Serotonin 1B receptors in the developing somatosensory and visual cortices are located on thalamocortical axons

Serotonin (5-HT)-immunoreactive axons are densely distributed in the primary visual and somatosensory cortices of rats, mice, and hamsters for the first 2 weeks of life, and a recent study from this laboratory has demonstrated that 5-HT1B receptors assume a pattern that exactly matches that of the serotoninergic axons. The differential distribution of these receptors is also transient. In the present study, we combined receptor binding autoradiography with neurochemical ablation of 5-HT axons or electrolytic lesions of the dorsal thalamus in an effort to determine the neural elements upon which the 5-HT1B receptors were located. Subcutaneous injections of the toxin 5,7-dihydroxytryptamine, made on the day of birth, totally eliminated the dense and patterned 5-HT innervation of the somatosensory and striate cortices of rats killed on postnatal day 8 but had no qualitative effect upon the distribution or density of 5-HT1B receptors in either of these cortical regions in animals killed at the same age. Conversely, electrolytic lesions of the dorsal thalamus made on postnatal day 6 resulted in a complete loss of the dense and patterned distribution of 5-HT1B receptors in rats killed on postnatal day 8. These results indicate that thalamocortical axons transiently express 5-HT1B receptors.