Basal forebrain grafts in the hippocampus and neocortex: regulation of acetylcholine release

Enhanced spatial memory and hippocampal long-term potentiation in p75 neurotrophin receptor knockout mice

Previous reports have described increases in the size and number of cholinergic neurons in the basal forebrain in p75 neurotrophin receptor (p75(NTR)) knockout mice. In an earlier study, we also found improved spatial memory in these mice, raising the possibility that p75(NTR) regulates hippocampal function by its effects on the cholinergic basal forebrain. We therefore investigated hippocampal long-term potentiation in p75(NTR) knockout mice that shared the same genetic background as control 129/Sv mice. We also investigated heterozygous mice, carrying just one functional p75(NTR) allele. The p75(NTR) knockout mice had enhanced long-term potentiation in the Schafer collateral fiber synapses of the hippocampus. Heterozygous mice had an intermediate level, greater than controls but less than knockout mice. Hippocampal choline acetyltransferase activity was also markedly elevated in p75(NTR) knockout mice, with a smaller increase in heterozygous mice. In the Barnes maze, p75(NTR) knockout mice displayed markedly superior learning to controls, and this was evident over the three age brackets tested. At each age, the performance of heterozygous mice was intermediate to the other groups. In the open field test, p75(NTR) knockout mice exhibited greater stress-related behavioral responses, including freezing, than did control animals. There were no differences between the three groups in a test of olfactory function. The dose-dependent effects of p75(NTR) gene copy number on hippocampal plasticity and spatial memory indicate that p75(NTR) has profound effects on hippocampal function. Bearing in mind that p75(NTR) is very sparsely expressed in the adult hippocampus and has a potent effect on hippocampal choline acetyltransferase activity, the effects of p75(NTR) on hippocampal function are likely to be mediated indirectly, by its actions on basal forebrain cholinergic neurons.

Biologic Transplantation and Neurotrophin-Induced Neuroplasticity After Traumatic Brain Injury

OBJECTIVE

In this review, we analyze progress in the treatment of traumatic brain injury with neurotrophins, growth factors and cell and tissue neurotransplantation. The primary objective of these therapies is to reduce neurologic deficits associated with the trauma by inducing neuroplasticity. These therapies are restorative and not necessarily neuroprotective.

MAIN OUTCOME MEASURES

An extensive literature on administration of neurotrophics factors and cell and tissue cerebral transplantation is reviewed. The effects of these therapeutic approaches on brain biochemical, molecular, cellular, and tissue responses are summarized.

CONCLUSION

The cumulative data indicate that cell therapy shows substantial promise in the treatment of neural injury.

Reduced evoked release of acetylcholine in the rodent hippocampus following traumatic brain injury

The chronic effects of traumatic brain injury on acetylcholine release were evaluated by using in vivo microdialysis. Acetylcholine release was measured in the hippocampus of anesthetized rats 2 weeks after lateral controlled cortical impact (n = 10) or sham surgery (n = 10). Prior to microdialysis, behavioral assessments of motor and spatial memory were performed. Cortical impact (6 meter/s, 2 mm deformation) produced beam balance deficits that persisted for 1 day and beam walking deficits that persisted for 3 days after injury. In addition, spatial memory, as measured by swim latencies in a Morris water maze, was compromised between 10-14 days after injury. Immediately following behavioral testing, the animals were anesthetized with halothane, and a microdialysis probe was placed into the dorsal hippocampus. After a 160 min equilibration period, extracellular levels of acetylcholine were measured prior to and after an intraperitoneal administration of scopolamine (1 mg/kg), which evokes acetylcholine release by blocking autoreceptors. Prior to scopolamine administration, there were no differences in extracellular levels of acetylcholine between injured and sham animals. However, there was a significant reduction of hippocampal acetylcholine release evoked by scopolamine in injured animals as compared to sham controls. In separate control groups, saline administration alone did not change hippocampal acetylcholine release in injured (n = 5) or sham (n = 5) animals. This study represents the first application of in vivo microdialysis to evaluate chronic neurotransmission deficits following TBI. The present study demonstrates that a magnitude of traumatic brain injury (TBI) sufficient to produce spatial memory deficits can result in a reduction in scopolamine-evoked release of acetylcholine within the hippocampus. The data further suggest that presynaptic mechanisms mediating release of acetylcholine could play a significant role in cholinergic neurotransmission deficits following TBI.