In addition, patients are asked to undergo a lumbar puncture (spinal tap) twice during the study to collect a sample of cerebrospinal fluid (CSF, the fluid that bathes the brain and spinal cord). The CSF is examined to try to understand how brain chemicals are related to depression and to the effects of ceftriaxone. A local anesthetic is given and a needle is inserted in the space between the bones in the lower back where the CSF circulates below the spinal cord. A small amount of fluid is collected through the needle. This test is optional.