In this study, we provide compelling evidence that sex differences in brain activity in the stress response circuitry are accounted for by women’s hormonal status. We suggest that gonadal hormones play a role in the regulation of arousal via the impact of subcortical brain activity on the cortical control of arousal. This has significant implications for elucidating the neurobiological mechanism of the control of stress as well as understanding basic physiological differences in the male and female brain in response to stress.
We demonstrated that, in the face of stressful stimuli, subcortical arousal circuitry, i.e. hypothalamic nuclei and left amygdala, along with anterior cingulate gyrus (ACG), showed greater activation in men than women, regardless of women’s cycle timing. That is, in comparing men to EF women, there was significantly greater activation in men of the left amygdala, hypothalamic nuclei (lateral area (LHA) and ventromedial nucleus (VMN)) and bilateral ACG. All other stress response regions activated similarly in men compared with EF women. In contrast, compared to LF/MC women, men exhibited greater activation in ACG, OFC, medial PFC, left amygdala, hippocampus, LHA and VMN, and brainstem regions (e.g., PAG and midbrain). Thus, we demonstrated that stress response circuitry activated in the male brain is more similar to women in the EF menstrual phase compared with women during LF/MC. The strongest evidence for this is seen in in which we averaged signal intensity values in voxels across a priori defined anatomic regions in the stress response circuitry. These findings suggest that compared to men, there is an attenuation of stress response circuitry activation in women, particularly evident during midcycle, in subcortical arousal regions that couple with attenuation of regions implicated in the cortical control of arousal. In fact, the largest sex difference effect sizes were in the ventral and medial prefrontal cortices.
Given that EF (when estrogen and progesterone are low) is hormonally more similar to men compared with LF/MC (when estrogen is high), we would argue that it is likely estrogen or ratio of estrogen: progesterone that accounts for activation differences. A recent study of reward circuitry, which shares some brain regions with the stress response (e.g., amygdala, hypothalamus, OFC and ACG), demonstrated region-specific gonadal hormonal regulation, particularly by estrogen unopposed by progesterone (Dreher et al., 2007
). Findings in Dreher (Dreher et al., 2007
) are not directly comparable, given they tested women at different points in the menstrual cycle than our study and did not directly test for hormonal effects on sex differences. However, the Dreher study does underscore the importance of hormonal regulation in related brain circuitry and the role of estrogen unopposed by progesterone (as would be similar to women during midcycle in our study).
Sex differences in negative affect were directly tested in two recent fMRI studies (Schienle et al., 2005
; Caseras et al., 2007
), suggesting greater activation of amygdala in men (Schienle et al., 2005
) and left ventrolateral PFC in women (Caseras et al., 2007
). However, gender groups were only matched on age and handedness, and stimulus and contrast images were not matched on faces and scenes, producing potential confounding influences on sex differences in brain activity. Further, lack of control for menstrual status may have attenuated sex differences in both studies. Finally, subjective valence and arousal ratings of images differed in men and women (Schienle et al., 2005
; Caseras et al., 2007
) and accounted for findings in (Caseras et al., 2007
), in contrast to our study demonstrating sex differences in brain activations given similar subjective ratings of arousal and valence. Our findings importantly suggest sex differences in the brain’s response to stress even in the context of similar clinical state and subjective behavioral ratings, suggesting a role for hormones in regulating homeostasis in the brain in response to stress.
Previous animal studies demonstrated that testosterone (T) regulated norepinephrine (NE) levels during prenatal development and differentially by sex (Stewart and Rajabi, 1994
), suggesting an association between HPG and HPA circuitry. That is, inhibiting aromatase (the enzyme that converts T to estrogen) during development produced a rise in NE in anterior frontal, insula and cingulate cortices (Stewart and Rajabi, 1994
). This is consistent with our findings in men compared with LF/MC women, in which we demonstrated a large sex difference in the LHA, a key region in the noradrenergic pathway (Luiten et al., 1987
), coupled with regions implicated in the cortical arousal circuitry (ACG, OFC, and vmPFC).
LHA has a primary role in arousal and is connected with cortical circuitry implicated in the inhibitory control of arousal and motivational shifts in behavior. Thus it contributes to maintaining homeostasis in the face of the fight/flight arousal response (Swanson, 1987
). Some of the densest connections of the LHA are with mPFC and OFC, demonstrated in rats (Luiten et al., 1987
) and cats (Room and Groenewegen, 1986
), and it is part of the reticular system regulating the autonomic nervous system (ANS) and somatomotor circuitry (Swanson, 1987
). In fact in our study, BOLD signal intensity values in activation of LHA correlated significantly and positively with those of vmPFC and OFC (r=.55 and .66 (p≤.005), respectively).
