Telencephalic afferent projections from the diencephalon and brainstem in the pigeon. A retrograde multiple-label fluorescent study.

The sites of origin of ascending projections to the telencephalon in the pigeon (Columba livia) were studied by means of the retrograde fluorescence technique following the injection of various tracers (Evans Blue, Fast Blue, Nuclear Yellow) concomitantly into different regions of topographically distinct areas of one hemisphere: hyperstriatum (HY), ectostriatum-paleostriatum (EP), posterior neostriatum (PN). The distribution of retrograde neuronal labeling observed within diencephalic and midbrain structures revealed systems providing unique projections to either a single or a multiple combination of the different telencephalic sites sampled. Single projections to HY arise from numerous dorsal thalamic nuclei including: ipsilaterally-dorsolateralis anterior pars medialis, dorsointermedius posterior, dorsomedialis anterior and posterior and bilaterally-dorsolateralis anterior and superficialis parvocellularis. Single ipsilateral projections upon either the ectostriatum or the caudal neostriatum arise from the nucleus (n.) rotundus/n. triangularis complex and the n. ovoidalis respectively. Multiple ipsilateral afferents to both HY and the neostriatum intermedium stem from rostral and caudal portions of the n. dorsolateralis posterior respectively, whereas the n. subrotundus projects to all of the telencephalic sites examined. Within midbrain/caudal brainstem regions, the area ventralis-Tsai sends afferents to HY and EP and multiple projections upon the latter in addition to the PN site originate in ipsilateral: zona perinervus oculomotorius, the lateral and medial mesencephalic reticular formation, n. tegmenti pedunculo-pontinus pars compacta and bilateral: zona peri-fasciculus longitudinalis medialis, substantia grisea centralis, n. locus coeruleus, n. annularis, n. linearis caudalis. In the case of the latter structures, the multiple-label procedure also made it possible to demonstrate differential distributions of the component populations of neurons whose axons terminate within distinct telencephalic sites and those which display multiple projections via collateral axon branching. Various neuronal systems projecting to the avian telencephalon are compared to those of other vertebrates and possible homologies in telencephalic afferent organization are discussed.