Role of GABA_B receptors in intracellular Ca²⁺ homeostasis and possible interaction between GABA_A and GABA_B receptors in regulation of transmitter release in cerebellar granule neurons

Abstract

The expression of GABAB receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [3H](S,R)-baclofen as well as in functional assessment of the ability of (R)-baclofen to interact with depolarization (15-40 mM KCl) coupled changes in intracellular Ca2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABAA and GABAB receptors was investigated. The binding studies showed that the granule cells express specific binding sites for (R)-baclofen. The number of binding sites could be increased by exposure of the cells to the GABAA receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABAB receptors are coupled to G-proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45Ca2+ uptake was measured or the intracellular Ca2+ concentration ([Ca2+]i) determined using the fluorescent Ca2+ chelator Fluo-3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABAB receptors as (R)-baclofen inhibited depolarization-induced increases in 45Ca2+ uptake and [Ca2+]i. (R)-Baclofen also inhibited K(+)-induced transmitter release from the neurons as monitored by the use of [3H]D-aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABAA receptor agonist isoguvacine it could be demonstrated that the GABAB receptors are functionally coupled to GABAA receptors in the neurons leading to a disinhibitory action of GABAB receptor agonists.

Hetero-oligomerization between GABAA and GABAB Receptors Regulates GABAB Receptor Trafficking

“…There are numerous reports describing examples of crosstalk between GABA A and GABA B receptors for which the molecular mechanisms are obscure, notably the ability of GABA A and GABA B receptors to mutually influence each other's ligand binding properties (23)(24)(25) and signaling activity (24, 26 -29). Numerous precedents in the literature illustrate that modulation of signaling, endocytosis, and/or pharmacology can result from direct interactions between receptors (30,31,(33)(34)(35).…”

“…Finally, numerous reports suggest that GABA B receptors participate in physiological cross-talk with other receptors through unknown mechanisms. Of particular note are functional interactions between GABA A and GABA B receptors in regulating each other's binding properties (23)(24)(25) and activity (24, 26 -29).…”

GABA_B Receptors in Anterior Pituitary Cells

“…First we addressed the question whether anterior pituitary GABA B receptors were coupled to guanine-nucleotide-binding proteins (G-proteins) of the i/0 kind, as extensively reported in the central nervous system [13, 15, 26, 27, 28]. Pertussis toxin (PTX) leads to ADP-ribosylation of the α-subunit of G i and G₀ and to G-protein receptor uncoupling [29].…”

The Loss of Glutamate-GABA Harmony in Anxiety Disorders

“…As an alternative to the ionotropic GABA A receptor, there is the GABA B receptor that is capable of exerting the slow and modulatory action of inhibitory neurotransmission, often in close association with the GABA A pentamer (Kardos et al, 1994). Similar to the above described ionotropic channel, the GABA B receptor was shown to be expressed both pre-and postsynaptically.…”