Development of rodent macrovibrissae: effects of neonatal whisker denervation and bilateral neonatal enucleation

Development of the whisker-to-barrel cortex system

This review provides an overview on the development of the rodent whisker-to-barrel cortex system from late embryonic stage to the end of the first postnatal month. During this period the system shows a remarkable transition from a mostly genetic-molecular driven generation of crude connectivity, providing the template for activity-dependent structural and functional maturation and plasticity, to the manifestation of a complex behavioral repertoire including social interactions. Spontaneous and sensory-evoked activity is present in neonatal barrel cortex and control the generation of the cortical architecture. Half a century after its first description by Woolsey and van der Loos the whisker-to-barrel cortex system with its unique and clear topographic organization still offers the exceptional opportunity to study sensory processing and complex behavior.

Prenatal thalamic waves regulate cortical area size prior to sensory processing

The cerebral cortex is organized into specialized sensory areas, whose initial territory is determined by intracortical molecular determinants. Yet, sensory cortical area size appears to be fine tuned during development to respond to functional adaptations. Here we demonstrate the existence of a prenatal sub-cortical mechanism that regulates the cortical areas size in mice. This mechanism is mediated by spontaneous thalamic calcium waves that propagate among sensory-modality thalamic nuclei up to the cortex and that provide a means of communication among sensory systems. Wave pattern alterations in one nucleus lead to changes in the pattern of the remaining ones, triggering changes in thalamic gene expression and cortical area size. Thus, silencing calcium waves in the auditory thalamus induces Rorβ upregulation in a neighbouring somatosensory nucleus preluding the enlargement of the barrel-field. These findings reveal that embryonic thalamic calcium waves coordinate cortical sensory area patterning and plasticity prior to sensory information processing.

How sensory maps are formed in the brain is only partially understood. Here the authors describe spontaneous calcium waves that propagate across different sensory nuclei in the embryonic thalamus; disrupting the wave pattern triggers thalamic gene expression changes and eventually alters the size of cortical areas.

Active vibrissal sensing in rodents and marsupials

In rats, the long facial whiskers (mystacial macrovibrissae) are repetitively and rapidly swept back and forth during exploration in a behaviour known as 'whisking'. In this paper, we summarize previous evidence from rats, and present new data for rat, mouse and the marsupial grey short-tailed opossum (Monodelphis domestica) showing that whisking in all three species is actively controlled both with respect to movement of the animal's body and relative to environmental structure. Using automatic whisker tracking, and Fourier analysis, we first show that the whisking motion of the mystacial vibrissae, in the horizontal plane, can be approximated as a blend of two sinusoids at the fundamental frequency (mean 8.5, 11.3 and 7.3 Hz in rat, mouse and opossum, respectively) and its second harmonic. The oscillation at the second harmonic is particularly strong in mouse (around 22 Hz) consistent with previous reports of fast whisking in that species. In all three species, we found evidence of asymmetric whisking during head turning and following unilateral object contacts consistent with active control of whisker movement. We propose that the presence of active vibrissal touch in both rodents and marsupials suggests that this behavioural capacity emerged at an early stage in the evolution of therian mammals.

The development of whisker control in rats in relation to locomotion

Adult rats sweep their large facial whiskers (macrovibrissae) back and forth in a rhythmic pattern known as "whisking". Here we examine how these whisker movements develop in relation to other aspects of exploratory behavior, particularly locomotion. We analyzed 963 high-speed video recordings of neonatal rats, from P1 (Post-natal day 1) to P21, and measured the emergence of whisker control and of head, body, and limb movements. Prior to P11, whisker movements were largely limited to unilateral retractions accompanying head turns. Between P11 and P13 bilateral whisking emerged alongside increased forward locomotion and improved control of the head. Contact-induced modulations of whisking symmetry, synchrony, and whisker spread emerge shortly thereafter but continue to develop until at least P18, coinciding with the emergence of adult-like locomotion patterns such as rearing. Overall, whisking develops alongside increasing locomotor competence indicating that active vibrissal sensing plays an important role in the exploratory behavior of the developing animal.

Effects of vibrissectomy during early postnatal ontogenesis in rat pups on behavioral development

The effects of daily trimming of the whiskers in rat pups from day 2 or day 9 to day 20 on eye opening time and the formation of early behavioral reactions, i.e., walking, rearing, grooming, and manipulatory activity, were studied in rat pups subjected to deprivation and siblings reared with them. Rat pups subjected to vibrissectomy from day 2 of life and their siblings showed acceleration of eye opening, maturation of grooming, and rearing without support; the formation of manipulatory activity was delayed, as were changes in the structures of correlational relationships between the times at which behavioral reactions formed and eye opening occurred. Vibrissectomy from day 9 of life eliminated most correlational relationships in deprived animals without impairing the dynamics of the development of early behavioral reactions. Siblings showed delayed formation of manipulatory activity, though changes in the structure of correlational relationships were less significant than in deprived animals.