Trophic factors as potential therapies for treatment of major mental disorders.

Notwithstanding major advances in psychotherapeutics, their efficacy and specificity remain limited. The slow onset of beneficial outcomes and numerous adverse effects of widely used medications remain of chief concern, warranting in-depth studies. The majority of frontline therapies are thought to enhance the endogenous monoaminergic drive, to initiate a cascade of molecular events leading to lasting functional and structural plasticity. They also involve alterations in trophic factor signalling, including brain-derived neurotrophic factor (BDNF), VGF (non-acronymic), vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF2), glial cell-derived neurotrophic factor (GDNF), and others. In several major mental disorders, emerging data suggest protective and restorative effects of trophic factors in preclinical models, when applied on their own. Antidepressant outcomes of VGF and FGF2, for instance, were shown in experimental animals, while BDNF and GDNF prove useful in the treatment of addiction, schizophrenia, and autism spectrum disorders. The main challenge with the effective translation of these and other findings in the clinic is the knowledge gap in action mechanisms with potential risks, as well as the lack of effective platforms for validation under clinical settings. Herein, we review the state-of-the-art and advances in the therapeutic use of trophic factors in several major neuropsychiatric disorders.

Molecular profile of the rat peri-infarct region four days after stroke: Study with MANF.

The peri-infarct region after ischemic stroke is the anatomical location for many of the endogenous recovery processes; however, -the molecular events in the peri-infarct region remain poorly characterized. In this study, we examine the molecular profile of the peri-infarct region on post-stroke day four, a time when reparative processes are ongoing. We used a multiomics approach, involving RNA sequencing, and mass spectrometry-based proteomics and metabolomics to characterize molecular changes in the peri-infarct region. We also took advantage of our previously developed method to express transgenes in the peri-infarct region where self-complementary adeno-associated virus (AAV) vectors were injected into the brain parenchyma on post-stroke day 2. We have previously used this method to show that mesencephalic astrocyte-derived neurotrophic factor (MANF) enhances functional recovery from stroke and recruits phagocytic cells to the peri-infarct region. Here, we first analyzed the effects of stroke to the peri-infarct region on post-stroke day 4 in comparison to sham-operated animals, finding that strokeinduced changes in 3345 transcripts, 341 proteins, and 88 metabolites. We found that after stroke, genes related to inflammation, proliferation, apoptosis, and regeneration were upregulated, whereas genes encoding neuroactive ligand receptors and calcium-binding proteins were downregulated. In proteomics, we detected upregulation of proteins related to protein synthesis and downregulation of neuronal proteins. Metabolomic studies indicated that in after stroke tissue there is an increase in saccharides, sugar phosphates, ceramides and free fatty acids and a decrease of adenine, hypoxantine, adenosine and guanosine. We then compared the effects of post-stroke delivery of AAV1-MANF to AAV1-eGFP (enhanced green fluorescent protein). MANF administration increased the expression of 77 genes, most of which were related to immune response. In proteomics, MANF administration reduced S100A8 and S100A9 protein levels. In metabolomics, no significant differences between MANF and eGFP treatment were detected, but relative to sham surgery group, most of the changes in lipids were significant in the AAV-eGFP group only. This work describes the molecular profile of the peri-infarct region during recovery from ischemic stroke, and establishes a resource for further stroke studies. These results provide further support for parenchymal MANF as a modulator of phagocytic function.

Galanin Administration Partially Restores Erectile Function After Cavernous Nerve Injury and Mediates Endogenous Nitrergic Nerve Outgrowth In Vitro.

