Abstracts

Rationale: The medial geniculate body (MGB) represents the major relay from the inferior colliculus to the auditory cortex. Its subdivisions differ in connectivity and functional properties. The ventral nucleus of the MGB (MGBv) is connected reciprocally with the primary auditory cortex (Te1), while the dorsal (MGBd), medial (MGBm) and suprageniculate (MGBsg) nuclei are reciprocally connected with the secondary auditory areas (Te2, Te3). Data about neuronal damage within MGB after status epilepticus (SE) are sparse, developmental data are missing. The present experiment was designed to characterize the severity of neuronal damage in the subdivisions of the MGB and to compare the neuronal damage of MGB subdivisions with damage of interconnected auditory cortical areas in immature animals after SE. Methods: Lithium chloride (3mmol/kg,i.p.) was injected 24 hours before pilocarpine (40 mg/kg, i.p.) to Wistar rat pups 12, 15, 18, 21 and 25 days old. Only animals exhibiting convulsive status epilepticus (SE) were included in this study. Control animals received saline instead of pilocarpine. The rats survived for 4, 8, 12, 24, 48 hours and 1 week after SE (4-5 animals for each survival interval). The animals were perfused under an overdose of urethane anesthesia. Coronal sections (40 m thick) were stained with cresyl violet or with a Fluoro-Jade B (FJB). Sections were examined with an epifluorescence microscope.Results: No degenerating neurons were detected in both the MGB and auditory cortex of 12 day old animals. A small number of degenerating neurons was found in 15 day-old pups in the MGBd only. Longer survival intervals (24h and more) were necessary for appearance of isolated degenerating neurons in the auditory cortex. The number of degenerating neurons in MGBd of 18-day-old rats increased; slight neuronal degenerations were present in the MGBv and MGBm. Minimal neuronal degenerations were found in all auditory cortical areas after longer intervals (48h, 1 w); degenerating neurons prevailed in the layer V. Number of degenerating neurons within MGBd and MGBv significantly increased in 21- and 25 day old animals especially after longer survival intervals. Number of degenerating neurons increased in layers II VI of all cortical auditory areas in 25-day-old rats. Conclusions: Comparison of neuronal degenerations in thalamic and cortical auditory structures after SE indicate that degenerative process within MGB started in 15-day-old animals while distinct cortical degenerations were evident in rats 21 and more days old. The thalamic level of the auditory circuits is more vulnerable than cortical areas. Neuronal damage increased with age and survival interval. Neuronal degeneration in the MGBd precedes that in the MGBv. Only layer V of the auditory cortex was affected in animals with SE at P15 and P18, while neuronal degenerations spread to all layers in older rats. Supported by grant No. P302/10/0971 of the Grant Agency of the Czech Republic.