The entorhinal cortex regulates blood glucose level in response to microinjection of neostigmine into the hippocampus.

OBJECTIVE: Microinjectin of neostigmine, an inhibitor of acetylcholine esterase, into the rat hippocampus elicited stress-like responses reflected by the release of adrenocorticotropic hormone (ACTH) and blood glucose elevations. The entorhinal cortex is regarded as an interface between the hippocampus and neocortex. The current study was designed to examine the role of the entorhinal cortex in regulation of blood glucose elevation induced by hippocampal neostigmine injection.

MATERIAL AND METHODS: We produced the entorhinal cortex lesions in 9 week-old male Wistar rats by the bilateral injections of the cell-selective neurotoxin, ibotenic acid (15microg/microl). Two weeks after the injections, neostigmine methylsulfate (sigma, 5x10(-8) mol) was microinjected into the rat hippocampus in a volume of 1microl for 1min using a CMA/100 microinjection pump. Plasma ACTH levels were measured by radioimmunoassay. Plasma glucose concentrations were determined by the immobilized enzyme membrane/H2O2 method with a compact glucose analyzer Antsense II (Bayer Medical Co.Ltd,Tokyo, Japan).

RESULTS: Compared with sham-operated control rats, the entorhinal lesions produced by ibotenic acid significantly attenuated the elevations of blood glucose evoked by the microinjection of neostigmine into the hippocampus. However, no significant difference of plasma ACTH in response to the injection was observed between the entorhinal-lesioned rats and controls.

CONCLUSION: The results of the present study indicate that the entorhinal cortex plays a role in the central nervous systems regulation of blood glucose and may be involved in a stress response presumably via an alternative pathway.