Characterizing the functional significance of the neonatal rat vibrissae prior to the onset of whisking

The present series of experiments assessed how information from the whiskers controls and modulates infant rat behavior during early learning and attachment. Passive vibrissal stimulation can elicit behavioral activity in pups throughout the first two postnatal weeks, although orienting to the source of stimulation is evident only after ontogenetic emergence of whisking. In addition, while pups were capable of demonstrating learning in a classical conditioning paradigm pairing vibrissa stimulation with electric shock, no corresponding changes were detected in the anatomy of the barrel cortex as determined by cytochrome oxidase (CO) staining. Finally, the role of whiskers in a more naturalistic setting was determined in postnatal day (PN)3-5 and PN11-12 pups. Our results showed that both nipple attachment and huddling were disrupted in whisker-clipped PN3-5 pups but only marginally altered in PN1I 1-12 pups. Together, these results suggest that the neonatal whisker system is behaviorally functional and relevant for normal mother-infant interactions, though it lacks the sophistication of a mature whisker system that evokes very specific and directed responses.     

Developmental onset of functional activity in the wallaby whisker cortex in response to stimulation of the infraorbital nerve

This study used the extrauterine development of a marsupial wallaby to investigate the onset of functional activity in the somatosensory pathway from the whiskers. In vivo recordings were made from the somatosensory cortex from postnatal day (P) 55 to P138, in response to electrical stimulation of the infraorbital nerve supplying the mystacial whiskers. Current source density analysis was used to localize the responses within the cortical depth. This was correlated with development of cortical lamination and the onset of whisker-related patches, as revealed by cytochrome oxidase. The earliest evoked activity occurred at P61, when layers 5 and 6 are present, but layer 4 has not yet developed. This activity showed no polarity reversal with depth, suggesting activity in thalamocortical afferents. By P72 synaptic responses were detected in developing layer 4 and cytochrome oxidase showed the first hint of segregation into whisker-related patches. These patches were clear by P86. The evoked response at this age showed synaptic activity first in layer 4 and then in deep layer 5/upper layer 6. With maturity, responses became longer lasting with a complex sequence of synaptic activity at different cortical depths. The onset of functional activity is coincident with development of layer 4 and the onset of whisker-related pattern formation. A similar coincidence is seen in the rat, despite the markedly different chronological timetable, suggesting similar developmental mechanisms may operate in both species.