Chemogenetic model of acute temporal lobe-like seizures in the area tempestas

The piriform cortex was identified as a crucial ictogenic trigger zone in the 1980s. Surgical outcome studies in humans likewise suggest that this region contributes to seizure generation. Within the anterior piriform cortex, the "area tempestas (AT)" has been identified as a particularly sensitive site for triggering seizures in rodents and non-human primates. While many studies have examined this region using focal pharmacological manipulations (e.g., drug microinjection) in rodents, within-subject reproducibility has been limited by the need for repeated drug injection into the brain parenchyma. To address this issue, here we developed a chemogenetic model to induce acute seizures in rats by targeting the AT. Twenty-two adult Wistar rats (Rattus norvegicus; 5 males, 17 females; approximately 4 months old, 215-490 g body weight) were used for this purpose. We first examined the efficacy of focal activation of the AT using a glutamatergic neuron targeted excitatory DREADD and compared this to disinhibition using an inhibitory DREADD targeted to GABAergic neurons. The hM4(Gi) construct targeted to GABAergic neurons, but not the Dq construct targeted to glutamatergic neurons induced seizures after clozapine-N-oxide (CNO) administration. CNO (5 or 10 mg/kg) rapidly induced seizures after unilateral or bilateral hM4(Gi) infusion into the AT. Electroencephalographic recordings demonstrated focal seizure onset in the AT which spread ipsilaterally to frontal cortex, then hippocampus, and finally to the contralateral frontal cortex. Synchronized spike bursts were correlated with stage 4 and generalized tonic-clonic seizures. GAD immunofluorescence revealed ≃87% of colocalization with hM4(Gi) receptors, consistent with seizures induced via reduction of inhibitory input in the AT. These results suggest that the present model provides controlled, localized, and reproducible seizure induction with low (‹10%) mortality rates.