Improvements for the anatomical characterization of insect neurons in whole mount: the use of cyanine-derived fluorophores and laser scanning confocal microscopy

Functional Recovery of a Locomotor Network after Injury: Plasticity beyond the Central Nervous System

Many animals depend on descending information from the brain for the initiation and proper execution of locomotion. Interestingly, after injury and the loss of such inputs, locomotor function can sometimes be regained without the regrowth of central connections. In the medicinal leech, Hirudo verbana, we have shown that crawling reemerges after removal of descending inputs. Here, we studied the mechanisms underlying this return of locomotion by asking if central pattern generators (CPGs) in crawl-recovered leeches are sufficient to produce crawl-specific intersegmental coordination. From recovered animals, we treated isolated chains of ganglia with dopamine to activate the crawl CPGs (one crawl CPG per ganglion) and observed fictive crawl-like bursting in the dorsal-longitudinal-excitor motoneuron (DE-3), an established crawl-monitor neuron. However, these preparations did not exhibit crawl-specific coordination across the CPGs. Although the crawl CPGs always generated bidirectional activation of adjacent CPGs, we never observed crawl-appropriate intersegmental phase delays. Because central circuits alone were unable to organize crawl-specific coordination, we tested the coordinating role of the peripheral nervous system. In transected leeches normally destined for recovery, we removed afferent information to the anterior-most (lead) ganglion located below the nerve-cord transection site. In these dually treated animals, overt crawling was greatly delayed or prevented. After filling the peripheral nerves with Neurobiotin tracer distal to the nerve-root lesion, we found a perfect correlation between regrowth of peripheral neuronal fibers and crawl recovery. Our study establishes that during recovery after injury, crawl-specific intersegmental coordination switches to a new dependence on afferent information.

Plasmon resonance and the imaging of metal-impregnated neurons with the laser scanning confocal microscope

The staining of neurons with silver began in the 1800s, but until now the great resolving power of the laser scanning confocal microscope has not been utilized to capture the in-focus and three-dimensional cytoarchitecture of metal-impregnated cells. Here, we demonstrate how spectral confocal microscopy, typically reserved for fluorescent imaging, can be used to visualize metal-labeled tissues. This imaging does not involve the reflectance of metal particles, but rather the excitation of silver (or gold) nanoparticles and their putative surface plasmon resonance. To induce such resonance, silver or gold particles were excited with visible-wavelength laser lines (561 or 640 nm), and the maximal emission signal was collected at a shorter wavelength (i.e., higher energy state). Because the surface plasmon resonances of noble metal nanoparticles offer a superior optical signal and do not photobleach, our novel protocol holds enormous promise of a rebirth and further development of silver- and gold-based cell labeling protocols.

Tyramine as a possible neurotransmitter/neuromodulator at the spermatheca of the African migratory locust, Locusta migratoria

Tyramine-like immunoreactivity was identified in neurons of the VIIIth abdominal ganglion and in axons projecting to the spermatheca of adult females of Locusta migratoria. Tyramine-like immunoreactive processes were also found throughout all regions of the spermatheca and tyramine-like immunoreactive bipolar or multipolar neurons were present on the spermathecal sac. HPLC coupled with electrochemical detection revealed more tyramine than octopamine present in spermathecal tissue. Electrical stimulation of the ventral ovipositor nerve resulted in a significant increase in calcium-dependent release of tyramine from the spermatheca. Both tyramine and octopamine increase the frequency and basal tonus of spermathecal contractions in a dose-dependent manner, with octopamine having a lower threshold. When tyramine is applied along with a half maximal octopamine dose, there is an additive effect on contractions of the spermatheca with slight synergistic effects at lower doses of tyramine. High concentrations of tyramine (10(-4)M) stimulated increases in cyclic AMP levels of the spermatheca; an effect blocked by phentolamine. Phentolamine has a higher affinity (and thus a lower IC(50) value congruent with 5.6 x 10(-8)M) than yohimbine (IC(50) congruent with 1.1 x 10(-4)M) in reducing tyramine-induced spermathecal contractions. Taken together, these results suggest that tyramine may be a co-transmitter with octopamine at the spermatheca, with both neuroactive chemicals acting on an octopamine receptor.

Hormone-dependent expression of fasciclin II during ganglionic migration and fusion in the ventral nerve cord of the moth Manduca sexta

The ventral nerve cord of holometabolous insects is reorganized during metamorphosis. A prominent feature of this reorganization is the migration of subsets of thoracic and abdominal larval ganglia to form fused compound ganglia. Studies in the hawkmoth Manduca sexta revealed that pulses of the steroid hormone 20-hydroxyecdysone (20E) regulate ganglionic fusion, but little is known about the cellular mechanisms that make migration and fusion possible. To test the hypothesis that modulation of cell adhesion molecules is an essential component of ventral nerve cord reorganization, we used antibodies selective for either the transmembrane isoform of the cell adhesion receptor fasciclin II (TM-MFas II) or the glycosyl phosphatidylinositol-linked isoform (GPI-MFas II) to study cell adhesion during ganglionic migration and fusion. Our observations show that expression of TM-MFas II is regulated temporally and spatially. GPI-MFas II was expressed on the surface of the segmental ganglia and the transverse nerve, but no evidence was obtained for regulation of GPI-MFas II expression during metamorphosis of the ventral nerve cord. Manipulation of 20E titers revealed that TM-MFas II expression on neurons in migrating ganglia is regulated by hormonal events previously shown to choreograph ganglionic migration and fusion. Injections of actinomycin D (an RNA synthesis inhibitor) or cycloheximide (a protein synthesis inhibitor) blocked ganglionic movement and the concomitant increase in TM-MFas II, suggesting that 20E regulates transcription of TM-MFas II. The few neurons that showed TM-MFas II immunoreactivity independent of endocrine milieu were immunoreactive to an antiserum specific for eclosion hormone (EH), a neuropeptide regulator of molting.

