Repeated, intermittent exposures to diisopropylfluorophosphate in rats: protracted effects on cholinergic markers, nerve growth factor-related proteins, and cognitive function.

Organophosphates (OPs) pose a constant threat to human health due to their widespread use as pesticides and their potential employment in military and terrorist attacks. The acute toxicity of OPs has been extensively studied; however, the consequences of prolonged or repeated exposure to levels of OPs that produce no overt signs of acute toxicity (i.e. subthreshold levels) are poorly understood. Further, there is clinical evidence that such repeated exposures to OPs lead to prolonged deficits in cognition, although the mechanism for this effect is unknown. In this study, the behavioral and neurochemical effects of repeated, intermittent, and subthreshold exposures to the alkyl OP, diisopropylfluorophosphate (DFP) were investigated. Rats were injected with DFP s.c. (dose range, 0.25-1.0 mg/kg) every other day over the course of 30 days, and then given a 2 week, DFP-free washout period. In behavioral experiments conducted at various times during the washout period, dose dependent decrements in a water maze hidden platform task and a spontaneous novel object recognition (NOR) procedure were observed, while prepulse inhibition of the acoustic startle response was unaffected. There were modest decreases in open field locomotor activity and grip strength (particularly during the DFP exposure period); however, rotarod performance and water maze swim speeds were not affected. After washout, DFP concentrations were minimal in plasma and brain, however, cholinesterase inhibition was still detectable in the brain. Moreover, the 1.0 mg/kg dose of DFP was associated with (brain region-dependent) alterations in nerve growth factor-related proteins and cholinergic markers. The results of this prospective animal study thus provide evidence to support two novel hypotheses: (1) that intermittent, subthreshold exposures to alkyl OPs can lead to protracted deficits in specific domains of cognition and (2) that such cognitive deficits may be related to persistent functional changes in brain neurotrophin and cholinergic pathways.

Repeated exposures to low-level chlorpyrifos results in impairments in sustained attention and increased impulsivity in rats.

Organophosphates such as chlorpyrifos (CPF) are among the most commonly used pesticides in the world. Therefore, it is not surprising that measurable levels of organophosphates (including CPF) are found in over 50% of fresh fruits, vegetables and grains that we consume and that approximately 80% of adults in the US have detectable levels of CPF metabolites in their urine. It is well known that acute exposure to organophosphates can cause cognitive deficits; however, the effects of daily or intermittent contact with low levels of organophosphates (often reflective of environmental exposures) are not well understood. The objective of this study was to determine if repeated low-level exposures to CPF impaired the performance of the 5-Choice Serial Reaction Time Task (5C-SRTT), an animal model of sustained attention. Adult rats were trained to stably perform the 5C-SRTT, then treated with vehicle or CPF 18.0 mg/kg daily for 14 consecutive days or every other day for 30 days. Behavioral testing occurred daily during the CPF-exposure period and throughout a 30 day washout period to assess recovery. All CPF-treated animals exhibited deficits in percent correct, an increase in omissions and premature responses without signs of impaired motivation or overt toxicity. Deficits in 5C-SRTT accuracy were apparent well into the 30 day washout period despite significant recovery of cholinesterase activity. These results indicate that repeated exposures to relatively low levels of chlorpyrifos lead to protracted impairments of sustained attention and an increase in impulsive behaviors in rats.

A 24-h access I.V. self-administration schedule of morphine reinforcement and the estimation of recidivism: Pharmacological modification by arecoline.

Central cholinergic neurons are known to play a role in the pharmacological actions of opiates. The purpose of the study was to determine whether the muscarinic receptor agonist arecoline, administered during morphine self-administration, would mitigate the subsequent return to self-administration behavior. Rats self-administered increasing concentrations of morphine in operant chambers according to a schedule that permitted unlimited access to lever-activated i.v. infusions on a continuous 24 h basis from 10 to 14 days. Abstinence was induced by discontinuation of the morphine solution and mild withdrawal symptoms were evident from 14 to 74 h. Thereafter the rats remained in their home cages for a 6-week period of protracted abstinence. They were then returned to the operant chambers where lever responding had no reward consequence. The cholinergic muscarinic agonist arecoline was administered twice daily (0.25 or 1 mg/kg, s.c.) throughout the self-administration schedule of morphine. Arecoline treatment partly decreased the self-administration of morphine, it prevented the abstinence-induced decrease in body weight, and it reduced lever responding after protracted withdrawal (by 56%). In animals already dependent on morphine, arecoline failed to alter ongoing self-administration behavior, but responding induced by lever reinstatement 6 weeks after withdrawal was significantly reduced (by 33%). There was a significant relationship between the degree of self-administration activity and the degree of lever responding during reinstatement after protracted abstinence. The results of this study support the role of cholinergic systems in self-administration behavior and context-induced post-withdrawal drug seeking.

