Rescue of NGF-deficient mice II: basal forebrain cholinergic projections require NGF for target innervation but not guidance

The effect of nerve growth factor on supporting spatial memory depends upon hippocampal cholinergic innervation

Nerve growth factor (NGF) gene therapy has been used in clinical trials of Alzheimer's disease. Understanding the underlying mechanisms of how NGF influences memory may help develop new strategies for treatment. Both NGF and the cholinergic system play important roles in learning and memory. NGF is essential for maintaining cholinergic innervation of the hippocampus, but it is unclear whether the supportive effect of NGF on learning and memory is specifically dependent upon intact hippocampal cholinergic innervation. Here we characterize the behavior and hippocampal measurements of volume, neurogenesis, long-term potentiation, and cholinergic innervation, in brain-specific Ngf-deficient mice. Our results show that knockout mice exhibit increased anxiety, impaired spatial learning and memory, decreased adult hippocampal volume, neurogenesis, short-term potentiation, and cholinergic innervation. Overexpression of Ngf in the hippocampus of Ngf gene knockout mice rescued spatial memory and partially restored cholinergic innervations, but not anxiety. Selective depletion of hippocampal cholinergic innervation resulted in impaired spatial memory. However, Ngf overexpression in the hippocampus failed to rescue spatial memory in mice with hippocampal-selective cholinergic fiber depletion. In conclusion, we demonstrate the impact of Ngf deficiency in the brain and provide evidence that the effect of NGF on spatial memory is reliant on intact cholinergic innervations in the hippocampus. These results suggest that adequate cholinergic targeting may be a critical requirement for successful use of NGF gene therapy of Alzheimer's disease.

The p75 neurotrophin receptor has nonapoptotic antineurotrophic actions in the basal forebrain

Because of controversy about the role of the p75 neurotrophin receptor (p75(NTR) ) in the cholinergic basal forebrain (CBF), we investigated this region in p75(NTR) third exon knockout mice that were congenic with 129/Sv controls. They express a shortened intracellular form of p75(NTR) , permitting detection of p75(NTR) -expressing cells. We performed separate counts of choline acetyltransferase (ChAT)-expressing and p75(NTR) -expressing neurons. In agreement with past reports, the number of ChAT-immunoreactive neurons in knockout mice was greater than in wild-type mice, and this was evident in each of the main anatomical divisions of the CBF. In contrast, the number of p75(NTR) -immunoreactive neurons did not differ between genotypes. The biggest increase in ChAT neurons (27%) was in the horizontal limb of the diagonal band of Broca (HDB), in which region the number of p75(NTR) -positive neurons was unchanged. Double staining revealed that some neurons in wild-type mice expressed p75(NTR) but not ChAT. In the knockout mice, all p75(NTR) -expressing neurons expressed ChAT. The increase in cholinergic neurons, therefore, was at least partially attributable to a higher proportion of ChAT immunoreactivity within the population of p75(NTR) -expressing neurons. Cholinergic neurons were also larger in knockout mice than in controls. In the hippocampal CA1 region, knockout mice had a greater number of cholinergic fibers. There was a 77% increase in hippocampal ChAT activity in knockout mice and a 38% increase in heterozygotes. The data do not support an apoptotic role but indicate a broad antineurotrophic role of p75(NTR) in the cholinergic basal forebrain.

Small molecule activators of the Trk receptors for neuroprotection

The neurotrophin signaling network is critical to the development and survival of many neuronal populations. Especially sensitive to imbalances in the neurotrophin system, cholinergic neurons in the basal forebrain are progressively lost in Alzheimer's disease. Therapeutic use of neurotrophins to prevent this loss is hampered, however, by a number of pharmacological challenges. These include a lack of transport across the blood-brain barrier, rapid degradation in the circulation, and difficulty in production. In this review we discuss the evidence supporting the neurotrophin system's role in preventing neurodegeneration and survey some of the pharmacological strategies being pursued to develop effective therapeutics targeting neurotrophin function.

Developmental changes of neurotrophin mRNA expression in the layers of rat visual cortex

Neurotrophins are essential factors for the structural, neurochemical and functional maturation of the brain including developmental and adult plasticity. Northern blots and polymerase chain reaction revealed the expression of neurotrophin 4 (NT4), neurotrophin 3 (NT3), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in the cortex. The cellular producers of NT3 and BDNF have been characterized by anatomical methods as being mostly pyramidal, and the tyrosine kinase B (TrkB) receptor is expressed by many cortical neurons. However, these methods have so far failed to identify the cells producing NT4 and NGF mRNA. These factors are much lower in expression than, e.g. BDNF, and apparently remain below detection levels of in situ hybridization. Given their specific actions on cell types and afferent systems, knowledge about the producing cell types is highly desirable. To narrow down on the producing cell types, we quantified by reverse transcriptase-polymerase chain reaction (RT-PCR) the developmental changes of BDNF, NT3, NT4, NGF and TrkB mRNA expression in total visual cortex lysates, and in the cortical layers dissected by tangential cryostat sectioning. We found dramatic changes in laminar expression of NT3 and NGF, mild changes of NT4, and no changes of BDNF and TrkB mRNA. For instance, NT3 is important early on for thalamocortical axons, and we found transient peaks of NT3 mRNA expression first in layer VI, then in layer IV. NT4 mRNA was in layers IV and VI, suggesting NT4 protein production in thalamorecipient layers, but peak expression gradually shifted to upper layers as did NGF expression. The layer-specific developmental expression shifts of neurotrophin mRNAs correlate with morphogenetic processes.

Cholinergic circuits and signaling in the pathophysiology of schizophrenia

Central cholinergic signaling has long been associated with aspects of memory, motivation, and mood, each affected functions in neuropsychiatric disorders such as schizophrenia. In this chapter, we review evidence related to the core hypothesis that dysregulation of central cholinergic signaling contributes to the pathophysiology of schizophrenia. Although central cholinergic circuits are resistant to simplification-particularly when one tries to parse the contributions of various classes of cholinergic receptors to disease related phenomena--the potential role of ACh signaling in Schizophrenia pathophysiology deserves careful consideration for prospective therapeutics. The established role of cholinergic circuits in attentional tuning is considered along with recent work on how the patterning of cholinergic activity may modulate corticostriatal circuits affected in schizophrenia.