Serotonergic Transmission in the Hippocampus Is Subject to Multifaceted Modulation by Other Neurotransmitters

Purpose: Serotonin (5-HT) release in the hippocampus is modulated by N-methyl-D-aspartate (NMDA) receptor activity, particularly under hypoxic conditions, but the roles of other neurotransmitter systems remain unclear. This study examined how several neurotransmitters affect spontaneous 5-HT release in rat hippocampal slices and whether these effects are direct or interneuron-mediated.Methods: Rat hippocampal slices (400 μm) were preloaded with [³H]5-HT and superfused with oxygenated buffer. Acetylcholine (ACh), norepinephrine (NE), dopamine (DA), NMDA, γ-aminobutyric acid (GABA), glycine, or neuropeptide Y (NPY) were applied for 20 minutes, and [³H]5-HT release was measured in 10-minute fractions. Tetrodotoxin (TTX) was used in parallel experiments to block action potential-dependent neurotransmission.Results: In control slices, spontaneous [³H]5-HT release reached a stable baseline after approximately 50 minutes of superfusion. ACh (10^-5M), GABA (10^-5M), glycine (10^-5M), and NPY (10^-6M) did not significantly affect 5-HT release relative to control. In contrast, NE (10^-5M) and DA (10^-5M) significantly increased 5-HT release (peaking at ~15%–37% above baseline, P<0.05), and these effects persisted in the presence of TTX, suggesting a direct action on serotonergic terminals. NMDA (10^-4M) also transiently enhanced 5-HT release (~22% above baseline, P<0.05), but this effect was completely abolished by TTX, indicating reliance on interneuronal activity.Conclusions: Hippocampal 5-HT release is differentially regulated by neurotransmitter systems: NE and DA act directly at serotonergic terminals, whereas NMDA’s effect requires interneuronal activity. These findings highlight complex neurochemical interactions that may underlie serotonergic modulation in neuropsychiatric disorders.