Research report
Long term depression is expressed during postnatal development in rat visual cortex: a role for visual experience

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Abstract

Long term forms of synaptic plasticity and in particular LTD/LTP are both present in the mammalian visual cortex. However, while LTP is not inducible in adulthood LTD can be elicited in the mature brain, but its developmental pattern is unknown. Aim of this work was to investigate whether LTD is expressed during postnatal development and if it is modulated by visual experience. To investigate these points we have used rat primary visual cortex slices taken at different stages of functional maturation process, i.e., postnatal day 17 (P17), P23 and P30–35. LTD was assessed by measuring the amplitude of extracellular field potentials recorded in cortical layers 2/3 and elicited by low frequency stimulation to the white matter. LTD was expressed at all ages investigated without significant differences between age groups. These data indicate that LTD developmental expression is not temporally related with the period of functional maturation of rat visual cortex. Dark rearing from birth to P23 resulted in a reduction of LTD amplitude while light deprivation from P17 to P30 did not affect LTD expression in comparison to age matched control values. We suggest that light imprinting is essential for a normal LTD expression during postnatal development.

Introduction

The functional maturation of mammalian visual cortex occurs during well described time periods of postnatal development. For example in the rat, as well as in other mammals, the functional properties of visual cortical neurons are immature soon after eye opening reaching progressively adult characteristics by P30–P40 7, 18. The underlying developmental processes are under the control of visual experience. Indeed, in light deprived animals from birth [7]the functional properties of visual cortical neurons remain immature. Synaptic plasticity has been implicated in the mechanisms responsible for the shaping of cortical circuitry occurring during postnatal development and for leading the visual system to be functionally competent 9, 16. Long term potentiation (LTP) and long term depression (LTD) are considered expression of synaptic plasticity and both have been extensively investigated in many different cortical areas 2, 3, 5, 8, 9, including the visual cortex. In the rat visual cortex both LTP and LTD can be induced and both forms of synaptic plasticity have been reported to be NMDA dependent 1, 10. As far as LTP is concerned its pattern of expression mostly overlaps visual functional maturation [11]and is no longer inducible in the adult animal. Visual experience controls the period during which LTP is present as shown by the observation that LTP can be elicited in adult rats which have been visually deprived, i.e., beyond the end of its natural period of expression 11, 12. In contrast, it has been shown that homosynaptic LTD is normally present in the adult rat [10]. However, whether LTD is present and/or its expression changes during the rat visual cortex maturation remains unknown. The goal of this paper is therefore to describe the developmental pattern of LTD expression after eye opening in slices of rats containing the primary visual cortex. To this aim we have recorded field potentials in layer II–III evoked by stimulation of the white matter. LTD was elicited at different postnatal ages (postnatal day 17, P17, P23, P30–35) using a low frequency stimulation protocol. These postnatal ages were chosen because they correspond to different stages of the period of functional maturation of rat visual cortex 7, 18. Furthermore, in order to examine the role of visual experience on LTD expression, similar experiments were repeated in slices taken from rats deprived of light either from birth to P23 or from P17 to P30, i.e., after eye opening.

Section snippets

Visual cortex slices preparation and electrophysiological recordings

Albino rats were deeply anesthetized with urethane and then decapitated. The brain was rapidly removed and visual cortex coronal slices (400 μm) were prepared and superfused in a submerged recording chamber at 33°C with artificial cerebro spinal fluid (ACSF) at a rate of 4 ml/min. An average of 2/3 slices per animal (the 2nd and 3rd slice from the occipital pole of each hemisphere) was used for experimental purpose. The ACSF was gassed with 95% O2 and 5% CO2 and had the following composition in

LTD expression during development

LTD was induced by low frequency stimulation (LFS, 900 pulses at 1 Hz) to the white matter and recorded extracellularly in layers 2/3 of primary visual cortex slices. LTD was reliably elicited at all ages investigated (Fig. 1), i.e., at P17 (2/3 days after eye opening, when the functional properties of visual cortical neurons are immature), at P23–P24 (intermediate stage in the functional maturation period) and at P30–35 which can be considered the end of the functional maturation period 7, 18

Discussion

The data reported in this paper indicate that LTD in layer 2/3 of the rat primary visual cortex elicited by stimulation of the white matter is present throughout postnatal development. In particular, LTD was found to be expressed soon after eye opening (P17) as well as during the functional maturation of the visual cortex. At P17 the functional circuitry of rat visual cortex is still undeveloped and properties of visual cortical neurons such as ocular dominance distribution, orientation

Acknowledgements

We thank Prof. E. Cherubini for critically reading the manuscript and for helpful suggestions. We thank W.W. Anderson for the on-line data acquisition and analysis program. Supported in part by a grant from the Human Frontier Science Program organization (RG 93/93) and from grant EC PSS* 0859.

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