Hypocretin-2-saporin lesions of the lateral hypothalamus produce narcoleptic-like sleep behavior in the rat

Hypocretins (Hcrts) are recently discovered peptides linked to the human sl eep disorder narcolepsy. Humans with narcolepsy have decreased numbers of H crt neurons and Hcrt-null mice also have narcoleptic symptoms. Hcrt neurons are located only in the lateral hypothalamus (LH) but neither electrolytic nor pharmacological lesions of this or any other brain region have produce d narcoleptic-like sleep, suggesting that specific neurons need to be destr oyed. Hcrt neurons express the Hcrt receptor, and to facilitate lesioning t hese neurons, the endogenous ligand hypocretin-2/orexin B (Hcrt2) was conju gated to the ribosome-inactivating protein saporin (SAP). In vitro binding studies indicated specificity of the Hcrt2-SAP because it preferentially bo und to Chinese hamster ovary cells containing the Hcrt/orexin receptor 2 (H crtR2/OX2R) or the Hcrt/orexin receptor 1 (HcrtR1/OX1R) but not to Kirsten murine sarcoma virus transformed rat kidney epithelial (KNRK) cells stably transfected with the substance P (neurokinin-1) receptor. Administration of the toxin to the LH, in which the receptor is known to be present, elimina ted some neurons (Hcrt, melanin-concentrating hormone, and adenosine deamin ase-containing neurons) but not others (a-melanocyte-stimulating hormone), indicating specificity of the toxin in vivo. When the toxin was administere d to the LH, rats had increased slow-wave sleep, rapid-eye movement (REM) s leep, and sleep-onset REM sleep periods. These behavioral changes were nega tively correlated with the loss of Hcrt-containing neurons but not with the loss of adenosine deaminase-immunoreactive neurons. These findings indicat e that damage to the LH that also causes a substantial loss of Hcrt neurons is likely to produce the multiple sleep disturbances that occur in narcole psy.