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

Maturation of dendritic architecture: lessons from insect identified neurons

The highly complex geometry of dendritic trees is crucial for neural signal integration and the proper wiring of neuronal circuits. The morphogenesis of dendritic trees is regulated by innate genetic factors, neuronal activity, and external molecular cues. How each of these factors contributes to dendritic maturation has been addressed in studies of the developing nervous systems of animals ranging from insects to mammals. This article reviews our current knowledge and understanding of the role of afferent input in the establishment of the architecture of mature dendritic trees, using insect neurons as models. With these model systems and using quantitative morphometry, it is possible to define the contributions of intrinsic and extrinsic factors in dendritic morphogenesis of identified neurons and to evaluate the impact of dendritic maturation on the integration of identified neurons into functional circuits subserving identified behaviors. The commonly held view of dendritic morphogenesis is that general structural features result from genetic instructions, whereas fine connectivity details rely mostly on substrate interactions and functional activity. During early dendritic maturation, dendritic growth cone formation produces new branches at all dendritic roots. The second phase is growth cone independent and afferent input dependent, during which branching is limited to high order distal dendrites. During the third phase, activity-dependent synaptic maturation occurs with limited or subtle remodeling of branching.

Behavioural correlates of phenotypic plasticity in mouthpart chemoreceptor numbers in locusts

Rearing locusts in an impoverished chemosensory environment leads to fewer chemoreceptors developing on the mouthparts and antennae as adults but the behavioural relevance of these changes remains unknown. To address this question, locusts were reared for the final two larval stadia on either a single, nutritionally near-optimal synthetic food ('plain' pretreatment), or a diet comprising two nutritionally complementary foods containing two added flavours ('mix' pretreatment). Insects reared on the 'mix' diet had a mean 20% more chemosensilla on the maxillary palps than those fed on the 'plain' diet. Following an equilibration period, when all newly moulted adults could feed on two nutritionally complementary foods, insects were food deprived for 2 or 6 h, and then given a test meal of a single balanced food at one of two dilutions whilst their behaviour was recorded. 'Mix'-pretreated locusts had a shorter latency to feed and were more likely to reject the test food upon first contact if deprived for only 2 h; but if they did take a meal it lasted longer and contained fewer pauses. Using sensilla number as a covariate removed the statistical significance of pretreatment regime, indicating that sensilla number, or some close correlate of it, can largely account for the variation in behaviour. This suggests that sensilla numbers are behaviourally relevant; particularly where locusts are not greatly food deprived and faced with marginally acceptable foods.

Genetic analysis of cholinergic nerve terminal function in invertebrates

Genetic analysis of nerve terminal function is proving fruitful and studies on invertebrates are making a substantial impact. In this survey, particular emphasis has been placed on cholinergic chemical synaptic transmission. The advanced genetics of Drosophila melanogaster and Caenorhabditis elegans with their rich diversity of behavioural and biochemical mutants is providing new insights into the functions of key molecular components of synapses. A 'space-invader' mutant of Periplaneta americana permits investigations of competition between neurons during synaptogenesis and its impact on neurotransmitter release. The growing importance of the C. elegans genome as a major research resource is emphasized in this survey.

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.

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.