Allopregnanolone analogs that positively modulate GABAA receptors protect against partial seizures induced by 6-Hz electrical stimulation in mice

Purpose: Low-frequency (6 Hz), long-duration (3 s) electrical stimulation in mice produces seizures characterized by immobility, focal clonus, and automatic behaviors reminiscent of human limbic epilepsy. Renewed interest has been expressed in this seizure model with the recognition that it is sensitive to a broad spectrum of anticonvulsants (AEDs) and may have distinct pharmacologic responsiveness from other in vivo tests of AED efficacy. Here we sought to determine whether the progesterone-derived neuroactive steroid allopregnanolone (5alpha,3alpha-P) and several structural analogues with varying degrees of activity as positive allosteric modulators of gamma-aminobutyric acid (GABA)(A) receptors are protective in the 6-Hz seizure model. Methods: Mice were pretreated with neuroactive steroids (15 min before) or clonazepam (CZP; 30 min before) to 6-Hz corneal stimulation (32 mA). Animals that failed to exhibit immobility were considered protected. Results: The neuroactive steroids prevented 6-Hz seizures with rank order of potencies (ED50 values): ganaxolone (6.3 mg/kg) > 5alpha,3alpha-P (14.2 mg/kg) greater than or equal to 5beta,3alpha-P (14.4 mg/kg) > 5alpha,3beta-P (>100 mg/kg). CZP also was protective (ED50 value, 0.075 mg/kg). The potencies of the neuroactive steroids and CZP are similar to their previously reported activities in the pentylenetetrazol (PTZ) seizure model. Conclusions: Neuroactive steroids have comparable potencies in the 6-Hz and PTZ models. Their structural specificity in both models corresponds with their activities as positive allosteric modulators of GABA(A) receptors, although ganaxolone is more potent than expected, probably because it has greater bioavailability. The 6-Hz model may be a valuable toot in drug development for the identification of GABAergic AEDs.