Soluble proteins, e.g. thrombospondins, can trigger synapse formation. Such proteins are synthesized in vitro and in vivo by astrocytes, which therefore have a role in synaptogenesis. These thrombospondins are only expressed in the normal brain exactly during the period of developmental synaptogenesis, being off in embryonic brain and adult brain but on at high levels in postnatal brain. Methods are provided for protecting or treating an individual suffering from adverse effects of deficits in synaptogenesis, or from undesirably active synaptogenesis. These findings have broad implications for a variety of clinical conditions, including traumatic brain injury, epilepsy, and other conditions where synapses fail to form or form inappropriately. Synaptogenesis is enhanced by contacting neurons with agents that are specific agonists or antagonists of thrombospondins. Conversely, synaptogenesis is inhibited by contacting neurons with inhibitors or antagonists of thrombospondins.