Eliminating afferent impulse activity does not alter the dendritic branching of the amphibian Mauthner cell

Presynaptic contact and activity opposingly regulate postsynaptic dendrite outgrowth

The organization of neural circuits determines nervous system function. Variability can arise during neural circuit development (e.g. neurite morphology, axon/dendrite position). To ensure robust nervous system function, mechanisms must exist to accommodate variation in neurite positioning during circuit formation. Previously, we developed a model system in the Drosophila ventral nerve cord to conditionally induce positional variability of a proprioceptive sensory axon terminal, and used this model to show that when we altered the presynaptic position of the sensory neuron, its major postsynaptic interneuron partner modified its dendritic arbor to match the presynaptic contact, resulting in functional synaptic input (Sales et al., 2019). Here, we investigate the cellular mechanisms by which the interneuron dendrites detect and match variation in presynaptic partner location and input strength. We manipulate the presynaptic sensory neuron by (a) ablation; (b) silencing or activation; or (c) altering its location in the neuropil. From these experiments we conclude that there are two opposing mechanisms used to establish functional connectivity in the face of presynaptic variability: presynaptic contact stimulates dendrite outgrowth locally, whereas presynaptic activity inhibits postsynaptic dendrite outgrowth globally. These mechanisms are only active during an early larval critical period for structural plasticity. Collectively, our data provide new insights into dendrite development, identifying mechanisms that allow dendrites to flexibly respond to developmental variability in presynaptic location and input strength.

Ear manipulations reveal a critical period for survival and dendritic development at the single-cell level in Mauthner neurons

Second-order sensory neurons are dependent on afferents from the sense organs during a critical period in development for their survival and differentiation. Past research has mostly focused on whole populations of neurons, hampering progress in understanding the mechanisms underlying these critical phases. To move toward a better understanding of the molecular and cellular basis of afferent-dependent neuronal development, we developed a new model to study the effects of ear removal on a single identifiable cell in the hindbrain of a frog, the Mauthner cell. Ear extirpation at various stages of Xenopus laevis development defines a critical period of progressively-reduced dependency of Mauthner cell survival/differentiation on the ear afferents. Furthermore, ear removal results in a progressively decreased reduction in the number of dendritic branches. Conversely, addition of an ear results in an increase in the number of dendritic branches. These results suggest that the duration of innervation and the number of inner ear afferents play a quantitative role in Mauthner cell survival/differentiation, including dendritic development.

Structural homeostasis: compensatory adjustments of dendritic arbor geometry in response to variations of synaptic input

As the nervous system develops, there is an inherent variability in the connections formed between differentiating neurons. Despite this variability, neural circuits form that are functional and remarkably robust. One way in which neurons deal with variability in their inputs is through compensatory, homeostatic changes in their electrical properties. Here, we show that neurons also make compensatory adjustments to their structure. We analysed the development of dendrites on an identified central neuron (aCC) in the late Drosophila embryo at the stage when it receives its first connections and first becomes electrically active. At the same time, we charted the distribution of presynaptic sites on the developing postsynaptic arbor. Genetic manipulations of the presynaptic partners demonstrate that the postsynaptic dendritic arbor adjusts its growth to compensate for changes in the activity and density of synaptic sites. Blocking the synthesis or evoked release of presynaptic neurotransmitter results in greater dendritic extension. Conversely, an increase in the density of presynaptic release sites induces a reduction in the extent of the dendritic arbor. These growth adjustments occur locally in the arbor and are the result of the promotion or inhibition of growth of neurites in the proximity of presynaptic sites. We provide evidence that suggest a role for the postsynaptic activity state of protein kinase A in mediating this structural adjustment, which modifies dendritic growth in response to synaptic activity. These findings suggest that the dendritic arbor, at least during early stages of connectivity, behaves as a homeostatic device that adjusts its size and geometry to the level and the distribution of input received. The growing arbor thus counterbalances naturally occurring variations in synaptic density and activity so as to ensure that an appropriate level of input is achieved.

