Calcium-binding protein immunoreactivity in the piriform cortex of the guinea-pig: Selective staining of subsets of non-gabaergic neurons by calretinin

Selective immunostaining for calcium-binding proteins identifies subpopulations of neurons with hypothetical distinct functional roles. The neuronal localization of calcium-binding proteins calretinin, parvalbumin and calbindin is here correlated to GABA and glutamate immunoreactivity in the guinea-pig piriform cortex. In the external border of the molecular layer, neurons positive for calretinin with morphological features of Cajal-Retzius cells were found. Rare GABA immunoreactive cells were observed in the same subpial region, whereas neurons containing GABA were abundant within layers la and Ib. Aspartate- and glutamate-immunoreactive cells were also found in the outer la layer. A distinct band of calretinin-immunoreactive fibres and terminals localized in layer Ia, where the afferent fibres originating from the olfactory bulb are segregated. In layer II the number of cells containing calretinin exceeded the number of neurons positive for the anti-GABA antibody. Part of the layer II calretinin-positive neurons with pyramidal shape and large apical dendrites directed toward the surface were found to be immunoreactive for the anti-glutamate antibody on adjacent sections. Neurons in layer II immunoreactive for either parvalbumin or calbindin showed morphological features of interneurons, and their number matched the count of GABA containing cells. Calretinin-positive neurons with the general morphological features of interneurons were scarcely represented in the deep piriform cortical layers, where large multipolar and small bipolar calbindin-positive cells prevailed. The present data show that in the piriform cortex of the guinea-pig calretinin, although expressed in Cajal-Retzius-like cells as in other cortical areas, also marks a subpopulation of glutamate containing pyramidal-like neurons in layer II.