Changes in human plasma nerve growth factor level after chronic alcohol consumption and withdrawal

Nerve Growth Factor, Stress and Diseases

Stress is a constant threat for homeostasis and is represented by different extrinsic and intrinsic stimuli (stressors, Hans Selye's "noxious agents"), such as aggressive behavior, fear, diseases, physical activity, drugs, surgical injury, and environmental and physiological changes. Our organisms respond to stress by activating the adaptive stress system to activate compensatory responses for restoring homeostasis. Nerve Growth Factor (NGF) was discovered as a signaling molecule involved in survival, protection, differentiation, and proliferation of sympathetic and peripheral sensory neurons. NGF mediates stress with an important role in translating environmental stimuli into physiological and pathological feedbacks since NGF levels undergo important variations after exposure to stressful events. Psychological stress, lifestyle stress, and oxidative stress are well known to increase the risk of mental disorders such as schizophrenia, major depressive disorders, bipolar disorder, alcohol use disorders and metabolic disorders such as metabolic syndrome. This review reports recent works describing the activity of NGF in mental and metabolic disorders related to stress.

Nerve Growth Factor in Alcohol Use Disorders

The nerve growth factor (NGF) belongs to the family of neurotrophic factors. Initially discovered as a signaling molecule involved in the survival, protection, differentiation, and proliferation of sympathetic and peripheral sensory neurons, it also participates in the regulation of the immune system and endocrine system. NGF biological activity is due to the binding of two classes of receptors: the tropomyosin-related kinase A (TrkA) and the low-affinity NGF pan-neurotrophin receptor p75. Alcohol Use Disorders (AUD) are one of the most frequent mental disorders in developed countries, characterized by heavy drinking, despite the negative effects of alcohol on brain development and cognitive functions that cause individual’s work, medical, legal, educational, and social life problems. In addition, alcohol consumption during pregnancy disrupts the development of the fetal brain causing a wide range of neurobehavioral outcomes collectively known as fetal alcohol spectrum disorders (FASD). The rationale of this review is to describe crucial findings on the role of NGF in humans and animals, when exposed to prenatal, chronic alcohol consumption, and on binge drinking.

Fetus morphology changes by second-trimester ultrasound in pregnant women drinking alcohol

Fetal alcohol spectrum disorders (FASDs) are a group of negative conditions occurring in children exposed to alcohol during gestation. The early discovery of FASD is crucial for mother and infant follow-ups. In this study, we investigated in pregnant women the association between urine ethylglucuronide (EtG-a biomarker of alcohol drinking) and indicators of the physical characteristics of FASD by prenatal ultrasound in the second trimester of gestation. We also correlated these data with the AUDIT-C, T-ACE/TACER-3, TWEAK, and food habit diary, screening questionnaires used to disclose alcohol drinking during pregnancy. Forty-four pregnant women were randomly enrolled and examined for ultrasound investigation during the second trimester of gestation. Urine samples were provided by pregnant women immediately after the routine interviews. EtG determinations were performed with a cutoff established at 100 ng/mL, a value indicating occasional alcohol drinking. Fifteen of the enrolled pregnant women overcame the EtG cutoff (34.09%). Analysis of variance (ANOVA) revealed that the fetuses of the positive EtG pregnant women had significantly longer interorbital distance and also significantly increased frontothalamic distance (P's < 0.02). Quite interestingly, no direct correlation was found between EtG data and both food diary and AUDIT-C. However, a significant correlation was observed between urinary EtG and T-ACE (r = 0.375; P = 0.012) and between urinary EtG and TWEAK (r = 0.512; P < 0.001) and a concordance with all questionnaire for EtG values higher than 500 ng/mL. This study provides clinical evidence that the diagnosis of maternal alcohol consumption during pregnancy by urine EtG may disclose FASD-related damage in the fetus.

NGF and BDNF Alterations by Prenatal Alcohol Exposure

BACKGROUND

It is now widely established that the devastating effects of prenatal alcohol exposure on the embryo and fetus development cause marked cognitive and neurobiological deficits in the newborns. The negative effects of the gestational alcohol use have been well documented and known for some time. However, also the subtle role of alcohol consumption by fathers prior to mating is drawing special attention.

