Glycogen phosphorylase reactivity in the entorhinal complex in familiar and novel environments: evidence for labile glycogenolytic modules in the rat.

Active and total glycogen phosphorylase were measured histochemically in the entorhinal complex of male Sprague-Dawley rats. Rats were sacrificed from their home cage, or after 5 min in a novel holeboard. Hemispheres from each group were paired, sectioned and processed together. Glycogen phosphorylase reactivity highlighted entorhinal cortex in contrast to less densely stained perirhinal cortex or neocortex. The presubiculum, but not parasubiculum, was strongly reactive for glycogen phosphorylase. Within medial and lateral entorhinal cortex, modularity of active glycogen phosphorylase reactivity was apparent. In inner Layer I there were small ( approximately 50 microm) intense patches of active glycogen phosphorylase. In Layer III there were both small and larger ( approximately 200 microm), patches of active glycogen phosphorylase. Lamina dessicans was reactive. Layers V and VI were relatively unreactive. Exposure to a holeboard intensified the small patches of active glycogen phosphorylase in inner Layer I, while attenuating active glycogen phosphorylase reactivity in Layer III. Total glycogen phosphorylase was unaffected by exposure to the novel environment and exhibited a pattern of continuous dense reactivity suggesting enzyme reserves, particularly in superficial layers of entorhinal cortex. These patterns confirm earlier evidence that glycogenolytic demand in Layers I and III of rat entorhinal cortex is organized in a modular fashion and show that such demand can be modified by brief exposure to a novel holeboard.