Impact of acute nicotine exposure on monoaminergic systems in adolescent and adult male and female rats.

Adolescence is a period of risk for beginning tobacco addiction. Differential neural response to nicotine in adolescents vs. adults may help explain the increased vulnerability to nicotine self-administration seen with adolescent onset. We indexed the effects of acute nicotine ditartrate (0.4 mg/kg, salt weight) administration on dopamine (DA) and serotonin (5HT) as well as the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in several brain regions (nucleus accumbens, striatum and frontal cortex) of 6-week old (adolescent) and 10-week old (young adult) Sprague-Dawley rats. When nicotine was administered DA concentrations in the accumbens were significantly higher in adults than in adolescents, whereas there was no age-related difference without nicotine. However neither age group showed a significant effect of nicotine vs. age-matched controls. DA turnover in the accumbens was significantly greater in adolescent females in response to nicotine, but adult females did not show this effect and neither did males of either age group. DA turnover in the striatum was significantly higher in adolescents than adults regardless of nicotine administration. In the frontal cortex, there was a more complex effect. Without nicotine, adult male rats had higher DA concentrations than adolescent males, whereas female rats did not differ from adolescent to adult ages. When given nicotine, the age effect was no longer seen in males. However, there was not a significant effect of nicotine vs. age-matched controls in either age group. No age or nicotine effects were seen in females. 5HT in the accumbens was significantly increased by nicotine administration in adults but not in adolescents. Altered neural responsivity of adolescents to nicotine-induced neural effects particularly in accumbens DA and 5HT may be related to the increased nicotine dose concentrations they self-administer.

Chronic infusions of mecamylamine into the medial habenula: Effects on nicotine self-administration in rats.

The habenula is an epithalamic structure through which descending connections go from the telencephalon to the brainstem, putting it in a key location to provide feedback control over the ascending projections from the brainstem to the telencephalon. The medial habenula has a high concentration of nicotinic receptors. We assessed the role of medial habenular nicotinic receptors for nicotine self-administration (SA) in female young adult Sprague-Dawley rats. The rats had bilateral chronic infusion cannulae placed into the medial habenula nucleus. Each cannula was connected to a slow delivery osmotic minipump to chronically infuse mecamylamine (100 µg/side/day) or vehicle for four consecutive weeks. The rats were tested for nicotine SA for the first two weeks of mecamylamine infusion. Then, they had one week of enforced abstinence, during which they had no access to the nicotine SA. Finally, they had one week of resumed nicotine SA access. There was a significantly differential mecamylamine effects in animals with lower and higher pretreatment baseline nicotine SA. Rats with lower baseline nicotine SA levels showed a nearly significant mecamylamine-induced reduction in SA while those with higher baseline levels of SA showed a significant mecamylamine-induced increase in nicotine SA. This study determined that medial habenular nicotinic receptors are important for nicotine reinforcement. Baseline level of performance makes a crucial difference for the involvement of habenular mechanisms in nicotine reinforcement with nicotinic activation being important for maintaining nicotine self-administration for those with lower levels of baseline self-administration and the opposite effect with subjects with higher levels of baseline self-administration.

Persisting neurobehavioral effects of developmental copper exposure in wildtype and metallothionein 1 and 2 knockout mice.

BACKGROUND: Metallothioneins (MT) are small proteins, which are crucial for the distribution of heavy and transition metals. Previously, we found in mice that knockout of MT 1 and 2 genes (MTKO) impaired spatial learning and potentiated the learning impairment caused by developmental mercury exposure. The current study examined the neurocognitive and neurochemical effects of MTKO with the developmental copper (Cu) supplementation. METHODS: Wildtype (WT) and MTKO mice were given supplemental Cu (0, 10 or 50 mg/l) in their drinking water during gestation and until weaning. When the mice were young adults they were trained on the win-shift 8-arm radial maze test of spatial learning and memory. After cognitive testing, their brains were analyzed for norepinepherine, dopamine and serotonin levels. RESULTS: In the spatial learning test, wildtype mice showed the normal sex difference with males performing more accurately than the females. This effect was eliminated by MTKO and restored by moderate Cu supplementation during development. In neurochemical studies, MTKO caused a significant overall increase in serotonin in all of the regions studied: the frontal cortex, posterior cortex, hippocampus, striatum, midbrain, and brainstem. MTKO also caused a significant increase in norepinepherine in the brainstem and hippocampus. In wildtype mice, Cu supplementation during development caused a significant decline in dopamine and norepinepherine in the midbrain and dopamine in the frontal cortex. These effects were blocked by MTKO. CONCLUSIONS: The normal sex difference in spatial working memory accuracy, which was eliminated by MTKO, was restored by moderate copper supplementation. MTKO increased serotonin across all brain areas studied and increased norepinepherine only in the hippocampus and brainstem. MTKO blocked copper-induced decreases in dopamine and norepinepherine in the midbrain and dopamine in the frontal cortex.