OBJECTIVE

Both paternal and maternal alcohol exposure has been shown to affect the neurotrophins' signalling pathways in the brain and in target organs of ethanol intoxication. Neurotrophins, in particular nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), are molecules playing a pivotal role in the survival, development and function of the peripheral and central nervous systems but also in the pathogenesis of developmental defects caused by alcohol exposure.

METHODS

New researches from the available literature and experimental data from our laboratory are presented in this review to offer the most recent findings regarding the effects of maternal and paternal prenatal ethanol exposure especially on the neurotrophins' signalling pathways.

RESULTS

NGF and BDNF changes play a subtle role in short- and long-lasting effects of alcohol in ethanol target tissues, including neuronal cell death and severe cognitive and physiological deficits in the newborns.

CONCLUSION

The review suggests a possible therapeutic intervention based on the use of specific molecules with antioxidant properties in order to induce a potential prevention of the harmful effects of the paternal and/or maternal alcohol exposure.

Nerve growth factor: basic studies and possible therapeutic applications

The nerve growth factor (NGF) belongs to a family of neurotrophic factors called neurotrophins. It was discovered as a molecule that stimulates the survival and maturation of developing neurons in the peripheral nervous system and has later been shown to protect adult neurons in the degenerating mammalian brain. Basic and clinical studies have been undertaken to use NGF as a therapeutic agent aimed at restoring and maintaining neuronal function in the central nervous system and to determine the mechanisms to safely deliver the molecule into the brain. Recent studies have also recognized that the role of NGF extends far beyond the horizon of nerve cells and even beyond the peripheral and central nervous system. Studies published from our laboratory have shown that topical application of NGF possesses a protective action on human pressure ulcer, corneal ulcer and glaucoma. Here, we will review these studies, supporting the therapeutic potential of NGF.

Control of arthritis pain with anti-nerve-growth factor: risk and benefit

Arthritis is characterized by pain and inflammation. Recently, attention has been focused on nerve-growth factor (NGF), a neurotrophin that is a key regulator of peripheral nociception because it mediates overexpression of proinflammatory neuron-derived molecules such as substance P, serotonin, and calcitonin gene-related peptide. Antibodies have been generated for NGF and its receptor that are effective in reducing pain in preclinical pain models, and clinical trials in patients with advanced knee and hip osteoarthritis and low-back pain. Results show pain reduction is rapid and sustained. Adverse events with anti-NGF included transient paraesthesia and edema, rapidly progressive OA, and, in a small number of patients treated with both anti-NGF and nonsteroidal anti-inflammatory drugs, osteonecrosis. Inhibition of the NGF-stimulated nociceptive pathway seems to be effective; however, the adverse effects require further investigation.

NGF, brain and behavioral plasticity

Nerve Growth Factor (NGF) was initially studied for its role as a key player in the regulation of peripheral innervations. However, the successive finding of its release in the bloodstream of male mice following aggressive encounters and its presence in the central nervous system led to the hypothesis that variations in brain NGF levels, caused by psychosocial stressor, and the related alterations in emotionality, could be functional to the development of proper strategies to cope with the stressor itself and thus to survive. Years later this vision is still relevant, and the body of evidence on the role of NGF has been strengthened and expanded from trophic factor playing a role in brain growth and differentiation to a much more complex messenger, involved in psychoneuroendocrine plasticity.

Differential patterns of serum brain-derived neurotrophic factor levels in alcoholic patients with and without delirium tremens during acute withdrawal

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is associated with alcohol addiction and withdrawal-related neurotoxicity. Delirium tremens (DT) is the most serious complication of alcohol withdrawal syndrome (AWS). In this study, we explored the differences in serum BDNF levels, measured at baseline and 1 week after alcohol withdrawal among alcoholic patients with and without DT.

METHODS

Sixty-five inpatients, fulfilling the DSM-IV criteria of alcohol dependence and admitted for alcohol detoxification, as well as 39 healthy control subjects were enrolled. The alcoholic patients were divided by the appearance of DTs into the DT group (n = 25) and non-DT group (n = 40). We collected blood samples of the patient groups on the first and seventh days of alcohol withdrawal and measured serum BDNF levels by sandwich enzyme-linked immunosorbent assay.