Microtubule-associated targets in chlorpyrifos oxon hippocampal neurotoxicity.

Prolonged exposure to organophosphate (OP) pesticides may produce cognitive deficits reflective of hippocampal injury in both humans and rodents. Recent work has indicated that microtubule trafficking is also adversely affected by exposure to the OP pesticide chlorpyrifos, suggesting a novel mode of OP-induced neurotoxicity. The present studies examined effects of prolonged exposure to chlorpyrifos oxon (CPO) on acetylcholinesterase (AChE) activity, immunoreactivity (IR) of microtubule-associated proteins, neuronal injury, and tubulin polymerization using in vitro organotypic slice cultures of rat hippocampus and bovine tubulin. Cultures were exposed to CPO (0.1-10 microM) in cell culture medium for 1-7 days, a regimen producing progressive reductions in AChE activity of 15-60%. Cytotoxicity (somatic uptake of the non-vital marker propidium iodide), as well as IR of alpha-tubulin and microtubule-associated protein-2 (a/b) [MAP-2], was assessed 1, 3, and 7 days after the start of CPO exposure. As early as 24 h after the start of exposure, CPO-induced deficits in MAP-2 IR were evident and progressive in each region of slice cultures at concentrations as low as 0.1 microM. CPO exposure did not alter alpha-tubulin IR at any time point. Concentration-dependent injury in the cornu ammonis (CA)1 pyramidal cell layer and to a lesser extent, CA3 and dentate cells, was evident 3 days after the start of CPO exposure (>or=0.1 microM) and was greatest after 7 days. Tubulin polymerization assays indicated that CPO (>or=0.1 microM) markedly inhibited the polymerization of purified tubulin and MAP-rich tubulin, though effects on MAP-rich tubulin were more pronounced. These data suggest that exposure to CPO produces a progressive decrease in neuronal viability that may be associated with impaired microtubule synthesis and/or function.

The induction of surface beta-amyloid binding proteins and enhanced cytotoxicity in cultured PC-12 and IMR-32 cells by advanced glycation end products.

During aging the non-enzymatic glycation of proteins and other molecules increases significantly, leading to the accumulation of advanced glycation end-products (AGEs). These AGEs enhance inflammatory and autoimmune reactions with resultant cytotoxicity. We noted in an earlier study that individuals with Alzheimer's disease exhibit enhanced expression of the receptor for advanced glycation end-products (RAGE) on the surface of their leukocytes. In order to better understand the relationship between AGEs and the cell surface binding of amyloid-beta protein (Abeta) 42 we studied the effect of two AGEs: glycated bovine serum albumin (BSA), and epsilon-carboxymethyllysine-BSA (CML), a glycoxidation product, on the binding of Abeta42 to rat PC-12 and IMR-32 cells. We measured the expression of three potential cell surface receptors binding Abeta42: RAGE, beta-amyloid precursor protein (beta-APP), and the alpha7 subtype of the nicotinic acetylcholine receptor (alpha7nAChR) by using specific antibody probes. Incubation of PC-12 or IMR-32 cells with bovine serum albumin-advanced glycation end-product (BSA-AGE) or with CML induced small but significant concentration-dependent increases in the expression of beta-APP, RAGE, and alpha7nAChRs as measured by flow cytometry or by ELISA. Incubation of the cells with 48 microM of either of the AGEs combined with varying concentrations (138-1100 nM) of Abeta42 resulted in the enhanced binding of the Abeta42 to the cell surface as compared with cells not exposed to the AGE co-treatment. The combination of AGE and Abeta treatment also resulted in the heightened expression of all three potential Abeta binding sites as well as their gene precursors. Exposure of cells to the same regimen of AGE plus Abeta resulted in the production of reactive oxygen species and mitochondrial toxicity. These results are consistent with the ability of AGEs to enhance the cell surface expression of diverse Abeta42 binding sites, a factor that can lead to the enhanced binding of amyloid and subsequent cell death.

CNS Targets for multi-functional drugs in the treatment of Alzheimer's and Parkinson's diseases.

