Evidence that large myelinated primary afferent fibers make synaptic contacts in lamina II of neonatal rats

Choleragenoid horseradish peroxidase (B-HRP) is a retrogradely transported marker that selectively labels large cutaneous myelinated primary afferent fibers. In adults, B-HRP labelled large afferent fibers are seen to enter laminae III-V, and to a lesser extent lamina I, whereas lamina II, which is the major termination site of unmyelinated primary afferents, remains unlabelled. In the neonate, however, there is extensive B-HRP label in lamina II. The present study shows that the B-HRP labelled fibers in the neonate make many synaptic contacts in lamina II. This supports the idea that large primary afferent fibers in neonatal animals make synaptic contact with post-synaptic targets that presumably process nociceptive information. Accordingly to ameliorate pain in neonates it may be more important to block low threshold sensory input whereas in adults it would be more important to block the high threshold inputs.