As the nervous system develops, an intricate web of connections forms between nerve cells, leading to the assembly of signalling networks that are capable of complex computations. However, the number and strength of connections formed between nerve cells varies. We ask how nerve cells deal with this variability so that the circuits they form are nicely matched to the functions they perform. Nerve cells are known to adjust their sensitivity to compensate for changes in the strengths of inputs they receive from other cells. In this study, we have identified a structural counterpart to this compensatory mechanism, and find that developing nerve cells respond to variation in the number of connections they receive by adjusting the size of their receiving structures (known as dendrites). Working with the same nerve cell in different embryos, we show that this cell reduces the size of its dendrites as the number of connections increases while allowing its dendrites to grow more extensively if inputs are reduced. These findings suggest that, at least during the early stages of wiring the nervous system, nerve cells regulate the growth of their dendrites, to compensate for variability and attain an optimal number of connections.

Structural homeostasis is defined as follows: developing neurons modify the growth of their dendrites to compensate for changes in synaptic density. This structural adjustment is mediated, at least in part, by postsynaptic PKA signalling.

Otolith endorgan input to the Mauthner neuron in the goldfish

The Mauthner (M-) cell of the goldfish, Carassius auratus, triggers the rapid escape response of the fish in response to various stimuli, including visual and auditory. The large size and accessibility of the M-cell make it an ideal model system for the study of synaptic transmission, membrane properties, and sensory-motor gating. Although physiological recordings have suggested that afferents from all three of the inner ear endorgans (the saccule, lagena, and utricle) synapse directly on the ipsilateral M-cell, the specific contacts and anatomical distributions of these inputs along the M-cell lateral dendrite remain unknown. We traced specific branches of the auditory (VIIIth) nerve from the three otolith organs of the fish inner ear to the M-cell. The goldfish sacculus gives rise to the vast majority of inputs that contact a large portion of the M-cell lateral dendrite, and these inputs vary greatly in size. In contrast to the ubiquitous distribution of saccular inputs, those from the lagena are segregated to distal regions of the M-cell and synapse on the distal dorsal branch of the lateral dendrite. Similarly, inputs from the utricle are also segregated to distal regions, synapsing on the ventral branch of the lateral dendrite. These results demonstrate that nerves from all three endorgans contact the M-cell, with input-specific segregation of synapses along the M-cell lateral dendrite.

Mechanisms of dendritic maturation

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 the developing nervous systems of animals ranging from insects to mammals. The results of such investigations have shown that the contribution of intrinsic and extrinsic factors and activity, however, appear to be weighted differentially in different types of neurons, in different brain areas, and especially in different species. Moreover, it appears that dozens of molecules have been found to regulate dendritic maturation, but it is almost certain that each molecule plays only a specific role in this formidable cooperative venture. This article reviews our current knowledge and understanding of the role of various factors in the establishment of the architecture of mature dendritic trees.

Afferent input regulates the formation of distal dendritic branches

During postembryonic development, the dendritic arbors of neurons grow to accommodate new incoming synaptic inputs. Our goal was to examine which features of dendritic architecture of postsynaptic interneurons are regulated by these synaptic inputs. To address this question, we took advantage of the cockroach cercal system where the morphology of the sensory giant interneurons (GIs) is uniquely identified and, therefore, amenable to quantitative analysis. We analyzed the three-dimensional architecture of chronically deafferented vs. normally developed dendritic trees of a specific identified GI, namely GI2. GI2 shows five prominent dendrites, four of which were significantly altered after deafferentation. De-afferentation induced an average of 55% decrease in metric measures (number of branch points, total length, and total surface area) on the entire dendritic tree. Sholl and branch order analysis showed a decrease in the most distal and higher order branches. We suggest that afferent input plays a specific role in shaping the morphology of dendritic trees by regulating the formation or maintenance of high-order distal branches.

Glutamate receptor activity is required for normal development of tectal cell dendrites in vivo

