DEVELOPMENTAL ADAPTATION OF WITHDRAWAL REFLEXES TO EARLY ALTERATION OF PERIPHERAL INNERVATION IN THE RAT

1. In adult decerebrate spinal rats whose plantar nerves (PLN) had bee n transected at-either postnatal day 1 (Pl) or P21 the nociceptive wit hdrawal reflexes (NWR) of musculi extensor digitorum longus (EDL), per oneus longus (PER) and semitendinosus (ST) were characterized with res pect to receptive field (RF) organization, magnitude and time course, using electromyography. Thermal (short CO, laser pulses) and mechanica l (calibrated pinch) stimulation were used. The innervation patterns i n normal and lesioned adult rats were assessed by acute nerve lesions. 2. The spatial organization of the mean mechano- and thermonociceptiv e RFs of all the muscles studied was similar to normal in both P1- and P21-lesioned rats, although in some P21-lesioned rats atypical EDL RE 's were encountered. 3. In P1-lesioned rats thermo-NWR of PER and EDL had normal magnitudes, while mechano-NWR were reduced. In P21-lesioned rats both thermo- and mechano-NWR of these muscles had reduced magnit udes. Except for thermo-NWR of ST in P1-lesioned rats, which were incr eased, NWR of ST had normal magnitudes in both P1- and P21-lesioned ra ts. The time course of thermonociceptive NWR of the muscles studied we re near normal in both P1- and P21-lesioned rats. 4. Acute nerve lesio ns in adult P1-lesioned rats revealed an essentially abolished contrib ution to NWR from the PLN. Instead, the contribution to NWR from other hindpaw nerves, such as the superficial and deep peroneal nerves, was dramatically increased. BS contrast, in P21-lesioned rats, the regene rated PLN contributed significantly to the NWR. 5. It is concluded tha t despite profound alterations of plantar hindpaw innervation induced by early PLN transection the cutaneous nociceptive input to NWR attain ed an essentially normal spatial organization. An experience-dependent mechanism is suggested to be instrumental in adapting the reflex conn ectivity to the peripheral innervation.