Multiple neurotransmitter systems are implicated in depressive disorders including the cholinergic systems, which has been found to be hypersensitive in depression whereby depressed patients show exaggerated neuroendocrine and pupillary responses to cholinergic agents, and experience forms of sleep disturbance (decreases latency to REM and increased REM density) that are consistent with increased cholinergic muscarinic sensitivity. Increasing cholinergic activity in manic bipolar (BP) patients induced depressive symptoms, and in major depressive disorder (MDD), worsened symptoms of depression. Clinical Studies: The role of cholinergic system in mood disorders has been highlighted recently through our demonstration that blocking cholinergic muscarinic activity with scopolamine produces rapid antidepressant effects. Our early studies identified an antidepressant effect of scopolamine in both MDD and BD patients. Significant (p<0.001) clinical improvement occurred following a single infusion of scopolamine. Over the course of the clinical trial, 70% of patients experience a full response (50% reduction in symptoms) and over 50% experience remission of symptoms. A replication of our original finding recently was published in an independent sample of patients with MDD. Moreover, we have reported that while men and women show significant clinical improvement following scopolamine, women show a larger response than do men (p<0.01). Recently, a pharmacokinetic study was initiated to identify an alternative route of administration of scopolamine. The NIH pharmacy developed a nasal spray formulation, and we currently are collecting pharmacokinetic data to identify the nasal spray dose that most closely approximates the area-under-the-curve obtained with the i.v. administration used to date. Clinical trials utilizing this route of administration will follow. Cognitive and Imaging Studies Behavioral and cognitive features of depression are associated primarily with the processing of affective information. A consistently reported finding is a mood congruent processing bias, which is defined as a tendency to show a bias for processing negative as compared to positive or neutral information. Memory studies show that MDD patients recall more negatively toned than positively toned information. In attention paradigms, depression-related words produce more interference on emotional stroop tasks than do happy or neutral words. The mood congruent processing bias observed in MDD can be characterized within the framework of cholinergic system and stimulus processing mechanisms. The biased processing of negative or sad information is consistent with an over-active cholinergic system in depression resulting in the over-representation of negative information. We would hypothesize that competition among competing stimuli in the environment engages cholinergic system, and the overactive system alters the bias preferentially towards negative stimuli in MDD. We are utilizing functional brain imaging methods to elucidate the role of the cholinergic system in stimulus processing biases. We characterized behaviorally a stimulus processing bias associated with a selective attention task where face and house stimuli are presented simultaneously. Faster reaction time when attending to faces (vs houses) reflects a stimulus process bias towards faces. Scopolamine selectively increased reaction time when attending to face stimuli with no change when attending to houses, reflecting a stimulus specific shift in the processing bias. Functional brain imaging results show that the brain regions in visual processing areas which show a bias towards faces (larger response to faces than to houses) during placebo, reduce the bias during scopolamine, a result that complements the behavioral finding. In brain regions that show a bias for processing houses during placebo, no change is observed following scopolamine. As the effects of cholinergic inhibition in the context of selective attention are stimulus specific, the cholinergic influence on selective attention occurs primarily via stimulus processing mechanisms. Acetylcholine also is important to working memory (WM) function and particularly is critical to the encoding of information through stimulus processing mechanisms. We evaluated the influence of blocking cholinergic muscarinic activity on neural responses during WM encoding as the attended stimulus feature is modulated. Healthy volunteers and patients with MDD participated in an fMRI study where a picture of a face was shown (encoding) followed by a delay component (maintenance) then another picture of a face (test/retrieval). Participants were instructed to match the test stimulus to either the identity or the emotional expression of the encoded face. Neural activity associated with encoding was estimated for the emotion and identity task conditions. We found that blocking cholinergic muscarinic receptors preferentially reduced BOLD response during encoding selectively when participants were attending to the emotional content of the stimulus, with no change when attending to identity. As emotion potentially is a more salient facial feature than identity, this finding suggests that the more salient stimulus feature is more susceptible to cholinergic modulation. Patient data currently are being analyzed. Biomarker of Treatment Response Clinical Markers of Treatment Response. We evaluated the potential for baseline clinical ratings to predict treatment response to scopolamine. We conducted a discriminant function analysis to determine if a linear combination of self-report mood-ratings could discriminate treatment responders from non-responders. Moreover, we used a validation procedure to determine if we could predict at baseline whether individual patients would be a responder or a non-responder following scopolamine treatment. The disciminant function identified a set of 4 or 5 clinical ratings that, when combined into a linear function, classified 86% of unipolar patients as responders or non-responders (p= 0.002);the discriminant function scores differed significantly between the responders and non-responders (p<0.001). The validation analysis was able to correctly predict classification in over 70% of MDD patients. Similarly, the discriminant function classified 88% of BP patients, and the discriminant fuction scores differed significantly between the responders and non-responders (p<0.001). The validation analysis in the BP group was able to correctly predict classification in over 85% of the patients. These results indicate that baseline self-report mood-ratings obtained prior to treatment can predict response outcome to scopolamine. Functional Imaging Markers of Treatment Response. We evaluated whether the functional brain response during WM may predict the antidepressant response to scopolamine. A group of MDD patients participated in a fMRI study using the WM task described above. Imaging data were obtained at baseline and following scopolamine. Neural activity (at baseline) associated with each separate task component was correlated with the magnitude of treatment response at study end. Correlations (p<0.01) were seen in visual processing areas for the encoding and retrieval components of WM, selectively during the emotion task with no correlation in the identity task. Following treatment with scopolamine, changes in neural response in these same brain regions also correlated with the magnitude of response. We show that baseline levels of neural activity in visual processing areas reflect potential for response to treatment with scopolamine. The cortical regions that show baseline predictive value for treatment response also respond to cholinergic modulation, which argues that baseline differences are cholinergically mediated.
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