Lamina Specific Postnatal Development of PKCγ Interneurons within the Rat Medullary Dorsal Horn

Protein kinase C gamma (PKC gamma) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKC gamma-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKC gamma-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKC gamma-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKC gamma-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (similar to 50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKC gamma-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKC gamma-ir interneurons. Together, the results show that: (i) the expression of PKC gamma-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKC gamma-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKC gamma-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKC gamma-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. (C) 2016 Wiley Periodicals, Inc.