The nicotine metabolite, cotinine, attenuates glutamate (NMDA) antagonist-related effects on the performance of the five choice serial reaction time task (5C-SRTT) in rats.

Cotinine, the most predominant metabolite of nicotine in mammalian species, has a pharmacological half-life that greatly exceeds its precursor. However, until recently, relatively few studies had been conducted to systematically characterize the behavioral pharmacology of cotinine. Our previous work indicated that cotinine improves prepulse inhibition of the auditory startle response in rats in pharmacological impairment models and that it improves working memory in non-human primates. Here we tested the hypothesis that cotinine improves sustained attention in rats and attenuates behavioral alterations induced by the glutamate (NMDA) antagonist MK-801. The effects of acute subcutaneous (dose range 0.03-10.0 mg/kg) and chronic oral administration (2.0 mg/kg/day in drinking water) of cotinine were evaluated in fixed and variable stimulus duration (VSD) as well as variable intertrial interval (VITI) versions of a five choice serial reaction time task (5C-SRTT). The results indicated only subtle effects of acute cotinine (administered alone) on performance of the 5C-SRTT (e.g., decreases in timeout responses). However, depending on dose, acute treatment with cotinine attenuated MK-801-related impairments in accuracy and elevations in timeout responses, and it increased the number of completed trials. Moreover, chronic cotinine attenuated MK-801-related impairments in accuracy and it reduced premature and timeout responses when the demands of the task were increased (i.e., by presenting VSDs or VITIs in addition to administering MK-801). These data suggest that cotinine may represent a prototype for compounds that have therapeutic potential for neuropsychiatric disorders (i.e., by improving sustained attention and decreasing impulsive and compulsive behaviors), especially those characterized by glutamate receptor alterations.

Effect of administration of the nicotinic acetylcholine receptor antagonist BTMPS, during nicotine self-administration, on lever responding induced by context long after withdrawal.

The use-dependent, nicotinic acetylcholine receptor antagonist bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (BTMPS) was studied for its potential to reduce the self-administration of nicotine in rats, as well as to reduce context-induced recidivistic-like behavior after a six-week period of cessation. Rats were allowed to self-administer nicotine (FR1 schedule) inside an operant chamber with a response lever active on a 24 h basis for 14 days. After the self-administration phase, the rats were returned to standard maintenance cages for a period of six weeks. At the end of six weeks the rats were returned to the operant chambers for 7 days and lever responses were recorded under conditions identical to the original self-administration phase, except that lever responses were not rewarded. Daily administration (s.c.) of BTMPS produced a dose-dependent decrease in the self-administration of nicotine 55-80% compared to control animals, and significantly decreased context-induced lever responding initiated six weeks after cessation (35-78% reduction vs. controls). Decreasing the BTMPS regimen to administration once every 3 days was not effective in reducing nicotine self-administration, but lever responding induced during the return to the operant chambers 6 weeks later was significantly decreased (40% reduction vs. controls). Therefore BTMPS can selectively reduce both self-administration of nicotine and long-term recidivistic-like behavior depending upon the dose regimen. Since BTMPS does not evoke anti-nicotinic effects under normal physiological conditions, these data support a proof of concept for the safe use of such compounds in the treatment of tobacco abuse.

Anomaly in aortic arch alters pathological outcome of transient global ischemia in Rhesus macaques.