Patients with mild forms of dementia and age-related memory impairment have just begun to benefit from pharmacotherapy developed over the last several years. However, current approaches do not significantly modify the course of neurodegeneration or of the aging process, and they offer limited and transient benefit to many patients. The goal of this review is to summarize new potential approaches in which molecules have been developed expressly to target multiple brain systems for the treatment of memory and cognition impairment. Some of these approaches include the development of single molecular entities that combine activity as cholinesterase inhibitors, muscarinic cholinergic M2 receptor antagonists, nicotinic acetylcholine receptor agonists, alpha(2)-adrenergic agonists, or monoamine oxidase inhibitors. Many of the bi-functional compounds discussed have improved efficacy as cognitive enhancing agents and/or they offer potential for neuroprotection and disease modification. It is likely that syndromes such as Alzheimer's disease will require multiple drug therapy to address the varied pathological aspects of the disease. Even if the strategy of combining drugs with different therapeutic targets is workable, the development of multi-functional compounds will obviate the challenge of administering multiple single drug entities with potentially different degrees of bioavailability, pharmacokinetics, and metabolism. Also, the simplification of the therapeutic regimen for individuals with AD who have difficulty with compliance is important.

The cholinergic hypothesis of age and Alzheimer's disease-related cognitive deficits: recent challenges and their implications for novel drug development.

The cholinergic hypothesis was initially presented over 20 years ago and suggests that a dysfunction of acetylcholine containing neurons in the brain contributes substantially to the cognitive decline observed in those with advanced age and Alzheimer's disease (AD). This premise has since served as the basis for the majority of treatment strategies and drug development approaches for AD to date. Recent studies of the brains of patients who had mild cognitive impairment or early stage AD in which choline acetyltransferase and/or acetylcholinesterase activity was unaffected (or even up-regulated) have, however, led some to challenge the validity of the hypothesis as well as the rationale for using cholinomimetics to treat the disorder, particularly in the earlier stages. These challenges, primarily based on assays of post mortem enzyme activity, should be taken in perspective and evaluated within the wide range of cholinergic abnormalities known to exist in both aging and AD. The results of both post mortem and antemortem studies in aged humans and AD patients, as well as animal experiments suggest that a host of cholinergic abnormalities including alterations in choline transport, acetylcholine release, nicotinic and muscarinic receptor expression, neurotrophin support, and perhaps axonal transport may all contribute to cognitive abnormalities in aging and AD. Cholinergic abnormalities may also contribute to noncognitive behavioral abnormalities as well as the deposition of toxic neuritic plaques in AD. Therefore, cholinergic-based strategies will likely remain valid as one approach to rational drug development for the treatment of AD other forms of dementia.

Potential cognitive actions of (n-propargly-(3r)-aminoindan-5-yl)-ethyl, methyl carbamate (tv3326), a novel neuroprotective agent, as assessed in old rhesus monkeys in their performance of versions of a delayed matching task.

(N-propargyl-(3R)-aminoindan-5-yl)-ethyl, methyl carbamate (TV3326), a known neuroprotective agent exhibiting the properties of both an inhibitor of monoamine oxidase (brain selective) and an inhibitor of acetylcholinesterase was administered to seven old rhesus monkeys well trained to perform versions of a delayed matching-to-sample (DMTS) task. An increasing dose regimen of TV3326 was administered orally according to a schedule that allowed the animals to perform the standard DMTS task and a self-titrating version of the DMTS task each week during the study. A distractor version of the task was administered during two of the doses of TV3326. Under the conditions of this experiment TV3326 failed to significantly affect accuracy on the standard DMTS task; however, the drug was very effective in improving the ability of subjects to titrate to longer-duration delay intervals in the titrating version of the task. The maximal drug-induced extension of the self-titrated delay interval amounted to a 36.7% increase above baseline. This increase in maximum delay duration occurred without a significant change in overall task accuracy. TV3326 also significantly improved task accuracy during distractor (interference) sessions. The compound was effective enough to return group performance efficiency to standard DMTS vehicle levels of accuracy. These results were independent of whether trials were associated with a distractor or non-distractor delay interval, and they were independent of delay interval. The lack of delay selectivity in task improvement by TV3326 may not be consistent with a selective effect on attention. TV3326 was not associated with any obvious side effect or untoward reaction of the animals to the drug. Thus, TV3326 may be expected to offer a significant positive cognitive outcome in addition to its reported neuroprotective action.

