A map of taste neuron projections in the Drosophila CNS

We provide a map of the projections of taste neurons in the CNS of Drosophila. Using a collection of 67 GAL4 drivers representing the entire repertoire of Gr taste receptors, we systematically map the projections of neurons expressing these drivers in the thoracico-abdominal ganglion and the suboesophageal ganglion (SOG). We define 9 categories of projections in the thoracico-abdominal ganglia and 10 categories in the SOG. The projection patterns are modular, and can be interpreted as combinations of discrete pattern elements. The elements can be interpreted in terms of the taste organ from which the projections originate, the structures from which they originate, and the quality of taste information that they represent. The extensive diversity in projection patterns provides an anatomical basis for functional diversity in responses elicited by different taste stimuli.     

Formation of retinotopic connections: Selective stabilization by an activity-dependent mechanism

During regeneration of the optic nerve in goldfish, the ingrowing retinal fibers successfully seek out their correct places in the overall retinotopic projection on the tectum. Chemospecific cell-surface interactions appear to be sufficient to organize only a crude retinotopic map on the tectum during regeneration. Precise retinotopic ordering appears to be achieved via an activity-dependent stabilization of appropriate synapses and is based upon the correlated activity of neighboring ganglion cells of the same receptive-field type in the retina. Four treatments have been found to block the sharpening process: (a) blocking the activity of the ganglion cells with intraocular tetrodotoxin (TTX), (b) rearing in total darkness, (c) correlating the activation of all ganglion cells via stroboscopic illumination and (d) blocking retinotectal synaptic transmission with alpha-bungarotoxin (alphaBTX). These experiments support a role for correlated visually driven activity in sharpening the diffuse projection and suggest that this correlated activity interacts within the postsynaptic cells, probably through the summation of excitatory postsynaptic potentials (EPSPs). Other experiments support the concept that effective synapses are stabilized: a local postsynaptic block of transmission causes a local disruption in the retinotectal map. The changes that occur during this disruption suggest that each arbor can move to maximize its synaptic efficacy. In development, initial retinotectal projections are often diffuse and may undergo a similar activity-dependent sharpening. Indirect retinotectal maps, as well as auditory maps, appear to be brought into register with the direct retinotopic projections by promoting the convergence of contacts with correlated activity. A similar mechanism may drive both the formation of ocular dominance patches in fish tectum and kitten visual cortex and the segregation of different receptive-field types in the lateral geniculate nucleus. Activity-dependent synaptic stabilization may therefore be a general mechanism whereby the diffuse projections of early development are brought to the precise, mature level of organization.     

Ectopic CNS projections guide peripheral neuron axons along novel pathways in leech embryos

Previous studies have indicated that the formation of stereotyped segmental nerves in leech embryos depends on the interactions between CNS projections and ingrowing afferents from peripheral neurons. Especially, CNS-ablation experiments have suggested that CNS-derived guidance cues are required for the correct navigation of several groups of peripheral sensory neurons. In order to directly test this hypothesis we have performed transplantations of CNS ganglia into ectopic sites in segments from which the resident ganglia have been removed. We find that the transplanted ganglia extend numerous axons distributed roughly equally in all directions. When these CNS projections reach and make contact with peripheral sensory axons they are used as guides for peripheral neurons to grow toward and into the ectopic ganglia even when this means following novel pathways that cross the midline and/or segmental boundaries. The peripheral sensory axons turn and grow toward the ectopic ganglia only when in physical contact with CNS axons, suggesting that diffusible chemoattractants are not a factor. These results demonstrate that the guidance cues provided by ectopic CNS projections are both necessary and sufficient to steer peripheral sensory neuron axons into the CNS.     

Sensory experience and the formation of a computational map of auditory space in the brain.

The basic wiring of the brain is first established before birth by using a variety of molecular guidance cues. These connections are then refined by patterns of neural activity, which are initially generated spontaneously and subsequently driven by sensory experience. In the superior colliculus, a midbrain nucleus involved in the control of orienting behaviour, visual, auditory, and tactile inputs converge to form superimposed maps of sensory space. Maps of visual space and of the body surface arise from spatially ordered projections from the retina and skin, respectively. In contrast, the map of auditory space is computed within the brain by tuning the neurons to different localization cues that result from the acoustical properties of the head and ears. Establishing and maintaining the registration of the maps in the face of individual differences in the size and relative positions of different sense organs is an activity-dependent process in which the synaptic circuits underlying the auditory representation are modified and calibrated under the influence of both auditory and visual experience. BioEssays 1999;21:900-911.     

