Hypoxia and hypercapnia inhibit hypothalamic orexin neurons in rats

Evidence of impaired function of orexin neurons has been found in individuals with cardiorespiratory disorders such as obstructive sleep apnea (OSA) and sudden infant death syndrome (SIDS), but the mechanisms responsible are unknown. Individuals with OSA and SIDS experience repetitive breathing cessations and/or rebreathing of expired air, resulting in hypoxia/hypercapnia (H/H). In this study we examined the responses of fluorescently identified rat orexin neurons in the lateral hypothalamus to acute H/H to test if and how these neurons alter their activity and function during this challenge. Experiments were conducted in an in-vitro slice preparation using voltage-clamp and current-clamp configurations. H/H (10 min) induced hyperpolarization accompanied by rapid depression and finally cessation of firing activity in orexin neurons. Hypoxia alone had similar but less potent effects. H/H did not alter the frequency of inhibitory glycinergic postsynaptic currents. The frequency of GABAergic currents was diminished but only at min 8-10 of H/H. In contrast, the frequency of excitatory glutamatergic postsynaptic events was diminished as early as min 2-4 of H/H. In the presence of glutamatergic receptor blockers the inhibitory effects of H/H on the firing activity and membrane potential of orexin neurons persisted, but to a lesser extent. In conclusion, both direct alteration of postsynaptic membrane properties and diminished glutamatergic neurotransmission likely contributes to the inhibition of orexin neurons by H/H. These mechanisms could be responsible for the decreased function of orexin in individuals at risk for OSA and SIDS.

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