Topography of four classes of Kenyon cells in the mushroom bodies of the cockroach

Parallel olfactory processing in a hemimetabolous insect

To represent specific olfactory cues from the highly complex and dynamic odor world in the brain, insects employ multiple parallel olfactory pathways that process odors with different coding strategies. Here, we summarize the anatomical and physiological features of parallel olfactory pathways in the hemimetabolous insect, the cockroach Periplaneta americana. The cockroach processes different aspects of odor stimuli, such as odor qualities, temporal information, and dynamics, through parallel olfactory pathways. These parallel pathways are anatomically segregated from the peripheral to higher brain centers, forming functional maps within the brain. In addition, the cockroach may possess parallel pathways that correspond to distinct types of olfactory receptors expressed in sensory neurons. Through comparisons with olfactory pathways in holometabolous insects, we aim to provide valuable insights into the organization, functionality, and evolution of insect olfaction.

Roles of octopamine neurons in the vertical lobe of the mushroom body for the execution of a conditioned response in cockroaches

Aminergic neurons mediate reward signals in mammals and insects. In crickets, we showed that blockade of synaptic transmission from octopamine neurons (OANs) impairs conditioning of an odor (conditioned stimulus, CS) with water or sucrose (unconditioned stimulus, US) and execution of a conditioned response (CR) to the CS. It has not yet been established, however, whether findings in crickets can be applied to other species of insects. In this study, we investigated the roles of OANs in conditioning of salivation, monitored by activities of salivary neurons, and in execution of the CR in cockroaches (Periplaneta americana). We showed that injection of epinastine (an OA receptor antagonist) into the head hemolymph impaired both conditioning and execution of the CR, in accordance with findings in crickets. Moreover, local injection of epinastine into the vertical lobes of the mushroom body (MB), the center for associative learning and control of the CR, impaired execution of the CR, whereas injection of epinastine into the calyces of the MB or the antennal lobes (primary olfactory centers) did not. We propose that OANs in the MB vertical lobes play critical roles in the execution of the CR in cockroaches. This is analogous to the fact that midbrain dopamine neurons govern execution of learned actions in mammals.

On the Role of the Head Ganglia in Posture and Walking in Insects

In insects, locomotion is the result of rhythm generating thoracic circuits and their modulation by sensory reflexes and by inputs from the two head ganglia, the cerebral and the gnathal ganglia (GNG), which act as higher order neuronal centers playing different functions in the initiation, goal-direction, and maintenance of movement. Current knowledge on the various roles of major neuropiles of the cerebral ganglia (CRG), such as mushroom bodies (MB) and the central complex (CX), in particular, are discussed as well as the role of the GNG. Thoracic and head ganglia circuitries are connected by ascending and descending neurons. While less is known about the ascending neurons, recent studies in large insects and Drosophila have begun to unravel the identity of descending neurons and their appropriate roles in posture and locomotion. Descending inputs from the head ganglia are most important in initiating and modulating thoracic central pattern generating circuitries to achieve goal directed locomotion. In addition, the review will also deal with some known monoaminergic descending neurons which affect the motor circuits involved in posture and locomotion. In conclusion, we will present a few issues that have, until today, been little explored. For example, how and which descending neurons are selected to engage a specific motor behavior and how feedback from thoracic circuitry modulate the head ganglia circuitries. The review will discuss results from large insects, mainly locusts, crickets, and stick insects but will mostly focus on cockroaches and the fruit fly, Drosophila.

Separate But Interactive Parallel Olfactory Processing Streams Governed by Different Types of GABAergic Feedback Neurons in the Mushroom Body of a Basal Insect

