Amygdaloid complex anatomopathological findings in animal models of status epilepticus

Temporal lobe epileptic seizures are one of the most common and well-characterized types of epilepsies. The current knowledge on the pathology of temporal lobe epilepsy relies strongly on studies of epileptogenesis caused by experimentally induced status epilepticus (SE). Although several temporal lobe structures have been implicated in the epileptogenic process, the hippocampal formation is the temporal lobe structure studied in the greatest amount and detail. However, studies in human patients and animal models of temporal lobe epilepsy indicate that the amygdaloid complex can be also an important seizure generator, and several pathological processes have been shown in the amygdala during epileptogenesis. Therefore, in the present review, we systematically selected, organized, described, and analyzed the current knowledge on anatomopathological data associated with the amygdaloid complex during SE-induced epileptogenesis. Amygdaloid complex participation in the epileptogenic process is evidenced, among others, by alterations in energy metabolism, circulatory, and fluid regulation, neurotransmission, immediate early genes expression, tissue damage, cell suffering, inflammation, and neuroprotection. We conclude that major efforts should be made in order to include the amygdaloid complex as an important target area for evaluation in future research on SE-induced epileptogenesis. This article is part of the Special Issue "NEWroscience 2018".

Distribution and morphology of calbindin neurons in the Amygdaloid Complex of the marmoset monkey (callithrix jacchus)

The location and distribution of the calcium-binding protein calbindin-D28k (CB) has been considered to be of great value as a neuronal marker for identifying distinct brain regions and discrete neuronal populations. In the amygdaloid complex (AC), the balance of excitatory and inhibitory inputs is controlled by CB immunoreactive interneurons. Alterations of inhibitory mechanisms in the AC may play a role in the emotional symptomatology of neurological diseases like Alzheimer's and psychiatric disorders like posttraumatic stress disorder. The present investigation examined the distribution and morphology of CB-containing neurons, neuropils and fibers in marmoset monkey ACs by using immunohistochemical and morphometrical methods. We recognized four types of CB cells in the AC: type 1 (multipolar), type 2 (spherical or bipolar), type 3 (pyramidal) and type 4 (halo cells), a cell type specific to the marmoset located in the basal and central nuclei. We detected CB cells in all nuclei and areas of the AC, where most of the cells were present in the deep nuclei (lateral, basal, accessory basal and paralaminar). In the superficial nuclei (the nucleus of the lateral olfactory tract, medial nucleus, periamygdaloid cortex and cortical nuclei), the CB cells were abundant in layers 2 and 3. The intercalated nuclei contained small densely packed cells. The CB neuropils were particularly dense in layer 1 of the superficial nuclei, in the deep nuclei and in the amygdalohippocampal area. Large CB immunoreactive neurons in the white matter and fibers with varicosities were found in the myelin tracts that surrounded the AC. These findings are the first step in determining whether some of these cells are specifically disrupted in pathological states.

Increases in dendritic spine density in BLA without metabolic changes in a rodent model of PTSD

Imaging studies have shown abnormal amygdala function in patients with posttraumatic stress disorder (PTSD). In addition, alterations in synaptic plasticity have been associated with psychiatric disorders and previous reports have indicated alterations in the amygdala morphology, especially in basolateral (BLA) neurons, are associated with stress-related disorders. Since, some individuals exposed to a traumatic event develop PTSD, the goals of this study were to evaluate the early effects of PTSD on amygdala glucose metabolism and analyze the possible BLA dendritic spine plasticity in animals with different levels of behavioral response. We employed the inescapable footshock protocol as an experimental model of PTSD and the animals were classified according to the duration of their freezing behavior into distinct groups: "extreme behavioral response" (EBR) and "minimal behavioral response". We evaluated the amygdala glucose metabolism at baseline (before the stress protocol) and immediately after the situational reminder using the microPET and the radiopharmaceutical ¹⁸F-FDG. The BLA dendritic spines were analyzed according to their number, density, shape and morphometric parameters. Our results show the EBR animals exhibited longer freezing behavior and increased proximal dendritic spines density in the BLA neurons. Neither the amygdaloid glucose metabolism, the types of dendritic spines nor their morphometric parameters showed statistically significant differences. The extreme behavior response induced by this PTSD protocol produces an early increase in BLA spine density, which is unassociated with either additional changes in the shape of spines or metabolic changes in the whole amygdala of Wistar rats.

D-Cycloserine acts via increasing the GluN1 protein expressions in the frontal cortex and decreases the avoidance and risk assessment behaviors in a rat traumatic stress model

D-cycloserine (DCS), an FDA approved anti-tuberculosis drug has extensively been studied for its cognitive enhancer effects in psychiatric disorders. DCS may enhance the effects of fear extinction trainings in animals during exposure therapy and hence we investigated the effects of DCS on distinct behavioral parameters in a predator odor stress model and tested the optimal duration for repeated daily administrations of the agent. Cat fur odor blocks were used to produce stress and avoidance and risk assessment behavioral parameters were used where DCS or saline were used as treatments in adjunct to extinction trainings. We observed that DCS facilitated extinction training by providing further extinction of avoidance responses, risk assessment behaviors and increased the contact with the cue in a setting where DCS was administered before extinction trainings for 3 days without producing a significant tolerance. In amygdala and hippocampus, GluN1 protein expressions decreased 72h after the fear conditioning in the traumatic stress group suggesting a possible down-regulation of NMDARs. We observed that extinction learning increased GluN1 proteins both in the amygdaloid complex and the dorsal hippocampus of the rats receiving extinction training or extinction training with DCS. Our findings also indicate that DCS with extinction training increased GluN1 protein levels in the frontal cortex. We may suggest that action of DCS relies on enhancement of the consolidation of fear extinction in the frontal cortex.

Increased nitric oxide-mediated neurotransmission in the medial prefrontal cortex is associated with the long lasting anxiogenic-like effect of predator exposure

Posttraumatic stress disorder (PTSD) is an anxiety disorder caused by the experience of a severe traumatic event. In rats this disorder has been modeled by exposure to a predator threat. PTSD has been associated to structural and functional changes in the medial prefrontal cortex (mPFC). Direct injections into this brain region of glutamate antagonists or inhibitors of the nitric oxide synthase (NOS) enzyme cause anxiolytic-like effects in rodents. In the present work we investigated if the behavioral changes induced by predator exposure are associated with changes in the mPFC nitrergic system. Since the hippocampus, amygdala and dorsal periaqueductal grey have also been associated to anxiety disorders, including PTSD, we also verified if this procedure would modify the nitrergic system in these regions. Male Wistar rats were exposed to a dummy or live cat for ten minutes and tested in the elevated plus maze test (EPM) seven days later. Immediately after the test their brains were removed for neuronal NOS (nNOS) immunohistochemistry detection and measurements of nitrite/nitrate (NOx) levels. Exposure to the live cat increased freezing responses. One week later the animals that froze when confronted with the cat presented a decreased percentage of entries in the open arms of the EPM and an increased number of nNOS positive neurons in the mPFC and basolateral nucleus of amygdala, but not in the hippocampus, central and medial nuclei of amygdaloid complex or dorsal-lateral periaqueductal grey. Moreover, cat exposed animals showed increased NOx levels in the mPFC but not in the hippocampus one week later. The number of nNOS neurons and NOx levels in the mPFC showed a significant correlation with freezing time during cat exposure. Our results suggest that plastic modifications of the nitrergic system in the mPFC could be related to long lasting behavioral changes induced by severe traumatic events such as predator exposure.