Distinctive hippocampal CA2 subfield of the Amazon rodent Proechimys

Robust Network Inhibition and Decay of Early-Phase LTP in the Hippocampal CA1 Subfield of the Amazon Rodent Proechimys

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 GABA_A antagonist in the Amazon rodents, even higher concentrations failed to facilitate LTP in Proechimys. Next, the effects of GABA_A 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 GABA_A 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.

Long-term Potentiation Decay and Poor Long-lasting Memory Process in the Wild Rodents Proechimys from Brazil's Amazon Rainforest

Proechimys are small terrestrial rodents from Amazon rainforest. Each animal species is adapted to a specific environment in which the animal evolved therefore without comparative approaches unique characteristics of distinct species cannot be fully recognized. Laboratory rodents are exceedingly inbred strains dissociated from their native habitats and their fundamental ecological aspects are abstracted. Thus, the employment of exotic non-model species can be informative and complement conventional animal models. With the aim of promoting comparative studies between the exotic wildlife populations in the laboratory and traditional rodent model, we surveyed a type of synaptic plasticity intimately related to memory encoding in animals. Using theta-burst paradigm, in vitro long-term potentiation (LTP) in the CA1 subfield of hippocampal slices was assessed in the Amazon rodents Proechimys and Wistar rats. Memory, learning and anxiety were investigated through the plus-maze discriminative avoidance task (PM-DAT) and object recognition test. In PM-DAT, both animal species were submitted to two test sessions (3-h and 24-h) after the conditioning training. Proechimys exhibited higher anxiety-like behavior in the training session but during test sessions both species exhibited similar patterns of anxiety-related behavior. After 3-h of the training, Proechimys and Wistar spent significantly less time in the aversive enclosed arm than in the non-aversive arm. But, at 24-h after training, Wistar rats remained less time in the aversive closed arm in comparison with the non-aversive one, while Proechimys rodents spent the same amount of time in both enclosed arms. In the object recognition test, both species were evaluated at 24-h after the acquisition session and similar findings than those of the PM-DAT (24-h) were obtained, suggesting that long-term memory duration did not persist for 24-h in the Amazon rodent. Field excitatory post-synaptic potentials recordings revealed that LTP decays rapidly over time reaching basal levels at 90 min after theta-burst stimulation in Proechimys, contrasting to the stable LTP found in the Wistar rats which was observed throughout 3-h recording period. These findings suggest a link between the LTP decay and the lack of 24-h long-lasting memory process in Proechimys. Nevertheless, why early-phase LTP in Proechimys decays very rapidly remains to be elucidated.

Rediscovering area CA2: unique properties and functions

Hippocampal area CA2 has several features that distinguish it from CA1 and CA3, including a unique gene expression profile, failure to display long-term potentiation and relative resistance to cell death. A recent increase in interest in the CA2 region, combined with the development of new methods to define and manipulate its neurons, has led to some exciting new discoveries on the properties of CA2 neurons and their role in behaviour. Here, we review these findings and call attention to the idea that the definition of area CA2 ought to be revised in light of gene expression data.

Enhanced nonsynaptic epileptiform activity in the dentate gyrus after kainate-induced status epilepticus

Understanding the mechanisms that influence brain excitability and synchronization provides hope that epileptic seizures can be controlled. In this scenario, non-synaptic mechanisms have a critical role in seizure activity. The contribution of ion transporters to the regulation of seizure-like activity has not been extensively studied. Here, we examined how non-synaptic epileptiform activity (NEA) in the CA1 and dentate gyrus (DG) regions of the hippocampal formation were affected by kainic acid (KA) administration. NEA enhancement in the DG and suppression in area CA1 were associated with increased NKCC1 expression in neurons and severe neuronal loss accompanied by marked glial proliferation, respectively. Twenty-four hours after KA, the DG exhibited intense microglial activation that was associated with reduced cell density in the infra-pyramidal lamina; however, cellular density recovered 7 days after KA. Intense Ki67 immunoreactivity was observed in the subgranular proliferative zone of the DG, which indicates new neuron incorporation into the granule layer. In addition, bumetanide, a selective inhibitor of neuronal Cl(-) uptake mediated by NKCC1, was used to confirm that the NKCC1 increase effectively contributed to NEA changes in the DG. Furthermore, 7 days after KA, prominent NKCC1 staining was identified in the axon initial segments of granule cells, at the exact site where action potentials are preferentially initiated, which endowed these neurons with increased excitability. Taken together, our data suggest a key role of NKCC1 in NEA in the DG.

