Synapse formation by sensory neurons after cross-species transplantation in crickets: the role of positional information

Expression of engrailed in an array of identified sensory neurons: comparison with position, axonal arborization, and synaptic connectivity

engrailed (en) is expressed in the posterior region of embryonic segments and appendages of the cockroach, Periplaneta americana. By 23% of embryogenesis En immunoreactivity is apparent in the dorsal half of the cercus, appendages of segment A11. By 40% of development, En staining is present in the dorsomedial half of the cercus. The nucleus of the medial filiform hair sensory neuron (M), born in this region, expresses en strongly. Staining is never seen in the lateral neuron (L). En is expressed in M as the sensory axons enter the terminal ganglion and begin to form their different arborizations and synaptic connections. This pattern of expression persists through development to the second instar. In mutant animals with supernumerary filiform hair sensilla, En immunoreactivity is only seen in the medial neurons. In second-instar and adult cerci en expression is also seen in medially located neurons. We compared the levels of En staining in the array of 25 second instar neurons with their position, axonal arbor type, and synaptic connections. Staining intensity correlates with distance from the cercal midline, suggesting that en is regulated by other circumferential positional determinants. The expression of en does not correlate with the formation of an M-type arbor. Although 10 to 12 sensory neurons that express en form synapses with giant interneuron 5, the correlation is not precise. These results suggest that, if En does form part of a combinatorial system of positional information in the cercus, its actions are modulated by other gene products.

Projection pattern of sensory neurons in the central nervous system of a homeotic mutation of the moth Manduca sexta

Octopod (Octo) is a mutation of the moth Manduca sexta, which transforms the first abdominal segment (A1) in the anterior direction. Mutant animals are characterized by the appearance of homeotic thoracic-like legs on A1. We exploited this mutation to determine what rules might be used in specifying the fates of sensory neurons located on the body surface of larval Manduca. Mechanical stimulation of homeotic leg sensilla did not cause reflexive movements of the homeotic legs, but elicited responses similar to those observed following stimulation of ventral A1 body wall hairs. Intracellular recordings demonstrated that several of the motoneurons in the A1 ganglion received inputs from the homeotic sensory hairs. The responses of these motoneurons to stimulation of homeotic sensilla resembled their responses to stimulation of ventral body wall sensilla. Cobalt fills revealed that the mutation transformed the segmental projection pattern of only the sensory neurons located on the ventral surface of A1, resulting in a greater number with intersegmental projection patterns typical of sensory neurons found on the thoracic body wall. Many of the sensory neurons on the homeotic legs had intersegmental projection patterns typical of abdominal sensory neurons: an anteriorly directed projection terminating in the third thoracic ganglion (T3). Once this projection reached T3, however, it mimicked the projections of the thoracic leg sensory neurons. These results demonstrate that the same rules are not used in the establishment of the intersegmental and leg-specific projection patterns. Segmental identity influences the intersegmental projection pattern of the sensory neurons of Manduca, whereas the leg-specific projections are consistent with a role for positional information in determining their pattern.

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.

Connectivity of identified central synapses in the cricket is normal following regeneration and blockade of presynaptic activity

Cercal sensory neurons in the cricket innervate interneurons in the central nervous system (CNS) and provide a model system for studying the formation of central synapses. When axons of the sensory neurons were transected during larval development, the cell bodies and the soma-bearing portion of axons, which are located within the cercus, survived but lost their excitability for 9-10 days. During this period, the sensory neurons grew new axons and reinnervated the terminal abdominal ganglion. Physiological recordings showed that sensory neurons of known identity reestablished monosynaptic contacts with their normal postsynaptic interneuron. Moreover, each synapse exhibited a characteristic strength indistinguishable from the intact synapse in an unoperated cricket. Since this selective connectivity was apparent immediately after the excitability of the axotomized sensory neurons was restored, action potentials in the sensory neurons appear to be unnecessary for normal synaptic regeneration to occur. Consistent with this, the reinnervation process was unaffected even when action potentials in the sensory neurons were blocked by tetrodotoxin (TTX) immediately following axotomy until just before testing. During the normal course of development, the characteristic strength of individual synapses changes systematically, resulting in the developmental rearrangement of these synapses (Chiba et al., 1988). This synaptic rearrangement was also unaffected when action potentials in the sensory neurons were blocked by TTX for the last 30% of larval development. Therefore, in the cricket cercal sensory system, both regeneration of the central synapses following axotomy of the presynaptic sensory neurons and the normal rearrangement of connectivity during larval development appear not to require axonal action potentials.

