S145. Association Between Impulsivity and Suicide Behaviors in Female Veterans Erin McGlade 1 , Elliott Bueler 2 , Chandni Sheth 2 , Margaret Legarreta 4 , and Deborah Yurgelun-Todd 5 1 University of Utah, MIRECC, 2 University of Utah, 4 MIR- ECC, Department of Veterans Affairs, 5 University of Utah, Department of Veteran Affairs, Rocky Mountain MIRECC Background: Women constitute the fastest-growing contin- gent of the military veteran population and report to the VA with a variety of physical and psychiatric symptoms. Suicidal be- haviors in female veterans are an area of national concern, yet limited research has focused on how impulsivity is related to suicidal behaviors by sex. Methods: Participants consisted of 48 female and 181 male veterans ages 18-55. Each completed the Columbia Suicide Severity Rating Scales, Barratt Impulsiveness Scale (BIS), Hamilton Anxiety Scale, and Hamilton Rating Scale for Depression. Results: Male veterans showed significant correlations be- tween lifetime history of suicidal behaviors (including ideation and attempts) and BIS Planning, Motor, Attention, and Total. Conversely, females only showed associations between his- tory of suicidal behaviors and BIS Motor and Total. Female and male veterans did not show significant differences in symp- toms of depression, anxiety, total impulsivity, or history of suicidal behaviors. Conclusions: Results suggest that impulsivity is related to suicidal behaviors differently by sex. For male veterans, difficulty planning, maintaining attention, and doing things without thinking were related to suicidal be- haviors. However, for female veterans saying things without thinking, difficulty sitting still, and acting on the spur of the moment also were related to suicidal behavior. Prior research has shown biological underpinnings related to sex differences in impulsivity, including differences in brain morphometry and resting state functional connectivity in the frontal cortex. Hor- mone levels including cortisol and testosterone also have been related to impulsivity and suicidal behaviors. Further research should focus on biological factors related to impulsivity and suicide by sex in veterans. Supported By: Rocky Mountain MIRECC, Salt Lake City VA Keywords: Sex Differences, Suicide Behavior, US Veterans S146. The Antidepressant Effects of Ketamine on Lateral Habenula Activity Require Endogenous Opioid Signaling Matthew Klein 1 , Joshua Chandra 2 , and Roberto Malinow 2 1 UCSD School of Medicine, 2 UCSD Background: Depression leads to significant impairment in daily function. The prolonged time course necessary for cur- rent pharmacological treatment may extended suffering and increase risk of suicide. Acute administration of ketamine rapidly alleviates depressive symptoms and reduces suici- dality. Ketamine is a non-competitive antagonist of the NMDA receptor but also displays activity on a wide variety of neuronal receptors. Recent studies suggest that the effects of ketamine may be mediated by a variety of systems including AMPA, and opiate receptors. We used an inbred line of congenital learned helplessness (cLH) rats, a validated model of Major Depressive Disorder (MDD), to examine the synaptic effects of ketamine adminis- tration. This model displays hyperactivity in the lateral habe- nula (LHb), an epithalamic nucleus that inhibits activity of the Ventral Tegmental Area (VTA). Hyperactivity in the habenula leads to decreased dopamine release from the VTA, and manifestation of anhedonia and avolition. In this model, acute ketamine administration results in a reduction in LHb activity, and improved performance in metrics of depressive-like symptoms. Methods: We used a variety of systems neuroscience tech- niques, including ex vivo calcium imaging, to examine alter- ations in habenular signaling resulting from ketamine administration Results: Our results demonstrate that ketamine administra- tion is effective in reducing depression-like symptoms, and LHb activity, in cLH rats. These beneficial effects on behavior and neural activity are blocked by prior administration of the opioid antagonist Naltrexone. Conclusions: The antidepressant effects of ketamine administration in cLH rats are likely mediated by an acute reduction in LHb activity that requires intact, endogenous opioid signaling. Supported By: R01-MH091119-07; NIMH; R. Malinow, A Synaptic Locus Controlling Behavioral Depression R25-MH101072-05; NIMH; N. Swerdlow, Psychiatric Research Residency Training Track Keywords: Ketamine, Treatment Resistant Depression, Lateral Habenula, Opioid System, Learned Helplessness, Systems Neuroscience S147. Evidence for the Involvement of the Serotonergic System in the Therapeutic Mechanism of Deep Brain Stimulation of the Subcallosal Cingulate (SCC-DBS) for Treating Depression Elliot Brown 1 , Darren Clark 1 , Andrea Protzner 2 , Rajamannar Ramasubbu 1 , and Zelma Kiss 1 1 Mathison Centre for Mental Health Research & Education, Hotchkiss Brain Institute, University of Calgary, 2 University of Calgary Background: Deep brain stimulation (DBS) of the subcallosal cingulate (SCC) is a promising investigational treatment for depression, but little is known about the therapeutic mecha- nism. Preclinical studies suggested a role for serotonin in the antidepressant effects of SCC-DBS. Here we aimed to deter- mine whether an electrophysiological indicator of central serotonergic activity, the loudness dependence of the auditory evoked potential (LDAEP), predicts outcome and whether it changes after chronic SCC-DBS. Methods: The LDAEP was measured with EEG at baseline in 12 patients who underwent SCC-DBS, and again at 6 and 12 months after surgery in 7 of these patients. Poster Abstracts Biological Psychiatry May 15, 2019; 85:S130eS379 www.sobp.org/journal S353 Biological Psychiatry: Celebrating 50 Years