Comparative Aspects of Forebrain Organization in the Ray-Finned Fishes: Touchstones or Not?

Abstract

The comparative interpretation of forebrain organization of the ray-finned fishes presents a number of challenges. The telencephalon develops by an eversion process, and much of its surface is ventricular rather than pial. A topological map of the ventricular surface shows that the area ventralis (presumed subpallium) is bordered dorsally by the dorsomedial (DM) zone of the area dorsalis, which is, in turn, bordered by the olfactory-recipient dorsoposterior (DP) zone of the area dorsalis. Within the diencephalon small dorsal and ventral thalami and a large group of migrated nuclei assigned to the posterior tuberculum (but of uncertain status) are present. Both the dorsal thalamus and the migrated nuclei of the posterior tuberculum project to the telencephalon. A review of the known connections of the four major zones of the area dorsalis together with their topological positions leads to the following tentative interpretations. Zone DP is homologous to the primary olfactory cortex. There is not enough information concerning the dorsodorsal zone (DD) to speculate on its comparative relationships. Zone DM is a limbic-like area lying between the subpallium and the olfactory pallium and is possibly homologous to the pallial amygdala. The dorsolateral zone (DL) displays a pattern of connectivity with the cerebellum and the optic tectum suggesting a possible homology with non-olfactory, non-limbic pallial areas. Caveats are raised concerning the above interpretations of DM and DL. One or both of them may have evolved independently of the telencephalic zones in tetrapods and may thus represent examples of homoplasy.

Subdivisions of the adult zebrafish pallium based on molecular marker analysis

“…In addition, it has been noted that Dc shows no significant immunoreactivity to Calretinin, Neuropeptide Y or Thyrosine hydroxylase separating it from the surrounding areas of the pallium 79 . Other models based mainly on topology suggest that Dm, Dd and Dld are homologous to the dorsal pallium 37 , whereas others have homologized Dd with the dorsal pallium 4, 29, 30, 33, 64 and have suggested that Dc is not a separate unit, but represents the deeper zone of the periventricular areas of Dm, Dd and Dl. Our gene expression data is consistent with the second part of the “modified pallial eversion model” of Mueller and colleagues.…”

“…However, we have not observed neuroblasts migrating laterally across the pallium to reach Dp in the adult by BrdU pulse chase studies or genetic lineage tracing 51, 76, 77 . The “eversion-rearrangement theory” by Northcutt and Braford suggests that differential expansion of the ventricular surface of some pallial zones and differential proliferation and migration of neuroblasts from the different ventricular zones might result in displacement or shifting of the different pallial subdivisions 33, 64, 75 . Similar to the “partial pallial eversion model”, they propose that a small stretch of Dp is still located between Dm and Dl.…”

Can fish suffer?: perspectives on sentience, pain, fear and stress

“…The amygdala of tetrapods contributes to olfactory input processing, arousal and emotions, such as fear (Maren, 2001). In fish, an amygdaloid complex exists and is located in the telencephalon (Munro and Dodd, 1983;Bradford, 1995;Yoshimoto et al, 1998;Carruth et al, 2000). Lesions or electrical stimulation of the amygdalar region in fish produces changes in aggression similar to those observed following amygdalar lesions or stimulation in tetrapods (Bradford, 1995;Portavella et al, 2002).…”

“…In fish, an amygdaloid complex exists and is located in the telencephalon (Munro and Dodd, 1983;Bradford, 1995;Yoshimoto et al, 1998;Carruth et al, 2000). Lesions or electrical stimulation of the amygdalar region in fish produces changes in aggression similar to those observed following amygdalar lesions or stimulation in tetrapods (Bradford, 1995;Portavella et al, 2002). The hypothalamus of tetrapods is involved in the integration and control of a variety of autonomic functions, sexual behaviour and emotions (Petrovich et al, 2001).…”

Identification of a brain center whose activity discriminates a choice behavior in zebrafish

“…Here, we report a study of an innate light/dark (L/D) choice behavior in zebrafish, a vertebrate genetic model organism with a brain similar to that of mammals but with significantly less complexity (13,14). Our findings show that the medial zone of the dorsal telencephalic region (Dm), the teleost anatomical homolog of the mammalian amygdala (14)(15)(16)(17), and the dorsal nucleus of the ventral telencephalic area (Vd), the zebrafish anatomical homolog of the mammalian striatum (18), are differentially activated between animals that display light avoidance and those that do not, whereas the brain nuclei involved in visual processing are similarly activated among these animals. Because Vd is likely downstream of Dm (15), we suggest that Dm serves as an internal center, the activity of which discriminates the outcome of this choice behavior.…”