We investigated a non-human primate (NHP) transient global ischemia (TGI) model which was induced by clipping the arteries originating from the aortic arch. Previously we demonstrated that our TGI model in adult Rhesus macaques (Macaca mulatta) results in marked neuronal cell loss in the hippocampal region, specifically the cornu Ammonis (CA1) region. However, we observed varying degrees of hippocampal cell loss among animals. Here, we report for the first time an anomaly of the aortic arch in some Rhesus macaques that appears as a key surgical factor in ensuring the success of the TGI model in this particular NHP. Eleven adult Rhesus macaques underwent the TGI surgery, which involved 10-15-minute clipping of both innominate and subclavian arteries. Animals were allowed to survive between 1 day and 28 days after TGI. Because of our experience and knowledge that Japanese macaques exhibited only innominate and subclavian arteries arising from the aortic arch, macroscopic visualization of these two arteries alone in the Rhesus macaques initially assured us that clipping both arteries was sufficient to produce TGI. During the course of one TGI operation, however, we detected 3 arterial branches arising from the aortic arch, which prompted us to subsequently search for 3 branches in succeeding TGI surgeries. In addition, we performed post-mortem examination of the heart to confirm the number of arterial branches in the aortic arch. Finally, in order to reveal the pathological effect of the aortic arch anomaly, we compared the hippocampal cell loss between animals found to have 3 arterial branches but had all or only two branches clipped during TGI operation. Post-mortem examination revealed that eight NHPs had the typical two arterial aortic branches, but three NHPs displayed an extra arterial aortic branch, indicating that about 30% of Rhesus macaques had 3 arterial branches arising from the aorta. Histological analyses using Nissl staining showed that in NHPs with the aortic arch anomaly clipping only two of three arterial branches led to a partial cell loss and minimal alteration in number of cell layers in the hippocampal region when compared with clipping all three branches, with the hippocampal cell death in the latter resembling the pathological outcome achieved by clipping the two arterial branches in NHPs displaying the typical two-artery aortic arch. The finding that 3 of 11 NHPs exhibited an extra arterial aortic branch recognizes this aortic arch anomaly in Rhesus macaques that warrants a critical surgical maneuver in order to successfully produce consistent TGI-induced hippocampal cell loss.

Desensitization of nicotinic acetylcholine receptors as a strategy for drug development.

The specific pharmacological response evoked by a nicotinic acetylcholine receptor (nAChR) agonist is governed by the anatomical distribution and expression of each receptor subtype and by the stoichiometry of subunits comprising each subtype. Contributing to this complexity is the ability of agonists that bind to the orthosteric site of the receptor to alter the affinity state of the receptor and induce desensitization and the observation that, at low doses, some nAChR antagonists evoke agonist-like nicotinic responses. Brain concentrations of nicotine rarely increase to the low-mid micromolar concentrations that have been reported to evoke direct agonist-like responses, such as calcium influx or neurotransmitter release. Low microgram per kilogram doses of nicotine administered to humans or to nonhuman primates to improve cognition and working memory probably result only in low nanomolar brain concentrations--more in line with the ability of nicotine to induce receptor desensitization. Here we review data illustrating that nicotine, its major metabolite cotinine, and two novel analogs of choline, JWB1-84-1 [2-(4-(pyridin-3-ylmethyl)piperazin-1-yl)ethanol] and JAY2-22-33, JWB1-84-1 [2-(methyl(pyridine-3-ylmethyl)amino)-ethanol], improve working memory in macaques. The effectiveness of these four compounds in the task was linearly related to their effectiveness in producing desensitization of the pressor response to ganglionic stimulation evoked by a nAChR agonist in rats. Only nicotine evoked an agonist-like action (increased resting blood pressure). Therefore, it is possible to develop new chemical entities that have the ability to desensitize nAChRs without an antecedent agonist action. Because these "silent desensitizers" are probably acting allosterically, an additional degree of subtype specificity could be attained.

Hippocampal CA1 cell loss in a non-human primate model of transient global ischemia: a pilot study.

