Shoykhet, Michael, Pranav Shetty, Brandon S. Minnery, and Daniel J. Simons. Protracted development of responses to whisker deflection in rat trigeminal ganglion neurons. J Neurophysiol 90: 1432-1437. First published June 11, 2003 10.1152/jn.00419.2003. The rodent whisker-to-barrel pathway constitutes a major model system for studying experience-dependent brain development. Yet little is known about responses of neurons to whisker stimulation in young animals. Response properties of trigeminal ganglion (NV) neurons in 2-, 3-, and 4-week-old and adult rats were examined using extracellular single-unit recordings and controlled whisker stimuli. We found that the receptive field size of NV neurons is mature in 2-week-old animals while response latencies, magnitudes, and angular tuning continue to develop between 2 weeks of age and adulthood. At the earliest time recorded, NV neurons respond to stimulation of only one whisker and can be characterized as slowly or rapidly adapting (SA, RA). The proportion of SA and RA neurons remains constant during development. Consistent with known on-going myelination of NV axons, response latencies decrease with age, becoming adult-like during the third and fourth postnatal weeks for RA and SA neurons, respectively. Unexpectedly, we found that evoked response magnitudes increase several-fold during development becoming adult-like only during the fourth postnatal week. In addition, RA neurons become less selective for whisker deflection angle with age. Maturation of response magnitude and angular tuning is consistent with developmental changes in the mechanical properties of the whisker, the whisker follicle, and the surrounding tissues. The findings indicate that whiskerderived tactile inputs mature during the first postnatal month when whisker-related cortical circuits are susceptible to long-term modification by sensory experience. Thus normal developmental changes in sensory input may influence functional development of cortical circuits.
Whisker plucking in developing and adult rats provides a convenient method of temporarily altering tactile input for the purposes of studying experience-dependent plasticity in the somatosensory cortex. Yet, a comprehensive examination of the effect of whisker plucking on the response properties of whisker follicle-innervating trigeminal ganglion (NVg) neurons is lacking. We used extracellular single unit recordings to examine responses of NVg neurons to controlled whisker stimuli in three groups of animals: (1) rats whose whiskers were plucked from birth for 21 days; (2) rats whose whiskers were plucked once at 21 days of age; and (3) control animals. After at least 3 weeks of whisker regrowth, NVg neurons in plucked rats displayed normal, single whisker receptive fields and could be characterized as slowly (SA) or rapidly adapting (RA). The proportion of SA and RA neurons was unaffected by whisker plucking. Both SA and RA NVg neurons in plucked rats displayed normal response latencies and angular tuning but abnormally large responses to whisker movement onsets and offsets. SA neurons were affected to a greater extent than RA neurons. The effect of whisker plucking was more pronounced in animals whose whiskers were plucked repeatedly during development than in rats whose whiskers were plucked once. Individual neurons in plucked animals displayed abnormal periods of prolonged rhythmic firing following deflection onsets and aberrant bursts of activity during the plateau phase of the stimulus. These results indicate that whisker plucking exerts a long-term effect on responses of trigeminal ganglion neurons to peripheral stimulation.
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