Preclinical characterization of selective phosphodiesterase 10A inhibitors: a new therapeutic approach to the treatment of schizophrenia.

We have recently proposed the hypothesis that inhibition of the cyclic nucleotide phosphodiesterase (PDE) 10A may represent a new pharmacological approach to the treatment of schizophrenia (Curr Opin Invest Drug 8:54-59, 2007). PDE10A is highly expressed in the medium spiny neurons of the mammalian striatum (Brain Res 985:113-126, 2003; J Histochem Cytochem 54:1205-1213, 2006; Neuroscience 139:597-607, 2006), where the enzyme is hypothesized to regulate both cAMP and cGMP signaling cascades to impact early signal processing in the corticostriatothalamic circuit (Neuropharmacology 51:374-385, 2006; Neuropharmacology 51:386-396, 2006). Our current understanding of the physiological role of PDE10A and the therapeutic utility of PDE10A inhibitors derives in part from studies with papaverine, the only pharmacological tool for this target extensively profiled to date. However, this agent has significant limitations in this regard, namely, relatively poor potency and selectivity and a very short exposure half-life after systemic administration. In the present report, we describe the discovery of a new class of PDE10A inhibitors exemplified by TP-10 (2-{4-[-pyridin-4-yl-1-(2,2,2-trifluoro-ethyl)-1H-pyrazol-3-yl]-phenoxymethyl}-quinoline succinic acid), an agent with greatly improved potency, selectivity, and pharmaceutical properties. These new pharmacological tools enabled studies that provide further evidence that inhibition of PDE10A represents an important new target for the treatment of schizophrenia and related disorders of basal ganglia function.

Synthesis, SAR and pharmacology of CP-293,019: a potent, selective dopamine D4 receptor antagonist.

A series of novel, potent and selective pyrido[1,2-a]pyrazine dopamine D4 receptor antagonists are reported including CP-293,019 (D4 Ki = 3.4 nM, D2 Ki > 3,310 nM), which also inhibits apomorphine-induced hyperlocomotion in rats after oral dosing.

Effects of central nicotinic cholinergic receptor blockade produced by chlorisondamine on learning and memory performance in rats.

The effects of chronic nicotinic receptor blockade on the performance of learning and memory tasks were determined using chlorisondamine, a compound which produces central nicotinic cholinergic receptor blockade that lasts for several weeks after a single icv administration. Chlorisondamine treatment did not affect the acquisition of spatial information in the Morris water maze or in the radial arm maze, tasks in which performance is reportedly disrupted by acute administration of the nicotinic antagonist, mecamylamine. Chlorisondamine also did not affect performance in the inhibitory avoidance task and did not alter the memory enhancement found in this task after post-training administration of nicotine. Mecamylamine, however, completely blocked the memory-enhancing effects of nicotine. In contrast to the differential ability to chlorisondamine and mecamylamine to block nicotine's memory-enhancing effects, these antagonists produced comparable blockade of nicotine's effects on open field behavior. It is unlikely that the different effects of systemically administered mecamylamine and centrally administered chlorisondamine on nicotine-induced memory enhancement are due to mecamylamine's peripheral effects, since hexamethonium, a peripherally active nicotinic antagonist, did not block nicotine-induced memory enhancement. The different pattern of effects of mecamylamine and chlorisondamine may be related to compensatory mechanisms being selectively induced by chronic blockade produced by chlorisondamine and not by acute blockade produced by mecamylamine. Alternatively, different effects of these two nicotinic cholinergic antagonists on the performance of learning and memory tasks might be related to selective actions of these compounds at nicotinic receptor subtypes or at nonnicotinic receptors.

Effects of lobeline, a nicotinic receptor agonist, on learning and memory.

The effects of (-)-lobeline were assessed in two learning and memory tasks in which nicotine-induced enhancement of performance has previously been demonstrated. Lobeline (19 mumol/kg, IP) administered immediately after inhibitory (passive) avoidance training improved retention performance assessed 24 h later, as rats that received this dose of lobeline took significantly longer to enter the shock compartment on the test day than rats that had been treated with vehicle. Pretraining lobeline treatment (1.9 mumol/kg, IP) significantly improved performance of rats with septal lesions in a spatial discrimination water maze, a finding confirmed when rats were retrained using new spatial locations and vehicle and lobeline treatments were reversed in a crossover design. The effective dose of lobeline in the inhibitory avoidance task was about 10-fold higher than that generally reported for nicotine, and direct comparison of the suppression of locomotor activity shortly after administration of nicotine or lobeline also revealed a 10-fold greater potency for nicotine. In contrast, no difference was found between the effective dose of lobeline in the current study and that we previously found with nicotine in the water maze. These findings suggest that lobeline's effects on the performance of learning and memory tasks may be similar to those of nicotine. Coupled with previous reports that lobeline does not produce the nicotine cue in drug discrimination experiments, this study also suggests that nicotinic receptors involved in the modulation of memory processes may be distinct from those involved in producing the nicotine cue.