Beyond the central pattern generator: amine modulation of decision-making neural pathways descending from the brain of the medicinal leech

The biological mechanisms of behavioral selection, as it relates to locomotion, are far from understood, even in relatively simple invertebrate animals. In the medicinal leech, Hirudo medicinalis, the decision to swim is distributed across populations of swim-activating and swim-inactivating neurons descending from the subesophageal ganglion of the compound cephalic ganglion, i.e. the brain. In the present study, we demonstrate that the serotonergic LL and Retzius cells in the brain are excited by swim-initiating stimuli and during spontaneous swim episodes. This activity likely influences or resets the neuromodulatory state of neural circuits involved in the activation or subsequent termination of locomotion. When serotonin (5-HT) was perfused over the brain, multi-unit recordings from descending brain neurons revealed rapid and substantial alterations. Subsequent intracellular recordings from identified command-like brain interneurons demonstrated that 5-HT, especially in combination with octopamine, inhibited swim-triggering neuron Tr1, as well as swim-inactivating neurons Tr2 and SIN1. Although 5-HT inhibited elements of the swim-inactivation pathway, rather than promoting them, the indirect and net effect of the amine was a reliable and sustained reduction in the firing of the segmental swim-gating neuron 204. This modulation caused cell 204 to relinquish its excitatory drive to the swim central pattern generator. The activation pattern of serotonergic brain neurons that we observed during swimming and the 5-HT-immunoreactive staining pattern obtained, suggest that within the head brain 5-HT secretion is massive. Over time, 5-HT secretion may provide a homeostatic feedback mechanism to limit swimming activity at the level of the head brain.

Hygienic behavior in the honey bee (Apis mellifera L.) and the modulatory role of octopamine

Honey bees, Apis mellifera, which perform hygienic behavior, quickly detect, uncap and remove diseased brood from the nest. This behavior, performed by bees 15-20 days old and prior to foraging, is likely mediated by olfactory cues. Because the neuromodulator octopamine (OA) plays a pivotal role in olfactory-based behaviors of honey bees, we examined whether bees bred for hygienic and nonhygienic behavior differed with regard to their OA expression and physiology. We compared the staining intensity of octopamine-immunoreactive (OA-ir) neurons in the deutocerebral region of the brain, medial to the antennal lobes, between hygienic and nonhygienic bees (based on genotype and phenotype). We also tested how the olfactory responses of the two lines, based on electroantennograms (EAGs), were affected by oral administration of OA and of epinastine, a highly specific OA antagonist. Our results revealed that bees expressing hygienic behavior (irrespective of genotype) possessed OA-ir neurons that exhibited more intense labeling than same-aged bees not performing the behavior. In bees bred for nonhygienic behavior, OA significantly increased the EAG response to low concentrations of diseased brood odor. Conversely, in bees bred for hygienic behavior, epinastine significantly reduced the magnitude of the EAG response, a reduction not observed in nonhygienic bees. Our results provide two lines of evidence that OA has the potential to facilitate the detection and response of honey bees to diseased brood. We discuss the contributions of OA for behavioral shaping and its ability to bias the nervous system to express one form of behavior over another.

Evidence for crustacean cardioactive peptide-like innervation of the gut in Locusta migratoria

Hindguts from female Vth instar larvae, young adults (1-2 days) and old adults (>10 days) are equally sensitive to the crustacean cardioactive peptide (CCAP), with changes in contraction occurring at a threshold concentration of 10(-9)M and maximal responses observed at concentrations ranging between 10(-7) and 5x10(-6)M. An immunohistochemical examination of the gut of Locusta migratoria with an antiserum raised against CCAP revealed an extensive network of CCAP-like immunoreactive processes on the hindgut and posterior midgut via the 11th sternal nerve arising from the terminal abdominal ganglion. Anterograde filling of the 11th sternal nerve with neurobiotin revealed extensive processes and terminals on the hindgut. Retrograde filling of the branch of the 11th sternal nerve which innervates the hindgut with neurobiotin revealed two bilaterally paired cells in the terminal abdominal ganglion which co-localized with CCAP-like immunoreactivity. Results suggest that a CCAP-like substance acts as a neurotransmitter/neuromodulator at the locust hindgut.

Cellular localization of bursicon using antisera against partial peptide sequences of this insect cuticle-sclerotizing neurohormone

Bursicon is the final neurohormone released at the end of the molting cycle. It triggers the sclerotization (tanning) of the insect cuticle. Until now, its existence has been verified only by bioassays. In an attempt to identify this important neurohormone, bursicon was purified from homogenates of 2,850 nerve cords of the cockroach Periplaneta americana by using high performance liquid chromatography technology and two-dimensional gel electrophoresis. Bursicon bioactivity was found in four distinct protein spots at approximately 30 kDa between pH 5.3 and 5.9. The protein of one of these spots at pH 5.7 was subsequently microsequenced, and five partial amino acid sequences were retrieved. Evidence is presented that two of these sequences are derived from bursicon. Antibodies raised against the two sequences labeled bursicon-containing neurons in the central nervous systems of P. americana. One of these antisera labeled bursicon-containing neurons in the crickets Teleogryllus commodus and Gryllus bimaculatus, and the moth Manduca sexta. A cluster of four bilaterally paired neurons in the brain of Drososphila melanogaster was also labeled. In addition, this antiserum detected three spots corresponding to bursicon in Western blots of two-dimensional gels. The 12-amino acid sequence detected by this antiserum, thus, seems to be conserved even among species that are distantly related.