We exposed adult Rhesus (Macaca mulatta) to a transient global ischemia, which was induced by clipping the innominate and subclavian arteries that originated from the aortic arch. NHP1 received 20-min, while NHP2 and NHP3, were exposed to a 15-min transient global ischemia and were euthanized at day 1 (NHP1), day 5 (NHP2) or day 30 (NHP3) after ischemia, respectively. NHP1 displayed severe paralysis and rigidity, and intermittent convulsions over the next 24 h. Although histological examination of the brain revealed no detectable gross brain damage (i.e., swelling) and only minimal cell loss in the hippocampus, the acute survival time after surgery likely prevented the cerebral ischemia to fully develop and to be morphologically manifested. Nonetheless, the 20-min ischemia might have been too severe and caused a systemic multiple organ collapse that produced the abnormal behavioral symptoms. On the other hand, NHP2 and NHP3 which received 15-min ischemia only exhibited minor hindlimb paralysis. Indeed, by 48 h after ischemia, both animals appeared fully recovered with only fine motor deficits. Immunohistochemical examination revealed that NHP2 and 3, but not NHP1, had a marked neuronal cell loss in the hippocampal region, specifically the cornu Ammonis (CA1) region. The cell loss in these two ischemic NHP hippocampi was further confirmed by direct comparison with a normal Rhesus brain. These findings replicate the brain pathology seen in Japanese macaques exposed to the same ischemia model [T. Tsukada, M. Watanabe, T. Yamashima, Implications of CAD and DNase II in ischemic neuronal necrosis specific for the primate hippocampus, J. Neurochem. 79 (2001) 1196-1206; T. Yamashima, Implication of cysteine proteases calpain, cathepsin and caspase in ischemic neuronal death of primates, Prog. Neurobiol. 62 (2000) 273-295; T. Yamashima, Y. Kohda, K. Tsuchiya, T. Ueno, J. Yamashita, T. Yoshioka, E. Kominami, Inhibition of ischemic hippocampal neuronal death in primates with cathepsin B inhibitor CA-074: a novel strategy for neuroprotection based on calpain-cathepsin hypothesis, Eur. J. Neurosci. 10 (1998) 1723-1733; T. Yamashima, T.C. Saido, M. Takita, A. Miyazawa, J. Yamano, A. Miyakawa, H. Nishijyo, J. Yamashita, S. Kawashima, T. Ono, T. Yoshioka, Transient brain ischemia provokes Ca2+, PIP2 and calpain responses prior to delayed neuronal death in monkeys, Eur. J. Neurosci. 8 (1996) 1932-1944; T. Yamashima, A.B. Tonchey, T. Tsukada, T.C. Saido, S. Imajoh-Ohmi, T. Momoi, E. Kominami, Sustained calpain activation associated with lysosomal rupture executes necrosis of the postischemic CA1 neurons in primates, Hippocampus 13 (2003) 791-800]. The present minimally invasive transient global ischemia model using Rhesus shows many histopathological symptoms seen in human patients who experienced global ischemia, and should allow translational validation of experimental therapeutics for ischemic injury. Additional studies are warranted to reveal behavioral deficits associated with this ischemia model.

MHP-133, a drug with multiple CNS targets: potential for neuroprotection and enhanced cognition.

MHP-133 is one of a novel series of compounds designed to target multiple brain substrates expected to have synergistic actions in the treatment of cognitive and neurodegenerative disorders such as Alzheimer's disease. The strategy was to develop compounds with multiple targets relevant for enhancing cognition and memory, but avoiding the serious side effects attributed to high potency cholinergic agonists. MHP-133 was shown to interact with subtypes of cholinergic, serotonergic, and imidazoline receptors and to weakly inhibit acetylcholinesterase activity. In vitro, the drug enhanced nerve growth factor (TrkA) receptor expression; it prevented excitotoxicity in a hippocampal slice preparation; and increased the secretion of soluble (non-toxic) amyloid precursor protein. MHP-133 also enhanced cognitive performance by rats and by non-human primate in tasks designed to assess working memory. The results of this study are consistent with the potential use of MHP-133 in the treatment of neurodegenerative disorders such as Alzheimer's disease.

Disconnection between activation and desensitization of autonomic nicotinic receptors by nicotine and cotinine.