Effects of nicotine on spatial memory deficits in rats with septal lesions.

Impaired septohippocampal function has been implicated in the memory deficits associated with Alzheimer's disease (AD), and septal lesions have been used to model the cognitive deficits associated with AD. In this study, we assessed the effects of systemic administration of nicotine on lesion-induced deficits in the acquisition of a spatial discrimination version of the Morris water maze. Rats with radiofrequency lesions of the medial septum were required to learn which of two visible platforms in a pool of water provided a means of escape. On each of the first 4 days of training, the rats received an injection of (-)nicotine (0, 0.1 or 0.3 mg/kg, i.p.) before training. Nicotine markedly improved the performance of septal rats. This enhanced performance was maintained in rats subsequently tested 1 and 15 days later without additional drug treatment. Septal rats initially trained under nicotine were impaired, however, when the platform locations were reversed and training was conducted under saline. Our findings suggest that nicotinic receptor stimulation might be useful in the treatment of cognitive deficits.

Chronic treatment with ethanol alters the physiological action of nicotine.

1. Ethanol decreases the release of acetylcholine through effects on presynaptic neurons at both muscarinic and nicotinic junctions. 2. Blockade of the release of acetylcholine should produce denervation supersensitivity at both muscarinic and nicotinic cholinergic junctions. 3. Chronic but not acute treatment with ethanol produces supersensitivity to the hypothermic effects of a muscarinic agonist in the rat. 4. The authors now report that chronic treatment with orally administered ethanol blunts (rather than enhances) the hypothermic response to nicotine in the rat. 5. This could have major public health implications. 6. Smoking and the use of ethanol containing beverages positively covary. 7. Ethanol induced reduction in sensitivity to nicotine suggests that the heavy consumption of ethanol may necessitate that one drink more than otherwise in order to obtain the desired effects of nicotine.

Effects of systemic and intracerebroventricular administration of mecamylamine, a nicotinic cholinergic antagonist, on spatial memory in rats.

The effects of both systemic and intracerebroventricular administration of mecamylamine, a nicotinic antagonist, were tested on the Morris water maze performance of rats. In experiment 1, mecamylamine (0, 3, and 10 mg/kg, IP) was administered before daily training sessions on the Morris water maze, a task in which rats use environmental cues to learn the location of an invisible escape platform in a large pool of water. The escape latencies of rats given the higher dose of mecamylamine were significantly longer than the latencies of rats given either saline or the peripherally-acting nicotinic antagonist hexamethonium (10 mg/kg). Analysis of search patterns during a free swim trial conducted in the absence of an escape platform confirmed the disruptive effects of the higher dose of mecamylamine. Similar drug effects were not observed when these rats were trained to a visible platform, and mecamylamine did not affect the retrieval of spatial information in well-trained rats. In experiment 2, similar effects were observed with ICV administration of mecamylamine (0, 10, 30, and 100 micrograms). The two higher doses increased escape latencies during the last day of place training and all three doses significantly impaired performance on a free swim. No significant effects were noted on subsequent training to a visible platform, and only the highest dose marginally impaired the retrieval of spatial information in well-trained animals. Thus, mecamylamine appears to impair the acquisition of spatial information in the Morris water maze but does not affect retrieval of previously acquired spatial information at comparable doses.

A nicotinic receptor antagonist enhances the hypothermic response to a muscarinic agonist.

1. Chronic treatment with amitriptyline produces supersensitivity to the hypothermic effects of the muscarinic agonist oxotremorine. 2. Chronic treatment with amitriptyline also produces supersensitivity to the hypothermic effects of nicotine. 3. Oxotremorine and other naturally occurring muscarinic agonists are also nicotinic agonists. 4. Chronic treatment with amitriptyline produces time-dependent and reversible supersensitivity to the hypothermic effects of nicotine. 5. The authors assessed the possibility that the development of supersensitivity to oxotremorine following chronic treatment with amitriptyline is related to an effect of this antidepressant on a nicotinic mechanism. 6. A nicotinic receptor antagonist would blunt (though not necessarily eliminate) enhanced sensitivity to the thermic effects of oxotremorine if the nicotinic effects of the latter are significant. 7. The simultaneous administration of mecamylamine (a peripherally and centrally active nicotinic receptor antagonist) greatly augments (rather than blunts) the hypothermic response to oxotremorine. 8. The data suggest that the oxotremorine may activate a nicotinic mechanism counterbalancing its effect on a muscarinic mechanism. 9. This is consistent with previously published reports that the activation of nicotinic and muscarinic mechanisms can exert opposite effects.

