Development of Thalamo-Telencephalic Circuits in a Reptile

Neuronal circuits are a core component required for proper brain function and organization. How these circuits are built remains incompletely understood. The vast majority of studies on forebrain development have investigated mammals. Whether the described observations found in mammals apply to other vertebrates is unknown. The present report examined the development of forebrain circuits that interconnect the dorsal thalamus and the telencephalon in Alligator mississippiensis. A tracer, neurobiotin, which is transported in both an anterograde and retrograde fashion, was used. Four circuits were identified after neurobiotin placement into either the diencephalon or in the forebrain bundles themselves. Three circuits ascended from the dorsal thalamus to the telencephalon. These were: the ipsilateral lateral forebrain bundle and the ipsilateral and contralateral medial forebrain bundles. The other circuit descended from the basal nuclei to connect with the dorsal thalamus. The first circuit to develop was the basal nuclei to dorsal thalamus circuit followed by the lateral forebrain bundle and lastly by the medial forebrain bundle. Because it preceded the formation of the three other circuits, the basal nuclei to the dorsal thalamus tract is suggested to serve as a scaffold for the other three circuits to follow. The formation of synapses by lateral forebrain bundle axons in the telencephalon was determined by immunohistochemistry using a synaptic vesicle glycoprotein. Synapse formation in the telencephalon from lateral forebrain bundle axons occurred before hatching. This suggested that information carried by the lateral forebrain bundle was critical for survival upon hatching. Medial forebrain bundle axon connections may become functional only after hatching.