Molecular characterization and cell-specific expression of a Manduca sexta FLRFamide gene

FMRFamide-related peptides (FaRPs) are a large group of neuropeptides containing a common RFamide C-terminus; they have been identified in vertebrates and invertebrates. We have isolated the cDNA that encodes three FaRPs in the tobacco hornworm, Manduca sexta, including the amidated decapeptide F10. The larger FaRPs are the partially processed precursors of F10, a neuropeptide belonging to the myosuppressin family of peptides. The presence of all three FaRPs in different tissues suggests differential utilization of typical dibasic processing sites and atypical processing sites C-terminal to leucine residues. F10 mRNA was detected in the brain, nerve cord, and midgut, and the mRNA levels in the nervous system are dynamically regulated during development. In situ hybridization analysis localized the F10 mRNA to a variety of cell types within the central nervous system (CNS), a peripheral neurosecretory cell (L1), and midgut endocrine cells, which suggests diverse functions. Distribution of the F10-containing neurons within the central nervous system is segment-specific, and the developmental profile suggests that the F10 gene products may have stage-specific functions. Molecular characterization of the F10 gene has provided insights into its regulation and cell-specific distribution that will enhance our understanding of how these FaRPs modulate different physiological systems and ultimately behavior.

Integration of endocrine signals that regulate insect ecdysis

The extremely large number of insects and members of allied groups alive today suggests that molting--shedding of an old cuticle--may be one of the most commonly performed behaviors on our planet. Removal of an old cuticle in insects is associated with stereotyped, species-specific patterns of behavior referred to as ecdysis. It has been recognized for decades that the initiation of ecdysis is under hormonal control, but until recently many of the key peptides that regulate ecdysis were unknown. The report in 1996 of a new ecdysis-triggering hormone (ETH) sparked an era of significant advances in our understanding of the regulation of molting. This article summarizes the current model of peptide regulation of ecdysis, a model that is based on a positive feedback loop between ETH and a brain peptide, eclosion hormone. Then the relationship of these regulatory peptides to the neural circuitry that is the ultimate driver of the behavior are described. Because insects can undergo both status quo (larval-larval) and metamorphic (larval-pupal and pupal-adult) molts, differences in ecdysis behavior at different life stages are described and potential sources of these differences are identified. Most of the work described is based on studies of ecdysis in the hawkmoth, Manduca sexta, but results from studies of ecdysis in the fruit fly Drosophila melanogaster are also discussed.

Distribution and development of dopamine- and octopamine-synthesizing neurons in the medicinal leech

Although the medicinal leech is a well-studied system in which many neurons and circuits have been identified with precision, descriptions of the distributions of some of the major biogenic amines, such as dopamine (DA) and octopamine (OA), have yet to be completed. In the European medicinal leech Hirudo medicinalis and the American medicinal leech Macrobdella decora,we have presented the first immunohistochemical study of DA neurons in the entire central nervous system, and of OA-immunoreactive (ir) neurons in the head and tail brains. Dopaminergic neurons were identified using the glyoxylic acid method and antisera to DA and its rate-limiting synthetic enzyme tyrosine hydroxylase (TH). Octopaminergic neurons were recognized using a highly specific antiserum raised against OA. An antibody raised against DA-beta-hydroxylase (DbetaH), the mammalian enzyme that converts DA to norepinephrine (NE), was found to immunostain OA-ir neurons. This antibody appears to cross-react with the closely related invertebrate enzyme tyramine-beta-hydroxylase, which converts tyramine to OA, suggesting that the OA-ir cells are indeed octopaminergic, capable of synthesizing OA. Because the DbetaH antiserum selectively immunostained the OA-ir neurons, but not the DA-synthesizing cells, our results also indicate that the DA-ir neurons synthesize DA and not NE as their end product. The expression of TH immunoreactivity was found to emerge relatively early in development, on embryonic day 9 (47-48% of development). In contrast, OA expression remained absent as late as embryonic day 20. Higher order processes of some of the dopaminergic and octopaminergic neurons in the adult brain were observed to project to a region previously described as a neurohemal complex. Several TH-ir processes were also seen in the stomatogastric nerve ring, suggesting that DA may play a role in the regulation of biting behavior. By mapping the distributions and developmental expression pattern of DA and OA neurons in the leech, we aim to gain a better understanding of the functional roles of aminergic neurons and how they influence behavior.

Dopamine-synthesizing neurons include the putative H-cell homologue in the moth Manduca sexta

The catecholamine dopamine (DA) plays a fundamental role in the regulation of behavior and neurodevelopment across animal species. Uncovering the embryonic origins of neurons that express DA opens a path for a deeper understanding of how DA expression is regulated and, in turn, how DA regulates the activities of the nervous system. In a well-established insect model, Manduca sexta, we identified the putative homologue of the embryonic grasshopper "H-cell" using intracellular techniques, laser scanning confocal microscopy, and immunohistochemistry. In both species, this neuron possesses four axons and has central projections resembling the letter H. The H-cell in grasshoppers is known to be derived from the midline precursor 3 cell (MP3) and to pioneer the pathways of the longitudinal connectives; in Drosophila, the H-cell is also known to be derived from MP3. In the current study, we demonstrate that the Manduca H-cell is immunoreactive to antibodies raised against DA and its rate-limiting synthetic enzyme, tyrosine hydroxylase (TH). In larvae and adults, one DA/TH-immunoreactive (-ir) H-cell per ganglion is present. In embryos, individual ganglia contain a single midline TH-ir cell body positioned along side its putative sibling. Such observations are consistent with the known secondary transformation (in grasshoppers) of only one of the two MP3 progeny during early development. Although a hallmark feature of invertebrate neurons is the fairly stereotypical position of neuronal somata, we found that the H-cell somata can "flip-flop" by 180 degrees between an anterior and posterior position. This variability appears to be random and is not restricted to any particular ganglion. Curiously, what is segment-specific is the absence of the DA/TH-ir H-cell in the metathoracic (T3) ganglion as well as the unique structure of the H-cell in the subesophageal ganglion. Because this is the first immunohistochemical study of DA neurons in Manduca, we have provided the distribution pattern and morphologies of dopaminergic neurons, in addition to the H-cells, within the ventral nerve cord during development.

