Neuroplastic effects of neonatal capsaicin on neurons in adult rat trigeminal nucleus principalis and subnucleus oralis. 
1.  The effects of C-fiber depletion induced by neonatal capsaicin treatment on the functional properties of low-threshold mechanoreceptive (LTM) neurons in the rat trigeminal (V) subnucleus oralis and nucleus principalis were examined.  Neonatal rats were injected with capsaicin within 48 h of birth.  The mechanoreceptive field (RF) and response properties of 184 oralis LTM neurons and 185 principalis LTM neurons were studied in adult capsaicin-treated rats.  These properties were compared with those of 200 oralis LTM neurons and 253 principalis LTM neurons from untreated or vehicle-treated (control) adult rats.  2.  The effectiveness of neonatal capsaicin in depleting C fibers was tested by determining the plasma extravasation of Evans blue dye that was induced in the hindlimb skin by the cutaneous application of the C-fiber excitant and inflammatory irritant mustard oil.  The amount of extravasation in capsaicin-treated rats was significantly less than that of control rats.  3.  Neonatal capsaicin treatment was associated with significant increases in neuronal RF size and in the percentage of neurons with convergent inputs from more than one type of peripheral tissue (e.g., nonsinus hair, vibrissae, glabrous skin/mucosa, subcutaneous structures such as joint or muscle, periodontal ligament) in both subnucleus oralis and nucleus principalis.  In subnucleus oralis, neonatal capsaicin treatment produced a significant increase in the percentage of neurons with a RF involving both V1 and V2 divisions, and a significant decrease in the percentage of neurons with a RF restricted to the V1 division.  Analogous changes were not observed in nucleus principalis, although for principalis vibrissa-sensitive neurons, neonatal capsaicin treatment was associated with significant increases in the total number of vibrissae per neuronal RF and in the maximal length of the vibrissal row (i.e., the number of vibrissae in the longest row of vibrissae, stimulation of which was effective in activating a given neuron).  4.  Neonatal capsaicin treatment did not significantly affect other oralis or principalis neuronal properties, including the percentage of neurons exhibiting spontaneous activity or abnormal response properties (such as habituating tap sensitivity, discontinuous RF, or mixed adaptation properties within the RF).  5.  The changes in the functional properties of oralis and principalis LTM neurons induced by neonatal capsaicin treatment are consistent with those previously reported at other levels of the rodent CNS.  They provide additional support to the view that C fibers may have an important role in shaping the functional properties of central LTM somatosensory neurons.