Maze performance in rats with hippocampal perforant paths lesions: Some aspects of functional recovery

Encoding versus retrieval of spatial memory: Double dissociation between the dentate gyrus and the perforant path inputs into CA3 in the dorsal hippocampus

The hippocampus is an essential neural structure for spatial memory. Computational models suggest that the CA3 subregion of the hippocampus plays an essential role in encoding and retrieval of spatial memory. The perforant path (PPCA3) and dentate gyrus (DG)-mediated mossy fibers (MFs) compose major afferent inputs into CA3. A possible functional dissociation between these afferent inputs was attempted using a simple navigation test (i.e., the modified Hebb-Williams maze). Behavioral testing was combined with electrolytic lesions of PPCA3 or neurotoxic lesions of the DG, to eliminate each afferent input into CA3. Lesions in either afferent input into CA3 affected learning of an effective navigational path on the maze. The contributions of the two CA3 afferent inputs, however, were different regarding encoding and retrieval of memory measured based on indices operationally defined for the behavioral paradigm (i.e., encoding, the number of errors reduced within a day; retrieval, the number of errors reduced between days). The DG-lesioned animals exhibited deficits regarding the encoding index, but not the retrieval index, whereas the PPCA3-lesioned rats displayed deficits regarding the retrieval index, but not the encoding index. The results suggest that the two major afferent inputs of CA3 may contribute differentially to encoding and retrieval of spatial memory.

Cholinergic modulation of the hippocampus during encoding and retrieval

The present experiments were aimed at determining whether acetylcholine (ACh) plays a role in encoding and retrieval of spatial information using a modified Hebb-Williams maze. In addition, the present experiments tested two computational models of hippocampal function during encoding and retrieval using a maze sensitive to hippocampal disruption. Thirty male, Long-Evans rats served as subjects. Chronic cannulae were implanted bilaterally into the CA3 (n=26) and CA1 (n=5) subregions of the hippocampus. Rats were tested using a modified Hebb-Williams maze. In the first experiment, rats were injected with either saline or scopolamine hydrobromide 10 min before testing for each day. The number of errors made per day per group was used as the measure of learning. Encoding was assessed by the average number of errors made on the first five trials of Day 1 compared to the last five trials of Day 1, whereas the average number of errors made on the first five trials of Day 2 compared to the last five trials of Day I was used to assess retrieval. No deficit was found for the saline group. The scopolamine group showed a deficit in encoding, but not retrieval. In the second experiment, rats were injected with either saline or physostigmine 10 min before testing each day. In contrast to the scopolamine groups, the physostigmine group showed a deficit in retrieval, but not encoding. To test whether the retrieval deficit was due to a disruption in storage or gaining access to the information two groups of rats received either saline on Day 1 and physostigmine on Day 2 or physostigmine on Day 1 and saline on Day 2. In addition, one group received physostigmine immediately after testing on Day 1. Data indicate that physostigmine causes a disruption of retrieval by means of a disruption in consolidation process. In conclusion, the cholinergic antagonist, scopolamine, disrupts encoding in both CA3 and CA1 subregions of the hippocampus. Furthermore, the cholinesterase inhibitor, physostigmine, boosts ACh action during a time when cholinergic levels need to decline for proper consolidation.

Involvement of non-neuronal cells in entorhinal-hippocampal reorganization following lesions

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main hippocampal termination zones. Subsequently, remaining fibers sprout and form new synapses on the denervated dendrites. This degeneration and reorganization is accompanied by sequential changes in glial morphology and function. Within a few hours following the lesion, amoeboid microglia migrate into the zone of denervation. Some hours later, signs of activation can be seen on astrocytes in the zone of denervation, where both cell types proliferate and remain in an activated state for more than two weeks. These activated glial cells might be involved in lesion-induced plasticity in at least two ways: (1) by releasing cytokines and growth factors which regulate layer-specific sprouting and (2) by phagocytosis of axonal debris, because myelin sheaths act as obstacles for sprouting fibers in the central nervous system. Whereas direct evidence for the former is still missing, the latter was investigated using phagocytosis-dependent labeling techniques. Both microglial cells and astrocytes incorporate axonal debris. Phagocytosing microglial cells develop the immune phenotype of antigen-presenting cells, whereas astrocytes strongly express FasL (CD95L), which induces apoptosis of activated lymphocytes. Thus, the interaction of glial cells with immune cells might be another, previously underestimated, aspect of reorganization following entorhinal lesion.

