Calbindin-D28k in cortical regions of the lizard Psammodromus algirus.

The morphology, distribution, and ultrastructural features of calbindin-D28k-immunoreactive neurons and fibers in the cortical regions of the lizard Psammodromus algirus, considered homologues to the mammalian hippocampal formation, were analyzed by using the peroxidase anti-peroxidase technique at the light and electron microscopic level. On the basis of staining properties and localization, two distinct populations of calbindin-D28k-immunoreactive neurons were observed in both the medial and dorsal cortices. Those located in the cell layer, namely principal neurons, were weakly immunostained, whereas a number of Golgi-like stained neurons were observed in plexiform layers. Double immunocytochemistry showed that all calbindin immunoreactive neurons in the deep plexiform layers were also gamma-aminobutyric acid immunoreactive. We consider them as a population of nonprincipal neurons different from those containing the calcium-binding proteins parvalbumin and calretinin. Two types of immunoreactive Boutons were revealed by electron microscopy on the basis of the synaptic specialization: Boutons making asymmetrical synapses were generally smaller in size and contacted on small dendritic profiles or cell bodies, whereas larger boutons established symmetrical synapses mainly on dendritic shafts. We propose that the first type of boutons arises from principal neurons and that the second type arises from nonprincipal ones. Finally, the staining pattern, localization, and the circuit in which nonprincipal calbindin-immunoreactive neurons and other neurochemically defined neurons could be involved in cortical regions of Psammodromus are compared with those of mammalian hippocampus.