Effects Of Intravenous Oxytocin On Peripheral Sensory Afferents Using Microneurography

This protocol will utilize a randomized, triple-masked study design to compare intravenous (IV) oxytocin to placebo on peripheral nerve response to acute pain sensitization in healthy volunteers. Equal numbers of adult men and women; ages 18-65 will be recruited. Participants, recruited from the regional community will report to the Linköping University Neurophysiologic Research Laboratory (Linköping, Sweden) for a 2-hour visit to complete eligibility screening, informed consent, pregnancy and drug use laboratory tests, and questionnaires regarding drug and alcohol use, and receive training in the tests to be used on the study day.

On a subsequent study day, participants will come initially to the Linköping University Medical Center where an IV catheter will be inserted and they will receive either IV oxytocin (20 IU) or an equal volume of placebo. Study drug will be randomized and administered in two 30-minute infusions separated by the minutes. Rapid rates of oxytocin infusion can cause acute adverse events in the first minutes of infusion. Although these adverse events rapidly resolve despite continuing the infusion, one event (facial flushing) could negate the blind of study personnel. For that reason, the first infusion will be given at 2 rates, a very slow rate for the first 5 minutes and a greater rate for the remaining 25 minutes. This has been demonstrated to reduce the incidence of facial flushing with oxytocin to a rate similar to placebo. Participants will be continuously monitored for vital signs and intermittently queried for any subjective sensations or adverse events during infusion.

After study drug infusion is complete, the IV catheter will be removed and the participant accompanied to the Linköping University Neurophysiologic Research Laboratory, adjacent to the Medical Center, where they will complete psychologic questionnaires and cognitive tests and undergo sensory testing of the skin on the arm of brushing using a computer controlled, robotic system. Then, a 200 micrometer tungsten electrode will be inserted in the radial or peroneal nerve under real-time ultrasound guidance and manipulated until a single nerve fiber and its area of skin innervation (receptive field) are identified. The nerve fiber will be categorized by its speed of conduction and firing pattern to a variety of non-noxious and noxious sensory stimuli to the receptive field. A temperature controlled thermode will be placed on the skin adjacent to and overlapping with the receptive field and acute inflammation induced by 10-second cycles of heating the probe to 50°C with 10 seconds at a non-noxious temperature (30°C) between each heat pulse for a total of 2 minutes. The response of the nerve fiber to noxious and non-noxious stimuli will be repeated.

The primary analysis will compare fiber firing in response to a fixed intensity punctate stimulus in the fiber's receptive field between oxytocin and placebo and will be performed separately on touch-sensitive fibers (fast conducting, low threshold mechanoreceptors (A-LTMRs)) and pain-sensitive fibers (fast conducting, low threshold mechanoreceptors (A-HTMRs)).

Based on numerous studies performed in rodents at Wake Forest University School of Medicine, we hypothesize that acute heat-induced inflammation will render A-LTMRs less responsive to a touch punctate stimulus and A-HTMRs more responsive to a noxious punctate stimulus and that both of these effects will be reduced by at least 50% in participants who receive IV oxytocin. If such results are obtained, they will challenge the existing paradigm that reduction in pain threshold (allodynia) after sensitization from peripheral injury is driven by increased A-LTMR input and support further exploration for the mechanisms by which oxytocin returns changes in touch and pain fibers towards normal to treat pain.

• Intravenous Oxytocin (Drug), also known as Oxytocin-Grindeks: Intravenous oxytocin, 20 IU will be administered via two 30-minute infusions separated by 30 minutes
• Intravenous Placebo (Drug), also known as Saline: Intravenous placebo will be infused in the same volume and timing as the intravenous oxytocin intervention

Primary outcomes

• Number of nerve fiber discharges with von Frey filament application [2-8 hours after start of the first IV infusion]: The number of nerve fiber discharges in response to a 5 second application of a 20 mN force von Frey filament and to a 1000 mN force von Frey filament to the area of skin responding to sensory stimulation. These are measured twice, once before and once after skin inflammation induced by heat pulses.

