Sculpting the visual map: The distribution and function of serotonin-1A and serotonin-1B receptors in the optic tectum of the frog

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT_1A and 5-HT_1B binding sites in the laminated optic tectum. 5-HT_1A binding sites, as labeled with [³H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT_1B binding sites were visualized using [¹²⁵I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT_1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT_1A receptors, refined it. We conclude that both 5-HT_1A and 5-HT_1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.