SYNAPTIC ORGANIZATION OF DAMAGED INFRAORBITAL NERVE AXONS IN PERINATAL RATS - DEMONSTRATION BY GALANIN IMMUNOCYTOCHEMISTRY

Neonatal transection of the infraorbital nerve (ION; the trigeminal, V , branch that supplies the mystacial vibrissae follicles) results in a n upregulation of galanin in the central arbors of primary afferent ax ons. The present study was undertaken to evaluate the synaptic organiz ation of these galanin-positive primary afferents and compare it with that of normal neurobiotin/biocytin labeled primary afferent axons fro m animals of the same age. Examination of 1200 neurobiotin/biocytin-la beled profiles in V nucleus principalis (PrV) of rats killed on postna tal day (P-) 7 indicated that 23.3% (n=279) of these profiles made syn aptic contacts: 87.4% were axodendritic, 8.9% were axoaxonic, 2.8% wer e axosomatic, and 0.7% were axospinous, Evaluation of 1200 galanin-pos itive profiles in PrV from rats that sustained transection of the ION on P-0 and were killed on P-7 indicated that only 64 (5.3%) of these p rofiles made synaptic contacts (P<0.05 compared with the intact animal s). Of the galanin-positive profiles that did make synapses in PrV, 81 .2% (n=52) were axodendritic and 18.8% (n=12) were axoaxonic. These re sults indicate that galanin released by damaged ION primary afferents in PrV is likely to affect the activity of second-order V neurons by a paracrine action rather than by acting at specific synapses.