Clinical implications of basic research; the role of orexin neurons in the central autonomic network.

Narcolepsy type 1 (NT1) and, to a lesser extent, neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, dementia with Lewy bodies and multiple system atrophy, entail the loss of the hypothalamic neurons that release the hypocretin/orexin (H/O) neuropeptides. NT1 has been associated with autonomic anomalies including alterations in temperature regulation and cardiovascular control, particularly during sleep. A spectrum of autonomic dysfunctions also characterizes neurodegenerative diseases. The central autonomic network (CAN) is an interconnected set of brain structures that are critical for the control of autonomic preganglionic neurons. The H/O neurons include pre-autonomic neurons that directly target preganglionic sympathetic neurons in the intermediolateral column of the spinal cord and parasympathetic neurons in the dorsal motor nucleus of the vagus nerve. The H/O neurons also project to and modulate the activity of other CAN structures that include pre-autonomic neurons, such as the rostral ventromedial medulla and caudal raphe nuclei, the rostral ventrolateral medulla and the hypothalamic paraventricular nucleus. In addition, the H/O neurons project to and modulate the activity of neurons in the nucleus of the solitary tract in the medulla, which receives and relays visceral afferent information, and in higher order structures of the CAN, such as the dorsomedial nucleus of the hypothalamus and the extended amygdala. The H/O neurons should, therefore, be regarded as a key component of the CAN. Functional alterations of the CAN due to H/O neuron deficiency might contribute to autonomic anomalies in patients with neurodegenerative diseases and are likely to underlie autonomic anomalies in patients with NT1.