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.

Assembly of the cricket cercal sensory system: genetic and epigenetic control

The cercal sensory system of the cricket is being examined using anatomical, physiological, and computer simulation techniques in order to better understand the assembly of sensory systems. This particular sensory system is of interest because it functions like numerically more complex vertebrate sensory systems but offers, to the neuroscientist, the technical advantages of a small number of large identified neurons. Two aspects of sensory processing are being examined in this system; the spatial aspects of a stimulus that tell an animal where a target is in its environment, and the qualities of a stimulus that help the animal to identify the stimulus. The spatial aspects of a stimulus are analyzed by a topographic mapping of the animal's sensory environment. The feature extraction machinery for other aspects of the stimulus lacks any obvious anatomical order and is embedded within the topographic map. We are attempting to tease apart the genetic and the epigenetic components of the assembly process for this sensory system. Here we review our progress with emphasis on the epigenetic aspects of its assembly. We describe previously published work on plasticity as well as new experiments focussed on the role of neuronal activity in the assembly of this neural circuit. Finally, we briefly describe simulation experiments that are helping us understand the role of various forms of synaptic plasticity in the determination of receptive fields.

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

Direct evidence for monosynaptic connections between filiform hair sensory axons and giant interneurons (GIs) in the first instar cockroach, Periplaneta americana, was obtained using intracellular recording and HRP injection followed by electron microscopy. GIs 1-6 all receive monosynaptic input from at least one filiform afferent axon. GI1, GI2 and GI5 receive input only from the medial (M) axon, while GI3, GI4 and GI6 receive input from both M and lateral (L) axons. The dendrites of GI3 and GI6 which are contralateral to the cell bodies receive input from both axons whereas the smaller ipsilateral dendritic fields have synapses only from the L axon. GI5 has M axon input only onto its contralateral dendrites. In 50% of preparations GI7 receives weak input from the ipsilateral L axon. There is no obvious relationship between the morphology of the giant interneurons and the pattern of input they receive from the filiform afferents.

Pre-axonogenesis migration of afferent pioneer cells in the grasshopper embryo

In insects, afferent neurons arise primarily from the ectodermal epithelium in the periphery and differentiate at the site of their precursor mitosis. Here we describe ectodermally derived cells that migrate away from their site of origin and initiate axonogenesis at a distant location. In embryonic grasshopper limb buds, the first two pairs of afferents to differentiate are the pair of Ti1 pioneers at the limb tip and the pair of Cx1 cells found at the base of the limb. While the Ti1 pioneers arise from the mitosis of a pioneer mother cell at the limb tip, the Cx1 cells are shown to emerge from the epithelium at circumferential positions that are approximately 150 degrees apart and that belong to different embryonic compartments. The cells migrate into contact with each other before initiating axonogenesis, and their axons then extend in a new direction that is orthogonal to the route of cell migration.

Genetic control of sexually dimorphic axon morphology in Drosophila sensory neurons

The mechanism by which orderly axonal projections are formed during development remains an important and largely unsolved problem in neurobiology. It may be possible to examine the control of axon growth in Drosophila and take advantage of genetic tools to better understand the phenomenon. We show here that some gustatory axons in Drosophila are sexually dimorphic and that genes involved in sex determination control the anatomy of these axons. Both males and females possess gustatory receptors on their legs but males possess more of these receptors than females. More significantly, the axons of the male receptors usually cross the midline and they never do so in females, indicating a central zone of bilateral input in the male but not in the female nervous system. In chromosomal females, expressing a tra or Sxl mutation, the gustatory system is transformed toward the male phenotype. Mutant XX adults resemble normal males externally, because they have more gustatory receptors, and internally, because their axons cross the midline. Gynandromorphs show that the sex of the sensory neuron, and apparently not the central nervous system, controls the growth of the axons. We conclude that the anatomical site of control for this dimorphism is the gustatory neurons.