Steroid regulation of octopamine expression during metamorphic development of the moth Manduca sexta

Octopamine (OA), a biogenic amine similar to norepinephrine, has profound and well-documented actions on the nervous systems of invertebrates. In the insect, Manduca sexta, we examined the developmental plasticity of OA synthesis, studied its endocrine regulation, and observed previously undescribed OA-immunoreactive (ir) neurons. We found that levels of tyramine beta-hydroxylase (TbetaH), an essential enzyme for the biosynthesis of OA, increase during metamorphosis. Based on the established and influential roles of the steroid hormone 20-hydroxyecdysone (20-HE) during development, we tested the hypothesis that increases in TbetaH levels and OA immunoreactivity are regulated by the rise in 20-HE occurring during pupal-adult development. We determined that the levels of TbetaH in the terminal abdominal ganglion (neuromeres 6-9) remain at a constant level during pupal development and the early stages of adult development. Beginning at ca. pupal stage 8, however, the levels of TbetaH begin to rise, reaching a maximum level by pupal stage 12. By removing the source of ecdysteroid hormone through ligation, and by subsequent replacement of 20-HE via infusion, we found evidence indicating that the preadult rise of 20-HE is both necessary and sufficient for the increased levels of TbetaH. During the course of our study, we also identified previously unreported OA-ir neurons. In particular, adult-specific OA-ir lateral cells were found, as were relatively small OA-ir dorsal median pairs that doubled in size during adult development. Abdominal ganglia not exposed to the preadult rise in 20-HE possessed neither the OA-ir lateral neurons nor the somatic growth of the smaller OA-ir median neurons. These newly described OA-ir neurons probably contribute to the steroid-induced elevations of TbetaH observed at the end of metamorphosis.

Correction methods for three-dimensional reconstructions from confocal images: I. Tissue shrinking and axial scaling

We show here, using locust wholemount ganglia as an example, that scaling artifacts in three-dimensional reconstructions from confocal microscopic images due to refractive index mismatch in the light path and tissue shrinking, can account for dramatic errors in measurements of morphometric values. Refractive index mismatch leads to considerable alteration of the axial dimension, and true dimensions must be restored by rescaling the Z-axis of the image stack. The appropriate scaling factor depends on the refractive indices of the media in the light path and the numerical aperture of the objective used and can be determined by numerical simulations, as we show here. In addition, different histochemical procedures were tested in regard to their effect on tissue dimensions. Reconstructions of scans at different stages of these protocols show that shrinking can be avoided prior to clearing when dehydrating ethanol series are carefully applied. Fixation and mismatching buffer osmolarity have no effect. We demonstrate procedures to reduce artifacts during mounting and clearing in methyl salicylate, such that only isometric shrinkage occurs, which can easily be corrected by rescaling the image dimensions. Glycerol-based clearing agents produced severe anisometric and nonlinear shrinkage and we could not find a way to overcome this.

The neural control of spermathecal contractions in the locust, Locusta migratoria

The innervation of the spermatheca and demonstration of neural control of spermathecal contractions in Locusta migratoria was illustrated using anterograde and retrograde fills, combined with electrophysiological stimulation and recording. The anterior portion of the spermatheca receives innervation via the receptaculum seminis nerve (N2B2) from two large ventral neurons and one dorsal neuron. All were bilaterally paired and situated in the VIIIth abdominal ganglion. Three ventral bilaterally paired neurons situated in the VIIIth abdominal ganglion also provide innervation to the posterior portion of the spermatheca via the ductus seminalis aperture nerve (N2B3). Six DUM neurons, located in the VIIIth abdominal ganglion, in addition to two centroposteriorly situated DUM neurons in the VIIth abdominal ganglion, are also associated with these two nerves. N2B4 also provides innervation to the posterior portion of the spermatheca. N2B6b is associated with sensory cells identified in the anterior lateral regions of the genital chamber. The spermatheca contracts spontaneously, with peristaltic contractions beginning at the spermathecal sac and continuing along the length of the spermathecal duct. However electrical stimulation of the ventral ovipositor nerve (VON or N2B), receptaculum seminis nerve (N2B2) and the ductus seminalis aperture nerve (N2B3) indicates that contractions are also under neural control. In particular contractions of the spermathecal sac, coil duct and anterior straight duct are initiated via motor projections from the receptaculum seminis nerve (N2B2) and posterior straight duct contractions are controlled by motor input from the ductus seminalis aperture nerve (N2B3). The results suggest that spermathecal contractions of the anterior and posterior portions of the spermatheca are under separate neural control.

