Trickle-down metabolomics: Saliva oxytocin measures do not reflect peripheral plasma concentrations
The neuropeptide oxytocin is thought to play an important role in social behavior. To better understand how the oxytocin system influences our thoughts and behaviors, researchers often collect blood samples to calculate levels of circulating oxytocin for comparison to various measures of psychological functioning.
As lower oxytocin levels have been associated with psychological dysfunction, oxytocin levels might provide a useful psychological biomarker. If it’s established that lower oxytocin levels are associated with dysfunction, then pharmacologically increasing oxytocin levels may help alleviate psychological and behavioral impairments.
In parallel to this, researchers also need to demonstrate that a dose of synthetic oxytocin increases peripheral concentrations, which have been shown to be representative of oxytocin concentrations in the brain.
Blood measures can be a useful tool, but blood collection isn’t a straightforward task for many laboratories. The collection and preparation of blood samples requires a specific set of expertise and infrastructure that many labs don’t possess, particularly those in psychology departments.
Also, some people don’t like needles.
Saliva measures have emerged as a popular alternative to blood measures. Based on the premise that circulating molecules in blood can transfer to salivary glands via surrounding capillaries, it’s assumed that saliva oxytocin concentrations can be used to index blood oxytocin concentrations. Saliva is much easier to collect than blood, as participants just need to spit into a tube or suck on a cotton swab.
Nevertheless, there are a few issues with using saliva to measure peripheral oxytocin concentrations. First, hormone concentrations in saliva are much less than what you find in blood. This isn’t a huge problem if you have a senstive measure and if your saliva measure is highly correlated with its corresponding blood measure. However, the relationship between blood and saliva oxytocin is relatively modest (r ∼ 0.5). For comparison, more established saliva measures like testosterone and cortisol have demonstrated much stronger correlations between blood and saliva (r ∼ 0.9).
Second, we don’t really understand the origin of saliva oxytocin concentration increases after intranasal administration. Clearance of intranasally delivered oxytocin from the nasal cavity down the throat (also known as “trickle-down” oxytocin) is a considerable limitation, as it’s unclear whether you’re measuring circulating oxytocin, or the synthetic oxytocin that was just administered.
I’ve run a few intranasal oxytocin trials in my short-ish career. One of the most common responses to intranasal oxytocin administration from research participants is something like, “Oh, that tastes a little bitter”.
That’s the spray being cleared from their nasal cavity and trickling past their taste buds.
Another related question is how long does trickle-down occur after intranasal administration? The use of radiolabelled oxytocin coupled with PET imaging is an obvious solution to this problem, but the development of an radiolabelled oxytocin has proven to be difficult.
An alternative to understanding the origin of salivary oxytocin is to compare salivary levels after intranasal oxytocin administration with salivary levels after intravenous (IV) oxytocin. If saliva oxytocin reflects peripheral concentrations, then IV oxytocin administration should also increase salivary concentrations.
Here’s what happens to plasma when you administer 1 international unit (IU) of IV oxytocin — which is roughly equivalent to a standard intranasal dose — compared to intranasal oxytocin and placebo.
There’s a five-fold jump in blood plasma oxytocin straight after IV administration (green line), and then these levels closely mirror intranasal administration (red and blue lines) from about 10 minutes after administration.
Research has yet to compare saliva and blood oxytocin concentrations after intranasal and intravenous administration. With this in mind, we collected both blood and saliva oxytocin concentration data as a part of a clinical trial. I eventually got the chance to analyse and write up this data, which is now published in the journal Hormones and Behavior (preprint here).
We used a crossover design, where we gave four treatments to 16 healthy males: low dose intranasal OT (8IU), typical dose intranasal OT (24IU), an equivalent IV dose (1IU), and placebo. This means that every participant visited the lab four times, and received one of treatments at each visit in a randomized sequence.
Because these treatments used different administration methods (i.e., intranasal vs. IV), we had to mask the treatment condition by using what’s called a double-dummy design. In each treatment condition, we administered both an intranasal and intravenous solution. For example, in the low-dose intranasal oxytocin condition, we also administered 8 IU of oxytocin and IV placebo.
Both saliva and blood plasma samples were collected for oxytocin concentration analysis at 6 timepoints: baseline, 0 mins (immediately after the completion of spray administration), 10 mins, 30 mins, 60 mins, and 120 mins.
What we found
Overall, we discovered that intranasal oxytocin (8IU and 24IU) administration increased saliva oxytocin concentrations in comparison to saliva oxytocin concentration levels after intravenous and placebo administration.
Immediately after intranasal oxytocin administration with a typical 24IU dose, there was almost 400 pg/mL of oxytocin detected in saliva. For reference, around 15 pg/mL is found in blood at this same time point after the same treatment. Normal levels in blood without any intervention are between 1–10 pg/mL.
We observed unusually high levels of salivary oxytocin for up to two hours after intranasal administration. It’s certainly possible that intranasal administration kickstarted the production of natural oxytocin in brain, but then this should probably also be have been reflected in blood plasma too — but it wasn’t.
We also found that saliva oxytocin concentrations were not related with plasma oxytocin concentrations after either intranasal or intravenous oxytocin administration at any of the these six timepoints. In other words, high levels of saliva oxytocin generally didn’t correspond with high levels of blood oxytocin.
If saliva oxytocin were representative of peripherally circulating oxytocin, then one would expect saliva levels to be similar after intranasal and IV oxytocin administration. However, as saliva oxytocin concentrations after intranasal oxytocin were larger compared to IV administration — even after two hours — it looks like that cleared exogenous oxytocin from the nasal cavity into the oropharynx largely contributes to these increases.
Bringing everything together, we concluded that saliva concentrations should not be used as a proxy measure of peripherally circulating oxytocin after intranasal exogenous oxytocin administration.
Method validation isn’t especially sexy, but it’s important to understand which measures can be used to accurately index measures of peripherally circulating oxytocin.
Paper citation: Quintana DS, Westlye LT, Smerud KT, Mahmoud RA, Andreassen OA, Djupesland PG (2018) Saliva oxytocin measures do not reflect peripheral plasma concentrations after intranasal oxytocin administration in men. Hormones and Behavior, DOI: 10.1016/j.yhbeh.2018.05.004
If you don’t have access to this journal, you can read the paper preprint.