Optogenetic-Mediated Release of Histamine Reveals Distal and Autoregulatory Mechanisms for Controlling Arousal

Abstract

Histaminergic neurons in the tuberomammillary nucleus (TMN) are an important component of the ascending arousal system and may form part of a "flip-flop switch" hypothesized to regulate sleep and wakefulness. Anatomical studies have shown that the wake-active TMN and sleep-active ventrolateral preoptic nucleus (VLPO) are reciprocally connected, suggesting that each region can inhibit its counterpart when active. In this study, we determined how histamine affects the two branches of this circuit. We selectively expressed channelrhodopsin-2 (ChR2) in TMN neurons and used patch-clamp recordings in mouse brain slices to examine the effects of photoevoked histamine release in the ventrolateral TMN and VLPO. Photostimulation decreased inhibitory GABAergic inputs to the ventrolateral TMN neurons but produced a membrane hyperpolarization and increased inhibitory synaptic input to the VLPO neurons. We found that in VLPO the response to histamine was indirect, most likely via a GABAergic interneuron. Our experiments demonstrate that release of histamine from TMN neurons can disinhibit the TMN and suppresses the activity of sleep-active VLPO neurons to promote TMN neuronal firing. This further supports the sleep-wake "flip-flop switch" hypothesis and a role for histamine in stabilizing the switch to favor wake states.

Histamine H3 receptor as a potential target for cognitive symptoms in neuropsychiatric diseases

“…Their activity peaks at high vigilance state and diminishes during sleep (Brown et al, 2001, Haas et al, 2008. Also, TMN histaminergic neurons have a mutual inhibitory interaction with the ventrolateral preoptic nuclei which is involved in sleep regulation [65].…”

Identification of histaminergic neurons through histamine 3 receptor-mediated autoinhibition

“…First, GABA co-released with histamine in the stiatum generates tonic inhibitory post-and pre-synaptic effects counteracting histamine action (Yu et al, 2015). Such GABA/histamine co-release is not evident in response to optogenetic stimulation of histaminergic neurons in the hypothalamus (Williams et al, 2014).…”

“…A recent study demonstrated that histamine release in TMNv triggered by optogenetic stimulation does not reduce the firing of histaminergic neurons (Williams et al, 2014), indicating that the function of H 3 R under these conditions might be compromised by an unknown mechanism. As the majority of histaminergic neurons express the TRPV1 (transient receptor potential vanilloid 1) channel (Kernder et al, 2014), whose activation by capsaicin leads to cell swelling (Pecze et al, 2013), which in turn may accelerate H 3 R internalisation (Osorio-Espinoza et al, 2014), we tested the response to the H 3 R ligand RAMH after capsaicin.…”

“…However for the whole application At 10nM D-22 did not produce significant effects on firing frequency per se, however at 100nM it significantly and progressively increased the firing of histaminergic neurons (to 120 ± 7.5 % of control by the end of application period and to 148 ± 4% during 7 min washout, n = 4). As a recent study attributed the major effect of histamine in TMNv triggered by optogenetic stimulation to a reduction of GABAergic input (Williams et al, 2014), we tested whether gabazine (GABA A R antagonist) affects exogenous histamine-mediated inhibition of firing under our conditions. Gabazine (10µM) did not affect firing frequency or eAPC amplitude ( Figure 8C).…”

“…Recent studies questioned the functionality of H 3 R in at least a subgroup of histaminergic neurons (Giannoni et al, 2009) and failed to detect autoinhibition after prolonged depolarization of histaminergic neurons transfected with channelrhodopsin 2 by light stimulation (Williams et al, 2014). We have therefore revisited the role of H 3 R in mouse TMN and addressed its reliability as an identification criterion.…”