Programmed cell death of an identified motoneuron examined in vivo: electrophysiological and morphological correlates

A paucity of information exists about the electrophysiological and anatomical correlates of neurons committed to die in vivo. Thus, we examined how an identified neuron, motoneuron MN-12, dies during development in the intact moth Manduca sexta. The developmental programmed cell death of this motoneuron was examined because of its defined commitment point of death. In addition, its ability to be unambiguously identified between animals and its accessibility to recording and dye injection facilitated our examination. MN-12 becomes committed to die approximately 28-30 h after adult emergence. At this time, MN-12 can no longer be saved by manipulations of steroid hormone levels, protein synthesis, or removal of descending inputs. Our initial prediction was that within a few hours after the commitment point, MN-12 would begin showing a gradual loss of central arbors and alterations in membrane properties. Contrary to our expectations, we found the MN-12 motoneuron to exhibit a stable central morphology and electrophysiological profile for approximately 12 h, followed by a rapid dismantling that occurred within a 1- to 2-h period. Several hours prior to the commitment point, the target muscle of MN-12 was no longer viable; yet, this did not affect the death of MN-12 or cause retraction of its motor terminals. We conclude that the delayed (12-h) onset of rapid cell death is not preceded by a slow accumulation of damages to the neuronal membrane (e.g., ion channels or cytoskeletal components) as both the electrical activity and morphology of MN-12 remained measurably unaltered during this 12-h lag.

Seeing what is coming: building collision-sensitive neurones

The image of a rapidly approaching object has to elicit a quick response. An animal needs to know that the object is approaching on a collision course and how imminent a collision is. The relevant information can be computed from the way that the image of the object grows on the retina of one eye. Firm data about the types of neurones that react to such looming stimuli and trigger avoidance reactions come from recent studies on the pigeon and the locust. The neurones responsible are tightly tuned to detect objects that are approaching on a direct collision course. In the pigeon these neurones signal the time remaining before collision whereas in the locust they have a crucial role in the simple strategy this animal uses to detect an object approaching on a collision course.

Expression of the Manduca sexta allatotropin gene in cells of the central and enteric nervous systems

Manduca sexta allatotropin (Mas-AT) was isolated and first characterized as a peptide that stimulated juvenile hormone biosynthesis in adult lepidopteran corpora allata and was subsequently shown to have cardioacceleratory activity in the pharate adult. In this study, we identified the cells in the nervous system of the insect that contain mRNA encoding Mas-AT and immunoreactivity against a polyclonal antiserum to Mas-AT. In larvae, Mas-AT mRNA and immunoreactivity was most abundant in two cells in the frontal ganglion, which project their axons down the recurrent nerve toward the gut, and in cells in the terminal abdominal ganglion. Lower levels of Mas-AT mRNA were detected in the brain and subesophageal ganglion. In the pupal and pharate adult stages, we detected Mas-AT mRNA and immunoreactivity in cells of the abdominal ganglia and in additional cells in the terminal abdominal ganglion. These additional cells in the ventral nerve cord that express Mas-AT during the pupal and pharate adult stages include cells that differentiate during metamorphosis as well as cells that exist in larvae but do not begin to express Mas-AT until these later developmental stages. Some of the cells that exhibit Mas-AT immunoreactivity lack Mas-AT mRNA, suggesting that the antisera used in this and previous studies recognizes other peptides in addition to Mas-AT. This pattern of expression suggests that Mas-AT may mediate multiple physiological functions during the life cycle of the insect, including the larval stage in which no function has yet been described for the peptide.

Tracing of a neuronal network in the locust by pressure injection of markers into a synaptic neuropil

Central neuronal circuits of vertebrates have often been investigated using injection of markers into synaptic neuropils, whereas similar techniques have rarely been applied in invertebrates. In this study we tested several neuroanatomical tracers for their ability to mark central neuronal circuits in insects, using the well described auditory network of the locust, Locusta migratoria. After physiological localization of an auditory neuropil various tracers were pressure injected. Horseradish peroxidase, dextrans (3 and 10 kDa) and especially biocytin and neurobiotin were effectively incorporated by auditory interneurons, which resulted in their extensive labeling. Postsynaptic regions turned out to be the major, if not exclusive sites of uptake of injected markers, which is deduced from two lines of evidence: (i) for labeling of identified auditory neurons it was necessary to apply the tracer to postsynaptic sites of the neuron; (ii) only a few non-auditory neurons were labeled (probably by lesioning axons during electrode impalement). No evidence could be found for an activity dependent uptake. We conclude that pressure injection of certain tracers into synaptic areas can be used to identify central nervous circuits in insects.

A rapid method for combined laser scanning confocal microscopic and electron microscopic visualization of biocytin or neurobiotin-labeled neurons

Intracellular recording and dye filling are widely used to correlate the morphology of a neuron with its physiology. With laser scanning confocal microscopy, the complex shapes of labeled neurons in three dimensions can be reconstructed rapidly, but this requires fluorescent dyes. These dyes are neither permanent nor electron dense and therefore do not allow investigation by electron microscopy. Here we report a technique that quickly and easily converts a fluorescent label into a more stable and electron-dense stain. With this technique, a neuron is filled with Neurobiotin or biocytin, reacted with fluorophore-conjugated avidin, and imaged by confocal microscopy. To permit long-term storage or EM study, the fluorescent label is then converted to a stable electron-dense material by a single-step conversion using a commercially available ABC kit. We find that the method, which apparently relies on recognition of avidin's excess biotin binding sites by the biotin-peroxidase conjugate, is both faster and less labor intensive than photo-oxidation procedures in common use. The technique is readily adaptable to immunocytochemistry with biotinylated probes, as we demonstrate using anti-serotonin as an example.

