Showing papers by "Nicholas J. Strausfeld published in 2010"

Dynamics of glutamatergic signaling in the mushroom body of young adult Drosophila

Abstract: The mushroom bodies (MBs) are paired brain centers located in the insect protocerebrum involved in olfactory learning and memory and other associative functions. Processes from the Kenyon cells (KCs), their intrinsic neurons, form the bulk of the MB's calyx, pedunculus and lobes. In young adult Drosophila, the last-born KCs extend their processes in the α/β lobes as a thin core (α/β cores) that is embedded in the surrounding matrix of other mature KC processes. A high level of L-glutamate (Glu) immunoreactivity is present in the α/β cores (α/βc) of recently eclosed adult flies. In a Drosophila model of fragile X syndrome, the main cause of inherited mental retardation, treatment with metabotropic Glu receptor (mGluR) antagonists can rescue memory deficits and MB structural defects. To address the role of Glu signaling in the development and maturation of the MB, we have compared the time course of Glu immunoreactivity with the expression of various glutamatergic markers at various times, that is, 1 hour, 1 day and 10 days after adult eclosion. We observed that last-born α/βc KCs in young adult as well as developing KCs in late larva and at various pupal stages transiently express high level of Glu immunoreactivity in Drosophila. One day after eclosion, the Glu level was already markedly reduced in the α/βc neurons. Glial cell processes expressing glutamine synthetase and the Glu transporter dEAAT1 were found to surround the Glu-expressing KCs in very young adults, subsequently enwrapping the α/β lobes to become distributed equally over the entire MB neuropil. The vesicular Glu transporter DVGluT was detected by immunostaining in processes that project within the MB lobes and pedunculus, but this transporter is apparently never expressed by the KCs themselves. The NMDA receptor subunit dNR1 is widely expressed in the MB neuropil just after eclosion, but was not detected in the α/βc neurons. In contrast, we provide evidence that DmGluRA, the only Drosophila mGluR, is specifically expressed in Glu-accumulating cells of the MB α/βc immediately and for a short time after eclosion. The distribution and dynamics of glutamatergic markers indicate that newborn KCs transiently accumulate Glu at a high level in late pupal and young eclosed Drosophila, and may locally release this amino acid by a mechanism that would not involve DVGluT. At this stage, Glu can bind to intrinsic mGluRs abundant in the α/βc KCs, and to NMDA receptors in the rest of the MB neuropil, before being captured and metabolized in surrounding glial cells. This suggests that Glu acts as an autocrine or paracrine agent that contributes to the structural and functional maturation of the MB during the first hours of Drosophila adult life.

Brain homology: Dohrn of a new era?

Abstract: swims ‘upside down’, suggested – at least to him – not a vertebrate ancestor but, like the ascidian tadpole, an evolutionary simplified or ‘degenerate’ fish. Anton Dohrn was convinced that annelids were the ancestors of chordates. He suggested that structures could evolve through time leading to a complete change in their structural and functional identity. In his argument for annelid ancestry, Dohrn proposed that a common ancestor to the vertebrates might have had a ringlike brain around the mouth, as does an annelid, whose dorsal brain is connected to the ventral nerve cord and ganglia by connectives that extend around the stomodeum (the front part of the gut). To achieve transformation from an annelid to a vertebrate nervous system, there must evolve in the latter a brain that was not perforated by the stomodeum. To obtain this condition, the annelid mouth would have closed, and a new mouth opened above the brain. Dohrn suggested that this new mouth would have arisen by functional modification of gill slits that would have been present in the ancestral annelid. As a result of this transformation, the inversion would have been that of the vertebrate, to account for its ventral mouth. Dohrn’s monograph, which has been translated by Michael Ghiselin, with a commentary, enunciates Dohrn’s crucial ideas about ‘Funktionswechsel’ or ‘the principle succession of functions’, meaning the evoluIn 1875, the great embryologist Karl von Baer, then in his 82nd year, received a monograph dedicated to him proposing an annelid ancestry of the vertebrates [Ghiselin, 1994]. The monograph was written by Anton Dohrn, best known as the creator and tireless promoter of the Naples Zoological Station, now known as the ‘Stazione Zoologica Anton Dohrn’. A few years earlier, in 1867 and 1871, one of the greatest Russian biologists of the time, Alexandr Kowalevsky, who had worked in the Zoological Station, concluded that Amphioxus is a chordate, as are tunicates, and that these were the most logical candidates for vertebrate ancestry [Kowalevsky, 1867]. Kowalevsky’s colleague, Philip Owsjannikow, working in St. Petersburg, had determined in 1868 that Amphioxus possessed a dorsal nerve cord and what he interpreted as a small rostral brain [Owsjannikow, 1867]. Kowalevsky and Ilja Metschnikoff, his friend and colleague, had also published their discoveries of commonality of embryonic cell organization (‘germ layers’) shared across the Metazoa, a finding that underpinned Charles Darwin’s hypothesis of a common metazoan ancestor [Kowalevsky, 1871; Levit, 2007]. Whether those discoveries encouraged Anton Dohrn to consider an alternative ancestry of the vertebrates is not known. What is known, however, is that Dohrn’s own studies of the lancelet Amphioxus lanceolatus , which typically Published online: December 22, 2010