BACKGROUND: Previously, we found that the neuropeptide galanin was strongly upregulated soon after bilateral cavernous nerve injury (BCNI) and that galanin and its receptors were expressed in nitrergic erectile innervation. Galanin has been observed to exert neuroregenerative effects in dorsal root ganglion neurons, but evidence for these effects in the major pelvic ganglion (MPG) after BCNI is lacking. AIM: To evaluate the neurotropic effects of galanin receptor agonists and antagonists in vitro in nitrergic neurons and MPG and in vivo in rats after BCNI. METHODS: Male Sprague-Dawley rats underwent BCNI and sham surgery. Organ culture and single-cell neuron culture of the MPG were performed. Osmotic pump treatment with the galanin agonist in vivo and measurement of erectile response to electrostimulation after BCNI, immunohistochemical localization of galanin and receptors in the human neurovascular bundle, and myographic analysis of rat corpus cavernosum smooth muscle relaxation to galanin receptor agonists were investigated. OUTCOMES: Neurite outgrowth in vitro and erectile response to electrostimulation after BCNI in vivo, immunohistochemical localization of galanin and receptors, and penile muscle relaxation in vitro. RESULTS: Galanin showed neurotrophic action in vitro and inhibition of endogenous galanin significantly impaired neurite outgrowth in nitrergic but not in sympathetic MPG neurons. In vivo administration of a selective galanin receptor-2 agonist, M1145, resulted in partial recovery of erectile function (EF) after BCNI. Galanin did not act as a direct vasodilator on corpus cavernosum muscle strips. CLINICAL TRANSLATION: Endogenous neurotrophins such as galanin could be used as a strategy to improve EF for patients after BCNI from radical prostatectomy. STRENGTHS AND LIMITATIONS: We evaluated the effect of galanin on nerve regeneration and EF recovery in vivo and in vitro. Limitations include the lack of washout period for the in vivo experiment and absence of differences in the expression of neuronal markers between treatment groups. CONCLUSIONS: We identified galanin as a potential endogenous mechanism for nerve regeneration after BCNI, which could play a physiologic role in EF recovery after radical prostatectomy. In vivo treatment with exogenous galanin was beneficial in enhancing EF recovery after BCNI, but further research is necessary to understand the underlying mechanisms. Weyne E, Hannan JL, Gevaert T, et al. Galanin Administration Partially Restores Erectile Function After Cavernous Nerve Injury and Mediates Endogenous Nitrergic Nerve Outgrowth In Vitro. J Sex Med 2018;15:480-491.

Influence of pigment epithelium-derived factor on outcome after striatal cerebral ischemia in the mouse.

We here suggest that pigment epithelium-derived factor (PEDF) does not have an effect on lesion size, behavioral outcome, cell proliferation, or cell death after striatal ischemia in the mouse. PEDF is a neurotrophic factor with neuroprotective, antiangiogenic, and antipermeability effects. It influences self-renewal of neural stem cells and proliferation of microglia. We investigated whether intraventricular infusion of PEDF reduces infarct size and cell death, ameliorates behavioral outcome, and influences cell proliferation in the one-hour middle cerebral artery occlusion (MCAO) mouse model of focal cerebral ischemia. C57Bl6/N mice were implanted with PEDF or artificial cerebrospinal fluid (control) osmotic pumps and subjected to 60-minute MCAO 48 hours after pump implantation. They received daily BrdU injections for 7 days after MCAO in order to investigate cell proliferation. Infarct volumes were determined 24 hours after reperfusion using magnetic resonance imaging. We removed the pumps on day 5 and performed behavioral testing between day 7 and 21. Immunohistochemical staining was performed to determine the effect of PEDF on cell proliferation and cell death. Our model produced an ischemic injury confined solely to striatal damage. We detected no reduction in infarct sizes and cell death in PEDF- vs. CSF-infused MCAO mice. Behavioral outcome and cell proliferation did not differ between the groups. However, we cannot exclude that PEDF might work under different conditions in stroke. Further studies will elucidate the effect of PEDF treatment on cell proliferation and behavioral outcome in moderate to severe ischemic injury in the brain.

Enhancement of nose-brain delivery of therapeutic agents for treating neurodegenerative diseases using peppermint oil.

The nose-brain pathway is a potential route for drug delivery as it bypasses the brain barriers. The main objective of this study was to investigate the efficacy of peppermint oil in enhancing the bioavailability of intranasally administered neurotrophins like nerve growth factor (NGF). The effect of different concentrations of peppermint oil (PO) on the delivery of NGF across bovine olfactory epithelium was studied in vitro using Franz diffusion cells. Trans-olfactory epithelial electrical resistance (TEER) was measured to assess the permeability status of the bovine olfactory epithelium. The bioavailability of intranasally administered formulations in rat hippocampus was studied by carrying out brain microdialysis in male Sprague-Dawley rats. Peppermint oil at concentrations of 0.05, 0.1 and 0.5% v/v enhanced the in vitro transport of NGF by 5, 7 and 8 fold, respectively. In vivo studies employing brain microdialysis in rats demonstrated that intranasal administration of NGF formulation with 0.5% PO enhanced the bioavailability by approximately 8 fold compared to rats administered with NGF alone. The bioavailability of NGF in the brain could be enhanced by intranasal administration of peppermint oil.