The basic organization of the olfactory system has been the subject of extensive studies in vertebrates and invertebrates. In many animals, GABA-ergic neurons inhibit spike activities of higher-order olfactory neurons and help sparsening of their odor representations. In the cockroach, two different types of GABA-immunoreactive interneurons (calyceal giants [CGs]) mainly project to the base and lip regions of the calyces (input areas) of the mushroom body (MB), a second-order olfactory center. The base and lip regions receive axon terminals of two different types of projection neurons, which receive synapses from different classes of olfactory sensory neurons (OSNs), and receive dendrites of different classes of Kenyon cells, MB intrinsic neurons. We performed intracellular recordings from pairs of CGs and MB output neurons (MBONs) of male American cockroaches, the latter receiving synapses from Kenyon cells, and we found that a CG receives excitatory synapses from an MBON and that odor responses of the MBON are changed by current injection into the CG. Such feedback effects, however, were often weak or absent in pairs of neurons that belong to different streams, suggesting parallel organization of the recurrent pathways, although interactions between different streams were also evident. Cross-covariance analysis of the spike activities of CGs and MBONs suggested that odor stimulation produces synchronized spike activities in MBONs and then in CGs. We suggest that there are separate but interactive parallel streams to process odors detected by different OSNs throughout the olfactory processing system in cockroaches.SIGNIFICANCE STATEMENT Organizational principles of the olfactory system have been the subject of extensive studies. In cockroaches, signals from olfactory sensory neurons (OSNs) in two different classes of sensilla are sent to two different classes of projection neurons, which terminate in different areas of the mushroom body (MB), each area having dendrites of different classes of MB intrinsic neurons (Kenyon cells) and terminations of different classes of GABAergic neurons. Physiological and morphological assessments derived from simultaneous intracellular recordings/stainings from GABAergic neurons and MB output neurons suggested that GABAergic neurons play feedback roles and that odors detected by OSNs are processed in separate but interactive processing streams throughout the central olfactory system.

Two Parallel Olfactory Pathways for Processing General Odors in a Cockroach

In animals, sensory processing via parallel pathways, including the olfactory system, is a common design. However, the mechanisms that parallel pathways use to encode highly complex and dynamic odor signals remain unclear. In the current study, we examined the anatomical and physiological features of parallel olfactory pathways in an evolutionally basal insect, the cockroach Periplaneta americana. In this insect, the entire system for processing general odors, from olfactory sensory neurons to higher brain centers, is anatomically segregated into two parallel pathways. Two separate populations of secondary olfactory neurons, type1 and type2 projection neurons (PNs), with dendrites in distinct glomerular groups relay olfactory signals to segregated areas of higher brain centers. We conducted intracellular recordings, revealing olfactory properties and temporal patterns of both types of PNs. Generally, type1 PNs exhibit higher odor-specificities to nine tested odorants than type2 PNs. Cluster analyses revealed that odor-evoked responses were temporally complex and varied in type1 PNs, while type2 PNs exhibited phasic on-responses with either early or late latencies to an effective odor. The late responses are 30–40 ms later than the early responses. Simultaneous intracellular recordings from two different PNs revealed that a given odor activated both types of PNs with different temporal patterns, and latencies of early and late responses in type2 PNs might be precisely controlled. Our results suggest that the cockroach is equipped with two anatomically and physiologically segregated parallel olfactory pathways, which might employ different neural strategies to encode odor information.

Complete identification of four giant interneurons supplying mushroom body calyces in the cockroach Periplaneta americana

Global inhibition is a fundamental physiological mechanism that has been proposed to shape odor representation in higher-order olfactory centers. A pair of mushroom bodies (MBs) in insect brains, an analog of the mammalian olfactory cortex, are implicated in multisensory integration and associative memory formation. With the use of single/multiple intracellular recording and staining in the cockroach Periplaneta americana, we succeeded in unambiguous identification of four tightly bundled GABA-immunoreactive giant interneurons that are presumably involved in global inhibitory control of the MB. These neurons, including three spiking neurons and one nonspiking neuron, possess dendrites in termination fields of MB output neurons and send axon terminals back to MB input sites, calyces, suggesting feedback roles onto the MB. The largest spiking neuron innervates almost exclusively the basal region of calyces, while the two smaller spiking neurons and the second-largest nonspiking neuron innervate more profusely the peripheral (lip) region of the calyces than the basal region. This subdivision corresponds well to the calycal zonation made by axon terminals of two populations of uniglomerular projection neurons with dendrites in distinct glomerular groups in the antennal lobe. The four giant neurons exhibited excitatory responses to every odor tested in a neuron-specific fashion, and two of the neurons also exhibited inhibitory responses in some recording sessions. Our results suggest that two parallel olfactory inputs to the MB undergo different forms of inhibitory control by the giant neurons, which may, in turn, be involved in different aspects of odor discrimination, plasticity, and state-dependent gain control. J. Comp. Neurol. 525:204-230, 2017. © 2016 Wiley Periodicals, Inc.

