An immature mossy fiber innervation of hilar neurons may explain their resistance to kainate-induced cell death in 15-day-old rats.

Recent studies in adult rodents have shown that mossy fibers, the axons of hippocampal granule cells, sprout into the inner molecular layer of adult rats when hilar cell death occurs following kainate-induced seizure activity. This pattern of hilar cell death and mossy fiber sprouting is not observed in young rats at 15 postnatal days of age. Since granule cells are generated postnatally, one may assume that a lack of a mature mossy fiber input to hilar neurons at 15 days of age is a possible cause for this observed difference. Neo-Timm preparations were made from rats at 5, 10, 12, 15, 20, 21, 25, 30 and 32 postnatal days of age to study the postnatal development of mossy fibers. The adult pattern of Timm-labeled mossy fiber innervation in the granule cell layer was observed by 25 days. The Timm reaction product forms large dense granules in CA3 of 15 day old rats but the hilus at this age lacks this type of large granule. Instead, the hilus displays only small labeled boutons, suggesting that mossy terminals have not yet reached a mature size. Electron microscopic preparations of the deep hilus and the subgranular zone of the hilus at 7, 12, 15, 21 and 30 days were analyzed to study the development of synapses formed by axons of granule cells. At 7 days the deep hilus showed only a few asymmetric synapses formed by the developing mossy fibers.(ABSTRACT TRUNCATED AT 250 WORDS

The inferior colliculus of GEPRs contains greater numbers of cells that express glutamate decarboxylase (GAD67) mRNA.

Previous studies have shown significantly greater GABA levels and numbers of GABAergic neurons in the central nucleus of the inferior colliculus (ICCN) of genetically epilepsy-prone rats (GEPR-9s). In the present study, in situ hybridization and emulsion autoradiographic techniques were used to determine whether there are also elevated numbers of ICCN cells that contain the 67-kD form of mRNA for the GABA synthesizing enzyme, glutamate decarboxylase (GAD), in GEPR-9s as compared to normal Sprague-Dawley (SD) rats. Hybridization with a 35S-labeled RNA probe complementary to a span of monkey GAD mRNA labeled cells throughout the brain including the ICCN. Labeled cells in the ICCN appeared to be of different sizes that corresponded with previous descriptions of GABAergic neurons from immunocytochemical studies. In the GEPR-9s, a larger number of GAD67 cRNA labeled neurons was observed in the ICCN as compared to SD rats. The external nucleus of the inferior colliculus was also found to contain significantly greater numbers of GAD67 cRNA labeled neurons whereas in the frontal cortex, a region of the brain that is not required for audiogenic seizure activity in GEPR-9s, there were no significant differences in hybridization between GEPR-9s and SD rats. Interestingly, within the superficial layers of the superior colliculus there was a higher density of hybridization in GEPR-9s than in SD rats indicating higher levels of GAD expression.(ABSTRACT TRUNCATED AT 250 WORDS

In situ hybridization for c-fos mRNA reveals the involvement of the superior colliculus in the propagation of seizure activity in genetically epilepsy-prone rats.

Previous work showed that bilateral lesions made between the inferior and superior colliculi reduced the severity of audiogenic seizures in genetically epilepsy-prone rats (GEPR-9s), and indicated that the connections between these two structures are vital for the propagation of seizure activity. To determine the involvement of the superior colliculus (SC) in seizure propagation, GEPR-9s were given four audiogenic seizures within 1 h by ringing a loud bell, and their brains were processed 30 min later for in situ hybridization for c-fos mRNA. Brain sections from such rats showed dense labeling in both the dorsal cortex and external nucleus of the inferior colliculus. Labeling continued rostrally into the intermediate and deep layers of the SC and the periaqueductal gray region. In addition, other brain regions such as the amygdala, piriform cortex and dorsal endopiriform nucleus showed dense labeling for c-fos mRNA. Comparable increases were not observed in the brains of Sprague-Dawley (SD) rats receiving auditory stimulation or in unstimulated GEPR-9s and SD rats, thereby indicating that increases in stimulated GEPR-9s are seizure-specific. This study provides further evidence that the SC is involved in the propagation of seizure activity in GEPR-9s, and also demonstrates the activation of other brain regions by audiogenic seizures

The inferior colliculus of GEPRs contains greater numbers of cells that express glutamate decarboxylase (GAD67) mRNA.

Previous studies have shown significantly greater GABA levels and numbers of GABAergic neurons in the central nucleus of the inferior colliculus (ICCN) of genetically epilepsy-prone rats (GEPR-9s). In the present study, in situ hybridization and emulsion autoradiographic techniques were used to determine whether there are also elevated numbers of ICCN cells that contain the 67-kD form of mRNA for the GABA synthesizing enzyme, glutamate decarboxylase (GAD), in GEPR-9s as compared to normal Sprague-Dawley (SD) rats. Hybridization with a 35S-labeled RNA probe complementary to a span of monkey GAD mRNA labeled cells throughout the brain including the ICCN. Labeled cells in the ICCN appeared to be of different sizes that corresponded with previous descriptions of GABAergic neurons from immunocytochemical studies. In the GEPR-9s, a larger number of GAD67 cRNA labeled neurons was observed in the ICCN as compared to SD rats. The external nucleus of the inferior colliculus was also found to contain significantly greater numbers of GAD67 cRNA labeled neurons whereas in the frontal cortex, a region of the brain that is not required for audiogenic seizure activity in GEPR-9s, there were no significant differences in hybridization between GEPR-9s and SD rats. Interestingly, within the superficial layers of the superior colliculus there was a higher density of hybridization in GEPR-9s than in SD rats indicating higher levels of GAD expression.(ABSTRACT TRUNCATED AT 250 WORDS

In situ hybridization for c-fos mRNA reveals the involvement of the superior colliculus in the propagation of seizure activity in genetically epilepsy-prone rats.

Previous work showed that bilateral lesions made between the inferior and superior colliculi reduced the severity of audiogenic seizures in genetically epilepsy-prone rats (GEPR-9s), and indicated that the connections between these two structures are vital for the propagation of seizure activity. To determine the involvement of the superior colliculus (SC) in seizure propagation, GEPR-9s were given four audiogenic seizures within 1 h by ringing a loud bell, and their brains were processed 30 min later for in situ hybridization for c-fos mRNA. Brain sections from such rats showed dense labeling in both the dorsal cortex and external nucleus of the inferior colliculus. Labeling continued rostrally into the intermediate and deep layers of the SC and the periaqueductal gray region. In addition, other brain regions such as the amygdala, piriform cortex and dorsal endopiriform nucleus showed dense labeling for c-fos mRNA. Comparable increases were not observed in the brains of Sprague-Dawley (SD) rats receiving auditory stimulation or in unstimulated GEPR-9s and SD rats, thereby indicating that increases in stimulated GEPR-9s are seizure-specific. This study provides further evidence that the SC is involved in the propagation of seizure activity in GEPR-9s, and also demonstrates the activation of other brain regions by audiogenic seizures