Vitamin D in OUD: Exploration of Alterations on the Dopamine D2/D3 Receptor System

Inclusion Criteria: 1. Age 18-50 years 2. Voluntary, written, informed consent 3. Physically healthy by medical history, physical, neurological, ECG, and laboratory examinations 4. Meets the criteria for OUD, as determined by the Structured Clinical Interview for DSM-5 (SCID) 5. Stable and receiving buprenorphine treatment for OUD 6. For females, non-lactating, with a negative serum or urine pregnancy (hCG) test 7. Lab results without clinically relevant findings (e.g., renal function, electrolytes, and vitamin D levels) 8. English speaking Exclusion criteria: 1. Medical contraindication to calcitriol administration (e.g., history of hypersensitivity to calcitriol or any component of the formulation, hypercalcemia or vitamin D toxicity) 2. History of substance dependence (e.g., alcohol, sedative hypnotics), except for nicotine and opiates. 3. A primary major DSM-5 psychiatric disorder (e.g., schizophrenia, bipolar disorder, major depression, etc.) as determined by the SCID, except for Opioid Use Disorder and related conditions. 4. A history of significant medical (e.g., cardiovascular, diabetic/metabolic) or neurological (e.g., cerebrovascular accidents, seizure, traumatic brain injury) illness 5. Positive answers on the cardiac history questionnaire that may place the subject at higher risk, as determined by the study physician's review of both the questionnaire responses and screening ECG. If there is concern for the subject's safety due to these assessments, research staff will consult a Yale PET Center affiliated cardiologist prior to including the subject for the study. 6. Current use of psychotropic and/or potentially psychoactive prescription medications 7. Receiving medications for OUD other than buprenorphine (e.g., methadone treatment) 8. For females, laboratory (β-HCG) or physical evidence of pregnancy/lactation 9. MRI-incompatible implants and other contraindications for MRI (i.e., aneurysm clip, metal fragments, internal electrical devices such as a cochlear implant, spinal cord stimulator or pacemaker) 10. History of claustrophobia or feeling of inability to lie still on his/her back for the PET or MRI scans 11. History of any bleeding disorder or current anticoagulant therapy 12. Donation or loss of 550 mL of blood or more (including plasmapheresis) or receipt of a transfusion of any blood product within 8 weeks prior to the first test day. 13. Use of any prescription medications and/or over-the-counter medications, vitamins and/or herbal supplements which could have a negative clinical interaction with calcitriol or which could confound scientific results of the study, within 2 weeks prior to each test day (e.g., thiazide diuretics, Mg based antiacids, digoxin, etc,). 14. Serum levels of 25(OH)D3 below 12 ng/ml. 15. Morbid obesity i.e., BMI over 35 (more prone to lower vitamin D levels) 16. Subjects with history of prior radiation exposure for research purposes within the past year such that participation in this study would place them over FDA limits for annual radiation exposure. This guideline is an effective dose of 5 rem received per year. 17. Subjects with current, past or anticipated exposure to radiation in the workplace 18. History of kidney stones within the past 5 years 19. Any degree of renal failure 20. History of parathyroid disorder (hyper or hypoparathyroidism) 21. History of osteoporosis or any pathologic fractures 22. Vitamin D supplementation in any form in the past 3 months 23. Known hypersensitivity to \[11C\]-PHNO or calcitriol 24. Malabsorption syndromes (i.e., Celiac sprue) 25. Serum corrected calcium \> 10.5 mg/dl or phosphate \> 4.2 mg/dL