Astrocytes and microglial cells incorporate degenerating fibers following entorhinal lesion: a light, confocal, and electron microscopical study using a phagocytosis-dependent labeling technique

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main termination zone in the outer molecular layers of the dentate gyrus. Concomitantly, astrocytes become hypertrophic, and microglial cells alter their phenotype, suggesting participation in anterograde degeneration. This study analyzes the involvement of these lesion-induced activated glial cells in the process of phagocytosis of degenerated axonal debris. We established a phagocytosis-dependent labeling technique that allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells. Stereotaxic application of small crystals of the biotin- and rhodamine-conjugated dextran amine Mini Ruby (MR) into the entorhinal cortex led to strong and stable axonal staining of perforant path axons. Following entorhinal lesion, labeled terminals and fibers condensed and formed small granules. Incorporation of these rhodamine-fluorescent granules resulted in a phagocytosis-dependent cell labeling. During the first 3 days, we were able to identify these cells as microglia by using double-fluorescence and confocal microscopy. The first unequivocally double-labeled astrocytes were found 6 days post lesion (dpl). Whereas in all stages a subpopulation of microglial cells remained devoid of MR-labeled granules, all astrocytes in the middle molecular layer were double-labeled after long survival times (20 dpl). On the ultrastructural level, labeled granules appeared to be perforant path axons containing the tracer. Both terminals and myelinated fibers could be seen inside the cytoplasm of microglial cells and astrocytes. Thus, anterograde degeneration is a sufficient stimulus to induce axon incorporation by both astrocytes and a subpopulation of microglial cells.

Evidence for recovery of spatial learning following entorhinal cortex lesions in mice

The influence of entorhinal cortex lesions on behaviour and concommitant changes in synaptophysin immunoreactivity (IR) in the denervated dentate gyrus was assessed. Male, C57/B6 mice received either bilateral (BI), unilateral (UNI), or no lesion (SHAM) to the entorhinal cortex. At various stages post-lesion the animals were evaluated in tests to examine neurological and cognitive (spatial and cued learning, Morris water maze) function. UNI lesioned animals from 6-36 days post-lesion showed no neurological nor marked cued learning deficit, yet a profound spatial learning deficit. However by 70 days post-lesion, spatial learning ability was clearly evident. In contrast, BI lesioned animals showed severe spatial learning deficits throughout the test period (6-70 days), cued learning was also impaired. In parallel groups of UNI lesioned mice, 6-36 days post-lesion there was a marked reduction (-40%) in synaptophysin IR in the dentate gyrus molecular layer. However by 70 days post-lesion a clear increase in this measure was noted. Changes in the expression of the growth associated protein, GAP43, were also noted over this period. Taken together, the present results suggest some recovery of spatial learning following unilateral entorhinal cortex lesions in mice. This behavioural recovery of a hippocampally dependant task may be associated with a recovery of function related to the synaptic remodelling and elevation of synapse number in the denervated hippocampus, as evidenced by changes in synaptophysin and GAP43 IR.

Transneuronal changes in dendrites of GABAergic parvalbumin-containing neurons of the rat fascia dentata following entorhinal lesion

