The relationship between tetanus intensity and the magnitude of hippocampal long-term potentiation in vivo.

In this study, we assessed the effects of varying tetanus and test-pulse intensity on the magnitude of long-term potentiation (LTP) in the perforant path-dentate gyrus projection of urethane-anaesthetized rats. We developed a novel within-subjects procedure in which test-pulse-stimulation intensity (60-1000 µA) was varied quasi-randomly under computer control throughout the recording period. After a baseline period, we applied a high-frequency tetanus, the intensity of which was varied over the same range as test-pulse intensity, but between subjects. The time-course of LTP was thus monitored continuously across a range of test-pulse intensities in each rat. Intense high-frequency tetanization at 1000 µA resulted in a paradoxical depression of the dentate field excitatory post-synaptic potential (fEPSP) slope at the lowest test intensity used (60 µA), but caused a potentiation at higher test intensities in the same animal. Moreover, intense tetanization induced less LTP than a moderate tetanus over most of the test-intensity range. Explanations for this pattern of data include a potentiation of feed-forward inhibition in conjunction with LTP of excitatory neurotransmission, or local tissue damage at the stimulation site. To address this issue, we conducted an additional experiment in which a second stimulating electrode was placed in the perforant path at a site closer to the dentate, in order to activate a common population of afferents at a location 'downstream' of the original stimulation site. After 1000-µA tetanization of the original ('upstream') site, fEPSPs were again depressed in response to test stimulation of the upstream site, but only potentiation was observed in response to stimulation of the downstream site. This is consistent with the idea that the depression induced by intense tetanization results from local changes at the stimulation site. In conclusion, while tetanus intensity must exceed the LTP induction threshold, intensities above 500 µA should be avoided; in the present study, tetanization at 250-500 µA yielded maximal levels of LTP.

Synaptic tagging and capture in the living rat.

In isolated hippocampal slices, decaying long-term potentiation can be stabilized and converted to late long-term potentiation lasting many hours, by prior or subsequent strong high-frequency tetanization of an independent input to a common population of neurons-a phenomenon known as 'synaptic tagging and capture'. Here we show that the same phenomenon occurs in the intact rat. Late long-term potentiation can be induced in CA1 during the inhibition of protein synthesis if an independent input is strongly tetanized beforehand. Conversely, declining early long-term potentiation induced by weak tetanization can be converted into lasting late long-term potentiation by subsequent strong tetanization of a separate input. These findings indicate that synaptic tagging and capture is not limited to in vitro preparations; the past and future activity of neurons has a critical role in determining the persistence of synaptic changes in the living animal, thus providing a bridge between cellular studies of protein synthesis-dependent synaptic potentiation and behavioural studies of memory persistence.

Mapping immediate-early gene activity in the rat after place learning in a water-maze: the importance of matched control conditions.

The expression of two immediate-early genes (IEGs), Zif268 and c-Fos, was quantified in hippocampal subregions and related structures following spatial learning in the Morris water-maze. A critical feature was the novel control protocol alongside more standard controls, the purpose of which was to test whether hippocampal activity is set automatically when traversing an environment or whether it is dependent on reaching a specific goal using learning that requires the hippocampus (i.e. task dependent). The new control protocol (Procedural Task) made it possible to match swim time, swim distance and learning to escape from water with that of the experimental (Working Memory) group. Unlike the Working Memory group, the Procedural Task animals showed no evidence of learning the absolute platform location during the test session. While the Working Memory rats showed c-Fos increases relative to the Procedural Task controls in the frontal and parahippocampal cortices, hippocampal levels did not differ. Again, for Zif268 there was no evidence of a relative increase of hippocampal activity in the Working Memory group. In fact, hippocampal Zif268 showed evidence of a relative decrease, even though the spatial working memory task is hippocampal dependent. The study not only highlighted the shortcomings of other control procedures used in water-maze studies (free-swimming or home cage control), but also indicated that the expression of these IEGs in the hippocampus is not a direct predictor of explicit spatial location learning. Rather, the activity in combinations of regions, including prefrontal cortex, provides a stronger correlate of water-maze learning.

Anterior thalamic lesions produce chronic and profuse transcriptional de-regulation in retrosplenial cortex: A model of retrosplenial hypoactivity and covert pathology.

Anterior thalamic lesions are thought to produce 'covert pathology' in retrosplenial cortex, but the causes are unknown. Microarray analyses tested the hypothesis that thalamic damage causes a chronic, hypo-function of metabolic and plasticity-related pathways (Experiment 1). Rats with unilateral, anterior thalamic lesions were exposed to a novel environment for 20 minutes, and granular retrosplenial tissue sampled from both hemispheres 30 minutes, 2h, or 8h later. Complementary statistical approaches (analyses of variance, predictive patterning and gene set enrichment analysis) revealed pervasive gene expression differences between retrosplenial cortex ipsilateral to the thalamic lesion and contralateral to the lesion. Selected gene differences were validated by QPCR, immunohistochemistry (Experiment 1), and in situ hybridisation (Experiment 2). Following thalamic lesions, the retrosplenial cortex undergoes profuse cellular transcriptome changes including lower relative levels of specific mRNAs involved in energy metabolism and neuronal plasticity. These changes in functional gene expression may be largely driven by decreases in the expression of multiple transcription factors, including brd8, c-fos, fra-2, klf5, nfix, nr4a1, smad3, smarcc2, and zfp9, with a much smaller number (nfat5, neuroD1, RXRγ) showing increases. These findings have implications for conditions such as diencephalic amnesia and Alzheimer's disease, where both anterior thalamic pathology and retrosplenial cortex hypometabolism are prominent.