IONOTROPIC GABA RECEPTOR FROM LOBSTER OLFACTORY PROJECTION NEURONS

This study reports an ionotropic GABA (gamma-aminobutyric acid) recept or in projection neurons acutely dissociated from the olfactory lobe o f the brain of the spiny lobster and analyzed by whole cell and cell-f ree patch-clamp recording. GABA evokes a macroscopic cur rent in the c ells that is linear from -100 to +100 mV, reverses at the imposed chlo ride equilibrium potential, has a permeability sequence of Cl- > aceta te > bicarbonate > phosphate > propionate and SCN- > Br- > I- Cl- > F- , and is reversibly blocked by the Cl channel blocker picrotoxin but n ot tert-butylbicyclophosphorothionate (TBPS). The current is bicuculli ne insensitive and activated by muscimol, isoguvacine, cis-4-aminocrot onic acid (CACA), and trans-aminocrotonic acid (TACA), as well as by t he GABA(c)-receptor antagonists 4,5,6,7-tetrahydroisoxazolo [5,4,-c]py ridin-3-ol (THIP), 3-amino-1-propanesulfonic acid (3-APS), and imidazo le-4-acetic acid (I-4AA), but not the GABAB-receptor agonists baclofen and 3-aminopropylphosphonic acid (3-APA). Agonist potency for the rec eptor is TACA > muscimol > GABA > I-4AA > isoguvacine > 3-APS > CACA > THIP. Unitary chloride currents in cell-free, outside-out patches fro m the cells share enough of these pharmacological properties to indica te that the channel underlies the macroscopic current. The receptor me diates an inhibitory current in the cells in vivo. The receptor is sim ilar, if not identical, to one from neurons cultured from the thoracic ganglia of the clawed lobster. The more extensive pharmacological cha racterization of the receptor reported here indicates that this lobste r CNS receptor is pharmacologically distinct from previously character ized ionotropic GABA receptors.