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Stress and alcohol context cues are each associated with alcohol-related behaviors, yet neural responses underlying these processes remain unclear. The present study investigated the neural correlates of stress and alcohol context cue experiences and examined sex differences in these responses. Using functional magnetic resonance imaging, brain responses were examined while 43 right-handed, socially drinking, healthy individuals (23 females) engaged in brief guided imagery of personalized stress, alcohol-cue and neutral-relaxing scenarios. Stress and alcohol-cue exposure increased activity in the cortico-limbic-striatal circuit (p<.01, corrected), encompassing the medial prefrontal cortex (mPFC), orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), left anterior insula, striatum and visuomotor regions (parietal and occipital lobe, and cerebellum). Activity in the right dorsal striatum increased during stress, while bilateral ventral striatum activity was evident during alcohol-cue exposure. Men displayed greater stress-related activations in the mPFC, rostral ACC, posterior insula, amygdala and hippocampus than women, whereas women showed greater alcohol-cue related activity in the superior and middle frontal gyrus (SFG/MFG) than men. Stress-induced anxiety was positively associated with activity in emotion modulation regions, including the medial OFC, ventromedial PFC, left superior-medial PFC and rostral ACC in men, but in women with activation in the SFG/MFG, regions involved in cognitive processing. Alcohol craving was significantly associated with the striatum (encompassing dorsal and ventral) in men, supporting its involvement in alcohol ‘urge’ in healthy men. These results indicate sex differences in neural processing of stress and alcohol-cue experiences, and have implications for sex-specific vulnerabilities to stress- and alcohol-related psychiatric disorders.
Stress is a key vulnerability factor in psychiatric disorders (Cohen, et al. 2007; Sinha 2009b), and sex differences in the prevalence of mood and anxiety disorders and addiction have been documented (Becker, et al. 2007; Kessler, et al. 1993). In individuals with alcohol use disorders, stress and alcohol-related cues are important factors increasing alcohol craving and relapse risk (Sinha and Li 2007). In epidemiological samples, stress increases alcohol consumption (Dawson, et al. 2005; Grzywacz and Almeida 2008), and greater alcohol use was also reported after exposure to the 911 attacks in New York (Boscarino, et al. 2006).
Stress provokes defensive motivation and avoidant behaviors (Nachmias, et al. 1996), while social alcohol consumption is associated with appetitive motivation and approach behaviors (Lukas, et al. 1986; Robinson and Berridge 1993). The defensive and appetitive systems are two parallel systems with common, integrative components (Cacioppo, et al. 1999; Carver 2001), suggesting the presence of specific yet overlapping brain systems underlying these two systems.
Neuroimaging studies on stress and aversive processing have identified a specific corticostriatal-limbic circuitry including the medial prefrontal cortex (PFC), anterior cingulate cortex (ACC), hippocampus, amygdala and striatum (Lopez, et al. 1999; Sinha, et al. 2004; Zhou, et al. 2008). Recent evidence also indicates that alcohol taste cue (Filbey, et al. 2008) and intravenous injection of alcohol (Gilman, et al. 2008) reliably activates brain reward circuits, specifically mesocortico-striatal structures such as the PFC and ventral striatum in healthy social drinkers. The ventral striatum has been associated with reward processing (Schott, et al. 2008) and the amygdala is associated with stress response (Zhou, et al. 2008), with some evidence in reward processing (Garavan, et al. 2001). These data indicate that the coticostriatal-limbic regions may represent the core circuits involved in both stress and alcohol cue-related processing, but no previous research has directly compared neural circuits associated with these processes in humans.
