PK Sampling After IV Oxytocin and Effects on Sensory Function in Healthy Volunteers

The primary goal of this protocol is to model change in oxytocin concentrations in plasma after intravenous administration, (oxytocin (Pitocin®), 10 IU) the investigators will also acquire tests of sensory function that could be modulated by oxytocin during later time periods when blood samples are widely separated by time. There are two measures which will assess two different aspects of sensory function.

MEASURE 1: Light Touch Frequency Threshold Light touch is subserved by a group of myelinated peripheral nerves with fast conduction in the Aβ range and which are capable of following high frequencies, being responsible for the sensation of vibration. This capability of individual nerve fibers to respond with high frequency also underlies our ability to sense a light touch moving across the body surface as in brushing.

In preclinical studies investigators have shown that, following injury, these light touch fibers lose their ability to follow high frequencies and, in some cases, no longer respond to mechanical stimulation at all. Oxytocin, when perfused around the neuronal cell bodies of these Aβ nerve fibers, partially restores their function, including their response to high frequency stimulation.

In order to determine the highest frequency that Aβ nerve fibers can respond to, the investigators have created a simple device that produces an oscillatory / vibratory stimulus which can be used on the hand. The subject places fingertips, palm, and volar aspect of the wrist sequentially and the device is set to slowly decrease the frequency of vibration from 1 kilohertz (kHz) until the participant first perceives this. This is repeated three times at each site and takes under 5 minutes in total.

The investigators anticipate that oxytocin will have no effect on normal Aβ fibers and that this measure will not change after oxytocin administration. This will provide important data to contrast with the expected efficacy of oxytocin in the setting of injury, to be studied in subsequent protocols.

MEASURE 2: Sustained Heat As part of a protocol used by the investigators in several studies, the surface of the skin of the forearm or leg is heated to 45°C using a computer-controlled thermode for a period of 5 minutes. This typically results in a slowly increasing pain experience, peaking at 5 minutes in the mild to low moderate range and has been tolerated by more than 150 subjects in studies over the past 14 years.

In an interim analysis of an ongoing clinical study in volunteers, the investigators noted that spinal injection of oxytocin, 15 μg (equivalent to 9 IU), that pain from this sustained heat was dramatically reduced over the 5 minutes of stimulus presentation. In contrast, spinal oxytocin had no effect on the stimulus-response relationship of pain to noxious heat over a 42-50°C range when the stimuli were presented for only 5 seconds. These data suggest that oxytocin reduces C-fiber mediated signaling of sustained heat pain, likely through a spinal site of action.

Drugs administered spinally have a much higher concentration in cerebrospinal fluid than plasma. Nonetheless, spinally administered drugs are typically absorbed to an extent and with a time course similar to that after intramuscular injection, and it is conceivable that the effect on sustained heat pain investigators are seeing in the spinal study reflect peripheral effects via systemic absorption. Thus, if the investigators see a similar reduction in response to sustained heat in this protocol of intravenous administration, the investigators will conclude that oxytocin is more likely activating peripherally than spinally.