GABAergic synaptic connections in mushroom bodies of insect brains.

Distribution and synaptic connections of GABA fibres in neuropil parts of the mushroom bodies in brains of crickets (Gryllus bimaculatus) and bees (Apis mellifera) were investigated by immuno-light and electron microscopy. In the inner calyx neuropil of cricket mushroom bodies, GABA fibres are pre- and post-synaptically connected with proximal Kenyon cell dendrites, indicating synaptic contacts differing from those of the Kenyon cell dendritic tips in the peripheral microglomeruli of the calyces. A more complex interaction of GABAergic fibres and Kenyon cell dendrites than assumed before is shown. In the mushroom bodies of bees, dendritic like strata of GABA fibre projections contribute to the subcompartmental layers of the vertical lobe. The GABA-immunostained fibre profiles exhibit pre- and postsynaptic sites as well and can therefore not be considered purely postsynaptic dendritic neuron parts. The micromorphology and synaptic contacts of the dendrites and dendritic like arborizations are seen as parts of local circuits within mushroom bodies.

Sprouting interneurons in mushroom bodies of adult cricket brains.

In crickets, neurogenesis persists throughout adulthood for certain local brain interneurons, the Kenyon cells in the mushroom bodies, which represent a prominent compartment for sensory integration, learning, and memory. Several classes of these neurons originate from a perikaryal layer, which includes a cluster of neuroblasts, surrounded by somata that provide the mushroom body's columnar neuropil. We describe the form, distribution, and cytology of Kenyon cell groups in the process of generation and growth in comparison to developed parts of the mushroom bodies in adult crickets of the species Gryllus bimaculatus. A subset of growing Kenyon cells with sprouting processes has been distinguished from adjacent Kenyon cells by its prominent f-actin labelling. Growth cone-like elements are detected in the perikaryal layer and in their associated sprouting fiber bundles. Sprouting fibers distant from the perikarya contain ribosomes and rough endoplasmic reticulum not found in the dendritic processes of the calyx. A core of sprouting Kenyon cell processes is devoid of synapses and is not invaded by extrinsic neuronal elements. Measurements of fiber cross-sections and counts of synapses and organelles suggest a continuous gradient of growth and maturation leading from the core of added new processes out to the periphery of mature Kenyon cell fiber groups. Our results are discussed in the context of Kenyon cell classification, growth dynamics, axonal fiber maturation, and function.

Separate distribution of deutocerebral projection neurons in the mushroom bodies of the cricket brain.

Deutocerebral projection neurones in the brain of the cricket (Gryllus bimaculatus) have been investigated by experimental dextran staining, viewed by light and electron microscopy. These neurones of two separate somata clusters innervate two separate primary glomerular neuropils of the deutocerebral segment, either the antennal lobe receiving only antennal nerve sensory input, or the glomerular lobe, receiving input from sensory neurones of lower segmental origin, including chemosensory fibres from mouth parts. Projection neurones of the antennal lobe only invade the anterior calyx of the mushroom body neuropil via the inner antenno glomerular tract, while glomerular relay neurones of the glomerular lobe innervate only the posterior calyx via the tritocerebral tract. All types of projection neurones give rise to presynaptic boutons. forming the central core of microglomeruli with patterned distribution. These projection neurons are cholinergic. The results are discussed in view of maintained segregated modal information, first processed in the separated primary deutocerebral neuropiles and further on in the second order input neuropils of the mushroom bodies. The large posterior calyces are proposed as a compartment for gustatory information.

F-actin at identified synapses in the mushroom body neuropil of the insect brain.

The distribution of f-actin stained by fluorescent phalloidin was investigated in the brain of several insect species, with a special focus on the mushroom body. For localizing f-actin in identified neurons and at synapses, additional staining with fluorescent dextrans and anti-synapsin I immunostaining was employed. Intense f-actin staining was consistently found in synaptic complexes of the mushroom body calyces (calycal microglomeruli [MG]). These MG contain a central core of presynaptic boutons, predominantly belonging to deutocerebral cholinergic excitatory projection neurons, which are surrounded by a shell of numerous Kenyon cell (KC) dendritic tips. In the cricket Gryllus bimaculatus, high-resolution confocal laser scanning imaging revealed colocalization of f-actin with KC dendritic spine parts within MG. Although presynaptic boutons appear to be mainly devoid of f-actin-phalloidin fluorescence, there appears to be an accumulation of f-actin in KC dendritic spines synaptically contacting the boutons. Electron microscopy of boutons and dextran-stained KC dendrites revealed their pre- and postsynaptic sites, with KCs being strictly postsynaptic elements. Their subsynaptic membrane appositions are considered to be associated with f-actin. Focal accumulation of f-actin in the dendritic tips of KCs was found to be a general feature of MG, with either spheroidal or indented boutons of different sizes, as encountered in the mushroom bodies of the cricket, honey bee, ant, and fruit fly. The structural similarities of calycal MG and f-actin accumulation in KC dendrites with cerebellar microglomeruli are considered comparatively. The accumulation of f-actin in KC dendrites is discussed in view of mushroom body plasticity and its potential role in learning and memory formation.