Presynaptic Inputs to Any CNS Projection Neuron Identified by Dual Recombinant Virus Infection

Although neuroanatomical tracing studies have defined the origin and targets of major projection neurons (PN) of the central nervous system (CNS), there is much less information about the circuits that influence these neurons. Recently, genetic approaches that use Cre recombinase-dependent viral vectors have greatly facilitated such circuit analysis, but these tracing approaches are limited by the availability of Cre-expressing mouse lines and the difficulty in restricting Cre expression to discrete regions of the CNS. Here, we illustrate an alternative approach to drive Cre expression specifically in defined subsets of CNS projection neurons, so as to map both direct and indirect presynaptic inputs to these cells. The method involves a combination of Cre-dependent transneuronal viral tracers that can be used in the adult and that does not require genetically modified mice. To trigger Cre-expression we inject a Cre-expressing adenovirus that is retrogradely transported to the projection neurons of interest. The region containing the retrogradely labeled projection neurons is next injected with Cre-dependent pseudorabies or rabies vectors, which results in labeling of poly- and monosynaptic neuronal inputs, respectively. In proof-of-concept experiments, we used this novel tracing system to study the circuits that engage projection neurons of the superficial dorsal horn of the spinal cord and trigeminal nucleus caudalis, neurons of the parabrachial nucleus of the dorsolateral pons that project to the amygdala and cortically-projecting neurons of the lateral geniculate nucleus. Importantly, because this dual viral tracing method does not require genetically derived Cre-expressing mouse lines, inputs to almost any projection system can be studied and the analysis can be performed in larger animals, such as the rat.     

Growth and guidance cues for regenerating axons: where have they gone?

Both attractive and repellent cues are required to guide developing axons to their targets in the central nervous system. Critical guidance molecules in the developing brain include the semaphorins, netrins, slits, and ephrins. Current research indicates that many of these molecules and their receptors are expressed in the adult central nervous system (CNS), and that injury can alter the levels of these ligands/receptors. Recent studies have begun the process of elucidating the functions of these receptors in adult mammals, and the effects that they have on the regeneration of adult neurons. This review addresses our current knowledge with respect to the response of adult CNS neurons to axonal injury, interventions for enhancing the survival and regeneration of injured neurons, and the expression of developmental axon guidance cues in the injured mature CNS, with specific focus on the retino-tectal projection.     

Nitric oxide modulates retinal ganglion cell axon arbor remodeling in vivo

Nitric oxide (NO) has been postulated to act as an activity-dependent retrograde signal that can mediate multiple aspects of synaptic plasticity during development. In the visual system, a role for NO in activity-dependent structural modification of presynaptic arbors has been proposed based on NO's ability to prune inappropriate projections and segregate axon terminals. However, evidence demonstrating that altered NO signaling does not perturb ocular dominance map formation leaves unsettled the role of NO during the in vivo refinement of visual connections. To determine whether NO modulates the structural remodeling of individual presynaptic terminal arbors in vivo we have: 1. Used NADPH-diaphorase histochemistry to determine the onset of NO synthase (NOS) expression in the Xenopus visual system. 2. Used in vivo time-lapse imaging to examine the role of NO during retinal ganglion cell (RGC) axon arborization. We show that NOS expression in the target optic tectum is developmentally regulated and localized to neurons that reside in close proximity to arborizing RGC axons. Moreover, we demonstrate that perturbations in tectal NO levels rapidly and significantly alter the dynamic branching of RGC arbors in vivo. Tectal injection of NO donors increased the addition of new branches, but not their stabilization in the long term. Tectal injection of NOS inhibitors increased the dynamic remodeling of axonal arbors by increasing branch addition and elimination and by lengthening pre-existing branches. Thus, these results indicate that altering NO signaling significantly modifies axon branch dynamics in a manner similar to altering neuronal activity levels (Cohen-Cory, 1999). Consequently, our results support a role for NO during the dynamic remodeling of axon arbors in vivo, and suggest that NO functions as an activity-dependent retrograde signal during the refinement of visual connections.     

