Elsevier

Neuroscience

Volume 148, Issue 1, 10 August 2007, Pages 34-43
Neuroscience

Behavioural neuroscience
The development of hypocretin (OREXIN) deficiency in hypocretin/ataxin-3 transgenic rats

https://doi.org/10.1016/j.neuroscience.2007.05.029Get rights and content

Abstract

Narcolepsy is linked to a widespread loss of neurons containing the neuropeptide hypocretin (HCRT), also named orexin. A transgenic (TG) rat model has been developed to mimic the neuronal loss found in narcoleptic humans. In these rats, HCRT neurons gradually die as a result of the expression of a poly-glutamine repeat under the control of the HCRT promoter. To better characterize the changes in HCRT-1 levels in response to the gradual HCRT neuronal loss cerebrospinal fluid (CSF) HCRT-1 levels were measured in various age groups (2–82 weeks) of wild-type (WT) and TG Sprague–Dawley rats. TG rats showed a sharp decline in CSF HCRT-1 level at week 4 with levels remaining consistently low (26%±9%, mean±S.D.) thereafter compared with WT rats. In TG rats, HCRT-1 levels were dramatically lower in target regions such as the cortex and brainstem (100-fold), indicating decreased HCRT-1 levels at terminals. In TG rats, CSF HCRT-1 levels significantly increased in response to 6 h of prolonged waking, indicating that the remaining HCRT neurons can be stimulated to release more neuropeptide. Rapid eye movement (REM) sleep in TG rats (n=5) was consistent with a HCRT deficiency. In TG rats HCRT immunoreactive (HCRT-ir) neurons were present in the lateral hypothalamus (LH), even in old rats (24 months) but some HCRT-ir somata were in various stages of disintegration. The low output of these neurons is consistent with a widespread dysfunction of these neurons, and establishes this model as a tool to investigate the consequences of partial hypocretin deficiency.

Section snippets

Animals

The TG rats are derived from the Sprague–Dawley strain. Two independent groups of rats were used in the present study, one maintained at the West Roxbury VA hospital and the other at Stanford University. However, both groups of rats were derived from rats obtained from the original colony in University of Texas Southwestern Medical Center, Dallas, TX, USA (Beuckmann et al., 2004). Genotype analysis of tail snips was used to confirm that the ataxin-3 transgene was present. Since the previous

Age-related changes in CSF and brain tissues

Fig. 1 summarizes the age-related changes in HCRT-1 level in CSF and brain tissues in WT and TG rats. Fig. 1A shows the CSF HCRT-1 levels in each rat. In WT rats, CSF HCRT-1 was detectable at 2 weeks, when the samples could be reliably collected. CSF HCRT-1 then increased gradually over the next few weeks to reach a stable level at week 5. In contrast, HCRT-1 levels in TG rats continuously decreased from week 2 to week 4, to stabilize at a low level after the fourth week. This decrease was

Discussion

The primary finding of this study is that the TG rats, which represent a rat model of the HCRT neuronal degeneration seen in human narcolepsy, have a 74% decline in CSF levels of HCRT-1. The TG rats showed a significant and precipitous decline in CSF HCRT-1 levels around 4 weeks of age, consistent with a previous study reporting cell death during this developmental period (Beuckmann et al., 2004). In the TG rats HCRT neurons were evident even in old (60–124 weeks) rats, but the rich axonal and

Acknowledgments

We thank Suraiya Begum for providing technical assistance, E. Winston for analyzing the sleep data, and Shelley Dixon for expert managerial assistance. Supported by NIH grants NS30140, NS52287, MH55772, MH01600, NS23724, MH073435 and Medical Research Service of the Department of Veterans Affairs. E.M. and M.Y. are HHMI investigators.

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