Synaptic specificity in the first instar cockroach: patterns of monosynaptic input from filiform hair afferents to giant interneurons

Neural circuit recording from an intact cockroach nervous system

The cockroach ventral nerve cord preparation is a tractable system for neuroethology experiments, neural network modeling, and testing the physiological effects of insecticides. This article describes the scope of cockroach sensory modalities that can be used to assay how an insect nervous system responds to environmental perturbations. Emphasis here is on the escape behavior mediated by cerci to giant fiber transmission in Periplaneta americana. This in situ preparation requires only moderate dissecting skill and electrophysiological expertise to generate reproducible recordings of neuronal activity. Peptides or other chemical reagents can then be applied directly to the nervous system in solution with the physiological saline. Insecticides could also be administered prior to dissection and the escape circuit can serve as a proxy for the excitable state of the central nervous system. In this context the assays described herein would also be useful to researchers interested in limb regeneration and the evolution of nervous system development for which P. americana is an established model organism.

Developmental expression of odorant-binding proteins and chemosensory proteins in the embryos of Locusta migratoria

We have investigated the development of chemosensilla and the secretion of odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) in the embryo of Locusta migratoria manilensis. We first report the changes of each sensillum in embryo just preceding hatch in detail and show that different sensilla have different developmental processes. Trichogen cells are first involved in forming the structure of pegs, and then, after retraction, they start secreting OBPs and CSPs in the sensillar lymph. The synthesis of LmigOBP1 starts during the embryogenesis about 0.5 h preceding hatching, specifically in sensilla trichodea and basiconica of the antenna. LmigOBP2, instead, was only found in the outer sensillum lymph (oSl) of sensilla chaetica of the antenna, while we could not detect LmigOBP3 in any type of sensilla of the antenna. The ontogenesis of CSPs in the embryos is similar to that of OBPs. Expression of CSPI homolog in Locusta migratoria is detected using the antiserum raised against SgreCSPI. CSPI is specifically expressed in the outer sensillum lymph of sensilla chaetica of the antenna, and anti-LmigCSPII dose not label any sensilla of the embryos. These data indicate that in locusts, OBPs and CSPs follow different temporal expression patterns, and also that OBPs are expressed in different types of sensilla.

Reflections on the specificity of synaptic connections

The principal focus of this treatise is the specificity of synaptic connectivity in the mammalian central nervous system. The occurrence of stereotypical patterns of connection at the macro level (e.g., the general consistency with which axonal pathways impinge on and originate within specific cortical areas and layers) implies that the cerebral cortex is a highly ordered structure. Order is seen also at the more micro level of synaptic connectivity, for instance, in the contrasting synaptic patterns of spiny vs. non-spiny neurons. Quantitative electron microscopic studies of synapses between identified neurons and correlative anatomical/electrophysiological investigations indicate that the high degree of order characterizing many aspects of cortical organization is mirrored by an equally ordered arrangement of synaptic connections between specific types of neurons. The recognition of recurring synaptic patterns has generated increased support for the notion of synaptic specificity as opposed to randomness, and we have begun now to understand the role of specificity in cortical function. At the core of cortical processing lie myriad possibilities for computation provided by the wealth of synaptic connections involving each neuron. Specificity, by limiting possibilities for connection, imposes an order on synaptic interactions even as processes of dynamic selection or synaptic remodeling ensure the constant formation and dissolution of cortical circuits. Collectively, these operations make maximal use of the richness of cortical synaptic connections to produce a highly flexible system, irrespective of the degree of hard-wiring, mutability, randomness or specificity that obtains for cortical wiring at any particular time. A brief, historical account of developments leading to our current understanding of cortical synaptic organization will precede the presentation of evidence for synaptic specificity.

