Human hippocampal slice preparations from patients with temporal lobe epilepsy (TLE) associated with hippocampal sclerosis (HS) are excellent material for the characterization of epileptiform-like activity. However, it is still unknown if hippocampal regions as cornu Ammonis (CA) 1, CA3 and CA4, generate population epileptiform-like activity. Here, we investigated epileptiform activities of the subiculum, CA1, CA2, CA3, CA4 (induced by elevation of extracellular potassium concentration) and the dentate gyrus (induced with hilar stimulation and elevation of potassium concentration) from sclerotic hippocampi of patients with drug-resistant TLE. Five types of epileptiform-like activity were observed: interictal-like events; periodic ictal spiking; seizure-like events; spreading depression-like events; tonic seizure-like events and no activity. Different susceptibilities to generate epileptiform activity among hippocampal regions were observed; the dentate gyrus was the most susceptible region followed by the subiculum, CA4, CA1, CA2 and CA3. The incidence of epileptiform activity pattern was associated with specific regions of the hippocampal formation. Moreover, it was observed that each region of the hippocampal formation exhibits frequency-specific ranges in each subfield of the sclerotic human tissue. In conclusion, this study demonstrates that epileptiform-like activity may be induced in different regions of the hippocampal formation, including regions that are severely affected by neuronal loss.

The types of epileptiform activity occurring in the sclerotic hippocampus with highest incidence are interictal-like events (II) and periodic ictal spiking (PIS). These activities are classified according to their event rates, but it is still unclear if these rate differences are consequences of underlying physiological mechanisms. Identifying new and more specific information related to these two activities may bring insights to a better understanding about the epileptogenic process and new diagnosis. We applied Poincaré map analysis and Recurrence Quantification Analysis (RQA) onto 35 in vitro electrophysiological signals recorded from slices of 12 hippocampal tissues surgically resected from patients with pharmacoresistant temporal lobe epilepsy. These analyzes showed that the II activity is related to chaotic dynamics, whereas the PIS activity is related to deterministic periodic dynamics. Additionally, it indicates that their different rates are consequence of different endogenous dynamics. Finally, by using two computational models we were able to simulate the transition between II and PIS activities. The RQA was applied to different periods of these simulations to compare the recurrences between artificial and real signals, showing that different ranges of regularity-chaoticity can be directly associated with the generation of PIS and II activities.

Background: Diverse forms of long-term potentiation (LTP) have been described, but one of the most investigated is encountered in the glutamatergic synapses of the hippocampal cornu Ammonis (CA1) subfield. However, little is known about synaptic plasticity in wildlife populations. Laboratory animals are extremely inbred populations that have been disconnected from their natural environment and so their essential ecological aspects are entirely absent. Proechimys are small rodents from Brazil’s Amazon rainforest and their nervous systems have evolved to carry out specific tasks of their unique ecological environment. It has also been shown that long-term memory duration did not persist for 24-h in Proechimys, in contrast to Wistar rats, when both animal species were assessed by the plus-maze discrimination avoidance task and object recognition test.Methods: In this work, different protocols, such as theta burst, single tetanic burst or multiple trains of high frequency stimulation (HFS), were used to induce LTP in hippocampal brain slices of Proechimys and Wistar rats.Results: A protocol-independent fast decay of early-phase LTP at glutamatergic synapses of the CA1 subfield was encountered in Proechimys. Long-term depression (LTD) and baseline paired-pulse facilitation (PPF) were investigated but no differences were found between animal species. Input/output (I/O) relationships suggested lower excitability in Proechimys in comparison to Wistar rats. Bath application of d-(-)-2-amino-5-phosphonopentanoicacid (D-AP5) and CNQX prevented the induction of LTP in both Proechimys and Wistar. However, in marked contrast to Wistar rats, LTP induction was not facilitated by the GABAA antagonist in the Amazon rodents, even higher concentrations failed to facilitate LTP in Proechimys. Next, the effects of GABAA inhibition on spontaneous activity as well as evoked field potentials (FPs) were evaluated in CA1 pyramidal cells. Likewise, much lower activity was detected in Proechimys brain slices in comparison to those of the Wistar rats.Conclusions: These findings suggest a possible high inhibitory tone in the CA1 network mediated by GABAA receptors in the Amazon rodents. Currently, neuroscience research still seeks to reveal molecular pathways that control learning and memory processes, Proechimys may prove useful in identifying such mechanisms in complement to traditional animal models.