Drugs that encourage the growth of new neurons in the brain are now headed for clinical trials. The drugs, which have already shown success in alleviating symptoms of depression and boosting memory in animal models, are being developed by BrainCells, a San Diego-based start-up that screens drugs for their brain-growing power. The company hopes the compounds will provide an alternative to existing antidepressants and says they may also prove effective in treating cognitive disorders, such as Alzheimer’s.