GABAA receptors are members of the Cys-loop family of ligand-gated ion channels which mediate most inhibitory neurotransmission in the central nervous system. These receptors are pentameric assemblies of individual subunits, including α1-6, β1-3, γ1-3, δ, ε, π, θ and ρ1-3. The majority of receptors are comprised of α, β and γ or δ subunits. Depending on the subunit composition, the receptors are located in either the synapses or extrasynaptic regions. The most abundant receptors are α1βγ2 receptors, which are activated and modulated by a variety of pharmacologically and clinically unrelated agents such as benzodiazepines, barbiturates, anaesthetics and neurosteroids, all of which bind at distinct binding sites located within the receptor complex. However, compared to αβγ, the binary αβ receptors lack a benzodiazepine α-γ2 interface. In pentameric αβ receptors, the third subunit is replaced with either an α1 or a β3 subunit leading to two distinct receptors that differ in subunit stoichiometry, 2α:3β or 3α:2β. The consequence of this is that 3α:2β receptors contain an α-α interface whereas 2α:3β receptors contain a β-β interface. Apart from the replacement of γ by α1 or β3 in binary receptors, the incorporation of ε subunit into GABAA receptors might be more complicated. As the ε subunit is not only capable of substituting the γ subunit, but also replacing the α/β subunits, receptors with altered stoichiometry and different pharmacological properties are produced. The different subunit arrangement of the receptors potentially constructs novel binding sites which may become new targets of the current or new drugs.
The hypothalamic paraventricular nucleus (PVN) is involved in the regulation of sympathetic outflow and particularly affects the heart. This study sets out to determine the role of GABA of the paraventricular nucleus (PVN) in cardiovascular regulation in streptozotocin-induced diabetic rats. Pharmacological stimulation of glutamatergic receptors with DL-Homocysteic acid (200 mM in 100 nL) in the PVN region showed a significant depression in both mean arterial pressure (MAP) and heart rate (HR) of diabetic rats (Diabetic vs. non-diabetic: MAP 15.0 ± 1.5 vs. 35.8 ± 2.8 mmHg; HR 3.0 ± 2.0 vs. 30.0 ± 6.0 bpm, P < 0.05). Microinjection of bicuculline methiodide (1 mM in 100 nL), a GABAA receptor antagonist, produced an increase in baseline MAP and HR in both non-diabetic and diabetic rats. In the diabetic rats, bicuculline injection into the PVN reduced the pressor and HR responses (Diabetic vs. non-diabetic: MAP 6.2 ± 0.8 vs. 25.1 ± 2.2 mmHg; HR 1.8 ± 1.1 vs. 25.4 ± 6.2 bpm, P < 0.05). A microinjection of muscimol (2 mM in 100 nL), which is a GABAA receptor agonist, in the PVN elicited decreases in MAP and HR in both groups. The diabetic group showed a significantly blunted reduction in HR, but not MAP (Diabetic vs. non-diabetic: MAP -15.7 ± 4.0 vs. -25.0 ± 3.8 mmHg; HR -5.2 ± 2.1 vs. -39.1 ± 7.9 bpm). The blunted vasopressor and tachycardic responses to bicuculline microinjection in the diabetic rats are likely to result from decreased GABAergic inputs, attenuated release of endogenous GABA or alterations in GABAA receptors within the PVN.
Novel treatments against migraine are an urgent medical requirement. The α6 subunit-containing GABAA receptors (α6GABAARs) are expressed in trigeminal ganglia (TG), the hub of the trigeminal vascular system (TGVS) that is involved in the pathogenesis of migraine. Here we reveal an unprecedented role of α6GABAARs in ameliorating TGVS activation using several pharmacological approaches in an animal model mimicking pathological changes in migraine. TGVS activation was induced by intra-cisternal (i.c.) instillation of capsaicin in Wistar rats. Centrally, i.c. capsaicin activated the trigeminal cervical complex (TCC) measured by the increased number of c-Fos-immunoreactive (c-Fos-ir) TCC neurons. Peripherally, it elevated calcitonin gene-related peptide immunoreactivity (CGRP-ir) in TG and depleted CGRP-ir in the dura mater. Pharmacological approaches included a recently identified α6GABAAR-selective positive allosteric modulator (PAM), the pyrazoloquinolinone Compound 6, two α6GABAAR-active PAMs (Ro15-4513 and loreclezole), an α6GABAAR-inactive benzodiazepine (diazepam), an α6GABAAR-selective antagonist (furosemide), and a clinically effective antimigraine agent (topiramate). We examined effects of these compounds on both central and peripheral TGVS responses induced by i.c. capsaicin. Compound 6 (3-10 mg/kg, i.p.) significantly attenuated the TCC neuronal activation and TG CGRP-ir elevation, and dural CGRP depletion induced by capsaicin. All these effects of Compound 6 were mimicked by topiramate, Ro15-4513 and loreclezole, but not by diazepam. The brain-impermeable furosemide antagonized the peripheral, but not central, effects of Compound 6. These results suggest that the α6GABAAR in TG is a novel drug target for TGVS activation and that α6GABAAR-selective PAMs have the potential to be developed as a novel pharmacotherapy for migraine.