The perforant path fibers from the entorhinal cortex form synapses with both granule cells and GABAergic, parvalbumin-containing (PARV) nongranule cells. The authors recently reported a persistent reduction of PARV-positive dendrites in the termination zones of entorhinal fibers in the hippocampus proper and fascia dentata after lesion of the entorhinal cortex. In the present study the authors analyzed the effects of de-entorhination on the ultrastructure of postsynaptic PARV-positive dendrites in the molecular layer of the fascia dentata. PARV immunocytochemistry was performed 2, 8, 55, and 360 days after an ipsilateral entorhinal lesion and, for comparison, 10 days after an ipsilateral fimbria-fornix transection that disconnects the hippocampus from its septal and commissural afferents. Two days after entorhinal lesion, the authors observed swelling of the tissue close to the hippocampal fissure. Adjacent distal dendritic tips of PARV-positive dentate neurons appeared bloated and reduced in number. Reduction of PARV-positive dendrites in the former perforant path termination zone persisted 55 days after entorhinal lesion and could still observed after postlesional survival times for 1 year. Degenerating axon terminals were still present 55 days following lesion and PARV-positive dendrites exhibited abnormal invaginations. Fimbria transection did not result in similar dendritic changes in PARV-positive neurons. The results indicate a long-lasting process of reorganization in the molecular layer of the fascia dentata following entorhinal lesion and persisting changes in the morphology of PARV-immunoreactive dendrites. Entorhinal fibers seem to play a specific role for the maintenance of these dendrites, since similar changes did not occur following removal of septal and commissural fibers.

Effects of tetrahydroaminoacridine (THA) on functional recovery after sequential lesion of the entorhinal cortex

Unilateral lesions of rat entorhinal cortex produce a transitory performance deficit on spatial learning tasks, such as reinforced alternation in a T-maze. Tetrahydroaminoacridine (THA), a cholinesterase inhibitor, was administered to determine its effects on behavioral recovery using a reinforced alternation task in a T-maze. Rate of recovery after unilateral entorhinal lesion was not affected by a low dose of THA (0.05 mg/kg), while a higher dose (5.0 mg/kg) impaired recovery. Behavioral recovery was subsequently evaluated in the same rats following lesions to the contralateral entorhinal cortex. Serial bilateral lesions of the entorhinal cortex are known to produce a prolonged performance deficit on the alternation task. The 0.05 mg/kg THA group exhibited an intermediate rate of recovery, between the undamaged control group and bilateral lesion-saline injected groups. The group receiving 5.0 mg/kg of THA after bilateral lesion did not differ from the bilateral lesion-saline group. The failure of THA to significantly improve functional recovery in rats with lesions of the entorhinal cortex indicates that the compound may have limited applicability in treating human neurodegenerative disorders such as Alzheimer's disease.

Selective working memory impairments following intradentate injection of colchicine: attenuation of the behavioral but not the neuropathological effects by gangliosides GM1 and AGF2

Bilateral injection of 3.5 micrograms of colchicine into the dentate gyrus produced specific learning and memory impairments together with a selective pattern of neuropathology. Animals injected with colchicine exhibited a significant impairment in their ability to perform the working memory, but not the reference memory, component of a multiple component T-maze task. These deficits were transient and over time all animals were able to reaquire the task to preoperative levels of performance. Histological analyses revealed that intradentate injection of colchicine produced 1) a significant decrease in the width of both the superior and inferior blades of the dentate gyrus reflecting the extensive loss of granule cells, 2) a related decrease in the size of the dentate molecular layer, and 3) a decrease in the number of cholinergic neurons in the medial septum. The second phase of the experiment demonstrated that gangliosides GM1 and AGF2 did not prevent the initial impairments in working memory performance induced by colchicine but rather accelerated the rate at which it recovered. The gangliosides did not decrease the extent of neuronal damage; there was no sparing of granule cells in the dentate gyrus or cholinergic neurons in the medial septum. These data further support a role for the hippocampus in working memory processes and they also indicate that gangliosides GM1 and AGF2 might be useful for treating the behavioral deficits induced by hippocampal damage.

d-Amphetamine enhances memory performance in rats with damage to the fimbria

Rats were preoperatively trained on a 5-unit linear maze and were then subjected to fimbria lesions. The animals were then retested on the same task with one group of rats with fimbria lesions and a control group being injected daily with 0.5 mg/kg d-amphetamine sulfate prior to testing. Lesions significantly impaired postoperative performance of the task, while amphetamine facilitated performance in fimbria lesioned rats. Due to an optimal learning of the task, performance of control animals was not significantly facilitated. These results raise several important issues including the mechanisms of functional recovery after brain lesions and the role of the hippocampal formation in learning and memory.