Two-laser dual-immunofluorescence confocal laser scanning microscopy using Cy2- and Cy5-conjugated secondary antibodies: unequivocal detection of co-localization of neuronal markers

The ability of the confocal laser scanning microscope (CLSM) to visualize in one focal plane the fluorescence associated with multiple markers renders this instrument extremely valuable for the study of co-localization of various markers in the somata and cellular processes of neurons. In the present protocol we deal with the question whether or not co-localization exists in neurons of two different neuronal markers. The conventionally used method towards answering this type of question is double-immunofluorescence microscopy. Fundamental to this approach, independent from whether the preparations are observed in a normal fluorescence microscope or in a CLSM, is that each of the applied fluorescent labels should not chemically interact with the other label or inadvertently be visible through the illumination/filter setup designed for the other fluorophore. In the field of double-label CLSM, three types of approach are distinguished: the single-laser, two-color approach, the two-laser, two-color approach, and the time-resolved approach (Brismar and Ulfhake, 1997). Each type of approach has its own advantages and disadvantages. In the instrument in our institute (a Zeiss LSM 410), combinations of fluorophores like fluorescein isothiocyanate (FITC) and tetramethyl rhodamine isothiocyanate (TRITC) are less useful, since TRITC produces a detectable signal in the FITC illumination/filter setup. Instead of experimenting with filter sets we have chosen to take two measures to eliminate this problem. Our first measure is to use fluorophores whose absorption/emission spectra overlap as little as possible. We have selected among the recently developed carbocyanine fluorophores one fluorescing in the visible range (Cy2) (green, in the same range as FITC and with much better resistance to fading than FITC; cf. Härtig et al., 1996), and another fluorescing in the near infrared range (Cy5, infrared; cf. Mesce et al., 1993). Our second measure to ensure excellent signal separation is the adoption of a two-laser, two-color approach. Co-localization of the calcium binding protein, calretinin, and a neurotransmitter, gamma-aminobutyric acid (GABA), in interneurons in the entorhinal cortex and the hippocampus of the rat was used as the principal test model. We compare the above two-laser, two-color approach with a single-laser, two-color CLSM approach using as markers Cy2 and the red fluorophore, Texas Red (physical characteristics resembling TRITC). In this paper considerable attention is paid to control experiments to verify the reliability of the staining procedure. The results show that our two-laser, two-color CLSM approach produces a complete and unambiguous separation of the fluorescent labels, Cy2 and Cy5. We are currently using this method to determine the degree of co-localization of neurochemical substances in CNS neurons.

Coactivation of putative octopamine- and serotonin-containing interneurons in the medicinal leech

Possible interactions between octopamine-immunoreactive (IR) and serotonergic neurons in the CNS of the medicinal leech were investigated. Simultaneous intracellular recordings of serotonin-containing neurons (either the Retzius neuron or cell 21) and the dorsolateral octopamine-IR (DLO) neuron demonstrated that both sets of neurons are coactive at times. Depolarization of either serotonergic cell 21 or the Retzius neuron did not alter the membrane potential of the DLO. Similarly, depolarization of the DLO did not affect the serotonergic neurons examined. Because it was found that the DLO and either the serotonergic cell 21 or Retzius neuron were at times coactive, we looked for possible sources of common excitatory inputs. The centrally located pressure (P)- and touch (T)-sensitive mechanosensory neurons excited the DLOs through a polysynaptic pathway. Stimulation of nociceptive (N) mechanosensory neurons did not cause a measurable depolarization in the membrane potential of the DLO. Through simultaneous recordings of the DLO, cell 21, and a particular identified mechanosensory neuron, it was demonstrated that activity in the T or P cells can excite both serotonergic cell 21 and the octopamine-IR DLO. These findings indicate that, in many instances, both serotonin and octopamine, biogenic amines with neuromodulatory actions in many different invertebrates, may be released simultaneously in the leech.

Allatostatin-like-immunoreactive neurons of the tobacco hornworm, Manduca sexta, and isolation and identification of a new neuropeptide related to cockroach allatostatins

The YXFGLamide C-terminus serves to define most members of a family of structurally related neuropeptides, the YXFGLamides. These peptides have been identified from the nervous system of various insects and include the allatostatins of cockroaches and crickets, the schistostatins of locusts, and the callatostatins of blowflies. The YXFGLamides have been shown to have various functions, including inhibition of juvenile hormone biosynthesis in cockroaches and crickets and inhibition of contraction of certain insect visceral muscles. We wanted to know if these peptides occur in Manduca sexta and what functions they might have. A new peptide, AKSYNFGLamide, was isolated and identified from M. sexta and has been named "lepidostatin-1"; this is the first YXFGLamide to be found in a lepidopteran, and there are indications that additional YXFGLamides occur in M. sexta. An antiserum to cockroach allatostatins (YXFGLamides) was shown to recognize lepidostatin-1 of M. sexta and was used to map YXFGLamide-immunoreactive neurons in larvae. Because immunoreactive interneurons were found to form an extensive neuropil, YXFGLamides probably function as neuromodulators in M. sexta. Neuroendocrine cells in the brain, abdominal ganglia, and their respective neurohemal organs were YXFGLamide immunoreactive and appear to release YXFGLamides as neurohormones. Immunoreactivity to YXFGLamides and M. sexta diuretic hormone were found to be colocalized and appear to be coreleased in these neuroendocrine cells, indicating that YXFGLamides may be involved in regulation of fluid transport. Innervation of the corpora allata by YXFGLamide-immunoreactive processes was very sparse, suggesting that this innervation does not play an important role in allatostasis. Many thoracic motor neurons were YXFGLamide immunoreactive, suggesting that YXFGLamides may have a myomodulatory or myotrophic function in larvae. However, this immunoreactivity disappeared early in metamorphosis and did not reappear in the adult. The YXFGLamide-immunoreactive neurons in the terminal abdominal ganglion were found to innervate the hindgut, indicating that YXFGLamides may be involved in the control of the rate of myogenic contractions of the larval hindgut.

