regulation in association with ANS function, treatment related changes and potential underlying mechanism (i.e. catecholamines). Conclusions: Altered ANS function provides a psychophysi- ological mechanism underlying difficulties in emotion regula- tion in adolescent NSSI. Measures of HRV in the study of developmental psychopathology may guide psychiatric assessment and the monitoring of treatment outcome. Keywords: Adolescence, Non-Suicidal Self-Injury, Borderline Personality Disorder, Heart Rate Variability S65. High Temporal Resolution Event-Related Func- tional Spectroscopy Reveals Grossly Abnormal Neuro- transmitter Responses in Pontine Reticular Nuclei in Autism Spectrum Disorder Brendon Nacewicz 1 , Olivia Surgent 1 , and Brittany Travers 1 1 University of Wisconsin School of Medicine and Public Health Background: Atypical excitation:inhibition has been posited as a core deficit of autism spectrum disorder (ASD). To test this in the amygdala, we previously adapted functional magnetic resonance spectroscopy to an event-related design with high temporal resolution (ht-fMRS). We found an inverted or delayed glutamate response and a relation between higher glutamate:GABA and more severe social impairments. In par- allel, our previous work mapped autistic social impairments onto quantitative measures of motor behavior. The micro- structural abnormality in the medial pons mediated this rela- tionship. We designed the current study to investigate excitation:inhibition in the pons with a motor-specific ht-fMRS paradigm. Methods: In the pons-focused ht-fMRS task (modeled after the amgydala task), participants used bilateral, MRI-compat- ible grip strength responders to indicate neutral (hard grip) or emotional (light grip) faces (intertrial interval ¼ 3s). Results: Control participants (3 typically-developing and 3 ADHD) showed an expected GABAergic response that rose during the 3s stimulus presentation and recovered in 1.5s after stimulus offset in the emotion-light grip condition. The neutral- hard grip condition evoked a lessor GABAergic response and a sustained glutamate elevation, similar to the amygdala gluta- mate response. The autism group (9 participants) showed no significant GABA response to either condition and a striking opposite glutamate deflection from controls that is evident in both conditions. Conclusions: Consistent with amygdala findings, grossly inverted glutamate responses, measurable with high-temporal resolution fMRS, are evident in pontine motor regions, including regions mediating emotion-modulated startle, and may be a broad subcortical or global abnormality in ASD. Future analyses will include counterbalanced conditions (neutral-light-grip and emotional-hard-grip). Supported By: The Hartwell Foundation’s Individual Biomedical Award (Travers, PI) Keywords: Magnetic Resonance Spectroscopy, Functional Imaging, Autism, Development, Biomarker S66. A Clinical Trial Investigating the Safety and Toler- ability of Floatation-Rest in Anorexia Nervosa Sahib Khalsa 1 , Scott Moseman 2 , Hung-wen Yeh 1 , W. Kyle Simmons 1 , Martin Paulus 1 , and Justin Feinstein 1 1 Laureate Institute for Brain Research, 2 Laureate Eating Disorders Program Background: Floatation-REST (Reduced Environmental Stimulation Therapy), a novel body-based intervention which attenuates exteroceptive sensory input to the nervous system, is being increasingly utilized as a non-pharmacological tool for reducing anxiety and stress. Anorexia nervosa (AN) is char- acterized by heightened anxiety, distorted body image, and disrupted interoception, raising the question of whether Floa- tation-REST might positively impact these symptoms, but there are no studies documenting the safety or tolerability of this procedure. Methods: This open-label study (clinicaltrials.gov: NCT02801084) examined the physiological and subjective ef- fects of Floatation-REST in weight-restored outpatients with AN. The primary aim was to evaluate the safety and tolerability of this intervention across four float sessions. Orthostatic blood pressure was measured after each session (primary outcome) using a wireless waterproof system. Participants also rated their affective state, body image, and interoceptive sensations before and after each session (exploratory outcomes). Results: Twenty-one AN patients completed the study (average EDE-Q: 2.3+/-1.4, average BMI 22+/-2.7. Primary outcome: there was no evidence of systolic or diastolic orthostatic hypotension after each float in any participant, and no adverse events. We observed significant improve- ments in anxiety (p<0.001, Cohen’sd>1), negative affect (p<0.01, Cohen’sd>0.5), heightened interoceptive aware- ness for cardiorespiratory (p<0.01, Cohen’s d 0.2-0.5) but not gastrointestinal sensations, and reduced body dissat- isfaction ratings (p<0.001, Cohen’s d>0.5) following floating. Conclusions: The findings from this initial trial suggest that individuals with AN can safely tolerate the Floatation-REST environment. The observed improvement in affect and body image disturbance suggests that this intervention might be investigated for potential clinical benefit in more acutely ill patients. Supported By: The William K. Warren Foundation Keywords: Anorexia Nervosa, Anxiety, Interoception, Body Image Disturbance, Negative Affect S67. Biomarker Change During Inpatient Treatment for Anorexia Nervosa and Associations With Weight Outcomes Jessica Baker 1 , Kimberly Brownley 2 , Laura Thornton 2 , Anna Bardone-Cone 2 , and Cynthia Bulik 2 1 University of North Carolina at Chapel Hill, School of Medicine, 2 University of North Carolina at Chapel Hill Poster Abstracts S372 Biological Psychiatry May 1, 2018; 83:S129eS455 www.sobp.org/journal Biological Psychiatry