γ-Aminobutyric acid modulation of acetylcholine-induced contractions of a smooth muscle from an echinoderm (Sclerodactyla briareus)

This study provides pharmacological evidence for the presence of GABAergic neurons innervating the longitudinal muscle of the body wall (LMBW) of holothurians. γ-Aminobutyric acid (GABA) A and B receptor subtypes were both present in this system and regulated spontaneous contractions as well as responses to acetylcholine (ACh) that stimulated contraction of the LMBW. GABA dose-dependently relaxed the resting tone of the LMBW. GABA (10^-5 M) inhibited ACh-induced (10^-4 M) contractions by 20%. GABA B agonist, baclofen, relaxed the LMBW, an effect potentiated by GABA. Pretreatment with baclofen (10^-4 M) inhibited ACh (10^-4 M) contractions of the LMBW by 50%. Phaclofen, a GABA receptor B antagonist, caused a dose-dependent increase in resting tension. Phaclofen-induced (10^-5 M) contractions were reversed by the addition of GABA or baclofen (10^-4 M) and potentiated by the addition of another GABA B receptor antagonist, 2-hydroxy-saclofen (10^-5 M). Pretreatment with phaclofen (10^-5 M) caused a marked potentiation of ACh-induced (10^-4 M) contractions by 101%. 2-Hydroxy-saclofen (10^-5 M) had a toxic effect on the LMBW, rendering it completely unresponsive either to ACh or to a second exposure to GABA, and so exhibiting cross-desensitization. Muscimol, a GABA A receptor agonist, had no effect on the resting tension of the LMBW. Curiously, pretreatment of the muscle with muscimol (10^-5 M) potentiated ACh-evoked (10^-4 M) contractions by nearly 20%. Bicuculline (10^-5 M), a GABA A receptor antagonist, generated large, sustained contractions and partially blocked GABA-induced (10^-4 M) relaxation. Like 2-hydroxy-saclofen, bicuculline (10^-5 M) had a profound cross-desensitizing effect on the LMBW to subsequent exposures to GABA and ACh. ACh was unable to potentiate the sustained contractions induced by bicuculline.