Eye-opening represents a turning point in the function of the visual cortex. Before eye-opening, the visual cortex is largely devoid of sensory inputs and neuronal activities are generated intrinsically. After eye-opening, the cortex starts to integrate visual information. Here we used in vivo two-photon calcium imaging to explore the developmental changes of the mouse visual cortex by analyzing the ongoing spontaneous activity. We found that before eye-opening, the activity of layer 2/3 neurons consists predominantly of slow wave oscillations. These waves were first detected at postnatal day 8 (P8). Their initial very low frequency (0.01 Hz) gradually increased during development to Ϸ0.5 Hz in adults. Before eye-opening, a large fraction of neurons (>75%) was active during each wave. One day after eye-opening, this dense mode of recruitment changed to a sparse mode with only 36% of active neurons per wave. This was followed by a progressive decrease during the following weeks, reaching 12% of active neurons per wave in adults. The possible role of visual experience for this process of sparsification was investigated by analyzing darkreared mice. We found that sparsification also occurred in these mice, but that the switch from a dense to a sparse activity pattern was delayed by 3-4 days as compared with normally-reared mice. These results reveal a modulatory contribution of visual experience during the first days after eye-opening, but an overall dominating role of intrinsic factors. We propose that the transformation in network activity from dense to sparse is a prerequisite for the changed cortical function at eye-opening.calcium waves ͉ cortical development ͉ mouse ͉ two-photon imaging ͉ up-down states
Previous studies of the ferret visual cortex indicate that the development of direction selectivity requires visual experience. Here, we used two-photon calcium imaging to study the development of direction selectivity in layer 2/3 neurons of the mouse visual cortex in vivo. Surprisingly, just after eye opening nearly all orientation-selective neurons were also direction selective. During later development, the number of neurons responding to drifting gratings increased in parallel with the fraction of neurons that were orientation, but not direction, selective. Our experiments demonstrate that direction selectivity develops normally in dark-reared mice, indicating that the early development of direction selectivity is independent of visual experience. Furthermore, remarkable functional similarities exist between the development of direction selectivity in cortical neurons and the previously reported development of direction selectivity in the mouse retina. Together, these findings provide strong evidence that the development of orientation and direction selectivity in the mouse brain is distinctly different from that in ferrets.
The zebrafish larva is a powerful model for the analysis of behaviour and the underlying neuronal network activity during early stages of development. Here we employ a new approach of "in vivo" Ca(2+) imaging in this preparation. We demonstrate that bolus injection of membrane-permeable Ca(2+) indicator dyes into the spinal cord of zebrafish larvae results in rapid staining of essentially the entire spinal cord. Using two-photon imaging, we could monitor Ca(2+) signals simultaneously from a large population of spinal neurons with single-cell resolution. To test the method, Ca(2+) transients were produced by iontophoretic application of glutamate and, as observed for the first time in a living preparation, of GABA or glycine. Glycine-evoked Ca(2+) transients were blocked by the application of strychnine. Sensory stimuli that trigger escape reflexes in mobile zebrafish evoked Ca(2+) transients in distinct neurons of the spinal network. Moreover, long-term recordings revealed spontaneous Ca(2+) transients in individual spinal neurons. Frequently, this activity occurred synchronously among many neurons in the network. In conclusion, the new approach permits a reliable analysis with single-cell resolution of the functional organisation of developing neuronal networks.
Cellular responses to GABAA receptor activation were studied in developing cerebellar Purkinje neurones (PNs) in brain slices obtained from 2‐ to 22‐day‐old rats. Two‐photon fluorescence imaging of fura‐2‐loaded cells and perforated‐patch recordings were used to monitor intracellular Ca2+ transients and to estimate the reversal potential of GABA‐induced currents, respectively. During the 1st postnatal week, focal application of GABA or the GABAA receptor agonist muscimol evoked transient increases in [Ca2+]i in immature PNs. These Ca2+ transients were reversibly abolished by the GABAA receptor antagonist bicuculline and by Ni2+, a blocker of voltage‐activated Ca2+ channels. Perforated‐patch recordings were used to measure the reversal potential of GABA‐evoked currents (EGABA) at different stages of development. It was found that EGABA was about −44 mV at postnatal day 3 (P3), it shifted to gradually more negative values during the 1st week and finally equilibrated at −87 mV at around the end of the 2nd postnatal week. This transition was well described by a sigmoidal function. The largest change in EGABA was −7 mV day−1, which occurred at around P6. The transition in GABA‐mediated signalling occurs during a period in which striking changes in PN morphology and synaptic connectivity are known to take place. Since such changes were shown to be Ca2+ dependent, we propose that GABA‐evoked Ca2+ signalling is one of the critical determinants for the normal development of cerebellar PNs.
Two-photon laser scanning microscopy has been used successfully for imaging activity-dependent changes of intracellular calcium and sodium levels. Here we introduce a simple technique for two-photon chloride imaging in intact neurons. It involves the use of the membrane-permeable Cl(-) indicator dye MQAE [N-(6-methoxyquinolyl) acetoethyl ester]. Brief incubation with MQAE produced a robust loading of cells in slices from various brain regions including hippocampus, cortex and cerebellum. In contrast to conventional fluorescence measurements using MQAE, two-photon imaging was not affected in a major way by dye bleaching and phototoxic damage. Instead, it allowed prolonged recordings of time-resolved changes in intracellular chloride concentration in somata and dendrites. As an example of an application we imaged GABA-mediated Cl(-) transients in pyramidal cells of cortical and hippocampal slices as well as in cerebellar Purkinje neurons. By combining Cl(-) imaging with the gramicidin-based perforated-patch-clamp technique we showed that changes in MQAE fluorescence are proportional to the magnitudes of GABA-evoked transmembrane Cl(-) fluxes. Thus, MQAE-based two-photon microscopy promises to be a valuable technique for many applications requiring chloride imaging in single cells.
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