Modeling Activity and Target-Dependent Developmental Cell Death of Mouse Retinal Ganglion Cells Ex Vivo

Programmed cell death is widespread during the development of the central nervous system and serves multiple purposes including the establishment of neural connections. In the mouse retina a substantial reduction of retinal ganglion cells (RGCs) occurs during the first postnatal week, coinciding with the formation of retinotopic maps in the superior colliculus (SC). We previously established a retino-collicular culture preparation which recapitulates the progressive topographic ordering of RGC projections during early post-natal life. Here, we questioned whether this model could also be suitable to examine the mechanisms underlying developmental cell death of RGCs. Brn3a was used as a marker of the RGCs. A developmental decline in the number of Brn3a-immunolabelled neurons was found in the retinal explant with a timing that paralleled that observed in vivo. In contrast, the density of photoreceptors or of starburst amacrine cells increased, mimicking the evolution of these cell populations in vivo. Blockade of neural activity with tetrodotoxin increased the number of surviving Brn3a-labelled neurons in the retinal explant, as did the increase in target availability when one retinal explant was confronted with 2 or 4 collicular slices. Thus, this ex vivo model reproduces the developmental reduction of RGCs and recapitulates its regulation by neural activity and target availability. It therefore offers a simple way to analyze developmental cell death in this classic system. Using this model, we show that ephrin-A signaling does not participate to the regulation of the Brn3a population size in the retina, indicating that eprhin-A-mediated elimination of exuberant projections does not involve developmental cell death

Cholinergic Activation of M2 Receptors Leads to Context-Dependent Modulation of Feedforward Inhibition in the Visual Thalamus

The temporal dynamics of inhibition within a neural network is a crucial determinant of information processing. Here, the authors describe in the visual thalamus how neuromodulation governs the magnitude and time course of inhibition in an input-dependent way

Relationship between lateral inhibitory connections and the topography of the orientation map in cat visual cortex

The functional and structural topography of lateral inhibitory connections was investigated in visual cortical area 18 using a combination of optical imaging and anatomical tracing techniques in the same tissue. Orientation maps were obtained by recording intrinsic signals in regions of 8.4-19 mm(2). To reveal the inhibitory connections provided by large basket cells, biocytin was iontophoretically injected at identified orientation sites guided by the pattern of surface blood vessels. The axonal and dendritic fields of two retrogradely labelled large basket cells were reconstructed in layer III. Their axonal fields extended up to 1360 mu m from the parent somata. In addition to single basket cells, the population of labelled basket cell axons was also studied. For this analysis anterogradely labelled basket axons running horizontally over 460-1280 mu m from the core of an injection site in layer III were taken into account. The distribution of large basket cell terminals according to orientation preferences of their target regions was quantitatively assessed. Using the same spatial resolution as the orientation map, a frequency distribution of basket cell terminals dependent on orientation specificity could be derived. For individual basket cells, the results showed that, on average, 43% of the terminals provided input to sites showing similar orientation preferences (+/-30 degrees) to those of the parent somata. About 35% of the terminals were directed to sites representing oblique-orientation [+/-(30-60)degrees], and 22% of them terminated at cross-orientation sites [+/-(60-90)degrees]. Furthermore, the possible impact of large basket cells on target cells at different distances and orientation preferences was estimated by comparing the occurrence of orientation preferences with the occurrence of basket terminals on the distance scale. It was found that a basket cell could elicit iso-orientation inhibition with a high impact between 100-400 and 800-1200 mu m, strong cross-orientation inhibition at similar to 400-800 mu m, and oblique-orientation inhibition between 300-500 and 700-900 mu m from the parent soma. The non-isotropic topography of large basket axons suggests a complex function for this cell class, possibly including inhibition related to orientation and direction selectivity depending on the location of the target cells and possible target selectivity

Excitatory and inhibitory neurotransmitter release at synapses in the visual cortex of heterozygous knock-out mice for brain-derived neurotrophic factor

Brain derived neurotrophic factor (BDNF) is a member of the nerve growth factor family (NGF), and involved in axonal and dendritic growth as well as synapse formation. BDNF is also reported to contribute to fast synaptic transmission and plasticity. The effect of BDNF on synaptic transmission seems to be region specific. In this study we further characterize the influence of BDNF on synaptic transmission in the mammalian visual cortex. We used 21-27 days old mice, which partially lack the BDNF coding gene (Heterozygous mice, HT, n=19). Age matched wild type littermates served as controls (n=20). Spontaneous postsynaptic currents were recorded from layer II/III pyramidal neurons of the visual cortex by using the whole-cell patch clamp method. We analyzed miniature postsynaptic excitatory currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSPs). Furthermore, the paired pulse modulation (PPM) ratio was calculated after electrical stimulation of afferent fibers in cortical layer IV. The frequency of both, mEPSCs and mIPSCs were found to be significantly reduced (p=0.019; p=0.0001, respectively) in the visual cortex of HT mice (n=9) when compared to results from wild type controls (n=9). The amplitude of mIPSCs was significantly decreased in HT (p=0.007, n=8) compared to recordings in slices from wild type mice (n=17). However, the amplitude of mEPSCs did not differ between HT and wild type animals. The paired pulse interaction ratio was significantly higher at inter stimulus intervals of 35 ms and 50 ms in HT animals compared to wild type controls (p=0.03 and p=0.04, respectively). These findings indicate that reduced level of BDNF effects both, excitatory and inhibitory neurotransmission in the neocortex. Furthermore, the reduced mEPSC frequency and increased PPF ratio in HT animals suggest a decreased presynaptic glutamate release. This study was supported by the IGSN and DFG (SBF 509, C4)