Sex differences in stress responses have also been previously reported. Recent neuroimaging evidence indicates that compared to women, men showed greater stress-related brain responses in fronto-limbic areas, especially in the medial PFC, ACC, hypothalamus and amygdala (Goldstein, et al. 2010). Greater physiological responses to stress in men are consistent with behavioral and neuroendocrine studies. For example, men show greater stress-related negative emotion and aggression responses, more robust fear conditioning and higher cortisol responses than women (Jackson, et al. 2006; Kudielka and Kirschbaum 2005; Verona, et al. 2007), while women under stress show greater tendencies to rumination and negative cognition (Nolen-Hoeksema 1987). Further, men showed greater stress-related increases in alcohol craving and consumption than women (Lindquist, et al. 1997; Tamres, et al. 2002). While this literature suggests sex differences in responses to stress and alcohol-related behaviors, and there is some evidence of sex differences in neuroimaging of stress, no previous research has examined sex differences when directly comparing the neural responses to emotional stress and alcohol context cues in healthy individuals.
To clarify sex-specific neural responses to stress and alcohol context cues, the current study utilized individually calibrated, personally relevant, guided imagery scripts of stress and alcohol context cue that were compared to personalized neutral relaxing scripts. It is a widely used, ecologically valid method of emotion, stress and craving provocation in laboratory and in neuroimaging studies (for a review, Sinha 2009a). Based on the previously cited research, we hypothesized that in healthy individuals, both stress and alcohol cues would activate the prefrontal and ACC regions known to be involved in stress and reward modulation. Further compared to women, men would show greater neural responses to stress in corticostriatal limbic regions, especially in the mPFC, ACC and amygdala. In terms of self-reported anxiety and craving, we expected significantly elevated stress-induced anxiety and alcohol cue-induced craving from the baseline. However, we did not expect that stress-induced craving or alcohol-cue induced anxiety would be increased in healthy individuals based on previous studies (Fox, et al. 2008; Sinha, et al. 2009). As the striatum is involved in motivation for alcohol (Heinz, et al. 2004; Schneider, et al. 2001; Wrase, et al. 2007), we expected it to be associated with subjective alcohol craving. We also hypothesized that the stress circuit involving the medial PFC, ACC, and the amygdala would be associated with stress-induced anxiety.
Forty-three healthy individuals between the ages of 19 and 50 were recruited from the community via newspaper, web advertising and flyers. All participants were right-handed and reported light to moderate levels of alcohol consumption (up to 25 drinks per month). Over the course of 2-3 sessions, participants completed demographic, psychiatric, cognitive, drug use and self assessments. To ensure healthy physical function, a medical evaluation was conducted including laboratory testing of renal, hepatic, pancreatic, hematopoietic, and thyroid functions. Participants were excluded if they had the following: (a) a history of head trauma, (b) pregnancy, (c) use of psychoactive medication, (d) current or lifetime substance abuse or dependence and (e) current or lifetime history of neurological or psychiatric disorders (as assessed by the Structured Clinical Interview for DSM-IV). Women were scheduled to participate in the laboratory sessions during the follicular or luteal phases of their menstrual cycle (as determined by sex steroid hormone measurements) and were excluded if they reported irregular menstrual cycle or were taking hormonal birth control, to control for the possible influence of steroid hormonal fluctuations on stress responses (Kirschbaum, et al. 1999). All participants were asked to refrain from alcohol for at least 72 hours prior to the scanning session and breathalyzer and urine toxicology screening was used to confirm drug and alcohol abstinence for each assessment session and on scanning day. Upon completion of the assessments, they participated in a 1.5 hour functional magnetic resonance imaging (fMRI) session. The Human Investigation Committee at the Yale University School of Medicine approved the study procedures and all participants signed an informed consent prior to study participation.
