BICUCULLINE BACLOFEN-INSENSITIVE GABA RESPONSE IN CRUSTACEAN NEURONS IN CULTURE/

Neurones were dissociated from thoracic ganglia of embryonic and adult lobsters and kept in primary culture. When gamma-aminobutyric acid (G ABA) was applied by pressure ejection, depolarizing or hyperpolarizing responses were produced, depending on the membrane potential. They we re accompanied by an increase in membrane conductance. When they were present, action potential firing was inhibited. The pharmacological pr ofile and ionic mechanism of GABA-evoked current were investigated und er voltage-clamp with the whole-cell patch-clamp technique. The revers al potential of GABA-evoked current depended on the intracellular and extracellular Cl- concentration but not on extracellular Naf and K+. B lockade of Ca2+ channels by Mn2+ was also without effect. The GABA-evo ked current was mimicked by application of the GABA(A) agonists muscim ol and isoguvacine with an order of potency muscimol>GABA>isoguvacine. cis-4-aminocrotonic acid (CACA), a folded and conformationally restri cted GABA analogue, supposed to be diagnostic for the vertebrate GABA( C) receptor, also induced a bicuculline-resistant chloride current, al though with a potency about 10 times lower than that of GABA. The GABA -evoked current was largely blocked by picrotoxin, but was insensitive to the GABA(A) antagonists bicuculline, bicuculline methiodide and SR 95531 at concentrations of up to 100 mu moll(-1). Diazepam and phenob arbital did not exert modulatory effects. The GABA(B) antagonist phacl ophen did not affect the GABA-induced current, while the GABA(B) agoni sts baclophen and 3-aminopropylphosphonic acid (3-APA) never evoked an y response. Our results suggest that lobster thoracic neurones in cult ure express a chloride-conducting GABA-receptor channel which conforms to neither the GABA(A) nor the GABA(B) types of vertebrates but shows a pharmacology close to that of the novel GABA(C) receptor described in the vertebrate retina.