Cotinine is the major metabolite of nicotine in humans, and the substance greatly outlasts the presence of nicotine in the body. Recently, cotinine has been shown to exert pharmacological properties of its own that include potential cognition enhancement, anti-psychotic activity, and cytoprotection. Since the metabolite is generally less potent than nicotine in vivo, we considered whether part of cotinine's efficacy could be related to a reduced ability to desensitize nicotinic receptors as compared with nicotine. Rats freely moving in their home cages were instrumented to allow ongoing measurement of mean arterial blood pressure (MAP). The ganglionic stimulant dimethylphenylpiperazinium (DMPP) maximally increased MAP by 25mmHg. Slow (20min) i.v. infusion of nicotine (0.25-1micromol) produced no change in resting MAP, but the pressor response to subsequent injection of DMPP was significantly attenuated in a dose-dependent manner by up to 51%. Pre-infusion of equivalent doses of cotinine produced the same maximal degree of inhibition of the response to DMPP. Discrete i.v. injections of nicotine also produced a dose dependent increase in MAP of up to 43mmHg after the highest tolerated dose. In contrast, injection of cotinine produced no significant change in MAP up to 13 times the highest dose of nicotine. These results illustrate the disconnection between nicotinic receptor activation and receptor desensitization, and they suggest that cotinine's pharmacological actions are either mediated through partial desensitization, or through non-ganglionic subtypes of nicotinic receptors.

Guidelines on nicotine dose selection for in vivo research.

RATIONALE: This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure. OBJECTIVES: This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models. RESULTS: Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses. CONCLUSIONS: The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Effect of amyloid peptides on the increase in TrkA receptor expression induced by nicotine in vitro and in vivo.

The ability of nicotine to induce a cytoprotective or neuroprotective action occurs through several downstream mechanisms. One possibility is that the drug increases the expression of tyrosine kinase A (TrkA) nerve growth factor (NGF) receptors. Certain beta-amyloid peptides (e.g., Abeta1-42) have been shown to bind with high affinity to alpha7 nicotinic receptors and thus interfere with a potentially neurotrophic influence. Treatment of differentiated PC-12 cells with nicotine produced a concentration-dependent increase in cell-surface TrkA receptors that occurred concomitantly with cytoprotection. The effect of nicotine was blocked by either of the alpha7 receptor antagonists alpha-bungarotoxin (alpha-BTX) or methyllycaconatine. The cytoprotective action of nicotine also was inhibited by pretreatment with 10-100 nM Abeta1-42. Nicotine also was administered (four injections of 30 microg, spaced evenly over 24 h) to rats by direct injection into a lateral cerebral ventricle. Brain TrkA expression was increased significantly in hippocampus and entorhinal cortex (up to 32% above control), with no changes found in cerebral cortex or hypothalamus. The nicotine-induced increases in TrKA expression in hippocampus and entorhinal cortex were significantly inhibited by 10 microg alpha-BTXor by 10 nmol Abeta1-42. Therefore, physiologically relevant concentrations of Abeta1-42 can prevent nicotine-induced TrkA receptor expression in brain regions containing cholinergic neurons susceptible to the neurotoxicity associated with Alzheimer's disease.

Long-lasting cognitive improvement with nicotinic receptor agonists: mechanisms of pharmacokinetic-pharmacodynamic discordance.

Agonists of nicotinic acetylcholine receptors (nAChRs) produce long-lasting cognitive effects in animal models and humans. The duration of these cognitive effects can outlast the presence of the agonists in the system, and the persistence of cognitive enhancement is increased further by repeated exposure. The basis for this discrepancy appears be the cellular and systemic mechanisms of learning and memory. Agonists of nAChRs induce long-term potentiation (LTP), which is a strengthening of synaptic connections that is associated with learning and memory formation. Some of the cellular effects of nAChR agonists overlap with the known cellular mechanisms of LTP, including long-lasting increases in intracellular concentrations of Ca2+, activation of second-messenger systems and transcription factors, elevated levels of gene products and enhanced neurotransmitter release. A better understanding of this phenomenon might shed new light on the role of nAChR systems in memory formation and retrieval.

Cotinine, a neuroactive metabolite of nicotine: potential for treating disorders of impaired cognition.

