The hippocampus is a component of the limbic system in the humanoid brain. It coordinates olfaction, autonomic functions, and aspects of emotional behavior.
In an alternate 2153, Captain Jonathan Archer's hippocampus was infected by interspatial parasites which prevented him from forming any new long-term memories. As a result, Archer woke up each day with no memories since his initial infection. Doctor Phlox designed a treatment using a subspace implosion to remove the parasites. After being removed, the parasites were erased from history, leaving Archer with no memory of their existence. (ENT: "Twilight")
Tampering by solanogen-based lifeforms caused elevated levels of serotonin to concentrate in the hippocampus of several crewmembers of the USS Enterprise-D. Doctor Beverly Crusher realized that this was caused by a neuro-sedative used by the aliens to sedate the victims they experimented on. (TNG: "Schisms")
In an alternate version of 2370 created by Q, Captain Jean-Luc Picard traveled back and forth through time. After one of these jumps, Doctor Crusher discovered a 13% increase of the acetylcholine level in his hippocampus, suggesting he had accumulated two days' worth of memories within a matter of minutes. (TNG: "All Good Things...")
In 2372, after changing his identity to free him from his exile from the Klingon Empire, Julian Bashir told "Rodek" that his hippocampus had been affected by a plasma discharge and was causing amnesia. (DS9: "Sons of Mogh")
In 2374, The Doctor told Seven of Nine that her hippocampus was in a state of agitation. This was due to the effects of a Borg resonance signal from the USS Raven. (VOY: "The Raven")