Electrical activity regulates dendritic reorganization in ganglion cells after neonatal retinal lesion in the cat

Long-term gene therapy causes transgene-specific changes in the morphology of regenerating retinal ganglion cells

Recombinant adeno-associated viral (rAAV) vectors can be used to introduce neurotrophic genes into injured CNS neurons, promoting survival and axonal regeneration. Gene therapy holds much promise for the treatment of neurotrauma and neurodegenerative diseases; however, neurotrophic factors are known to alter dendritic architecture, and thus we set out to determine whether such transgenes also change the morphology of transduced neurons. We compared changes in dendritic morphology of regenerating adult rat retinal ganglion cells (RGCs) after long-term transduction with rAAV2 encoding: (i) green fluorescent protein (GFP), or (ii) bi-cistronic vectors encoding GFP and ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF) or growth-associated protein-43 (GAP43). To enhance regeneration, rats received an autologous peripheral nerve graft onto the cut optic nerve of each rAAV2 injected eye. After 5–8 months, RGCs with regenerated axons were retrogradely labeled with fluorogold (FG). Live retinal wholemounts were prepared and GFP positive (transduced) or GFP negative (non-transduced) RGCs injected iontophoretically with 2% lucifer yellow. Dendritic morphology was analyzed using Neurolucida software. Significant changes in dendritic architecture were found, in both transduced and non-transduced populations. Multivariate analysis revealed that transgenic BDNF increased dendritic field area whereas GAP43 increased dendritic complexity. CNTF decreased complexity but only in a subset of RGCs. Sholl analysis showed changes in dendritic branching in rAAV2-BDNF-GFP and rAAV2-CNTF-GFP groups and the proportion of FG positive RGCs with aberrant morphology tripled in these groups compared to controls. RGCs in all transgene groups displayed abnormal stratification. Thus in addition to promoting cell survival and axonal regeneration, vector-mediated expression of neurotrophic factors has measurable, gene-specific effects on the morphology of injured adult neurons. Such changes will likely alter the functional properties of neurons and may need to be considered when designing vector-based protocols for the treatment of neurotrauma and neurodegeneration.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Cellular positioning and dendritic field size of cholinergic amacrine cells are impervious to early ablation of neighboring cells in the mouse retina

We have examined the role of neighbor relationships between cholinergic amacrine cells upon their positioning and dendritic field size by producing partial ablations of this population of cells during early development. We first determined the effectiveness of L-glutamate as an excitotoxin for ablating cholinergic amacrine cells in the developing mouse retina. Subcutaneous injections (4 mg/g) made on P-3 and thereafter were found to produce a near-complete elimination, while injections at P-2 were ineffective. Lower doses on P-3 produced only partial reductions, and were subsequently used to examine the effect of partial ablation upon mosaic organization and dendritic growth of the remaining cells. Four different Voronoi-based measures of mosaic geometry were examined in L-glutamate-treated and normal (saline-treated) retinas. Partial depletions of around 40% produced cholinergic mosaics that, when scaled for density, approximated the mosaic geometry of the normal retina. Separate comparisons simulating a 40% random deletion of the normal retina produced mosaics that were no different from those experimentally depleted retinas. Consequently, no evidence was found for positional regulation in the absence of normal neighbor relationships. Single cells in the ganglion cell layer were intracellularly filled with Lucifer Yellow to examine the morphology and dendritic field extent following partial ablation of the cholinergic amacrine cells. No discernable effect was found on their starburst morphology, and total dendritic field area, number of primary dendrites, and branch frequency were not significantly different. Cholinergic amacrine cells normally increase their dendritic field area after P-3 in excess of retinal expansion; despite this, the present results show that this growth is not controlled by the density of neighboring processes.

Compensatory dendritic growth of CA1 pyramidal cells following growth impairment in the neonatal period

In the neonatal rat, inhibition of hippocampal neural activity in vivo by tetanus toxin results in a severe growth impairment of the basal dendrites of CA1 pyramidal cells. Here we tested whether this early growth impairment results in a permanent reduction of the basal dendritic tree or whether recovery processes are recruited later in development when synaptic activity has fully recovered. Quantitative analysis of dendritic parameters and spine density from reconstructed CA1 pyramidal cells showed that young adult CA1 pyramidal (postnatal day 31-34) cells that were exposed to activity deprivation in the neonatal period were almost indistinguishable from control cells of the same age. These results suggest that the early hippocampal activity controls the growth rate but is not necessary for the generation of an adult normal basal dendritic tree.

Metamorphic changes in GABA immunoreactivity in the brainstem of the bullfrog, Rana catesbeiana

We examined immunoreactivity for gamma-aminobutyric acid (GABA) in auditory and vestibular brainstem nuclei of the bullfrog, Rana catesbeiana, across metamorphosis, a developmental period featuring significant anatomical and functional remodeling of the nervous system. In the early larval period, GABA-immunoreactive cell somata were visible in the vestibular nucleus complex and the torus semicircularis, as well as in the spinal cord, cerebellum and optic tectum. Fiber bundles such as the medial longitudinal fasciculus and the lemnsical pathways also exhibited intense label at these early stages. In contrast, only diffuse neuropil label was visible in the dorsolateral nucleus and the superior olivary nucleus at the same stages. This diffuse immunoreactivity became progressively more reduced over larval development, and stained somata were visible in these medullary nuclei by metamorphic climax stages. In the torus semicircularis, the numbers of labeled somata in both the developing laminar and principal nuclei increased over metamorphic development, and became progressively more organized into distinct layers. The adult pattern of GABA-like immunoreactivity in the auditory brainstem was reached by metamorphic climax stages, coincident with the maturation of the opercularis system, and preceding the final development of the external tympanum and the tympanic conduction pathway. The relatively earlier maturation of vestibular, compared to auditory, areas in the medulla might reflect the behavioral importance of vestibular-mediated motor reactions during tadpole life. The distribution of GABA in auditory brainstem nuclei in both developing and adult frogs is comparable to that observed in mammals and birds.

Development of retinal ganglion cell structure and function

In this review, we summarize the main stages of structural and functional development of retinal ganglion cells (RGCs). We first consider the various mechanisms that are involved in restructuring of dendritic trees. To date, many mechanisms have been implicated including target-dependent factors, interactions from neighboring RGCs, and afferent signaling. We also review recent evidence showing how rapidly such dendritic remodeling might occur, along with the intracellular signaling pathways underlying these rearrangements. Concurrent with such structural changes, the functional responses of RGCs also alter during maturation, from sub-threshold firing to reliable spiking patterns. Here we consider the development of intrinsic membrane properties and how they might contribute to the spontaneous firing patterns observed before the onset of vision. We then review the mechanisms by which this spontaneous activity becomes correlated across neighboring RGCs to form waves of activity. Finally, the relative importance of spontaneous versus light-evoked activity is discussed in relation to the emergence of mature receptive field properties.