Relationship between the perifornical hypothalamic area and oral pontine reticular nucleus in the rat. Possible implication of the hypocretinergic projection in the control of rapid eye movement sleep

Abstract

The perifornical (PeF) area in the posterior lateral hypothalamus has been implicated in several physiological functions including the regulation of sleep-wakefulness. Some PeF neurons, which contain hypocretin, have been suggested to play an important role in sleep-wake regulation. The aim of the present study was to examine the effect of the PeF area and hypocretin on the electrophysiological activity of neurons of the oral pontine reticular nucleus (PnO), which is an important structure in the generation and maintenance of rapid eye movement sleep. PnO neurons were recorded in urethane-anesthetized rats. Extracellular recordings were performed by means of tungsten microelectrodes or barrel micropipettes. Electrical stimulation of the ipsilateral PeF area elicited orthodromic responses in both type I (49%) and type II (58%) electrophysiologically characterized PnO neurons, with a mean latency of 13.0 +/- 2 and 8.3 +/- 5 ms, respectively. In six cases, antidromic spikes were evoked in type I PnO neurons with a mean latency of 3.2 +/- 0.4 ms, indicating the existence of PnO neurons that projected to the PeF area. Anatomical studies showed retrogradely labeled neurons in the PeF area from the PnO. Some of these neurons projecting to the PnO contained hypocretin (17.8%). Iontophoretic application of hypocretin-1 through a barrel micropipette in the PnO induced an inhibition, which was blocked by a previous iontophoretic application of bicuculline, indicating that the inhibitory action of hypocretin-1 may be due to activation of GABA(A) receptors. These data suggest that the PeF area may control the generation of rapid eye movement sleep through a hypocretinergic projection by inhibiting the activity of PnO neurons.

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

“…Considered with data showing that delivering hypocretin-1 to the PnO increases GABA levels in the PnO 13 and causes inhibition of some PnO neurons, 10 and with evidence that GABAergic transmission in the PnO is wakefulness promoting, 13,[19][20][21][22][23][24][25] the present results support the interpretation that increasing GABAergic transmission in the PnO is one mechanism by which hypocretin-1 increases wakefulness. The results are discussed below relative to the functional roles of hypocretin-1 in rat PnO and the mechanisms by which hypocretin-1 administered to the PnO causes an increase in wakefulness.…”

“…Extracellular recording studies of PnO neurons in urethane-anesthetized rat show that iontophoretic application of hypocretin-1 causes a hyperpolarization that is blocked by prior application of bicuculline. 10 This finding indicates that the hypocretin-1-induced inhibition of PnO neurons is mediated by GABA A receptors. Identified GABAergic neurons in brainstem slices of mouse PnO have been shown to be excited by hypocretin-1, 18 and intracellular recording studies in halothane-anesthetized cat show that hypocretin-1 can also cause direct depolarization of PnO neurons and an increase in PnO neuronal firing rate.…”

“…17 Hcrt-r2 is present on GABAergic neurons in rat PnO, 54 suggesting that hcrt-r2 mediates the increase in PnO GABA levels caused by hypocretin-1, 13 as well as the hypocretin-1-induced hyperpolarization of PnO neurons. 10 SB-334867 has a 50-fold higher affinity for hcrt-r1 than for hcrt-r2, 55 and the finding that the hypocretin-1-induced increase in wakefulness was blocked by SB-334867 ( Figure 6A) receptors. This finding, therefore, supports the interpretation that the hypocretin-1-induced increase in wakefulness ( Figure 3) results from enhanced GABAergic transmission in the PnO.…”

“…1 One such area is the pontine reticular nucleus, oral part (PnO). 1,10 The PnO is the rostral portion of the rodent pontine reticular formation 11 and contributes to the generation of wakefulness and rapid eye movement (REM) sleep. 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.…”

Modulation of neurotransmitter release in orexin/hypocretin‐2 receptor knockout mice: A microdialysis study

“…With the assumption that OX2 receptors are coupled exclusively to G q proteins, a disinhibitory action through their presynaptic regulation of inhibitory GABAergic neurons might be a probable mechanism (Kukkonen et al, 2002). Studies have also suggested an excitatory role of OX2 receptors in modulating activity of GABAergic neurons in the medial septum/diagonal band of Broca (Wu et al, 2002) and arcuate nucleus (Burdakov et al, 2003), indicating that, at least in other brain areas, activation of OX2 receptors could result in an increase of GABA release and a subsequent inhibitory action on other neurotransmitters in the area via GABA A receptors (Nuñez et al, 2006;Brevig et al, 2010). An alternative mechanism could involve a direct action through OX2 receptors coupled to G i/o proteins and localized on cholinergic, histaminergic, and norepinephrinergic nerve terminals.…”