Secondary outcomes

• Verbal pain intensity score with von Frey filament application [2-8 hours after start of the first IV infusion]: Verbal pain intensity score in response to a 5 second application of a 20 mN force von Frey filament and to a 1000 mN force von Frey filament to the area of skin responding to sensory stimulation. Pain scores are obtained twice, once before and once after skin inflammation induced by heat pulses. Pain intensity is rated on a 0-10 scale with 0 being no pain at all and 10 being the worst imaginable pain.
• Pattern of nerve fiber discharges with von Frey filament application [2-8 hours after start of the first IV infusion]: The pattern of spikes in response to a 5 second application of 20 mN force von Frey filament and to a 1000 mN force von Frey filament to the area of skin responding to sensory stimulation. Pattern is categorized as one of 4 choices (rapidly adapting, slow adapting, continuous, or on-off) according to standard neurophysiological definitions. These will be measured twice, once before and once after heat-induced inflammation
• Number of nerve fiber burst discharges with tuning fork application [2-8 hours after start of the first IV infusion]: The number of spike bursts in response to a 2 second application of a 128 Hz tuning fork to the receptive field. If there are any bursts to 128 Hz, then the test will be repeated with, separately, a 75 Hz tuning fork and a 512 Hz tuning fork to the area of skin responding to sensory stimulation. Spike bursts are defined as multiple spikes coinciding with each vibration of the tuning fork. These will be measured twice, once before and once after heat-induced inflammation
• Number of nerve fiber discharges with warming and cooling [2-8 hours after start of the first IV infusion]: The number of discharges in response to slow warming to 50 degrees C and slow cooling to 0 degrees C by application of a Peltier-controlled thermode to the area of skin responding to sensory stimulation. Spike bursts are defined as multiple spikes coinciding with each vibration of the tuning fork. These will be measured twice, once before and once after heat-induced inflammation
• Number of nerve fiber discharges with soft brushing [2-8 hours after start of the first IV infusion]: The number of discharges in response to brushing in the receptive field using a computer controlled robotic device at various pressures and speeds. Total discharges with each brush will be measured. The brushing tests will be performed twice, once before and once after heat-induced inflammation
• Number of nerve fiber discharges with von Frey filament application: stimulus response relationship [2-8 hours after start of the first IV infusion]: The number of discharges in response to 5 sec application of von Frey filaments of 8 different forces from 4-3000 mN, presented in random order to the area of skin responding to sensory stimulation. Threshold will be determined as the lowest force resulting in any discharge. A stimulus response curve will be created using number of discharges to 5 second von Frey application at each of the 8 forces. This stimulus response is measured twice, once before and once after skin inflammation induced by heat pulses.
• Inflammation response to skin heating [2-8 hours after start of the first IV infusion]: The area of erythema immediately after skin heating to 6 cycles of 50 degrees C for 10 sec over a total of 2 minutes will be calculated by measurement of height and width in mm, with values expressed in mm squared.

