Pharmacology of nerve growth factor in the brain

Hippocampal region-specific endogenous neuroprotection as an approach in the search for new neuroprotective strategies in ischemic stroke. Fiction or fact?

Ischemic stroke is the leading cause of death and long-term disability worldwide, and, while considerable progress has been made in understanding its pathophysiology, the lack of effective treatments remains a major concern. In that context, receiving more and more consideration as a promising therapeutic method is the activation of natural adaptive mechanisms (endogenous neuroprotection) - an approach that seeks to enhance and/or stimulate the endogenous processes of plasticity and protection of the neuronal system that trigger the brain's intrinsic capacity for self-defence. Ischemic preconditioning is a classic example of endogenous neuroprotection, being the process by which one or more brief, non-damaging episodes of ischemia-reperfusion (I/R) induce tissue resistance to subsequent prolonged, damaging ischemia. Another less-known example is resistance to an I/R episode mounted by the hippocampal region consisting of CA2, CA3, CA4 and the dentate gyrus (here abbreviated to CA2-4, DG). This can be contrasted with the ischemia-vulnerable CA1 region. There is not yet a good understanding of these different sensitivities of the hippocampal regions, and hence of the endogenous neuroprotection characteristic of CA2-4, DG. However, this region is widely reported to have properties distinct from CA1, and capable of generating resistance to an I/R episode. These include activation of neurotrophic and neuroprotective factors, greater activation of anti-excitotoxic and anti-oxidant mechanisms, increased plasticity potential, a greater energy reserve and improved mitochondrial function. This review seeks to summarize properties of CA2-4, DG in the context of endogenous neuroprotection, and then to assess the potential utility of these properties to therapeutic approaches. In so doing, it appears to represent the first such addressing of the issue of ischemia resistance attributable to CA2-4, DG.

Intracerebroventricular administration of nerve growth factor induces gliogenesis in sensory ganglia, dorsal root, and within the dorsal root entry zone

Previous studies indicated that intracerebroventricular administration of nerve growth factor (NGF) leads to massive Schwann cell hyperplasia surrounding the medulla oblongata and spinal cord. This study was designed to characterize the proliferation of peripheral glial cells, that is, Schwann and satellite cells, in the trigeminal ganglia and dorsal root ganglia (DRG) of adult rats during two weeks of NGF infusion using bromodeoxyuridine (BrdU) to label dividing cells. The trigeminal ganglia as well as the cervical and lumbar DRG were analyzed. Along the entire neuraxis a small number of dividing cells were observed within these regions under physiological condition. NGF infusion has dramatically increased the generation of new cells in the neuronal soma and axonal compartments of sensory ganglia and along the dorsal root and the dorsal root entry zone. Quantification of BrdU positive cells within sensory ganglia revealed a 2.3- to 3-fold increase in glial cells compared to controls with a similar response to NGF for the different peripheral ganglia examined. Immunofluorescent labeling with S100β revealed that Schwann and satellite cells underwent mitosis after NGF administration. These data indicate that intracerebroventricular NGF infusion significantly induces gliogenesis in trigeminal ganglia and the spinal sensory ganglia and along the dorsal root entry zone as well as the dorsal root.

Neurotrophins

Nerve growth factor was the first identified protein with anti-apoptotic activity on neurons. This prototypic neurotrophic factor, together with the three structurally and functionally related growth factors brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4/5 (NT4/5), forms the neurotrophin protein family. Target T cells for neurotrophins include many neurons affected by neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and peripheral polyneuropathies. In addition, the neurotrophins act on neurons affected by other neurological and psychiatric pathologies including ischemia, epilepsy, depression and eating disorders. Work with cell cultures and animal models provided solid support for the hypothesis that neurotrophins prevent neuronal death. While no evidence exists that a lack of neurotrophins underlies the etiology of any neurodegenerative disease, these studies have spurred on hopes that neurotrophins might be useful symptomatic-therapeutic agents. However first clinical trials led to variable results and severe side effects were observed. For future therapeutic use of the neurotrophins it is therefore crucial to expand our knowledge about their physiological functions as well as their pharmacokinetic properties. A major challenge is to develop methods for their application in effective doses and in a precisely timed and localized fashion.

