Elsevier

Neuroscience

Volume 134, Issue 3, 2005, Pages 1081-1090
Neuroscience

Systems neuroscience
Hypothalamic and zona incerta neurons expressing hypocretin, but not melanin concentrating hormone, project to the hamster intergeniculate leaflet

https://doi.org/10.1016/j.neuroscience.2005.03.062Get rights and content

Abstract

The hypocretins (Hcrt; also known as orexins) and melanin-concentrating hormone comprise distinct families of neuropeptides synthesized in cells located in the lateral hypothalamus and adjacent areas. The Hcrts are thought to modulate food intake and sleep/wake patterns in mammals. Melanin-concentrating hormone has a well-documented role in energy metabolism. A moderate plexus of Hcrt immunoreactive terminals has been described in the hamster intergeniculate leaflet, part of the circadian rhythm system. This study investigated the origin of Hcrt-immunoreactive terminals in the intergeniculate leaflet and determined whether melanin-concentrating hormone neurons also project to the intergeniculate leaflet. The tracer, cholera toxin β-subunit, was injected into the intergeniculate leaflet of the golden hamster. Double-label fluorescent immunohistochemistry for cholera toxin β-subunit and Hcrt or melanin-concentrating hormone was then performed to identify retrogradely labeled cells also containing immunoreactive peptide. Most cholera toxin β-subunit-labeled cells were detected in the medial zona incerta and sub-incertal zone, with few observed in the lateral hypothalamus. Hcrt-immunoreactive cells were abundant and found largely in the lateral hypothalamus and adjacent nuclei. Melanin-concentrating hormone cells were also abundant in the medial zona incerta, in close proximity to cholera toxin β-subunit-labeled cells, but ventral to them. Cells containing both cholera toxin β-subunit and Hcrt-immunoreactive, were present in the dorsal aspect of the lateral hypothalamus. The number observed was small, ≤1% of the total number of Hcrt cells counted in the hamster. No cholera toxin β-subunit-immunoreactive cells also contained melanin-concentrating hormone and no melanin-concentrating hormone-immunoreactive processes were evident in the intergeniculate leaflet. The results show that a small number of lateral hypothalamus cells containing Hcrt-immunoreactivity project to the intergeniculate leaflet, but they are scattered rather than collected into a discrete group. At the present time there is no information regarding the function of these cells, although they may contribute to the regulation of sleep/arousal, circadian rhythmicity, or vestibulo-oculomotor function.

Section snippets

Animals

Adult, outbred male golden hamsters (Charles River, Wilmington, MA, USA) were maintained with free access to food and water under a 14-h light/10-h dark photoperiod in individual, polypropylene cages. All experimental procedures were approved by the Institutional Animal Care and Use Committee of Stony Brook University. Care was taken to use the least number of animals possible and to minimize their discomfort. All experimental procedures were approved by the Institutional Animal Care and Use

Injection sites

Brains that received injections centered in the IGL (n=11) with minimal spread to adjacent structures were chosen for analysis (Fig. 1). Typical injection sites were centered in the medial aspect of the mid-caudal IGL with very little functional spread into the dorsal lateral geniculate nucleus and almost no spread into the ventral lateral geniculate. Successful CTB injections to the IGL were further verified by the presence of retrogradely labeled neurons in the pretectum and contralateral IGL

Discussion

An Hcrt-ir terminal plexus has been reported in the hamster IGL (Mintz et al., 2001). The present data confirm this projection and identify the cells of origin which are located in the ZIM and LH. The data also show the anatomical distribution of MCH-ir cells in the hamster hypothalamus and their relationship to Hcrt-ir and IGL projecting cells. The results demonstrate that, as in the rat (Broberger et al 1998, Elias et al 1998), Hcrt- and MCH-ir neurons have overlapping distributions in the

Acknowledgment

The authors wish to thank Dr. Seth Horowitz for his assistance with earlier versions of the manuscript. Grant Sponsor: National Institute of Mental Health; Grant Number MH6447101. Grant Sponsor: National Institute of Neurological Disease and Stroke; Grant Number NS22168.

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