NEUROPLASTIC EFFECTS OF NEONATAL CAPSAICIN ON BARREL CORTEX OF ADULT-RAT - AN ELECTROPHYSIOLOGICAL AND AUTORADIOGRAPHIC STUDY

Capsaicin (50 mg/kg s.c./25 mu l) was administered to rats on the 2nd and 5th days after birth. The animals were raised, and from the age of 3 months the properties of the evoked activity were tested in the con tralateral barrelfield. This neonatal capsaicin treatment was found to induce profound changes in the responsiveness of the barrel cortex in the adult rats: (1) the receptive field of the neurons in the C1 barr el was expanded; units within a particular barrel were driven by a sig nificantly larger number of vibrissae than in the controls. (2) The ra te of discharge evoked by the related vibrissa deflection was enhanced , while (3) the angular sensitivity of the neurons was decreased. (4) The most prominent change in cortical activity was observed by autorad iography: capsaicin-treated rats exhibited an enhanced labelling of di fferent types of neurons throughout the hemisphere (surpassing the cor tical representation of stimulated vibrissae). The present observation s indicate that neonatal capsaicin affects the functional activity of the rat somatosensory cortex. It is suggested that unmyelinated sensor y afferents play a role in the development of the rat somatosensory sy stem.