You have seen that ion channels can function as receptors in cell signaling (see Figure 7.5). In Chapters 44 and 45, you will learn how signaling affects ion channels in the central nervous system and in senses, where stimulation of a receptor by light, sound, or touch can lead to opening of an ion channel. A different type of ion channel response lies within some cells. For example, signal transduction using the IP₃–DAG pathway can result in the opening of Ca²⁺ channels in the ER membrane.

Oxytocin, which we described in the opening of this chapter, combines the two ion channel responses. Oxytocin binds to a G protein-coupled receptor (see Figure 7.7), which is expressed in brain tissue as well as muscles involved in birth and lactation. Activation of the receptor results in signal transduction through the IP₃–DAG pathway, releasing Ca²⁺ into the cytoplasm via ion channels. In the brain, the effect of Ca²⁺ is to indirectly stimulate nerve cell activity by the opening of ion channels for Na⁺, as you will learn in Chapter 44. To summarize:

Oxytocin → Receptor → G protein activation → IP₃ signal transduction→ Ca²⁺ channel opening → Na⁺ channel opening

You saw in the opening of this chapter that the behavioral effect of oxytocin in voles involves bonding between them. Does this also happen in people? Neuroscientist Paul Zak at the Center for Neuroeconomics Studies thinks so. He has done experiments with human volunteers and shown that oxytocin signaling is important in trusting behavior (Investigating Life: Is Oxytocin a “Trust” Signal in Humans?). Recently, it was shown that people who have an inherited defect in the oxytocin receptor act in some ways like the montane voles that lack oxytocin receptors: they are less trusting.