The pharmacological effects of the tobacco-derived alkaloid nicotine have been widely studied in humans and animals for decades. However, relatively little attention has been given to the potential actions of its major metabolite, cotinine. After nicotine consumption the duration of cotinine's presence in blood and brain greatly exceeds that of nicotine. Therefore, cotinine could mediate the more protracted pharmacological effects of nicotine. The studies described in this report were thus designed to further investigate certain neuropharmacological actions of cotinine. Behavioral tests (e.g., delayed matching-to-sample) were conducted in aged rhesus monkeys to assess the effects of cotinine on working memory and attention. In rats a prepulse inhibition (PPI) procedure was used to assess the effects of the compound on auditory gating - a method for predicting the potential antipsychotic properties of drugs. Cotinine exhibited significant effectiveness in these tasks. The drug was also cytoprotective in differentiated PC-12 cells with a potency equivalent to that of nicotine. The effects of chronic cotinine treatment on the expression of nicotinic and muscarinic acetylcholine receptors in rat brain were measured by [125I]epibatidine, [125I]alpha-bungarotoxin ([125I]BTX), [3H]pirenzepine ([3H]PRZ), and [3H]AFDX-384 ([3H]AFX) autoradiography. Unlike nicotine, cotinine failed to upregulate the expression of brain nicotinic receptors. Based on its relative safety in man, cotinine should prove useful in the treatment of diseases of impaired cognition and behavior without exhibiting the toxicity usually attributed to nicotine.

Neuronal nicotinic receptor subtypes: defining therapeutic targets.

Humans have appreciated the beneficial properties of the tobacco plant for thousands of years. These effects include alertness, reduced anxiety, muscle relaxation, and analgesia. Yet it has been less than two decades since the central actions of nicotine have been examined in earnest for potential therapeutic applications. In fact, the cholinergic systems, in comparison to other neurotransmitter systems of the body, have been relatively poorly exploited in terms of therapeutic agents, and the muscarinic cholinergic systems have been relegated mainly to the treatment of gastrointestinal disorders and glaucoma; for the nicotinic system, antagonists are used to induce muscle paralysis during certain surgical procedures. For both families of cholinergic receptors, widespread exploitation in terms of therapeutics has been limited by significant side effect profiles associated with available cholinergic drugs.

Effect of beta-amyloid peptide 1-42 on the cytoprotective action mediated by alpha7 nicotinic acetylcholine receptors in growth factor-deprived differentiated PC-12 cells.

Brain deposition of beta-amyloid peptide (Abeta1-42)-containing senile plaques is a consistent finding in Alzheimer's disease (AD). However, the link between Abeta1-42 and neuronal degeneration remains unclear. It has been reported that Abeta peptides bind with selectivity to alpha7 nicotinic acetylcholine receptors (alpha7nAChRs) and that the two proteins are associated in human AD brain tissue. A potential functional interaction between alpha7nAChRs and Abeta1-42 also has been suggested through the ability of nicotine to inhibit Abeta1-42-induced cytotoxicity. Differentiated PC-12 cells share several features in common with cholinergic basal forebrain neurons. The cells express alpha7nAChRs, they require growth factor stimulation for their maintenance and survival, and nicotine protects against cytotoxicity subsequent to growth factor withdrawal. Using these cells as a model system, we designed experiments to more directly determine whether Abeta peptides (Abeta1-42 and Abeta1-40) interfere with a potential nicotinic cytoprotective action and with the ability of nicotine to increase intracellular Ca2+. Differentiated PC-12 cells were preloaded with fura 2/acetoxymethyl ester and intracellular free Ca2+ levels were determined by fluorescent imaging. Nicotine-induced Ca2+ signals were inhibited by pretreatment with the alpha7nAChR-selective antagonists alpha-bungarotoxin and methyllycaconitine, and they were completely absent in cells maintained in Ca2+-free medium. The nicotine response also was blocked by pretreatment with 100 nM Abeta1-42. Nicotine (1-1000 muM) produced a concentration-dependent increase in cell viability in differentiated PC-12 cells that underwent nerve growth factor withdrawal for 24 h. Cell viability was maintained near 100% by 100 muM nicotine. The cytoprotective action of nicotine was efficiently antagonized by cotreatment with alpha7nAChR antagonists. A concentration-dependent inhibition of the cytoprotective action of nicotine also was produced by cotreatment with Abeta1-42 (1-100 nM), but not with Abeta40 -1. It is possible, therefore, that in AD, as growth factor support to basal forebrain cholinergic neurons declines, the interaction of Abeta peptides with alpha7nAChRs may enhance toxicity by interfering with an important nicotinic signal for neuronal viability.

