Comparison of melanin-concentrating hormone and hypocretin/orexin mRNA expression patterns in a new parceling scheme of the lateral hypothalamic zone

A high-resolution spatial distribution analysis of hypothalamic neurons expressing melanin-concentrating hormone or hypocretin/orexin was performed in adult male rats with in situ hybridization cytochemistry. For the analysis, a new parcellation of the lateral zone with some two-dozen regions was used, and distributions were plotted on 15 transverse reference levels through the hypothalamus. Qualitatively the results confirm earlier, much lower resolution mapping studies, although some discrepancies are clarified. Previous work indicates that each of these cell populations is far from homogeneous, and the present results should help establish a framework for clarifying more precisely how they are differentiated and organized in terms of axonal input-output relationships and gene expression patterns, and for defining precise relationships with other hypothalamic neuron populations.