Other outcomes

• Exploratory outcome: Score on Digit Symbol Substitution Test (DSST) [1 hour before to 2 hr after start of the first IV infusion]: DSST is a measure of cognitive information processing speed. DSST is comprised of a paper sheet featuring a key at the top with numbers 1 to 9, each corresponding to a unique symbol. Below this key are five rows containing 25 randomly placed numbers without their associated symbols. Participants are given pencils to write the symbols, and are instructed to substitute as many numbers with their corresponding symbols as possible within a 90-second period, commencing with the leftmost symbol in the first row and proceeding from left to right. The test score is determined by accuracy: enumerating the quantity of correctly inserted symbols, with a range of 0 (no information processing) to 25 (highest information processing speed). It will be performed twice, once before and once after study drug administration in order to explore effects of oxytocin on information processing speed.
• Exploratory outcome: Digit Span Test (DST) [1 hour before to 2 hr after start of the first IV infusion]: DST is a cognitive test of verbal working memory. Sequences are constructed of seven digits for forward recall and six digits for backward recall. Digits are not repeated within a sequence. A forward sequence is first read to the participant and they respond be repeating them in order. A backward sequence is first read to the participant and they respond by repeating them in reverse order, starting with the last digit. The maximum scored without making a mistake on the sequence is scored with a range 0 = no memory to 7 or 6 (maximum memory in forward and backward sequences, respectively) and the sum of these two (0-13) is the outcome score. DST will be performed twice, once before and once after study drug administration in order to explore effects of oxytocin on information processing speed.
• Exploratory outcome: Rey Auditory Verbal Learning Test (RAVLT) [30-60 minutes after completing the second IV infusion]: RAVLT assesses verbal learning and memory. A list of 15 words (list A) is read 5o participants 5 times. After each reading (trial), participants are asked to recall as many words as they can (trials 1-5). Then a different list of 15 words is read to the participants, and they are asked to recall words from this list, (trial 6). Participants then are asked to recall words they had learned from List A without cues 5 min (short delayed recall; trial 7) and 30 min (long delayed recall; trial 8) after trial 6. Scores are calculated for (1) Total Immediate Recall (sum of words recalled in trials 1 to 5: range 0 = no recall, 15 = maximum recall); (2) Rate of Learning (trial 5 minus trial 1: range 0 = no learning, 15 = maximum learning); (3) Short Delayed Recall and Long Delayed Recall (at 5 and 30 min after trial 6, respectively) with range 0 - no long delayed recall to 15 - maximum delayed recall.
• Exploratory outcome: Perceptual Aberrations Scale (PAS) [1 hour before to 2 hr after start of the first IV infusion]: PAS is a 35-item self-report questionnaire measuring body image and visual perception aberrations. It is scored on a True/False scale. Its total score ranges from 0 to 35 and it is calculated as the sum of the 'True' responses to all items, except four (i.e. items 6, 13, 24, 25) that are reverse-keyed. Scores range from 0 (no aberrations) to 35 (extreme aberrations).
• Exploratory outcome: State-Trait Anxiety Scale (STAI) [1 hour before to 2 hr after start of the first IV infusion]: STAI is a 6-item self-report questionnaire measuring anxiety. It is scored on a 4 point Likert scale with positive items (calm, relaxed, content) reverse scored. Total score ranges from 20 (no anxiety) to 80 (severe anxiety)
• Blood pressure [0 to 6 hr after start of first IV infusion]: Blood pressure will be measured non-invasively using an automated blood pressure cuff and values for systolic and diastolic pressure measured in mm Hg.
• Heart rate [0 to 6 hr after start of first IV infusion]: Heart rate will be measured by the same device used to non-invasively measure blood pressure and will be expressed as beats per minute.

Sex: All, ages: 18 Years to 65 Years, accepts healthy volunteers

Inclusion Criteria:

* In good health as determined by the Principal Investigator (PI) or co-investigators based on prior medical history, current psychiatric assessment, and clinical assessment of lab tests.
* Female participants of child-bearing potential including those < 1 year post-menopausal, must be practicing highly effective methods of birth control such as hormonal methods (e.g., combined oral, implantable, injectable, or transdermal contraceptives), double barrier methods (e.g., condoms, sponge, diaphragm, or vaginal ring plus spermicidal jellies or cream), or abstinence from heterosexual intercourse for a minimum of 1 full cycle before study drug administration.
* Normal blood pressure (systolic 90-140 mmHg; diastolic 50-90 mmHg) resting heart rate 45-100 beats per minute) without medication.
* Proficient in Swedish or English

Exclusion Criteria:

* Hypersensitivity, allergy, or a history of significant reaction to any ingredients of the active drug (Oxytocin Grindeks® 8.7 microgram/ml solution) or the placebo (Sodium Chloride , ATC code: V07AB).
* Any disease, diagnosis, or condition (medical or surgical) that, in the opinion of the PI, would place the participant at increased risk (active gynecologic disease in which increased uterine tone would be detrimental e.g., uterine fibroids with ongoing bleeding), compromise the participant's compliance with study procedures, or compromise the quality of the data.
* Women who are pregnant (positive result for urine pregnancy test at screening visit), women who are currently nursing or lactating, women that have been pregnant within 2 years.
* Body Mass Index (BMI) > 30
* Participants with neuropathy, chronic pain, diabetes mellitus, or taking benzodiazepines or pain medications on a daily basis.
* Participants with current or history of ventricular tachycardia, atrial fibrillation or prolonged QT interval.
* Participants with past or current history of hyponatremia or at risk for hyponatremia; anyone taking thiazide diuretics, loop diuretics, combination diuretics, lithium, carbamazepine, enalapril, ramipril, celecoxib, temazepam, gliclazide, glimepiride, glibenclamide, glipizide, omeprazole, pantoprazole, desmopressin, antidepressants (SSRI's or MAOI), or the recreational drug ecstasy.
* Participants with a known latex allergy.
* Participants with substances use disorders or any other psychiatric disorder(s).
* Participant with high risk of fainting episodes.

• James Eisenach, MD, Study chair, Atrium Health Wake Forest University School of Medicine

NIH grants: P01NS119159