Abstracts

During conditions of increased glutamate release such as seizures, Group I metabotropic glutamate receptors (mGluR 1 [amp] 5) activate many second messenger pathways and may contribute to epileptogenesis. In hippocampal slices the group I mGluR agonist 3,5-dihyroxyphenylglycine (DHPG) results in the induction of epileptiform discharges that persist for hours after DHPG is removed. We investigated the sensitivity of persistent epileptiform activity to mGluR 1 [amp] 5 antagonists, and the need for phospholipase C (PLC), a second messenger activated by DHPG, in producing long term changes in excitability.
Hippocampal slices were prepared from Sprague-Dawley rats (100-200 g), and incubated either in the presence of DHPG (100 [mu]M) alone or in an artificial cerebrospinal fluid (ACSF) with a specific antagonist followed by DHPG for 90-120 min. Slices were transferred to an interface chamber and bathed in ACSF ([K⁺][sub]o[/sub] = 5 mM) for 1 hour and then monitored extracellularly in the CA3 region for spontaneously occurring epileptiform activity that consisted of either brief interictal ( [lt] 500 ms) or prolonged ([gt]2s) ictal-like discharges. Following evaluation slices were exposed to bicuculline (BMI, 10 [mu]M) and assessed for epileptiform activity.
The combined blockade of mGluR1 with LY367385 (30 [mu]M) and the mGluR5 with MPEP (50 [mu]M) prevented the induction of ictal activity by DHPG exposure in all 30 slices with 6 slices having interictal discharges. Control slices prepared from the same animals demonstrated ictal discharges in 46% of slices and interictal activity in 38% (n = 26, p [lt] 0.05, Chi-square). In slices exposed to group I mGluR blockade, BMI produced interictal activity in all slices with three displaying ictal discharges. Exposure to LY367385 alone prevented DHPG-induction of ictal discharges in ACSF but 68% of slices had interictal activity (n = 19). MPEP alone also blocked the DHPG-induced generation of ictal discharges with only 4% of slices displaying ictal patterns and 36% interictal patterns (n = 28), compared to control slices that had 21% ictal and 61% interictal patterns (p [lt] 0.05). Blockade of PLC with U73122 (1 [mu]M) also prevented ictal discharges that result from DHPG exposure although 50% of 16 slices demonstrated interictal activity in ACSF compared to control slices that displayed ictal discharges in 30% and interictal discharges in 65 % (n = 20, p [lt] 0.05). In U73122-exposed slices, BMI application resulted in interictal activity in 94% of slices and 6% had ictal activity compared to the controls slices that had 45% interictal and 55% ictal activity (n = 16, p [lt] 0.05).
The induction of persistent ictal epileptiform activity by DHPG was dependent on activation of mGluR1, mGluR5, and PLC. The induction of interictal discharges was less sensitive to blockade by group I mGluR or PLC antagonists. These results point to the cooperative effect of group I mGluRs and the necessity of PLC second messenger activation in the epileptogenic effects of DHPG.
[Supported by: VA Research]