Assessing the effects of chronic sazetidine-A delivery on nicotine self-administration in both male and female rats.

RATIONALE: Sazetidine-A is a selective α4ß2 nicotinic receptor desensitizing agent and partial agonist. It has been shown in previous studies to significantly reduce nicotine self-administration in rats after acute or repeated injections. However, the effects of continuous chronic infusions of sazetidine-A on maintenance of nicotine self-administration and relapse after abstinence have yet to be examined. OBJECTIVES: This study evaluated the efficacy of continuous sazetidine-A infusions (sc) over a period of 4 weeks to reduce nicotine self-administration in male and female Sprague-Dawley rats. METHODS: Sazetidine-A was administered via Alzet osmotic minipumps to young adult female and male rats at doses of 0, 2 or 6 mg/kg/day for 4 weeks. The effects of sazetidine-A on IV nicotine self-administration were examined in repeated 3-h sessions over the first 2 weeks of infusion followed by 1 week of forced abstinence from nicotine and 1 week of resumed nicotine access. RESULTS: The 6 mg/kg/day sazetidine-A dose significantly reduced overall nicotine self-administration compared with vehicle control across the sessions for both male (p < 0.001) and female (p < 0.05) rats. The lower 2 mg/kg/day sazetidine-A infusion dose was effective in reducing nicotine self-administration for male (p < 0.001), but not female rats. No attenuation in sazetidine-A effectiveness was seen over the course of the 4-week treatment. In the vehicle control group, male rats self-administered significantly (p < 0.001) more nicotine than females. CONCLUSIONS: The continuing effectiveness of sazetidine-A in reducing nicotine self-administration in both male and female rats supports its promise as a new treatment to help people successfully quit smoking.

Threshold of adulthood for the onset of nicotine self-administration in male and female rats.

The great majority of tobacco addiction begins during adolescence. More heavily addicted smokers begin smoking earlier, but differentiating the neurobehavioral impact of nicotine self-administration during adolescence from self-selection bias (whereby people more prone to heavy addiction also begin earlier) cannot be ethically unconfounded in humans. The goals of this research were to determine the age threshold for the adult-like nicotine self-administration and determine sex differences. Male and female Sprague-Dawley rats were tested for nicotine self-administration starting at 4, 5, 6, 7, and 8 weeks of age in an operant FR1 schedule for IV nicotine (0.03 mg/kg/infusion) in 45-min sessions for 2 weeks, with 1 week of enforced abstinence and 1 week of resumed access. This study replicated our earlier work that nicotine self-administration was increased in adolescent vs. adult rats and that the effect was more pronounced in adolescent males, but the increased nicotine self-administration was more persistent in adolescent-onset females. The age threshold for adult-like behavior was 6-7 weeks of age. Adolescent-onset nicotine self-administration had persisting effects of eggaurated increases of nicotine self-administration when fixed-ratio requirements for self-administration were lowered. Female rats that had begun nicotine self-administration during adolescence showed exaggerated increases in nicotine self-administration after a switch back to FR1 from FR8, indicating a lessened control over their self-administration. Adolescent-onset nicotine self-administration was not found to potentiate cocaine self-administration. Adolescent-onset nicotine self-administration causes persistent increases in nicotine self-administration in female rats even after they reach adulthood and disrupts control over self-administration behavior.

Critical duration of exposure for developmental chlorpyrifos-induced neurobehavioral toxicity.

