The serotonergic somatosensory projection to the tectum of normal and eyeless salamanders

The spinotectal somatosensory projection was compared in normal, genetically eyeless, and embryonically manipulated salamanders. In normal animals, serotonin fluorescence was restricted to the intermediate tectalneuropil. This same region showed both high levels of serotonin uptake and somatosensory single unit electrical activity. In mutant eyeless salamanders and in normal animals enucleated early in development, serotonin fluorescence, serotonin uptake, and somatosensory activity were present in the superficial tectal neuropil. One-eyed animals, either genetically normal axolotls with one eye enucleated embryonically or genetically eyeless animals in which a normal eye had been transplanted, showed normal intermediate serotonin fluroescence and somatosensory physiology in the visually innervated half-tectum. In the visually uninnervated half-tectum, they showed superficial serotonin fluorescence and somatosensory physiology. In normal animals, 5,7-dihydroxytryptamine (5,7-DHT), a specific poison for serotonergic fibers, eliminated physiological responses in the contralateral somatosensory tectal region. The 5,7-DHT poisoning also abolished U.V.-induced serotonin fluorescence in the intermediate tectal neuropil. These results are discussed in terms of (1) evidence for serotonin as a central neurotransmitter for somatosensory information in the tectum, (2) the effects of eyelessness on tectal organization, and (3) related results in other animals.