Intracerebroventricularly administered neurotrophins attenuate blood cerebrospinal fluid barrier breakdown and brain pathology following whole-body hyperthermia: an experimental study in the rat using biochemical and morphological approaches.

Previous studies from our laboratory show that apart from blood-brain barrier (BBB) disruption, the blood-cerebrospinal fluid (CSF) barrier (BCSFB) for proteins is also broken down following whole-body hyperthermia (WBH) in a rat model. Breakdown of the BCSFB alters brain homeostasis and adversely affects the structure and function of the central nervous system (CNS). Since neurotrophins and growth factors (e.g., brain-derived growth factor [BDNF], glial cell line-derived neurotrophic factor [GDNF], and insulin-like growth factor 1 [IGF-1]) are known neuroprotective agents in traumatic and ischemic brain injuries, a possibility exists that these neurotrophins will also attenuate neuronal and choroidal injury in WBH. Subjection of adult rats to 4 h of WBH at 38 degrees C in a biological oxygen demand (BOD) incubator exhibited a profound increase in BCSFB permeability to Evans blue and radioiodine. Degeneration of choroidal epithelial cells and underlying ependyma, dilatation of the lateral ventricular space, and degenerative changes in the adjacent neuropil were frequent. The hippocampus, caudate nucleus, thalamus, and hypothalamus showed profound BBB disruption and brain edema formation. Intracerebroventricular (i.c.v.) administration of BDNF, GDNF, and IGF-1 into the right lateral cerebral ventricle (1, 2, or 5 microg in 30 microL, 24 h before WBH) significantly reduced the BCSFB and BBB breakdown, brain edema formation, and cellular/tissue injuries. These beneficial effects were most pronounced in GDNF- or IGF-1-pretreated animals. These novel observations suggest that neurotrophins administered into ventricular CSF can attenuate BCSFB and BBB damage following WBH and thereby confer neuroprotection. Stabilization of BCSFB function is thus one of the crucial factors in achieving neuroprotection in WBH.

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system.

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Neurotrophin-eluting hydrogel coatings for neural stimulating electrodes.

Improved sensory and motor prostheses for the central nervous system will require large numbers of electrodes with low electrical thresholds for neural excitation. With the eventual goal of reducing stimulation thresholds, we have investigated the use of biodegradable, neurotrophin-eluting hydrogels (i.e., poly(ethylene glycol)-poly(lactic acid), PEGPLA) as a means of attracting neurites to the surface of stimulating electrodes. PEGPLA hydrogels with release rates ranging from 1.5 to 3 weeks were synthesized. These hydrogels were applied to multielectrode arrays with sputtered iridium oxide charge-injection sites. The coatings had little impact on the iridium oxide electrochemical properties, including charge storage capacity, impedance, and voltage transients during current pulsing. Additionally, we quantitatively examined the ability of neurotrophin-eluting, PEGPLA hydrogels to promote neurite extension in vitro using a PC12 cell culture model. Hydrogels released neurotrophin (nerve growth factor, NGF) for at least 1 week, with neurite extension near that of an NGF positive control and much higher than extension seen from sham, bovine serum albumin-releasing boluses, and a negative control. These results show that neurotrophin-eluting hydrogels can be applied to multielectrode arrays, and suggest a method to improve neuron-electrode proximity, which could result in lowered electrical stimulation thresholds. Reduced thresholds support the creation of smaller electrode structures and high density electrode prostheses, greatly enhancing prosthesis control and function.

A PEDF N-terminal peptide protects the retina from ischemic injury when delivered in PLGA nanospheres.