Activity-dependent mapping in the retinotectal projection

It is now 15 years since the discovery that N-methyl-d-aspartate receptor activity is required to maintain the refined topographic organization of retinotectal projections. Recent studies have identified additional components of the signaling pathways required for activity-dependent map formation and maintenance. Nitric oxide and brain-derived neurotrophic factor, candidate retrograde messengers, and serotonin and acetylcholine, modulators of neuronal excitability, all affect mapping. These studies indicate that the mapping process intersects with other processes fundamental to visual system development and function, such as process outgrowth, synaptic turnover and neuromodulation.     

Selective stabilization of retinotectal synapses by an activity-dependent mechanism

How does each ingrowing retinal fiber select the right spot in the overall retinotopic projection? Chemospecific surface interactions appear to be sufficient only to organize a crude retinotopic map on the tectum during regeneration of the optic nerve of goldfish. Precise retinotopic ordering is achieved via an activity-dependent stabilization of appropriate synapses, based on the correlated activity of neighboring ganglion cells of the same receptive field type in the retina. Four treatments have been found to block the sharpening process: 1) blocking activity of the ganglion cells with intraocular tetrodotoxin (TTX); 2) rearing in total darkness; 3) correlated activation of all ganglion cells via stroboscopic illumination in a featureless environment; 4) block of retinotectal synaptic transmission with alpha-bungarotoxin. These experiments support a role for normal visually driven activity in sharpening the diffuse projection, and demonstrate that the correlated activity of the optic fibers interacts within the postsynaptic cells, probably through the summation of excitatory postsynaptic potentials. Intraocular TTX experiments suggest that a similar mechanism may drive both the formation of ocular dominance patches in fish tectum and kitten visual cortex and the segregation of different receptive field types in the lateral geniculate nucleus. Thus, it may be a general mechanism whereby the diffuse projections of early development are brought to a mature level of organization.     

Planarian homologs of netrin and netrin receptor are required for proper regeneration of the central nervous system and the maintenance of nervous system architecture

Conserved axon guidance mechanisms are essential for proper wiring of the nervous system during embryogenesis; however, the functions of these cues in adults and during regeneration remain poorly understood. Because freshwater planarians can regenerate a functional central nervous system (CNS) from almost any portion of their body, they are useful models in which to study the roles of guidance cues during neural regeneration. Here, we characterize two netrin homologs and one netrin receptor family member from Schmidtea mediterranea. RNAi analyses indicate that Smed-netR (netrin receptor) and Smed-netrin2 are required for proper CNS regeneration and that Smed-netR may mediate the response to Smed-netrin2. Remarkably, Smed-netR and Smed-netrin2 are also required in intact planarians to maintain the proper patterning of the CNS. These results suggest a crucial role for guidance cues, not only in CNS regeneration but also in maintenance of neural architecture.     

Developmental origin of wiring specificity in the olfactory system of Drosophila

In both insects and mammals, olfactory receptor neurons (ORNs) expressing specific olfactory receptors converge their axons onto specific glomeruli, creating a spatial map in the brain. We have previously shown that second order projection neurons (PNs) in Drosophila are prespecified by lineage and birth order to send their dendrites to one of approximately 50 glomeruli in the antennal lobe. How can a given class of ORN axons match up with a given class of PN dendrites? Here, we examine the cellular and developmental events that lead to this wiring specificity. We find that, before ORN axon arrival, PN dendrites have already created a prototypic map that resembles the adult glomerular map, by virtue of their selective dendritic localization. Positional cues that create this prototypic dendritic map do not appear to be either from the residual larval olfactory system or from glial processes within the antennal lobe. We propose instead that this prototypic map might originate from both patterning information external to the developing antennal lobe and interactions among PN dendrites.     

