The termination pattern of sciatic nerve afferents in the substantia gelatinosa of neonatal rats.

A study was carried out to determine at what stage of development the adult pattern of primary afferent projections to the substantia gelatinosa (SG) is first established. Horseradish peroxidase conjugated wheat-germ agglutinin (WGA-HRP) was applied to cut ends of sciatic nerves at mid-thigh level in adult and neonatal rats aged 1-16 days. The distribution of transganglionically transported label in primary afferent terminals in SG was reconstructed from transverse sections of spinal cord segments L1 to S1. The distinctive U-shaped projection field of the sciatic nerve in SG of the adult was found to exist in miniature form already in the second postnatal day 24 h after WGA-HRP had been applied. The highly specific somatotopic projections of primary afferents to SG are probably established before birth. Any delay in the maturation of functional specificity in the dorsal horn of the neonate cannot be attributed to the absence or disorganization of anatomical projections to SG of primary afferent inputs.