During the session prior to the fMRI scan, six individually-tailored imagery scripts were developed from participants’ descriptions of 2 alcohol context cue, 2 stressful and 2 neutral-relaxing experiences using Scene Construction Questionnaires (Li, et al. 2005; Sinha 2009a), based on previously described standardized methods (Sinha 2009a). For stress scripts, participants identified, “a situation that made them sad, mad, upset and which in the moment they could do nothing to change it.” Examples of highly stressful situations include loss of a job, death of or conflict with a significant other and loss of an important relationship. As a manipulation check, the situations were rated by the participants on a 10-point Likert scale (1=not at all stressful and 10=the most stressful) and only those situations that were rated as 8 or above were found appropriate for stimulus provocation and used for script development. Alcohol context cue scripts were developed from individual experiences of alcohol anticipation and consumption (e.g., birthday celebration, meeting friends at a bar) and scenarios occurring in the context of negative affect or psychological distress were excluded. Specifically, the alcohol cue script was developed in response to the query, “please tell us about a recent situation when you really wanted an alcoholic drink and then you went ahead and had one.” Thus each subject provided their preferred individual situation of wanting and consuming an alcoholic beverage. Neutral scripts were based on the personal experiences of commonly experienced neutral-relaxing situations, such as laying on the beach or reading at the park. While individual stimulus and response content specific to an experience was included in each script, the script style, content format and length were standard across conditions and subjects, as described previously (Sinha 2009a). Each script was 2 minutes in length and was audio-taped in random order for presentation during the scanning session. During the scanning session, all research staff and fMRI technicians were blind to content, order and type of the script stimuli.
The following procedures were implemented to ensure efficacy of the imagery manipulation. First, all participants completed the Questionnaire on Mental Imagery (Sheehan 1967) that measures individual difference in mental imagery ability, and individuals reporting average or above levels of imagery ability were included (Sinha 2009a). There were no statistical differences in scores of QMI between male (M=70.4, SD=20.2) and female (M=65.2, SD=22.3) participants. Additionally the scores of Toronto Alexithymia Scale (Bagby, et al. 1988) indicated that there were no statistical differences between men (M=58.1, SD=9.6) and women (M=58.6, SD=15.4), suggesting that men and women were equivalent in ability to reflect upon and rate their emotions.
Second, a structured relaxation and imagery training procedure, known to minimize variability in imagery ability (see (Sinha 2009a) for details), was implemented prior to the scanning session. Finally, each participant rated imagery vividness on a 10-point Likert scale (1=cannot visualize the image and 10=extremely clear, ‘as if’ it were happening right now) following each of the stress, alcohol cue and neutral trials, and there were no significant differences between men and women, or across conditions in vividness of the imagery for each trial (Men: M=8.2, SD=1.3, Women: M=8.8, SD=1.0).
The stress scripts were rated for the type of stress as either: Interpersonal (men: 72%, women: 72.5%; personal violation, relationship, betrayal), Environmental (men: 9 %, women: 2.5%; housing, legal, financial), Achievement (men: 17%, women: 22.5%; job, career), Medical (men: 2%, women: 2.5%; injury, illness). There were no differences in type of stressor by gender, χ2= 1.71, p = NS.
The 3-T Siemens Trio MRI system equipped with a single-channel, standard-quadrature head coil collected the images using T2*-sensitive gradient-recalled single shot echo-planar pulse sequence. Anatomical images were acquired with spin-echo imaging in the axial-plane parallel to the AC-PC line (repetition time (TR) = 300 msec, echo time (TE) = 2.5 msec, bandwidth = 300 Hz/pixel, flip angle = 60 degrees, field-of-view = 220×220 mm, matrix = 256×256, 32 slices, slice thickness = 4mm, no gap). Functional images were obtained using a single-shot gradient echo-planar imaging sequence with 32 axial slices parallel to the AC-PC line covering whole brain (TR = 2,000 msec, TE = 25 msec, bandwidth = 2004 Hz/pixel, flip angle = 85 degrees, field of view = 220×220 mm, matrix = 64×64, slice thickness = 4mm with no gap, 190 measurements). Once the functional images were collected, a high resolution 3D Magnetization Prepared Rapid Gradient-Echo sequence was used to acquire sagittal images for multi-subject registration. (TR = 2530 ms; TE = 3.34 ms; bandwidth = 180 Hz/pixel; flip angle = 7°; slice thickness = 1mm; field-of-view = 256 × 256 mm; matrix = 256 × 256).
