PACAP-PAC1 Receptor Activation Is Necessary for the Sympathetic Response to Acute Intermittent Hypoxia.

Repetitive hypoxia is a key feature of obstructive sleep apnoea (OSA), a condition characterized by intermittent airways obstruction. Patients with OSA present with persistent increases in sympathetic activity and commonly develop hypertension. The objectives of this study were to determine if the persistent increases in sympathetic nerve activity, known to be induced by acute intermittent hypoxia (AIH), are mediated through activation of the pituitary adenylate cyclase activating polypeptide (PACAP) signaling system. Here, we show that the excitatory neuropeptide PACAP, acting in the spinal cord, is important for generating the sympathetic response seen following AIH. Using PACAP receptor knockout mice, and pharmacological agents in Sprague Dawley rats, we measured blood pressure, heart rate, pH, PaCO2, and splanchnic sympathetic nerve activity, under anaesthesia, to demonstrate that the sympathetic response to AIH is mediated via the PAC1 receptor, in a cAMP-dependent manner. We also report that both intermittent microinjection of glutamate into the rostroventrolateral medulla (RVLM) and intermittent infusion of a sub-threshold dose of PACAP into the subarachnoid space can mimic the sympathetic response to AIH. All the sympathetic responses are independent of blood pressure, pH or PaCO2 changes. Our results show that in AIH, PACAP signaling in the spinal cord helps drive persistent increases in sympathetic nerve activity. This mechanism may be a precursor to the development of hypertension in conditions of chronic intermittent hypoxia, such as OSA.

GABA allosteric modulators: An overview of recent developments in non-benzodiazepine modulators.

γ-Aminobutyric acid (GABA) is the major inhibitory transmitter controlling synaptic transmission and neuronal excitability. It is present in a high percentage of neurons in the central nervous system (CNS) and also present in the peripheral nervous system, and acts to maintain a balance between excitation and inhibition. GABA acts via three subclasses of receptors termed GABAA, GABAB, and GABAC. GABAA and GABAC receptors are ligand-gated ion channels, while GABAB receptors are G-protein coupled receptors. Each class of GABA receptor has distinct pharmacology and physiology. GABAA receptors are heteropentameric transmembrane protein complexes made up of α1-6, ß1-3, γ1-3, δ, ε, θ, π subunits, giving rise to numerous allosteric binding sites and have thus attracted much attention targets for the treatment of conditions such as epilepsy, anxiety and sleep disorders. The development of ligands for these binding sites has also led to an improved understanding of the different physiological functions and pathological processes and offers the opportunity for the development of novel therapeutics. This review focuses on the medicinal chemistry aspects including drug design, structure-activity relationships (SAR), and mechanism of actions of GABA modulators, including non-benzodiazepine ligands at the benzodiazepine binding site and modulators acting at sites other than the high-affinity benzodiazepine binding site. Recent advances in this area their future applications and potential therapeutic effects are also highlighted.

An Investigation of the Differential Effects of Ursane Triterpenoids from Centella asiatica, and Their Semisynthetic Analogues, on GABAA Receptors.

The ursane triterpenoids, asiatic acid 1 and madecassic acid 2, are the major pharmacological constituents of Centella asiatica, commonly known as Gotu Kola, which is used traditionally for the treatment of anxiety and for the improvement of cognition and memory. Using the two-electrode voltage-clamp technique, these triterpenes, and some semisynthetic derivatives, were found to exhibit selective negative modulation of different subtypes of the GABAA receptor expressed in Xenopus laevis oocytes. Despite differing by only one hydroxyl group, asiatic acid 1 was found to be a negative modulator of the GABA-induced current at α1 ß2 γ2L, α2 ß2 γ2L and α5 ß3 γ2L GABAA receptors, while madecassic acid 2 was not. Asiatic acid 1 exhibited the greatest effect at α1 ß2 γ2L (IC50 37.05 µm), followed by α5 ß3 γ2L (IC50 64.05 µm) then α2 ß2 γ2L (IC50 427.2 µm) receptors. Conversion of the carboxylic acid group of asiatic acid 1 to a carboxamide group (2α,3ß,23-trihydroxy-urs-12-en-28-amide 5) resulted in enhanced inhibition at both the α1 ß2 γ2L (IC50 14.07 µm) and α2 ß2 γ2L receptor subtypes (IC50 28.41 µm). The results of this study, and the involvement of α5 -containing GABAA receptors in cognition and memory, suggest that asiatic acid 1 may be a lead compound for the enhancement of cognition and memory.

