Serotonin transporter in substance P (neurokinin 1) receptor knock-out mice

We recently demonstrated that mice lacking the gene for substance P (neurokinin 1) receptors (NK1-/-) show improved cortical dialysate serotonin (5-HT) responses to paroxetine [J. Neurosci. 21 (2001) 8188]. To test for changes that may involve the 5-HT transporter (5-HTT) in these mutant mice, in vivo/in vitro studies were performed. Autoradiographic quantification of 5-HTT was performed: [3H]citalopram binding did not reveal any modification of 5-HT binding sites in the dorsal raphe nucleus (DRN) of wild-type NK1+/+ control and mutant NK1-/- mice. These results were further confirmed by 5-HTT mRNA quantification using competitive reverse transcription and polymerase chain reaction (RT-PCR) assay, which showed similar messenger levels in the DRN of both mice genotypes. The functional status of 5-HTT in vivo was tested by using the zero net flux method of quantitative microdialysis in two serotonergic nerve terminal regions, the frontal cortex and ventral hippocampus, of wild-type NK1+/+ and NK1-/- mice. Neither basal extracellular 5-HT levels nor the 5-HT extraction fraction of the probe (Ed an index of 5-HT uptake in vivo) differed between wild-type and mutant mice in the two brain regions studied. These results suggest that no compensatory response to the constitutive deletion of the tachykinin NK1 receptor involving changes in the activity of the selective 5-HT transporter occurred in the DRN, frontal cortex and ventral hippocampus in mice.