FIG. 2. Schematic representation of the three known transduction pathways for 5-HT2 receptors: a decrease in certain resting and voltage-activated K+ currents (IK); positive coupling to the phosphoinositide (PI) second messenger pathway; and an increase in the hyperpolarization-activated cationic current Ih. The dashed lines indicate a proposed negative feedback between the activation of protein kinase C (PKC) and 5-HT2 receptors, presumably through phosphorylation of the receptor. Activation of the PI pathway is presumably mediated by a member of the Gq subfamily (62); ion channel coupling may also be through Gq, but there are no data as yet on this point. This scheme is intended to apply to both the 5-HT2A and 5-HT2C receptors (formerly known as 5-HT2 and 5-HT1C, respectively).