In vivo chronoamperometric measures of extracellular serotonin clearance in rat dorsal hippocampus: contribution of serotonin and norepinephrine transporters.

The effects of blockade of serotonin (5-HT) and norepinephrine (NE) transporters (SERT and NET, respectively) on the removal of locally applied 5-HT from extracellular fluid (ECF) were examined using in vivo chronoamperometry. Male Sprague-Dawley rats were anesthetized with chloralose/urethane, and a Nafion-coated, carbon fiber electrode attached to a multibarrel micropipette was positioned into either the dentate gyrus or CA3 region of the dorsal hippocampus. Pressure ejection of 5-HT elicited reproducible electrochemical signals of similar peak amplitude and time course in both structures. Local application of the selective serotonin reuptake inhibitors (SSRI) fluvoxamine and citalopram prolonged the clearance of 5-HT in both brain regions and also increased signal amplitude in the CA3 region. These effects were abolished in rats pretreated with 5, 7-dihydroxytryptamine (5,7-DHT), a selective 5-HT neurotoxin. The NE uptake inhibitors desipramine (DMI) and protriptyline did not alter the 5-HT signal in the CA3 region but prolonged the clearance of 5-HT in the dentate gyrus; this effect was absent in rats pretreated with 6-hydroxydopamine (6-OHDA), a selective catecholamine neurotoxin. The prolongation of the removal of 5-HT from the ECF in the dentate gyrus caused by fluvoxamine or desipramine was of comparable magnitude and was dose dependent. Furthermore, per picomole of 5-HT applied, the signal amplitude and clearance time were significantly increased in the dentate gyrus of rats lesioned with either 5,7-DHT or 6-OHDA. Only 5,7-DHT treatment caused this effect in the CA3 region. From these data, it is inferred that in certain regions of brain (dentate gyrus), both the SERT and NET contribute to the active clearance of exogenously applied 5-HT.