A block design was used where each block comprised of a 5.5 minute (min) fMRI run that entailed a 1.5 min quiet baseline period followed by a 2.5 min imagery period (2 min of read-imagery and 0.5 min of quiet-imagery) and a 1-min quiet recovery period. During the baseline period, participants were asked to stay still in the scanner without engaging in any mental activity. During the recovery period, participants were instructed to stop imagining and lay still for another minute.
The order of all three script types were counterbalanced and randomized across subjects. Scripts from the same condition were never presented consecutively and each script was presented only once, such that different scripts were used per trial.
Before and after each BOLD trial, participants were instructed to rate anxiety and craving levels using a 10-point verbal analog scale (VAS: 1=not at all, 10=extremely high). Anxiety rating refers to how “tense, anxious and/or jittery” they felt, and craving ratings indicate their “desire to drink alcohol at that moment”. To decrease and normalize any residual anxiety or craving from prior trials, participants engaged in a brief exposure of progressive relaxation between fMRI trials for 2 minutes. Participants were instructed to progressively relax muscles in each part of the body (e.g., arm, leg, stomach muscles). This technique is mainly focused on relaxing physiological muscle tension, and does not involve mental relaxation or imagery. After relaxation, anxiety and craving ratings returned back to baseline and there were no baseline differences across trials in these ratings.
The raw imaging data was converted from Digital Imaging and Communication in Medicine format to Analyze format using XMedCon (Nolfe 2003). To achieve a steady-state equilibrium between radio-frequency pulsing and relaxation, the first ten images were discarded from the beginning of each functional run, leaving 180 measurements. Images were motion-corrected for three translational and three rotational directions (Friston, et al. 1996) discarding any trial with linear motion exceeding 1.5 mm or having a rotation greater than 2 degrees. At the individual level, General Linear Model (GLM) on each voxel in the entire brain volume was used with a regressor (time during imagery) for each trial per condition and the baseline for each trial was included separately as a regressor. The recovery period is excluded from the data analysis and was not used as the baseline regressor, due to the possibility of carryover effects from the imagery period. Each functional trial was spatially smoothed using a 6 mm Gaussian kernel and individually normalized to create beta-maps (3.44mm × 3.44mm × 4mm).
In order to adjust for individual anatomical differences, three registrations were sequentially conducted using the Yale BioImage Suite software package (Duncan, et al. 2004; Papademetris 2006); linear registration of raw data into 2D anatomical image, the 2D to 3D (1×1×1 mm) linear registration, a non-linear registration to a reference 3D image. The reference image was the Colin27 Brain (Holmes, et al. 1998) in Montreal Neurological Institute (MNI) space (Evans, et al. 1993).
The second level group analysis was conducted with Analysis of Functional NeuroImages software (Cox 1996) using random mixed effects models. A 2×3 ANOVA (sex by condition) was carried out with sex as the between-subject fixed-effect factor, condition (neutral/alcohol cues/stress) as the within-subject fixed-effect factor, and subject (N=43) as the random-effect factor. A FamilyWise Error rate (FWE) correction for multiple comparisons was applied using Monte Carlo simulations (Xiong, et al. 1995) conducted with AlphaSim in AFNI (Cox 1996) and set at p<.01 for the overall factorial analysis (Main effects and Interaction terms), and at p<.05 (FWE corrected) for simple effect analysis to understand source of the significant main effects and interactions.
Whole-brain correlation analyses with anxiety and alcohol craving were conducted using BioImage Suite (Papademetris 2006) with the application of AFNI AlphaSim FWE correction for multiple comparisons. To reduce the influence of any possible extreme values (outliers) in the correlation analyses, the Winsorization method (Chen and Dixon 1972; Dixon 1960) for values with a Cook’s Distance score greater than 1 was used. The correlation r value was presented after extreme values were reigned in to the value of the next highest score to reduce their influences. However, scatter plots (Figure 4) displayed all values without the Winsorization to show the overall patterns of the data.