Combining laser scanning confocal microscopy and electron microscopy in studies of the insect nervous system

Experimentally determining the synaptic interconnections between neurons in the nervous system is laborious and difficult in any animal species, but especially so in many invertebrates, including insects, where neurons generally have large, finely branching neuritic trees that form both pre- and postsynaptic specializations in dense neuropils with other neuritic trees. Electron microscopy is needed to identify synapses, but correlation of synapse type and location with the overall branching patterns of neurons, which are visible readily only in the light microscope or through extensive reconstruction of serial electron-microscope sections, is very difficult. In this paper, we present a simple method that we have developed (Sun et al. (1995) J. Histochem. Cytochem., 43: 329-335) that combines laser scanning confocal microscopy and electron microscopy for the study of synaptic relationships of neurons in the antennal lobe, the first central neuropil in the olfactory pathway, of the moth Manduca sexta. Briefly, neurons are labeled by intracellular injection with neurobiotin or biocytin, and then processed with a gold-particle tag for electron microscopic study and a fluorescent tag for confocal microscopy, and embedded in plastic. The fluorescence of the labeled neuron in the plastic blocks is imaged in three dimensions with laser scanning confocal microscopy and then the neuron is thin-sectioned at precisely chosen depths for electron microscopic study. The fluorescence pattern can be monitored repeatedly between episodes of thin-sectioning, and subtraction of a fluorescence image from the previous fluorescence image reveals which fluorescent processes have been sectioned. In this way, electron microscopic detail can be mapped onto a three-dimensional light microscopic image of the neuron.

Novel mouse IgG-like immunoreactivity expressed by neurons in the moth Manduca sexta: developmental regulation and colocalization with crustacean cardioactive peptide

Immunoglobulin-related molecules have been shown to play important roles in cell-cell recognition events during the development of both vertebrate and invertebrate nervous systems. In the moth, Manduca sexta, we report the presence of novel, mouse, immunoglobulin G (mIgG)-like immunoreactivity in a discrete population of identified neurosecretory neurons (the NS-Ls also known as the cell 27s) and interneurons (the IN-704s). A number of polyclonal anti-mIgG antibodies were used to immunostain these cells in wholemount. The mIgG-like-immunoreactive (IR) neurons were present during embryogenesis through the developing adult stages, but disappeared in the postemerged adult. Biochemical analysis of M. sexta ventral nerve cords revealed that the mIgG-like antigen is a membrane-associated 27-kDa protein which is likely responsible for the mIgG-like immunostaining observed. Unambiguous identification of the mIgG-like-IR neurons was based on neuronal morphology and our ability to demonstrate conclusively that these neurons expressed immunoreactivity to an antiserum against crustacean cardioactive peptide (CCAP). The NS-Ls and IN-704s were both shown to colocalize the CCAP and mIgG-like immunoreactivities. The mIgG-like and CCAP-IR neurons were identical to a subset of CCAP-IR neurons recently described by Davis et al. [(1993) J. Comp. Neurol., 338:612-627] in pupae. We found these CCAP-IR neurons, however, also to be present in larvae. The mIgG-like- and CCAP-IR neurons included the NS-L pair of the subesophageal maxillary neuromere, which projected anteriorly to the corpora cardiaca, and the NS-L of the labial neuromere whose axons projected out the dorsal nerve of the next posterior ganglion. The mIgG-like and CCAP-IR NS-Ls were also observed throughout the three thoracic ganglia, and all shared strikingly similar structural features. These cells exited out the dorsal nerve of the next posterior ganglion and eventually projected to the neurohemal release sites of the perivisceral organs. These neurons appear to be the homologues of the abdominal CCAP-IR NS-Ls, neurons that in the adult switch their neurotransmitter and release the neuropeptide bursicon. Our description of the distribution and developmental expression of this novel mIgG-like immunoreactivity may provide new insights into the regulation of neurotransmitter plasticity and/or recognition-signaling events involved in the embryonic and postembryonic assembly of the nervous system.

A motoneuron spared from steroid-activated developmental death by removal of descending neural inputs exhibits stable electrophysiological properties and morphology

Neurons die during the development of nervous systems. The death of specific, identified motoneurons during metamorphosis of the tobacco hornworm, Manduca sexta, provides an accessible model system in which to study the regulation of postembryonic neuronal death. Hormones and descending neural inputs have been shown to influence the survival of abdominal motoneurons during the first few days of adult life in this insect. Motoneurons prevented from undergoing the normal process of developmental degeneration by removal of neural inputs were examined at the physiological and structural levels using several cell imaging techniques. Although these neurons lost their muscle targets and experienced the endocrine cue that normally triggers death, they showed no overt electrophysiological or morphological signs of degeneration. Thus, by appropriate intervention, the MN-12 motoneuron can be spared from developmental neuronal death and remain as a functioning supernumerary element in the mature nervous system.

Distribution and developmental expression of octopamine-immunoreactive neurons in the central nervous system of the leech

