A Stickiness Response System in Rats

The perception of stickiness is known to everyone who interacts with the world. While eating, walking, and navigating diverse environments including crowded subways, forests, fields and lunchrooms, stickiness is a common and old sensation. Responses to sticky stimuli have been measured in animal and human brains; however, precise behavioral responses and the underlying neural mechanisms are not well understood. We applied sticky stimuli to three-week-old rat pups and found the effects vary greatly across the animal's body: Sticky stimuli are quickly removed from forepaws and nose, but often evoke only little reaction from hindpaws. When we applied sticky (marshmallow, mochi) and non-sticky stimuli (water, oil) to forepaws, we observed stimulus unspecific behaviors (licking and grooming) with variable response onsets as well as three fast-onset sticky-specific behaviors. Sticky-specific behaviors were exclusively triggered by sticky stimuli and included paw shaking and paw swiping (behaviors presumably aiming at stickiness removal) and paw tapping. In tapping, animals gently tap their forepaws onto each other or on the ground; we wondered if the resulting paw-substrate detachments serve stickiness sensing. Blocking of forepaw skin sensation reduced sticky-specific responses to sticky stimuli compared to control conditions (Ringer's injections). To assess central representations of stickiness, we obtained in vivo whole-cell recordings of neurons in forepaw-somatosensory-cortex while presenting sticky and non-sticky stimuli to anesthetized rat pups. While responses were heterogeneous across the population, we observed individual neurons that had significantly different responses to stimulus detachment for sticky and non-sticky stimuli. In summary, we describe a fast-onset, body-part-specific stickiness response system in rats, which is strongly driven by forepaw skin afferents.