DescriptionThe hippocampus is required in the mammalian brain for memory formation. The hippocampal homolog or analog of bony fish has not been definitively established and is the subject of this study. Neuroanatomical differences during development of the prosencephalon within the neural tube between bony fish and mammals make comparative anatomy difficult. Therefore, inferring homologous divisions of the neural tube between bony fish and mammals is an inference and an assumption.
We know fish make memories. Spatial memories, memories of one’s environment in relation to spatial orientation, are vital for the survival of an individual and a species. Historic studies have suggested that the homologous region of the mammalian hippocampus is the dorsolateral pallium pars ventralis (Dlv) of the teleost’s prosencephalon. In this experimental study I found that dorsomedial pallium (Dm), not Dlv, may be more important in establishing spatial memory in fish. Previous studies suggest Dm to be the homolog of the amygdala.
To investigate this theory, a spatially orientated task was designed to facilitate the formation of a spatial memory to induce changes in regional activity in the brains of trained fish. Regions analyzed were dorsomedial pallium (Dm), dorsolateral pallium pars dorsalis (Dld), dorsolateral ventral pallium pars dorsalis (Dlv-d) and dorsolateral ventral pallium pars ventralis (Dlv-v). Four conditions were constructed: naïve, control, experimental and recall. The brains of Individuals within these groups were stained for the activity of cytochrome c oxidase (CO) to visualize regional activity by comparison after the appropriate condition because CO activity is an indicator of neural activity. Six ratios were constructed using the data: Dm/Dld, Dm/Dlv-d, Dm/Dlv-v, Dld/Dlv-v, Dld, Dlv-d and Dlv-d/Dlv-v.
Two of the ratios were found to be significant across conditions. These ratios were Dm/Dld and Dm/Dlv-d. In the experimental group, Dm was upregulated in the ratio Dm/Dlv-d nearly twice as the recall group. Inversely, the recall group was upregulated in Dm in the ratio Dm/Dld nearly twice as much as the experimental group. This suggests that spatial memory formation and recall in teleost is Dm dependent.