Challenges for measuring oxytocin: The blind men and the elephant?

Highlights

• Common methods of measuring oxytocin often yield uncorrelated results.

• Reported concentrations often differ by an order of magnitude.

• These discrepancies do not mean than some methods are valid and others are not.

• We highlight reasons why multiple valid methods may produce divergent results.

• Current debates may be analogous to the parable of the blind men and the elephant.

Abstract

Since its discovery more than a century ago, oxytocin has become one of the most intensively studied molecules in behavioral biology. In the last five years, Psychoneuroendocrinology has published more than 500 articles with oxytocin in the title, with many of these articles including measures of endogenous oxytocin concentrations. Despite longstanding interest, methods of measuring endogenous oxytocin are still in active development. The widely varying oxytocin concentrations detected by different approaches to measurement – and lack of correlation among these techniques – has led to controversy and confusion. We identify features of oxytocin that may help to explain why various approaches may be differentially sensitive to diverse conformational states of the oxytocin molecule. We propose that discrepancies in data generated by different methods of measurement are not necessarily an indicator that some methods are valid whereas others are not. Rather, we propose that current challenges in the measurement of oxytocin may be analogous to the parable of the blind men and the elephant, with different methods of sample preparation and measurement being sensitive to different states in which the oxytocin molecule can exist.

Introduction

Since its discovery more than a century ago, oxytocin has become one of the most intensively studied molecules in behavioral biology. Once thought of primarily as a female reproductive hormone, oxytocin is now recognized for having diverse roles in the mammalian nervous system, and is implicated in processes ranging from social monogamy and theory of mind to regulation of appetite, the immune and autonomic systems, and bone mass and cardiomyocyte differentiation (Blackburn et al., 1992; Carter, 2014; Carter and Perkeybile, 2018; Feldman et al., 2007; Li et al., 2017; MacKinnon et al., 2018; Paquin et al., 2002; Peltola et al., 2018; Tamma et al., 2009; Wai et al., 2018). In the last five years, Psychoneuroendocrinology has published more than 500 articles with oxytocin in the title, with many of these articles including measures of endogenous oxytocin concentrations. Despite this longstanding interest, methods of measuring endogenous oxytocin are still in active development, and questions about the validity of various techniques have spurred controversy and confusion (Carter et al., 2007; Carter, 2014; Jurek and Neumann, 2018; Leng and Sabatier, 2016; McCullough et al., 2013).

Specifically, two of the most common approaches for plasma sample preparation (use of extracted or non-extracted samples; see below for details) lead to entirely uncorrelated results (Leng and Sabatier, 2016); even when using the same protocols, different commercially available assays can produce markedly divergent measurements (Lefevre et al., 2017; MacLean et al., 2018). Questions about endogenous oxytocin concentrations have been further complicated by recent findings that the vast majority of endogenous oxytocin may be bound to other molecules in plasma and serum. In many cases oxytocin may evade detection unless these bonds are broken prior to measurement (Brandtzaeg et al., 2016; Martin and Carter, 2013). Although efforts to develop reliable measures / confirm the validity of bound oxytocin have proven challenging (Franke et al., 2019), multiple techniques have been developed in recent years, with utility for both immunoassay and mass spectrometry applications (Brandtzaeg et al., 2016; Liu et al., 2019). The widely varying oxytocin concentrations detected by different approaches to sample preparation and measurement – and lack of correlation between these techniques – has led to the suggestion that particular approaches may be ‘no more than a random number generator’ (Leng and Sabatier, 2016). Reflecting on the poor agreement between various approaches, others have suggested that it is urgent that we adopt a single approach to be implemented as the standard in the field (McCullough et al., 2013). Although we agree that progress will depend on a better understanding of why different methods produce such different results, we suggest that it is premature to accept any single approach as a gold standard, since discrepancies between methods are not necessarily an indicator that some methods are valid whereas others are not. Instead, we suggest that the current challenges in the measurement of oxytocin may be analogous to the parable of the blind men and the elephant.

In this parable, a group of blind men conceptualize what an elephant is like by touching its body. Each man touches a different part of the elephant, and afterward recounts his perspective. Of course, all of the men find themselves in utter disagreement about what an elephant is like, having been exposed to only partial information about a complex system. We propose that the current diversity of approaches to measuring oxytocin may be analogous to the blind men in this parable. Varying approaches to sample preparation and measurement of oxytocin may yield different and often conflicting information due to differential sensitivity to diverse conformational states of the oxytocin molecule. We propose that this phenomenon arises because oxytocin, like an elephant, is biologically complex.