The effects of morphine on diet selection are dependent upon baseline diet preferences.

It has been reported that morphine causes a selective increase in the intake of dietary fat. Because we have noted considerable variability among rats in their preferences for carbohydrate and fat, we reasoned that the effect of morphine on diet selection may differ in fat-preferring vs. carbohydrate-preferring rats. Male Sprague-Dawley rats were given ad lib access to separate sources of carbohydrate, fat and protein (Experiment 1), or to a carbohydrate/protein and a fat/protein diet (Experiment 2). After daily baseline intakes of the diets were determined, all rats were tested for feeding responses to subcutaneous injections of morphine (0, 2 and 10 mg/kg). Significant positive correlations were found between baseline daily intake of a given diet and the effect of morphine on the intake of that diet. Generally, morphine increased carbohydrate intake in carbohydrate-preferring rats, and increased fat intake in fat-preferring rats. These results suggest that the effect of morphine is to increase intake of a preferred diet rather than to increase intake of a specific macronutrient.

The anorectic effects of CRH and restraint stress decrease with repeated exposures.

Intracerebroventricular (icv) administration of corticotropin-releasing hormone (CRH) or exposure to a restraint stressor causes acute anorexia in rats. However, the effects on food intake of repeated injections of CRH or repeated exposures to restraint stress have not been previously reported. As the effects of these more chronic CRH and stress treatments may be of greater relevance to emerging hypotheses of the pathogenesis of human eating and affective disorders, we measured the changes in food intake and body weight of rats after repeated central injections of CRH. In two experiments using two different daily dosages of CRH and two different schedules of administration, we found that the anorectic effect of CRH decreased over repeated injections. Weight gain was slowed significantly only in the high-dose experiment. Rats may become tolerant to the anorectic effects of CRH delivered by repeated icv injections. These findings have important implications for hypothesized mechanisms of anorexia nervosa and/or depression.

Effects of preferential delta and kappa opioid receptor agonists on the intake of hypotonic saline.

A previous study has implicated central mu opioid receptors in the preference for salt solutions. Because mu, kappa and delta receptors are all thought to play a role in food intake and/or the mediation of palatability, we performed a series of experiments to determine whether preferential agonists at kappa and delta receptors might also stimulate the intake of salt solutions. When injected centrally into nondeprived rats, two selective agonists at delta receptors caused increases in the intake of 0.6% saline; the intake of concurrently available water was either unchanged or slightly increased. The selective kappa agonist U-50,488H had no effect on water or saline intake, whereas the preferential kappa agonist DAFPHEDYN caused a delayed increase in saline intake. These results indicate a role for central delta receptors in the preference for salt solutions, and are consistent with the suggestion that opioids play a role in the mediation of palatability.

Telemetric measurement of core temperature in pharmacological research: validity and reliability.

1. The authors present data establishing the reliability and validity of a method for telemetrically measuring core temperature. 2. The method is designed to be of particular utility to psychobiological researchers.

Chronic treatment with ethanol produces supersensitivity to oxotremorine.

1. The effect of chronic treatment with ethanol (14% v:v in drinking water) on the physiological endpoint core temperature which is partially regulated by a muscarinic mechanism was measured in adult male rats (n = 8). 2. One and two weeks of treatment were associated with enhancement of the hypothermic response to oxotremorine, 0.25 mg/kg ip (p = 0.0005 and p = 0.0001, respectively). 3. The sample remained supersensitive to this muscarinic agonist 48 and 96 hours after the discontinuation of treatment (p = 0.0014 and p = 0.013 respectively). 4. Repeated injections of oxotremorine, 0.25 mg/kg ip, every other day for 10 days did not produce carry-over effects in a control experiment. 5. The results suggest that ethanol renders muscarinic mechanisms supersensitive during chronic treatment and that supersensitivity remains up to 96 hours following withdrawal.

Effects of placebo (saline) injections on core temperature in the rat.

1. Core temperature was telemetrically measured in 15 rats before (i.e., at baseline) and at 10-min intervals for 120 min following the injection of normal saline (1 ml/kg ip) or "no injection." 2. The sample exhibited a mean temperature increase of 0.60 +/- 0.10 degree C (mean +/- SEM) following injection. 3. This differed significantly from the mean increase of 0.13 +/- 0.03 degree C following "no injection" (p less than 0.001). 4. The injection of saline (1 ml/kg) affected a mean rise in core temperature of 0.55 +/- 0.07 degree C (p greater than 0.000001) in 46 animals in a second experiment. 5. These data indicate that routine handling and a simple injection comprise significant and measurable stress which must be controlled in neuropharmacological studies employing a thermoregulation paradigm.

Relationship between nicotine tolerance questionnaire scores and plasma cotinine.

