Activation of the basal forebrain by the orexin/hypocretin neurones

Abstract

The orexin neurones play an essential role in driving arousal and in maintaining normal wakefulness. Lack of orexin neurotransmission produces a chronic state of hypoarousal characterized by excessive sleepiness, frequent transitions between wake and sleep, and episodes of cataplexy. A growing body of research now suggests that the basal forebrain (BF) may be a key site through which the orexin-producing neurones promote arousal. Here we review anatomical, pharmacological and electrophysiological studies on how the orexin neurones may promote arousal by exciting cortically projecting neurones of the BF. Orexin fibres synapse on BF cholinergic neurones and orexin-A is released in the BF during waking. Local application of orexins excites BF cholinergic neurones, induces cortical release of acetylcholine and promotes wakefulness. The orexin neurones also contain and probably co-release the inhibitory neuropeptide dynorphin. We found that orexin-A and dynorphin have specific effects on different classes of BF neurones that project to the cortex. Cholinergic neurones were directly excited by orexin-A, but did not respond to dynorphin. Non-cholinergic BF neurones that project to the cortex seem to comprise at least two populations with some directly excited by orexin-A that may represent wake-active, GABAergic neurones, whereas others did not respond to orexin-A but were inhibited by dynorphin and may be sleep-active, GABAergic neurones. This evidence suggests that the BF is a key site through which orexins activate the cortex and promote behavioural arousal. In addition, orexins and dynorphin may act synergistically in the BF to promote arousal and improve cognitive performance.

Figure 1

The ascending arousal systems are diffusely projecting neurons (blue) that use acetylcholine, monoamines, or neuropeptides to produce broad changes in neuronal activity. The pedunculopontine (PPT) and laterodorsal tegmental (LDT) nuclei are the major cholinergic inputs to the thalamus. The key monoaminergic nuclei include the locus coeruleus (LC) which is a major source of noradrenaline (NA) to the hypothalamus and cortex, the dorsal and median raphe nuclei which produce serotonin (5-HT), the A10 cell group of the ventral periaqueductal gray matter (vPAG) which produces dopamine (DA), and the tuberomammillary nucleus (TMN) which produces histamine. In addition, peptidergic neurons in the lateral hypothalamus (LH) produce orexins and melanin-concentrating hormone (MCH). All these regions innervate the BF, and BF neurons send descending projections back to the lateral hypothalamus (red), thalamus, and brainstem.

Figure 2

Cholinergic neurons of the MCPO and SI are excited by orexin-A but do not respond to dynorphin. (a) Two SI cholinergic neurons labeled with Cy3-p75-IgG (left) and the same neurons under infrared differential interference contrast (IR-DIC) visualization. (b) Firing properties of MCPO/SI neurons during depolarizing (left) and hyperpolarizing current pulses (in TTX 1 μM, right), showing low threshold Ca²⁺, delayed firing followed by hyperpolarizing potentials due to activation of I_K(A) (arrowhead) and a small I_h. (c) MCPO/SI neurons do not respond to dynorphin (10 μM), but orexin-A (300 nM) activates an inward current (Vh = -60 mV). (d) A SI cholinergic neuron that projects to the mPFC is double labeled with retrograde fluorescent beads (green) and Cy3-p75-IgG (red) and has a sustained increased in firing with orexin-A (trace below). (e) Orexin-A potentiates EPSCs evoked by local electrical stimulation (Vh =-60 mV)

Figure 3

Non-cholinergic, cortically-projecting neurons in the MCPO and SI have two types of responses to orexin-A and dynorphin. (a) Two SI neurons retrogradely labeled with green fluorescent beads from mPFC. The lower cell is also labeled with red Cy3-p75-IgG, a marker for the BF cholinergic neurons; the upper cell is a non-cholinergic. (b) A subset of these neurons have pronounced depolarizing sags during negative current pulses (arrowhead) due to the activation of Ih. (c) Ih recorded in voltage clamp mode (Vh = -50mV; -10 mV pulses). (d) Spontaneous firing is increased by orexin-A (300 nM) but is unaffected by dynorphin (10 μM). (e) Inward current activated by orexin-A (Vh = -60 mV). (f) Evoked EPSCs are potentiated by orexin-A and inhibited by dynorphin (Vh = -60 mV). (g) A second subset of non-cholinergic cortically-projecting neurons in the MCPO/SI have burst discharges, no Ih, and no IK_A. (h) These type of neuron is inhibited by dynorphin (dotted line = -60 mV).

Figure 4

Pathways through which the orexin neurons may activate the BF to promote wakefulness. Orexins excite wake-promoting cholinergic and non-cholinergic neurons (most of which probably contain GABA). Orexins also enhance release of glutamate in the BF. In contrast, dynorphin released from the orexin neurons acts through κ opiate receptors (KOR) to inhibit sleep-active cells, including GABAergic interneurons. Solid lines indicate pathways active during wake; dashed lines indicate pathways active during sleep. Arrows indicate excitatory inputs; bars indicate inhibitory inputs. Not shown are the descending projections to the thalamus, hypothalamus and brainstem.