EFFECTS OF EARLY ISOLATION ON LAYER II NEURONS IN THE ENTORHINAL CORTEX OF THE GUINEA PIG R. BARTESAGHI,* M. RAFFI AND S. SEVERI Dipartimento di Fisiologia Umana e Generale, Universita ` di Bologna, Piazza di Porta San Donato 2, I-40127 Bologna, Italy Abstract—Previous studies showed that early environmental conditions severely affect the morphology of the granule cells in the hippocampal dentate gyrus and pyramidal neu- rons in fields CA3 and CA1. The aim of the present study was to determine whether early isolation affects neuron morphol- ogy in layer II of the entorhinal cortex, from which the per- forant path to the dentate gyrus and CA3 takes its origin. Male and female guinea pigs were assigned at 6 –7 days of age to either a control (social) or an isolated environment where they remained for 80 –90 days. The brains were Golgi–Cox stained and neurons were sampled from layer II of the ento- rhinal cortex. Morphometric analysis was carried out on star cells, the most abundant neuron population. Isolated males had star cells with less dendritic branches, a shorter den- dritic length and a smaller dendritic spine density than con- trol males. In contrast, isolated females had more dendritic branches than control females, though this difference was of small magnitude. While isolated males had star cells with a smaller soma than control males, isolated females had a soma larger than control females. In both environments sex differences were found in the star cell morphology. In the control environment males had more dendritic branches, a greater dendritic length, a larger soma but a smaller spine density than females. In the isolated environment males had less branches, a shorter dendritic length, a smaller spine density and a smaller soma than females. The results indicate that early isolation affects the struc- ture of the star cells in the entorhinal cortex and that males and females react to isolation in an opposite manner. A sim- ilar sexually dimorphic response to early isolation was pre- viously observed in the dentate gyrus and fields CA3 and CA1. The presence of widespread effects of isolation in the entorhinal cortex and numerous hippocampal structures suggests that the outcome of early isolation might be a change in learning and memory functions requiring the hip- pocampal region. © 2003 IBRO. Published by Elsevier Sci- ence Ltd. All rights reserved. Key words: environment, hippocampal region, star cells, per- forant pathway, dendritic plasticity. Though the process of neurogenesis in most brain regions is completed before birth, neuron maturation occurs in the early postnatal period. During this critical period of post- natal development neurons undergo dendritic develop- ment (dendritic proliferation and regression) and establish synaptic contacts with afferent pathways. The process of postnatal dendritic maturation and synapse formation has been shown to be strongly influenced by numerous neu- robiological factors. Consequently, neurobiological alter- ations occurring during the early phases of life might en- danger the establishment of a regular neuron pattern. It is well known that during a critical period of brain maturation the arrival of synaptic signals plays a major role in the process of dendritic development and synapse formation in the visual cortex, whose final wiring has been shown to be dramatically shaped by visual experience in the cat (Hubel and Wiesel, 1970) and monkey (Blakemore et al., 1978). Extending these data to other brain systems, it ensues that changes in the amount of information reaching a given brain region during the early phases of life are likely to be followed by the improper development of its neurons and their connections and, hence, the physiology of that region. In view of the importance of afferent signals in brain maturation, changes in the degree of environmental com- plexity during the early stages of life are likely to have widespread effects on brain development. Indeed, data in nonhuman primate models (Harlow and Harlow, 1966; Chamove et al., 1973; Sackett, 1984) and humans show that children raised in impoverished environmental condi- tions exhibit deficits in several behaviors, including learn- ing and memory (Spitz, 1945; Hoffman-Plotkin and Twen- tyman, 1984; Feldman and Walton-Allen, 1997; Gottfried et al., 1998; McGue and Bouchard, 1998; Rutter, 1998; O’Connor et al., 2000; Croft et al., 2001; Kreppner et al., 2001). In line with this, data in the rat (Fiala et al., 1978; Juraska et al., 1985) and mouse (Kempermann et al., 1997, 1998) show that the complexity of environment in the first postnatal period affects the dendritic morphology and the number of neurons in the hippocampal formation, a brain region that plays a fundamental role in some types of long term memory (encoding of current and the retention of past experience) in the mammalian brain (McNaughton et al., 1989; Squire and Zola, 1998; Wallenstein et al., 1998; Burgess et al., 2002). Moreover, it has been shown in the rat (Juraska et al., 1984; Daniel et al., 1999) and mouse (Kempermann et al., 1997, 1998; Kogan et al., 2000) that learning tasks requiring the hippocampal formation are strongly influenced by environmental complexity. Most studies on the impact of environment on the hippocampal formation have been performed in the rat, an animal whose brain is very immature at birth (Altman and Bayer, 1975, 1990). The consequences of environmental complexity might be less prominent in species whose brain is at a more advanced stage of maturation at birth. To obtain information about this issue, in previous studies we *Corresponding author. Tel: +39-051-209-1727; fax: +39-051-251- 731. E-mail address: rbart@biocfarm.unibo.it (R. Bartesaghi). Abbreviations: ANOVA, analysis of variance. Neuroscience 120 (2003) 721–732 0306-4522/03$30.00+0.00 © 2003 IBRO. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0306-4522(03)00354-3 721