LAMINAR DISTRIBUTION AND SOURCES OF CATECHOLAMINERGIC INPUT TO THE OPTIC TECTUM OF THE PIGEON (COLUMBA-LIVIA)

A combined immunohistochemical and retrograde tracing approach was use d to characterize the catecholaminergic innervation of the optic tectu m (TeO), the major target of retinal projections in many avian species . Giemsa counterstaining was employed to determine precisely the lamin ar localization of immunoreactive fibers and presumptive terminals. Th e 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 dor sal-anterior tectum, TH-immunoreactive processes are particularly dens e in retinorecipient layers 4 and 7 and in layer 5a. As in the mammali an superior colliculus, the hulk of the catecholaminergic innervation of the pigeon TeO reflects inputs, presumably noradrenergic, originati ng in the locus coeruleus and nucleus subcoeruleus. However, the catec holaminergic innervation of the pigeon TeO shows several features dist inct from those reported for the mammalian superior colliculus. These include an input from a pretectal TH-positive cell group unknown in ma mmals and the presence of residual TH immunoreactivity after administr ation of the noradrenergic neurotoxin DSP-4. Moreover, the pattern of TH-immunoreactive fibers in pigeon TeO indicates more laminar and regi onal specialization within this structure than has been reported for t he catecholaminergic innervation of the superior colliculus in mammals . (C)1995 Wiley-Liss, Inc.