Serotonin immunoreactive neurons are present in the superficial layers of the hamster's, but not the rat's, superior colliculus.

Immunocytochemistry for serotonin (5-HT) was carried out in both hamsters and rats in order to determine whether or not 5-HT-positive cells existed in the superior colliculus (SC) of either species. In both hamster and rat, the superficial and deep SC laminae contained dense networks of 5-HT-positive fibers. The rat's SC contained no 5-HT-positive neurons. In hamster, numerous 5-HT-immunoreactive cells were visible throughout the depth of the stratum griseum superficiale (SGS). These neurons had a variety of morphological characteristics and included marginal cells, horizontal cells, and neurons with vertically oriented dendritic trees. No 5-HT-positive neurons were found in any other portion of the hamster's SC. 5-HT-positive SC cells were observed with antisera from two different sources and they were not seen in animals that were pretreated with reserpine. Pretreatment with fluoxetine (an inhibitor of 5-HT uptake) also resulted in a disappearance of 5-HT-positive neurons in the hamster's SC. This result indicated that “serotonergic” cells in the colliculus of this species are capable of taking up, but probably not synthesizing, this indoleamine. The dorsal and ventral lateral geniculate nuclei (LGNd and LGNv, respectively) both contain numerous 5-HT-positive fibers and both of these structures receive input from the SGS. Combination of retrograde tracing with fluorogold and immunocytochemistry indicated that 5-HT-accumulating SC neurons were not the source of these fibers. Unilateral ablation of the superficial SC laminae also failed to reduce 5-HT immunoreactivity in either the LGNd or LGNv. These results are consistent with the possibility that 5-HT-accumulating cells in the hamster's SC may be interneurons that take up this transmitter after it is released by afferents to this nucleus.