Control of central synaptic specificity in insect sensory neurons

Synaptic specificity is the culmination of several processes, beginning with the establishment of neuronal subtype identity, followed by navigation of the axon to the correct subdivision of neuropil, and finally, the cell-cell recognition of appropriate synaptic partners. In this review we summarize the work on sensory neurons in crickets, cockroaches, moths, and fruit flies that establishes some of the principles and molecular mechanisms involved in the control of synaptic specificity. The identity of a sensory neuron is controlled by combinatorial expression of transcription factors, the products of patterning and proneural genes. In the nervous system, sensory axon projections are anatomically segregated according to modality, stimulus quality, and cell-body position. A variety of cell-surface and intracellular signaling molecules are used to achieve this. Synaptic target recognition is also controlled by transcription factors such as Engrailed and may be, in part, mediated by cadherin-like molecules.

Post-embryonic development of cuticular caps of campaniform sensilla of the cockroach leg: potential implications in scaling force detection

All animals generate progressively larger forces as they increase in size and mass. Their abilities to detect these forces must be similarly adjusted. In insects, campaniform sensilla monitor strains in the exoskeleton and provide information about forces acting upon the legs. Each sensory neuron possesses a dendrite that inserts into a cuticular cap in the exoskeleton. The cap is the site of mechanotransduction. We measured the sizes and numbers of receptor caps on the cockroach hindleg at different developmental stages. Our goal was to identify morphological features that could be correlated with the range of forces that must be detected. As cockroaches increase in size through successive molts, the number of cuticular caps in each group increases. The tibial group, for example, has two sensilla in first instar animals and 10-12 in the adult. There is also an increase in the range of cap sizes within each group. Observations of animals and their molted exoskeletons suggest that this increase occurs as the caps of existing receptors increase in size and smaller ones are added with each molt. These changes may be important in increasing the range of forces the receptors can signal while retaining sensitivity to low levels of force.

Differential roles of engrailed paralogs in determining sensory axon guidance and synaptic target recognition

The transcription factor Engrailed (En) controls axon pathfinding and synaptic target choice in an identified neuron (6m) of the cockroach cercal sensory system. Knock-out of En using double-stranded RNA interference (dsRNAi) transforms 6m so that it resembles a neighboring neuron that normally does not express the en gene, has a different arbor anatomy, and makes different connections. Like many animals, the cockroach has two En paralogs, Pa-En1 and Pa-En2. In this study we tested the hypothesis that the paralogs have different effects on axon guidance and synaptic target recognition, using RNAi to knock out each one individually. Using dye injections into 6m and intracellular recordings from target interneurons, we obtained evidence that both Pa-En1 and Pa-En2 determine the axonal arborization, but only Pa-En1 controls synaptic connections. However, because immunocytochemical quantification of En protein in 6m after RNAi showed that Pa-En1 represents 65% of the total En activity and Pa-En2 only 35%, our results could be caused by dosage effects. We measured the effects of diluting the mixture of both dsRNAs on the amounts of En protein. From this dose-response curve, we calculated the appropriate dilutions of the dsRNA mixture that would titrate total En protein to levels equivalent to knock-out of either paralog. RNAi using these dilutions showed that Pa-En1 and Pa-En2 both contribute toward the control of axonal guidance and confirmed that Pa-En1 has the paralog-specific function of controlling synaptic target recognition.

Persistent engrailed expression is required to determine sensory axon trajectory, branching, and target choice

The transcription factor Engrailed (En) directs, in the cockroach cercal system, the shape of the axonal arborization and the choice of postsynaptic partners of an identified sensory neuron (6m). Knock-out of En using double-stranded RNA interference transforms 6m so that it resembles a neighboring neuron that normally does not express the en gene, has a different arbor anatomy, and makes different connections. We characterized the development of 6m and perturbed en expression at different stages. Our results show that En is not required before birth for 6m to become a neuron, but that it is required in the postmitotic neuron to control axonal arborization and synaptic specificity. Knock-out of En after 6m has entered the CNS does not change the axonal trajectory and has minor effects on axonal branches but causes the formation of synaptic connections typical of an En-negative cell. This suggests that En controls target recognition molecules independently from those guiding the axon. In contrast, double-stranded RNA injection 1 d later does not have any effects on the phenotype of 6m, suggesting that the period of synapse formation is over by the time En levels have fallen or, if synapse turnover occurs, that En is not required to maintain the specificity of synaptic connections. We conclude that persistent en expression is required to determine successive stages in the differentiation of the neuron, suggesting that it is not far upstream from those genes encoding axon guidance and synaptic recognition molecules.