Enhanced visuospatial memory following intracerebroventricular administration of nerve growth factor

The present work assessed the effects of intracerebroventricular injections of rh recombined human nerve growth factor (rh NGF) (5 micrograms/2.5 microl) at postnatal days 12 and 13 upon the development of spatial learning capacities. The treated rats were trained at the age of 22 days to escape onto an invisible platform at a fixed position in space in a Morris navigation task. For half of the subjects, the training position was also cued, a procedure aimed at facilitating escape and at reducing attention to the distant spatial cues. Later, at the age of 6 months, all the rats were trained in a radial-arm maze task. Treatment effects were found in both immature and adult rats. The injection of NGF improved the performance in the Morris navigation task in both training conditions. There was a significant reduction in the escape latency and an increased bias toward the training platform quadrant during probe trials. The most consistent effect was the precocious development of an adult-like spatial memory. In the radial-arm maze, the NGF-treated rats made significantly fewer reentries than vehicle rats and this effect was particularly marked in the treated female rats. Taken together, these experiments reveal that the development and the maintenance of an accurate spatial representation are tightly related to the development of brain structures facilitated by the action of NGF. Moreover, these experiments demonstrate that an acute pharmacological treatment that leads to a transient modification in the choline acetyltransferase activity can induce a behavioral change long after the treatment.

Pharmacology of neurotrophic factors

The field of neurotrophic factor pharmacology emerged during the past decade with the discovery that these proteins can counteract neuronal atrophy and death in the adult nervous system. These concepts are being tested in clinical trials. Therapeutic use of neurotrophic proteins seems practical for diseases of the peripheral nervous system (PNS), where they can be given by systemic administration. For diseases of the CNS, special administration strategies will have to be developed to deliver the neurotrophic factors into the brain. The development of small molecule mimetics represents an alternative approach that is actively pursued to provide brain-penetrant neurotrophics.

Reversible Schwann cell hyperplasia and sprouting of sensory and sympathetic neurites after intraventricular administration of nerve growth factor

Substantial dysfunction and loss of cholinergic neurons occur in Alzheimer's disease (AD). Nerve growth factor (NGF) is a potent neurotrophic factor for cholinergic basal forebrain neurons, and the use of NGF to stimulate residual dysfunctional cells in AD is being considered. To define the effects of NGF on other cell populations in the brain, NGF was continuously infused into the lateral ventricle of rats for 7 weeks. At the end of treatment, Schwann cell hyperplasia and abundant sensory and sympathetic neurite sprouting were observed in the subpial region of the medulla oblongata and the spinal cord. Following withdrawal of NGF, the Schwann cell hyperplasia and sprouting of sensory and sympathetic neurites disappeared completely. These findings suggest that better temporal and spatial delivery systems for NGF must be explored to limit potential undesirable side effects while maintaining the survival and function of diseased basal forebrain cholinergic neurons.

Selective effects of nerve growth factor on spatial recent memory as assessed by a delayed nonmatching-to-position task in the water maze

Nerve growth factor (NGF) ameliorates age-related deficits in certain types of memory in rats. Although the effects of NGF on reference memory are well documented, the influence of NGF on recent memory is less well understood. The issue of recent memory is of primary importance in the design of therapies for cognitive disorders, because this type of memory is impaired in elderly humans and is severely affected early in the course of Alzheimer's disease (AD). The present study was designed to evaluate the efforts of NGF on recent memory in a task that used escape from water as the motivating stimulus and used the same design as forced-choice recognition tasks given to humans. Fischer-344 rats, 4 months old (4MO) or 23 months old (23MO), were pretested in a new spatial recent memory task designed for the Morris water maze, a delayed nonmatching-to-position task, and infused intraventricularly with recombinant human NGF or vehicle. After 2 weeks of NGF infusion, no substantial changes in behavior were observed in either age group. However, NGF treatment extended over 4 weeks improved considerably the choice accuracy of 23MO rats to a level similar to the performance of 4MO rats. These results, together with our previous work (Markowska et al., 1994), indicate that the effects of NGF on spatial recent memory are more intense than on spatial reference memory. NGF suppressed the body weight gain in 4MO rats but did not affect 23MO rats. In 23MO rats, NGF mildly counteracted age-related deficits in inhibitory avoidance, but did not have an effect in young rats.

NGF deprivation of adult rat brain results in cholinergic hypofunction and selective impairments in spatial learning

Cholinergic hypofunction has often been correlated with a variety of behavioural impairments. In the present study, adult Wistar rats were intraventricularly infused with antibodies to nerve growth factor (anti-NGF) to examine the effects on cholinergic neurons of the basal forebrain, and on behavioural performance. Immunocytochemical techniques indicated that chronically infused anti-NGF penetrates into the basal forebrain, cortex, striatum, corpus callosum and hippocampus, confirming previous findings after a single injection. Treatment with anti-NGF for 1 or 2 weeks resulted in a significant decrease of 27-33% in density of choline acetyltransferase immunostaining of the cholinergic cell bodies in the medial septum and vertical diagonal band, and a 26% reduction in choline acetyltransferase enzyme activity in the septal area. An array of spatial learning Morris water maze tasks was used to distinguish between acquisition skills and the flexible use of learned information in novel tests. Rats subjected to the spatial learning paradigm received anti-NGF infusion for 2 weeks prior to and for another 2 weeks during the behavioural testing. The anti-NGF-treated animals were found to be no different from those receiving control serum in the Morris water maze acquisition task, either in the latency to find the platform or in the time spent searching in the training quadrant when the platform was removed. However, in consecutive extinction trials, anti-NGF rats continued to search in the empty training quadrant, suggesting the occurrence of perseveration; control rats expanded their search over other areas of the pool.(ABSTRACT TRUNCATED AT 250 WORDS)