Repeated exposures to subthreshold doses of chlorpyrifos in rats: hippocampal damage, impaired axonal transport, and deficits in spatial learning.

Organophosphorus (OP) compounds are detectable in the environment for years after use and endanger many populations. Although the effects of acutely toxic doses of many OP compounds are well described, much less is known about repeated low-level exposures. The purpose of these studies was to further evaluate potential toxicological effects of the extensively used OP pesticide chlorpyrifos (CPF) in rats. CPF, across a range of subthreshold doses (i.e., for acute toxicity), reduced rearing and sniffing activity and the magnitude of weight gain over 14 days of repeated exposure. Performance in a spatial learning task was impaired after 14 days of exposure to CPF (18.0 and 25.0 mg/kg) when testing was initiated 24 h after the last injection but not after a 14-day washout. However, inhibition of both fast anterograde and retrograde axonal transport was observed for up to 20 days after exposure to 25.0 mg/kg CPF. Studies using hippocampal cultures indicated that 8 days of continuous exposure to the parent compound, CPF (> or =100 micro M), resulted in cell toxicity and death. Furthermore, a dose (2.5 mg/kg) of CPF that had no effects on weight gain or memory performance when administered 5 days per week over 38 days impaired forelimb grip strength in the later days of testing. Collectively, these results indicate that repeated exposures to subthreshold doses of CPF may lead to growth retardation, behavioral abnormalities, and muscle weakness. Some of these symptoms may be attributed to effects of the OP on axonal transport.

Sex dimorphisms in the cognitive-enhancing action of the Alzheimer's drug donepezil in aged Rhesus monkeys.

Brain acetylcholinesterase has been targeted for the development of novel treatments for memory deficits associated with Alzheimer's disease (AD) and other neurodegenerative disorders. The long-acting AChE inhibitor donepezil (Aricept) is used to improve memory and other aspects of cognition in AD patients. Because donepezil and other cholinesterase inhibitors are effective in a restricted population of AD patients, this study was to designed to determine whether aged females monkeys receive the same level of benefit to the mnemonic action of donepezil as do males. In this study, six male and six female rhesus monkeys (>20 years) who were proficient in the performance of a delayed matching-to-sample task each received an ascending series of four doses of donepezil (0.01-0.1 mg/kg) over 5 weeks. As a group, male subjects exhibited improvement in task accuracy across the three highest doses, with the maximum effect occurring after the 0.025 mg/kg dose. However, the females exhibited increased task accuracy only after the highest dose. When data were combined for sessions run 10 min after drug administration and for sessions run 24 h later (in the absence of drug), improvements in task accuracy were greater on average for males. Most of this difference was attributed to the fact that task accuracy by females actually declined during sessions run after the two lowest doses of donepezil. When task performance after donepezil was determined as the individualized Best Dose, as a group, males responded maximally to less than half the dose that was maximal for females. These findings support the concept that aged males and females respond differently to this class of agents, perhaps representing fundamental sex-related differences in memory processing, or in the manner that age affects these processes.

Effects of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a selective ligand for nicotinic acetylcholine receptors, in tests of visual attention and distractibility in rats and monkeys.

Nicotine, a nonselective ligand for nicotinic acetylcholine receptors (nAChRs), has been shown to improve attention and reduce distractibility in humans. Although the numerous side effects induced by nicotine prevent its use as a therapeutic agent, it is hypothesized that subtype-selective nAChR ligands may offer a potential therapeutic benefit to humans with attention deficits. In this study, we evaluated the attention-enhancing properties of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a ligand selective for neuronal nAChRs with predominant activity at the human beta 4 subtype. SIB-1553A was evaluated in a test of attention (i.e., five-choice serial reaction time task or SRTT) and distractibility (i.e., delayed matching to sample task with distractor or DMTS-D) in adult rats and monkeys, respectively. SIB-1553A did not improve SRTT performance in normal rats, but reversed deficits induced by the N-methyl-D-aspartate (NMDA) antagonist dizocilpine. In the DMTS-D, SIB-1553A improved accuracy across several doses at the short delay intervals, which were affected most by distracting stimuli in adult monkeys. Subsequent testing with optimal doses for each monkey was also associated with significant improvements in DMTS-D accuracy at short delays, indicating the reproducibility of the drug effect. In both species, SIB-1553A had no significant effects on latencies for sample or choice selection and was not associated with adverse effects at efficacious doses. Although it remains to be further demonstrated, SIB-1553A may act through combined nicotinic and non-nicotinic mechanisms. Collectively, the present data suggest that in specific conditions SIB-1553A may improve certain aspects of attentional function in young adult rats and nonhuman primates without adverse side effects.