Double-stranded RNA interference shows that Engrailed controls the synaptic specificity of identified sensory neurons

The transcription factor Engrailed (En) controls the topography of axonal projections by regulating the expression of cell-adhesion molecules [1] [2] [3] [4] but it is not known whether it also controls the choice of individual synaptic target cells. In the cercal sensory system of the larval cockroach (Periplaneta americana), small numbers of identified wind-sensitive sensory neurons form highly specific synaptic connections with 14 identified giant interneurons [5] [6], and target-cell choice is independent of the pattern of axonal projections [6]. En is a putative positional determinant in the array of cercal sensory neurons [7]. In the present study, double-stranded RNA (dsRNA) interference [8] was used to abolish En expression. This treatment changed the axonal arborisation and synaptic outputs of an identified En-positive sensory neuron so that it came to resemble a nearby En-negative cell, which was itself unaffected. We thus demonstrate directly that En controls synaptic choice, as well as axon projections.

Specificity of identified central synapses in the embryonic cockroach: appropriate connections form before the onset of spontaneous afferent activity

The mechanisms by which neurons recognize the appropriate postsynaptic cells remain largely unknown. A useful approach to this problem is to use a system with a few identifiable neurons that form highly specific synaptic connections. We studied the development of synapses between two identified cercal sensory afferents and two giant interneurons (GIs) in the embryonic cockroach Periplaneta americana. By 46% of embryonic development, the axons of the filiform hair sensory neurons have entered the terminal ganglionic neuropil and grow alongside the GI primary dendrites, although they do not form synapses. From 50% of development, the GI dendrites grow outward from the center of the neuropil to contact the presynaptic axons and their branches. The sensory neurons begin to spike at 52% of development, and, from 55% of development, these action potentials evoked excitatory postsynaptic potentials in the GIs. Synaptic contacts were first seen at this time. The pattern of synaptic connections was highly specific from the outset. G12 had strong input from the medial (M) afferent and had almost negligible input from the lateral (L) afferent, whereas G13 had input from both. This specificity was present before bursts of spontaneous activity began in the sensory neurons at 59% of development. G12 filopodia selectively formed synaptic contacts with the M axon rather than the L axon. The few contacts made by G12 with the L axon had a normal morphology but fewer presynaptic densities. Filopodial insertions were not involved in selective synapse formation. In this system, highly specific synaptic recognition appears to be activity independent.

Transplantation of neurons reveals processing areas and rules for synaptic connectivity in the cricket nervous system

In order to assess the nature of spatial cues in determining the characteristic projection sites of sensory neurons in the CNS, we have transplanted sensory neurons of the cricket Acheta domesticus to ectopic locations. Thoracic campaniform sensilla (CS) function as proprioceptors and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a ventral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific projection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neurons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when transplanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each segmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encounter after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the host ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of neuropil in each segmental ganglion, or with novel targets, which lack dendrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molecular markers are utilized for cell-cell recognition in each segmentally homologous ganglion. Regenerating sensory neurons can recognize novel postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segmentation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity.

Ectopic sensory neurons in mutant cockroaches compete with normal cells for central targets

The cercus of the first instar cockroach, Periplaneta americana, bears two filiform hairs, lateral (L) and medial (M), each of which is innervated by a single sensory neuron. These project into the terminal ganglion of the CNS where they make synaptic connections with a number of ascending interneurons. We have discovered mutant animals that have more hairs on the cercus; the most typical phenotype, called “Space Invader” (SI), has an extra filiform hair in a proximo-lateral position on one of the cerci. The afferent neuron of this supernumerary hair (SIN) “invades the space” occupied by L in the CNS and makes similar synaptic connections to giant interneurons (GIs). SIN and L compete for these synaptic targets: the size of the L EPSP in a target interneuron GI3 is significantly reduced in the presence of SIN. Morphometric analysis of the L afferent in the presence or absence of SIN shows no anatomical concomitant of competition. Ablation of L afferent allows SIN to increase the size of its synaptic input to GI3. Less frequently in the mutant population, we find animals with a supernumerary medical (SuM) sensillum. Its afferent projects to the same neuropilar region as the M afferent, makes the same set of synaptic connections to GIs, and competes with M for these synaptic targets. The study of these competitive interactions between identified afferents and identified target interneurons reveals some of the dynamic processes that go on in normal development to shape the nervous system.

