Transient expression of the serotonin transporter in the developing mouse thalamocortical system.

The serotonin transporter was visualized in sections through the developing mouse thalamus by autoradiography of [3H]citalopram binding. In late gestation, a high density of transporter expression appeared in the ventrobasal thalamic complex and medial geniculate body. During the first postnatal week, binding in these areas decreased to low levels. A similar pattern of transient [3H]citalopram binding was observed in the somatosensory cortex, although the rise and decline of labeling occurred some days later. The density of the serotonergic innervation in the ventrobasal thalamic complex is known to be very low during the entire developmental period. Therefore, these data suggest that the serotonin transporter may be expressed transiently by thalamic neurons projecting to the cerebral cortex (as a "heterocarrier") which are capable of taking up serotonin in the somatosensory cortex. We propose that serotonin may act temporarily as a "false" transmitter in thalamocortical axons.