Characterization of the sleep-wake cycle of the Neotropical rodent Proechimys guyannensis

Objectives:

To better understand the sleep–wake cycle characteristics in the female Neotropical rodent Proechimys guyannensis related to comparative neurobiology.

Methods:

Surface neocortical and hippocampal electrodes were chronically implanted in the brains of female Wistar and Proechimys animals. In addition, electrodes for the study of muscle activity were implanted into the neck muscle of both species. After surgical recovery and a period of adaptation, animals were continuously registered for periods as long as 48 h.

Results:

In both the light and dark phases of the cycle, significant differences in some electrographic patterns were observed between the Proechimys and Wistar animals. Although Proechimys has nocturnal activities and a pattern of polyphasic sleep similar to Wistar rats, the analysis of its sleep-wakefulness cycle indicates that the Neotropical rodent sleeps less with consequent longer periods of wakefulness when compared to Wistar rats.

Conclusions:

Together with previous findings of different neuroanatomical, neurophysiologic and behavioral characteristics, this study allow us to better understand adaptive differences of the Neotropical rodent Proechimys.

Beneficial influence of physical exercise following status epilepticus in the immature brain of rats

Studies in adult animals have demonstrated a beneficial effect of physical exercise on epileptic insults. Although the effects of physical exercise on the mature nervous system are well documented, its influence on the developing nervous system subjected to injuries in childhood has been little explored. The purpose of our study was to investigate whether a physical exercise program applied during brain development could influence the hippocampal plasticity of rats submitted to status epilepticus (SE) induced by pilocarpine model at two different ages of the postnatal period. Male Wistar rats aged 18 (P18) and 28 (P28) days were randomly divided into four groups: Control (CTRL), Exercise (EX), SE (SE) and SE Exercise (SE/EX) (n=17 per group). After the aerobic exercise program, histological and behavioral (water maze) analyses were performed. Our results showed that only animals subjected to pilocarpine-induced SE at P28 presented spontaneous seizures during the observational period. A significant reduction in seizure frequency was observed in the SE/EX group compared to the SE group. In adulthood, animals submitted to early-life SE displayed impairment in long-term memory in the water maze task, while the exercise program reversed this deficit. Reduced mossy fiber sprouting in the dentate gyrus was noted in animals that presented spontaneous seizures (SE/EX vs SE). Exercise increased cell proliferation (Ki-67 staining) and anti-apoptotic response (bcl-2 staining) and reduced pro-apoptotic response (Bax staining) in animals of both ages of SE induction (P18/28). Exercise also modified the brain-derived neurotrophic factor (BDNF) levels in EX and SE/EX animals. Our findings indicate that in animals subjected to SE in the postnatal period a physical exercise program brings about beneficial effects on seizure frequency and hippocampal plasticity in later stages of life.

Malnutrition in infancy as a susceptibility factor for temporal lobe epilepsy in adulthood induced by the pilocarpine experimental model