Dopamine- and Tyrosine Hydroxylase-Immunoreactive Neurons in the Brain of the American Cockroach, Periplaneta americana

The catecholamine dopamine plays several vital roles in the central nervous system of many species, but its neural mechanisms remain elusive. Detailed neuroanatomical characterization of dopamine neurons is a prerequisite for elucidating dopamine’s actions in the brain. In the present study, we investigated the distribution of dopaminergic neurons in the brain of the American cockroach, Periplaneta americana, using two antisera: 1) an antiserum against dopamine, and 2) an antiserum against tyrosine hydroxylase (TH, an enzyme required for dopamine synthesis), and identified about 250 putatively dopaminergic neurons. The patterns of dopamine- and TH-immunoreactive neurons were strikingly similar, suggesting that both antisera recognize the same sets of “dopaminergic” neurons. The dopamine and TH antibodies intensively or moderately immunolabeled prominent brain neuropils, e.g. the mushroom body (memory center), antennal lobe (first-order olfactory center) and central complex (motor coordination center). All subdivisions of the mushroom body exhibit both dopamine and TH immunoreactivity. Comparison of immunolabeled neurons with those filled by dye injection revealed that a group of immunolabeled neurons with cell bodies near the calyx projects into a distal region of the vertical lobe, which is a plausible site for olfactory memory formation in insects. In the antennal lobe, ordinary glomeruli as well as macroglomeruli exhibit both dopamine and TH immunoreactivity. It is noteworthy that the dopamine antiserum labeled tiny granular structures inside the glomeruli whereas the TH antiserum labeled processes in the marginal regions of the glomeruli, suggesting a different origin. In the central complex, all subdivisions excluding part of the noduli and protocerebral bridge exhibit both dopamine and TH immunoreactivity. These anatomical findings will accelerate our understanding of dopaminergic systems, specifically in neural circuits underlying aversive memory formation and arousal, in insects.

Participation of NO signaling in formation of long-term memory in salivary conditioning of the cockroach

The molecular and neural basis of protein synthesis-dependent long-term memory (LTM) has been the subject of extensive studies in vertebrates and invertebrates. In crickets and honey bees, it has been demonstrated that nitric oxide (NO) signaling plays critical roles in LTM formation, but no experimental system appropriate for electrophysiological study of neural mechanisms by which production of NO leads to LTM formation has been established in insects. We have reported that cockroaches, as do dogs and humans, exhibit conditioning of salivation, i.e., they exhibit an increased level of salivation in response to an odor paired with sucrose reward. Salivary conditioning can be monitored by activity changes of salivary neurons in rigidly immobilized animals and thus is useful for the study of brain mechanisms of learning and memory. In the present study, we found that injection of cycloheximide, a protein synthesis inhibitor, into the hemolymph before multiple conditioning trials impairs formation of 1-day memory, but not that of 30-min memory. This indicates that formation of 1-day memory requires protein synthesis but that of earlier memory does not. We also found that injection of l-NAME, an inhibitor of NO synthase, before multiple conditioning impairs formation of 1-day memory but not that of 30-min memory. We thus conclude that NO signaling participates in the formation of protein synthesis-dependent LTM but not that of earlier memory in salivary conditioning. Salivary conditioning in cockroaches should become a pertinent system for the study of neural mechanisms by which activation of NO synthase leads to LTM formation.

Visual and olfactory input segregation in the mushroom body calyces in a basal neopteran, the American cockroach

The cockroach Periplaneta americana is an evolutionary basal neopteran insect, equipped with one of the largest and most elaborate mushroom bodies among insects. Using intracellular recording and staining in the protocerebrum, we discovered two new types of neurons that receive direct input from the optic lobe in addition to the neuron previously reported. These neurons have dendritic processes in the optic lobe, projection sites in the optic tracts, and send axonal terminals almost exclusively to the innermost layer of the MB calyces (input site of MB). Their responses were excitatory to visual but inhibitory to olfactory stimuli, and weak excitation occurred in response to mechanosensory stimuli to cerci. In contrast, interneurons with dendrites mainly in the antennal lobe projection sites send axon terminals to the middle to outer layers of the calyces. These were excited by various olfactory stimuli and mechanosensory stimuli to the antenna. These results suggest that there is general modality-specific terminal segregation in the MB calyces and that this is an early event in insect evolution. Possible postsynaptic and presynaptic elements of these neurons are discussed.

Stratification and synaptogenesis in the mushroom body of the honeybee, Apis mellifera

Stratification is a basic anatomical feature of central brain in both vertebrates and many invertebrates. The aim of this study was to investigate the relationship between stratification and synaptogenesis in the developing mushroom bodies of the honeybee. During metamorphosis, the vertical lobe of mushroom body shows progressive stratification with three thick primary strata and more secondary strata and laminae. Three primary strata are formed at the metamorphic stage P1, before the youngest generation of the mushroom body intrinsic neurons, Kenyon cells, is produced. Thus, the primary strata within the lobe are unlikely to represent three major subpopulations of the Kenyon cells sequentially produced in the mushroom bodies. Formation of laminae starts at the stage P2 and culminates at the end of metamorphosis. The laminae appear within the lobe rather than being added sequentially from the ingrowth stratum. Alternating dark and light lamina (lamina doublets) are formed in the vertical lobe in late metamorphosis (stages P6-P9), but they are not visible in adults. The pattern of stratification is not continuous along the vertical lobe at the same developmental stage, and resorting of axons of the Kenyon cells is likely to occur within dark laminae. In the developing vertical lobe, dark laminae show lower synaptic density and exhibit an ultra structure that is indicative for a delay in synaptogenesis relative to the primary strata. A local transient block of synaptogenesis within the dark laminae may provide correct targeting of Kenyon cells by extrinsic mushroom body neurons.

