Study of a Model of Focal Limbic Epilepsy in Rats

Epilepsy is a neurological disorder characterized by the recurrence of spontaneous, unprovoked epileptic seizures. Mesial temporal lobe epilepsy (more briefly, MTLE) is a very common form of epilepsy which is featured by the occurrence of focal limbic seizures, and associated to a specific neuropathological alteration, the so-called Ammon’s horn sclerosis (AHS, from now on abbreviated as AHS), whose main features are a selective loss of the CA1 and CA3/4 section of the Ammon’s horn (CA, from Latin Cornu Ammonis, abbreviated as CA), a selective cell loss of inhibitory interneurons in the hilus of the dentate gyrus (DG), and the abnormal sprouting of granule cells mossy fibers (the so called mossy fiber sprouting, MFS). The onset of spontaneous recurrent seizures (SRS) is the hallmark of a good model of epilepsy. For temporal lobe epilepsy (TLE), the most used models consist in administering systemically chemoconvulsants inducing limbic status epilepticus (i.e. seizures lasting for more than 30’, SE) and evaluating the occurrence of SRS. However, in these models, the widespread involvement of different structures which complicates the interpretation of experimental findings obtained with this experimental approach, since any morphological/functional effect of these models might be due either to the direct action of the chemoconvulsant or to the SE. The morphological features of many structures of the limbic system are highly phylogenically conserved through the evolution from rodents to primates and humans; it has been recently shown that it is possible to evoke limbic seizures and SE from a small structure, the deep extent of the anterior piriform cortex (from now on abbreviated as APC) by focally infusing picomolar concentration of chemoconvulsants; this structure roughly corresponds to the periamygdaloid cortex in humans. It is the brain region most densely innervated by the noradrenergic fibers originating from the nucleus locus coeruleus (LC), and we recently showed that microinfusing bicuculline (a GABA A receptor antagonist) into the APC of rats with a lesion of LC (induced by a selective neurotoxin, DSP-4, i.p.), induces SE, similarly to the SE obtained by microinfusing into the APC of rats with an intact noradrenergic system, cychlotiazide+ bicuculline. LC plays a critical role in modulating several models of seizures, and it lays a critical role in plastic mechanisms and neuroprotection in the brain. Thus, we compared the group DSP-4+bicuculline and cyclotiazide+bicuculline, to evaluate whether the focal SE evoked from the APC is capable of inducing SRS and AHS, and whether LC plays a significant role in this phenomena. We found that: a) despite a similar duration and severity of SE in the two models of SE, in the group DSP-4+bicuculline there was a higher incidence of SRS; b) the cell loss in the hippocampal DG hilus and CA3 was higher in the group DSP-4+bic, while MFS was more intense in the group cyclotiazide+bicuculline; also the loss of parvalbumin-positive neurons was more represented in the DSP-4+bicuculline group, while GFAP expression (an index of reactive gliosis), was similar in the two groups. In conclusion, our study confirms that focal induction of SE from the APC represents a good model of TLE, and that NE released from the fibers originating from the LC plays a significant role both in the hippocampal damage occurring after SE, and in the incidence of SRS. Differently from what observed in other models, our findings challenge a prominent role of MFS in the occurrence of SRS, since this phenomenon was less intense in the group with more frequent SRS (DSP-4+bicuculline) than in the one with an intact LC.