Effects of Early Hyperthyroidism on Shuttle Box Behavior and Hippocampal Mossy Fiber Distribution
By means of 3 genetic approaches we have shown that rat and mouse strains that perform poorly in a two-way avoidance task have comparatively more mossy fiber synapses upon the basal dendrites of the hippocampal pyramidal neurons than strains that show higher levels of performance in that task (Schwegler and Lipp, 1983). There is a strong negative correlation between these two parameters. Early hyperthyroidism is known to increase the number of these synapses in a dose-dependent way (Lauder and Mugnaini, 1977). Therefore, we injected thyroxine during the first two postnatal weeks in pups of rat and mouse strains (Roman High-avoidance rats and DBA/2 mice). Thyroxine was applied in three different groups: daily in group 1; every second day in group 2; every third day in group 3; a control group was treated with saline. At the age of three months the animals were tested for two-way avoidance acquisition (rats) and performance (mice). After behavioral testing, the animals were killed and their brains were cut in horizontal sections, stained by means of the Timm method, and analyzed on a graphics tablet connected to a desk computer. Thyroxine treated animals showed both a decreased acquisition and performance of the shuttle-box task, and these effects were dose-dependent in rats.