Coexistence of GABAA and GABAB receptors on Aδ and C primary afferents

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Abstract
1 Intracellular recordings from adult rat dorsal root ganglion neurones were performed in vitro and the coexistence of two γ-aminobutyric acid (GABA) receptors on the membrane of identified Aδ and C primary afferents was demonstrated. 2 Transient applications of GABA (10−6 − 10−2m) evoked dose-dependent depolarizations and increased membrane conductance. The responses were mimicked by muscimol, isoguvacine, THIP and 3 amino propane sulphonic acid (3 APS); they were blocked by bicuculline and picrotoxin. Pentobarbitone induced an increase of GABA-induced depolarizations. 3 Perfusion of tetraethylammonium (TEA, 7.5 mm) and intracellular injection of Cs+ ions unmasked the Ca2+ component of action potentials, which appeared as long-lasting plateau depolarizations. Such action potentials were shortened in the presence of methoxyverapamil (D600, 5 × 10−6- 10−5m) and in a medium without Ca+ ions. 4 Prolonged (5–10 min) perfusion of GABA (10−9-10−5m) shortened the Ca2+ component of action potentials. This effect was mimicked by baclofen (10−7-5 × 10−6m) and muscimol (5 × 10−7-10−5m) and was not affected by bicuculline perfusion (5 × 10−6-10−5m). Isoguvacine (2.5 × 10−5m) did not affect action potential duration. 5 It is concluded that two GABA receptors coexist on the membrane of slow conducting primary afferents: the bicuculline-sensitive GABAA receptor mediates depolarizations and the bicuculline-insensitive GABAB receptor shortens the calcium component of action potentials.