Developmental exposure of rats to the pesticide chlorpyrifos (CPF) causes persistent neurobehavioral impairment. In a parallel series of studies with zebrafish, we have also found persisting behavioral dysfunction after developmental CPF exposure. We have developed a battery of measures of zebrafish behavior, which are reliable and sensitive to toxicant-induced damage. This study determined the critical duration of developmental CPF exposure for causing persisting neurobehavioral effects. Tests of sensorimotor response (tap startle response and habituation), stress response (novel tank diving test) and learning (3-chamber tank spatial discrimination) were conducted with adult zebrafish after early developmental CPF exposure. The CPF exposure level was 100 ng/ml with durations of 0-1, 0-2, 0-3, 0-4 and 0-5 days after fertilization. Developmental CPF exposure had persisting behavioral effects in zebrafish tested as adults. In the tactile startle test, CPF exposed fish showed decreased habituation to startle and a trend toward increased overall startle response. In the novel tank exploration test, exposed fish showed decreased escape diving response and increased swimming activity. In the 3-chamber learning test, the 0-5 day CPF exposure group had a significantly lower learning rate. There was evidence for persisting declines in brain dopamine and norepinepherine levels after developmental CPF exposure. In all of the measures the clearest persistent effects were seen in fish exposed for the full duration of five days after fertilization. In a follow-up experiment there were some indications for persisting behavioral effects after exposure during only the later phase of this developmental window. This study demonstrated the selective long-term neurobehavioral alterations caused by exposure to CPF in zebrafish. The zebrafish model can facilitate the determination of the molecular mechanisms underlying long-term neurobehavioral impairment after developmental toxicant exposure.

Persistent behavioral impairment caused by embryonic methylphenidate exposure in zebrafish.

As more adults take the stimulant medication methylphenidate to treat attention deficit hyperactivity disorder (ADHD) residual type, the risk arises with regard to exposure during early development if people taking the medication become pregnant. We studied the neurobehavioral effects of methylphenidate in zebrafish. Zebrafish offer cellular reporter systems, continuous visual access and molecular interventions such as morpholinos to help determine critical mechanisms underlying neurobehavioral teratogenicity. Previously, we had seen that persisting neurobehavioral impairment in zebrafish with developmental chlorpyrifos exposure was associated with disturbed dopamine systems. Because methylphenidate is an indirect dopamine agonist, it was thought that it might also cause persistent behavioral impairment after developmental exposure. Zebrafish embryos were exposed to the ADHD stimulant medication methylphenidate 0-5 days post fertilization (12.5-50mg/l). They were tested for long-term behavioral effects as adults. Methylphenidate exposure (50mg/l) caused significant increases in dopamine, norepinepherine and serotonin on day 6 but not day 30 after fertilization. In the novel tank diving test of predatory avoidance developmental methylphenidate (50mg/l) caused a significant reduction in the normal diving response. In the three-chamber spatial learning task early developmental methylphenidate (50mg/l) caused a significant impairment in choice accuracy. These data show that early developmental exposure of zebrafish to methylphenidate causes a long-term impairment in neurobehavioral plasticity. The identification of these functional deficits in zebrafish enables further studies with this model to determine how molecular and cellular mechanisms are disturbed to arrive at this compromised state.

Lorcaserin, a 5-HT2C agonist, decreases nicotine self-administration in female rats.

Lorcaserin, a selective 5-hydroxytryptamine(2C) (5-HT(2C)) agonist, has been shown to facilitate weight loss in obese populations. It was assessed for its efficacy in reducing nicotine self-administration in young adult female Sprague-Dawley rats. The effect of short-term doses (subcutaneous) on nicotine self-administration (0.03 mg/kg per infusion) with a fixed ratio 1 schedule was assessed in 3-h sessions. Short-term lorcaserin doses (0.3125-20 mg/kg) were administered in a counterbalanced order. Significant reduction of nicotine self-administration was achieved with all of the short-term doses in this range. Tests of lorcaserin on locomotor activity detected prominent sedative effects at doses greater than 1.25 mg/kg with more modest transient effects seen at 0.625 to 1.25 mg/kg. Long-term effects of lorcaserin on locomotor activity were tested with repeated injections with 0.625 mg/kg lorcaserin 10 times over 2 weeks. This low lorcaserin dose did not cause an overall change in locomotor activity relative to that of saline-injected controls. Long-term lorcaserin (0.625 mg/kg) significantly reduced nicotine self-administration over a 2-week period of repeated injections. Long-term lorcaserin at this same dose had no significant effects on food self-administration over the same 2-week period of repeated injections. These studies support development of the 5-HT(2C) agonist lorcaserin to aid tobacco smoking cessation.

