Sleep‐wakefulness effects after microinjections of hypocretin 1 (orexin A) in cholinoceptive areas of the cat oral pontine tegmentum

Abstract

Hypocretinergic/orexinergic neurons, which are known to be implicated in narcolepsy, project to the pontine tegmentum areas involved in the control of rapid eye movement (REM) sleep. Here, we report the effects on sleep-wakefulness produced by low-volume microinjections of hypocretin (Hcrt)1 (20-30 nL, 100, 500 and 1000 microm) and carbachol (20-30 nL, 0.1 m) delivered in two areas of the oral pontine tegmentum of free-moving cats with electrodes for chronic sleep recordings: in the dorsal oral pontine tegmentum (DOPT) and in the ventral part of the oral pontine reticular nucleus (vRPO). Carbachol in the DOPT produced dissociate polygraphic states, with some but not all REM sleep signs. In contrast, carbachol in the vRPO produced a shift with short latency from wakefulness (W) to REM sleep with all of its polygraphic and behavioral signs. Hcrt-1 in the DOPT increased W and decreased both slow-wave sleep (SWS) and REM sleep during the first 3 h post-drug. The same doses of Hcr-1 in the vRPO produced a significant suppression of REM sleep without a definitive trend for changes in the other states. Both groups showed significant decreases in the number of transitions from SWS to REM sleep. Thus, Hcrt-1 produced distinct effects in cholinoceptive areas of the oral pontine tegmentum; in the DOPT it promoted W, suppressed SWS and probably defacilitated REM sleep, and in the vRPO it directly inhibited REM sleep. Hypocretinergic/orexinergic signaling is lost in narcoleptics and this absence would mean that pontine defacilitation/inhibition of REM sleep would also be absent, explaining why these patients can fall directly into REM sleep from W.

Hypocretin1/orexinA-immunoreactive axons form few synaptic contacts on rat ventral tegmental area neurons that project to the medial prefrontal cortex

“…Thus, Hcrt1/OxA could enhance the actions of an excitatory neurotransmitter during longer periods of time by rising excitability of common postsynaptic target neurons and/or temporal/spatial summation of individual actions of Hcrt1/OxA and the excitatory transmitter. Moreover, Hcrt/Ox inhibit brain structures involved in non-REM sleep generation, such as the preoptic area [20, 60, 61] and also blocks structures involved in REM sleep such as the ventral part of oral pontine tegmentum [62, 63].

Figure 9 Schematic drawing depicting the possible cellular mechanisms whereby Hcrt1/OxA activates neurons in the ventral tegmental area (VTA).

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Hypocretin and GABA Interact in the Pontine Reticular Formation to Increase Wakefulness

“…Rat microinjection sites in the present study (Figures 1 & 4) are in homologous regions of cat pontine reticular formation that produced either increases in wakefulness when hypocretin-1 was injected during wakefulness 16 or selective decreases in REM sleep. 15 Hypocretin-1 did significantly decrease REM sleep in the present study and also decreased NREM sleep (Figure 3). The reasons for these differences are not known but could include differences in microinjection sites, hypocretin receptors, and afferent hypocretin terminals, in addition to the use of different microinjection volumes and amounts of hypocretin-1.…”

“…An additional novel finding is that the hypocretin-1-induced increase gion of cat pontine reticular formation (referred to as peri-locus coeruleus α) during wakefulness causes a concentration-dependent increase in wakefulness and decrease in NREM sleep and REM sleep, whereas the same concentrations of hypocretin-1 delivered to the ventral part of cat pontine reticular formation cause a selective inhibition of REM sleep and no change in the amount of wakefulness or NREM sleep. 15 The dissimilar findings between these 2 studies in cat are reconciled by noting that different brain regions were microinjected with different amounts of hypocretin-1 in different microinjection volumes. 15,50 Species-specific responses to PnO microinjection of hypocretin-1 in rat and cat have been discussed in detail.…”

“…15 The dissimilar findings between these 2 studies in cat are reconciled by noting that different brain regions were microinjected with different amounts of hypocretin-1 in different microinjection volumes. 15,50 Species-specific responses to PnO microinjection of hypocretin-1 in rat and cat have been discussed in detail. 13 For the present report and a previous study 13 using rat, all microinjections were made during wakefulness, and hypocretin-1 caused an increase in wakefulness (Figure 3).…”

“…12 Microinjection of hypocretin-1 into rat PnO causes an increase in wakefulness, 13 and microinjection of hypocretin-1 into cat pontine reticular formation increases the cortically activated states of REM sleep 14 or wakefulness. 15,16 Administering hypocretin-1 into the PnO may increase wakefulness by modulating the release of arousal-promoting neurotransmitters within the PnO. Direct administration of hypocretin-1 to the PnO of isoflurane-anesthetized rat causes a concentration-dependent increase in both acetylcholine (ACh) release 17 and GABA levels 13 within the PnO.…”