RESULTS

Serum BDNF levels differed significantly among the three groups: (i) control group 14.8 ± 4.7 ng/ml; (ii) non-DT group 12.3 ± 3.3 ng/ml; (iii) DT group 6.2 ± 2.6 ng/ml (p < 0.001). One week after alcohol withdrawal, the BDNF levels increased significantly for both alcoholic groups. While non-DT group had comparable BDNF levels (13.4 ± 3.5 ng/ml) with controls, the DT group still exhibited lower levels (8.9 ± 4.4 ng/ml).

CONCLUSIONS

This study suggests chronic drinking leads to a reduction in BDNF levels, and patients with more deficient BDNF expression are vulnerable to the development of DTs. Additionally, BDNF levels elevated after prompt alcohol detoxification treatment. These findings indicate that BDNF could involve modifying the phenotypes of AWS as well as the pertinent neuroadaptive processes of alcohol dependence.

NGF plasma levels increase due to alcohol intoxication and decrease during withdrawal

Recent studies show that alcohol dependence is associated with alterations in plasma levels of nerve growth factor (NGF). The aim of this study was to further elucidate reported alterations in NGF plasma levels during alcohol intoxication and withdrawal. Therefore, we assessed NGF plasma levels by enzyme-linked immunosorbent assay (ELISA) on admission (day 0) and day 7 of alcohol withdrawal in male alcohol dependent patients (n=75) in comparison to healthy controls (n=44). We found significant higher (U=1005.0, p<0.001) NGF plasma levels in the alcohol-dependent patients. Subgroup analysis showed significant higher (U=-2.934, p=0.003) NGF plasma levels in patients suffering from acute alcohol intoxication (group A) than in early abstinent patients (group B). From day 0 to day 7 of alcohol withdrawal NGF plasma levels decreased significantly in both groups (group A: Z=-3.118, p=0.002, group B: Z=-2.103, p=0.035). Our results suggest that acute alcohol intoxication is associated with an increase in NGF plasma levels, which decrease during alcohol withdrawal. These results suggest that NGF plasma levels increase as part of a regulation mechanism that counteracts alcohol intoxication.

Nerve growth factor in serum is a marker of the stage of alcohol disease

Long-term alcohol abuse has deleterious effects on the peripheral and central nervous system. Nerve growth factor (NGF) is a pleiotropic neurotrophic protein involved in development, maintenance of function and regeneration of nerve cells. We examined patients in different stages of alcohol disease and measured their NGF serum concentrations based on the hypothesis that these reflect the state of disease. We examined 57 patients suffering from alcohol-dependence for more than 2 years (DSM IV) on day 8 of a qualified withdrawal, 18 patients with Korsakoff's syndrome and 40 healthy controls. In addition to clinical examination, careful history taking and a standard neuropsychological test battery, serum NGF concentrations were measured by a highly sensitive enzyme-immunoassay. Of the 57 patients 9 had suffered from severe withdrawal delirium in the past, other clinical parameters were alike. Cognitive test performance did not differ from the control group. Mean NGF levels of controls amounted to 42.1pg/ml (S.D. 68.0); mean levels of patients with alcohol dependence were raised significantly to 401.5pg/ml (S.D. 932.6) without delirium in the past and even further to 3292.5pg/ml (S.D. 4879.6) with former withdrawal delirium. By contrast, patients with persistent amnestic disorder (Korsakoff's syndrome) showed values identical to the controls. NGF serum levels were significantly elevated in alcohol-dependent patients, more so in those with prior delirium. Their cognitive tests being normal, this possibly reflects the activity of NGF as an endogenous repair mechanism for damaged neurons. In accordance with this hypothesis, NGF values are "normal" in patients with persistent alcohol-related cognitive decline.