D-cycloserine selectively decreases nicotine self-administration in rats with low baseline levels of response.

Expanding the variety of treatments available to aid smoking cessation will allow the treatments to be customized to particular types of smokers. The key is to understand which subpopulations of smokers respond best to which treatment. This study used adult female Sprague-Dawley rats to evaluate the efficacy of D-cycloserine, a partial NMDA glutamate receptor agonist, in reducing nicotine self-administration. Rats were trained to self-administer nicotine (0.03 mg/kg/infusion, i.v.) via operant lever response (FR1) with a secondary visual reinforcer. Two studies of D-cycloserine effects on nicotine self-administration were conducted: an acute dose-effect study (0, 10, 20 and 40 mg/kg, s.c.) and a chronic study with 40 mg/kg given before each test session for two weeks. Effects on rats with low or high pretreatment baseline levels of nicotine self-administration were assessed. In the acute study there was a significant interaction of D-cycloserine×baseline level of nicotine self-administration. In the low baseline group, 10 mg/kg D-cycloserine significantly decreased nicotine self-administration. In the high baseline group, 40 mg/kg significantly increased nicotine self-administration. In the repeated injection study, there was also a significant interaction of d-cycloserine×baseline level of nicotine self-administration. Chronic D-cycloserine significantly reduced nicotine self-administration selectively in rats with low baseline nicotine use, but was ineffective with the rats with higher levels of baseline nicotine self-administration. NMDA glutamate treatments may be particularly useful in helping lighter smokers successfully quit smoking, highlighting the need for diverse treatments for different types of smokers.

Histamine H(1) antagonist treatment with pyrilamine reduces nicotine self-administration in rats.

Nicotine has been definitively shown to be critically involved in the neural bases of tobacco addiction. However, nicotine releases a wide variety of neurotransmitters. Nicotine-induced dopamine release has been shown to play a key role in facilitating nicotine self-administration. Other transmitter systems may also play important roles in the pharmacological effects of nicotine and may provide important leads for combating nicotine self-administration. Clozapine, an antipsychotic drug, which blocks a variety of different transmitter receptors including serotonin 5HT(2) and histamine H(1) receptors, has been found to decrease smoking. Previously we found that the serotonin 5HT(2) antagonist, ketanserin, significantly reduced nicotine self-administration. In the current study, we assessed histamine H(1) receptor interaction with nicotine self-administration. Young adult female Sprague-Dawley rats were fitted with IV catheters and trained to self-administer nicotine (0.03mg/kg/infusion). Acute doses of 40mg/kg of pyrilamine, a histamine H(1) antagonist, significantly reduced nicotine self-administration. We also found that repeated injections (20mg/kg) or chronic infusion via osmotic minipumps (50mg/kg/day) of pyrilamine also significantly decreased nicotine self-administration. The peripherally restricted H(1) antagonist ebastine was ineffective in reducing nicotine self-administration, pointing to central H(1) receptor blockade as key for the effectiveness of pyrilamine. H(1) antagonists may be a promising avenue to explore for new treatments to aid smoking cessation.

Effects of sazetidine-A, a selective alpha4beta2 nicotinic acetylcholine receptor desensitizing agent on alcohol and nicotine self-administration in selectively bred alcohol-preferring (P) rats.