Intrinsic Membrane Properties and Inhibitory Synaptic Input of Kenyon Cells as Mechanisms for Sparse Coding?

The insect mushroom bodies (MBs) are multimodal signal processing centers and are essential for olfactory learning. Electrophysiological recordings from the MBs' principal component neurons, the Kenyon cells (KCs), showed a sparse representation of olfactory signals. It has been proposed that the intrinsic and synaptic properties of the KC circuitry combine to reduce the firing of action potentials and to generate relatively brief windows for synaptic integration in the KCs, thus causing them to operate as coincidence detectors. To better understand the ionic mechanisms that mediate the KC intrinsic firing properties, we used whole cell patch-clamp recordings from KCs in the adult, intact brain of Periplaneta americana to analyze voltage- and/or Ca2+-dependent inward ( ICa, INa) and outward currents [ IA, IK(V), IK,ST, IO(Ca)]. In general the currents had properties similar to those of currents in other insect neurons. Certain functional parameters of ICaand IO(Ca), however, had unusually high values, allowing them to assist sparse coding. ICahad a low-activation threshold and a very high current density compared with those of ICain other insect neurons. Together these parameters make ICasuitable for boosting and sharpening the excitatory postsynaptic potentials as reported in previous studies. IO(Ca)also had a large current density and a very depolarized activation threshold. In combination, the large ICaand IO(Ca)are likely to mediate the strong spike frequency adaptation. These intrinsic properties of the KCs are likely to be supported by their tonic, inhibitory synaptic input, which was revealed by specific GABA antagonists and which contributes significantly to the hyperpolarized membrane potential at rest.

Modular subdivision of mushroom bodies by Kenyon cells in the silkmoth

In insects, olfactory information in the glomeruli of the antennal lobe, the first olfactory center, is transmitted to the lateral protocerebrum and the calyx of the mushroom body via projection neurons. In male silkmoths (Bombyx mori), arborization patterns in the calyx differ markedly between projection neurons that respond to sex pheromones and those that respond to general odors. However, little is known about the organization of the mushroom body's intrinsic neurons, called Kenyon cells (KCs), which receive the inputs from the projection neurons. We investigated the silkmoth mushroom body and identified four parallel subdivisions in the lobes and pedunculus by immunolabeling with antibodies against the catalytic subunit of protein kinase A in Drosophila melanogaster (DC0) and the neuromodulatory peptide FMRFamide. To further understand the detailed organization of the mushroom body, we performed extensive labeling of individual KCs. We identified four morphological types whose axonal projections corresponded to the subdivisions in the lobes, and found that each type of KC had a characteristic dendritic morphology in the calyx. These results show a correlation between the axonal projections of KCs in the lobes and dendritic morphology in the calyx, and indicate different functional roles for the subdivisions.

Critical role of nitric oxide-cGMP cascade in the formation of cAMP-dependent long-term memory

Cyclic AMP pathway plays an essential role in formation of long-term memory (LTM). In some species, the nitric oxide (NO)-cyclic GMP pathway has been found to act in parallel and complementary to the cAMP pathway for LTM formation. Here we describe a new role of the NO-cGMP pathway, namely, stimulation of the cAMP pathway to induce LTM. We have studied the signaling cascade underlying LTM formation by systematically coinjecting various "LTM-inducing" and "LTM-blocking" drugs in crickets. Multiple-trial olfactory conditioning led to LTM that lasted for several days, while memory induced by single-trial conditioning decayed away within several hours. Injection of inhibitors of the enzyme forming NO, cGMP, or cAMP into the hemolymph prior to multiple-trial conditioning blocked LTM, whereas injection of an NO donor, cGMP analog, or cAMP analog prior to single-trial conditioning induced LTM. Induction of LTM by injection of an NO donor or cGMP analog paired with single-trial conditioning was blocked by inhibitors of the cAMP pathway, but induction of LTM by a cAMP analog was unaffected by inhibitors of the NO-cGMP pathway. Inhibitors of cyclic nucleotide-gated channel (CNG channel) or calmodulin-blocked induction of LTM by cGMP analog paired with single-trial conditioning, but they did not affect induction of LTM by cAMP analog. Our findings suggest that the cAMP pathway is a downstream target of the NO-cGMP pathway for the formation of LTM, and that the CNG channel and calcium-calmodulin intervene between the NO-cGMP pathway and the cAMP pathway.