Correlation of filiform hair position with sensory afferent morphology and synaptic connections in the second instar cockroach

An attempt is made to relate the distribution of filiform hairs on the cercus of the second instar cockroach, Periplaneta americana, to the morphology and patterns of synaptic connectivity of their afferents. We studied the most distal 25 of the 39 filiform hairs which are commonly present. Filiform afferent arborizations were stained by cobalt filling from the cell body in the cercus. Three fundamental arbor types were found, two similar to those of the first instar medial (M) and lateral (L) afferents, and a third, novel type. L-type arbors could be divided into four subtypes. The most obvious correlate of arbor type is the circumferential position of the hair on the cercus. The proximodistal position of the sensillum within each cercal segment is also a determinant of its arbor. By comparison of hair positions and afferent morphologies, we were able to ascribe homologies between the second instar hairs and members of adult longitudinal hair columns. The patterns of monosynaptic connections between afferents and giant interneurons (GIs) 1, 2, 3, 5, and 6 were determined by recording synaptic potentials in GIs evoked by direct mechanical displacement of individual filiform hairs. Latency from stimulus onset to the rise phase of the first excitatory postsynaptic potential (EPSP) was used as the criterion of monosynapticity. The EPSP amplitudes of the two original L and M afferents are halved in the second instar, in the absence of a significant decrease in GI input resistance. The other afferents can be divided into two basic classes: those which input to GI5 (M-type), and those which input to GI3 and GI6 (L-type). The former is correlated with a central or medial position, while the latter is associated with a group of afferents situated laterally on the cercus. In segments 3 and 4, input to GIs 1 and 2 also correlates with a medial cercal position, however, in the more proximal segments 5 and 6, afferents at all positions input to these interneurons. The occurrence of afferents of identical morphology and similar connectivity in equivalent positions in different segments suggests that each sensory neuron is determined by its two-dimensional position within a segment. The presence of afferents with the same morphology which display proximodistal differences in synaptic connectivity, and of other afferents which have M-type connectivity despite L-type morphology, means that anatomy is generally a poor predictor of synaptic connectivity.

Specificity of filiform hair afferent synapses onto giant interneurons in Periplaneta americana: anatomy is not a sufficient determinant

The synapses between the filiform hair sensory afferents and giant interneurons (GIs) 1-6 of embryonic and first instar cockroaches, Periplaneta americana, were used to investigate the role of neuronal anatomy in determining synaptic specificity. The pattern of afferent-to-GI synapses was first determined by intracellular recording of excitatory postsynaptic potentials (EPSPs). The lateral (L) axon synapses only with GIs 3, 4, and 6, while the medial (M) axon synapses with the contralateral dendrites of all six GIs but with the ipsilateral dendrites only of GIs 1, 2, and 4. The three-dimensional anatomy of the filiform afferents and GIs was determined by injection of cobalt. There is little anatomical segregation of the filiform afferents; consequently, there is no correlation between the anatomy of the GIs and their synaptic inputs. The M axon and ipsilateral GI3 were studied in more detail by light and electron microscopy. Despite the presence of an anterior M axon branch which loops around the ipsilateral GI3 neurite at a distance of 2 microns, no synapses are formed between them. This lack of synapses is not due to the presence of physical barriers. Investigation of filiform afferents and GIs in embryonic ganglia shows that at no stage are the afferents sufficiently separated for their anatomy to be an important factor in determining the specificity of the synaptic inputs of the GIs. It was postulated that two pairs of complementary cell surface labels would be sufficient to code for this specificity, and that, in GIs 3, 5, and 6, spatial differences in the expression of these labels allow the M axon to distinguish ipsilateral dendrites from contralateral.