Neuropeptide Y innervation of retinorecipient layers of chick optic tectum

A dense laminar network of varicose neuropeptide Y immunopositive fibres, but not cells, was found to cover the retinorecipient layers of the entire optic tectum of 10- to 21-day-old domestic chicks. Unilateral enucleation resulted in no apparent loss of neuropeptide Y immunopositive fibres in the contralateral tectum, suggesting that they are not of retinal origin. To identify possible sources of neuropeptide Y immunopositive tectal input, the distribution of neuropeptide Y immunopositive perikarya was investigated in the meso-diencephalic region of the chick. A virtually continuous network of neuropeptide Y immunopositive cells and fibres was observed stretching from rostro-lateral thalamus to the pretectum in close apposition to the perirotundal belt. These neuropeptide Y immunopositive structures did not seem to respect the borders of known anatomical regions but partially co-localized with the nucl. dorsolateralis anterior pars magnocellularis and pars medialis, nucl. pretectalis diffusus, nucl. lentiformis mesencephali pars parvocellularis and pars magnocellularis, nucl. principalis precommissuralis, nucl. lateralis precommissuralis, nucl. superficialis magnocellularis (SM), nucl. posteroventralis thalami Kühlenbeck and the nucl. subrotundus. In the nucleus of the basal optic root, neuropeptide Y immunopositive perikarya were observed only within or adjacent to its dorsal and lateral subdivisions although all subdivisions were enmeshed with neuropeptide Y immunopositive fibres. The neuropeptide Y immunopositive tectal input is likely to derive from tectothalamic--presumably perirotundal--neuronal groups. The extent of this tectal afferent projection, not reported earlier in the domestic chick, suggests a powerful neuropeptide-Yergic control of retinotectal relay function.