Abstracts

RATIONALE: At the end of this activity you should be able to describe the potential role of zinc in dentate gyrus disinhibition during epilepsy. Feed-forward inhibition in the dentate gyrus (DG) is normally able to subdue synchronous activity entering the hippocampus via the perforant path. In temporal lobe epilepsy zinc co-released from recurrent mossy fibers is hypothesized to disrupt this inhibition, reducing the DG[ssquote]s ability to protect downstream targets: particularly CA3. This hypothesis hinges on two points: in epilepsy recurrent mossy fiber connections onto granule cells are prevalent and synaptic GABA[sub]A[/sub] receptors become zinc sensitive. It is not known if zinc can affect DG function or whether these effects are specific to changes in the DG associated with epilepsy. We therefore tested zinc[scquote]s ability to disrupt DG gatekeeper function in control and epileptic animals and compared these results to partial disinhibition with the GABA[sub]A[/sub] antagonists picrotoxin or bicuculline.
METHODS: Adult epileptic rats were generated using a pilocarpine model, with epilepsy defined as at least two observed seizures. Hippocampal slices (400 [mu]m) were stained with the voltage sensitive dye RH-795, and placed in an interface chamber at 34[degree]C. Imaging DG and CA3 activity in response to perforant path stimulation was recorded with a 6400 pixel CCD camera at .2 to 2 kHz. Concurrent electrophysiological field responses were also recorded from the dentate or CA3. Pharmacological disinhibition was generated with 3 - 5 [mu]M picrotoxin or 20 [mu]M bicuculline.
RESULTS: In normal conditions, stimulation of the perforant path produced a decrease in fluorescence over the whole molecular layer of the DG indicating a strong depolarization throughout the synaptic field of the perforant path. This stimulation generated little or no activity from the CA3 in slices from either group, showing that, under normal conditions, gatekeeper function of the DG remains intact even in epileptic animals. The presence of 0.3 mM zinc significantly increased CA3 activity following perforant path stimulation, but only in slices from epileptic animals, with zinc having no effect on slices form control rats. In contrast, directly disinhibiting the DG by applying GABA[sub]A[/sub] antagonists allowed robust CA3 responses to be evoked in both groups.
CONCLUSIONS: These data show that zinc can act to specifically increase DG excitability in the DG of epileptic animals. Because synaptic GABA[sub]A[/sub] receptors on granule cells are not normally zinc sensitive but become so in epilepsy, and zinc effects are mimicked by partial disinhibition this data suggests that these epilepsy-associated zinc effects are due to disinhibition in the DG. This further supports the hypotheis that endogenous release of zinc in epilepsy may disrupt gatekeeper function of the DG.
[Supported by: NIH-NIMD grants NS-32403 and NS-38572]