Modifications of the serotonergic system in mice lacking serotonin transporters: an in vivo electrophysiological study.

The serotonin transporter (5-HTT) plays a key role in the regulation of serotonin (5-hydroxytryptamine, 5-HT) transmission in the pathophysiology and therapeutics of several psychiatric disorders. The mean spontaneous firing rate of midbrain dorsal raphe 5-HT neurons was recorded in chloral hydrate-anesthetized mice. The serotonin transporter (5-HTT), which plays a key role in the regulation of serotonin was significantly decreased in homozygous mice lacking the 5-HT transporter (5-HTT -/-) by 66% and in heterozygous (5-HTT +/-) mice by 36% compared with their normal littermates (5-HTT +/+). Systemic injection of the selective 5-HT(1A) receptor antagonist WAY 100635 enhanced 5-HT neuronal firing by 127% in 5-HT -/- mice, thus indicating an enhanced synaptic availability of 5-HT at inhibitory 5-HT(1A) receptors. Nevertheless, the cell body 5-HT(1A) autoreceptors were desensitized in both 5-HTT -/- and 5-HTT +/- mice. At the postsynaptic level, the recovery time (RT(50)) of the firing rate of hippocampus CA(3) pyramidal neurons following iontophoretic applications of 5-HT was significantly prolonged only in 5-HTT -/- mice. The selective 5-HT reuptake inhibitor paroxetine significantly prolonged the RT(50) in 5-HTT +/+ and 5-HTT +/- mice, without altering the maximal inhibitory effect of 5-HT. These neurons in 5-HTT -/- mice showed an attenuated response to the 5-HT(1A) agonist 8-hydroxy-2-diproplyaminotetralin, but not to 5-HT itself. These results establish that the lack of 5-HTT causes a prolonged recovery of firing activity following 5-HT applications. The genetic deletion of the 5-HTT plays a key role on 5-HT(1A) receptor adaptation: a desensitization at pre- and postsynaptic levels in 5-HTT -/- mice, but to a different extent, and only at the presynaptic level in the 5-HTT +/- group.