Importantly, a correlation with the amygdala's activation level Before by which the amygdala and hippocampus exert their differential effects. Differential activation of amygdala, dorsal and ventral hippocampus .. (C) Correlation between ERK2 activation in the BLA and ERK2. Differential activation of hippocampus and amygdala following spatial learning under the hippocampus and the amygdala following spatial learning under low or high levels of stress, interaction in relation to memory. Mol. Neurobiol.,
Human and animal studies that have assessed its effects on subsequent memory processing reported that stress may impair, not affect or enhance, memory performance. Exposure to a variety of acute stressors or corticosterone can impede hippocampal-dependent memory processes, such as acquisition Oitzl and de Kloet, ; Kirschbaum et al.
In addition, acute stress can impair long-term potentiation LTP Foy et al. On the other hand, exposure to acute stressful experience or stress hormones can either facilitate or leave unaffected hippocampal memory processes and synaptic plasticity in different hippocampal subregions Servatius and Shors, ; Akirav et al, ; Shors, ; Domes et al.
Studies suggest that the amygdala may mediate stress-related alterations of hippocampal memory processes and LTP Akirav and Richter-Levin, ; Kim et al.
Particularly, the basolateral amygdala complex BLA is believed to be essential for both stress-induced impairment and enhancement of hippocampal function Liang et al. Furthermore, electrical stimulation of the amygdala increases the magnitude of hippocampal dentate gyrus DG LTP Ikegaya et al. Akirav and Richter-Levinelaborated on these effects and reported that BLA activation could induce either an excitatory or an inhibitory effect on DG LTP, depending on the temporal profile of stimulation.
Most recently, Nakao et al. The stress-induced alteration of hippocampal functioning has often been generalized to the hippocampal formation. However, studies indicated that hippocampal subregions display distinct functional profiles. For instance, the DG and CA1 have been shown to be involved in different aspects of spatial learning Gilbert et al.
Differential activation of hippocampus and amygdala following spatial learning under stress.
In fact, studies have shown that the DG and CA1 display a distinct susceptibility to acute stress Gerges et al. Such dissociation in hippocampal subregions may be relevant to the understanding of the twofold effect of stress on hippocampal-dependent learning and memory.
The purpose of this study was to elaborate on the mechanisms underlying these observations and further to assess the effects of amygdala activation on LTP in the CA1 and DG subregions.
They were housed in Plexiglas cages five rats per cage and were maintained on a free-feeding regimen with a All electrophysiological testing was performed at least 1 week after their arrival, during the light phase of the cycle. The experiments were carried out in accordance with the guidelines of the University of Haifa Ethics and Animal Care Committee. The scalp was incised and retracted, and the head position was adjusted to place bregma and lambda in the same horizontal plane.
The amygdaloid complex is reciprocally connected with the hippocampus, mainly through the BLA and posterior cortical nuclei Pitkanen et al.
The hippocampus mediates declarative memory functions and plays an important role in the integration of memory elements at the time of retrieval by assigning significance for events within space and time Squire and Zola-Morgan, It has been suggested that during an emotional experience the amygdala interprets the emotional value of the incoming information while attaching emotional significance to its different aspects Richter-Levin and Akirav, Thus, intensity of the incoming input from the amygdala correlates with the intensity of memory encoding in the hippocampus Canli et al.
The functional connections between the hippocampus and amygdala seem to be centralized to the ventral parts of the hippocampus and the BLA of the amygdala Pitkanen et al. In accordance, gene expression in the dorsal hippocampus DH was shown to correlate with cortical regions involved in information processing, while genes expression in the ventral hippocampus VH correlate with regions involved in emotion and stress such as the amygdala.
These findings had led to the suggestion that the DH performs primarily cognitive functions while the VH relates to stress, emotion, and affect for review see Fanselow and Dong, In regard to stress response, a recent hypothesis by Segal et al. According to this hypothesis, under normal state the hippocampus is linked to the rest of the brain primarily via its dorsal efferents, connected primarily with paleo- and neocortical areas.
However, during a stressful experience the hippocampal functioning is modified such that it emphasizes more its ventral pole, and synchronizes its activity with other stress-related areas of the brain, such as the amygdala. This hypothesis is partially supported by gene expression studies of the DH and VH in rodents Caudal et al.
However, there is no current support to this hypothesis in regard to VH and amygdala co-activation involvement in the retrieval of stressful memory. Extracellular signal-regulated kinase ERK is suggested to represent an essential component of the signal transduction mechanisms sub-serving memory formation.
Its activation was found to be required for the expression of long-term memory LTM induced by fear-conditioning paradigms and spatial learning in the hippocampus and amygdala Adams and Sweatt, ; Sweatt, In accordance with that, studies in our lab have shown that exposure to a contextual reminder of a stressful experience was accompanied by ERK2 activation in the BLA of exposed rats Ilin and Richter-Levin, ; Ardi et al.
We hypothesized that rats exposed to the combination of a contextual reminder, 24 h following an exposure to UWT, will show higher levels of ERK activation in the ventral regions of the hippocampus and in the BLA compared to rats exposed to the UWT or the reminder alone. They had ad libitum access to standard rodent chow pellets and water. This procedure lasted 5 consecutive days. Rats were placed back in the plastic tank the same one that was used in the 5 days of swim experience and in the UWT stress and were given a 30 s swim session.
Following a 2 min period of drying in a neutral cage with dry sawdust rats were returned to their home cage until time for decapitation.
Experimental design The experiment was conducted through four sequential sets of trail runs. UWT and Swim rats were exposed to 5 consecutive days of swim experience. On the 7th day, half of rats from both UWT and Swim groups were exposed to the reminder while the rest of the rats remained in their home cage.