RATIONALE: Manipulations of nicotinic cholinergic receptors have been shown to influence both alcohol and nicotine intake. Sazetidine-A [6-(5(((S)-azetidine-2-yl)methoxy)pyridine-3-yl)hex-5-yn-1-ol] is a novel compound that potently and selectively desensitizes alpha4beta2 nicotinic receptors with only modest receptor activation. OBJECTIVES: The goal of the present study was to examine the effects of sazetidine-A on alcohol and nicotine self-administration in alcohol-preferring (P) rats. METHODS: P rats were given the choice of water or alcohol. Once stable baselines were established, the acute (0, 0.1, 0.3, 1, and 3 mg/kg, s.c.) and chronic (3 mg/kg for 10 days) effects of sazetidine-A on alcohol intake were assessed. Naltrexone (2.5 mg/kg) served as a positive control. The effect of sazetidine-A (3 mg/kg) and naltrexone (4 mg/kg) on saccharin (0.2%) preference was also assessed. In addition, the acute effects of sazetidine-A (3 mg/kg) and naltrexone (4 mg/kg) on alcohol intake after alcohol deprivation were evaluated. In another experiment, the effects of sazetidine-A (0, 1, or 3 mg/kg) on i.v. nicotine self-administration in P and NP rats were assessed. RESULTS: Sazetidine-A caused a dose-dependent reduction in alcohol intake. Chronic sazetidine-A also effectively reduced alcohol intake until the seventh day of treatment, when partial tolerance appeared to develop. In the post-deprivation study, sazetidine-A significantly reduced alcohol intake and preference. Sazetidine-A at 3 mg/kg significantly reduced nicotine self-administration in both lines. CONCLUSIONS: Sazetidine-A significantly reduced alcohol and nicotine intake in P rats that self-administer higher levels of both drugs. Sazetidine-A may hold promise for the treatment of alcohol and nicotine addiction.

Early postnatal parathion exposure in rats causes sex-selective cognitive impairment and neurotransmitter defects which emerge in aging.

Developmental exposure of rats to the organophosphate (OP) pesticides leads to altered neurobehavioral function in juvenile and young adult stages. The current study was conducted to determine whether effects of neonatal parathion exposure on cognitive performance persist in older adult and aged rats, and the relationship of behavioral changes to underlying cholinergic and serotonergic mechanisms. We administered parathion to rat pups on postnatal days 1-4, at doses spanning the threshold for the initial signs of systemic toxicity and for barely detectable cholinesterase inhibition (0.1 or 0.2 mg/kg/day). Beginning at 14 months of age and continuing until 19 months, the rats were trained in the 16-arm radial maze. Controls showed the normal sex difference in this spatial learning and memory task, with the males committing significantly fewer working memory errors than females. Neonatal parathion exposure eliminated the sex difference primarily by causing impairment in males. In association with the effects on cognitive performance, neonatal parathion exposure elicited widespread abnormalities in indices of serotonergic (5HT) and cholinergic synaptic function, characterized by upregulation of 5HT(2) receptors and the 5HT transporter, deficits in choline acetyltransferase activity and nicotinic cholinergic receptors, and increases in hemicholinium-3 binding to the presynaptic choline transporter. Within-animal correlations between behavior and neurochemistry indicated a specific correlation between working memory performance and hippocampal hemicholinium-3 binding; parathion exposure eliminated this relationship. Like the behavioral effects, males showed greater effects of parathion on neurochemical parameters. This study demonstrates the sex-selective, long-term behavioral alterations caused by otherwise nontoxic neonatal exposure to parathion, with effects increasingly expressed with aging.

Sazetidine-A, a selective alpha4beta2 nicotinic receptor desensitizing agent and partial agonist, reduces nicotine self-administration in rats.

Adequate treatment of tobacco addiction remains problematic. Part of the problem with treatment is a poor understanding of the pharmacologic aspects of nicotine contributing to addiction. In addition to activating nicotinic acetylcholine receptors, nicotine also desensitizes them. It is currently not known how much of each of nicotine's actions contribute to its particular behavioral effects. Sazetidine-A (saz-A) is a novel nicotinic receptor-desensitizing agent and partial agonist with high selectivity for alpha4beta2 receptors. The current experiments were conducted to determine whether saz-A would reduce nicotine self-administration in rats and to characterize its ancillary effects. Adult male Sprague-Dawley rats were allowed to self-administer nicotine. After initial food pellet training followed by 10 sessions of nicotine self-administration training, the rats were administered saz-A (0.1-3 mg/kg s.c.) or the saline vehicle in a repeated-measures counterbalanced design. Saz-A at the 3 mg/kg dose significantly decreased nicotine self-administration relative to performance of the same rats after saline injections. In a second study, long-term administration of this dose of sazetidine-A over the course of 10 sessions significantly reduced nicotine self-administration with no apparent diminution of effect. Saz-A in this dose range had only modest effects on locomotor activity, without any overall decrease in activity over a 1-h-long session. Saz-A significantly reduced food self-administration, but this effect was smaller than its effect on nicotine self-administration. Saz-A, which is a selective alpha4beta2-desensitizing agent and partial agonist, effectively reduces nicotine self-administration. This type of treatment holds promise for a new therapy to aid smoking cessation.

