A Buttery Taste to Vascular Biology

Peripheral GABAA receptors - Physiological relevance and therapeutic implications

The role of γ- aminobutyric acid (GABA) and GABAA receptors is not only essential for neurotransmission in the central nervous system (CNS), but they are also involved in communication in various peripheral tissues such as the pancreas, liver, kidney, gastrointestinal tract, trachea, immune cells and blood vessels. GABAA receptors located outside the CNS ("peripheral GABAA receptors") enable both neuronal and non-neuronal GABA-ergic signaling in various physiological processes and are generally thought to have similar properties to the extrasynaptic receptors in the CNS. By activating these peripheral receptors, GABA and various GABAA receptor modulators, including drugs such as benzodiazepines and general anesthetics, may contribute to or otherwise affect the maintenance of general body homeostasis. However, the existing data in the literature on the role of non-neuronal GABA-ergic signaling in insulin secretion, glucose metabolism, renal function, intestinal motility, airway tone, immune response and blood pressure regulation are far from complete. In fact, they mainly focus on the identification of components for the local synthesis and utilization of GABA and on the expression repertoire of GABAA receptor subunits rather than on subunit composition, activation effects and (sub)cellular localization. A deeper understanding of how modulation of peripheral GABAA receptors can have significant therapeutic effects on a range of pathological conditions such as multiple sclerosis, diabetes, irritable bowel syndrome, asthma or hypertension could contribute to the development of more specific pharmacological strategies that would provide an alternative or complement to existing therapies. Selective GABAA receptor modulators with improved peripheral efficacy and reduced central side effects would therefore be highly desirable first-in-class drug candidates. This review updates recent advances unraveling the molecular components and cellular determinants of the GABA signaling machinery in peripheral organs, tissues and cells of both, humans and experimental animals.

Vasodilatory effects of a variety of positive allosteric modulators of GABAA receptors on rat thoracic aorta

Different subtypes of GABAA (gamma-aminobutyric acid A) receptors, through their specific regional and cellular localization, are involved in the manifestation of various functions, both at the central and peripheral levels. We hypothesized that various non-neuronal GABAA receptors are expressed on blood vessels, through which positive allosteric modulators of GABAA receptors exhibit vasodilatory effects. This study involved two parts: one to determine the presence of α1-6 subunit GABAA receptor mRNAs in the rat thoracic aorta, and the other to determine the vasoactivity of the various selective and non-selective positive GABAA receptor modulators: zolpidem (α1-selective), XHe-III-074 (α4-selective), MP-III-022 (α5-selective), DK-I-56-1 (α6-selective), SH-I-048A and diazepam (non-selective). Reverse transcription-polymerase chain reaction (RT-PCR) analysis data demonstrated for the first time the expression of α1, α2, α3, α4 and α5 subunits in the rat thoracic aorta tissue. Tissue bath assays on isolated rat aortic rings revealed significant vasodilatory effects of diazepam, SH-I-048A, XHe-III-074, MP-III-022 and DK-I-56-1, all in terms of achieved relaxations (over 50% of relative tension decrease), as well as in terms of preventive effects on phenylephrine (PE) contraction. Diazepam was the most efficient ligand in the present study, while zolpidem showed the weakest vascular effects. In addition, diazepam-induced relaxations in the presence of antagonists PK11195 or bicuculline were significantly reduced (P < 0.001 and P < 0.05, respectively) at lower concentrations of diazepam (10-7 M and 3 × 10-7 M). The present work suggests that the observed vasoactivity is due to modulation of "vascular" GABAA receptors, which after further detailed research may provide a therapeutic target.