Mispositioned Neurokinin-1 Receptor-Expressing Neurons Underlie Heat Hyperalgesia in Disabled-1 Mutant Mice

Projecting neurons in spinal dorsal horn send collateral projections to dorsal midline/intralaminar thalamic complex and parabrachial nucleus

Itch is an annoying sensation that always triggers scratching behavior, yet little is known about its transmission pathway in the central nervous system. Parabrachial nucleus (PBN), an essential transmission nucleus in the brainstem, has been proved to be the first relay station in itch sensation. Meanwhile, dorsal midline/intralaminar thalamic complex (dMITC) is proved to be activated with nociceptive stimuli. However, whether the PBN-projecting neurons in spinal dorsal horn (SDH) send collateral projections to dMITC, and whether these projections involve in itch remain unknown. In the present study, a double retrograde tracing method was applied when the tetramethylrhodamine-dextran (TMR) was injected into the dMITC and Fluoro-gold (FG) was injected into the PBN, respectively. Immunofluorescent staining for NeuN, substance P receptor (SPR), substance P (SP), or FOS induced by itch or pain stimulations with TMR and FG were conducted to provide morphological evidence. The results revealed that TMR/FG double-labeled neurons could be predominately observed in superficial laminae and lateral spinal nucleus (LSN) of SDH; Meanwhile, most of the collateral projection neurons expressed SPR and some of them expressed FOS in acute itch model induced by histamine. The present results implicated that some of the SPR-expressing neurons in SDH send collateral projections to the dMITC and PBN in itch transmission, which might be involved in itch related complex affective/emotional processing to the higher brain centers.

Reelin dorsal horn neurons co-express Lmx1b and are mispositioned in disabled-1 mutant mice

Mice missing either Reelin or Disabled-1 (Dab1) exhibit dorsal horn neuronal positioning errors and display heat hypersensitivity and mechanical insensitivity. Reelin binds its receptors, apolipoprotein E receptor 2 and very low-density lipoprotein receptor, leading to the recruitment and phosphorylation of Dab1 and activation of downstream pathways that regulate neuronal migration. Previously, we reported that 70% of Dab1 laminae I-II neurons co-expressed LIM-homeobox transcription factor 1-beta (Lmx1b). Here, we asked whether Reelin-expressing dorsal horn neurons co-express Lmx1b, are mispositioned in dab1 mutants, and contribute to nociceptive abnormalities. About 90% of Reelin-labeled neurons are Lmx1b-positive in laminae I-II, confirming that most Reelin and Dab1 neurons are glutamatergic. We determined that Reelin-Lmx1b and Dab1-Lmx1b dorsal horn neurons are separate populations, and together, comprise 37% of Lmx1b-positive cells within and above the Isolectin B4 (IB4) layer in wild-type mice. Compared to wild-type mice, dab1 mutants have a reduced area of laminae I-II outer (above the IB4 layer), more Reelin-Lmx1b neurons within the IB4 layer, and fewer Reelin-Lmx1b neurons within the lateral reticulated area of lamina V and lateral spinal nucleus. Interestingly, both Reelin- and Dab1-labeled dorsal horn neurons sustain similar positioning errors in mutant mice. After noxious thermal and mechanical stimulation, Reelin, Lmx1b, and Reelin-Lmx1b neurons expressed Fos in laminae I-II and the lateral reticulated area in wild-type mice and, therefore, participate in nociceptive circuits. Together, our data suggest that disruption of the Reelin-signaling pathway results in neuroanatomical abnormalities that contribute to the nociceptive changes that characterize these mutant mice.