Increased Sensitivity of Mice Lacking Extrasynaptic δ-Containing GABAA Receptors to Histamine Receptor 3 Antagonists

Abstract

Histamine/gamma-aminobutyric acid (GABA) neurons of posterior hypothalamus send wide projections to many brain areas and participate in stabilizing the wake state. Recent research has suggested that GABA released from the histamine/GABA neurons acts on extrasynaptic GABA A receptors and balances the excitatory effect of histamine. In the current study, we show the presence of vesicular GABA transporter mRNA in a majority of quantified hypothalamic histaminergic neurons, which suggest vesicular release of GABA. As histamine/GABA neurons form conventional synapses infrequently, it is possible that GABA released from these neurons diffuses to target areas by volume transmission and acts on extrasynaptic GABA receptors. To investigate this hypothesis, mice lacking extrasynaptic GABA A receptor d subunit (Gabrd KO) were used. A pharmacological approach was employed to activate histamine/GABA neurons and induce histamine and presumably, GABA, release. Control and Gabrd KO mice were treated with histamine receptor 3 (Hrh3) inverse agonists ciproxifan and pitolisant, which block Hrh3 autoreceptors on histamine/GABA neurons and histamine-dependently promote wakefulness. Low doses of ciproxifan (1 mg/kg) and pitolisant (5 mg/kg) reduced locomotion in Gabrd KO, but not in WT mice. EEG recording showed that Gabrd KO mice were also more sensitive to the wake-promoting effect of ciproxifan (3 mg/kg) than control mice. Low frequency delta waves, associated with NREM sleep, were significantly suppressed in Gabrd KO mice compared with the WT group. Ciproxifan-induced wakefulness was blocked by histamine synthesis inhibitor a-fluoromethylhistidine (aFMH). The findings indicate that both histamine and GABA, released from histamine/ GABA neurons, are involved in regulation of brain arousal states and d-containing subunit GABA A receptors are involved in mediating GABA response.

Co-release of histamine and GABA in prefrontal cortex excites fast-spiking interneurons and causes divisive gain change in pyramidal cells; an effect that is enhanced in older mice

“…Therefore, GABA diffusion through the extracellular space is sufficiently unhindered to enable rapid coupling between the GABA released from TMNHDC axons and alterations in the gain of the surrounding PyrNs. This is similar to the mode of action reported in other cortical and striatal regions following TMNHDC activation (Yu et al, 2015), and consistent with observations following the knockout of high-affinity d subunit-containing GABAA receptors (Abdurakhmanova et al, 2020).…”

Mice Lacking GABAA Receptor δ Subunit Have Altered Pharmaco-EEG Responses to Multiple Drugs

“…These findings were not limited to the previously shown blunted response to THIP (Winsky-Sommerer et al, 2007), but the δ-KO mice were also less sensitive to 4-MMC-induced suppression of pharmaco-EEG across multiple frequency bands. Furthermore, we observed similarly altered baseline EEG band powers as in our previous experiments with the same δ-KO mouse line (Abdurakhmanova et al, 2020), indicating that cortical EEG is a well reproducible method to study drug-induced alterations in cortical field potentials.…”

“…Signal power analyses were done using the same pipeline structure used in our previous publication (Abdurakhmanova et al, 2020). However, the MATLAB scripts were reimplemented using Python (Van Rossum and Drake, 2009).…”

“…With this approach, the differential effect of THIP has been detected as a robust slow-wave pharmaco-EEG activation in the δ-WT mice, which was lacking in δ-KO mice ( Winsky-Sommerer et al, 2007 ). More recently, we tested the hypothesis of concurrent GABA release from histaminergic neurons leading to an increase in ambient GABA level after the blockade of histamine H 3 autoreceptors ( Yu et al, 2015 ; Scammell et al, 2019 ) and found that the δ-KO mice were more sensitive to wake-promoting effects of H 3 antagonists than the δ-WT mice ( Abdurakhmanova et al, 2020 ), indicating that δ-GABA A Rs are involved in restricting the excessive brain excitation induced by the histaminergic system.…”

The Histaminergic System in Neuropsychiatric Disorders

“…Additionally, some histamine neurons have the capacity to express the glutamic acid decarboxylase-67 (GAD67, GABA-synthesizing enzyme) and the vesicular GABA transporter (Vgat) gene [ 12 , 54 ]. Abdurakhmanova et al show nice double-label in situ hybridization of HDC, Gad67 and Vgat mRNAs in the TMN and find that both histamine and GABA, released from histamine/GABA neurons, are involved in regulation of brain arousal states [ 55 ]. Selective deletion of Vgat gene expression from histaminergic TMN neurons increased wake during the night and locomotion [ 12 ].…”