Relationship between the increased cell surface alpha7 nicotinic receptor expression and neuroprotection induced by several nicotinic receptor agonists.

Nicotine and other nicotinic acetylcholine receptor agonists have been shown to exert neuroprotective actions in vivo and in vitro by an as yet unknown mechanism. Even the identification of the subtype of nicotinic receptor(s) mediating this action has not been determined. In neural cell lines, the induction of cytoprotection often requires exposure to nicotine for up to 24 hr to produce a full protective effect. One phenomenon associated with chronic exposure of neural cells to nAChR agonists is the increased expression of nAChRs (upregulation), possibly as a response to desensitization. Because nicotinic receptors desensitize rapidly in the continuous presence of agonist, we investigated whether the neuroprotective actions produced by different nicotinic receptor agonists was related to their ability to induce nicotinic receptor upregulation. Differentiated PC12 cells were preincubated for 24 hr with various nAChR ligands, and the cells were subsequently deprived of both NGF and serum to induce cytotoxicity. Under control conditions cell viability was reduced to 66.5 +/- 5.4% of control by trophic factor withdrawal. For those cells pretreated with nicotine (1 nM-100 microM) cell viability increased from 74.2 +/- 1.5 to 97.3 +/- 4%. The neuroprotective action of nicotine was blocked by co-treatment with either 5 microM mecamylamine or 10 nM methyllycaconitine (MLA). The high potency blockade by MLA suggested that neuroprotection was mediated through the alpha7 nicotinic receptor subtype. For the seven agonists examined for neuroprotective activity, only nicotine was capable of evoking a near maximal (near 100% cell viability) neuroprotective action. The next most effective group included epibatidine, 4OHGTS-21, methycarbamylcholine, and 1,1-dimethyl-4-phenyl-piperazinium iodide. These least effective group included cytisine and tetraethylammonium. Incubation of differentiated PC12 cells with 10 microM nicotine increased the number of [(125)I]alpha bungarotoxin ([(125)I]alphaBGTbinding sites by 41% from 82.6 +/- 3.67 to 117 +/- 10.3 fmol/mg protein). Under similar conditions of incubation, the nicotinic receptor agonist cytisine (that was least effective in terms of neuroprotection) failed to increase the number of [(125)I]alphaBGT binding sites. Cells expressing increased levels of cell surface [(125)I]alphaBGT binding sites received added neuroprotective benefit from nicotine. Thus the induced upregulation of the alpha7 subtype of nicotinic receptors during chronic exposure to nicotine may be responsible for the drug's neuroprotective action.

SIB-1553A, (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride, a subtype-selective ligand for nicotinic acetylcholine receptors with putative cognitive-enhancing properties: effects on working and reference memory performances in aged rodents and nonhuman primates.

Preclinical and clinical data have suggested the potential use of nicotinic acetylcholine receptor (nAChR) ligands for treating cognitive dysfunction associated with neurodegenerative diseases, such as Alzheimer's disease. SIB-1553A, (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride, a novel nAChR ligand with predominant agonist subtype selectivity for beta4 subunit-containing human neuronal nAChRs, was tested in a variety of cognitive paradigms in aged rodents and nonhuman primates after acute and repeated administration. Subcutaneous administration of SIB-1553A improved delayed nonmatching to place performance in aged mice. In aged rhesus monkeys, intramuscular and oral administration of SIB-1553A improved choice accuracy in a delayed matching to sample task. SIB-1553A improved performances in these spatial and nonspatial working memory tasks but was less effective at improving performances in spatial reference memory tasks (i.e., aged rodents exposed to a discrimination task in a T-maze or trained to locate a hidden platform in a water maze). These data suggest that SIB-1553A has a predominant effect on attention/working memory processes. SIB-1553A also induced the release of acetylcholine in the hippocampus of aged rats and was equally effective whether administered acutely or repeatedly (6 weeks of daily subcutaneous administration). Thus, rats repeatedly treated with SIB-1553A exhibit neither tolerance nor sensitization to the effects of the compound. The SIB-1553A-induced cognitive improvement may be in part related to an increase in cholinergic function. The present study provides additional support for the use of subtype-selective nAChR ligands as a potential therapy for the symptomatic treatment of specific cognitive deficits (such as attention/working memory deficits) associated with aging and neurological diseases.