Table 1 presents demographic characteristics and current levels of alcohol consumption in men and women. There were no significant differences in age, education and race by gender. As expected and consistent with general trends (Kessler, et al. 1994; Office for National Statistics 2006), women drank less than men on average, but there were no significant differences between genders in current levels of alcohol consumption.
Condition and sex effects on self-report measures of craving and anxiety in each were assessed using linear mixed model with Condition (stress, alcohol, neutral) and Time-point (baseline, imagery) as within-subject factors and Sex as a between-subject factor (see Figure 1). Post-hoc t-tests were corrected for multiple comparisons using a modified Bonferroni procedure (Hochberg 1988).
For the anxiety ratings, significant main effects of Condition (F(2 82)=26.6, p<.0001), Time-point (F(1 41) = 58.04, p<.0001) and a Condition X Time-point interaction (F(2, 80)=33.37, p<.0001) were observed. No other effects were significant including Sex main effect, Sex X Condition and Sex X Condition X Time-point interactions. There were no differences in anxiety ratings across the baseline period in each trial. However as expected, in the imagery period, anxiety during stress was greater than in the neutral (t=10.33, p<.0001) and in the alcohol (t=8.36, p<.001) conditions. When the imagery period was compared with the baseline period, anxiety ratings during imagery was significantly elevated from baseline only in the stress condition (t=10.98, p<.0001) and not in the neutral and alcohol cue conditions.
For the craving ratings, significant main effects of Condition (F(2 82)=4.24, p<.05), Time-point (F(1 41)=15.52, p<.001) and a Condition X Time-point (F(2 80)=10.3, p<0.0001) interaction were observed. No other effects were significant including Sex main effect, Sex X Condition and Sex X Condition X Time-point interactions. There were no differences in craving ratings across the baseline period in each trial. However, during the imagery period, craving in the alcohol-cue condition was greater than in the neutral (t=5.24, p<0.001) and stress (t=3.16, p<0.05) conditions. When the imagery period was compared with the baseline period, craving during imagery was greater than the baseline for the alcohol cue condition (t=5.59, p<.001) and not during the neutral and stress condition.
A significant condition main effect (p<.01, whole-brain FWE corrected) on brain activation was observed for the medial and lateral OFC, ventromedial PFC, lateral PFC, superior/middle frontal gyrus (SFG/MFG), anterior and posterior cingulate cortex (ACC and PCC) and left anterior insula. Additionally activation in specific areas of superior/middle/inferior temporal lobe, superior/inferior parietal lobe, angular gyrus, occipital lobe and cerebellum were observed (see Figure 2). These regions were strongly activated for both Stress-Neutral and Alcohol cue-Neutral contrasts (see Table 2). Activations in the dorsal striatum were present only during stress exposure, while the ventral striatum, occipital lobe, bilateral MFG/precentral gyrus were evident only during the alcohol-cue exposure (see Table 2). Greater activity during alcohol-cue relative to stress were seen bilaterally in the pre-/post- central gyrus at p<0.01, whole-brain corrected (Left: t=2.81, 1592mm3, X=−52, Y=−6, Z=35; Right: t=2.97, 1967mm3, X=62, Y=−11, Z=25).
A significant Sex X Condition interaction was evident in several regions of the cortico-striatal-limbic circuit (p<0.01, whole-brain FWE corrected) (see Table 3 and Figure 3). In the Stress-Neutral condition, men showed greater activations in brain regions involved in emotional modulation compared to women. These regions include the medial PFC (dorsomedial, ventromedial), rostral ACC, posterior insula, putamen, as well as limbic regions such as the amygdala, hippocampus, and parahippocampal gyrus. Additionally selected areas of superior/middle/inferior temporal lobe, superior parietal lobe, lingual gyrus and cerebellum were more activated in men compared to women. In the Alcohol cue-Neutral contrast, women displayed greater activation in the left superior/middle frontal gyrus (BA 6) compared with men (see Table 3, Figure 3).
Secondary analyses including days of alcohol used per week (frequency) and amount of current alcohol use as covariates were also conducted and the above results were unchanged.