Relationship between the induction of RAGE cell-surface antigen and the expression of amyloid binding sites.

Purified human brain neurofilament protein was subjected to glycating conditions to produce an advanced glycation end product (HNF-AGE). Two mice were immunized with this HNF-AGE. Unexpectedly, the animals generated IgGs against a peptide immunogenic fragment of the receptor for advanced glycation end products (RAGE) and against human amyloid beta peptide (Abeta). In leukocyte populations, 30-52% of lymphocytes were positive for cell-surface RAGE, and 20-25% were positive for the Abeta peptide. A monoclonal antibody (MAb) directed against the sequence of human RAGE was reactive against a 35-kDa protein band that was highly expressed in blood cells, plasma proteins, lung, liver, spleen, and brain derived from the immunized mice. A MAb directed against Abeta proteins also identified the same 35-kDa band. Thus, the time-dependent formation of AGEs might play a role within the context of the immune system in the expression of binding sites for amyloid peptides, possibly resulting in enhancing cellular toxicity.

Enhanced attention in rhesus monkeys as a common factor for the cognitive effects of drugs with abuse potential.

RATIONALE: One of the common neurochemical features of many drugs of abuse is their ability to directly or indirectly enhance dopaminergic activity in the brain, particularly within the ventral tegmental-nucleus accumbens pathway. Dopaminergic pathways in the frontal and limbic cortex also may be targets for these agents, where pharmacological effects could result in heightened attention and/or support self-administration behavior. OBJECTIVES: The purpose of this study was to determine whether drugs from differing pharmacological classes that exhibit abuse potential would share the ability to counter distractability in the delayed matching task. METHODS: Well trained mature macaques performed a computer-assisted delayed matching-to-sample task which included trials associated with three delay intervals and randomly interspersed task-relevant distractors. Drug regimens included four to five doses and subjects were tested no more than twice per week. RESULTS: All but one of the six compounds (tomoxetine), on average, increased task accuracy for either non-distractor or distractor trials. It was evident that for several compounds, doses required to improve accuracy for non-distractor trials were routinely greater than the doses required to improve accuracy for distractor trials. Data for the individualized Best dose (based upon the subject's optimal level of accuracy during distractor trials) revealed statistically significant distractor-related improvements in task accuracy for the same five compounds. The relative efficacy for reversing distractor-induced decrements in task accuracy was estimated by the level of improvement with respect to baseline: nomifensine (31%)>nicotine (22%) approximately morphine (19%) approximately caffeine (19%) approximately methylphenidate (22%) >tomoxetine (9%). Tomoxetine (noradrenergic preferring) was the only compound that did not produce a significant improvement in accuracy. CONCLUSIONS: These results provide pharmacological support for the concept that attentional mechanisms may play an important role in the "environmental" associative aspects of drug seeking behavior, and as such they may provide the basis for treatment strategies aimed at preventing relapse in detoxified addicts.

The potential role of cotinine in the cognitive and neuroprotective actions of nicotine.

Cotinine is a primary metabolite of nicotine that has been suggested in many studies in animals and in humans to exert measurable effects on aspects of on-going behavior or on cognitive function. Much of the interest in cotinine derives from its long pharmacological half-life (15-19 hours) relative to nicotine (2-3 hours). Despite decades of study focusing on nicotine as the predominant behaviorally active component of tobacco, there continue to be aspects of the pharmacology of the drug that have yet to be explained. For example, nicotine can evoke a protracted behavioral response, i.e., in great excess of the presence of the drug in the plasma. Also, there is often a striking differential between the potency for nicotine-induced behavioral responses in humans and animals, and its potency as a cholinergic agonist, neurochemically. One possibility that may explain one or more of these properties of nicotine is the presence of a long-lived bioactive metabolite or breakdown product of nicotine such as cotinine. Preliminary data in support of this hypothesis are consistent with the ability of cotinine to improve performance accuracy on delayed matching task by macaque monkeys, and in reversing apomorphine-induced deficits in prepulse inhibition of acoustic startle in rats. The drug also was shown to be as potent as nicotine in the ability to act as a cytoprotective agent in cells that express a neuronal cholinergic phenotype. This new appreciation for the role of cotinine in nicotine's actions, and as a pharmacological agent in its own right, particularly in aspects of cognitive function and for neuroprotection, ultimately may be applied towards the treatment of Alzheimer's disease and related disorders, and for various psychiatric syndromes.

