Immunoglobulin G is a natural oxytocin carrier which modulates oxytocin receptor signaling: relevance to aggressive behavior in humans

Abstract

Oxytocin is a neuropeptide produced mainly in the hypothalamus and secreted in the CNS and blood. In the brain, it plays a major role in promoting social interactions. Here we show that in human plasma about 60% of oxytocin is naturally bound to IgG which modulates oxytocin receptor signaling. Further, we found that IgG of violent aggressive inmates were characterized by lower affinity for oxytocin, causing decreased oxytocin carrier capacity and reduced receptor activation as compared to men from the general population. Moreover, peripheral administration of oxytocin together with human oxytocin-reactive IgG to resident mice in a resident-intruder test, reduced c-fos activation in several brain regions involved in the regulation of aggressive/defensive behavior correlating with the attack number and duration. We conclude that IgG is a natural oxytocin carrier protein modulating oxytocin receptor signaling which can be relevant to the biological mechanisms of aggressive behavior.

Keywords: Aggressive behavior; Autoantibodies; Brain; Human; Intracellular signaling; Mice; Neuroendocrinology; Neuropeptides; Oxytocin; Oxytocin receptor; c-fos.

Fig. 1

OT peptide and OT-reactive immunoglobulins. OT concentrations assayed in different OT fractions labeled by capital letters in (a), including native plasma (A), IgG free (B) and bound OT (C) as well as total OT (A + B) are shown in (b). Estimation of IgG OT carrier capacity by analyzing total/plasma OT ratios (c) and IgG unbound OT (d). Percentage of IgG bound and unbound OT (e). Plasma levels of OT-reactive IgM and IgG (f). Affinity kinetics of IgG for OT including the dissociation equilibrium constant, K_D (g) and the rates of association, K_a (h) and dissociation, K_d (i). b, c, f, Student’s t-test, d, g, i, Mann–Whitney test, *p < 0.05, **p < 0.01

Fig. 2

Activation of human OT-R in HEK293 cells in vitro. a Stimulation of intracellular Ca²⁺ mobilization using OT (10⁻⁷M), as well as adenosine triphosphate (ATP, 10⁻⁴M) and HBSS buffer as positive and negative controls, respectively. b Intracellular Ca²⁺ mobilization by OT/IgG IC from the aggressor or control groups or with IgG alone. c Comparison of OT/IgG IC-induced total Ca²⁺ release between aggressors and control groups using the area under curve (AUC). d OT dose–response for OT/IgG IC-induced of Ca²⁺ release in OTR-expressing cells. c ****p < 0.0001 Student’s t-test

Fig. 3

Dynamics of GFP-labeled human OT-R cellular internalization in vitro. a Upper line, OT alone, 2nd line OT/IgG Contr IC, and 3rd line OT/IgG Aggr IC. Vertical columns correspond to the time-points of microscopy- before (0 min), and 5 or 20 min after application of OT alone or OT/IgG IC. Pseudocolored fluorescence signal corresponds to the GFP-labeled human OT-R which relative intensity level is shown by a color-map. Scale bar 10 μm. b OT-R internalization rates were quantified by ratios between membrane and cytoplasmic OT-R fluorescence, dashed lines reflect the data fit by exponential decay model. Two-way repeated measurement ANOVA p < 0.0001, Bonferroni post-tests *p < 0.05, **p < 0.01, ***p < 0.001 and ****p < 0.0001, a OT vs. OT + IgG Contr, b OT vs. OT + IgG Aggr, c, OT + IgG Contr vs. OT + IgG Aggr

Fig. 4

Resident intruder test (RIT) and brain c-fos immunohistochemistry in mice. a First contact latency. b First attack latency. c. Number of attacks. d Individual attack duration. Immunohistochemical detection of c-fos protein (green) in the brain after the RIT in the septum (e), ventromedial nucleus of the hypothalamus (VMN, h) and the paraventricular nucleus of the thalamus (PVNT, k), each panel subdivided in 4 images corresponding to the groups of 0.9% NaCl (A), OT (B), OT/IgG Contr IC (C) and OT/IgG Aggr IC (D). Quantification of c-fos positive cells in the septum (f) VMN (i) and PVNT (l). Significant Spearman’s correlations between c-fos-cell number and behavior are shown in g, j and m, in the septum (g) for number of attacks, and in VMN (j) and PVNT (m) for attack duration. Student’s t-test *p < 0.05 (d), ANOVA p < 0.0001 (f), p = 0.003 (i) and p = 0.009 (I), Tukey’s post-tests *p < 0.05, **p < 0.01 and ***p < 0.001, Student’s t-test #p < 0.05 (l). c-fos number (f, i, l) was calculated bi-laterally, n = 20

Fig. 5

Correlations of the BS-rAQ aggressivity scores with OT signalling-related biomarkers. Correlation values (Pearson’s r) are illustrated by the diameter of the bubble and direction by colour for negative (blue) and positive (red) correlations. Surrounding colour indicates significant differences as specified in the legend. For exact p-values see the Supplementary Table 2