Tonically Active cAMP-Dependent Signaling in the Ventrolateral Medulla Regulates Sympathetic and Cardiac Vagal Outflows.

The ventrolateral medulla contains presympathetic and vagal preganglionic neurons that control vasomotor and cardiac vagal tone, respectively. G protein-coupled receptors influence the activity of these neurons. Gα s activates adenylyl cyclases, which drive cyclic adenosine monophosphate (cAMP)-dependent targets: protein kinase A (PKA), the exchange protein activated by cAMP (EPAC), and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. The aim was to determine the cardiovascular effects of activating and inhibiting these targets at presympathetic and cardiac vagal preganglionic neurons. Urethane-anesthetized rats were instrumented to measure splanchnic sympathetic nerve activity (sSNA), arterial pressure (AP), heart rate (HR), as well as baroreceptor and somatosympathetic reflex function, or were spinally transected and instrumented to measure HR, AP, and cardiac baroreflex function. All drugs were injected bilaterally. In the rostral ventrolateral medulla (RVLM), Sp-cAMPs and 8-Br-cAMP, which activate PKA, as well as 8-pCPT, which activates EPAC, increased sSNA, AP, and HR. Sp-cAMPs also facilitated the reflexes tested. Sp-cAMPs also increased cardiac vagal drive and facilitated cardiac baroreflex sensitivity. Blockade of PKA, using Rp-cAMPs or H-89 in the RVLM, increased sSNA, AP, and HR and increased HR when cardiac vagal preganglionic neurons were targeted. Brefeldin A, which inhibits EPAC, and ZD7288, which inhibits HCN channels, each alone had no effect. Cumulative, sequential blockade of all three inhibitors resulted in sympathoinhibition. The major findings indicate that Gα s-linked receptors in the ventral medulla can be recruited to drive both sympathetic and parasympathetic outflows and that tonically active PKA-dependent signaling contributes to the maintenance of both sympathetic vasomotor and cardiac vagal tone.

The Differential Effects of Resveratrol and trans-ε-Viniferin on the GABA-Induced Current in GABAA Receptor Subtypes Expressed in Xenopus Laevis Oocytes.

PURPOSE: The natural products resveratrol and trans-ε-viniferin have been reported to have many beneficial effects, which include the enhancement of cognition and memory. There have been no studies which have reported the effects of these compounds on the different GABAA receptor subtypes and this study aimed to address this. METHODS: The effects of both resveratrol, and its dimer, trans-ε-viniferin, have been investigated on different GABAA receptor subtypes expressed in Xenopus laevis oocytes, using the two-electrode voltage clamp technique. RESULTS: Resveratrol induced a current of 22 ± 3.53 nA in the α1ß2γ2L subtype of the GABAA receptor (but not in the α5ß3γ2L and α2ß2γ2L subtypes) when applied alone. It also positively modulated the GABA-induced current (IGABA) in α1ß2γ2L receptors, in adose-dependent manner (EC50 58.24 µM). The effects of resveratrol were not sensitive to the benzodiazepine antagonist flumazenil. trans-ε-Viniferin exhibited a different pattern of activity to resveratrol; it alone had no effect on any of the subtypes, but it did negatively modulate the GABA-induced current (IGABA) in all three subtypes. The greatest inhibition was found in the α1ß2γ2L subtype (IC50 5.79 µM), with the inhibition in the α2ß2γ2L (IC50 of 19.08 µM) and α5ß3γ2L (IC50 of 21.05 µM) subtypes being similar. The effects of trans-ε-viniferin in α1ß2γ2L and α2ß2γ2L receptors werealso not sensitive to the benzodiazepine antagonist flumazenil while, in the α5ß3γ2L subtype the effect was not sensitive to the inverse agonist L-655,708, indicating different binding sites for this molecule. CONCLUSIONS: The results of the present study indicate that both resveratrol and trans-ε-viniferin modulate the GABA-induced current in different ways, and that trans-ε-viniferin may be a lead compound for the discovery of agents which selectively inhibit the GABA-induced current in α1-containing subtypes.This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.