Nicotine effects on learning in zebrafish: the role of dopaminergic systems.

RATIONALE: Nicotine improves cognitive function in a number of animal models including rats, mice, monkeys, and recently, zebrafish. The zebrafish model allows higher throughput and ease in discovering mechanisms of cognitive improvement. MATERIALS AND METHODS: To further characterize the neural bases of nicotine effects on learning in zebrafish, we determined changes in dopaminergic systems that accompany nicotine-enhanced learning. RESULTS: Nicotine improved learning and increased brain levels of dihydroxyphenylacetic acid (DOPAC), the primary dopamine metabolite. There was a significant correlation between choice accuracy and DOPAC levels. The nicotinic antagonist mecamylamine blocked the nicotine-induced increase in DOPAC concentrations, in line with our previous finding that mecamylamine reversed nicotine-induced learning improvement. CONCLUSIONS: Dopamine systems are related to learning in zebrafish; nicotine exposure increases both learning rates and DOPAC levels; and nicotinic antagonist administration blocks nicotine-induced rises in DOPAC concentrations. Rapid cognitive assessment of drugs with zebrafish could serve as a useful screening tool for the development of new therapeutics for cognitive dysfunction.

Nicotinic alpha7- or beta2-containing receptor knockout: effects on radial-arm maze learning and long-term nicotine consumption in mice.

Classically, it has been thought that high-affinity nicotinic receptors-containing beta2 subunits are the most important receptor subtypes for nicotinic involvement in cognitive function and nicotine self-administration, while low affinity alpha7-containing nicotinic receptors have not been thought to be important. In the current study, we found that knockout of either beta2 or alpha7 subunits caused significant deficits in spatial discrimination in mice. The character of the impairment in the two knockouts was different. The beta2 knockout preferentially impaired cognition in males while the alpha7 caused impairment regardless of sex. Both beta2- and alpha7-containing nicotinic receptors also are important for nicotine self-administration, also in different ways. Most animal model studies of nicotine self-administration are relatively short-term whereas the problem of tobacco addiction is considerably longer-term. To better model the impact of nicotinic receptor subtypes on nicotine self-administration over the long-term, we studied the impact of genetic knockout of low affinity alpha7 receptors vs. high-affinity beta2-containing nicotinic receptors. Mice with knockouts of either of these receptors and their wildtype counter parts were given free access to a choice of nicotine-containing and nicotine-free solution in their home cages on a continuous basis over a period of 5 months. During the first few weeks, the beta2-containing nicotinic receptor knockout mice showed a significant decrease in nicotine consumption relative to wildtype mice, whereas the alpha7 knockout mice did not significantly differ from wildtype controls at the beginning of their access to nicotine. Interestingly, in the longer-term after the first few weeks of nicotine access, the beta2 knockout mice returned to wildtype mouse levels of nicotine consumption, whereas the alpha7 knockout mice developed an emergent decrease in nicotine consumption. The alpha7 receptor knockout-induced decrease in nicotine consumption persisted for the 5-month period of the study. Both alpha7- and beta2-containing nicotinic receptors play critical roles in cognitive function and nicotine self-administration. Regarding cognitive function, beta2-containing receptors are important for maintaining normal sex differences in spatial learning and memory, whereas alpha7 receptors are important for cognitive function regardless of sex. Regarding nicotine self-administration high-affinity beta2-containing nicotinic receptors are important for consumption during the initial phase of nicotine access, but it is the alpha7 nicotinic receptors that are important for the longer-term regulation of nicotine consumption.

Ketanserin, a 5-HT2 receptor antagonist, decreases nicotine self-administration in rats.

