This investigation yielded three main results. First, lorazepam did not affect risky behavior at the doses tested. Second, lorazepam dose-dependently attenuated activation in (a) the amygdala and medial prefrontal cortex during the decision phase and (b) in the bilateral insular cortex and amygdala during the outcome (i.e., rewarded or punished) phase. Third, a lorazepam-induced activation increase in insular cortex was associated with less risky responses. Taken together, this investigation supports the idea that GABAergic modulation in limbic and paralimbic structures, which include bilateral amygdala, insula and medial prefrontal cortex, is important during both the response selection (i.e., decision) and outcome phase of risk-taking decision-making.
Both lorazepam doses tested in this study did not affect risk-taking behavior. These results contrasts with a previous study, which showed that alprazolam dose-dependently decreased risky responses (Lane, Tcheremissine, Lieving, Nouvion, and Cherek, 2005
). These apparently contradictory findings may be reconciled by highlighting several differences between that study and the current investigation. First, in the Lane et al. (2005b) study, alprazolam did not robustly affect risk-taking unless a dose of 2 mg was used, which would correspond to approximately 4 mg of lorazepam. Thus, the doses used in the current study may not have been sufficiently high to affect behavior. It should be noted that higher doses of lorazepam affect alertness (Greenblatt, Harmatz et al., 1988
) and other cognitive functions such as memory (Sperling, Greve et al., 2002
). Therefore, one may not be able to differentiate the behavioral effects on risk-taking from those on sedation. Second, subjects in this study had no history of any DSM-IV Axis I diagnosis. In contrast, several subjects in the above mentioned study (Lane, Tcheremissine, Lieving, Nouvion, and Cherek, 2005
) were reported to have a history of drug abuse/dependence. We have previously shown that individuals who are at higher risk for stimulant use show increased risk-taking behavior (Leland and Paulus, 2005
). This suggests that individuals in Lane and colleagues' study may have had a higher baserate of selecting risky responses, and/or that their risk-taking propensity may interact with the effects of a benzodiazepine. Third, there are significant task-related differences between the paradigm used by Lane and colleagues and the risky-gains task. Although it is important to probe risk-taking using different types of decision-making tasks due to the variability of such behavior in different contexts (Monterosso, Ehrman et al., 2001
), these behaviors may be differentially sensitive to GABAergic modulation. Finally, we did not find a selective dose-dependent difference to punishment, which may be due to the fact that the overall effect of punishment on decreasing subsequent risky responses was modest. Lastly, there may have been the possibility that some risk-taking preferences may have been in transition across the three testing sessions. Thus, if risk levels were unstable during the primary dosing and testing, it could have obscured the acute drug effects on risk-taking behavior.
Lorazepam had the expected effect on decision (risk)-related processing in the amygdala and the medial prefrontal cortex / anterior cingulate. However, in contrast to our hypothesis, lorazepam dose-dependently increased
risk-related activation in the insular cortex which, in the left hemisphere, was inversely associated with the rate of risky responses. Consistent with our previous findings (Paulus, Rogalsky, Simmons, Feinstein, and Stein, 2003
), heightened insular cortex activation may be related to increased anticipation of aversive outcomes when processing the consideration of selecting a risky response. In contrast, the dose-dependent attenuation of amygdala activation during the response selection phase may reflect an altered appraisal of inherent value of the risky choice (Kahn, Yeshurun, Rotshtein, Fried, Ben Bashat, and Hendler, 2002
). Other investigators have emphasized the importance of the amygdala for the subjective assessment of incentive value of the choice (Arana, Parkinson et al., 2003
), as opposed to the role of the insular cortex, which is thought to instantiate somatic markers (Bechara, 2001
). According to the somatic marker hypothesis (Damasio, 1996
), certain stimuli initiate a state that is associated with pleasurable or aversive somatic markers which serve as a guide to bias the selection towards certain (pleasurable) actions. Moreover, the amygdala, in the context of choice selection, has also been implicated in processing emotional consequences of the possibility of alternative outcomes, which is often referred to as regret (Coricelli, Critchley et al., 2005
), and is related to choice-related processing of missing or unavailable information (Hsu, Bhatt et al., 2005
). We speculate that lorazepam-related dose-dependent attenuation of the amygdala during response-related processing may have counterbalanced the increase in insular cortex activation and result in no observable behavioral changes of risky response selection. Future studies may need to examine the effect of lorazepam on the functional connectivity between the amygdala and insular as well as the amygdala and medial prefrontal cortex to better delineate the role of GABA in risk-related behavior.