Dahl salt-sensitive and salt-resistant rats: examination of learning and memory performance, blood pressure, and the expression of central nicotinic acetylcholine receptors.

Substantial human and animal data suggest a correlation between hypertension and memory impairment that may appear prior to overt manifestations of cerebrovascular pathology. It is unclear, however, whether hypertension plays a causal role in these memory deficits, whether hypertension and cognitive impairment are each based in family history and not interdependent, or whether a combination of these factors is important. The purpose of this study was to assess whether deficits in memory performance and nicotinic acetylcholine receptors were present in Dahl salt-sensitive rats (as observed previously in spontaneously hypertensive rats) and whether the presence of hypertension per se (induced with an 8% Na(+) diet) contributed to the deficits. Memory was assessed in a passive avoidance task, an eight-arm radial arm maze and in a water maze task, and nicotinic receptors were measured via quantitative receptor autoradiography utilizing [125I]alpha-bungarotoxin and [3H]epibatidine. Salt-sensitive rats exhibited impaired performance in both spatial learning tasks, but not the passive avoidance task, compared to controls (salt-resistant strain) and they exhibited reductions in nicotinic receptors labeled by [125I]alpha-bungarotoxin but not [3H]epibatidine in some brain regions, including some areas important for memory (e.g. the hippocampus and amygdala). In the radial arm maze, the degree of memory impairment and in binding studies the reduced expression of nicotinic receptors each failed to correlate with the highest blood pressures, and the salt-sensitive animals were impaired relative to controls whether or not the high Na(+) diet was administered. In contrast, higher blood pressures did correlate with inferior task performance in the water maze. These findings may suggest that the genetics of the subjects were critical for performance when appetitive drives were involved, but diet (and perhaps hypertension) were key to performance when memory did not involve appetitive drives or mechanisms. Overall, the data obtained from Dahl rats appear to support the role of family history (selective breeding in rats) as underlying the reductions in central nicotinic acetylcholine receptors, whereas both family history and hypertension may contribute to poor cognitive performance.

Inhibition of nerve growth factor signaling by peroxynitrite.

The reactive oxygen species peroxynitrite has been implicated in mediating oxidative damage within the brain, and in particular in those regions associated with the pathology of Alzheimer disease. Evidence for peroxynitrite damage includes the abundance of nitrated tyrosine residues within proteins of neural cells. Potential sites for peroxynitrite-induced cytotoxicity are the tyrosine residues of tyrosine kinase receptors that are crucial for the maintenance of cholinergic neurons. The peroxynitrite generator 3-morpholinosydnonmine (SIN-1) was used to examine the effects of peroxynitrite generation on nerve growth factor (NGF)/TrkA signaling in PC12 pheochromocytoma cells that express a cholinergic phenotype. NGF produced a concentration-dependent increase in PC12 cellular metabolism (EC(50) = 15.2 ng/ml) measured in a microphysiometer. This action of NGF was inhibited in a concentration-dependent manner up to 67% of control by a brief (20 min) exposure of the cells to SIN-1. This inhibition of the NGF cellular response by SIN-1 was not related to generalized cellular toxicity. In fact, the peroxynitrite scavenger uric acid significantly attenuated the inhibitory actions of SIN-1. Pretreatment with SIN-1 also resulted in a decrease in the NGF-induced phosphorylation of TrkA protein. Furthermore, SIN-1 treatment reduced the activity of mitogen activated protein kinase (MAPK), a downstream kinase activated by TrkA receptor stimulation. These data suggest that SIN-1 treatment inhibits NGF signaling by inactivating TrkA receptors through the formation of nitrotyrosine residues on the receptor. The inactivation of TrkA receptors may contribute to the initial insult that eventually leads to neuronal cell death.

Protractive effects of chronic treatment with an acutely sub-toxic regimen of diisopropylflurophosphate on the expression of cholinergic receptor densities in rats.

