The small GTP-binding protein RhoA regulates serotonin-induced Na+-current response in the neurons of Aplysia.

Abstract Application of serotonin (5-HT) induces a slow inward current response in identified neurons of Aplysia ganglia under voltage clamp. The 5-HT-induced current response was depressed in Na + -free media, but augmented in Ca 2+ -free media, and unaffected by a change in external K + . The 5-HT-induced response was markedly blocked by intracellular injection of guanosine 5′- O -(2-thiodiphosphate) (GDPβS). After the injection of guanosine 5′- O -(3-thiotriphosphate) (GTPγS), the responses to 5-HT gradually and significantly increased at the initial period, reached its plateau, and finally decreased. Intracellular injection of Clostridium difficile toxin B, a blocker of small G-protein Rho family members such as Rho (RhoA, RhoB and RhoC), Rac and Cdc42, markedly depressed the 5-HT-induced response. Intracellular injection of Clostridium botulinum C3 exoenzyme, a specific blocker of RhoA, RhoB, RhoC, exhibited a similar depressing effect observed with toxin B. In contrast, intracellular injection of recombinant L63RhoA, a constitutively active form of RhoA, significantly augmented the 5-HT-induced response without affecting the resting membrane. These results suggested that the 5-HT-induced Na + -current response might be facilitated by the activation of Aplysia Rho which is closely homologous to RhoA, RhoB or RhoC in mammalian neuron.