Origins of serotonin‐like immunoreactivity in the optic tectum of rana pipiens

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Abstract
We have previously identified a population of serotonin‐like immunoreactive (5‐HT‐ir) retinal ganglion cells in Rana pipiens. In this study, we examined serotonin‐like immunoreactivity (5‐HTLI) in a probable target of those cells, the optic tectum. We observed both 5‐HT‐ir fibers and cell bodies in this structure. 5‐HT‐ir cells were located in the cellular layers of the tectum, layers 2, 4, and 6, and scattered in its superficial layers. 5‐HT‐ir fibers in the tectum displayed a laminated organization and were located in tectal layers 3, 5, 6, 7, and 9. Retrograde labelling experiments showed that 5‐HT‐ir retinal ganglion cells projected to the optic tectum. However, these experiments also demonstrated that serotonergic neurons in the midbrain tegmentum, the nucleus isthmi, and the medulla did so as well. 5‐HT‐ir fibers seen in lamina A of layer 9 were very much reduced in density in animals in which the optic nerve had been lesioned for 3–6 months. Immunoreactive fibers in lamina B of layer 9 were not affected by the lesion. Our results suggest that 5‐HT‐ir fibers in lamina A of layer 9 are mainly of retinal origin, whereas those in lamina B originate from other brain areas. The 5‐HT‐ir tectal cells located in the cellular layers probably contribute the 5‐HT‐ir fibers seen in layers 3, 5, 6, and 7.