Ontogeny of the entorhinal cortex

Ontogeny and neural substrates of the context preexposure facilitation effect

Contextual fear conditioning emerges around post-natal day (PD) 23 in the rat. This is thought to reflect hippocampus-dependent conjunctive learning, which binds the individual features of the context into a unified representation (Rudy, 1993). However, context conditioning can also be supported by hippocampus-independent, feature-based simple associations (Rudy, 2009) and these may operate at PD 23-24 (Pugh & Rudy, 1996). To address this issue, we studied the ontogeny of a variant of contextual fear conditioning, termed the context-preexposure-facilitation-effect (CPFE), in which exposure to context and (immediate) foot shock occur on successive occasions. This variant requires conjunctive as opposed to feature-based simple associations (Rudy, 2009). We tested PD 17, 24, and 31 rats on the CPFE vs. conventional context conditioning (Exp. 1) and on the CPFE with stronger reinforcement (Exp. 2). The CPFE emerged on PD 24 regardless of reinforcer strength and in parallel with context conditioning. Infusions of the NMDA-receptor antagonist, MK-801, into the dorsal hippocampus just before pre-exposure on PD 24 eliminated the CPFE, whereas infusions occurring after pre-exposure had no effect (Exp. 3). These findings demonstrate a role of hippocampal NMDA receptors in the CPFE as early as PD 24 and implicate conjunctive learning mechanisms in the ontogeny of contextual fear conditioning.

Immunolabelling of spliceosomes in sections and cultured astrocytes of human fetal brain tissue

In the cell nucleus first large pre-mRNAs are synthesized which contain protein coding as well as non-coding sequences. The latter are removed in a process called splicing which takes place in nuclear spliceosomes. These spliceosomes consist among others of protein factors, such as the splicing factor SC35 being abundant in speckled regions of the cell nucleus. This study aims at determining immunostaining patterns using anti-SC35 in sections of the human fetal prosencephalon and cultured human astrocytes. Within the allocortical entorhinal region of the fifth gestational month the number, size and distribution of SC35-positive speckles varies considerably among the laminae which can, thus, clearly be delineated. The immature isocortical plate, however, does not display a laminar arrangement at this developmental stage. Differential immunostaining patterns can be seen in subcortical areas. Cultured human astrocytes reveal numerous speckles occupying a large portion of the nucleoplasm. On account of the SC35-immunostaining patterns no distinction of subpopulations of astrocytes is possible. The results demonstrate that SC35-immunoreactive speckles show lamina and area-specific characteristics of human fetal brain sections. Conspicuous differences in number, size and distribution of speckles are visible in different cytoarchitectonic structures; thus, architectonic borders stand out clearly in SC35-immunopreparations. The occurrence of area-specific immunolabelling of nuclear speckle domains reflects neuronal differentiation at the pre-translational level. It may be assumed that a distinct set of proteins, generated by a definite nerve cell type, can be correlated with a distinct morphology of spliceosomes. The in vitro finding indicates that anti-SC35 may well be used as a tool to study possible alterations of the speckles after, for instance, application of growth factors.

Expression of a novel murine phospholipase D homolog coincides with late neuronal development in the forebrain

Members of the phospholipase D (PLD) superfamily are defined by the conserved HXKXXXXD motif, which is essential for the catalytic function of mammalian PLD. PLD enzymes are thought to play roles in signal transduction and membrane vesicular trafficking in mammalian cells. Here we describe a 54-kDa novel murine polypeptide (designated SAM-9) that is predicted to be a membrane-associated member of the PLD superfamily. SAM-9 shares 40, 30, and 29% amino acid identity with potential orthologs, in vaccinia virus, Caenorhabditis elegans, and Dictyostelium discoideum, respectively, and belongs to a subclass of PLD homologs in which the second HXKXXXXD motif is imperfect and harbors a conserved Asp to Glu substitution. The sam-9 gene has more than eight exons, and the two HXKXXXXD motifs are encoded by two highly conserved exons. The expression of the sam-9 gene is greater in the brain than in non-nervous tissue and appears to be predominantly of neuronal origin. sam-9 expression is pronounced in mature neurons of the forebrain and appears to be turned on at late stages of neurogenesis as revealed by in situ hybridization analysis of sam-9 expression during postnatal development of the hippocampal formation and the primary somatosensory cortex.