Postnatal development of inner lamina II interneurons of the rat medullary dorsal horn

Pain processing in young mammals is immature. Despite the central role of the medullary dorsal horn (MDH) in processing orofacial sensory information, the maturation of the neurons within the MDH has been largely overlooked. Combining in vitro electrophysiological recordings and 3D morphological analysis over the first postnatal month in rats, we investigated the age-dependent development of the neurons within the inner lamina II (IIi) of the MDH. We show the lamina IIi neuronal population transition into a more hyperpolarized state, with modification of the action potential waveform, and a shift from single spiking, at early postnatal ages, to tonic firing and initial bursting at later stages. These physiological changes are associated with a strong structural remodelling of the neuronal morphology with most of the modifications occurring after the third postnatal week. Among the lamina IIi neuronal population, the subpopulation of intemeurons expressing they isoform of the protein kinase C (PKC gamma(-)) are key elements for the circuits underlying facial mechanical allodynia. How do they develop from the rest of the lamina Ili constitute an important question that remained to be addressed. Here, we show that PKC gamma(+) intemeurons display electrophysiological changes over time comparable with the PKC gamma(-) population. However, they show a distinctive increase of the soma volume and primary branches length, as opposed to the PKC gamma(-) population. Together, our data demonstrate a novel pattern of late postnatal maturation of lamina Ili intemeurons, with a spotlight on PKC gamma(-) intemeurons, that may be relevant for the development of orofacial sensitivity.