Vandana Zaman

Hippocampal neurotrophin levels after injury: Relationship to the age of the hippocampus at the time of injury.

Aging impairs the competence of the hippocampus for synaptic reorganization after injury. This potentially is due to the inability of the aging hippocampus to up‐regulate the critical neurotrophic factors for prolonged periods after injury to levels at which they can stimulate neurite outgrowth and facilitate synaptic reorganization. We hypothesize that the concentrations of neurotrophins in the hippocampus after injury depend on the age at the time of injury. We quantified the concentrations of brain‐derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin‐3 (NT‐3) in the hippocampus of young, middle‐aged, and aged Fischer 344 rats at 4 days after kainic acid (KA)‐induced injury. In comparison with the age‐matched intact hippocampus, the KA‐lesioned hippocampus exhibited increased levels of BDNF and NGF in all three age groups. In contrast, the NT‐3 concentration was unaltered after KA lesion. Notwithstanding similar percentage increases in BDNF after injury, the lesioned middle‐aged and aged hippocampus contained 45–52% less BDNF than the lesioned young hippocampus. NGF and NT‐3 levels after injury were comparable across the three age groups, however. Furthermore, lower BDNF concentration in the injured aging hippocampus was associated with normal astrocytic response but significantly diminished microglial reaction. Thus, in comparison with the injured young hippocampus, the injured aging hippocampus contains considerably less BDNF but similar levels of NGF and NT‐3. Lower BDNF levels in the injured aging hippocampus might underlie the diminished spontaneous healing response observed in the aging hippocampus after injury, particularly in terms of synaptic reorganization and dentate neurogenesis.

Hippocampal neurotrophin levels in a kainate model of temporal lobe epilepsy: a lack of correlation between brain‐derived neurotrophic factor content and progression of aberrant dentate mossy fiber sprouting

A significant upregulation of neurotrophins particularly brain‐derived neurotrophic factor (BDNF) is believed to be involved in the initiation of epileptogenic changes such as the aberrant axonal sprouting and synaptic reorganization in the injured hippocampus. However, it is unknown which of the neurotrophins are upregulated during the peak period of aberrant mossy fiber sprouting in the chronically injured hippocampus. We measured chronic changes in the levels of BDNF, nerve growth factor (NGF) and neurotrophin‐3 (NT‐3) in the adult hippocampus using enzyme‐linked immunosorbent assay (ELISA) after a unilateral intracerebroventricular administration of kainic acid (KA), a model of temporal lobe epilepsy. For comparison, neurotrophins were also measured from the control intact hippocampus. Further, to see the association between changes in neurotrophin levels and the progression of mossy fiber sprouting, chronic changes in the mossy fiber distribution within the dentate supragranular layer (DSGL) were quantified. In the KA‐lesioned hippocampus, the neurotrophins BDNF and NGF were upregulated at 4 days post‐lesion, in comparison to their levels in the intact hippocampus. However, the concentration of BDNF reached the baseline level at 45 days post‐lesion and dramatically diminished at 120 days post‐lesion. In contrast, the upregulation of NGF observed at 4 days post‐lesion was sustained at both 45 days and 120 days post‐lesion. The concentration of NT‐3 was upregulated at 45 days post‐lesion but remained comparable to baseline levels at 4 days and 120 days post‐lesion. Interestingly, analysis of mossy fiber sprouting revealed that most of the aberrant sprouting in the lesioned hippocampus occurs between 45 days and 120 days post‐lesion. Taken together, these results suggest that the period of robust mossy fiber sprouting does not correlate with the phase of post‐lesion BDNF upregulation. Rather, it shows a relationship with the time of upregulation of neurotrophins NGF and NT‐3.

Repair of the injured adult hippocampus through graft-mediated modulation of the plasticity of the dentate gyrus in a rat model of temporal lobe epilepsy.