For stress-induced anxiety, whole brain correlation analysis revealed differential associations for men and women (p<0.05, whole-brain FWE corrected). In men, significant positive correlations with anxiety were observed in the medial OFC, ventromedial PFC, left superior-medial PFC and rostral ACC. In women, stress-induced anxiety was significantly associated with brain activity in the middle and superior frontal regions (see Table 4 and Figure 4). There were no outliers in the relation between brain activity and stress-induced anxiety in men. In women, there was one extreme value (greater than 1 using Cook’s distance) in the relationship between female anxiety and SFG/MFG activity. The correlation was still significant even after removing this value. To reduce the influence of this value, the r value was presented after the Winsorization in Table 4 (r=.62).
For alcohol cue-induced craving, male alcohol craving was positively associated with activity in the striatum, right dorsolateral and lateral PFC, ACC, and middle/inferior temporal lobe (p<0.05, whole-brain corrected; see Table 4 and Figure 4). The striatum cluster encompassing both the ventral and dorsal portion was significantly positively correlated with alcohol craving (Figure 4). No outlier scores were present in associations between craving and the striatum as well as other brain regions (Table 4). There were no brain regions associated with female alcohol craving that survived multiple comparisons.
The current findings indicate overlapping neural responses to stress and alcohol context cues in healthy individuals with increased activation of the corticostriatal-limbic circuit encompassing the medial and lateral PFC, ACC, PCC, anterior insula, and striatum, areas known to be involved in processing of emotions, stress and rewarding stimuli. Furthermore, significant sex differences in neural processing of stress and alcohol context cues were observed, suggesting sex-specific functional responses to stress and alcohol cues. These differences in neural responses likely contribute to the well known sex differences in stress-related coping and in vulnerabilities to stress-related psychiatric disorders.
During stress and alcohol-cue exposure, strong medial and lateral prefrontal activation along with increased activity in ACC was evident. The PFC is involved in executive control and emotion regulation (Ochsner, et al. 2004), and the ACC is associated with online conflict monitoring (MacDonald, et al. 2000). Both these regions were active during stress and alcohol-cue exposures, indicating activation of neuroregulatory systems for on-line modulation of stress and alcohol cue experiences. Furthermore, the anterior insula was activated, which is known to be involved in interoceptive processing of emotion, pain and reward (Harris, et al. 2009; Hollander, et al. 2008). Ventral striatum activation was seen during alcohol-cue exposure, while the right dorsal striatum increased during stress exposure. Increases in dorsal striatal activity during stress have been reported previously (Sinha and Li 2007) and chronic stress is known to alter dorsal striatal projections to the frontal cortex (Rossi, et al. 2009), supporting the notion that stress influences habit-based decision making involving fronto-striatal pathways (Dias-Ferreira, et al. 2009). The ventral striatum (VS) has been associated with reward processing (Haber and Knutson 2010), and our findings of specific activation in the ventral striatum during alcohol context cue exposure is consistent with this observation and previous studies showing VS activation with an alcohol taste cue (Filbey, et al. 2008) and intravenous injection of alcohol (Gilman, et al. 2008).
In comparing the alcohol cue with the stress condition, greater activity in the premotor region was seen in the alcohol cue relative to the stress condition, suggesting that action-specific components of emotion may be more activated during this condition. This is consistent with previous research indicating that pleasant emotions facilitate approach-related behaviors and psychomotor activation, whereas unpleasant and stressful emotion promotes defensive urges (Lang 2000; Lang, et al. 1997). Alcohol is also shown to generate positive appetitive states in healthy social drinkers (Lukas, et al. 1986) and it is suggested that motor control is an essential part of appetitive processing (Haber and Knutson 2010).
