
As described in the introduction, a similar effect is observed in Cushing's syndrome in which cortisol levels are elevated due to a hormone-secreting tumor. Mania or hypomania is reported in about 30% of Cushing's syndrome patients and depression in about 50–60% of the patients 38. In both Cushing's syndrome and steroid treatment there is evidence that mania tends to occur earlier than depression 37,38.
Along with susceptibility to glucocorticoid-induced mood episodes, our proposed mechanism also requires special conditions which promote prolonged excess cortisol that can cross the mania and depression threshold. One such condition is a neuropsychological trait called emotional hyper-reactivity 68, which is the generation of stronger-than-typical emotional responses to stimuli and is often reported in bipolar disorder 69,70. We operationalize emotional hyper-reactivity by defining it as greater hypothalamic sensitivity leading to more pronounced input signals from daily stress than those of the typical population. A related variable is deregulation of circadian cycles as evident by dysregulated melatonin dynamics in BD. Circadian rhythm is an important input to the HPA axis. Both emotional reactivity and circadian dysregulation have fast-scale dynamics (hours-day) that can, in our model, excite slow timescale fluctuations (months) along the lines of Fig. 4.
The heart of the proposed mechanism is the ability of the HPA axis to generate months-scale fluctuations of cortisol. This timescale of months is provided by the growth and shrinkage of the pituitary and adrenal functional masses. This months-scale property of the HPA axis converts input stresses that fluctuate rapidly over hours and days to large months-scale fluctuations. We posit that these months-scale fluctuations are larger in those with emotional hyper-reactivity because of their enhanced daily stress inputs. This coincides with the hair cortisol fluctuations measured here. As a result of prolonged periods of high cortisol, there is an increased probability of triggering mood episodes.
The prevalence of BD in the general population is about 2%. This may correspond to the intersection of the two traits mentioned above. Cortisol-induced mania appears in about 30% of Cushing's patients, so that susceptibility to cortisol-induced mania may characterize about 30% of the general population. Emotional hyper-reactivity appears in at least 10% of the general population (see SI). If these traits are independent, one may expect about 3% to have both (Fig. 5A).