Abstract

In rats and mice, ascending and descending axons from neurons producing melanin-concentrating hormone (MCH) reach the cerebral cortex and spinal cord. However, these ascending and descending projections originate from distinct sub-populations expressing or not “Cocaine-and-Amphetamine-Regulated-Transcript” (CART) peptide. Using a BrdU approach, MCH cell bodies are among the very first generated in the hypothalamus, within a longitudinal cell cord made of earliest delaminating neuroblasts in the diencephalon and extending from the chiasmatic region to the ventral midbrain. This region also specifically expresses the regulatory genes Sonic hedgehog (Shh) and Nkx2.2. First MCH axons run through the tractus postopticus (tpoc) which gathers pioneer axons from the cell cord and courses parallel to the Shh/Nkx2.2 expression domain. Subsequently generated MCH neurons and ascending MCH axons differentiate while neurogenesis and mantle layer differentiation are generalized in the prosencephalon, including telencephalon. Ascending MCH axons follow dopaminergic axons of the mesotelencephalic tract, both being an initial component of the medial forebrain bundle (mfb). Netrin1 and Slit2 proteins that are involved in the establishment of the tpoc and mfb, respectively attract or repulse MCH axons.We conclude that first generated MCH neurons develop in a diencephalic segment of a longitudinal Shh/Nkx2.2 domain. This region can be seen as a prosencephalic segment of a medial neurogenic column extending from the chiasmatic region through the ventral neural tube. However, as the telencephalon expends, it exerts a trophic action and the mfb expands, inducing a switch in the longitudinal axial organization of the prosencephalon.

Highlights

  • Neurons producing melanin-concentrating hormone form a very conspicuous cell population in the dorsal and lateral hypothalamus [1]

  • Differentiation of melanin-concentrating hormone (MCH) neurons in the primary diencephalic mantle layer In the rat, the peak of birth of MCH neurons that project into the spinal cord is at E11, but the MCH phenotype differentiates only two to three days later (E13/14) [17]

  • First MCH cell bodies are clearly generated within this cell cord at this early stage, and are among the very first to be generated in the diencephalon, along with other unidentified cells

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Summary

Introduction

Neurons producing melanin-concentrating hormone form a very conspicuous cell population in the dorsal and lateral hypothalamus [1]. They are involved in sleep/wake cycle, and project throughout the central nervous system [1,2,3], as other diffusely projecting hypocretin (Hcrt)-, histamin- or serotonincontaining cell groups [4,5,6,7]. The MCH peptide modulates the dopaminergic mesotelencephalic system in the ventral tegmental area and accumbens nucleus [12]. It acts in these structures at least partly in concert with CART (‘cocaine and amphetamine related transcript’ peptide) and GABA [12,13]

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