5-HT4 receptors exert a frequency-related facilitatory control on dorsal raphe nucleus 5-HT neuronal activity

We investigated, using single-unit recordings in chloral hydrate-anaesthetized rats, the role of serotonin(4) (5-HT4 ) receptors in the control of dorsal raphe nucleus (DRN) 5-HT neuron activity. About one-half (36) of the 76 neurons recorded were affected by either the preferential 5-HT4 agonist cisapride (500 and 1000 mug/kg, i.v.) or the selective 5-HT4 antagonist, GR 125487 (200- 2000 mug/kg, i.v.). Responding neurons displayed a significantly higher mean basal firing rate (1.93 +/- 0.1 Hz) than non-responders (1.31 +/- 0.1 Hz). The firing rate of responding 5-HT neurons was enhanced dose-dependently by cisapride (+47 and +94% at 500 and 1000 mug/kg, respectively), an effect abolished by GR 125487 (500 mug/kg) and reduced by the 5-HT4 antagonist, SDZ 205557 (500 mug/kg, i.v). Conversely, GR 125487 induced a dose-dependent inhibition of responders activity, which was almost completely suppressed at the dose of 2000 mug/kg. In a separate set of experiments, the selective 5-HT4 agonist, prucalopride (500 mug/kg, i.v), increased the firing activity (+35%) of 5-HT neurons displaying a high basal firing rate; subsequent injection of GR 125487 (500 mug/kg, i.v.) suppressed this effect. These results indicate that 5-HT4 receptors exert both a tonic and a phasic, positive, frequency-related control on DRN 5-HT neuronal activity. The existence of such a control might open new avenues for therapeutic research in the antidepressant field.