Synaptogenesis in the mushroom body calyx during metamorphosis in the honeybeeApis mellifera: An electron microscopic study

The goals of this study are to determine relationships between synaptogenesis and morphogenesis within the mushroom body calyx of the honeybee Apis mellifera and to find out how the microglomerular structure characteristic for the mature calyx is established during metamorphosis. We show that synaptogenesis in the mushroom body calycal neuropile starts in early metamorphosis (stages P1-P3), before the microglomerular structure of the neuropile is established. The initial step of synaptogenesis is characterized by the rare occurrence of distinct synaptic contacts. A massive synaptogenesis starts at stage P5, which coincides with the formation of microglomeruli, structural units of the calyx that are composed of centrally located presynaptic boutons surrounded by spiny postsynaptic endings. Microglomeruli are assembled either via accumulation of fine postsynaptic processes around preexisting presynaptic boutons or via ingrowth of thin neurites of presynaptic neurons into premicroglomeruli, tightly packed groups of spiny endings. During late pupal stages (P8-P9), addition of new synapses and microglomeruli is likely to continue. Most of the synaptic appositions formed there are made by boutons (putative extrinsic mushroom body neurons) into small postsynaptic profiles that do not exhibit presynaptic specializations (putative intrinsic mushroom body neurons). Synapses between presynaptic boutons characteristic of the adult calyx first appear at stage P8 but remain rare toward the end of metamorphosis. Our observations are consistent with the hypothesis that most of the synapses established during metamorphosis provide the structural basis for afferent information flow to calyces, whereas maturation of local synaptic circuitry is likely to occur after adult emergence.

Classical Olfactory Conditioning in the Cockroach Periplaneta americana

We established a classical conditioning procedure for the cockroach, Periplaneta americana, by which odors were associated with reward or punishment. Cockroaches underwent differential conditioning trials in which peppermint odor was associated with sucrose solution and vanilla odor was associated with saline solution. Odor preference of cockroaches was tested by allowing them to choose between peppermint and vanilla sources. Cockroaches that had undergone one set of differential conditioning trials exhibited a significantly greater preference for peppermint odor than did untrained cockroaches. Memory formed by three sets of differential conditioning trials, with an inter-trial interval of 5 min, was retained at least 4 days after conditioning. This conditioning procedure was effective even for cockroaches that had been harnessed in plastic tubes. This study shows, for the first time in hemimetaborous insects, that both freely moving and harnessed insects are capable of forming olfactory memory by classical conditioning procedure. This procedure may be useful for future electrophysiological and pharmacological studies aimed at elucidation of neural mechanisms underlying olfactory learning and memory.

Development and evolution of the insect mushroom bodies: towards the understanding of conserved developmental mechanisms in a higher brain center

The insect mushroom bodies are prominent higher order neuropils consisting of thousands of approximately parallel projecting intrinsic neurons arising from the minute basophilic perikarya of globuli cells. Early studies described these structures as centers for intelligence and other higher functions; at present, the mushroom bodies are regarded as important models for the neural basis of learning and memory. The insect mushroom bodies share a similar general morphology, and the same basic sequence of developmental events is observed across a wide range of insect taxa. Globuli cell progenitors arise in the embryo and proliferate throughout the greater part of juvenile development. Discrete morphological and functional subpopulations of globuli cells (or Kenyon cells, as they are called in insects) are sequentially produced at distinct periods of development. Kenyon cell somata are arranged by age around the center of proliferation, as are their processes in the mushroom body neuropil. Other aspects of mushroom body development are more variable from species to species, such as the origin of specific Kenyon cell populations and neuropil substructures, as well as the timing and pace of the general developmental sequence.