LHA is also a primary region in the noradrenergic pathway with the PVN (Luiten et al., 1987
) (hypothalamic nuclei with the greatest concentration of corticotropin releasing hormone), thus providing an anatomical basis of prefrontal control of the HPA axis. In fact, LHA and VMN were qualitatively different in the male and female response circuitry, i.e., LHA and VMN were significantly activated in men, and, compared with women, not dependent on cycle phase. Sex differences in hypothalamic activations may be one reason for sex differences in cortisol levels in response to stress, regulated in part by the PVN in releasing adrenocorticotropin hormone (Schaeffer and Baum, 1984
) which interacts with LHA and VMN.
Increases in hypothalamic activations in men may have necessitated compensatory increases in prefrontal circuitry in order to provide cortical control and homeostasis, in part reflected in the positive correlations of LHA with vmPFC and OFC (noted above). In fact, BOLD signal intensity values in activations of vmPFC and OFC were also significantly and positively correlated with the amygdala (r=.61 and .65 (p≤.001), respectively) and hippocampus (r=.72 and .65 (p≤.005), respectively). Previous literature has argued that BOLD signal is mainly associated with excitatory inputs to cells rather than their output spikes (Logothetis, 2008
), and BOLD changes can be associated with increased inhibition depending on the region being inhibited and experimental conditions (Buzsaki et al., 2007
; Logothetis, 2008
). In our study, BOLD activity in subcortical regions was expected to represent both an early increase in excitatory inputs, signaling arousal, followed by an increase in cortical inhibitory inputs to inhibit arousal. This would result in an effect that was consistent with a net increase in BOLD activity (i.e., positive correlations between cortical and subcortical activity). Positive correlations were also found between vmPFC and hippocampus and amygdala during recall of fear (Milad et al., 2007
), which was interpreted as consistent with the top-down inhibitory control of subcortical arousal by vmPFC (Milad et al., 2007
). In fact, the associations of cortical arousal circuitry to regulation of subcortical arousal regions have been disrupted in disorders such as anxiety and major depression (Shin et al., 2001
; Dougherty et al., 2004
) and autism (Bachevalier and Loveland, 2006
), for which stress response and fear circuitries have been implicated.
The modulatory role of cortical arousal circuitry fits with our previous findings (Goldstein et al., 2005
) and others (Protopopescu et al., 2005
) in women and with studies demonstrating an inhibitory role of estradiol on arousal circuitry (Best, 1992
; Lindheim et al., 1994
; Kirschbaum et al., 1996
). Although some preclinical studies have demonstrated excitatory roles for estradiol (Woolley and McEwen, 1993
; Segal and Murphy, 2001
; Lund et al., 2005
), this does not negate an inhibitory role for estradiol, which has been associated with estrogen receptor beta (Ostlund et al., 2003
; Shansky et al., 2004
; Bao et al., 2005b
; Lund et al., 2005
) while the excitatory role may be associated with estrogen receptor alpha (Lund et al., 2006
). These opposing effects of ERα
; and ERβ
in response to stress have been demonstrated at the level of behavior (Lund et al., 2005
) and the level of the anterior hypothalamus (Lund et al., 2006
Thus, our findings present a potential mechanism for the regulation of the stress response by circulating hormones in women. We also showed that there is not a general effect of hormones on overall blood flow or brain activity, but hormonal effects are region-specific. This makes sense given that these brain regions control the HPA and HPG axes, involve brainstem regions implicated in ANS function and provide frontal cortical influence over autonomic and endocrine function (Price, 1999
). They are also dense in sex steroid receptors (McEwen, 1981
; Keverne, 1988
; Ostlund et al., 2003
; Bao et al., 2005a
; Lund et al., 2005
), noradrenergic receptors (among other monoaminergic receptors), and vasopressin and oxytocin (Keverne, 1988
; De Vries and Al-Shamma, 1990
; Feldman et al., 1995
; Pacak et al., 1995
; Tobet and Hanna, 1997
; Price, 1999
; McEwen, 2000
; Swaab, 2004
), thus underscoring their adrenal and ovarian functions.
In summary, findings in this study demonstrated that the male and female brain differs in brain activity in response to stressful stimuli, even given the same clinical or behavioral response. Hormonal changes in women during midcycle result in an attenuation of subcortical arousal coupled with attenuation in cortical arousal circuitry that differs from men. This may have important clinical implications for understanding sex differences in clinical disorders associated with stress response circuitry (McEwen, 2000
) and high rates of co-morbid endocrine disorders.
From an evolutionary point of view, it is important for the female during midcycle to have a heightened cortical capacity, unencumbered by excessive arousal, to optimally judge whether a potentially threatening stimulus, such as an approaching male, is an opportunity for successful mating or for fight or flight. Thus, females have been endowed with a natural hormonal capacity to regulate the stress response that differs from males. This mechanism may have been maladaptive or unnecessary from an evolutionary point of view for the male, who had primary responsibility for protection of the species thus necessitating a constant fight or flight behavioral response. Historically, these complementary sex-specific social roles have been dynamic and this may reflect sex-specific plasticity in these arousal circuitry neural systems. Thus, although there are sex differences in these neurobiologic systems, they may now support some of the same social functions.