Co-expression of calretinin and γ-aminobutyric acid in neurons of the entorhinal cortex of the common marmoset monkey

The γ‐aminobutyric acid (GABA)‐containing interneuron population in the entorhinal cortex has been shown to consist of several subpopulations. In addition to GABA, these neurons contain another neurochemical substance, such as a neuropeptide or a calcium binding protein. In the present study, we examined the co‐localization of calretinin and GABA in the entorhinal cortex of the common marmoset Callithrix jacchus, a New World monkey. Although the function of calretinin remains unclear, there are indications that it might have a protective role against cell death in a number of neuropathological diseases. Furthermore, it might have a regulatory role in the neurotransmission of GABAergic neurons. In contrast to the rat brain, sparse data exist regarding the degree of co‐expression of these two markers in the monkey brain. Using immunofluorescence and confocal laser scanning microscopy, we found that an average of 56% of the calretinin‐positive neurons in the monkey entorhinal cortex contained GABA, whereas about 27% of the GABA‐positive neurons co‐expressed calretinin. Interestingly, these numbers were higher in the superficial layers of the entorhinal cortex in comparison with the deep layers. However, no differences were found in co‐localization percentages between the different entorhinal subfields. In general, the degree of co‐localization was higher in comparison to findings in the rat entorhinal cortex. The higher amount of co‐localization observed in the present study might reflect species differences between the primate and the non‐primate brain. © 2004 Wiley‐Liss, Inc.