Neurotrophin ligand-receptor systems in somatosensory cortex of adult rat are affected by repeated episodes of ethanol

Ethanol exposure profoundly affects learning and memory and neural plasticity. Key players underlying these functions are neurotrophins. The present study explored the effects of ethanol on the distribution of neurotrophins in the cerebral cortex of the adult rat. Age- and weight-matched pairs of adult male, Long-Evans rats were fed a liquid, ethanol-containing (6.7% v/v) diet or pair-fed an isocaloric control diet three consecutive days per week for 6, 12, 18, or 24 weeks. Brains were processed immunohistochemically for nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) expression and for the expression of three neurotrophin receptors, p75, trkA, and trkB. Total numbers of immunolabeled neurons in specific layers of somatosensory cortex of ethanol- and control-fed animals were determined stereologically. Ethanol exposure induced an increase in the numbers of NGF- or BDNF-expressing neurons and in neurotrophin content per somata. These changes were (a) time and (b) laminar dependent. In contrast, the number of receptor-expressing neurons did not change due to ethanol exposure or to length of time on the ethanol diet. Thus, ethanol induces the recruitment of cortical neurons to express neurotrophins and an increase in the amount of neurotrophin expression per neuron.

Possible role of nerve growth factor in the pathogenesis of alcohol dependence

BACKGROUND

Recent studies have raised the possibility that nerve growth factor (NGF) is abnormally regulated in the central nervous system (CNS) of animal models of chronic ethanol treatment. The goals of this study were to determine whether prolonged alcohol consumption is associated with the plasma NGF levels and to assess the effect of a positive family history of alcohol dependence on plasma NGF levels in the alcohol-dependent patients.

METHODS

We used the enzyme-linked immunosorbent assay (ELISA) to determine the concentrations of peripheral NGF in patients with alcohol dependence and in a control group.

RESULTS

The plasma NGF concentrations in the alcohol-dependent patients were significantly lower than in the controls (71.9 vs 110.5 pg/mL, respectively). Moreover, the alcohol-dependent patients with positive family histories showed a greater decrease in their NGF levels than those subjects with negative family histories (64.7 vs 83.3 pg/mL, respectively).

CONCLUSIONS

Our study suggests that the NGF levels may be a trait marker for the development of alcohol dependence.

Repeated episodic exposure to ethanol affects neurotrophin content in the forebrain of the mature rat

Chronic exposure to ethanol can cause deficits in learning and memory. It has been suggested that withdrawal is potentially more damaging than the ethanol exposure per se. Therefore, we explored the effect of repeated episodic exposure to ethanol on key regulators of cortical activity, the neurotrophins. Rats were exposed to ethanol via a liquid diet for 3 days per week for 6-24 weeks. Control rats were pair-fed an isocaloric liquid diet or ad libitum fed chow and water. The concentrations of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined using enzyme-linked immunosorbant assays (ELISAs). Five telencephalic structures were examined: parietal cortex, entorhinal cortex, hippocampus, the basal nucleus, and the septal nuclei. All five areas expressed each of the three neurotrophins; BDNF was most abundant and NGF the least. The parietal cortex was susceptible to ethanol exposure, NGF and BDNF content increased, and NT-3 content fell, whereas no changes were detectable in the entorhinal cortex. In the hippocampus, the amount all three neurotrophins increased following episodic ethanol exposure. Neurotrophin content in the two segments of the basal forebrain was affected; NGF and NT-3 content in the basal forebrain was reduced and NGF and BDNF content in the septal nuclei was increased by ethanol exposure. In many cases where ethanol had an effect, the change was transient so that by 24 weeks of episodic exposure, no significant changes were evident. Thus, the effects of ethanol are site- and time-dependent. This pattern differs from changes caused by chronic ethanol exposure, hence, neurotrophins must be vulnerable to the effects of withdrawal. Furthermore, the ethanol-induced changes do not appear to fit a model consistent with retrograde regulation, rather they suggest that neurotrophins act through autocrine/paracrine systems.

Epigenetic control of neurobehavioural plasticity: the role of neurotrophins

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are two neurotrophins involved in the differentiation, growth and maintenance of selected peripheral and central populations of neuronal cells, during development and at adulthood. Furthermore, neuronal activity enhances expression and action of these neurotrophins, modifying synaptic transmission and connectivity. Neurotrophin production has been shown to be experience-dependent. In particular, during early developmental phases, experiences such as maternal deprivation or exposure to an enriched environment markedly affect NGF and BDNF levels. At adulthood, psychosocial stress has been shown to markedly alter NGF and BDNF levels, both in plasma and selected brain areas, including the hypothalamus and hippocampus. These results have been extended to humans, showing that NGF levels are enhanced by emotional stress induced by parachute jumping. Overall, these findings suggest a role of neurotrophins as factors mediating both short- and long-term effects of experience on brain structure and function.

