Nucleus ruber is known as an important supraspinal center that controls forelimb movements in tetrapods, and the rubral homologue may serve similar functions in fishes (motor control of pectoral fin). However, two apparently different structures have been identified as ‘nucleus ruber' in actinopterygians. One is nucleus ruber of Goldstein (1905) (NRg), and the other nucleus ruber of Nieuwenhuys and Pouwels (1983) (NRnp). It remains unclear whether one of these nuclei (or perhaps both) is homologous to tetrapod nucleus ruber. To resolve this issue from a phylogenetic point of view, we have investigated the distribution of tegmental neurons retrogradely labeled from the spinal cord in eight actinopterygian species. We also investigated the presence/absence of the two nuclei with Nissl- or Bodian-stained brain section series of an additional 28 actinopterygian species by comparing the morphological features of candidate rubral neurons with those of neurons revealed by the tracer studies. Based on these analyses, the NRg was identified in all actinopterygians investigated in the present study, while the NRnp appears to be absent in basal actinopterygians. The phylogenetic distribution pattern indicates that the NRg is the more likely homologue of nucleus ruber, and the NRnp may be a derived nucleus that emerged during the course of actinopterygian evolution.

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Immunohistochemical distribution of [D-Ala2]deltorphin I (DADTI), a highly selective ligand for ƒÂ opioid receptors, was investigated in rat retina by using a specific antiserum. The antiserum against DADTI was raised in a rabbit by immunizing with a synthetic peptide (Tyr-D-Ala-Phe-Asp-Val-Val-Gly•ENH2) conjugated to poly-L-glutamate with water soluble carbodiimide. When examined by immunospot assay, the major epitope of the antiserum appeared to be in the C-terminal region containing Asp-Val-Val-Glu•ENH2, the sequence known to be responsible for opioid receptor selectivity. Immunohistochemically, DADTIlike-positive staining was found in some retinal amacrine cells situated in the inner nuclear layer. Their stained processes were distributed in the inner plexiform layer. In addition, a few positive cells, probably ectopic amacrine cells, were seen in the ganglion cell layer. Double immunostaining for DADTI and other neurotransmitters or neuropeptides revealed that 12.8% and 27.7% of DADTI-positive cells contained GABA and neuropeptide Y, respectively. However, DADTI-positive cells were seldom colocalized with somatostatin or tyrosine hydroxylase. The present study suggests that DADTI in retinal amacrine cells play role(s) functioning as an endogenous opioid peptide.