The neuroprotective effects of small pigment epithelium-derived factor (PEDF) peptides injected intravitreally as free peptides or delivered in poly(lactide-co-glycolide) (PLGA) nanospheres, were tested in retinal ischemic injury. We induced transient ischemia in C57BL/6 mice by elevating the intraocular pressure to the equivalent of 120 mmHg for 60 min, then injected these eyes with one of the following: PBS, full-length native PEDF, N-terminal peptides-PEDF(136-155) and PEDF(82-121), blank PLGA nanospheres or PLGA loaded with PEDF(82-121) (PLGA-PEDF(82-121)). Morphometric analysis and TUNEL assays were used to determine the extent of retinal damage. Transient ischemia caused a rapid reduction in the number of viable cells in the retinal ganglion cell (RGC) layer over 48h as compared to non-ischemic retinas. About 76% surviving cells in the RGC layer were observed in the full-length PEDF protein treated group, whereas only 32% of cells survived in the PBS group. Thus, PEDF prevented approximately 44% of the cell death in the RGC layer resulting from transient ischemia. PEDF(82-121) peptide was as effective as full-length PEDF when injected as either a free peptide or delivered in PLGA nanospheres. PLGA-PEDF(82-121) showed longer-term protection of the RGC layer with no noticeable side effects at 7days. PEDF and PEDF(82-121) lessened damage to the IPL as measured by layer thickness. PEDF and PEDF(82-121) also delayed retinal responses to ischemic injury as measured by GFAP immunolabeling in Müller cells. PEDF(82-121) is an effective neuroprotective peptide in retinal ischemia. PLGA-PEDF(82-121) offers greater protection to the retina suggesting that this peptide and the method of delivering therapeutically active drugs have potential clinical advantages for longer-term treatments of retinal diseases.

New advances promising in treating neuropathy. Expert explain what's the latest and best.

Neuropathies among HIV patients have continued to increase in recent years, particularly in the case of antiretroviral toxic neuropathy. Research data suggest that about one-third of HIV patients experience some form of neuropathy symptoms.

Intravenous TAT-GDNF is protective after focal cerebral ischemia in mice.

BACKGROUND AND PURPOSE: Delivery of therapeutic proteins into tissues and across the blood-brain barrier is severely limited by their size and biochemical properties. The 11-amino acid human immunodeficiency virus TAT protein transduction domain is able to cross cell membranes and the blood-brain barrier, even when coupled with larger peptides. The present studies were done to evaluate whether TAT-glial line-derived neurotrophic factor (GDNF) fusion protein is protective in focal cerebral ischemia. METHODS: Anesthetized male C57BL/6j mice were submitted to intraluminal thread occlusion of the middle cerebral artery. Reperfusion was initiated 30 minutes later by thread retraction. Laser Doppler flow was monitored during the experiments. TAT-GDNF, TAT-GFP (0.6 nmol each), or vehicle was intravenously applied over 10 minutes immediately after reperfusion. After 3 days (30 minutes of ischemia), animals were reanesthetized and decapitated. Brain injury was evaluated by histochemical stainings. RESULTS: Immunocytochemical experiments confirmed the presence of TAT-GDNF protein in the brains of fusion protein-treated nonischemic control animals 3 to 4 hours after TAT fusion protein delivery. TAT-GDNF significantly reduced the number of caspase-3-immunoreactive and DNA-fragmented cells and increased the number of viable neurons in the striatum, where disseminated tissue injury was observed, compared with TAT-GFP- or vehicle-treated animals. CONCLUSIONS: Our results demonstrate that TAT fusion proteins are powerful tools for the treatment of focal ischemia when delivered both before and after an ischemic insult. This approach may be of clinical interest because such fusion proteins can be intravenously applied and reach the ischemic brain regions. This approach may therefore offer new perspectives for future strategies in stroke therapy.

Neurotrophic actions of novel compounds designed from cyclopentenone prostaglandins.