Neurotrophic factors and CNS disorders: findings in rodent models of depression and schizophrenia

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are proteins involved in neuronal survival and plasticity of dopaminergic, cholinergic and serotonergic neurons in the central nervous system (CNS). Loss of neurons in specific brain regions has been found in depression and schizophrenia, and this chapter summarizes the findings of altered neurotrophins in animal models of those two disorders under baseline condition and following antidepressive and antipsychotic treatments. In a model of depression (Flinders sensitive line/Flinders resistant line; FSL/FRL rats), increased NGF and BDNF concentrations were found in frontal cortex of female, and in occipital cortex of male 'depressed' FSL compared to FRL control rats. Using the same model, the effects of electroconvulsive stimuli (ECS) and chronic lithium treatment on brain NGF, BDNF and glial cell line-derived neurotrophic factors were investigated. ECS and lithium altered the brain concentrations of neurotrophic factors in the hippocampus, frontal cortex, occipital cortex and striatum. ECS mimic the effects of electroconvulsive therapy (ECT) that is an effective treatment for depression and also schizophrenia. Since NGF and BDNF may also be changed in the CNS of animal models of schizophrenia, we investigated whether treatment with antipsychotic drugs (haloperidol, risperidone, and olanzapine) affects the constitutive levels of NGF and BDNF in the CNS. Both typical and atypical antipsychotic drugs altered the regional brain levels of NGF and BDNF. Other studies also demonstrated that these drugs differentially altered neurotrophin mRNAs. Overall, these studies indicate that alteration of brain level of NGF and BDNF could constitute part of the biochemical alterations induced by antipsychotic drugs.

Chronic cannabis abuse raises nerve growth factor serum concentrations in drug-naive schizophrenic patients

Long-term cannabis abuse may increase the risk of schizophrenia. Nerve growth factor (NGF) is a pleiotropic neurotrophic protein that is implicated in development, protection and regeneration of NFG-sensitive neurones. We tested the hypothesis that damage to neuronal cells in schizophrenia is precipitated by the consumption of cannabis and other neurotoxic substances, resulting in raised NGF serum concentrations and a younger age for disease onset. The NGF serum levels of 109 consecutive drug-naive schizophrenic patients were measured and compared with those of healthy controls. The results were correlated with the long-term intake of cannabis and other illegal drugs. Mean (+/- SD) NGF serum levels of 61 control persons (33.1 +/- 31.0 pg/ml) and 76 schizophrenics who did not consume illegal drugs (26.3 +/- 19.5 pg/ml) did not differ significantly. Schizophrenic patients with regular cannabis intake (> 0.5 g on average per day for at least 2 years) had significantly raised NGF serum levels of 412.9 +/- 288.4 pg/ml (n = 21) compared to controls and schizophrenic patients not consuming cannabis (p < 0.001). In schizophrenic patients who abused not only cannabis, but also additional substances, NGF concentrations were as high as 2336.2 +/- 1711.4 pg/ml (n = 12). On average, heavy cannabis consumers suffered their first episode of schizophrenia 3.5 years (n = 21) earlier than schizophrenic patients who abstained from cannabis. These results indicate that cannabis is a possible risk factor for the development of schizophrenia. This might be reflected in the raised NGF-serum concentrations when both schizophrenia and long-term cannabis abuse prevail.

Stress and nerve growth factor: findings in animal models and humans

Stress is elicited by environmental, social or pathological conditions occurring during the life of animals and humans that determine changes in the nervous, endocrine and immune systems. In the present review, we present data supporting the hypothesis that stress-related events both in animal models and humans are characterized by modifications of endogenous nerve growth factor (NGF) synthesis and/or utilization. Stress inducing alteration in NGF synthesis and/or utilization appears to be more severe during neurogenesis and in early postnatal life. However, NGF endogenously released during stress may promote remodeling of damaged tissues following acute and/or chronic stressful events.

Changes in plasma nerve growth factor levels in older adults associated with chronic stress

Evidence indicates that the actions of nerve growth factor (NGF) reach beyond the nervous system and might modulate immune function. Based on reports that blood NGF rises following the acute stress of parachute jumping, we investigated whether exposure to a chronic stressor, caregiving for a cognitively impaired spouse, could alter the levels of blood NGF. High perceived stress and depression in caregivers (vs. well-matched controls) were associated with elevated blood NGF. These data suggest that exposure to this chronic stressor can alter the concentrations of circulating NGF, and that psychological stress can induce changes in NGF concentrations in older adults.