The dose-dependent attenuation of risk-related activation in the medial prefrontal cortex / anterior cingulate is consistent with the idea that lorazepam decreases conflict processing. Various types of conflict tests have been used in animal studies of anxiety-related behaviors (Millan, 2003
). In particular, a recent study using a new conflict procedure with rhesus monkeys showed a high correlation between therapeutic potencies and the ability of benzodiazepines to increase suppressed responding. The cingulate cortex has been hypothesized to contribute to the internal monitoring of planning and controlling functions of the prefrontal cortex as well as the regulation of affective signals coming from limbic areas (Frith and Done, 1988
). Others have attributed a conflict-monitoring function to this structure (Botvinick, Nystrom et al., 1999
) due to its role implementing strategic processes to reduce cognitive conflicts (Carter, Macdonald et al., 2000
). The anterior cingulate cortex is also important for assessing the value of an action for obtaining a predicted outcome (Richmond, Liu et al., 2003
). Moreover, this medial prefrontal structure may have an important role in maintaining action-outcome associations, when the action is only probabilistically associated with an outcome (Rushworth, Walton et al., 2004
). Taking into account the prominent role of the anterior cingulate during cognitive and action monitoring, GABAergic modulation of this structure during risk-related response processing may reflect the degree to which a response alternative signals conflict and initiates the utilization of strategic processes to reduce such state (i.e., conflict). In contrast to animal conflict procedures, processing of risk-related responding during the risky gains decision-making task incorporates both conflict (the possibility of a punished outcome) and uncertainty. Recent studies indicate that risk (probabilistic aversive outcomes) and ambiguity (incomplete information) may depend on partially distinct neural substrates (Hsu, Bhatt, Adolphs, Tranel, and Camerer, 2005
;Huettel, Stowe et al., 2006
). Therefore, future investigations of GABAergic modulation may need to utilize paradigms that disambiguate uncertainty from punished responding to better delineate the effects of benzodiazepines on these processes in humans.
Fear is an adaptive response to potentially dangerous, i.e., risky, stimuli that threaten to perturb homeostasis (Millan, 2003
). GABAergic neurons constitute the major mode of inhibitory transmission throughout the CNS, acting as a “brake” under conditions of stress (Millan, 2003
). Interestingly, the highest densities of benzodiazepine receptors in human brain have been reported in cortical, limbic and paralimbic areas (Zezula, Cortes et al., 1988
;Marowsky, Fritschy et al., 2004
;Quirk and Gehlert, 2003
). Acute administration of lorazepam has strong dose-related effects on sleep, mood, drug liking, and abuse liability (Roache and Griffiths, 1987
) as well as cognitive functions such as learning (Rush, Higgins et al., 1993
) and attention (Rush, Higgins et al., 1994
), which is associated with attenuation of fronto-temporal activation (Coull, Frith et al., 1999
). However, the effects of benzodiazepines on fear-related experimental paradigms have been mixed (Riba, Rodriguez-Fornells et al., 2001
;Baas, Grillon et al., 2002
).We have previously shown that lorazepam dose-dependently attenuates amygdala and insular cortex during emotional face processing (Paulus, Feinstein, Castillo, Simmons, and Stein, 2005
) and the current study extends this finding to include attenuation of risk-related processing.