Distribution of dendrites of descending neurons and its implications for the basic organization of the cockroach brain

To determine precisely the brain areas from which descending neurons (DNs) originate, we examined the distribution of somata and dendrites of DNs in the cockroach brain by retrogradely filling their axons from the cervical connective. At least 235 pairs of somata of DNs were stained, and most of these were grouped into 22 clusters. Their dendrites were distributed in most brain areas, including lateral and medial protocerebra, which are major termination areas of output neurons of the mushroom body, but not in the optic and antennal lobes, the mushroom body, the central complex, or the posteroventral part of the lateral horn. The last area is the termination area of major types of olfactory projection neurons from the antennal lobe, i.e., uni- and macroglomerular projection neurons, so these neurons have no direct connections with DNs. The distribution of axon terminals of ascending neurons overlaps with that of DN dendrites. We propose, based on these findings, that there are numerous parallel processing streams from cephalic sensory areas to thoracic locomotory centers, many of which are via premotor brain areas from which DNs originate. In addition, outputs from the mushroom body, central complex, and posteroventral part of the lateral horn converge on some of the premotor areas, presumably to modulate the activity of some sensorimotor pathways. We propose, based on our results and documented findings, that many parallel processing streams function in various forms of reflexive and relatively stereotyped behaviors, whereas indirect pathways govern some forms of experience-dependent modification of behavior.

A unique mushroom body substructure common to basal cockroaches and to termites

The mushroom bodies of the cockroach Periplaneta americana are made up of intrinsic neurons (class I and class II Kenyon cells) with dendrites in a dorsal calyx and axons that bifurcate into medial and vertical lobes. Here, we describe a substructure of the cockroach mushroom bodies composed of a previously unrecognized class of Kenyon cells with distinct morphologies. The embryonically produced class III Kenyon cells form a separate accessory calyx below the calyx proper. The medial branches of class III Kenyon cell axons form the previously described "gamma bulb," whereas the vertical branches leave the vertical lobe to form a toroidal "lobelet" around the posterior surface. Taking advantage of the morphologically and immunochemically distinct nature of the lobelet, we have attempted to determine the distribution of this unique structure in other insects of the taxon Dictyoptera (cockroaches, mantises, and termites). Our data indicate that the lobelet is present only in basal cockroaches and in termites, supporting existing theories of a close phylogenetic relationship between these groups. Higher termites possess a duplicated lobe structure due to immense elaboration of the processes of class III Kenyon cells. The degree of complexity in the mushroom body lobes of termites agrees with current taxonomic arrangements of the Isoptera based on non-neural morphological and DNA sequence analyses. It thus appears that the evolution of the Dictyoptera has been accompanied by increasing complexity of the mushroom bodies, achieved in part through the further specialization and elaboration of a subset of Kenyon cells.

Development of cricket mushroom bodies

Mushroom bodies are recognized as a multimodal integrator for sensorial stimuli. The present study analyzes cricket mushroom body development from embryogenesis to adulthood. In the house cricket, Kenyon cells were born from a group of neuroblasts located at the apex of mushroom bodies. Our results demonstrate the sequential generation of Kenyon cells: The more external they are, the earlier they were produced. BrdU treatment on day 8 (57% stage) of embryonic life results, at the adult stage, in the labelling of the large Kenyon cells at the periphery of the mushroom body cortex. These cells have specific projections into the posterior calyx, the gamma lobe, and an enlargement at the inner part of the vertical lobe; they represent a part of mushroom bodies of strictly embryonic origin. The small Kenyon cells were formed from day 9 (65% stage) of the embryonic stage onward, and new interneurons are produced throughout the entire life of the insect. They send their projections into the anterior calyx and into the vertical and medial lobes. Mushroom body development of Acheta should be considered as a primitive template, and cross-taxonomic comparisons of the mushroom body development underscore the precocious origin of the gamma lobe. As a result of continuous neurogenesis, cricket mushroom bodies undergo remodeling throughout life, laying the foundation for future studies of the functional role of this developmental plasticity.

Nitric oxide synthase histochemistry in insect nervous systems: Methanol/formalin fixation reveals the neuroarchitecture of formaldehyde-sensitive NADPH diaphorase in the cockroach Periplaneta americana