Glutamatergic retinotectal inputs mediated principally by NMDA receptors can be recorded from optic tectal neurons early during their morphological development in Xenopus tadpoles. As tectal cell dendrites elaborate, retinotectal synaptic responses acquire an AMPA receptor-mediated synaptic component, in addition to the NMDA component. Here, we tested whether glutamatergic activity was required for the elaboration of dendritic arbors in Xenopus optic tectal neurons. In vivo time-lapse imaging of single DiI-labeled neurons shows that the NMDA receptor antagonist APV (100 microM) blocked the early development of the tectal cell dendritic arbor, whereas the AMPA receptor antagonist CNQX (20 microM) or the sodium channel blocker TTX (1 microM) did not. The decreased dendritic development is attributable to failure to add new branches and extend preexisting branches. These observations indicate that NMDA-type glutamatergic activity promotes the initial development of the dendritic arbor. At later stages of tectal neuron development when AMPA receptor-mediated synaptic transmission is strong, both APV and CNQX decrease dendritic arbor branch length, consistent with a role for glutamatergic synaptic transmission in maintaining dendritic arbor structure. These results indicate that AMPA and NMDA receptors can differentially influence dendritic growth at different stages of neuronal development, in correlation with changes in the relative contribution of the receptor subtype to synaptic transmission.

Dark-rearing fails to affect the basal dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex

The development of the cat's visual cortex is incomplete at birth and is influenced by the cat's early visual experience. We have previously demonstrated that the basal dendritic fields of layer 3 pyramidal cells grow substantially during the first 5 weeks after birth and that stripe-rearing affects their orientation. In this paper we determined the effects on these dendritic fields of visual deprivation (dark-rearing) during the first 3 months of life. The visual cortices of both normally reared and dark-reared cats were impregnated by the Golgi method, sectioned in the tangential plane and counterstained. The basal dendritic fields of completely impregnated pyramidal cells from layer 3 were drawn with the aid of a camera lucida, and compared in terms of number and length of primary dendrites, branching, size, elongation, and distribution of dendritic field orientations. Surprisingly, we observed no significant differences in any parameter measured. Thus, although stripe-rearing can specifically alter the orientation of the dendritic fields of the layer 3 pyramidal cells, and dark-rearing has been shown by others to alter the size of layer 4 stellate cells, dark-rearing failed to affect the dendritic fields of layer 3 pyramidal cells.

Development of locomotion in the rat: the significance of early movements

The development of the nervous system is determined by an interaction between genetic and epigenetic factors. We investigated the possible role of proprioceptive afferent input in the development of locomotion in the rat. Kinematic analysis of locomotion in normal rats reveals a marked transition from immature overground locomotion into the adult pattern around the 15th postnatal day. Around this age, the timing of EMG activity in the tibialis anterior and the gastrocnemius muscle in the hindpaw gains accuracy and the intensity of the EMG increases. In the soleus muscle we observed an increased regularity in the shape of individual motor unit potentials. Neuroanatomical research revealed the occurrence of dendrite bundles in the motoneuronal pool of the soleus muscle pool from the 16th day as the result of a reorganisation. Immobilisation of one hindlimb in an extended position by casts around the leg from the first till the 20th postnatal day does not interfere with the development of dendrite bundles in the soleus motoneuronal pool. However, we demonstrated long lasting abnormalities in the timing of the EMG activation patterns in the gastrocnemius and the tibialis anterior muscles. It is conceivable that the experimental interference with afferent feed back is the causative factor of these functional abnormalities.

Structural and functional consequences of neonatal deafferentation in the superficial layers of the hamster's superior colliculus

Intracellular recording and horseradish peroxidase (HRP) injection techniques were used to evaluate the effects of neonatal enucleation upon the structural and functional properties of cells in the superficial retinorecipient laminae of the hamster's superior colliculus (SC). The physiological recordings confirmed previous results that normally visual superficial layer neurons develop somatosensory receptive fields in the enucleated animals. This study further showed that all of the physiological subclasses of somatosensory neurons normally encountered in the deep layers were present in the superficial laminae. With the exception of marginal cells, all of the morphological classes of neurons in the superficial SC laminae of sighted hamsters (narrowfield vertical cells, widefield vertical cells, stellate cells, horizontal cells, and giant stellate cells) were recovered from the blinded animals. Quantitative comparison of neurons within a given morphological class demonstrated only slight differences between cells from blind and sighted hamsters. However, there was a significant reduction in the percentage of neurons with dorsally directed dendrites in the neonatally enucleated animals. Additional experiments with the Golgi technique also demonstrated that neonatal enucleation altered the distribution of morphological cell types in the superficial SC laminae. These results suggest that enucleation in the hamster may result in relative reductions in specific cell types in the superficial SC laminae rather than dendritic changes in all of the cell classes present in these layers.