C-tactile afferents: Cutaneous mediators of oxytocin release during affiliative tactile interactions?

Abstract

Low intensity, non-noxious, stimulation of cutaneous somatosensory nerves has been shown to trigger oxytocin release and is associated with increased social motivation, plus reduced physiological and behavioural reactivity to stressors. However, to date, little attention has been paid to the specific nature of the mechanosensory nerves which mediate these effects. In recent years, the neuroscientific study of human skin nerves (microneurography studies on single peripheral nerve fibres) has led to the identification and characterisation of a class of touch sensitive nerve fibres named C-Tactile afferents. Neither itch nor pain receptive, these unmyelinated, low threshold mechanoreceptors, found only in hairy skin, respond optimally to low force/velocity stroking touch. Notably, the speed of stroking which c-tactile afferents fire most strongly to is also that which people perceive to be most pleasant. The social touch hypothesis posits that this system of nerves has evolved in mammals to signal the rewarding value of physical contact in nurturing and social interactions. In support of this hypothesis, in this paper we review the evidence that cutaneous stimulation directly targeted to optimally activate c-tactile afferents reduces physiological arousal, carries a positive affective value and, under healthy conditions, inhibits responses to painful stimuli. These effects mirror those, we also review, which have been reported following endogenous release and exogenous administration of oxytocin. Taken together this evidence suggests c-tactile afferent stimulation may mediate oxytocin release during affiliative tactile interactions.

Oxytocin increases the pleasantness of affective touch and orbitofrontal cortex activity independent of valence

“…-22 -This finding is also in line with previous reports that both positive pleasant massage (Li et al, 2019) and painful/aversive responses (Boll et al, 2018) to tactile stimuli can increase endogenous OT release. Furthermore, OT can have analgesic effects and reduce the perceived unpleasantness of stimuli (see Boll et al, 2018;Walker et al, 2017). Given the role of OFC in encoding hedonic pleasure (Lamm et al, 2015;Mcglone et al, 2012;Rolls et al, 2003) and the increased pleasantness experienced by the subjects, OT may exert similar effects on affective touch by either potentiating the rewarding value of positive touch or reducing the perceived aversion of negative touch.…”

Maternal caregiving and DNA methylation in human infants and children: Systematic review

“…C‐tactile fibers activate specifically in response to gentle and affectionate maternal touch—like the one observed in a subgroup of the studies included in the present systematic review (eg, stroking and sensitive touch). Although there is no study to date assessing the concurrent stimulation of C‐tactile fibers and methylation changes in infants and children, it should be noted that C‐tactile activation by gentle touch is a potential environmental proxy for oxytocin release during affiliative interactions and C‐fiber dysfunction in chronic pain syndromes is linked with epigenetic gene silencing of the opioid receptor gene . Although we are far from sketching an exhaustive model of the pathways through which specific maternal caregiving behavior may lead to altered methylation regulation in human infants and children, the cross‐species findings that specific maternal interpersonal touches associate with changes in the methylation status of target genes appear to a promising direction for future studies.…”

Subject sex and partner sex modulate social touch responses across multiple cortical areas

“…Light touch and caress activates a particular class of Ctactile mechanosensory afferents in hairy skin 3,8 . Activation of these fibers leads to decreased levels of the stress hormone cortisol, mediates release of the neuropeptide oxytocin and modulates the activity of insular cortex, a key region for emotional regulation [9][10][11][12] . However, two major lines of evidence also point to a major role of 'top-down' modulation in the processing of social touch.…”