Divergent axon collaterals originate in the estrogen receptive ventromedial nucleus of hypothalamus in the rat

The ventromedial nucleus of the hypothalamus (VMH) plays a crucial role in the mediation of lordosis by integrating predominantly inhibitory limbic signals with cyclic variation of ovarian steroids and sending a stimulatory output to the midbrain, especially the periaqueductal gray (PAG). Tract-tracing studies have established projections of the VMH and Golgi studies have shown these neurons to frequently give rise to axon collaterals, but the anatomical pattern of shared projections has not been explored. We have used a combination of retrograde tracers to map VMH projections to the medial division of the medial preoptic nucleus (MPNm), posterodorsal division of the medial nucleus of the amygdala (MeApd), and the PAG. Neurons with dual projections were mainly confined to the VMHvl and represented 31%–37% of each projection subset. Neurons simultaneously projecting to the MPNm, MeApd, and PAG represented 7%–9% of each projection subset. By combining tract-tracing with steroid autoradiography, we found that approximately one-quarter of each projection subset in the VMHvl concentrated ³H-estradiol. Thus, some of the VMHvl neurons that communicate a facilitatory signal to the PAG may also act to stimulate lordosis through a feedback suppression of the net inhibition formed by efferent signals from the forebrain. The even distribution of estrogen binding among projection subsets suggests a lack of compartmentalization of estrogen-regulated processes that are relevant to lordosis. 1994 John Wiley & Sons, Inc.     

Position,guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography. © 1993 John Wiley & Sons, Inc.     

Multiple strategies for directed growth cone extension and navigation of peripheral neurons

Leeches have a diverse constellation of peripheral neural elements that are challenged to extend growth cones in highly specific ways in a constantly changing embryonic environment. Two major systems are reviewed here. In one, peripheral afferents extend growth cones toward the central nervous system (CNS), forming common pathways, and then segregate into particular tracts within the CNS. A majority of these afferents depend on CNS-derived guidance cues and projections from the CNS to guide their way. However, not all of the nerves are established this way and at least one of the peripheral nerves is likely to be pioneered by sensillar sensory afferents. The distribution of particular antigens (such as the lan3–2 antigen) suggests the identity of molecules involved in homophilic adhesion along common pathways, whereas others (such as the lan4–2 and 3–6 antigens) are candidates for mediating specific pathway choices. In the second system, the myo-organizing Comb cell (C cell) projects multiple growth cones simultaneously along oblique trajectories not influenced by segmental or midline boundaries. Its parallel growth cones exhibit space-filling as well as directional growth and are guided by local cues to extend in discrete phases that are coordinated with the development of the environment. Both systems exhibit highly directed outgrowth orchestrated by a hierarchy of cues, establish patterns of neurites used to direct later migrating cells, and seem to be regulated temporally and spatially by interactions with the embryonic environment. These systems illustrate the strengths of examining neural development in vivo across several levels of analysis. © 1995 John Wiley & Sons, Inc.     

Neural projection patterns from homeotic tissue of Drosophila studied in bithorax mutants and mosaics.

The central projection patterns of sensory cells from the wing and haltere of Drosophila, as revealed by filling their axons with cobalt, consist of dorsal components arising from small campaniform sensilla and ventral components arising from large campaniform sensilla and from bristles. All of the bristles of the wing are innervated, some singly and some multiply. All three classes of sensilla are strongly represented on the wing, but the haltere carries primarily small campaniform sensilla and has a correspondingly minute ventral projection. In bithorax mutants in which the haltere is transformed into wing, ventral components are added to the projection pattern, while the dorsal components appear as if haltere tissue were still present. Thus, the three classes of receptors not only produce different projection patterns when they develop in their native mesothoracic segment, but also behave differently in the homeotic situation. Consequently, different developmental programs are inferred for each class. When somatic recombination clones of bithorax tissue are generated in phenotypically wild-type flies, they also produce ventral projections. However, these projections of mutant fibers into wild-type ganglia differ in certain details from the projections of mutant fibers into mutant ganglia. Thus, homeotic changes are inferred to occur in the CNS of mutant flies, but these are not required for the execution of those developmental instructions carried in the genome of large campaniform and bristle sensory cells which specify that their axons should grow ventrad in the CNS.     

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. © 1993 John Wiley & Sons, Inc.     

