Tactile mechanisms and afferents underlying the rat pup transport response

Juvenile rodents and other altricial mammals react with calming, immobility, and postural modifications to parental pickup, a set of behaviors referred to as the transport response.^1,2,3,4,5 Here, we investigate sensory mechanisms underlying the rat transport response. Grasping rat pups in anterior neck positions evokes strong immobility and folding up of feet, whereas more posterior grasping has lesser effects on immobility and foot position. Transport responses are enhanced by slow (1 Hz), and even more so by fast (4 Hz), gentle shaking and translation, features consistent with parental transport. With lateral grasping, the forepaw below the grasping position points downward and the forepaw lateral to the grasping position points upward and medially. Such forepaw adjustments put the pup's center of gravity below the grasping point, optimizing pup transportability. Tactile stimuli on the back, belly, tail, whisker, dorsal forepaws, and dorsal hind-paws do not significantly affect behavior of anterior-neck-held pups. Instead, ground contact, or paw stimulation consistent with ground contact, disrupts transport responses. We identify afferents mediating transport response by examining membrane labeling with FM 1-43⁶ following anterior neck grasping. We observe a dense innervation of the anterior-neck-skin region (∼30 terminals/mm²). We find an age-related decrease of cytochrome oxidase reactivity in the rat somatosensory cortical neck representation, a possible correlate to developmental decrease in pup transport response. We conclude that anterior neck grasping and loss of ground contact trigger calming and postural adjustments for parental transport in rat pups, responses putatively driven from the densely innervated anterior neck skin.