Malnutrition during the earliest stages of life may result in innumerable brain problems. Moreover, this condition could increase the chances of developing neurological diseases, such as epilepsy. We analyzed the effects of early-life malnutrition on susceptibility to epileptic seizures induced by the pilocarpine model of epilepsy. Wistar rat pups were kept on a starvation regimen from day 1 to day 21 after birth. At day 60, 16 animals (8 = well-nourished; 8 = malnourished) were exposed to the pilocarpine experimental model of epilepsy. Age-matched well-nourished (n = 8) and malnourished (n = 8) rats were used as controls. Animals were video-monitored over 9 weeks. The following behavioral parameters were evaluated: first seizure threshold (acute period of the pilocarpine model); status epilepticus (SE) latency; first spontaneous seizure latency (silent period), and spontaneous seizure frequency during the chronic phase. The cell and mossy fiber sprouting (MFS) density were evaluated in the hippocampal formation. Our results showed that the malnourished animals required a lower pilocarpine dose in order to develop SE (200 mg/kg), lower latency to reach SE, less time for the first spontaneous seizure and higher seizure frequency, when compared to well-nourished pilocarpine rats. Histopathological findings revealed a significant cell density reduction in the CA1 region and intense MFS among the malnourished animals. Our data indicate that early malnutrition greatly influences susceptibility to seizures and behavioral manifestations in adult life. These findings suggest that malnutrition in infancy reduces the threshold for epilepsy and promotes alterations in the brain that persist into adult life.

Morphological and electrophysiological properties of pyramidal-like neurons in the stratum oriens of Cornu ammonis 1 and Cornu ammonis 2 area of Proechimys

Proechimys (Rodentia: Echimyidae) is a neotropical rodent of the Amazon region that has been successfully colonized in the laboratory and used for experimental medicine. Preliminary studies indicated that Proechimys (casiragua) rodents express an atypical resistance to developing a chronic epileptic condition in common models of temporal lobe epilepsy. Moreover, previous investigation of our laboratory described a remarkably different Proechimy's cytoarchitecture organization of the hippocampal CA2 subfield. In the present study, we investigated the intrinsic neuronal properties and morphological characteristics of the Proechimys's hippocampal pyramidal neurons of the CA1 and CA2 areas. A comparative approach was performed using neurons recorded in Wistar rats. A striking finding in Proechimys rodents was the presence of large pyramidal-like neurons throughout the stratum oriens from CA2 to CA1 area. In order to confirm such distinctive feature of the Proechimys's hippocampus, we performed Nissl staining and immunohistochemistry for neurofilament protein SM311. CA2 pyramidal neurons in the stratum pyramidale of Proechimys exhibited a significantly higher input resistance and lower time constant when compared to corresponding cell groups in the same area of the Wistar rat's. This newly identified population of pyramidal-shaped neurons in stratum oriens of Proechimys exhibited distinct electrophysiological and morphological properties. This included larger capacitance, lower input resistance, larger rheobase, long latency to first action potential and slower firing frequency. In addition, the apical dendrites of these neurons were oriented in parallel to apical dendrites of regular pyramidal neurons in stratum pyramidale. Moreover, these neurons were immunoreactive to SM311 as the majority of the neurons of the pyramidal layer. The functional role of these hippocampal neurons of the rodent Proechimys deserves further investigation.

Early exercise promotes positive hippocampal plasticity and improves spatial memory in the adult life of rats

There is a great deal of evidence showing the capacity of physical exercise to enhance cognitive function, reduce anxiety and depression, and protect the brain against neurodegenerative disorders. Although the effects of exercise are well documented in the mature brain, the influence of exercise in the developing brain has been poorly explored. Therefore, we investigated the morphological and functional hippocampal changes in adult rats submitted to daily treadmill exercise during the adolescent period. Male Wistar rats aged 21 postnatal days old (P21) were divided into two groups: exercise and control. Animals in the exercise group were submitted to daily exercise on the treadmill between P21 and P60. Running time and speed gradually increased over this period, reaching a maximum of 18 m/min for 60 min. After the aerobic exercise program (P60), histological and behavioral (water maze) analyses were performed. The results show that early-life exercise increased mossy fibers density and hippocampal expression of brain-derived neurotrophic factor and its receptor tropomyosin-related kinase B, improved spatial learning and memory, and enhanced capacity to evoke spatial memories in later stages (when measured at P96). It is important to point out that while physical exercise induces hippocampal plasticity, degenerative effects could appear in undue conditions of physical or psychological stress. In this regard, we also showed that the exercise protocol used here did not induce inflammatory response and degenerating neurons in the hippocampal formation of developing rats. Our findings demonstrate that physical exercise during postnatal development results in positive changes for the hippocampal formation, both in structure and function.