Sex differences in neural responses were also observed. During stress, men showed greater activation in brain regions including the ventromedial PFC, rostral ACC, posterior insula, amygdala and hippocampus than women. The neural circuit connecting the medial PFC, rostral ACC, and amygdala is regarded as the core circuit for emotional modulation (Davidson, et al. 2000; Ochsner, et al. 2004), especially stress experiences (Li and Sinha 2008; Sinha, et al. 2006). Male-specific hyperactivity in emotion-related brain regions are consistent with prior research indicating greater emotional and physiological responses in men following stress compared to women, including higher cortisol levels (Kudielka and Kirschbaum 2005) and diastolic blood pressure responses (Chaplin, et al. 2008) and greater negative and aggressive emotions (Verona, et al. 2007). Greater sensitivity to stress-related conditioning effects has also been reported in males relative to females in animal (Wood, et al. 2001; Wood and Shors 1998) and in human studies (Jackson, et al. 2006). In contrast, females showed greater activity in the SFG and MFG during alcohol-cue exposures. The SFG/MFG is involved in high-level cognitive processing such as reasoning (Goel and Dolan 2003), working memory (Smith and Jonides 1999) and attention during language processing (Cabeza and Nyberg 2000). Greater activations in these regions during alcohol cues reflect that healthy women may utilize attention and language-based processing of alcohol context cues. Interestingly, these sex differences in neural responses to stress and alcohol cues are consistent with previous psychobiological and learning studies indicating a tendency in males towards utilizing interoceptive and associative cues to identify emotions and in learning tasks, while females utilizing external, context-dependent stimuli to identify their emotions and in learning tasks (Herman and Wallen 2007; Roberts and Pennebaker 1995; Sandstrom 2007; Sandstrom, et al. 1998). Thus, males and females appear to rely on different types of information in emotion, stress and learning contexts, which in turn, are likely to activate different brain regions in a sex-specific manner, as observed in the current data.
Sex-specific patterns were also evident in the relationship between brain activity and stress-induced anxiety ratings. In men, stress-induced anxiety was positively correlated with activity in the ventromedial PFC, medial OFC, left superior-medial PFC and ACC, areas showing greater stress-related activation in men. Associations with these emotion modulatory regions (Davidson, et al. 2000; Ochsner, et al. 2004) suggest more emotion-focused processing in males while experiencing stress-induced anxiety. In females, SFG/MFG activity was positively correlated with stress-induced anxiety. The SFG/MFG are involved in cognitive processing (Cabeza and Nyberg 2000; Goel and Dolan 2003) and also closely interact with anterior parts of PFC to guide emotional behaviors (Koechlin, et al. 2000). Recent evidence indicates that the SFG/MFG contributes to response correction during reasoning processes (Kalbfleisch, et al. 2007), and activity in these regions increased in individuals with anxious tendencies (Karch, et al. 2008). These findings suggest greater utilization of cognitive processes in experiencing stress-induced anxiety in women.
For alcohol-cue induced craving, significant neural correlates of male craving were found dominantly in the striatum and also selected regions of right dorsolateral/lateral PFC, ACC and middle/inferior temporal lobe. The association of striatal activity with craving is consistent with previous studies with alcohol-dependent individuals (Heinz, et al. 2004; Modell and Mountz 1995; Myrick, et al. 2004) as well as other addictive disorders (Rothemund, et al. 2007; Vanderschuren, et al. 2005; Volkow, et al. 2006). We found significant correlations between craving and brain activity in both ventral and dorsal striatum. In reinforcement learning, the ventral striatum is thought to be engaged in learning reward values, and the dorsal striatum maintains reward values to guide decision and behaviors (Kahnt, et al. 2009). Studies showed that the dorsal striatum is specifically involved in action initiation and in habit learning, including chronic drug use habits (Kahnt, et al. 2009; Porrino, et al. 2004; Volkow, et al. 2006). It is also known that the dorsal striatum is associated with craving when conditioned stimuli (e.g, cues) are presented to human subjects (Volkow, et al. 2006). Concurrent correlated activity of the ventral and dorsal striatum with craving in our data suggests that desire for alcohol involves processing and actively pursuing reward values in the presence of alcohol context cues. It is notable that we found this association in healthy men, since previous studies have shown correlations between striatal activity and craving in individuals with addictive disorders or in heavy drinkers, but not in healthy individuals. This suggests the involvement of the striatum in motivational aspects of wanting in both healthy and in clinical samples.
