LIM homeobox 6 (Lhx6)+ neurons in the ventral zona incerta project to the core portion of the lateral supramammillary nucleus in the rat

There was a recent report suggesting that LIM homeobox 6 (Lhx6)+ GABA-releasing neurons of the ventral zona incerta (ZI) promote sleep. We demonstrated in the previous study that Lhx6+ ZI neurons are activated during paradoxical sleep (PS) hypersomnia which was induced by 48-hour PS deprivation, implying their roles in the control of PS like melanin-concentrating hormone (MCH) cells. Since the core portion of the lateral supramammillary nucleus (SUMl) is the major hypothalamic area activating the dentate gyrus as well as other limbic cortices during PS, we examined in the present study whether Lhx6+ ZI cells provide efferent projections to the SUMl, using the retrograde-tracing method. The majority of Lhx6+ neurons projecting to the SUMl occupied the ventral border (or ventral one-third) of the ventral ZI. Based on the quantitative analysis, the mean number of retrogradely-labeled Lhx6+ neurons was comparable to that of retrogradely-labeled MCH cells in the ZI. However, the total (i.e., single- plus double-labeled) number of Lhx6+ cells was approximately three times larger than that of MCH cells in the ZI. Thus, the proportion (about 7.8%) of retrogradely-labeled Lhx6+ neurons over the total Lhx6+ cells was approximately one-third of the percentage (about 20.9%) of retrogradely-labeled MCH neurons over the total MCH cells. On the other hand, a combination of retrogradely-labeled, Lhx6 and MCH cells occupied approximately 43.7% of the total retrogradely-labeled neurons in the ventral ZI. The present observations suggested that Lhx6+ neurons in the ventral ZI might play an important role in the regulation of PS, partly via the neural network involving the SUMl.