Fly photoreceptor synapses: Their development, evolution, and plasticity

Abstract

Recent studies are reviewed on the synapses of photoreceptor terminals in the first optic neuropile of the flies, Musca and Drosophila. Afferent synaptic contacts are of uniform dimensions; they have a postsynaptic tetrad with a membrane organization of P-face particles, resembling other inhibitory synapses. A distributed population of such contact sites forms progressively during synaptogenesis by the selective, sequential accretion of identified postsynaptic elements at the receptor terminal. The comparative anatomy of this synapse indicates that elements have also been added during its phylogeny from an ancestral dyad. All cells are homologs of those in other species of Diptera. The number of synaptic sites is regulated by both pre- and postsynaptic cells, in proportion to their cell surfaces; an independent size increase in the receptor terminals (procured in the Drosophila mutant gigas) produces an increase in their synaptic population. The number of sites declines with age, however, accompanied by an increase in size of those synaptic sites remaining; this occurs for both afferent and feedback photoreceptor synapses. Lastly, the number of sites changes with visual experience; the frequency of feedback synapses is larger following dark rearing during early adult life than following visual experience.

Types, numbers and distribution of synapses on the dendritic tree of an identified visual interneuron in the brain of the locust

“…Chemical synapses in both vertebrate (Pfenninger 1973) and invertebrate nervous systems (Wood et al 1977;Schürmann 1980;Watson 1984;Meinertzhagen 1989) share a number of unifying morphological characters that allow the discrimination of the presynaptic from the postsynaptic element and thus the identification of their structural polarity. Structural specializations of identified neurons are seen in the form, size and number of synaptic vesicles and pre-and subsynaptic densities and in the appearence of the synaptic cleft.…”

Acetylcholinesterase genes within the Diptera: takeover and loss in true flies

“…There have been several innovations in true flies that may have triggered a selected change in ace-2. For example, adults have a particularly complex organization of the photoreceptor synapses, characterized by a cholinergic presynaptic platform acting as a sort of signal amplification device (Meinertzhagen 1989;Edwards & Palka 1991;Yasuyama & Salvaterra 1999;Buschbeck 2000). This is interpreted as an adaptation to fast flying, which requires particularly efficient and fast receptor-neuron and neuron-neuron communication.…”

Morphological plasticity of motor axons in Drosophila mutants with altered excitability

“…In recent years, however, studies of long-term physiological and anatomical changes in Aplysia during behav-ioral conditioning (Kandel et al, 1986;Bailey and Chen, 1988a,b), as well as studies of regeneration (Murphey and Lemere, 1984;Shepherd and Murphey, 1986;Loer et al, 1987;Loer and Kristan, 1989) have changed this view. Instead, it has become apparent that, even though the behavioral repertoire of invertebrates is less complex than that of vertebrates, developmental and physiological plasticity is a common feature (reviewed in Lnenicka and Murphey, 1989; see also Technau, 1984;Meinertzhagen, 1989). The mechanisms underlying neuronal plasticity remain unknown in most cases.…”