Octopamine, a biogenic amine analogous to norepinephrine, plays an important role in the orchestration and modulation of invertebrate behavior. In the leech, the behavioral actions of octopamine have been demonstrated; however, identification of octopaminergic neurons had not been determined by using immunohistochemical techniques. Thus, we used an antibody highly specific to octopamine to examine the distribution of octopamine-immunoreactive neurons in the segmental ganglia of American and European medicinal leeches (Macrobdella decora and Hirudo medicinalis). One pair of octopamine-immunoreactive neurons was located in the dorsolateral ganglionic region of anterior ganglia 1-6 and posterior ganglia 15-21. No corresponding octopamine-immunoreactive neurons were found in midbody ganglia 7-14. Using Neutral Red staining in combination with intracellular Neurobiotin injections and octopamine immunostaining, we determined the identity of the dorsolateral octopamine-immunoreactive cells. The dorsolateral octopamine-immunoreactive neuron (the DLO) was not cell 21, the only previously reported Neutral Red staining neuron in the dorsolateral position. We also determined that the Leydig neuron was not octopamine immunoreactive in either of the two medicinal leech species. Octopamine immunostaining in the sex ganglia revealed hundreds of immunoreactive neurons in sexually mature leeches. Such neurons were not observed in juvenile leeches. The developmental time course of octopamine immunoreactivity in the dorsolateral octopamine-immunoreactive neurons was also investigated by staining embryonic Hirudo medicinalis. Octopamine expression occurred relatively late as compared with the detectable onset of serotonin expression. Octopamine expression in the dorsolateral octopamine-immunoreactive cells was not detectable at early to mid-embryonic stages, and must commence during late embryonic to early juvenile stages. The identification of octopamine-immunoreactive cells now sets the stage for further investigations into the functional role of octopamine in leech behavior and the development of behavior.

Utility of intensely fluorescent cyanine dyes (Cy3) for assay of gap junctional communication by dye-transfer

Utilization of a class of fluorescent cyanine dyes (Cy3) for the assay of gap junctional communication by the dye transfer method was examined. When compared with Lucifer Yellow (LY), a commonly used tracer, microinjected Cy3 dye was found to yield similar degrees of cell coupling. Blockade of the transfer of both tracers by 12-O-tetradecanoylphorbol-13 acetate (TPA), which is known to cause closure of communicating channels, confirmed gap junctional mediation of dye movement. The fixability of a microinjected amine derivative of Cy3 dye demonstrated its compatibility with immunostaining protocols involving fluorescein isothiocyanate (FITC)-conjugated reagents. These results together with the brilliant fluorescence of Cy3 dyes suggest the potential of Cy3 reagents as additional tools to study gap junction function.

Evidence for an endogenous neurocidin in the Manduca sexta ventral nerve cord

Half of the neurons in the abdominal nervous system of the moth Manduca sexta die after adult eclosion. Two physiological signals regulate post-eclosion neuronal death in adult moths. The first is endocrine: a decline in blood ecdysteroids is necessary for the death of neurons in the segmental ganglia. The second signal, which is highly specific for a pair of motoneurons found at the posterior midline in each of the three unfused abdominal ganglia, originates in the nervous system. It is transmitted from the fused pterothoracic ganglion to abdominal ganglion A3 via the intersegmental connectives. To characterize the signal of neural origin, we have developed an in vitro bioassay for neuron-killing factors ("neurocidins"). Aqueous extracts of pterothoracic ganglia were prepared and applied to cultured ventral nerve cords. These extracts exhibited concentration-dependent effectiveness in killing motoneurons. The active component of the extract was heat-stable and protease-sensitive. Size fractionation studies suggested that the active component has a molecular mass between 10 and 30 kD. This is the first report of an endogenous neuron-killing protein from an insect nervous system.

Leucokinin and diuretic hormone immunoreactivity of neurons in the tobacco hornworm, Manduca sexta, and co-localization of this immunoreactivity in lateral neurosecretory cells of abdominal ganglia

Because leucokinins stimulate diuresis in some insects, we wished to identify the neurosecretory cells in Manduca sexta that might be a source of leucokinin-like neurohormones. Immunostaining was done at various stages of development, using an antiserum to leucokinin IV. Bilateral pairs of neurosecretory cells in abdominal ganglia 3-7 of larvae and adults are immunoreactive; these cells project via the ipsilateral ventral nerves to the neurohemal transverse nerves. The immunoreactivity and size of these lateral cells greatly increases in the pharate adult, and this change appears to be related to a period of intensive diuresis occurring a few days before adult eclosion. Relationships of these neurons to cells that are immunoreactive to a M. sexta diuretic hormone were also investigated. Diuretic hormone and leucokinin immunoreactivity are co-localized in the lateral neurosecretory cells and their neurohemal projections. A median pair of leucokinin-immunoreactive, and a lateral pair of diuretic hormone-immunoreactive neurons in the larval terminal abdominal ganglion project to neurohemal release sites within the cryptonephridium. The immunoreactivity of these cells is lost as the cryptonephridium is eliminated during metamorphosis. This loss appears to be related to the change from the larval to adult pattern of diuresis.

Proctolin and excitation of the crayfish swimmeret system

The ventral nerve cord of crayfish contains axons of five pairs of excitatory interneurons, each of which can activate the swimmeret system. Perfusion of the ventral nerve cord with the neuropeptide proctolin also activates the swimmeret system. The experiments reported here were conducted to test the hypothesis that one or more of these excitatory interneurons uses proctolin as a transmitter. Each of the five excitatory axons was located and stimulated separately in an individual crayfish, and similar motor activity was elicited by stimulating each of them. Quantitative comparison of spontaneous swimmeret motor patterns with activity caused by stimulating one of these excitatory axons, EC, or by perfusing with proctolin solutions showed that the motor patterns produced under these three conditions were not significantly different (P > 0.05). By using a new, affinity-purified proctolin antiserum, we labeled axons in the connective tissue between the last thoracic and first abdominal ganglion and compared the positions of labeled axons with the previously described positions of the excitatory axons. About 0.3% of the axons in these connective tissues showed proctolin-like immunoreactivity, but heavily labeled pairs of axons did occur bilaterally in the regions of excitatory swimmeret axons. The projections of these labeled axons into the abdominal ganglia were traced in serial plastic sections. Labeled processes were abundant in the lateral neuropils, the loci of the swimmeret pattern-generating circuitry. From this evidence, we propose that three of these excitatory swimmeret interneurons use proctolin as a transmitter, but that a fourth does not. The evidence for the fifth axon is ambiguous.