The Fagerström Tolerance Questionnaire (TQ) is often used in both research and treatment contexts to evaluate nicotine tolerance and physiological dependence in cigarette smokers. Recently, however, questions about its validity and its usefulness in comparison to other easily collected measures have been raised. In the present study, 100 male subjects reporting for experimental sessions (Sample I) and 50 male and female subjects entering a smoking cessation clinic program (Sample II) were administered the TQ and determinations of plasma cotinine during ad libitum smoking were made. TQ scores were found to be correlated with cotinine levels in both samples, and several of the individual items proved to have statistically significant discriminatory value. Other schemes for determining degree of dependence were considered and found not to be superior to the TQ. Suggestions for further refining the TQ are reviewed.

Effects of a selective mu opioid receptor agonist and naloxone on the intake of sodium chloride solutions.

Endogenous opioid peptides are thought to play a role in mediating the palatability or rewarding aspects of sweet tastes. There is also evidence, however, which suggests that opioids may influence the preference for the taste of salt as well. In the present studies, we measured the effects of central administration of naloxone and the mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on the ingestion of salt solutions. In non-deprived rats given a choice of water and 0.6% saline, ICV injections of DAGO (1 and 3 nmol) significantly increased the intake of 0.6% saline; baseline water intake was minimal and was unaffected by DAGO. When rats were given a choice between water and 1.7% saline, DAGO stimulated both water and saline intake. Because 1.7% saline is a hypertonic solution, the increase in water intake may have been secondary to saline intake. In rats on a deprivation schedule in which water and 0.6% saline were available for only 2-3 h/day, there was a tendency for DAGO to increase 0.6% saline intake and decrease water intake, though these effects were not significant. In rats given water and 1.7% saline, DAGO increased saline intake and had no effect on water intake. Naloxone was also tested in water-deprived rats. Naloxone (20 and 50 micrograms) significantly decreased 0.6% saline intake; baseline water intake was low (3-5 ml) and was unaffected by naloxone. When rats were given a choice between water and 1.7% saline, naloxone (50 micrograms) significantly reduced water intake, while intake of 1.7% saline was slightly increased. These results suggest a role for central mu opioid receptors in mediating the preference for salt solutions.

Centrally administered opioid peptides stimulate saccharin intake in nondeprived rats.

Endogenous opioid peptides are thought to play a role in mediating the pleasurable or rewarding aspects of the ingestion of certain foods and liquids. We therefore measured the effects of central administration of selective opioid agonists and naloxone on the intake of two concentrations of saccharin solution. All tests were performed on nondeprived rats, such that the taste of the solutions provided the primary incentive to consume. Intracerebroventricular (ICV) administration of the selective mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and the selective delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (3 nmol) increased intake of a 0.15% saccharin solution by approximately 10 ml over 3 hr. Water was available simultaneously, but intake was minimal. The selective kappa agonist U-50,488H did not increase intake of the saccharin solution. Naloxone (30 and 100 micrograms, ICV) caused a 44% reduction in saccharin solution intake in the first hour; two- and three-hour cumulative intakes were not different from control. DAGO and DTLET were also tested when rats were given a weaker saccharin solution (0.006%) along with water. Both agonists caused small increases in saccharin and water intake, but the increases above baseline were much smaller than those observed with the more palatable 0.15% saccharin solution. These results are consistent with reports by others which suggest that endogenous opioids influence taste preferences or palatability. Further, they indicate a role for central mu and delta opioid receptors in the mediation of this influence.

Paced puffing as a method for administering fixed doses of nicotine.

Smokers' ability to regulate nicotine intake by varying topographical parameters such as depth of inhalation and number of puffs makes it difficult to administer standardized doses of nicotine as delivered from smoking. A number of studies have claimed to control these parameters without confirming the effectiveness of such procedures by measures of plasma nicotine. The purpose of the present study was to determine whether specifying onset and duration of each puff would result in accurate dosing. Plasma nicotine boosts for five "paced puffers" were compared across two sessions and with similar data for five "free smokers." Neither between-subject consistency nor within-subject reproducibility was improved by this paced puffing procedure, despite apparent topographical control.

Nicotine dependence and the Fagerström Tolerance Questionnaire: a brief review.

The Fagerström Tolerance Questionnaire (TQ) is widely used in both clinical and research settings as a measure of physiological dependence on nicotine. In the light of claims that the efficacy of nicotine gum is related to degree of dependence and that outcomes can be improved by tailoring gum strength to degree of dependence, the availability of a simple, noninvasive test of dependence assumes additional importance. Recently, a number of articles have expressed reservations about various aspects of the TQ: 1) Several TQ items suffer from flaws or ambiguities that undermine their usefulness, and 2) the TQ may not measure what it purports to measure. The present paper reviews these issues, assembles data and observations that bear on the problems, and makes suggestions that may help in refining the TQ or producing a better instrument.