Interneuronal and glial-neuronal gap junctions in the lamina ganglionaris of the compound eye of the housefly, Musca domestica

Drosophila Vision Depends on Carcinine Uptake by an Organic Cation Transporter

Recycling of neurotransmitters is essential for sustained neuronal signaling, yet recycling pathways for various transmitters, including histamine, remain poorly understood. In the first visual ganglion (lamina) of Drosophila, photoreceptor-released histamine is taken up into perisynaptic glia, converted to carcinine, and delivered back to the photoreceptor for histamine regeneration. Here, we identify an organic cation transporter, CarT (carcinine transporter), that transports carcinine into photoreceptors during histamine recycling. CarT mediated in vitro uptake of carcinine. Deletion of the CarT gene caused an accumulation of carcinine in laminar glia accompanied by a reduction in histamine, resulting in abolished photoreceptor signal transmission and blindness in behavioral assays. These defects were rescued by expression of CarT cDNA in photoreceptors, and they were reproduced by photoreceptor-specific CarT knockdown. Our findings suggest a common role for the conserved family of CarT-like transporters in maintaining histamine homeostasis in both mammalian and fly brains.

Long-distance mechanism of neurotransmitter recycling mediated by glial network facilitates visual function in Drosophila

Neurons rely on glia to recycle neurotransmitters such as glutamate and histamine for sustained signaling. Both mammalian and insect glia form intercellular gap-junction networks, but their functional significance underlying neurotransmitter recycling is unknown. Using the Drosophila visual system as a genetic model, here we show that a multicellular glial network transports neurotransmitter metabolites between perisynaptic glia and neuronal cell bodies to mediate long-distance recycling of neurotransmitter. In the first visual neuropil (lamina), which contains a multilayer glial network, photoreceptor axons release histamine to hyperpolarize secondary sensory neurons. Subsequently, the released histamine is taken up by perisynaptic epithelial glia and converted into inactive carcinine through conjugation with β-alanine for transport. In contrast to a previous assumption that epithelial glia deliver carcinine directly back to photoreceptor axons for histamine regeneration within the lamina, we detected both carcinine and β-alanine in the fly retina, where they are found in photoreceptor cell bodies and surrounding pigment glial cells. Downregulating Inx2 gap junctions within the laminar glial network causes β-alanine accumulation in retinal pigment cells and impairs carcinine synthesis, leading to reduced histamine levels and photoreceptor synaptic vesicles. Consequently, visual transmission is impaired and the fly is less responsive in a visual alert analysis compared with wild type. Our results suggest that a gap junction-dependent laminar and retinal glial network transports histamine metabolites between perisynaptic glia and photoreceptor cell bodies to mediate a novel, long-distance mechanism of neurotransmitter recycling, highlighting the importance of glial networks in the regulation of neuronal functions.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

Gap junctions in Drosophila: developmental expression of the entire innexin gene family

Invertebrate gap junctions are composed of proteins called innexins and eight innexin encoding loci have been identified in the now complete genome sequence of Drosophila melanogaster. The intercellular channels formed by these proteins are multimeric and previous studies have shown that, in a heterologous expression system, homo- and hetero-oligomeric channels can form, each combination possessing different gating characteristics. Here we demonstrate that the innexins exhibit complex overlapping expression patterns during oogenesis, embryogenesis, imaginal wing disc development and central nervous system development and show that only certain combinations of innexin oligomerization are possible in vivo. This work forms an essential basis for future studies of innexin interactions in Drosophila and outlines the potential extent of gap-junction involvement in development.

FMRFamide-like and allatostatin-like immunoreactivity in the lateral heart nerve of Periplaneta americana: colocalization at the electron-microscopic level

Both allatostatin immunoreactivity (AS-IR) and FMRFamide immunoreactivity (FMRFa-IR) have been demonstrated light-microscopically in the lateral heart nerve of Periplaneta americana. The identical labeling of some fibers suggests the coexistence of the two antigens. Electron-microscopically, six granule types in the peripheral part of the lateral heart nerve can be distinguished according to their size and density (types 1-6). These granule types can be subdivided immunocytochemically by means of a new mirror-section technique. Granules of types 4 and 5 always exclusively show FMRFa-IR. In the populations of fibers containing granules of types 1 and 6, axon profiles can be found that contain granules colocalizing FMRFa-IR and AS-IR. Other axon profiles of these populations only contain immunonegative granules of the same ultrastructure. Granules of type 2 can be differentiated immunocytochemically in three forms in the same section: In some fibers, they are nonreactive; in other fibers of the same section, they show FMRFa - IR, whereas in a third fiber type, granules show AS - IR. Finally, granules of type 3 can be observed with FMRFa-IR. In other fibers, they occur with the same ultrastructure but exhibit no immunoreactivity. Two soma types occur in the lateral heart nerve. Soma type I is characterized by the production of electron-dense granules that show FMRFa-IR. Type II is in close contact with various fibers, forming different types of axosomatic synapses, hitherto unknown in Insecta.

Analog of vertebrate anionic sites in blood-brain interface of larval Drosophila

The blood-brain barrier ensures brain function in vertebrates and in some invertebrates by maintaining ionic integrity of the extraneuronal bathing fluid. Recent studies have demonstrated that anionic sites on the luminal surface of vascular endothelial cells collaborate with tight junctions to effect this barrier in vertebrates. We characterize these two analogous barrier factors for the first time on Drosophila larva by an electron-dense tracer and cationic gold labeling. Ionic lanthanum entered into but not through the extracellular channels between perineurial cells. Tracer is ultimately excluded from neurons in the ventral ganglion mainly by an extensive series of (pleated sheet) septate junctions between perineurial cells. Continuous junctions, a variant of the septate junction, were not as efficient as the pleated sheet variety in blocking tracer. An anionic domain now is demonstrated in Drosophila central nervous system through the use of cationic colloidal gold in LR White embedment. Anionic domains are specifically stationed in the neural lamella and not noted in the other cell levels of the blood-brain interface. It is proposed that in the central nervous system of the Drosophila larva the array of septate junctions between perineurial cells is the physical barrier, while the anionic domains in neural lamella are a "charge-selective barrier" for cations. All of these results are discussed relative to analogous characteristics of the vertebrate blood-brain barrier.

Photoreceptor maintenance and degeneration in the norpA (no receptor potential-A) mutant of Drosophila melanogaster

The norpA (no receptor potential) mutant of Drosophila melanogaster has a visual transduction deficit. This study determines whether lack of function leads to structural repercussions in photoreceptor cells of the compound eye and their synapses. For this purpose, we examined thin sections and freeze fracture replicas of norpA using transmission electron microscopy. Ultrastructurally, retinula cells in the compound eye and all aspects of the first optic neuropil (lamina ganglionaris) are essentially normal in newly emerged flies. However, as expected, intraretinular pigment granules fail to show their light elicited aggregation; further, the P face particle density is somewhat lower than in wild type. We confirm that there are unusual membrane specializations on the plasmalemma of the retinula cell dubbed "zippers." Zippers appear to increase with age and can cause a distorted geometry of ommatidia. Only a few retinula cells ultimately degenerate in norpA, and the proportion may not differ from that of wild type. Despite the absence of the receptor potential in norpA, many aspects of the turnover of rhabdomeric membrane appear to be as in wild type.