Laminar distribution and sources of catecholaminergic input to the optic tectum of the pigeon (Columba livia)

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Abstract
A combined immunohistochemical and retrograde tracing approach was used to characterize the catecholaminergic innervation of the optic tectum (TeO), the major target of retinal projections in many avian species. Giemsa counterstaining was employed to determine precisely the laminar localization of immunoreactive fibers and presumptive terminals. The TeO of the pigeon is densely innervated by fibers immunoreactive for tyrosine hydroxylase (TH), which are most heavily distributed to the superficial layers of its dorsal and anterior portions. Within the dorsal‐anterior tectum, TH‐immunoreactive processes are particularly dense in retinorecipient layers 4 and 7 and in layer 5a. As in the mammalian superior colliculus, the bulk of the catecholaminergic innervation of the pigeon TeO reflects inputs, presumably noradrenergic, originating in the locus coeruleus and nucleus subcoeruleus. However, the catecholaminergic innervation of the pigeon TeO shows several features distinct from those reported for the mammalian superior colliculus. These include an input from a pretectal TH‐positive cell group unknown in mammals and the presence of residual TH immunoreactivity after administration of the noradrenergic neurotoxin DSP‐4. Moreover, the pattern of TH‐immunoreactive fibers in pigeon TeO, indicates more laminar and regional specialization within this structure than has been reported for the catecholaminergic innervation of the superior colliculus in mammals.