Telencephalic projections from midbrain and isthmal cell groups in the pigeon. I. Locus coeruleus and subcoeruleus.

Horseradish peroxidase (HRP) and amino acid autoradiography were used in pigeon to determine the trajectories and projection patterns of neurons within the locus coeruleus and subcoeruleus nuclei upon the cerebral hemispheres. The specific cell groups investigated include the locus coeruleus (LoC), nucleus subcoeruleus dorsalis, and nucleus subcoeruleus ventralis. Efferents from each of these nuclei ascend to the telencephalon via the medial and lateral forebrain bundles, ansa lenticularis, and the quintofrontal and occipitomesencephalic tracts. A separate dorsally situated bundle derived from LoC neurons reaches many dorsal thalamic nuclei. The telencephalic projections of the LoC and subcoeruleus nuclei are bilateral and symmetrical, although projections to the contralateral hemisphere are sparse. Crossing fibers project to contralateral targets primarily via the dorsal supraoptic decussation and along the dorsal and ventral margins of the anterior commissure. Within the telencephalon, the following neural structures receive input from neurons in the LoC and subcoeruleus cell groups: the paleostriatal complex including the paleostriatum augmentatum and lobus parolfactorius, septal nuclei, nucleus accumbens, olfactory tubercle, hippocampus and parahippocampal area, nucleus taeniae, dorsal archistriatum, lateral neostriatum, hyperstriatum dorsale, hyperstriatum ventrale, and preoptic area. Large portions of the cerebral hemispheres including the hyperstriatum accessorium, much of the neostriatum and hyperstriatum ventrale, and all but the dorsal portion of the archistriatum receive little or no input from either the locus coeruleus or subcoeruleus cell groups. This is apparently different from the condition in mammals in which virtually all cortical fields receive input from neurons within the LoC. Moreover, the pattern of projections of the subcoeruleus nuclei upon telencephalic fields described here as well as recent histochemical data suggest that these cell groups are comparable to the lateral tegmental (A8) cell group of mammals rather than to the mammalian subcoeruleus nuclei.