Preclinical research indicates acute opioid administration evokes an immune response in the periphery and brain. Here, we will translate those preclinical findings to healthy human volunteers and quantify the neuroimmune response to a morphine challenge using positron emission tomography (PET) imaging with [11C]PBR28.

Subjects will be recruited from the local community via media advertisements, flyers, and word-of-mouth. Interested individuals will undergo a phone screen and in-person medical and psychiatric examination. Up to 20 eligible individuals (see Inclusion/Exclusion criteria) will be invited to participate in this study.

In a single day, subjects will complete behavioral and physiological testing, a [11C]PBR28 PET scan, and report subjective drug effects before and after a morphine challenge. Subjects will complete either a 'High' or 'Low' morphine dose condition (single-blind): 0.07mg/kg i.m. vs. 0.04mg/kg i.m., respectively. To measure the neuroimmune response to morphine, we will use [11C]PBR28 PET imaging (120-minute scans on a High Resolution Research Tomograph with Vicra motion correction). [11C]PBR28 binds with high affinity and specificity to the 18kDa translocator protein (TSPO), which is highly expressed in microglia and has been shown to respond to inflammatory challenges. TSPO volumes of distribution (VT), i.e., TSPO availability, will be quantified in brain regions of interest using multilinear analysis-1 (MA-1) with the metabolite-corrected arterial input function. The post-morphine [11C]PBR28 PET scan will occur 2-hours after the morphine challenge.

Specific Aim 1: To determine whether an acute morphine administration increases brain TSPO availability in healthy volunteers.

Hypothesis 1: Relative to pre-morphine levels, morphine will significantly increase TSPO availability across brain regions of interest, consistent with a neuroimmune response.

Specific Aim 2: To determine whether morphine evokes a dose-dependent increase in brain TSPO availability in healthy volunteers.

Hypothesis 2: Relative to pre-morphine levels, morphine will dose-dependently increase TSPO availability across brain regions of interest.

Specific Aim 3: To determine whether morphine administration increases peripheral markers of inflammation, e.g., cytokine/chemokine concentration in plasma.

Hypothesis 3: Relative to pre-morphine levels, morphine will increase cytokine/chemokine concentrations in plasma, including IL-1B, IL-2, IL-6, IL-10, TNF-a, IFNy, MCP-1, and GM-CSF, consistent with a peripheral immune response. Exploratory Hypotheses: 1) IL-1B, IL-6, TNF-a, IFNy, MCP-1, and GM-CSF will exhibit morphine dose-dependent increases in plasma. 2) The change in IL-6, TNF-a, IFN-y, and GM-CSF levels will be positively correlated with the change in brain TSPO VT levels.

Specific Aim 4: To determine whether morphine administration alters pain sensitivity, pain tolerance, cognitive function, and reward responsiveness.

Hypothesis 4: Relative to pre-morphine levels, morphine will enhance pain tolerance and impair verbal learning/memory proficiency and impair reward responsiveness. Morphine will not alter pain sensitivity, visual attention, psychomotor processing speed, or working memory proficiency. Exploratory hypotheses: 1) The change in TSPO availability in thalamus will be positively correlated with the change in pain tolerance. 2) The change in TSPO availability in hippocampus will be inversely correlated with the change in verbal learning/memory proficiency. 3) The change in TSPO availability in caudate, ACC, and OFC will be inversely correlated with the change in reward responsiveness.

Specific Aim 5: To determine whether morphine administration alters vital signs.

Hypothesis 5: Relative to pre-morphine levels, morphine will dose-dependently reduce systolic and diastolic blood pressure. Relative to pre-morphine levels, morphine will lower heart rate.

Specific Aim 6: To measure the subjective response to morphine administration.

Hypothesis 6: Relative to pre-morphine levels, morphine will dose-dependently increase subjective ratings of 'high', 'good drug effect', 'nausea', and 'bad drug effect'. Exploratory hypotheses: 1) The change in TSPO availability in caudate, ACC, and OFC will be positively correlated with the change in 'high' and 'good drug effect'. 2) The change in TSPO availability in the insula and thalamus will be positively correlated with the change in 'nausea' and 'bad drug effect'.