Intracerebroventricular kainate administration in rat, a model of temporal lobe epilepsy (TLE), causes degeneration of the hippocampal CA3 pyramidal and dentate hilar neurons. This leads to a robust but aberrant sprouting of the granule cell axons (mossy fibers) into the dentate supragranular layer and the CA3 stratum oriens. Because this plasticity is linked to an increased seizure susceptibility in TLE, strategies that restrain the aberrant mossy fiber sprouting (MFS) are perceived to be important for preventing the TLE development after the hippocampal injury. We ascertained the efficacy of fetal hippocampal CA3 or CA1 cell grafting into the kainate-lesioned CA3 region of the adult rat hippocampus at early post-kainic acid injury for providing a lasting inhibition of the aberrant MFS. Analyses at 12 months after grafting revealed that host mossy fibers project vigorously into CA3 cell grafts but avoid CA1 cell grafts. Consequently, in animals receiving CA3 cell grafts, the extent of aberrant MFS was minimal, in comparison with the robust MFS observed in both “lesion-only” animals and animals receiving CA1 cell grafts. Analyses of the graft axon growth revealed strong graft efferent projections into the dentate supragranular layer with CA3 cell grafting but not with CA1 cell grafting. Thus, the formation of reciprocal circuitry between the dentate granule cells and the grafted CA3 pyramidal neurons is likely the basis of inhibition of the aberrant MFS by CA3 cell grafts. The results also underscore that grafting of cells capable of differentiating into CA3 pyramidal neurons is highly efficacious for a lasting inhibition of the abnormal mossy fiber circuitry development in the injured hippocampus.

Pattern of long-distance projections from fetal hippocampal field CA3 and CA1 cell grafts in lesioned CA3 of adult hippocampus follows intrinsic character of respective donor cells

Fetal hippocampal grafts transplanted to the lesioned CA3 of adult hippocampus can extend axonal projections to many regions of the host brain. However, the identity of grafted cells that project to specific host regions is unknown. We hypothesize that the pattern of long-distance axonal projections from distinct fetal hippocampal cells grafted to lesioned CA3 is specified by the intrinsic nature of respective donor cells rather than characteristics of the host graft region. We grafted fetal hippocampal CA3 or CA1 cells into kainic acid lesioned CA3 of adult hippocampus at four days post-lesion. Neurons projecting to either the contralateral hippocampus or the ipsilateral septum were then measured in these grafts at four months post-grafting using Fluoro-Gold and DiI tract tracing. CA3 grafts located close to the degenerated CA3 cell layer showed a high propensity for establishing projections into the contralateral hippocampus (commissural projections) compared to similarly located CA1 grafts, which exhibited negligible commissural projections. Similar distinction was observed between the two graft types even when they were located only partially in the lesioned CA3. Among CA3 grafts, those placed near the degenerated CA3 cell layer established significantly greater commissural projections than those placed only partially in the CA3 region. Septal projections, in contrast, were robust from both CA3 and CA1 grafts. This differential projection pattern between CA3 and CA1 grafts resembles projections of CA3 and CA1 cells in intact hippocampus.These results demonstrate that the intrinsic character of grafted fetal cells determines the type of efferent projections from fetal grafts into different targets in the lesioned adult host brain. However, the extent of efferent projections from specific grafts is also influenced by the location of grafted cells within the host region. Thus, graft-mediated appropriate reconstruction of damaged circuitry in the lesioned brain may require grafting of homotopic donor cells. Further, the robust and specific projections observed from CA3 grafts is likely beneficial for functional recovery of hippocampus following CA3 injury and hence of significance towards developing a graft-mediated therapy for human temporal lobe epilepsy.

Survival of fetal hippocampal CA3 cell grafts in the middle-aged and aged hippocampus: Effect of host age and deafferentation

The potential application of neural transplantation to many neurodegenerative disorders at early stages of disease progression would involve middle‐aged and aged persons. Hence, it is important to examine critically the extent of graft cell survival in both intact and partially deafferented middle‐aged and aged brain. We investigated the degree of survival of 5′‐bromodeoxyuridine (BrdU)‐labeled fetal hippocampal CA3 cells after grafting into both intact hippocampus and partially deafferented hippocampus (i.e., hippocampus contralateral to intracerebroventricular administration of kainic acid) of middle‐aged and aged Fischer 344 rats. Absolute cell survival within these grafts was rigorously analyzed using BrdU immunostaining of serial sections and the optical fractionator cell counting method. In the intact hippocampus, graft cell survival was 23% of injected cells for middle‐aged rats and 18% for aged rats, which is consistent with the survival of fetal hippocampal cells in the intact young adult hippocampus reported earlier (Shetty and Turner [1995] Neuroscience 67:561–582). A partial deafferentation at the time of grafting significantly enhanced the degree of graft cell survival to 35% of injected cells in the middle‐aged hippocampus and 27% in the aged hippocampus. However, the overall graft cell survival after deafferentation was significantly (30%) greater in the middle‐aged hippocampus compared with the aged hippocampus. These results reveal that 1) the degree of survival of fetal neural cells in the intact mature brain remains constant with aging and 2) a partial deafferentation of the mature host brain at the time of grafting enhances survival of grafted fetal cells, regardless of the host age. However, the overall extent of graft cell survival after deafferentation depends on the age of the mature brain at the time of deafferentation.