Developmental changes in CSF hypocretin-1 (orexin-A) levels in normal and genetically narcoleptic Doberman pinschers

Abstract

Loss of hypocretin cells or mutation of hypocretin receptors causes narcolepsy. In canine genetic narcolepsy, produced by a mutation of the Hcrtr2 gene, symptoms develop postnatally with symptom onset at 4 weeks of age and maximal symptom severity by 10-32 weeks of age. Canine narcolepsy can readily be quantified. The large size of the dog cerebrospinal fluid (CSF) cerebellomedullary cistern allows the withdrawal of sufficient volumes of CSF for accurate assay of hypocretin levels, as early as postnatal day 4. We have taken advantage of these features to determine the relation of CSF hypocretin levels to symptom onset and compare hypocretin levels in narcoleptic and normal dogs. We find that by 4 days after birth, Hcrtr2 mutants have significantly higher levels of Hcrt than normal age- and breed-matched dogs. These levels were also significantly higher than those in adult narcoleptic and normal dogs. A reduction followed by an increase in Hcrt levels coincides with symptom onset and increase in the narcoleptics. The Hcrtr2 mutation alters the normal developmental course of hypocretin levels.

Figure 1. Developmental changes in CSF Hcrt-1 concentration in normal (A) and narcoleptic puppies (B)

CSF Hcrt-1 levels measured from 4 days to 32 weeks after birth. The number of dogs used for each time point is indicated in parentheses.

Figure 2. Developmental changes in CSF Hcrt-1 concentration and cataplexy

A, comparison of changes in Hcrt-1 concentration (means + s.e.m.) in normal and narcoleptic puppies. *P < 0.05, **P < 0.01 compared to values of normal and narcoleptic dog at each time point (same age group); †P < 0.05, ††P < 0.01 compared with 32-week-old narcoleptic dog; and ‡P < 0.01, ‡‡P < 0.01 compared with 32-week-old normal dog. B, developmental changes in cataplexy and Hcrt-1 monitored in a subgroup of narcoleptic dogs (n = 8). The reduced level of CSF Hcrt-1 concentration compared to age-matched normal dogs (A) coincides with the onset of cataplexy (•) between the second and fourth postnatal week. The severity of cataplexy reaches its peak and remains high from the tenth postnatal week onward, continuing into adulthood.

Figure 3. Developmental variation of CSF Hcrt-1 concentration in male and female dogs

Hcrt-1 levels measured from 4 days to 32 weeks after birth. A, normal dogs; B, narcoleptic dogs. The variation of Hcrt-1 throughout development is similar in male and female puppies in both the normal and narcoleptic group.

Figure 4. Developmental changes in body weight in normal and narcoleptic Doberman Pinchers

There was no significant difference in body weight between normal and narcoleptic animals over development. Although normal dogs had somewhat higher weights between 7 and 12 months, the number of dogs available at these ages was reduced and this difference was significant only at 8 months. **P < 0.01 compared to normal.