DISTINCTIVE HIPPOCAMPAL CA2 SUBFIELD OF THE AMAZON RODENT PROECHIMYS C. A. SCORZA, a * B. H. S. ARAUJO, a R. M. ARIDA, b F. A. SCORZA, a L. B. TORRES, a,c H. A. AMORIM a AND E. A. CAVALHEIRO a a Disciplina de Neurologia Experimental, Universidade Federal de São Paulo/Escola Paulista de Medicina (UNIFESP/EPM), Rua Botucatu 862, 04023-900 São Paulo, Brasil b Departamento de Fisiologia, Universidade Federal de São Paulo/ Escola Paulista de Medicina (UNIFESP/EPM), 04023-900 São Paulo, Brasil c Instituto Evandro Chagas, IEC/Centro Nacional de Primatas–CENP, 67000-000, Ananindeua, Pará, Brasil Abstract—Previous data of our laboratory have shown that the Amazonian rodents Proechimys do not present sponta- neous seizures in different models of epilepsy, suggesting endogenous inhibitory mechanisms. Here, we describe a re- markably different Proechimy’s cytoarchitecture organiza- tion of the hippocampal cornu Ammonis 2 (CA2) subfield. We identified a very distinctive Proechimy=s CA2 sector exhibit- ing disorganized cell presentation of the pyramidal layer and atypical dispersion of the pyramidal-like cells to the stratum oriens, strongly contrasting to the densely packed CA2 cells in the Wistar rats. Studies showed that CA2 is the only cornu ammonis (CA) subfield resistant to the extensive pyramidal neural loss in mesial temporal lobe epilepsy (MTLE) associ- ated to hippocampal sclerosis. Thus, in order to investigate this region, we used Nissl and Timm staining, stereological approach to count neurons and immunohistochemistry to neuronal nuclei (NeuN), parvalbumin (PV), calbindin (CB) and calretinin (CR). We did not notice statistically significant dif- ferences in the total number of neurons of the CA2 region between Proechimys and Wistar. However, Proechimys ro- dents presented higher CA2 volume than Wistar rats. Further- more, no significant difference in the optical density of par- valbumin-immunoreactivity was found between subject groups. On the other hand, Proechimys presented significant higher density of calbindin and calretinin-immunoreactivity when compared to Wistar rats. In this context, this unique CA2 subfield seen in Proechimys opens up a new set of possibilities to explore the contribution of CA2 neurons in normal and pathological brain circuits. © 2010 IBRO. Pub- lished by Elsevier Ltd. All rights reserved. Key words: hippocampus, calcium binding proteins. The basic cytoarchitectonic scheme of the hippocampus was originally established by Cajal (1911) and his student Lorente de No= (1934). Their findings are the keystone of knowledge of the archicortex till the present day. Lorente de Nó (1934) used the term cornu ammonis (CA) for the hippocampus proprius and defined the CA2 subfield as a narrow zone of cells interposed between CA3 and CA1 that had large cell bodies like CA3 but did not receive mossy fiber innervations. However, CA2 has remained somewhat of an anomaly, sometimes considered a sepa- rate region and sometimes simply as an intermingling of cells from CA1 and CA3 (Grove and Tole, 1999; Woodhams et al., 1993). Evidence suggests that CA2 is an anatomically and functionally distinct region, although the boundaries of this region are unclear (Lein et al., 2005). In the Nissl stained sections, most authors rarely identify the CA2 (Schwerdtfeger, 1984; El Falougy et al., 2008). Thus, possibly because of its small size and difficulty encoun- tered in defining its borders, CA2 has been little studied (Mercer et al., 2007). Recently, there has been evidence in favour of distinguishing features of the CA2 field relative to rest of the hippocampus. For example, a relative resis- tance to CA2 pyramidal cell loss has been noted in head trauma (Maxwell et al., 2003), refractory epilepsy (Mathern et al., 1995), ischemia (Sadowski et al., 1999), and early isolation (Bartesaghi and Severi, 2004). Moreover, CA2 possesses a unique pathology in schizophrenic patients (Benes et al., 1998). Thus, CA2 appears to be dissimilar from other CA regions with respect to death and survival from injury or age (Zhao et al., 2007). Studies have also considered whether hippocampal electrical signal propa- gation should include flow through the CA2 region in ad- dition to the traditional dentate gyrus-CA3-CA1 tri-synaptic circuit (Sekino et al., 1997; Sekino and Shirao, 2001). Indeed, a novel circuit in the hippocampal network has been proposed, which contains the CA3-CA2-CA1 path- way in which the CA2 field functions as a gate controlled by the activity of the supramammillary nucleus to control sig- nal propagation in memory formation (Sekino and Shirao, 2007). Moreover, the CA2 region is unique in receiving inputs from the hypothalamic supramammillary nucleus and the interruption of supramammillohipocampal affer- ents prevents the genesis and spread of limbic seizures in the hippocampus (Saji et al., 2000). Functionally, the CA2 area has been proposed as a pacemaker region for syn- chrony (Wong and Traub, 1983). Taken together, the evi- dence points to an active participation of CA2 neurons in the hippocampal network. In this line of evidence, a great deal of the knowledge that has improved our understanding of the brain disorders has derived from appropriate animal models (Curia et al., 2008). This is certainly the case in mesial temporal lobe epilepsy (MTLE), the most common type of partial complex seizure in adulthood (Wieser, 2004). Thus, understanding *Corresponding author. Tel: 55-11-5576-4508; fax: 55-11-5573-9304. E-mail address: carlascorza.nexp@epm.br (C. A. Scorza). Abbreviations: CA, cornu ammonis; CB, calbindin; CE, Coefficient of error; CR, calretinin; IR, immunoreaction; MTLE, mesial temporal lobe epilepsy; PBS, phosphate buffered saline; PV, parvalbumin; SE, sta- tus epilepticus. Neuroscience 169 (2010) 965–973 0306-4522/10 $ - see front matter © 2010 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2010.05.079 965