In addition, the correlation with alcohol craving was found in neural circuit of reward regulation, connecting DLPFC, ACC and the striatum. The DLPFC and ACC are regarded as key regulatory regions for reward processing, with the dorsal ACC showing an involvement in monitoring reward and the DLPFC evaluating these stimuli for reward-based decision-making (Haber and Knutson 2010). The DLPFC is also involved in context-dependent processing associated with drug/reward cues (Wilson, et al. 2004). Individuals having alcohol use disorder showed increased activity in DLPFC (George, et al. 2001) compared to healthy social drinkers. Research suggests that altered functional connectivity between DLPFC and striatum significantly contributes to increased craving in individuals with alcohol dependence (Park, et al. 2010). Further, increased activity in the ACC and striatum was associated with high levels of alcohol craving in alcoholic participants (Myrick, et al. 2004). These results, along with our finding, suggests that the neural circuit connecting the striatum, ACC, and DLPFC plays an important role in the modulation of alcohol-cue induced motivation.
Unlike in men, we did not find significant association of brain activity with alcohol craving that survived whole-brain correction in women. This result is similar to previous studies showing alcohol-cue induced craving only in men, but not in healthy socially drinking women (Willner, et al. 1998). As alcohol craving is known to be influenced by hormonal and mood fluctuations (Epstein, et al. 2006; Kraus, et al. 2004; Rubonis, et al. 1994) in women, it is possible that these factors affected the lack of strong neural associations with craving in women. Furthermore, women were drinking at lower levels than men, albeit not significantly less so, and hence recruitment of women with moderate to heavy drinking habits in future studies would be important to identify neural correlates of alcohol craving in women.
Taken together, the current findings provide important neural insights into sex differences in stress-related coping and alcohol-related behaviors. Previous research indicates that men show more automatic and behaviorally-oriented emotional expression and instrumental coping responses, whereas women tend to verbally express their emotion and utilize verbal coping strategies (Barrett, et al. 2000; Brody 1993). Further, women are more likely to engage in conversation, prayer and rumination (Nolen-Hoeksema, et al. 1999; Tamres, et al. 2002) following stress consistent with current data showing engagement of cognitive and verbal brain regions in female stress-induced anxiety. Men’s tendencies of instrumental and action-oriented stress coping, such as smoking and drinking (Lindquist, et al. 1997; Tamres, et al. 2002), are consistent with current data on significant association between striatal activity and alcohol craving as well as greater reactivity in the emotion-action brain regions during stress. It further supports research showing greater male-specific vulnerability for developing alcohol use disorders (Kessler, et al. 1994) and greater female-specific vulnerability to rumination and mood disorders (Kessler, et al. 1993). Thus, the current data on sex-specific neural response to stress and alcohol context cues provide novel insights for understanding sex differences in stress-related vulnerabilities in the development of stress related disorders such as depression and alcohol use disorders.
This research was supported by grants from the NIH Roadmap for Medical Research Common Fund, its Office of Research on Women’s Health and the National Institutes of Drug Abuse (NIDA) and the National Institute of Alcohol Abuse and Alcoholism (NIAAA): UL1-DE019586 (RS), P50-DA016556 (RS), R01-AA13892 (RS), PL1-DA024859 (RS), and RL5-DA024858 (PI: Carolyn Mazure). We also thank Adam K. Hong for his technical assistance for this study.
FINANCIAL DISCLOSURES: All authors do not have direct or indirect financial or personal relationships, interests, and affiliations relevant to the subject matter of the manuscript that have occurred over the last two years, nor that are expected in the foreseeable future. Dr. Sinha is on the Scientific Advisory Board for Embera Neurotherapeutics and is also a consultant for Glaxo-Smith Kline, Pharmaceuticals.