Abstracts

Control of seizures by the substantia nigra pars reticulata (SNR) in the rat is age- and sex-dependent. We have previously shown that GABA[sub]A[/sub] receptor - mediated transmission in the SNR is sex-dependent. The GABA[sub]A[/sub] agonist muscimol induces depolarization and intracellular [Ca²⁺] elevation in SNR neurons from male PN15 slices (response typical of immaturity in other studied structures) in contrast to hyperpolarization and lack of [Ca²⁺][sub]i[/sub] elevation in SNR neurons of female PN15 slices (mature type response). These cellular properties depend on intracellular [Cl^-] and may play a role in seizure control. In the current study we tested whether prior seizures can alter cellular responses to GABA[sub]A[/sub] receptor activation.
Status epilepticus (SE) was induced by systemic injections of kainic acid in Sprague Dawley rats at ages PN4 (2mg/kg), PN5 (3mg/kg) and PN6 (3.5mg/kg). Coronal slices from kainate-treated and control PN7-15 male and female rats were cut with a vibratome and perfused in oxygenated (95% O2/ 5% CO2) ACSF. Perforated whole cell patch clamp recordings from SNR neurons were performed with the cation-permeable ionophore gramicidin to avoid alteration of intracellular chloride by the electrode. Changes in resting membrane potential were measured and I-V curves were constructed in response to bath-applied muscimol (10[micro]M). In other experiments, slices were loaded with the membrane-permeable [Ca²⁺][sub]i[/sub] indicator fura-2 AM. Changes in the 360nm/380nm fluorescence ratio, signifying [Ca²⁺][sub]i[/sub] elevation, in response to muscimol were measured.
Each of the 3 consecutive kainate injections induced SE lasting 2 to 3 h with [lt]5% overall mortality. In slices from control female rats, electrical responses to muscimol changed from depolarizing to hyperpolarizing between the ages PN9 and PN12. Muscimol-evoked [Ca²⁺][sub]i[/sub] elevations were present at early ages and gradually subsided in the same age range (PN9-12). Control male rats exhibited depolarization and [Ca²⁺][sub]i[/sub] elevation (immature type response) throughout the ages PN7 to 15. In contrast to slices from control rats, slices from male and female rats that had received 3 kainate injections exhibited muscimol-induced hyperpolarizations or significantly smaller depolarizations and absent or significantly smaller [Ca²⁺][sub]i[/sub] elevations throughout the ages PN7 to 15.
A series of 3 episodes of kainate-induced status epilepticus induced a premature switch of GABA[sub]A[/sub] responses from depolarizing and [Ca²⁺][sub]i[/sub]-elevating to hyperpolarizing and non-[Ca²⁺][sub]i[/sub]-elevating in both sexes. The depolarizing and [Ca²⁺][sub]i[/sub]-elevating effects of GABAA receptors in the SNR of immature brain may play a role in its susceptibility to convulsant stimuli, whereas the seizure-induced switch may affect the developmental maturation of the structure and its long-term role in seizure control.
[Supported by: Heffer Family Medical Foundation; NS 20253]