NeuroVoices: Jerome Siegel, PhD, on Locus Coeruleus Neuron Loss as a Key Pathologic Feature of Narcolepsy

Prior research has linked narcolepsy with cataplexy primarily to the loss of hypocretin neurons in the hypothalamus.¹ In a newly published postmortem analysis, investigators reported that patients with narcolepsy with cataplexy also demonstrated substantial degeneration of norepinephrine-producing neurons in the locus coeruleus, including an average 46% reduction in neuronal number and an 18% increase in cell size across 11 examined brains.² The study additionally identified microglial clustering surrounding both hypocretin and locus coeruleus neurons, suggesting inflammatory involvement in the degeneration of these neuronal populations.

The study, conducted by senior author Jerome Siegel, PhD, and colleagues, noted that these findings expand the current understanding of narcolepsy pathophysiology beyond hypocretin deficiency alone. According to the investigators, evidence of microglial activation in both the hypothalamus and locus coeruleus supports the possibility that autoimmune or neuroimmune-related mechanisms that may contribute to neuronal loss across multiple brain regions involved in arousal and muscle tone regulation. Thus, the researchers suggested that further investigation into microglial activity may help clarify the mechanisms underlying degeneration in narcolepsy.

In a new iteration of NeuroVoices, Siegel, professor of psychiatry at the David Geffen School of Medicine at UCLA, emphasized that locus coeruleus neuronal deficits appeared more consistent than hypocretin neuronal loss in examined postmortem tissue. He also noted that some patients with narcoleptic symptoms present with normal hypocretin levels, indicating that additional mechanisms may contribute to disease pathology. Siegel highlighted the potential therapeutic relevance of norepinephrine-targeting agents, including norepinephrine reuptake inhibitors, and underscored the need for future research evaluating autoimmune pathways, broader neuronal involvement, and neuroimmune interactions in narcolepsy.

NeurologyLive: Can you provide an overview of your narcolepsy study and explain the main objective behind investigating hypocretin and locus ceruleus neuron loss?

Jerome Siegel, PhD: We want to understand narcolepsy, and it's important to appreciate that we're working with postmortem tissue. The initial discovery here was made by our group, and also slightly later by other groups, showed that hypocretin neurons were lost in narcolepsy. But a problem with this finding, which initiated the idea that the loss of hypocretin neurons was responsible for narcolepsy, and indeed this does occur in narcolepsy. A test was developed for measuring hypocretin levels in the cerebrospinal fluid, and that now is done routinely in people suspected of being narcoleptic. In fact, a large percentage of people who have all the symptoms of narcolepsy have absolutely normal hypocretin levels, and this is reviewed in the introduction to our paper. So, there's something missing here.

Knowing that locus coeruleus neurons are very well established as parts of the ascending reticular activating system, that they're related to arousal, and there's other data indicating that they're related to muscle tone control. We looked at a total of 11 brains in people with narcolepsy to see if there were any deficits in the locus coeruleus. In fact, the deficits are much more consistent than those in the hypocretin neuronal group. So certainly, presumably, an autoimmune process is affecting both groups. That wouldn't be surprising, but the emphasis on it being just a hypocretin deficit seems to have missed the larger effect or the more significant damage, which is the loss of locus coeruleus cells in the brainstem.

Based on your findings, what are the key clinical takeaways and treatment implications for patients with narcolepsy?

The important clinical implication is in terms of treatments. There's a recent paper by a Chinese group, in which they compared treatments that have been used in narcolepsy without any real theoretical limitation on what they were using. There are stimulants of various kinds, everything from caffeine to amphetamine. They also had a group that were using noradrenergic reuptake inhibitors, which showed the best treatment outcome for narcolepsy, even though that had no logical connection with the loss of hypocretin cells.

Now that we have established that there is, in fact, a loss of locus coeruleus cells, which release norepinephrine, this complete picture is present. The loss of these cells is correlated with narcolepsy and that the norepinephrine reuptake inhibitors, which raise norepinephrine level, are the most effective treatment for narcolepsy, which, of course, is what clinicians and patients are most concerned about.

What additional research is still needed to better understand the autoimmune mechanisms and broader neurologic effects associated with narcolepsy?

From a neuroscientist perspective, as opposed to a neurologist perspective, you want to know exactly what is triggering this autoimmune response. I think most people have accepted for a long time because there's a certain HLA linkage in narcolepsy, which is almost perfect. So, how is it that these two different groups of neurons are destroyed, and how might that be prevented and detected in a more effective way?

People with narcolepsy are also consistently depressed. The argument has been made that this goes beyond what you would expect for a chronic illness, although that's very hard to quantify. But you need to know that both of these systems are damaged and maybe treat both systems to produce the optimal effect.

Also, we want to understand how this happens. Are there other neuronal groups that are affected? Is this a general phenomenon that might affect people who are not diagnosed with narcolepsy, or who have a different magnitude of changes in these two systems? In fact, does the immune system shape the development of the brain in normal individuals? We're all exposed to immunoactive substances, and you think of the brain as being protected from that, but maybe not.

Transcript edited for clarity. Click here to view more NeuroVoices.

REFERENCES
1. Mahoney CE, Cogswell A, Koralnik IJ, Scammell TE. The neurobiological basis of narcolepsy. Nat Rev Neurosci. 2019;20(2):83-93. doi:10.1038/s41583-018-0097-x
2. Thannickal TC, Wu MF, Cornford ME, Siegel JM. Human narcolepsy is linked to degeneration of both locus coeruleus and hypocretin neurons. Nat Commun. Published online March 28, 2026. doi:10.1038/s41467-026-70899-x