Individuals chronically exposed to low levels of organophosphate insecticides may present with subtle impairments in cognition. In addition, low level diisopropylflurophosphate (DFP) exposure (0.25 mg/kg per day for 2 weeks) in rats resulted in protracted working memory impairment [29]. The current studies attempt to show a temporal relationship between the DFP-induced impairment in performance of a spatial memory task and the protracted decrease in the expression of cholinergic receptors and acetylcholinesterase in specific brain regions. Cholinergic receptors labeled with the ligands [(3)H]epibatidine and [(3)H]AFDX-384 were affected to a much greater extent and for a longer period of time than were both acetylcholinesterase activities and cholinergic receptors labeled with [(3)H]QNB. Pre-testing administration of nicotine was shown to completely reverse this DFP-induced impairment in memory-related task performance. Additionally, prophylaxis with pyridostigmine bromide (PB) caused DFP-treated animals to exhibit near normal levels of memory-related task performance. These results are consistent with the development of a protracted phase of learning impairment to sub-acute DFP exposure, which may involve the loss of hippocampal nicotinic receptors, and may be prevented or reversed by PB or nicotine, respectively.

Deficits in spatial learning and nicotinic-acetylcholine receptors in older, spontaneously hypertensive rats.

Spontaneously hypertensive rats are often used as models of attention deficit hyperactivity disorder and to investigate the effects of hypertension on cognitive function. Along with the wide variety of cardiovascular anomalies, these animals as young adults also exhibit deficits in memory and attention and central nicotinic-acetylcholine receptor sites. These findings may have particular significance since nicotinic receptors appear to be involved in the regulation of cerebral circulation and mnemonic function. Furthermore, a lack of high affinity nicotinic receptors (in knockout mice) has also been shown to accelerate both the structural and cognitive degeneration associated with age, findings that may be especially relevant to age-related memory disorders such as Alzheimer's Disease where large deficits in nicotinic receptors are observed. Since spontaneously hypertensive rats appear to be both memory-impaired and deficient in nicotinic receptors at a young age (compared to the non-hypertensive phenotype, Wistar-Kyoto rats), we were interested to learn if these conditions were exacerbated in older animals with particular interest in specific nicotinic receptor subtypes in memory areas of the brain. Spatial learning was assessed in 15-month-old subjects of each phenotype (i.e. hypertensive and non-hypertensive) using a two-phase water maze paradigm, and nicotinic receptors were measured via autoradiography with [125I]-alpha-bungarotoxin and [3H]-epibatidine. In the water maze, both groups learned to locate a hidden platform as indicated by progressively shorter latencies across training days, however, Wistar-Kyoto rats were more efficient in both phases. While the number of both bungarotoxin and epibatidine binding sites was lower in the hypertensive rats across several brain regions, in the case of epibatidine binding, the magnitude of the difference and the number of areas affected was generally greater and included areas important for spatial learning (e.g. frontal and entorhinal cortex). In a direct comparison between 3-month-old and 15-month-old rats of each phenotype, epibatidine sites were markedly reduced by age (i.e. by greater than 50% in some cases) across multiple brain regions in both groups, although Wistar-Kyoto rats appeared to be more substantially affected by age. These data further support the use of the spontaneously hypertensive rat as model for studying learning-impairment and reduced central nicotinic receptors and also indicate that these characteristics persist and (in the case of high affinity nicotinic receptor cites) worsen with age.

Multiple central nervous system targets for eliciting beneficial effects on memory and cognition.

The development of drugs for the treatment of disorders of cognition has benefited from a more precise knowledge of the loss of specific neural pathways associated with certain neurodegenerative diseases such as Alzheimer's disease (AD). The loss of basal forebrain cholinergic neurons in AD has engendered the development of new compounds that target various aspects of the cholinergic system. However, limitations in the effectiveness of the most common of these, the anticholinesterases, have fueled the race to provide more efficacious compounds. In an attempt to avoid side effects and improve efficacy, other neuronal targets have been considered, including receptors for norepinephrine, dopamine, serotonin, excitatory amino acids, neural peptides, and others. Our laboratory has had the opportunity to study the memory-enhancing potential of many of the compounds developed expressly for these neuronal targets in macaques. Upon reviewing 21 such studies it was evident that: 1) To varying degrees, pharmacological manipulation of each target yielded improved task performance. 2) Combining pharmacological targets could lead to additive or synergistic effects on task performance. 3) Mature adult and aged monkeys provided equivalent estimates of drug effectiveness. 4) There appeared to be no limiting level of task improvement for compounds tested in aged and younger monkeys. 5) Certain of these agents also exhibited potential disease-modifying actions. Thus, certain memory-enhancing agents may prove more useful when implemented early in the course of a disease such as AD, and they also may enjoy a wide application for the treatment of the memory decline associated with normal aging.