Development of 5-HT(1B), SERT and thalamo-cortical afferents in early nutrionally restricted rats: an emerging explanation for delayed barrel formation.

Barrel formation is delayed in nutritionally restricted rats. The underlying cause of such delay is yet unclear. Because barrels appear upon the arrival of somatosensory thalamo-cortical afferents and the reorientation of the dendritic arborizations of cortical spiny stellate neurons, it is likely that at least one of these processes is altered by nutritional restriction. Also, the serotoninergic afferent system has been implicated in regulating barrel segregation and growth during early postnatal life. We then evaluated the pattern of immunostaining of the serotonin transporter (SERT) and of the serotonin receptor 1B (5-HT(1B)), as well as the growth and arrival time of somatosensory thalamo-cortical afferents, to infer the contribution of these elements in the delayed formation of barrels observed in nutritionally restricted rats. It was found that the rates of development and the segregation of thalamo-cortical fibers were normal in nutritionally restricted rats. SERT, but not 5-HT(1B) immunoreactivity, was decreased in the primary somatosensory cortex during barrel specification. The availability of both proteins in nutritionally restricted rats was lower than that observed in their well fed counterparts at later developmental times. It is concluded that the delayed formation of barrels observed in nutritionally restricted rats is due to a retarded reorientation of dendritic arbors of cortical neurons. This might happen as a secondary effect of decreasing the availability of SERT and/or increasing the availability of 5-HT(1B) receptor early in postnatal life.