The origin and distribution of catecholaminergic axon terminals in the cerebral cortex of the turtle ( Chrysemys picta)

Abstract

Numerous catecholaminergic (CA) axon terminals have been visualized in the dorsal cortex of the rostral half of the turtle ( Chrysemys picta) telencephalic vesicle by means of the Falck-Hillarp fluorescence histochemical method. The highest density of CA axon terminals is found within the outer 100 μm of the molecular layer which contains the superficial segments of the apical dendrites of the cortical cells. The CA terminals gradually become less abundant towards the hippocampal cortex medially and are scarce in the molecular layer of the piriform cortex laterally. The few CA axons ending in the piriform cortex are mainly arborized around the neuronal somata. Unilateral brain stem transections produced either at caudal midbrain or isthmus level result in a marked loss of CA terminals within the cerebral cortex and hypothalamic area of the turtle, on the lesioned side. In contrast, brain stem hemisections placed at various levels caudally to the isthmus produce a severe loss of CA terminals in the hypothalamic area but not in the cerebral cortex. It appears, therefore, that the cortical CA terminals in the turtle arise mainly from CA perikarya located at the isthmus level (nucleus dorsolateralis tegmenti) whereas the hypothalamic CA terminals originate principally from CA neurons located in the medulla. In the present study, both the ‘isthmo-cortical’ and ‘medullo-hypothalamic’ CA pathways of the turtle brain have been tentatively related to the mammalian ‘dorsal and ventral noradrenergic pathways’. As the morphological organization of ascending CA pathways to the cerebral cortex and hypothalamus is very similar in the turtle and mammals, it is postulated that these CA neuronal systems are phylogenetically ancient and thus constitute fundamental components of the vertebrate central nervous system.