Selective action of orexin (hypocretin) on nonspecific thalamocortical projection neurons

Abstract

As is evident from the pathological consequences of its absence in narcolepsy, orexin (hypocretin) appears to be critical for the maintenance of wakefulness. Via diffuse projections through the brain, orexin-containing neurons in the hypothalamus may act on a number of wake-promoting systems. Among these are the intralaminar and midline thalamic nuclei, which project in turn in a widespread manner to the cerebral cortex within the nonspecific thalamocortical projection system. Testing the effect of orexin in rat brain slices, in two nuclei of this system, centromedial (CM) nuclei and rhomboid nuclei, we found that it depolarized and excited all neurons tested through a direct postsynaptic action. An additional analysis of this effect in CM neurons indicates that it results from the decrease of a potassium conductance. By a detailed comparison of the effects of orexin A and B, we established that orexin B was more potent than orexin A, indicating the probable mediation by orexin type 2 receptors. In contrast to its effect on the nonspecific thalamocortical projection neurons, orexin had no effect on the specific sensory relay neurons of the somatic, ventral posterolateral, and visual dorsal lateral geniculate nuclei. Orexin differs in this regard from norepinephrine and acetylcholine, to which neurons in the specific and nonspecific systems are sensitive. Orexin may thus act in the thalamus to promote wakefulness by exciting neurons of the nonspecific thalamocortical projection system, which, through widespread projections to the cerebral cortex, stimulate and maintain cortical activation.

Fig. 1.

Localization of orexin (hypocretin)-responsive neurons in the CM intralaminar thalamic nucleus. A, B, Characterization of a CM neuron with its responses to a depolarizing current step delivered from rest (A) or from a hyperpolarized level (B), showing the typical strong LTS (asterisk). C, Depolarizing and excitatory action of orexin A (Ox A).D, Three CM neurons that have responded to orexins are shown (enlarged in inset) within a single slice.E, All neurobiotin-filled neurons (n= 13, dots, asterisks, and triangle), including the one (red triangle) illustrated in Figure2D and the three (asterisks) shown in D, are within the limits of the CM neurons. Scale bars: D, 250 μm; inset inD, 25 μm; E, 500 μm. MD, Mediodorsal thalamic nucleus; Sub, submedius thalamic nucleus.

Fig. 2.

Effects of orexin (hypocretin) on CM neurons.A, B, Effects of orexin A (Ox A) and orexin B (Ox B) on the same CM neuron.C, Dose–response curves to Ox A and Ox B (for the two peptides at consecutive concentrations: n = 4 and 4 at 4 nm, 7 and 9 at 10 nm, 8 and 10 at 20 nm, 6 and 11 at 30 nm, 3 and 6 at 50 nm, 15 and 27 at 100 nm, and 4 and 4 at 200 nm, respectively). D, Comparison of hyperpolarizing pulses before (asterisk) and during (square) the effect of orexin (see bottom inset enlargement of the pulses demonstrating the increase in membrane resistance in the presence of orexin). Top inset enlargements (corresponding to positions 1 and 2 of the original trace) illustrate the rebound burst after a hyperpolarizing pulse delivered from the resting level under control conditions (1) and the tonic firing during the effect of orexin (2). E, The depolarizing effects of orexin (left) are reversibly suppressed (right) when neurons are held atE_K (middle).

Fig. 3.

Actions of orexin (hypocretin) on Rh and VPL neurons. A, Localizations of the Rh and VPL nuclei with all injected cells (dots, asterisks, andtriangles). IAM, Interanteromedial thalamic nucleus; ic, internal capsule; MD, mediodorsal thalamic nucleus; Sub, submedius thalamic nucleus; VPM, ventral posteromedial thalamic nucleus;Rt, reticular thalamic nucleus. Red triangles correspond to the injected cells shown inB and C. B, C, Neurobiotin-filled neurons in the Rh and VPL nuclei (insets showing characteristic responses to hyperpolarizing pulses). D, Depolarizing and excitatory effect of orexin in the Rh neurons. E, Absence of effect of orexin in the VPL. Scale bars: A, 500 μm; B, C, 20 μm.