Abstracts

RATIONALE: Studies have shown that interneurons which express somatostatin (SOM) are highly vulnerable to excitotoxic damage that accompanies status epilepticus. Other results suggest dendrites are the primary sites of neuronal injury which is characterized by the formation of varicose swellings (dendritic beads). Transgenic mice that selectively express green fluorescent protein (GFP) in SOM-expressing interneurons were used to study the dynamics of dendritic bead formation. METHODS: Hippocampal explant slice cultures were prepared from mice of transgenic line GIN. Time-lapse videomicroscopy was use to examine dendritic changes in GFP-expressing interneurons. Kainate was focally applied to specific dendritic regions via iontophoresis using glass micropipettes. RESULTS: Within seconds of brief focal applications, kainate induced dendritic beading at application sites. Beading then progressed in dendritic segments distal to the application site, and reversed within an hour of washout. Surprisingly, beading did not occur on the proximal side of the application site. However, higher intensity kainate application would first cause beading at, and distal to, the stimulation site, and subsequently and progressively would encompass the dendritic branches on the proximal side. Neurons having undergone this type of beading were unable to recover. Bath application of lidocaine suppressed the formation of dendritic beads at all but the site of kainate application. Upon washout, "normal" beading could be induced. CONCLUSIONS: In contrast to bath application studies, brief focal application of kainate may more closely mimic interneuronal injury during seizures, since neurons would receive strong focal excitatory input from specific presynaptic inputs. Our results show that dendritic damage (bead formation) occurs in a very discrete manner, with "primary" beading occurring at, and "secondary" beading outside of, the stimulation site. Secondary beading may depend on back-propagating action potentials, since lidocaine blocks their formation. Supported by NIH and Texas Advanced Technology Program grants.