Rewiring brain circuits to block cataplexy in murine models of narcolepsy

Abstract

Narcolepsy was first identified almost 130 years ago, but it was only 15 years ago that it was identified as a neurodegenerative disease linked to a loss of orexin neurons in the brain. It is unclear what causes the orexin neurons to die, but our strategy has been to place the gene for orexin into surrogate neurons in the validated mouse models of narcolepsy, and test whether it can block narcolepsy symptoms, such as cataplexy. In both the orexin knockout and the orexin-ataxin-3 mouse models of narcolepsy we have found that cataplexy can be blocked if the surrogate neurons are part of the circuit responsible for cataplexy. We have also determined that the orexin gene can be inserted into surrogate neurons in the amygdala to block emotion-induced cataplexy. Through the use of optogenetics we anticipate that it will be possible to preemptively block cataplexy.

Fig. 1. A neural circuit model of narcolepsy and our strategy to pre-emptively block cataplexy from being triggered by emotions

Normal muscle tone is regulated by the activity of pontine neurons (Brooks and Peever, 2008). During emotions the orexin excitatory signal from the lateral hypothalamus neutralizes the inhibitory signal from the amygdala and maintains the activity of pontine neurons (A). In narcoleptic mice the orexin excitatory signal from the lateral hypothalamus is absent. Therefore, the inhibitory signal from the amygdala overwhelms and inhibits pontine neurons controlling muscle tone, resulting in muscle atonia during waking (i.e. cataplexy) (B). When we insert orexin gene into surrogate neurons in the amygdala (C) or block the inhibitory input (D), the pons will be disinhibited and be able to maintain muscle tone. But if we excite the amygdala GABA pathway there will be more inhibitory input to pons that will exacerbate cataplexy (E).

Fig 2

Illustration of WGA-Cre circuit mapping tool and preliminary behavioral data that placing orexin into a specific circuit reduces cataplexy. Photo A illustrates that injection of AAV-mCherry-WGA-Cre into pons and AAV-DIO-orexin injection into the amygdala can identify CeA neurons projecting to the pons. WGA-Cre (red dots) transports retrogradely (trans-synaptic) to neuronal somata in amygdala, where Cre drives orexin expression (green). Photo B depicts extent of injection of WGA-Cre-mCherry in the vlPAG/LC area three weeks after injection. Photo C depicts neurons containing Cre-driven Orexin expression in the CeA. Figure D summarizes the behavioral data that Cre-driven orexin expression in CeA neurons projecting to the pons significantly suppresses predator odor-induced cataplexy (N=7/each group) (*=p<0.05).