Many of the biological actions of progesterone are mediated through the progesterone receptor (PR), a nuclear transcription factor. Progesterone is well recognized to protect against seizures in animal models. Although this activity has been attributed to the progesterone metabolite allopregnanolone, a GABA A receptor-modulating neurosteroid with anticonvulsant properties, PRs could also play a role. Here, we used PR knockout (PRKO Ϫ/Ϫ ) mice bearing a targeted deletion of the PR gene that eliminates both isoforms of the PR to investigate the contribution of the PR to the anticonvulsant activity of progesterone. The protective activity of progesterone was examined in female and male homozygous PRKO mice and isogenic wild-type controls in the pentylenetetrazol (PTZ), maximal electroshock, and amygdala-kindling seizure models. In all three models, the anticonvulsant potency of progesterone was undiminished in PRKO mice compared with control mice. On the contrary, there was a substantial increase in the anticonvulsant potency of progesterone in the PTZ and kindling models. The antiseizure activity of progesterone in PRKO mice was reversed by pretreatment with finasteride, a 5␣-reductase inhibitor that blocks the metabolism of progesterone to allopregnanolone. Unlike progesterone, the neurosteroids allopregnanolone and allotetrahydrodeoxycorticosterone exhibited comparable anticonvulsant potency in PRKO and wild-type mice. The basis for the heightened progesterone responsiveness of PRKO mice is not attributable to pharmacokinetic factors, because the plasma allopregnanolone levels achieved after progesterone administration were not greater in the PRKO mice. These studies provide strong evidence that the PR is not required for the antiseizure effects of progesterone, which mainly occurs through its conversion to the neurosteroid allopregnanolone.

The amygdala is a critical brain region for limbic seizure activity, but the mechanisms underlying its epileptic susceptibility are obscure. Several lines of evidence implicate GluR5 (GLUK5) kainate receptors, a type of ionotropic glutamate receptor, in the amygdala's vulnerability to seizures and epileptogenesis. GluR5 mRNA is abundant in temporal lobe structures including the amygdala. Brain slice recordings indicate that GluR5 kainate receptors mediate a portion of the synaptic excitation of neurons in the rat basolateral amygdala. Whole‐cell voltage‐clamp studies demonstrate that GluR5 kainate receptor‐mediated synaptic currents are inwardly rectifying and are likely to be calcium permeable. Prolonged activation of basolateral amygdala GluR5 kainate receptors results in enduring synaptic facilitation through a calcium‐dependent process. The selective GluR5 kainate receptor agonist ATPA induces spontaneous epileptiform bursting that is sensitive to the GluR5 kainate receptor antagonist LY293558. Intra‐amygdala infusion of ATPA in the rat induces limbic status epilepticus; in some animals, recurrent spontaneous seizures occur for months after the ATPA treatment. Together, these observations indicate that GluR5 kainate receptors have a unique role in triggering epileptiform activity in the amygdala and could participate in long‐term plasticity mechanisms that underlie some forms of epileptogenesis. Accordingly, GluR5 kainate receptors represent a potential target for antiepileptic and antiepileptogenic drug treatments. Most antiepileptic drugs do not act through effects on glutamate receptors. However, topiramate at low concentrations causes slow inhibition of GluR5 kainate receptor‐mediated synaptic currents in the basolateral amygdala, indicating that it may protect against seizures, at least in part, through suppression of GluR5 kainate receptor responses.

Fast excitatory synaptic responses in basolateral amygdala (BLA) neurons are mainly mediated by ionotropic glutamate receptors of the a-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtype. AMPA receptors containing an edited GluR2 subunit are calcium impermeable, whereas those that lack this subunit are calcium permeable and also inwardly rectifying. Here, we sought to determine the extent to which synapses in the rat BLA have AMPA receptors with GluR2 subunits. We assessed GluR2 protein expression in the BLA by immunocytochemistry with a GluR2 subunitspecific antiserum at the light and electron microscopic level; for comparison, a parallel examination was carried out in the hippocampus. We also recorded from amygdala brain slices to examine the voltage-dependent properties of AMPA receptormediated evoked synaptic currents in BLA principal neurons. At the light microscopic level, GluR2 immunoreactivity was localized to the perikarya and proximal dendrites of BLA neurons; dense labeling was also present over the pyramidal cell layer of hippocampal subfields CA1 and CA3. In electron micrographs from the BLA, most of the synapses were asymmetrical with pronounced postsynaptic densities (PSD). They contained clear, spherical vesicles apposed to the PSD and were predominantly onto spines (86%), indicating that they are mainly with BLA principal neurons. Only 11% of morphological synapses in the BLA were onto postsynaptic elements that showed GluR2 immunoreactivity, in contrast to hippocampal subfields CA1 and CA3 in which 76% and 71% of postsynaptic elements were labeled (p < 0.001). Synaptic staining in the BLA and hippocampus, when it occurred, was exclusively postsynaptic, and particularly heavy over the PSD. In whole-cell voltage clamp recordings, 72% of BLA principal neurons exhibited AMPA receptor-mediated synaptic currents evoked by external capsule stimulation that were inwardly rectifying. Although BLA principal neurons express perikaryal and proximal dendritic GluR2 immunoreactivity, few synapses onto these neurons express GluR2, and a preponderance of principal neurons have inwardly rectifying AMPA-mediated synaptic currents, suggesting that targeting of GluR2 to synapses is restricted. Many BLA synaptic AMPA receptors are likely to be calcium permeable and could play roles in synaptic plasticity, epileptogenesis and excitoxicity.

Two pathways that have been shown to mediate cerebral ischemic damage are the MEK/ERK cascade and the pro-apoptotic δPKC pathway. We investigated the relationship between these pathways in a rat model of focal ischemia by observing and modifying the activation state of each pathway. The ERK1/2 inhibitor, U0126, injected at ischemia onset, attenuated the increase in phosphorylated ERK1/2 (P-ERK1/2) after reperfusion. The δPKC inhibitor, δV1-1, delivered at reperfusion, did not significantly change P-ERK1/2 levels. In contrast, the δPKC activator, ψδRACK, injected at reperfusion, reduced ERK1/2 phosphorylation measured 4 h after reperfusion. Additionally, U0126 pretreatment at ischemia onset reduced infarct size compared with vehicle, but U0126 injected at the onset of reperfusion had no protection. Finally, combination of U0126 injection at ischemia onset plus δV1-1 injection at reperfusion further reduced infarct size, while combination of U0126 delivered at ischemia onset with ψδRACK injected at reperfusion increased infarct size compared with U0126 alone. In conclusion, we find that inhibiting both the MEK/ERK and the δPKC pathways offers greater protection than either alone, indicating they likely act independently.