Long-term serotonin reinnervation of the suprachiasmatic nucleus after 5,7-dihydroxytryptamine axotomy in the adult rat.

We present here immunohistochemical evidence that serotonin (5-HT) reinnervation in the suprachiasmatic nucleus (SCN) of adult rats, after intraventricular administration of 5,7-dihydroxytryptamine, is more extensive than formerly believed. The first fibers appeared at 4 months and developed until more than 7 months. However, comparison with age-matched controls showed that, even 24 months post-lesion, the density of 5-HT innervation remained abnormally low. At the electron microscopic level, many synaptic contacts were found to have been re-established. The cellular mechanisms possibly responsible for incomplete 5-HT reinnervation in the SCN are discussed.