The olfactory bulb as an independent developmental domain

The olfactory system is a good model to study the mechanisms underlying guidance of growing axons to their appropriate targets. The formation of the olfactory bulb involves differentiation of several populations of cells and the initiation of the central projections, all under the temporal and spatial patterns of gene expression. Moreover, the nature of interactions between the olfactory epithelium, olfactory bulb and olfactory cortex at early developmental stages is currently of great interest. To explore these questions more fully, the present review aims to correlate recent data from different developmental studies, to gain insight into the mechanisms involved in the specification and development of the olfactory system. From our studies in the pax6 mutant mice (Sey(Neu)/Sey(Neu)), it was concluded that the initial establishment of the olfactory bulb central projections is able to proceed independently of the olfactory sensory axons from the olfactory epithelium. The challenge that now remains is to consider the validity of the olfactory bulb as an independent development domain. In the course of evaluating these ideas, we will review the orchestra of molecular cues involved in the formation of the projection from the OB to the olfactory cortex.     

Activity-dependent regulation of BRINP family genes

We previously identified a family of novel developmentally regulated genes: BRINP1, 2, and 3, which are predominantly and widely expressed in the CNS from earlier developmental stages to adulthood. In the present study, we investigated the activity-dependent regulation of BRINP expression in the CNS. Among the three BRINP genes, BRINP1-mRNA was specifically up-regulated in the dentate gyrus of mouse hippocampus by kainic acid treatment. In cultured hippocampal neurons, the induction of BRINP1-mRNA was also observed by the activation of glutamate receptors. Although BDNF-mRNA is up-regulated in a similar activity-dependent manner, BDNF itself did not induce BRINP1-mRNA. From these results, the physiological roles of the activity-dependent induction of BRINP1-mRNA are discussed.     

Getting into shape: optimal ligand gradients for axonal guidance

During neural development, neurons from downstream, presynaptic regions of the nervous system (such as the retina) send spatially patterned axonal projections to upstream, target regions (the tectum or superior colliculus). A servomechanism model has been proposed to explain the pattern and time-course of axonal growth between these two regions [Honda, H., 1998. Topographic mapping in the retinotectal projection by means of complementary ligand and receptor gradients: a computer simulation study. J. Theor. Biol., 192, 235-246]. Here, we show that a modification of this model incorporating a different criterion for axonal decision-making, called the local optimum rule, is guaranteed to converge to a topographic map under a wide range of conditions encountered during neural development. A theoretical investigation of these conditions leads to new hypotheses regarding map formation.     

Exploring consensus in 21st century projections of climatically suitable areas for African vertebrates

Africa is predicted to be highly vulnerable to 21st century climatic changes. Assessing the impacts of these changes on Africa's biodiversity is, however, plagued by uncertainties, and markedly different results can be obtained from alternative bioclimatic envelope models or future climate projections. Using an ensemble forecasting framework, we examine projections of future shifts in climatic suitability, and their methodological uncertainties, for over 2500 species of mammals, birds, amphibians and snakes in sub‐Saharan Africa. To summarize a priori the variability in the ensemble of 17 general circulation models, we introduce a consensus methodology that combines co‐varying models. Thus, we quantify and map the relative contribution to uncertainty of seven bioclimatic envelope models, three multi‐model climate projections and three emissions scenarios, and explore the resulting variability in species turnover estimates. We show that bioclimatic envelope models contribute most to variability, particularly in projected novel climatic conditions over Sahelian and southern Saharan Africa. To summarize agreements among projections from the bioclimatic envelope models we compare five consensus methodologies, which generally increase or retain projection accuracy and provide consistent estimates of species turnover. Variability from emissions scenarios increases towards late‐century and affects southern regions of high species turnover centred in arid Namibia. Twofold differences in median species turnover across the study area emerge among alternative climate projections and emissions scenarios. Our ensemble of projections underscores the potential bias when using a single algorithm or climate projection for Africa, and provides a cautious first approximation of the potential exposure of sub‐Saharan African vertebrates to climatic changes. The future use and further development of bioclimatic envelope modelling will hinge on the interpretation of results in the light of methodological as well as biological uncertainties. Here, we provide a framework to address methodological uncertainties and contextualize results.