The topography and neuroanatomy of fibers connecting the deutocerebrum to the protocerebrum in the brain of the American cockroach Periplaneta americana were investigated by staining single or multiple deutocerebral neurons with cobalt, Lucifer Yellow, or biocytin. Five tracts are distinguished on the basis of their routes from origins in the antennal lobe to the protocerebral neuropil: the inner antenno-cerebral tract (IACT); antenno-cerebral tracts II, III, and IV (ACT II, III, IV), and the outer antenno-cerebral tract (OACT). These tracts are largely composed of the axons of four classes of deutocerebral projection neurons, which have been identified morphologically; the neuronal arborizations in the glomeruli of the antennal lobe and in the protocerebral projection regions have been examined. Projection neurons with processes in the inner antenno-cerebral tract and in the antenno-cerebral tract II each innervate a single glomerulus in the antennal lobe, and both types have terminals in the calyces of the mushroom bodies and in the lateral lobe of the protocerebrum. The axons of pheromone-sensitive projection neurons with dendritic trees in the male-specific macroglomerulus seem to run exclusively in the inner antenno-cerebral tract. Subgroups of these pheromone sensitive neurons differ in relative sensitivity to the two female attractant components as well as in the arborization pattern of their dendrites in the macroglomerulus. The projection neurons of two other classes each innervate many glomeruli in the antennal lobe, those of one class sending their axons into the protocerebrum in the antenno-cerebral tract IV and the other, in the outer antenno-cerebral tract. The neurons of antenno-cerebral tract IV innervate not only the mushroom body calyces and the lateral lobe but also neuropil regions not previously described in the cockroach. Neurons with axons in the outer antenno-cerebral tract have no terminals in the calyces but innervate the lateral lobe and the neuropil surrounding the tract. The morphological findings presented here show that, in addition to the tracts previously documented in the cockroach brain, there are other, presumably olfactory, connections between the deutocerebrum and the protocerebrum.

Mitral cells are the primary output neurons of the vertebrate olfactory bulb and are major recipients of sensory input from the periphery. The morphogenesis of mitral cell dendrites was followed to elucidate their early spatial and temporal interactions with olfactory receptor neurons and glia during the construction of olfactory glomeruli. Monodelphis domestica, a marsupial born at an extremely immature stage, and rats were examined. Mitral cells were retrogradely labeled by application of the lipophilic dye 1,1' dihexadecyl-3,3,3'3'-tetramethylin-docarbocyanine perchlorate (DiI) to the lateral olfactory tract. In double-labeling experiments, olfactory receptor neurons were stained with 3,3' dihexadecyloxacarbocyanine perchlorate (DiO), or olfactory nerve Schwann cells were visualized using S-100 protein immunohistochemistry. Tissue was examined with a confocal laser scanning microscope. Some preparations were subsequently investigated with an electron microscope. In Monodelphis, differentiation of mitral cells starts with an outgrowth of numerous, uniform, and widespread dendrites. As soon as terminals of olfactory receptor axons coalesce into glomerular knots within the presumptive glomerular layer, dendrites of individual mitral cells innervate several adjacent glomeruli where they receive sensory synaptic input. With maturation, supernumerary mitral cell dendrites retract, leaving one primary dendrite bearing a terminal glomerular tuft. Simultaneously, secondary dendrites begin to arise. The formation of glomeruli begins earlier and progresses faster in the rat compared to Monodelphis. Nevertheless, mitral cell differentiation in both species follows a common sequence: overproduction of dendrites, selection of usually one primary apical dendrite, and elimination of supernumerary processes. Since olfactory receptor neurons form synaptic contacts with the widespread mitral cell dendrites, considerable synaptic rearrangement must occur within the olfactory glomeruli during maturation.

In honeybees (Apis mellifera), the biogenic amine octopamine has been shown to play a role in associative and non-associative learning and in the division of labour in the hive. Immunohistochemical studies indicate that the ventral unpaired median (VUM) neurones in the suboesophageal ganglion (SOG) are putatively octopaminergic and therefore might be involved in the octopaminergic modulation of behaviour. In contrast to our knowledge about the behavioural effects of octopamine, only one neurone (VUMmx1) has been related to a behavioural effect (the reward function during olfactory learning). In this study, we have investigated suboesophageal VUM neurones with fluorescent dye-tracing techniques and intracellular recordings combined with intracellular staining. Ten different VUM neurones have been found including six VUM neurones innervating neuropile regions of the brain and the SOG exclusively (central VUM neurones) and four VUM neurones with axons in peripheral nerves (peripheral VUM neurones). The central VUM neurones innervate the antennal lobes, the protocerebral lobes (including the lateral horn) and the mushroom body calyces. Of these, a novel mandibular VUM neurone, VUMmd1, exhibits the same branching pattern in the brain as VUMmx1 and responds to sucrose and odours in a similar way. The peripheral VUM neurones innervate the antennal and the mandibular nerves. In addition, we describe one labial unpaired median neurone with a dorsal cell body, DUMlb1. The possible homology between the honeybee VUM neurones and the unpaired median neurones in other insects is discussed.

Glomerular organization of the antennal (olfactory) lobe is initiated by the arrival of sensory axons from the antenna. Bundles of axon terminals coalesce into spheroidal knots of neuropil called protoglomeruli. Previous studies have suggested that the protoglomeruli form a template for the mature glomerular array, but an early role for projection neurons in establishing the template has not been excluded. We examined with the confocal laser scanning microscope the morphological development of the uniglomerular projection neurons during the stages in which glomeruli are constructed. Groups of projection neurons were stained with the lipophilic dye DiI to assess the development of the population as a whole; individual neurons were filled intracellularly with Lucifer Yellow to examine in detail the development of shape. In some preparations, sensory axons and glial cells also were labeled by using different fluorescent dyes to reveal possible interactions between projection neuron dendrites and sensory axons or glial cells. Protoglomeruli form in a wave beginning at the entry point of the antennal nerve and proceeding across the lobe to the opposite pole. A second wave follows in which projection neurons become tufted and innervate the newly formed glomeruli, sometimes extending into the glial border surrounding the protoglomeruli. In animals deprived of sensory axons, some projection neurons still form tufted dendritic trees and, in one region of the neuropil, a glomerulus-like structure. The early presence of projection neuron processes in the protoglomeruli and the formation of at least one glomerulus-like structure in unafferented lobes suggest that uniglomerular projection neurons play an active role in the construction of olfactory glomeruli.

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