Brain Research, 290 (1984) 201-210 201 Elsevier Research Reports Visual Receptive Thalamopetal Neurons in the Optic Tectum of Teleosts (Holocentridae) HIRONOBU ITO I and HORACIO VANEGAS2 1Department of Anatomy, Osaka University Medical School, Nakanoshima 4, Kitaku, Osaka 530 (Japan) and 2Centro de Biofisica y Bioquimica, Instituto Venezolano de Investigaciones Cwntificas (IVIC), Caracas (Venezuela) (Accepted May 17th, 1983) Key words: tectum opticum - - thalamus - - telencephalon - - visual system - - teleosts Tectal neurons previously known to receive retinofugal input were herein shown to project to the nucleus prethalamlcus. Following HRP injections into the nucleus prethalamicus, pyriform neurons in the stratum perlventriculare and stratum album centrale, and fu- siform neurons in the stratum griseum centrale, were retrogradely labeled. Because the labeled types of neurons have been character- ized as the main visual receptive neurons of the optic tectum, and because the nucleus prethalamicus of teleosts projects to the telen- cephalon, this nucleus can now be considered homologous to the nucleus rotundus of reptiles and birds and to the nucleus lateralis pos- terior-pulvinar complex of mammals, that is, it provides a relay for retinotectal visual input to the telencephalon. Orthogradely la- beled terminals as well as retrogradely labeled neurons were also found in the dorsal area of the telencephalon. The tecto-prethalamo- telencephalic projections are only ipsilateral. INTRODUCTION The route for visual input to the teleostean telen- cephalon has been the subject of some uncertainty. For examplb, in spite of an analogous name to that of mammals, the corpus geniculatum ispum of Mead- er 15, which receives considerable retinofugal in- put 1,26, does not project to the telencepha- l o n 3,9,10,20,26. However, neurons in the nucleus pre- thalamicus, which has no retinofugal input1, 26 but re- ceives massive tectofugal projections4,13, 26, showed retrograde degeneration following telencephalic ab- lations zS, and thus it was suggested that retinofugal information may ascend to the telencephalon via the tectum and the nucleus prethalamicus. Two indepen- dent studies using retrograde HRP transport subse- quently demonstrated a projection from the nucleus prethalamicus to the telencephalon3.10, and the re- gion of termination of this projection has recently been characterized 20 as the area dorsalis pars lateralis of Nieuwenhuys 21. The telencephalon, in turn, pro- jects onto the nucleus prethalamicus11,13,20,25, 26. The findings reviewed above have provided the necessary experimental support to the proposal 2 that the nucleus prethalamicus of teleosts is homologous to the nucleus rotundus of reptiles and birds and the lateralis posterior-pulvinar complex of mammals. However, since the teleostean tectum receives many modalities of sensory information 7,8.12.22.25,26, it is im- portant to determine whether the tecto-prethalamic projection arises from the neurons that receive ret- inofugal information. We have herein used retro- grade transport of HRP to show that this projection arises from neuron types previously known to be syn- apsed upon by retinofugal terminals. MATERIALS AND METHODS Fifteen holocentrid fish (8 Holocentrus ascensionis and 7 Adioryx spinosus) were used in the present study. Animals were anesthetized by immersion in an. approximately 1:10,000 solution of tricaine methane- sulfonate (MS 222) and held rigidly in a fish holder. Operative procedures were performed during gill perfusion with aerated water containing MS 222 at ratios of 1:20,000. A part of the cranium was remov- 0006-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.