Time course of expression and function of the serotonin transporter in theneonatal rats primary somatosensory cortex

Immunocytochemical and autoradiographic techniques were employed to determi ne the time course of expression of the serotonin (5-HT) transporter (SERT) on thalamocortical afferents in the rat's primary somatosensory cortex (S- I), and to correlate this expression to the transient vibrissae-related pat terning of 5-HT immunostaining previously described. In additional in vivo and in vitro experiments, 5-HT and H-3-5-HT were applied directly to the co rtices of untreated and 5,7-dihydroxytryptamine-treated (5,7-DHT) rats in o rder to determine the period during which SERT functions on thalamocortical axons to take up 5-HT. In postnatal rats, SERT immunohistochemistry reveal ed a somatotopic patterning in S-I that persisted until P-15, which is 6 da ys after the disappearance of the vibrissae-related 5-HT immunostaining. H- 3-citalopram autoradiography revealed a vibrissae-related pattern in layer IV of S-I until at least P-30. Following destruction of raphe-cortical affe rents with 5,7-DHT on the day of birth, this binding pattern remained visib le until at least P-25, indicating that SERT located on thalamocortical axo ns is responsible for the 3H-citalopram patterning observed in S-I. Tissue from 5,7-DHT-treated rats that had 5-HT applied directly to their cortices revealed a normal vibrissae-related pattern of 5-HT immunostaining in S-I a t P-7 and P-11 but only a faint pattern at P-13 and none at P-14. In additi on, 3H-5-HT injected directly into S-I labeled layer TV barrels at P-6 and P-12 but not at P-18. The results of these experiments demonstrate that SER T is expressed by thalamocortical afferents and remains functional long aft er the vibrissae-related 5-HT immunostaining in cortex disappears.