Effects of a lengthy period of undernutrition from birth and subsequent nutritional rehabilitation on the synapse: granule cell neuron ratio in the rat dentate gyrus.

Recent evidence showing alterations in spatial memory due to a period of undernutrition during early life has implicated the hippocampus as one of the brain centres that may be particularly adversely affected. However, there are very few quantitative morphological studies that have examined the neuronal and synaptic populations of the hippocampi from undernourished animals. We decided to carry out such experiments, paying particular attention to the granule cell of the dentate gyrus. Male rats were undernourished from the 18th day of gestation until 21, 75, or 150 days of age. Some of these previously undernourished rats were nutritionally rehabilitated between 150 and 250 days of age. Groups of well-fed control and experimental rats were killed by intracardiac perfusion with 2.5% sodium-cacodylate-buffered glutaraldehyde. The right hippocampus from each rat was dissected out and processed for electron microscopy. Stereological procedures at the light and electron microscopical levels were used to estimate the numerical densities of granular cell neurons and molecular layer synapses in the dorsal lip of the dentate gyrus. These estimates were used to calculate synapse: neuron ratios. There were 5,056 +/- 347 (mean +/- SE) and 5,002 +/- 190 synapses per neuron in 21-day-old control and undernourished rats, respectively. By 75 days these values had increased to 9,215 +/- 588 and 6,683 +/- 237. The difference was statistically significant. By 150 days of age the value for control animals had fallen once again to 6,518 +/- 209 whereas undernourished rats had increased slightly to 7,689 +/- 288 (P less than .01); 250-day-old rats, previously undernourished from birth to 150 days of age, showed a substantial increase in the synapse: neuron ratio during the period of nutritional rehabilitation. Thus these nutritionally rehabilitated rats had 9,407 +/- 365 synapses per neuron whereas age-matched controls had only 6,323 +/- 239 (P less than .01). These results indicate that the rat dentate gyrus is vulnerable to undernutrition even during the postweaning period and that a lengthy period of undernutrition can alter the developmental growth curve for synapse: neuron ratios.