Nicotine intake constitutes a principal mechanism for tobacco addiction. In addition to primary effects on nicotinic acetylcholine receptors, nicotine has cascading effects, which may also underlie its neurobehavioral actions. Nicotine induces serotonin (5-HT) release, which has not classically been thought to be involved in tobacco addiction as dopamine has. However, addiction can be characterized more as a disorder of compulsion than a disorder of enjoyment. 5-HT mechanisms play key roles in compulsive disorders. Nicotine-induced 5-HT release may be a key to tobacco addiction. Ketanserin, a 5-HT2a and 5-HT2c receptor antagonist, significantly attenuates nicotine effects on attention and memory. These studies were conducted to determine if ketanserin would reduce nicotine self-administration in rats. Male Sprague-Dawley rats (N=12) were given initial food pellet training and then 10 sessions of nicotine self-administration training (0.03 mg/kg/infusion, i.v.). Then the rats were administered ketanserin (1 or 2 mg/kg, s.c.) or the saline vehicle. Ketanserin (2 mg/kg) significantly decreased nicotine self-administration. This did not seem to be due to sedative or amnestic effects of ketanserin. In a second study, the effects of repeated administration of 2 mg/kg ketanserin (N=11) vs. saline injections (N=10) were examined. In the initial phase, the acute effectiveness of ketanserin in significantly reducing nicotine self-administration was replicated. The effect became attenuated during the following several sessions, but the significant effect became re-established during the final phases of this two-week study. 5-HT mechanisms play critical roles in the maintenance of nicotine self-administration. Better understanding of those roles may help lead to new 5-HT-based treatments for tobacco addiction.

Mercury-induced cognitive impairment in metallothionein-1/2 null mice.

Metallothioneins are central for the metabolism and detoxification of transition metals. Exposure to mercury during early neurodevelopment is associated with neurocognitive impairment. Given the importance of metallothioneins in mercury detoxification, metallothioneins may be a protective factor against mercury-induced neurocognitive impairment. Deletion of the murine metallothionein-1 and metallothionein-2 genes causes choice accuracy impairments in the 8-arm radial maze. We hypothesize that deletions of metallothioneins genes will make metallothionein-null mice more vulnerable to mercury-induced cognitive impairment. We tested this hypothesis by exposing MT1/MT2-null and wild-type mice to developmental mercury (HgCl(2)) and evaluated the resultant effects on cognitive performance on the 8-arm radial maze. During the early phase of learning metallothionein-null mice were more susceptible to mercury-induced impairment compared to wildtype mice. Neurochemical analysis of the frontal cortex revealed that serotonin levels were higher in metallothionein-null mice compared to wild-type mice. This effect was independent of mercury exposure. However, dopamine levels in mercury-exposed metallothionein-null mice were lower compared to mercury-exposed wild-type mice. This work shows that deleting metallothioneins increase the vulnerability to developmental mercury-induced neurocognitive impairment. Metallothionein effects on monoamine transmitters may be related to this cognitive effect.

Adolescent vs. adult-onset nicotine self-administration in male rats: duration of effect and differential nicotinic receptor correlates.

Adolescence is the life stage when tobacco addiction typically begins. Adolescent neurobehavioral development may be altered by nicotine self-administration in a way that persistently potentiates addiction. Previously, we showed that female adolescent rats self-administer more nicotine than do adults and that the increased nicotine intake then persists through the transition to adulthood [E.D. Levin, A. Rezvani, D. Montoya, J. Rose, H. Swartzwelder, Adolescent-onset nicotine self-administration modeled in female rats, Psychopharmacology 169 (2003) 141-149.]. In the current study, male Sprague-Dawley rats were given access to nicotine via the standard operant IV self-administration procedure (nicotine bitartrate dose of 0.03 mg/kg/infusion). One group of male rats started during adolescence the other group started in young adulthood. After the end of the four-week period of self-administration brain regions of the rats were assessed for alpha4beta2 nicotinic receptor binding. We found that male rats, like females, show higher nicotine self-administration when starting during adolescence as compared to starting in adulthood (p<0.001). Indeed, the effect in adolescent males was even greater than that in females, with more than triple the rate of nicotine self-administration vs. the adult-onset group during the first 2 weeks. The adolescent onset nicotine-self-administering rats also had significantly greater high affinity nicotinic receptor binding in the midbrain and the striatum, whereas hippocampal binding did not differ between the age groups. Striatal values significantly correlated with nicotine self-administration during the first 2 weeks in the adult-onset group but not the adolescent-onset rats, suggesting that the differences in self-administration may depend in part on underlying disparities in synaptic responses to nicotine. After the initial 2 weeks, nicotine self-administration in male rats declined toward adult-like levels, as the adolescent rats approached adulthood. This study showed that adolescent male rats self-administer significantly more nicotine than do male adult rats, but that adolescent-onset nicotine self-administration in male rats declines over weeks of continued use to approach adult-onset levels. In a previous study, we found that female rats also show greater nicotine self-administration with adolescent onset vs. adult onset, but that the females continued higher rates of self-administration into adulthood. Our results thus reinforce the concept that the adolescent brain is unusually receptive to the effects of nicotine in a manner that reinforces the potential for addiction.