Studies in animal models and humans suggesting a role of nerve growth factor in schizophrenia-like disorders

Neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), are known to play a crucial role in growth, differentiation and function in a variety of brain neurons during development and in adult life. We have recently shown that environmental changes, aggressive behavior and anxiety-like responses alter both circulating and brain basal NGF levels. In the present review, we present data obtained using animal models which suggest that neurotrophic factors, particularly NGF and BDNF, might be implicated in mechanism(s) leading to a condition associated with schizophrenic-like behaviors. The hypothesis that neurotrophins of the NGF family can be implicated in some maldevelopmental aspects of schizophrenia is supported by findings indicating that the constitutive levels of NGF and BDNF are affected in schizophrenic patients.

Prenatal ethanol effects on NGF level, NPY and ChAT immunoreactivity in mouse entorhinal cortex: a preliminary study

It has been reported that maternal ethanol consumption leads to deficits in the limbic areas involved in cognitive functions and interferes with synthesis and utilization of neurotrophins. In the present study, it was hypothesized that prenatal alcohol intake might induce neuroanatomical alterations in the entorhinal cortex (EC). We also investigated the possible EC involvement of brain nerve growth factor (NGF), the first neurotrophin to be isolated, during such pathological events. To test this hypothesis, we used pregnant mice exposed to ethanol during EC neurogenesis (starting about gestational day 8). Our data show that prenatal alcohol intake in male mice alters the EC neuronal growth and differentiation. These morphological alterations are accompanied by an altered NGF level in the EC of prenatal alcohol-treated mice. We also found a decrease in choline acetyltransferase- and neuropeptide Y-immunopositive neurons in the EC of alcohol-exposed mice. However, the relationship between neuronal damage induced in the EC by ethanol, low presence of NGF, and the possible functional and behavioral consequences remains to be elucidated.

Neurobehavioral alteration in rodents following developmental exposure to aluminum

Aluminum (Al) is one of the most abundant metals in the earth's crust, and humans can be exposed to it from several sources. It is present in food, water, pharmaceutical compounds, and in the environment, e.g., as a result of acid rain leaching it from the soil. Exposure to Al has recently been implicated in a number of human pathologies, but it has not yet been definitely proved that it plays a major causal role in any of them. In this paper we review the effects of developmental exposure of laboratory animals to Al salts as a model for human pathological conditions. The data presented show behavioral and neurochemical changes in the offspring of AL-exposed mouse dams during gestation, which include alterations in the pattern of ultrasonic vocalizations and a marked reduction in central nervous system (CNS) choline acetyltransferase activity. Prenatal Al also affects CNS cholinergic functions under Nerve Growth Factor (NGF) control, as shown by increased central NGF levels and impaired performances in a maze learning task in young-adult mice. The need for more detailed studies to evaluate the risks for humans associated with developmental exposure to Al, as well as the importance of using more than one strain of laboratory animal in the experimental design, is emphasized.

The expanding role of nerve growth factor: from neurotrophic activity to immunologic diseases

Numerous studies published in the last 10-15 years have shown that nerve growth factor (NGF), a polypeptide originally discovered in connection with its neurotrophic activity, also acts on cells of the immune system. NGF has been found in various immune organs including the spleen, lymph nodes, and thymus, and cells such as mast cells, eosinophils, and B and T cells. The circulating levels of NGF increase in inflammatory responses, in various autoimmune diseases, in parasitic infections, and in allergic diseases. Stress-related events both in animal models and in man also result in an increase of NGF, suggesting that this molecule is involved in neuroendocrine functions. The rapid release of NGF is part of an alerting signal in response to either psychologically stressful or anxiogenic conditions in response to homeostatic alteration. Thus, the inflammation and stress-induced increase in NGF might alone or in association with other biologic mediators induce the activation of immune cells during immunologic insults. A clearer understanding of the role of NGF in these events may be useful to identify the mechanisms implicated in certain neuroimmune and immune dysfunctions.