Abstracts

RATIONALE: Previous studies showed that peptide neuromodulators play an important role in status epilepticus. Tachykinins are proconvulsants, while galanin, dynorphin and , to a lesser extent, somatostatin and neuropeptide Y act as endogenous anticonvulsants. In this study we examined the mechanism by which the tachykinin neurokinin B triggers seizures and status epilepticus. We found that intrahippocampal injection of neurokinin B (NKB) induces extensive receptor trafficking which mobilizes excitatory glutamate receptors to the cell surface. Blocking these receptors was very effective in stopping status epilepticus.
METHODS: Status epilepticus (SE) was induced either by intrahippocampal injection of NKB or by intermittent electrical stimulation of the perforant path. Seizures were monitored by EEG telemetry-videotape. Double-label immunofluorescence staining used fluo-neurokinin B for imaging of NK3 receptors, or antibodies to synaptophysin, a marker of the presynaptic apparatus, or to NMDA receptors. Confocal microscopy was utilized to reveal the subcellular distribution of receptors.
RESULTS: Intrahippocampal injection of NKB or its agonist senktide caused dose-dependent seizures. One nmole induced electrographic seizures for 24[plusminus]5 min. In perforant path-induced status epilepticus, pretreatment with the selective NK3 receptor antagonists SR142,801 (1 nmol) or L-769,927 (2 nmol) prevented the development of SE. Treatment of established SE with 100-200 nmoles reduced Se duration from 485 [plusminus]45 min. to a few min. Immunocytochemical studies showed that, in control hippocampi, most NMDA receptors in CA3 pyramidal cells did not colocalize with synaptophysin, which outlined the cell surface, but were internalized in cell soma or proximal dendrites. Thirty min. after NKB injection, most of these spare receptors had been mobilized to the cell surface and colocalization with synaptophysin had increased five-fold. Treatment of established, diazepam-refractory SE with NMDA blockers (either systemic or intrahippocampal) was very effective in terminating SE..
CONCLUSIONS: These results suggest that NKB plays an important proconvulsant role in SE. NKB or SE-associated trafficking of NMDA receptors may increase the number of active excitatory receptors on the neuronal surface and play a key role in the maintenance phase of SE. Other data suggest that NKB-associated receptor trafficking alters GABAA receptors in the opposite direction (endocytosis), reducing their number during the initiation phase of SE. The effectiveness of NMDA blockers in stopping diazepam-refractory SE suggests that agents that block NMDA receptor trafficking of function should be investigated as therapeutic agents in SE.
[Supported by: VA Research Service and by grant NS13515 from NINDS]