Relative levels of cytoprotection produced by analogs of choline and the role of alpha7-nicotinic acetylcholine receptors.

Several analogs of the acetylcholine precursor molecule choline have been widely studied as potential false cholinergic neurotransmitters with the therapeutic goal of using them to limit cholinergic neurotransmission. More recently, choline itself has been shown to act as a full, if low potency, agonist at the alpha7 subtype of the nicotinic acetylcholine receptor. This pharmacological property has been associated with the ability of nicotine and other related alpha7 receptor agonists to offer neuroprotection in a variety of experimental models. We confirm here that choline offers a significant degree of protection against the cytotoxicity induced by growth factor deprivation in differentiated PC-12 cells. Choline-induced cytoprotection ( approximately 1 mM) was about 3 orders of magnitude less potent than that for nicotine (EC(50) = 0.7 microM). Choline also exhibited only about 40% of the full cytoprotective effect of nicotine. Ethyl substitution for choline's N-methyl groups did not result in a significant improvement over choline as a cytoprotective agent. In contrast, pyrrolidinecholine exhibited much greater potency (EC(50) = 20 microM) and increased efficacy (about 55% of nicotine's effect) than choline. Like choline and nicotine, pyrrolidinecholine fully displaced [(125)I]alpha-bungarotoxin binding (K(i) = 33 microM) and chronic exposure to the analog increased cell surface binding sites. The cytoprotective effects of the analog were completely inhibited by coincubation with methyllycaconitine (MLA), a selective alpha7-nicotinic receptor antagonist. These findings are consistent with the possibility that the choline structure may serve as a template for the development of novel agents with both alpha7-nicotinic agonist activity and potential neuroprotective ability, as many of these compounds, including pyrrolidinecholine, are transported along with choline into the central nervous system.

Nicotine increases the expression of high affinity nerve growth factor receptors in both in vitro and in vivo.

Impairment in nerve growth factor (NGF)-mediated support to basal forebrain cholinergic neurons may represent an initial insult to certain neural cells in Alzheimer's disease (AD). High affinity NGF receptor (TrkA) levels are decreased in AD brains as compared to age-matched control brains. One of the approaches suggested for the treatment of AD exploits the ability of small molecular substances to enhance the expression of endogenous growth factors and/or their receptors. The purpose of this study was to determine whether treatment with nicotine in both in vitro and in vivo settings would increase the neural expression of TrkA receptors. Using a differentiated PC12 neuronal-like system, chronic nicotine treatment increased cell surface TrkA receptor expression. Nicotine's action was blocked by co-treatment with either the non-competitive nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine or with the alpha7 nAChR-selective antagonist methyllycaconitine. Surprisingly, certain low doses of mecamylamine alone also increased TrkA receptor levels. Rats prepared with chronic indwelling intravenous catheters were continuously infused with nicotine to deliver a total dose of 12 mg/kg over 24 hr. This treatment resulted in a significant 44% increase in TrkA receptor expression in the hippocampus. As in the cell experiments, mecamylamine also increased hippocampal TrkA receptor expression. In fact, the ratio of the maximal mecamylamine response to the maximal nicotine response that was measured in vitro, i.e., 0.43 was remarkably similar to that for the in vivo experiment, i.e., 0.47. Since in our previous studies the increase in TrkA expression produced by nicotine was shown to be related to its cytoprotective actions, these results suggest that nicotine's neuroprotective actions might also be mediated through the drug's interaction with central alpha7 nAChRs and subsequent increase in TrkA receptor expression.