Specific Aim 7: To determine whether morphine administration alters peripheral stress markers of the autonomic nervous system and HPA-axis.

Hypothesis 7: Relative to pre-morphine levels, morphine will dose-dependently increase plasma levels of epinephrine, norepinephrine, and cortisol.

Specific Aim 8: To determine whether morphine administration alters peripheral markers of neurosteroids.

Hypothesis 8: Relative to pre-morphine levels, morphine will dose-dependently increase plasma levels of allopregnanolone and pregnenolone.

Specific Aim 9: To determine whether morphine administration alters peripheral markers of the metabolic hormone ghrelin.

Hypothesis 9: Relative to pre-morphine levels, morphine will dose-dependently increase plasma levels of ghrelin.

PBR_Morphine Study

Inclusion Criteria:

1. Men and women aged between 21 and 50 years (driver's license or valid state ID).
2. Physically healthy by medical history, physical, neurological, EKG and laboratory examinations (reviewed by the Study Physician).
3. Normal weight, as indicated by a body mass index (BMI) and body weight ≤ 250lbs.
4. Read, comprehend, and write English at a sufficient level to complete study-related materials.
5. Able to provide voluntary and written informed consent.
6. Eligibility and willingness to participate in study procedures, including MRI and PET scanning.
7. Previous medical use of opioids without adverse reactions (≥2 lifetime uses).
8. Medically eligible to receive up to 0.1 mg/kg deltoid intramuscular morphine (based on medical history, medical exams, and not meeting any exclusion criteria below).
9. Medically eligible to receive 10mg of oral metoclopramide based on medical history, medical exams, and current medications.

Exclusion criteria:

1. Any DSM-5 Axis I disorder diagnosis based on Structured Clinical Interview for DSM-5 (SCID-5), including meeting criteria for substance dependence.

2. Any current psychotropic medication use, including MAOI use within the past 14 days

3. Recent (past 6 months) medical or non-medical opioid-use.

4. Prior medical use prescription opioids for >14 consecutive days (self-report)

5. Prior non-medical use of any opioid (i.e., recreational opioid use will be excluded).

6. Positive result on a urine drug screen (excluding marijuana).

7. Current or previous chronic pain disorder (>6 months of continuous pain).

8. 'Low affinity binding' individuals based on rs6971 polymorphism (<10% of the population).

9. For females, pregnancy (positive urine test).

10. Current use of non-steroidal anti-inflammatory medications or statins.

11. Medical contraindication to receive up to 0.1 mg/kg intramuscular morphine administration as determined by Study Physician. This includes:

    1. known hypersensitivity/allergy to morphine;
    2. acute or severe bronchial asthma;
    3. known or suspected gastrointestinal obstruction, including paralytic ileus;
    4. seizure disorder;
    5. concomitant use of a benzodiazepine or any other CNS depressant;
    6. any other significant medical condition, that in the opinion of the Study Physician and Investigators, could: put the patient at risk because of participation in the study, or influence the results of the study, or cause concern about the patient's ability to successfully complete in the study.

12. Known family history (first-degree relative) of opioid-use disorder or alcohol-use disorder.

13. MRI contraindications, including metal in body (or work in metal/machine shop), pacemaker, claustrophobia, or inability to tolerate MRI scanning.

14. Medical contraindications to metoclopramide, as determined by the Study Physician, including:

    1. known hypersensitivity/allergy to metoclopramide;
    2. mechanical gastrointestinal obstruction, perforation, or hemorrhage;
    3. seizure disorder;
    4. history of tardive dyskinesia;
    5. or concomitant use of medications/agents likely to increase extrapyramidal reactions.

De-identified participant data will be shared upon reasonable request.

Study supporting information will be available upon reasonable request after publication of the primary manuscript.

Available to research scientists and clinicians for academic uses only.

Subjects are informed that they will receive a single morphine injection. Subjects are blinded to the morphine dose condition. Investigators and assessors are not blinded to morphine dose condition.