Previously we found that some cyclopentenone prostaglandin derivatives promoted neurite outgrowth from PC12 cells and dorsal root ganglia explants in the presence of nerve growth factor; and so we referred to them as neurite outgrowth-promoting prostaglandins (NEPPs). In this study, NEPPs protected HT22 cells against oxidative glutamate toxicity. NEPP6, one of the most effective promoters of neurite outgrowth in PC12 cells, protected the cells most potently among NEPPs 1--10. Several derivatives, NEPPs 11--19, were newly synthesized based on the chemical structure of NEPP6. NEPP11 had a more potent neuroprotective effect than NEPP6. NEPP11 also prevented the death of cortical neurons induced by various stimuli and reduced ischemic brain damage in mice. Biotinylated compounds of NEPPs were synthesized to investigate their cellular accumulation. NEPP6-biotin protected the cells and emitted potent signals from the cells. In contrast, biotinylated non-neuroprotective derivatives emitted much weaker signals. These results suggest that NEPPs are novel types of neurotrophic compounds characterized by their dual biological activities of promoting neurite outgrowth and preventing neuronal death and that their accumulation in the cells is closely associated with their neuroprotective actions.

NT-3 attenuates functional and structural disorders in sensory nerves of galactose-fed rats.

The present study investigated the effect of NT-3, a neurotrophin expressed in nerve and skeletal muscle, on myelinated fiber disorders of galactose-fed rats. Adult, female Sprague-Dawley rats were fed diets containing complete micronutrient supplements and either 0% D-galactose (control) or 40% D-galactose. Treated controls received 20 mg/kg NT-3 and treated galactose-fed rats received 1, 5, or 20 mg/kg NT-3 three times per week by subcutaneous injections. After 2 months, sciatic and saphenous sensory nerve conduction velocity (SNCV) and sciatic motor nerve conduction velocity (MNCV) were measured and the sciatic, sural, peroneal and saphenous nerves and dorsal and ventral roots processed for light microscopy. Treatment of control animals with NT-3 had no effect on any functional or structural parameter. Compared to control values, galactose feeding induced a sensory and motor nerve conduction deficit and a reduction in axonal caliber. Treatment with 5 and 20 mg/kg NT-3 ameliorated deficits in sciatic and saphenous SNCV in galactose-fed rats but had no effect on the MNCV deficit. NT-3 treatment also attenuated the decrease in mean axonal caliber in the dorsal root and sural nerve but not in the saphenous nerve, ventral root and peroneal nerve. These observations show that NT-3 can selectively attenuate the sensory conduction deficit of galactose neuropathy in a dose-dependent manner that depends only in part on restoration of axonal caliber of large-fiber sensory neurons.

Leptin and ciliary neurotropic factor (CNTF) inhibit fasting-induced suppression of luteinizing hormone release in rats: role of neuropeptide Y.

Periods of chronic undernutrition and short periods of fasting suppress pituitary luteinizing hormone (LH) secretion and upregulate hypothalamic neuropeptide Y (NPY), the orexigenic peptide. The effect of suppression of NPY upregulation with ciliary neurotropic factor (CNTF), a cytokine, and leptin, an adipocyte hormone, on pituitary LH secretion was evaluated in fasted rats. In the first experiment, daily injection of CNTF (0.2 nmol) intracerebroventricularly (i.c.v.) for 4 days drastically reduced food intake and body weight gain similar to the weight loss seen in pair-fed rats. Food deprivation (FD) also decreased body weight. Despite drastic loss in body weight, plasma LH was reduced in FD and pair-fed rats, but not in CNTF-treated rats. In the second experiment, FD rats received either control vehicle, CNTF (0.2 nmol) or leptin (0.2 nmol) daily for 4 days. FD increased steady state levels of preproNPY mRNA in the hypothalamus over the control freely-fed rats. However, both CNTF and leptin suppressed hypothalamic gene expression and significantly attenuated LH suppression in response to FD. Taken together, these results support the hypothesis that the upregulation of hypothalamic NPY system may underlie diminution in pituitary gonadotropin secretion and that the NPYergic pathway may serve as a communication bridge between the neural processes that regulate reproduction and those that maintain energy balance.

Ciliary neurotrophic factor stimulates the expression of glial fibrillary acidic protein by brain astrocytes in vivo.

