This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Major depressive disorder (MDD) has been associated with abnormally reduced function of central serotonergic systems. The tryptophan depletion (TD) paradigm has been widely used to study serotonergic function in depression. TD, achieved by oral loading with all essential amino acids except the serotonin (5-HT) precursor tryptophan, causes a transient reduction in the synthesis of brain 5-HT leading to a temporary lowering of mood, which may be a trait abnormality in depression. The proposed pilot study will compare the mood lowering response to TD in a total of 10 healthy subjects with and without a family history of mood disorders, designated as high and low risk for MDD respectively. We will test the hypothesis that TD will induce a greater mood lowering response in high-risk (n=5) than in low-risk individuals (n=5). We have also obtained preliminary evidence that the s allele of the serotonin transporter gene promoter polymorphism (5-HTTLPR) and a positive family history of depression are additive risk factors for the development of depression during TD.
We aim to replicate these findings in a larger female sample and extend it to males. In addition, whenever a high-risk subject has a sibling with unmedicated remitted MDD, the sibling will also be enrolled in the study (n=1 for this pilot study). Recent findings of increased amygdala reactivity to emotional faces in healthy carriers of the s allele of the 5-HTTLPR polymorphism may indicate increased susceptibility to mood disorders. We propose to examine the effects of TD on amygdala reactivity to emotional faces in high- and low-risk subjects and in subjects grouped by genotype, as measured by functional magnetic resonance imaging (fMRI). We also plan to follow subjects longitudinally to determine whether a mood lowering response to TD in the high-risk group can predict increased risk for first-onset of MDD over time. The principal goal of this project is obtain pilot data for a grant proposal. The bulk of this study will take place during the first year, with procedures including a screening interview with medical clearance and neuropsychological testing, genotyping, two test days (TD and sham depletion), and fMRI on both test days, and clinical ratings taken on each visit. Due to the nature of a longitudinal study however, subjects will be followed biannually for 2 years. For this reason, we are applying for a 4-year study through the GCRC. Hypothesis:
Specific Aim #1 : To compare the behavioral response to TD in healthy subjects with a positive family history of mood disorders (at 'high risk' for MDD) and healthy subjects with a negative family history of mood disorders (at 'low risk' for MDD), and to explore potential gender differences in behavioral response to TD. Hypothesis #1: TD will induce a greater mood lowering response in subjects at high-risk for MDD than in subjects at low-risk for MDD.
Specific Aim #2 : To assess the relationship between behavioral response to TD and the 5-HTTLPR polymorphism in healthy subjects at high and low risk for MDD.
This aim i s exploratory due to the preliminary nature of data on associations between response to TD and the 5-HTTLPR. Hypothesis #2: We hypothesize that the behavioral response to TD will differ based on genotype. In addition, we will explore the relationship between behavioral response to TD and polymorphisms in other serotonergic genes implicated in depression, including the 5HT2A and 5HT2C receptor genes, and the monoamine oxidase type A enzyme (MAOA) gene promoter.
Specific Aim #3 : Using fMRI, assess amygdala reactivity to presentation of sad and fearful faces during TD in healthy subjects at high and low risk for MDD.
This aim i s exploratory because there have been no prior fMRI studies of amygdala reactivity to emotional faces during TD in healthy subjects at high and low risk for MDD. We will explore the relationship between family history, the 5-HTTLPR polymorphism, and amygdala reactivity during TD.Hypothesis #3: During TD, subjects at high risk for MDD will exhibit greater amygdala reactivity to sad and fearful faces than subjects at low risk for MDD.
Specific Aim #4 : To determine whether behavioral response to TD will predict increased risk for a first episode of MDD in high-risk subjects during prospective longitudinal follow-up. Hypothesis: #4: High-risk subjects who exhibit a significant mood lowering response during TD (at least a 10-point decrease on the depression subscale of the Profile of Mood States -POMS) will be at increased risk for a first episode of MDD during the follow-up period, compared to individuals with mild or no mood lowering response during TD.
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