The most widely prescribed antidepressants — medicines such as Prozac, Lexapro and Paxil — work by blocking the serotonin transporter, a brain protein that normally clears away the mood-regulating chemical serotonin. Or so the current thinking goes.

That theory about how selective serotonin reuptake inhibitors (SSRIs) work can now be put to the test with a new mouse model developed by neuroscientists at Vanderbilt University.

These mice, described in the online edition of the Proceedings of the National Academy of Sciences (PNAS), express a serotonin transporter that has been genetically altered so that it does not respond to many SSRIs or cocaine.