The Disordered Mind: What Unusual Brains Tell Us About Ourselves

by Eric R. Kandel

Unlike other medications used to treat psychiatric illnesses, lithium is a salt; consequently, it does not bind to a receptor on the surface of a neuron. Rather, it is actively transported into the neuron through sodium ion channels in the cell membrane that open in response to an external stimulus (see chapter 1). When a sodium ion channel opens, both sodium and lithium move into the cell. The sodium is subsequently pumped out, but the lithium remains inside. There, lithium may stabilize mood swings by affecting the action of neurotransmitters, either directly or through interaction with a second messenger system. As we have seen, neurotransmitters bind to receptors on the cell membrane. This activates second messenger systems, which transmit signals from the receptors to molecules inside the neuron. Lithium may blunt the activation of second messenger systems, thus reducing signal transmission. Lithium may also damp down a neuron’s responsiveness to neurotransmitters inside the cell. This could explain why lithium works so effectively in bipolar disorder: it may decrease a neuron’s sensitivity to both external and internal stimuli. In addition, lithium affects the modulatory neurotransmitters serotonin and dopamine as well as the mediating neurotransmitter GABA. Thus, its efficacy may be attributable to its wide-ranging neurobiological effects rather than to a single mechanism.