Development of putative inhibitory neurons in the embryonic and postnatal mouse superficial spinal dorsal horn.

The superficial spinal dorsal horn is the first relay station of pain processing. It is also widely accepted that spinal synaptic processing to control the modality and intensity of pain signals transmitted to higher brain centers is primarily defined by inhibitory neurons in the superficial spinal dorsal horn. Earlier studies suggest that the construction of pain processing spinal neural circuits including the GABAergic components should be completed by birth, although major chemical refinements may occur postnatally. Because of their utmost importance in pain processing, we intended to provide a detailed knowledge concerning the development of GABAergic neurons in the superficial spinal dorsal horn, which is now missing from the literature. Thus, we studied the developmental changes in the distribution of neurons expressing GABAergic markers like Pax2, GAD65 and GAD67 in the superficial spinal dorsal horn of wild type as well as GAD65-GFP and GAD67-GFP transgenic mice from embryonic day 11.5 (E11.5) till postnatal day 14 (P14). We found that GABAergic neurons populate the superficial spinal dorsal horn from the beginning of its delineation at E14.5. We also showed that the numbers of GABAergic neurons in the superficial spinal dorsal horn continuously increase till E17.5, but there is a prominent decline in their numbers during the first two postnatal weeks. Our results indicate that the developmental process leading to the delineation of the inhibitory and excitatory cellular assemblies of pain processing neural circuits in the superficial spinal dorsal horn of mice is not completed by birth, but it continues postnatally.