Nuclei and tracts in the epithalamus of crocodiles.

ABSTRACT

The epithalamus, an area of the dorsal diencephalon found in all vertebrates, consists of the habenula, the subhabenular nuclei, and associated tracts. The habenula is itself divisible into two parts-a medial and a lateral nucleus differing in their inputs, outputs, and cellular morphology. The medial component is related to the limbic system and serotonergic raphe, while the lateral nucleus is more interconnected with the basal ganglia and midbrain dopamine systems. These findings, which come from experiments mainly done on mammals, serve as a basis for comparison with other vertebrates. However, similar studies in other amniotes, such as reptiles, are few. To fill this gap in knowledge, two species of crocodiles were examined utilizing a variety of histological methods in various planes of section. The following results were obtained. First, the habenula was divided into medial and lateral parts based on its cytoarchitecture. Neurons in the medial habenula were small, were closely packed, and had a limited dendritic arbor characterized by unusual distal dendritic appendages, whereas neurons in the lateral habenula were larger, were more loosely packed, and had longer dendritic processes that were commonly beaded. Second, the stria medullaris, the major input to the habenula, was identified by its immunoreactivity to parvalbumin. Third, the fasciculus retroflexus (habenulointerpeduncular tract), the primary output of the habenula, was visualized by staining with acetylcholinesterase. Fourth, nuclei associated with the habenula, the subhabenular nuclei, have been identified and characterized. These features provide a means to recognize the major nuclei and tracts in the epithalamus in crocodiles and are likely applicable to other reptiles.