Final answer:
The sentence means that the levels of mRNA for serotonin receptors are not affected by acute LSD use.
Step-by-step explanation:
The sentence 'Serotonin Receptor mRNA Levels Are Unchanged by Acute LSD' means that the levels of mRNA, which is a molecule involved in protein synthesis, for serotonin receptors are not affected by short-term exposure to LSD. Serotonin receptors are proteins on cell surfaces that bind to serotonin and mediate its effects. This suggests that acute LSD use does not directly impact the production of serotonin receptors at the mRNA level.