Effects of the 5-HT4 receptor agonist RS67333 and paroxetine on hippocampal extracellular 5-HT levels

The 5-HT4 receptor modulates activity of serotonergic neurons and is a new potential target for antidepressant treatment. This microdialysis study evaluated the effect of the 5-HT4 receptor agonist, RS67333, on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-HIAA levels in rat ventral hippocampus during chloral hydrate anaesthesia, and explored the ability of RS67333 to augment the effect of the selective serotonin reuptake inhibitor paroxetine. The effect of RS67333 was examined after acute and subchronic (3 days) administration. Acute RS67333 (1.5 mg/kg i.v.) had no effect on extracellular 5-HT or 5-HIAA levels, while acute paroxetine (0.5 mg/kg i.v.) increased 5-HT levels by 299 ± 16% and decreased 5-HIAA levels by 25 ± 4%. Administration of RS67333 80 min after paroxetine caused an additional transient increase in 5-HT levels (to 398 ± 52% of baseline). Subchronic RS67333 administration (1.5 mg/kg i.p.) increased basal 5-HT levels by 73 ± 15% and decreased 5-HIAA levels by 27 ± 13%. In conclusion, the 5-HT4 receptor agonist RS67333 augmented the acute effect of paroxetine on extracellular 5-HT levels in the ventral hippocampus, and after 3 days increased basal hippocampal 5-HT levels.