From Erdem Tuzun
I read with interest Eugenie Samuel’s recent news item on American research showing that
people suffering from narcolepsy do so because their brains have fewer hypocretin neurons
(9 September, p 8).
Samuel says that the neurotransmitter hypocretin, also known as orexin, normally keeps
mind and body active by suppressing activity in the part of the brain known as the medial
medulla. She would do better to refer to the locus coeruleus.
The locus coeruleus is clearly connected with loss of muscle tone during
narcoleptic fits (called cataplexies), and orexin neurons connect this region
with the hypothalamus. Although some weak orexin receptor re-activity can be
observed in the medial medulla too, the significance of this for an
understanding of sleep or narcolepsy-cataplexy has not yet been identified. So
although the significance of the medial medulla in cataplexy has been
established, its accurate place in the orexin story is not known.
There are two major possibilities. The medial medulla may be passively
hyperactivated because of the lack of activation in the locus coeruleus. Or
orexin may play a role and directly activate the medial medulla, causing
cataplexy as a result.
Istanbul