Of interest, lorazepam did not affect striatal activation (either dorsal or ventral) related to gain versus loss when contrasting the outcome-related activation for selecting a risky response and receiving 40 or 80 points versus being punished by losing 40 or 80 points. This observation supports the general idea that lorazepam did not indiscriminately attenuate brain activation. This finding is consistent with our previous results, which suggested that lorazepam did not affect primary visual cortex activation (Paulus, Feinstein, Castillo, Simmons, and Stein, 2005
This investigation had several limitations. First, the lack of a behavioral effect of lorazepam on risk-related response selection makes it more difficult to interpret dose-dependent activation differences because we were unable to relate a lorazepam-induced behavioral change to a neural activation change. However, activation differences that are not accompanied by behavioral differences can be very informative because they speak to the neural processes that may underlie the overtly observed behavior. Although we acknowledge that the observed neural activation differences may be due to a number of different factors (which cannot be disambiguated with the current experimental design and with BOLD-fMRI technology) they are, nevertheless, important. In particular, some investigators have proposed a “risk-as-feelings” hypothesis, which highlights the role of affect experienced at the moment of decision making (Loewenstein, Weber et al., 2001
). The differential activity in limbic and paralimbic structures is consistent with the notion that lorazepam altered the affective state associated with risk-related behavior. Therefore, lorazepam may not change risky behaviors (on this task) but may affect the feelings associated with selecting a risky alternative. Future studies will need to better delineate these effects. Moreover, the lack of behavioral differences eliminates possible performance confounds on the effects of lorazepam. The results presented in this manuscript are limited to explain risk-related brain processes and future investigations may apply other risk-taking decision-making paradigms better suited to produce significant interactions between lorazepam and behavior. Second, our experimental design lacked of a baseline risk-taking session prior to dosing, which raises the possibility that some risk-taking preferences may have been temporally unstable. These instabilities could obscure the acute drug effects on risk-taking behavior. Third, the risky gains decision-making task does not allow for differentiation between uncertainty-related processing and reward or punishment-related processing. A recently published article reported several regions (including the anterior insula) that showed a selective increase in activation uncertainty when compared to risky decision making (Huettel, Stowe, Gordon, Warner, and Platt, 2006
). Future studies should develop adequate paradigms to clarify the commonalities and discrepancies between the uncertainty and risk-taking neural networks. Fourth, the AFNI 3dDeconvolve program used to calculate the response amplitude assumes that the overlapping BOLD responses accumulate with linear additivity and, potentially, nonlinear effects may contribute to the altered hemodynamic responses. However, in computational simulation, we found little accumulation of the hemodynamic response. This is primarily due to the fact that our regressors are based on the subjects' response latency and the individual variations from trial to trial. Moreover, if nonlinearities (due to the task set up) are contributing to the effect, this would be similar across the different treatment conditions. Fifth, the Monte-Carlo threshold adjustment method (Forman, Cohen, Fitzgerald, Eddy, Mintun, and Noll, 1995
) used to extract the activation clusters has been reported to possibly underestimate the spatial extent of autocorrelation or the variability of the autocorrelation function, thus, possibly interfering with the critical levels for a significant cluster size (Petersson, Nichols et al., 1999
). Finally, the lack of jittering in our paradigm may cause a certain degree of overlap between the decision and outcome periods. Therefore, other paradigms will need to be used in the future to better delineate whether lorazepam differentially affects uncertainty or reward/punishment processing.
In summary, this investigation used three techniques - human psychopharmacology, functional neuroimaging, and an experimental paradigm targeted to probe risk-related responding - to elucidate the role of GABAergic modulation on brain structures important for processing risk. The results are consistent with the hypothesis that GABAergic modulation attenuates activation of neural substrates signaling aversive outcomes, which may help to explain why these drugs are anxiolytic. Future investigations will need to determine whether these effects are specific to benzodiazepines, are characteristic of other anxiolytic drugs, and can be used to predict potential anxiolytic efficacy of novel compounds.