Quantitative study of the development of neurons and synapses in rats reared in the dark during early postnatal life. 1. Superior colliculus
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Early dark rearing in rats did not alter synapse-to-neuron ratios at 30 days. However, by 65 days, dark-reared rats showed fewer neurons in the superior colliculi compared to controls.
Area of Science:
- Neuroscience
- Developmental Biology
- Visual System Research
Background:
- Early postnatal visual experience is crucial for the development of the visual system's structure and function.
- Dark rearing during critical developmental periods can lead to significant alterations in neural circuits.
Purpose of the Study:
- To investigate the impact of early-life dark rearing on synapse-to-neuron ratios in the superior colliculi of male rats.
- To assess the effects of subsequent rehabilitation in controlled lighting conditions on these neural parameters.
Main Methods:
- Male rats were reared in darkness from birth to 30 days, with some undergoing a 35-day rehabilitation period.
- Synapse-to-neuron ratios were determined by estimating numerical densities of neurons and synapses using the 'disector' method at light and electron microscopic levels.
- Neuronal nuclei and paramembranous densities served as counting units for neurons and synapses, respectively.
Main Results:
- No significant differences in neuron density, synapse density, or synapse-to-neuron ratios were observed between dark-reared and control rats at 30 days of age.
- At 65 days, rats with prior dark rearing exhibited a significantly lower numerical density of neurons compared to light-reared controls.
- Statistical analysis revealed significant effects of age on neuron and synapse densities, but no significant main effects of lighting conditions or interactions.
Conclusions:
- Early dark rearing does not immediately affect synapse-to-neuron ratios in the superior colliculi but leads to a reduced neuronal density by later developmental stages.
- Rehabilitation in standard light conditions did not fully rescue the observed reduction in neuronal density in adulthood.
- These findings highlight the long-term impact of early visual deprivation on neuronal development within the superior colliculi.
Abstract:
Rearing animals in dark conditions during early postnatal life has been shown to affect both the morphology and the normal functioning of the visual system. We have investigated the effects on the synapse-to-neuron ratios in the superior colliculi of rearing male rats in the dark from birth until 30 days of age, followed in some cases by a 35 day period of rehabilitation in control lighting conditions. Control lighting conditions consisted of a room on a 12 hour light/12 hour dark cycle. Synapse-to-neuron ratios were calculated from estimates of the numerical densities of neurons and synapses. These estimates were made using the 'disector' method at the light and electron microscopical levels. Neuronal nuclei were used as the counting unit for neurons and paramembranous densities for synapses. There were no significant differences in the numerical densities of neurons, synapses or synapse-to-neuron ratios between dark-reared and control rats at 30 days of age. Sixty five days old rats, previously raised in the dark, had a significantly smaller numerical density of neurons than light-reared controls. Two-way analysis of variance techniques showed significant effects of age on the estimates of the numerical densities of neurons and synapses. However, there were no significant main effects of the lighting conditions, nor any significant interaction for any of the measures.