To determine the functional development of neurons, we applied nerve growth factor (NGF) or 8-bromo-cyclic-adenosine monophosphate (8-Br-cAMP) to PC12 cells and recorded the 5-hydroxytryptamine (5-HT)-induced response by the use of a patch-clamp technique. Cultured PC12 cells expressed 5-HT- sensitive receptors, which are almost absent in untreated cells, in the continuous presence of NGF or 8-Br-cAMP for a period of 10 days. Activation of the receptors by 5-HT produced a transient inward current. In a K+-free solution, the reversal potential (E(5-HT)) of I(5-HT) was + 10.3 mV, and the current-voltage (I-V) relation showed inward rectification at positive potentials. The permeability ratio for monovalent cations was Na+:Li+:K+:Rb+:Cs+ = 1:1.19:0.89:0.94:0.91, indicating that a 5-HT- induced current is passing through the ligand-gated large cation channel. 2- Methyl-5-HT, a specific 5-HT3 agonist, induced a similar inward current, even though the current amplitude was smaller and the activation and inactivation kinetics were slower than those of 5-HT. ICS-205-930, a specific 5-HT3 antagonist, inhibited the 5-HT-induced current in a concentration- dependent manner with a noncompetitive inhibition profile. Spiperone, a 5- HT1 and 5-HT2 families antagonist, and ketanserine, 5-HT2 family antagonist, did not affect the 5-HT-induced response. The time to peak (t(p)) as well as fast and slow time constants (τ(if) and τ(is)) decreased with increasing 5-HT concentration. It was concluded that PC12 cells in the presence of either NGF or 8-Br-cAMP expressed 5-HT3 receptors, although the latter effect was weaker. This suggests that intracellular adenosine 3',5'- cyclic monophosphate (cAMP) plays an important role in the expression of 5- HT3 receptors in PC12 cells.
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