Ciliary neurotrophic factor is a cytokine that has effects on neuronal survival and phenotype in vitro and in vivo. Ciliary neurotrophic factor has also been shown to have effects on microglia and oligodendrocytes in vitro and in vivo. In this study, we demonstrate in vivo effects of ciliary neurotrophic factor on astrocytes in both the injured and uninjured central nervous system. Ciliary neurotrophic factor increases the expression of glial fibrillary acidic protein and induces concomitant morphological changes in central nervous system astrocytes. Messenger RNA for both ciliary neurotrophic factor and the alpha-component of the ciliary neurotrophic factor receptor is demonstrated in the optic nerve, an essentially pure population of central nervous system glia. We also report here that the promoter region of the glial fibrillary acidic protein gene contains sequences thought to confer direct ciliary neurotrophic factor modulation of glial fibrillary acidic protein gene transcription. Although it is thought that astrocytes are a source of endogenous ciliary neurotrophic factor in the central nervous system and that neurons express the alpha-component of the ciliary neurotrophic factor receptor, the results of the present investigation suggest that astrocytes themselves respond to ciliary neurotrophic factor and that ciliary neurotrophic factor may also be important in glial cell-cell interactions.

Potential of neurotrophic factors in therapy of Parkinson's disease.

Neurotrophic factors of dopaminergic neurons may represent a potential neuroprotective therapy for PD. This article reviews published experiments that demonstrate the effects of neurotrophic factors on dopaminergic neurons in vitro and in vivo. At present this issue is predominantly investigated in basic neuroscientific research. Its possible future clinical relevance is discussed.

Polymer-encapsulated Schwannoma cells expressing human nerve growth factor promote the survival of cholinergic neurons after a fimbria-fornix transection.

Many investigators have recently used genetically modified primary fibroblasts of fibroblast cell lines (e.g., 3T3, 208F, or BHK cells) to deliver recombinant nerve growth factor (NGF) into the CNS. In the current study, SCT-1 cells, a Schwannoma cell line derived from a transgenic mouse, were transfected with a human NGF (hNGF) cDNA. After selection, these cells were encased within a polymer capsule and implanted into the ventricles of fimbria-fornix lesioned rats. Encapsulated, non-transfected cells served as controls. Results demonstrated that the hNGF transgene is expressed for at least 3 weeks after implantation. Moreover, the cells did not overgrow the capsule. Recombinant hNGF was able to save > 70% of lesioned cholinergic neurons, as assessed by NGF-receptor (NGFr) and choline acetyltransferase (ChAT) immunohistochemistry, from cell death. The number of cholinergic neurons in animals that received control capsules (i.e., nontransfected SCT-1 cells) was similar to lesion only animals (i.e., approximately 27% and approximately 33% for NGFr- and ChAT-positive neurons, respectively. These results show that SCT-1 cells can be used to deliver biologically active hNGF into the lesioned rat brain.

NGF restores decrease in catalase and increases glutathione peroxidase activity in the brain of aged rats.

The effects of subchronic administration of nerve growth factor (NGF) into the lateral ventricle on catalase and selenium-dependent glutathione-peroxidase (GSH-Px) activity in several areas of the brain in 3-, 12- and 24-month-old rats were studied. NGF given daily (1 microgram for 28 consecutive days) produced in all brain areas studied a significant increase in catalase activity in 12- and 24-month-old rats. The most important finding was a complete restoration in 12- and 24-month-old rats of catalase activity to levels similar to those occurring in young (3-month-old) rats. In addition, NGF produced in comparison to 3-month-old rats and to same age vehicle-treated rats a significant increase in selenium-dependent GSH-Px in all the brain areas studied in 12- and 24-month-old animals, whereas selenium-independent GSH-Px was unaffected. In conclusion, the present results show that long-term administration of NGF into the lateral ventricle significantly increases in old animals the activity of key enzymes involved in the metabolic degradation of hydrogen peroxide.

Hypothalamic self-stimulation in rats following immunosympathectomy or central nerve growth factor antiserum injection.

In Experiment 1, immunosympathectomized rats self-stimulated at a much lower rate on high variable ratio schedules of reinforcement than injected controls. No differences were found for responding on continuous reinforcement or low variable ratio reinforcement schedules. In Experiment 2, a similar reduction in varialbe ratio response rate was found for subjects centrally injected with nerve growth factor-antiserum relative to controls. The results suggest a reduction in central catecholamine levels as a result of antiserum treatment.