Persistent neurobehavioral effects of early postnatal domoic acid exposure in rats.

Domoic acid (DA) is a marine biotoxin, produced by the diatom Pseudo-nitzchia spp., which has been shown to cause cognitive impairment in adults who are exposed via contaminated seafood. The neurobehavioral consequences of developmental exposure are much less well understood. In a previous study, we showed that a single prenatal exposure to DA in rats at mid-gestation caused neurobehavioral changes that persist into adulthood including increased susceptibility to the benchmark amnestic drug scopolamine. In the current study, we examined the lasting neurobehavioral consequences of DA exposure on the first day of postnatal life, a time in rats marking the completion of the major phase of neuroproliferation and corresponding to week 24 of human gestation. The effects of DA exposure at doses from 0.025-0.1 mg/kg (s.c.) twice per day on each of postnatal days 1 and 2 were compared with vehicle-treated controls and rats treated by the same protocol with 1 mg/kg of kainic acid. Following kainic acid exposure, a sex-selective effect was seen with females but not males showing a significant slowing of response latency in the radial-arm maze. The high DA dose of 0.1 mg/kg was quite toxic causing lethality in all of the offspring exposed and this group was excluded from further analysis. When the offspring in the 0.05 mg/kg DA dose group were tested, significant hypoactivity in the Figure-8 maze was observed during adolescence. No significant DA effects were seen in response latency or choice accuracy on the radial-arm maze during either acquisition or with challenge of the amnestic drug scopolamine. Early postnatal DA exposure in the rat can be lethal and sublethal exposure can cause neurobehavioral effects manifest in modest hypoactivity during the adolescent period. However, the sublethal persistent neurobehavioral toxicity appears to be less pervasive than reported effects following DA administered mid-gestation.

Increased nicotine self-administration following prenatal exposure in female rats.

There is a significant association between maternal cigarette smoking during pregnancy and greater subsequent risk of smoking in female offspring. In animal models, prenatal nicotine exposure causes persistent alterations in cholinergic and monoaminergic systems, both of which are important for nicotine actions underlying tobacco addiction. Accordingly, the current study was conducted to determine if there is a cause-and-effect relationship between prenatal nicotine exposure and nicotine self-administration starting in adolescence. Pregnant rats were administered nicotine (6 mg/kg/day) by osmotic minipump infusion throughout gestation and then, beginning in adolescence and continuing into adulthood, female offspring were given access to nicotine via a standard operant IV self-administration procedure (0.03 mg/kg/infusion). Gestational nicotine exposure did not alter the initial rate of nicotine self-administration. However, when animals underwent one week of forced abstinence and then had a second opportunity to self-administer nicotine, the prenatally-exposed animals showed a significantly greater rate of self-administration than did the controls. Prenatal nicotine exposure causes increased nicotine self-administration, which is revealed only when the animals are allowed to experience a period of nicotine abstinence. This supports a cause-and-effect relationship between the higher rates of smoking in the daughters of women who smoke cigarettes during pregnancy and implicates a role for nicotine in this effect. Our results further characterize the long-term liabilities of maternal smoking but also point to the potential liabilities of nicotine-based treatments for smoking cessation during pregnancy.