Enkephalin-mediated basal ganglia influences over the optic tectum: immunohistochemistry of the tectum and the lateral spiriform nucleus in pigeon.

By using immunohistochemical techniques with antisera directed against either leucine-enkephalin or methionine-enkephalin (generously supplied by K.-J. Chang), four distinct bands of fibers with enkephalinlike immunoreactivity were demonstrated in the pigeon tectum: (1) a thin band of thick fibers and tightly clustered bulbous swellings in layer 3, (2) a broader band of fibers with less tightly clustered bulbous swellings in layer 5, (3) a broad band of numerous obliquely and radially oriented fibers that spanned layers 8-13, and (4) a band of sinuous fibers in layer 15. In addition, numerous enkephalinergic cell bodies with radially ascending processes were seen in layers 8-10. Since the neurons of the avian lateral spiriform nucleus (SpL) of the pretectum are known to contain enkephalin (Davis et al., '80; De Lanerolle et al., '81) and project to the tectum (Brecha et al., '76; Reiner et al., '82), unilateral electrolytic lesions were made of SpL. In birds with unilateral lesions of SpL, layers 8-13 of the ipsilateral tectum were nearly devoid of enkephalinergic fibers, but no alterations were seen in layers 3, 5, and 15. Since no other neurons in the vicinity of SpL are enkephalinergic and project to the tectum, the loss of enkephalin-immunoreactive fibers in the ipsilateral tectal layers 8-13 is attributable to the destruction of SpL. Although the source of the enkephalinergic fibers in tectal layers 3, 5, and 15 is unclear, part of the enkephalin pattern in layers 3 and 5 may derive from the ascending processes of the enkephalinergic neurons of layer 8-10. The present results indicate that SpL has an enkephalinergic projection to layers 8-13 of the ipsilateral tectum. The avian SpL receives its major input from the ascending processes of the enkephalinergic neurons of layers 8-10, nuclei that themselves receive major basal ganglia inputs (Reiner et al., '82) and projects to the tectal layers 8-13 (Reiner et al., '82), the layers of origin of the major tectal efferent projections (Reiner and Karten, '82). The enkephalinergic fibers in layers 8-13 may, thus, have some influence upon the motor output functions of the avian tectum.