Formaldehyde-insensitive NADPH diaphorase (NADPHd) activity is used widely as a histochemical marker for neuronal nitric oxide synthase (NOS). However, in several insects including the cockroach Periplaneta americana, NOS is apparently formaldehyde-sensitive; NADPHd fails to reveal neuron morphology and results in faint generalized staining. Here we have used a novel fixative, methanol/ formalin (MF), to reveal for the first time the neuroarchitecture of NADPHd in the cockroach, with intense selective staining occurring in neurons throughout the brain and thoracic ganglia. Immunocytochemical and histochemical analysis of cockroach and locust nervous systems indicated that neuronal NADPHd after MF fixation can be attributed to NOS. However, NADPHd in locust glial and perineurial cells was histochemically different from that in neurons and may thus be due to enzymes other than NOS. Histochemical implications of species-specific enzyme properties and of the transcriptional complexity of the NOS gene are discussed. The present findings suggest that MF fixation is a valuable new tool for the comparative analysis of the neuroarchitecture of NO signaling in insects. The Golgi-like definition of the staining enabled analysis of the NADPHd architecture in the cockroach and comparison with that in the locust. NADPHd in the tactile neuropils of the thoracic ganglia showed a similar organization in the two species. The olfactory glomeruli of the antennal lobes were in both species densely innervated by NADPHd-positive local interneurons that correlated in number with the number of glomeruli. Thus, the NADPHd architectures appear highly conserved in primary sensory neuropils. In the cockroach mushroom bodies, particularly dense staining in the gamma-layer of the lobes was apparently derived from Kenyon cells, whereas extrinsic arborizations were organized in domains across the lobes, an architecture that contrasts with the previously described tubular compartmentalization of locust mushroom bodies. These divergent architectures may result in different spatiotemporal dynamics of NO diffusion and suggest species differences in the role of NO in the mushroom bodies.

Three-dimensional mapping of brain neuropils in the cockroach, Diploptera punctata

Herein, we present a complete three-dimensional (3D) map of major neuropil structures in the central brain of the cockroach Diploptera punctata. The positions of the structures have been ascertained by confocal microscopy, which, until now-for reasons of tissue opacity and nonhomogeneity-has been thought impractical in imaging fluorescently labeled structures thicker than 150 microm. In this report, however, we have used digestive enzymes and microwave-aided fixation to stain, clear, and optically section, in its entirety, an intact central brain more than 500 microm thick. The central brain from an adult female cockroach was stained thoroughly with the membrane probe NBD-ceramide and the DNA probe propidium iodide. The central brain as well as such neuropil regions as mushroom bodies, central complex, antennal glomeruli, and lobus glomerulati were individually outlined, segmented, and reconstructed in three dimensions to a spatial resolution of approximately 1 microm in the X-Y plane and 3 microm in the Z plane. The volume and surface area of each neuropil compartment were determined, and Kenyon cells of the mushroom bodies were counted. We determined that each brain hemisphere contains about 230,000 Kenyon cells, 99 antennal lobe glomeruli, and 40 lobus glomerulatus glomeruli. Segmented compartments were assigned as separate channels and merged into a single data base to reconstruct a 3D central brain containing eight different channels. This is the first 3D map at submicron resolution of an entire animal's brain that measures more than 500 microm in thickness.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

Development of laminar organization in the mushroom bodies of the cockroach: Kenyon cell proliferation, outgrowth, and maturation

The mushroom bodies of the insect brain are lobed integration centers made up of tens of thousands of parallel-projecting axons of intrinsic (Kenyon) cells. Most of the axons in the medial and vertical lobes of adult cockroach mushroom bodies derive from class I Kenyon cells and are organized into regular, alternating pairs (doublets) of pale and dark laminae. Organization of Kenyon cell axons into the adult pattern of laminae occurs gradually over the course of nymphal development. Newly hatched nymphs possess tiny mushroom bodies with lobes containing a posterior lamina of ingrowing axons, followed by a single doublet, which is flanked anteriorly by a gamma layer composed of class II Kenyon cells. Golgi impregnations show that throughout nymphal development, regardless of the number of doublets present, the most posterior lamina serves as the "ingrowth lamina" for axons of newborn Kenyon cells. Axons of the ingrowth lamina are taurine- and synaptotagmin-immunonegative. They produce fine growth cone tipped filaments and long perpendicularly oriented collaterals along their length. The maturation of these Kenyon cells and the formation of a new lamina are marked by the loss of filaments and collaterals, as well as the onset of taurine and synaptotagmin expression. Class I Kenyon cells thus show plasticity in both morphology and transmitter expression during development. In a hemimetabolous insect such as the cockroach, juvenile stages are morphologically and behaviorally similar to the adult. The mushroom bodies of these insects must be functional from hatching onward, while thousands of new neurons are added to the existing structure. The observed developmental plasticity may serve as a mechanism by which extensive postembryonic development of the mushroom bodies can occur without disrupting function. This contrasts with the more evolutionarily derived holometabolous insects, such as the honey bee and the fruit fly, in which nervous system development is accomplished in a behaviorally simple larval stage and a quiescent pupal stage.

