Three patterns of pretectal organization can be discerned morphologically in teleosts. The taxonomic distribution of these pretectal patterns suggests that the intermediately complex pattern (seen in most teleost groups) has given rise to both the elaborate pattern (seen in percomorphs) and the simple pattern (seen in cyprinids). Two pretectal patterns (intermediately complex and elaborate) form part of similar, homologous visual pathways to the hypothalamus; the third pattern is involved in a nonhomologous pathway to the hypothalamus. Acetylcholinesterase (AChE) histochemistry was used in the present study in order to characterize these pretectal patterns further. It is demonstrated that AChE is a highly selective and reliable interspecific marker for all divisions of the superficial pretectum, the nucleus corticalis, the posterior pretectal nucleus (or nucleus glomerulosus) and portions of the inferior lobe. Therefore, the histochemical data support the hypothesis of a homology between the three patterns of pretectal organization in teleosts. Furthermore, the present data provide a basis for more specific investigations regarding the involvement of acetylcholine as a neurotransmitter within the visual pathways to the hypothalamus in teleosts.

Abstract. Among vertebrates, telencephalo-pontine systems exist only in birds and mammals. However, three nuclei in the diencephalon and mesencephalon of teleost fishes have been indicated analogous to the pons to represent relay stations between telencephalon and cerebellum. Since two of these nuclei (dorsal preglomerular nucleus, dorsal tegmental nucleus) have only been described in the highly derived, electrosensory mormyrids, we investigated telencephalic connections in two nonelectrosensory teleosts, the goldfish Carassius auratus and the freshwater butterflyfish Pantodon buchholzi, and cerebellar connections only in the latter species, since for C. auratus these connections are already established. Horseradish peroxidase tracing reveals that C. auratus has a dorsal tegmental nucleus and a paracommissural nucleus both of which are telencephalo-recipient and project to the cerebellum, and that P. buchholzi has a dorsal preglomerular nucleus with such connections. These results extend our knowlegde of the distribution and, therefore, the phylogeny of telencephalo-cerebellar systems in teleosts. Similar to tetrapods, teleosts appear to have developed telencephalo-cerebellar systems several times independently.

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The central connections of the goldfish olfactory bulb were studied with the use of horseradish peroxidase methods. The olfactory bulb projects bilaterally to ventral and dorsolateral areas of the telencephalon; further targets include the nucleus praeopticus periventricularis and a caudal olfactory nucleus near the nucleus posterior tuberis in the diencephalon, bilaterally. The contralateral bulb and the anterior commissure also receive an input from the olfactory bulb. Contralateral projections cross in rostral and caudal portions of the anterior commissure and in the habenular commissure. Retrogradely labeled neurons are found in the contralateral bulb and in three nuclei in the telencephalon bilaterally; the neurons projecting to the olfactory bulb are far more numerous on the ipsilateral side than in the contralateral hemisphere. Afferents to the olfactory bulb are found to run almost entirely through the lateral part of the medial olfactory tract, while the bulb efferents are mediated by the medial part of the medial olfactory tract and the lateral olfactory tract. Selective tracing of olfactory sub-tracts reveals different pathways and targets of the three major tract components. Reciprocal connections between olfactory bulb and posterior terminal field suggest a laminated structure in the dorsolateral telencephalon.