The oxytocin system regulates tearing

Abstract

Tears are an exocrine physiological fluid secreted onto the ocular surface from the lacrimal apparatus in all mammals. Limited research has been conducted on the functional neuronal circuitry of tear production. In particular, the neuronal mechanisms of emotional tearing, which is a physiological reaction harmonized with enhanced emotional arousal and assumed to be unique to humans, remain unclear. We identified that the oxytocin neurons in the paraventricular hypothalamus is functionally projected to the oxytocin receptor-expressing neurons in the lacrimation center of the superior salivatory nucleus. Optogenetic activation or inhibition of these neurons and/or receptors can modulate the superior salivatory nucleus dependent tear secretion mediated through oxytocin. Moreover, we identified that maternal behavior, nociceptive behavior stimulation, and aversive memory retrieval are linked to tearing in mice, and that these emotional linked tearing are suppressed by optogenetic inhibition of the corresponding oxytocin system. Thus, tearing could be regulated through functional connections between central oxytocin systems in the paraventricular hypothalamus and the superior salivatory nucleus.

Oxytocin and Emotional Tear in Dogs

“…In this study, we found that tearing dog face can facilitate human caring emotion to dogs, which is similar function observed in human children. We also discovered that emotional tear can be also observed in mice, maternal reunion, nociceptive stimulation, and aversive memory retrieval are linked to tearing in mice 14 . The social function of tearing in animals needs to be elucidated in future.…”

“…Recently we discovered the OXT can facilitated the synthesis and release of tears in the mouse brain; OXT neurons located in the paraventricular hypothalamus send the fibers to the superior salivary nucleus and increase the tear secretion. Therefore, OXT increase during the reunion with the owner can act on both central and peripheral organs to stimulate tear secretion 14 .…”

“…Even in humans, tear fluid contained some chemosignals and tears is used as socio-sexual signals 16 Therefore, one important function of tear fluid is chemo-signaling in mammals. In humans, social and emotional status can be transmitted by tear 14 . Infants use tears to communicate feelings, especially negative ones such as hunger, pain, and discomfort, to their parents 15 .…”

Increase of tear volume in dogs after reunion with owners is mediated by oxytocin

“…Another study in mice demonstrated that oxytocin neurons located in the paraventricular hypothalamus send fi bers to the superior salivary nucleus and are involved in increasing tear production. Therefore, oxytocin increase related to positive and negative emotional arousal can facilitate tear secretion, acting on both central and peripheral organs 7 . One point to note is that oxytocin administration also acts on vasopressin receptors, so it will be necessary to clarify in the future which receptors were affected by the results of this administration experiment.…”