Thalamus of Reptiles and Mammals: Some Significant Differences.

Background: Most studies comparing forebrain organization between reptiles and mammals have focused on similarities. Equally important are the differences between their brains. While differences have been addressed infrequently, this approach can highlight the evolution of brains in relation to their respective environments.

Summary: This review focuses on three key differences between the dorsal and ventral thalamus of reptiles and mammals. One is the organization of thalamo-telencephalic interconnections. Reptiles have at least three circuits that transmit information between the dorsal thalamus and telencephalon, whereas mammals have just one. A second is the number and distribution of local circuit neurons in the dorsal thalamus. Most reptilian dorsal thalamic nuclei lack local circuit neurons, whereas these same nuclei in mammals contain varying numbers. The third is the organization of the thalamic reticular nucleus. In crocodiles, at least, the neurons in the thalamic reticular nucleus are heterogeneous with two separate nuclei each being associated with a different circuit. In mammals, the neurons in the thalamic reticular nucleus, which is a single structure, are homogeneous.

Key messages: Transcriptomics and development are suggested to be the most likely approaches to explain these differences between reptiles and mammals. Transcriptomics can reveal which neuron types are "new" or "old" and whether neurons and their respective circuits have been re-purposed to be used differently. Examination of the development and connections of the dorsal and ventral thalamus will determine whether their formation is similar or different from what has been described for mammals.