NGF suppression of weight gain in adult female rats correlates with decreased hypothalamic cholecystokinin levels

Effects of chronic intraventricular administration of nerve growth factor (NGF, 1 microgram qod for 21 days) on weight gain, hypothalamic neuropeptide levels and choline acetyltransferase (ChAT) activity were determined in adult female Wistar rats. Rats chronically treated with cytochrome c (cc) gained 163 g over the 21 day treatment schedule, whereas NGF-treated rats only gained 110 g. Thus, NGF-treated rats gained 53 g less; this change in weight gain is equivalent to approximately a 20% decrease of total weight gain compared to the cc-treated control rats. Chronic NGF treatment significantly decreased hypothalamic cholecystokinin (CCK) levels by 24% (P = 0.0070), but did not alter either hypothalamic neuropeptide Y (NPY) or bombesin (BOMB) levels (98% and 105% of cc-treated control levels, respectively). In addition, chronic NGF treatment did not significantly alter hypothalamic ChAT activity (95% of cc-treated control rats). The results of the present study suggest that NGF-induced decreases in weight gain are not the result of alterations of hypothalamic cholinergic function. However, it is possible that NGF-induced alterations of hypothalamic CCK synthesis and release may be involved in the NGF-induced decrease in weight gain.

Effects of chronic nerve growth factor treatment on hippocampal [3H]cytisine/nicotinic binding sites and presynaptic nicotinic receptor function following fimbrial transections

Recent studies with nerve growth factor (NGF) have identified the pharmacological actions of this neurotrophin in a variety of animal models that mimic some of the neurotransmitter deficits that occur in Alzheimer's disease (AD, for reviews see Refs 7, 15, 17, 19). Based upon extensive pharmacological studies, NGF has been characterized as a crucial maintenance factor for adult cholinergic neurons of the septo-hippocampal and basalo-cortical pathways. Among the reported actions of NGF is an attenuation of lesion-induced decrements in presynaptic and postsynaptic cholinergic markers and functions in the hippocampal formation. Thus, in studies that used partial fimbriectomies to parallel the cholinergic neurodegeneration that occurs in AD, intraventricularly administered nerve growth factor prevented the loss of choline acetyltransferase (ChAT) and acetylcholinesterase immunoreactivity in the septum and increased a variety of presynaptic cholinergic markers involved in the synthesis, storage and release of the neurotransmitter acetylcholine (for reviews see Refs 7, 17, 19). More specifically, chronic NGF treatment attenuates lesion-induced reductions in hippocampal ChAT activity and high-affinity choline uptake, the end-result of which is an enhanced capacity to synthesize acetylcholine. This increased acetylcholine synthesis, in turn, appears to translate directly into augmented vesicular storage and release of the neurotransmitter. For instance, not only does NGF treatment reverse lesion-induced reductions in maximal binding densities of the acetylcholine vesicular transport marker [3H]vesamicol, but it also enhances acetylcholine release and turnover rate. NGF treatment also appears to restore the sensitivity of postsynaptic muscarinic receptors to agonist-induced stimulation following partial fimbriectomies.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased neuronal activity in the nucleus basalis of Meynert in Alzheimer's disease as suggested by the size of the Golgi apparatus

In order to study changes in neuronal activity in the nucleus basalis of Meynert in aging and Alzheimer's disease, we applied a polyclonal antibody directed against the Golgi apparatus on formalin-fixed, paraffin-embedded material. Subsequently, an image analysis system was used to measure the size of the Golgi apparatus in (i) all nucleus basalis neurons and also separately in (ii) the remaining large cells (perikaryonal diameter > 30 microns). A significant reduction of 49% in the size of the Golgi apparatus was found in the entire population of nucleus basalis neurons in Alzheimer's disease. Furthermore, although there was no significant decrease in the size of the persisting large neurons in the nucleus basalis of Meynert, a significantly decreased size of the Golgi apparatus was found in these neurons in Alzheimer's disease. These results suggest that the overall activity of nucleus basalis neurons is severely decreased in Alzheimer's disease. Furthermore, these data support the idea that atrophy and decreased activity are the main phenomena in the nucleus basalis in Alzheimer's disease; they also indicate that the size of the Golgi apparatus is a sensitive parameter to follow this process.