Differential roles of dorsal hippocampal subregions in spatial working memory with short versus intermediate delay

Towards an immuno-precipitated neurodevelopmental animal model of schizophrenia

Epidemiological studies have indicated an association between maternal bacterial and viral infections during pregnancy and the higher incidence of schizophrenia in the resultant offspring post-puberty. One hypothesis asserts that the reported epidemiological link is mediated by prenatal activation of the foetal immune system in response to the elevation of maternal cytokine level due to infection. Here, we report that pregnant mouse dams receiving a single exposure to the cytokine-releasing agent, polyriboinosinic-polyribocytidilic acid (PolyI:C; at 2.5, 5.0, or 10.0 mg/kg) on gestation day 9 produced offspring that subsequently exhibited multiple schizophrenia-related behavioural deficits in adulthood, in comparison to offspring from vehicle injected or non-injected control dams. The efficacy of the PolyI:C challenge to induce cytokine responses in naïve non-pregnant adult female mice and in foetal brain tissue when injected to pregnant mice were further ascertained in separate subjects: (i) a dose-dependent elevation of interleukin-10 was detected in the adult female mice at 1 and 6h post-injection, (ii) 12 h following prenatal PolyI:C challenge, the foetal levels of interleukin-1beta were elevated. The spectrum of abnormalities included impairments in exploratory behaviour, prepulse inhibition, latent inhibition, the US-pre-exposure effect, spatial working memory; and enhancement in the locomotor response to systemic amphetamine (2.5 mg/kg, i.p.) as well as in discrimination reversal learning. The neuropsychological parallels between prenatal PolyI:C treatment in mice and psychosis in humans, demonstrated here, leads us to conclude that prenatal PolyI:C treatment represents one of the most powerful environmental-developmental models of schizophrenia to date. The uniqueness of this model lies in its epidemiological and immunological relevance. It is, sui generis, ideally suited for the investigation of the neuropsychoimmunological mechanisms implicated in the developmental aetiology and disease processes of schizophrenia.

Activity and plasticity in the CA1, the dentate gyrus, and the amygdala following controllable vs. uncontrollable water stress

The level of controllability has been shown to modulate the effects of stress on physiology and behavior. In the present study, we investigated the effects of controllable vs. uncontrollable stressors on plasticity in hippocampal CA1, the dentate gyrus (DG), and basal amygdala nucleus (B) in the rat, using the electrophysiological procedure of long-term potentiation (LTP). A naive group was left undisturbed until the electrophysiological recording commenced. Rats of the two controllable stress groups were trained in the Morris water maze to locate an invisible underwater platform (the first group), or visible platform (the second group), thus escaping from the water, before the recording. The uncontrollable stress group underwent the same procedure (exposure time to water was adjusted to the averaged exposure time of the first controllable group) without the escape platform. We first assessed the effects of stress and controllability on LTP in CA1. Both controllable stressors and the uncontrollable stress impaired CA1 LTP, with a more robust effect induced by the uncontrollable stress. We further assessed the effects of the same procedures on LTP in DG and B. The uncontrollable stress enhanced LTP in DG and increased baseline responses (suggesting uncontrollable stress-induced plasticity) in the amygdala. All the stressors decreased amygdalar LTP. An assessment of plasma levels of corticosterone (CORT), following the behavioral procedures, revealed an enhancement in CORT release following the uncontrollable, but not controllable stress, indicating the uncontrollable condition as the most stressful. These findings provide insight into the differential effects of stress and stress controllability on different hippocampal subregions and the amygdala.

The Role of CA1 in the Acquisition of an Object-Trace-Odor Paired Associate Task

This experiment was designed to determine whether adding a temporal component to an object-odor association task would recruit the hippocampus. The rats were given CA1, CA3, or control lesions prior to learning the object-trace-odor task. Rats were presented with an object for 10 s, after which the object was removed, followed by a 10-s trace period, followed by the presentation of an odor 50 cm away. If the odor and the object were paired, rats were to dig in the odor cup for a reward. If unpaired, rats were to refrain from digging. Rats that had CA1 lesions were unable to make the association, whereas rats that had CA3 lesions performed as well as controls. These results support the idea that the hippocampus is involved in forming arbitrary associations that do not necessarily involve space as long as they involve a temporal component.

Hippocampal CA3-region is crucial for acquisition and memory consolidation in Morris water maze task in mice

This experiment investigated the involvement of the dorsal hippocampal CA3-region in the different phases of learning and memory in spatial and non-spatial tasks. To do so, we temporarily inactivated the CA3-subfield by a focal injection of diethyldithiocarbamate (DDC) which chelates most of the heavy metals present in this region. The effects of temporary inactivation of the CA3-region were examined in an associative task, the Morris water maze (MWM). To study the different phase of memory we used a new behavioural massed-procedure founded on four massed training sessions in the spatial and the non-spatial (cue) version of this task. In the spatial version, we showed that a bilateral injection of DDC into the CA3-region impairs the acquisition but not the recall of spatial information. The main result of this study is that the same injection performed immediately after the training session also perturbed memory consolidation. In the cue version of the MWM, we found no difference between the DDC-injected mice and their controls in acquisition or memory consolidation of non-spatial information. These results suggest that the hippocampal CA3-region is essential for spatial memory processes and specifically in memory consolidation of spatial information.

Knowing Where Things Are in the Second Year of Life: Implications for Hippocampal Development

Prior data have revealed striking contrasts between 18 and 24-month-old children in place learning, an ability knownto dependon the hippo campus(Newcombe, Huttenlocher, Drummey, & Wiley, 1998). The current research examined the development of three other basic abilities of mature spatial competence: the representation of multiple locations, the learning of relations among objects, and the recall of a single location after a substantial filled delay. Results indicated a transition from 18 to 24 months in all three abilities. This evidence supports a general transition in spatial representation that occurs towards the end of infancy. Existing neurobehavioral data suggest that a corresponding change in hippocampal functioning underlies this development.

A Behavioral Assessment of Hippocampal Function Based on a Subregional Analysis

The purpose of this review is to determine whether specific subregions (dentate gyrus [DG], CA3, and CA1) of the hippocampus provide unique contributions to specific processes associated with intrinsic information processing exemplified by novelty detection, encoding, pattern separation, pattern association, pattern completion, retrieval, short-term memory and intermediate-term memory. Based on anatomical neural network organization, electrophysiology of cellular activity, lesions, early gene activation, and computational modeling, it can be shown that there exists extensive cooperation among the three subregions of the hippocampus, but there also exists reliable specificity of function for each of the subregions of the hippocampus. The primary process supported by the DG subregion of the hippocampus can be characterized by orthogonalization of sensory inputs to create a metric spatial representation. Furthermore the DG participates in conjunction with CA3 in supporting spatial pattern separation. The CA3 subregion of the hippocampus supports processes associated with spatial pattern association, spatial pattern completion, novelty detection, and short-term memory. The CA1 subregion of the hippocampus supports processes associated with temporal pattern association, temporal pattern completion, and intermediate-term memory. Furthermore, the CA3 in conjunction with CA1 supports temporal pattern separation. All the above-mentioned processes are assumed to reflect intrinsic processing of information within the hippocampus. The diversity of functions associated with the different subregions of the hippocampus suggests that one should not treat the hippocampus as a single entity, but rather that one should concentrate on elucidating further the functions of both dorsal and ventral subregions of the hippocampus and pathways that directly connect each of the subregions as well as their connections with the entorhinal cortex.