Fate of neuroblast progeny during postembryonic development of mushroom bodies in the house cricket, Acheta domesticus

Mushroom bodies represent the main sensory integrative center of the insect brain and probably play a major role in the adaptation of behavioral responses to the environment. Taking into account the continuous neurogenesis of cricket mushroom bodies, we investigated ontogenesis of this brain structure. Using BrdU labeling, we examined the fate of neuroblast progeny during the postembryonic development. Preimaginal Kenyon cells survived throughout larval and imaginal moults and persisted during adulthood. Our results indicate that the location of labelled Kenyon cells in the cortex of the adult cricket mainly depends upon the period when they were produced during development. The present data demonstrate that cricket mushroom bodies grow from the inside out and that, at any developmental stage, the center of the cortex contains the youngest Kenyon cells. This study also allowed us to observe the occurrence of quiescent neuroblasts. Kenyon cell death during postembryonic and adult life seems to be reduced. Although preimaginal Kenyon cells largely contribute to adult mushroom body structure, a permanent remodeling of the mushroom body occurs throughout the whole insect life due to the persistence of neurogenesis in the house cricket. Further studies are needed to understand the functional significance of these findings.

Representation of the calyces in the medial and vertical lobes of cockroach mushroom bodies

Previous studies of honey bee and cockroach mushroom bodies have proposed that afferent terminals and intrinsic neurons (Kenyon cells) in the calyces are arranged according to polar coordinates. It has been suggested that there is a transformation by Kenyon cell axons of the polar arrangements of their dendrites in the calyces to laminar arrangements of their terminals in the lobes. Findings presented here show that cellular organization in the calyx of an evolutionarily basal neopteran, Periplaneta americana, is instead rectilinear, as it is in the lobes. It is shown that each calyx is divided into two halves (hemicalyces), each supplied by its own set of Kenyon cells. Each calyx is separately represented in the medial lobe where the dendritic trees of some efferent neurons receive inputs from one calyx only. Kenyon cell dendrites are arranged as narrow elongated fields, organized as rows in each hemicalyx. Dendritic fields arise from 14 to 16 sheets of Kenyon cell axons stacked on top of each other lining the inner surface of the calyx cup. A sheet consists of approximately 60 small bundles, each containing 5-15 axons that converge from the rim of the calyx to its neck. Each sheet contributes to a pair oflaminae, one dark one pale, called a doublet, that extends through the mushroom body. Dark laminae contain Kenyon cell axons packed with synaptic vesicles. Axons in pale laminae are sparsely equipped with vesicles. By analogy with photoreceptors, and with reference to field potential recordings, it is speculated that dark laminae are continuously active, being modulated by odor stimuli, whereas pale laminae are intermittently activated. Timm's silver staining and immunocytology reveal a second type of longitudinal division of the lobes. Five layers extend through the pedunculus and lobes, each composed of subsets of doublets. Four layers represent zones of afferent endings in the calyces. A fifth (the y layer) represents a specific type of Kenyon cell. It is concluded that the mushroom bodies comprise two independent modular systems, doublets and layers. Developmental studies show that new doublets are added at each instar to layers that are already present early in second instar nymphs. There are profound similarities between the mushroom bodies of Periplaneta, an evolutionarily basal taxon, and those of Drosophila melanogaster and the honey bee.

Topography of modular subunits in the mushroom bodies of the cockroach

The mushroom body (MB), a conspicuous neuropil structure in the insect brain, is implicated in associative memory and in some aspects of motor control. Intrinsic neurons of the MB (Kenyon cells) extend dendrites into the calyx, and their axons run through the pedunculus and then bifurcate to form the alpha and the beta lobes. At the pedunculus and the lobes, Kenyon cells make synaptic connections with dendrites of extrinsic (output) neurons. Previously, we reported that the alpha lobe of the cockroach MB consists of repetitive modular subunits (Mizunami et al. [1997] Neurosci. Lett. 229:153-156). Each subunit is composed of a dark layer and a light layer, and the layers are refereed to as slabs. Each slab is composed of axons of a specific subset of Kenyon cells. In the present study, we examined serial sections of reduced silver preparations and found that each dark and light slab continues throughout the length of the pedunculus and the alpha and beta lobes. We also found that Golgi-impregnated Kenyon cells often exhibit a characteristic grouping, forming a thin sheet interlaced by dozens or hundreds of axons. The sheet is much thinner than the slab, and each sheet remains within a particular slab throughout the length of the pedunculus and the lobes. Thus, the sheet is a component forming the slab. In the pedunculus and the beta lobe, a class of Golgi-impregnated extrinsic neurons exhibit segmented dendritelike arbors that interact with every other slab, i.e., either with only dark or light slabs. Because each neuron of this class interacts with each particular set of dark or light slabs, we conclude that the slabs are units for transmitting output signals from the MB.