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

Abstract

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.

Figure 1

Whisker stimulation produced an increase in behavioral activity in pups as indicated by cumulative response of eight 30-s whisker stimulations (N = 10–16 pups/group). Asterisk represents significant age differences (p < 0.05).

Figure 2

(A) Neonatal rat pups given a whisker learning paradigm (whisker stimulation paired with 0.5 mA shock) for the first 8 days of life exhibited conditioned behavioral activity as measured by acquisition curves during training (N = 6–8 pups/training group). Acquisition curves represent the first 29 s of whisker stimulation, and nonwhisker stimulated conditions such as SHOCK only are not shown. No corresponding plastic changes were found in the anatomy of the barrel cortex for (B) area and (C) perimeter of the barrels (N = 3–4 pups/group). Additional analysis by cortical barrel row yielded consistent nonsignificant results. Asterisk represents a significant difference between the experimental PAIRED group and each of the control groups (p < 0.05).

Figure 3

Removing pups’ whiskers disrupted (A) nipple attachment when the mother was on her side (mother-on-side, N = 8 pups/group), (B) nipple attachment when the mother was suspended above the pup (mother-above, N = 4–5 pups/group) and (C) huddling responses to a target pup (N = 7 pups/group). Asterisk represents a significant difference between groups (p < 0.05).