Responding maintained by primary reinforcing visual stimuli is increased by nicotine administration in rats

Nicotine has been shown to increase responding maintained by turning off a houselight. To examine whether this effect extends to other primary reinforcing visual stimuli, the present study assessed whether nicotine would increase responding maintained by the illumination, and not just the darkening, of a visual stimulus. One group of rats (n = 4) was initially trained to press two levers, using food as a consequence, while a separate group of rats (n = 4) was initially trained to press one lever. After training, all rats pressed an active lever to turn on or turn off a houselight for 10 s, while presses on an inactive lever had no programmed consequences. A third group of rats (n = 4) were never trained to press either of the two levers and did not experience any programmed consequences for pressing. Although nicotine slightly increased lever pressing on both levers in the third group, nicotine resulted in much greater increases in responding maintained by the visual stimuli in the first two groups. Nicotine selectively increased responding maintained by visual stimuli, regardless of which levers were originally trained and regardless of whether those stimuli consisted of turning on or turning off a houselight, suggesting that nicotine enhances the value of primary reinforcing visual stimuli.     

Effects of acute and chronic nicotine on elevated plus maze in mice: involvement of calcium channels

The current experiments examined the anxiety-related effects of acute and repeated nicotine administration using the elevated plus maze test in mice. Nicotine (0.1 mg/kg s.c., 5 and 30 min after injection; 0.5 mg/kg, s.c., 5 min after injection) had an anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open arm entries. Tolerance developed to this anxiogenic action after 6 days of daily nicotine administration (0.1 mg/kg, s.c.). Five minutes after the seventh injection, an anxiolytic effect was observed, i.e., specific increases in the percentage of time spent on the open arms and in the percentage of open arm entries. L-type voltage-dependent calcium channel antagonists nimodipine (5 and 10 mg/kg, i.p.), flunarizine (5 and 10 mg/kg, i.p.), verapamil (5, 10, 20 mg/kg) and diltiazem (5, 10, 20 mg/kg, i.p.) were also injected prior to an acute low dose of nicotine or to each injection of chronic nicotine. Our results revealed that calcium channel blockers dose-dependently attenuated both an anxiogenic effect of nicotine as well as the development of tolerance to this effect. Our results suggest that neural calcium-dependent mechanisms are involved in the anxiety-related responses to acute and chronic nicotine injection that may ultimately lead to addiction and smoking relapse in human smokers.     

Influence of nicotine and caffeine on rat embryonic development

The influence on embryonic development of nicotine and caffeine at dose levels approximating human consumption was investigated in Sprague-Dawley rats. One group of animals received nicotine administered subcutaneously by an Alzet mini-osmotic pump from gestational day 6 through 12 (25 mg over 7 days; rate 149 micrograms/hr). Control animals received physiological saline in a similar manner. A second group received a single intravenous injection of caffeine (25 mg/kg) on gestational day 6. Control animals were treated with physiological saline. A further group received both nicotine and caffeine on gestational day 6 as described for the two previous groups. There were no significant differences among any of the groups with respect to maternal weight gain, litter size, embryolethality, fetal weight, or crown-rump length. The offspring of nicotine treated animals showed a significantly higher incidence of hydrocephaly when compared to the controls, but in the combined treatment group no malformed fetuses were observed. Light microscopic examination of maternal liver, kidney and placentas revealed changes in the hepatic sinusoids, glomeruli and intervillous spaces after nicotine and combined treatment. In addition, the decidua basalis was poorly developed compared to the controls. Chorionic villi and fetal kidney appeared normal in all groups. A coteratogenic effect is not evident from these findings.     

Effect of repeated nicotine exposure on core temperature and motor activity in male and female rats

Comparatively little is known about the thermoregulatory effects of single and repeated administration of nicotine. Nicotine is a relatively fast acting drug that induces transient changes in core temperature (T_c) of rodents. The development of radio telemetry allows one to detect subtle and rapid changes in T_c that otherwise are difficult to measure with conventional colonic probe techniques. To this end, T_c and motor activity (MA) were monitored by radio telemetry in male and female Sprague-Dawley rats following five daily injections of saline or nicotine tartrate (0.5 mg/kg; sc). The first injection of saline led to a transient increase in T_c that was attributed to the handling and injection procedures. Rats dosed with nicotine for the first time were hypothermic for approximately 2 h after injection. The hypothermia was attributed to an impaired increase in T_c from handling and injection. A transient hyperthermic response began to develop with each successive injection of nicotine. After the fourth injection of nicotine, T_c of male rats increased by 0.5°C above controls; T_c of females increased by 0.25°C above controls after the fifth injection. MA also increased transiently with each injection of saline and nicotine. The MA response of females was significantly greater than the males for the second through fifth injections of nicotine. Overall, the thermoregulatory system develops sensitization with 4–5 repeated injections of nicotine. The mediation of a hyperthermic response to a systemically administered cholinergic agonist is a novel effect. It may aid in understanding the delayed hyperthermic response to organophosphate pesticides. The sensitization of the thermoregulatory system to nicotine may shed light on the neuropharmacological mechanisms of this drug.     

Ethanol withdrawal tremor potentiates the tremorogenic action of nicotine

The tremorogenic effect of nicotine was studied in control rats and in rats withdrawn for 16-48 h from six to nine days' ethanol administration. The frequency and the intensity of tremor were measured electronically. A single dose of nicotine 5 mg/kg caused shortlasting (2-3 min) convulsions within 1 min after injection in control rats. Tremor occurred first after five repeated injections of nicotine (5 mg/kg) at half-hour intervals. This tremor encompassed peak frequencies of 6, 10 and 15 Hz. Hexamethonium at a dose of 10 mg/kg did not inhibit the tremor but eliminated the highest peak frequency (15 Hz) and tended to increase the intensity. Propranolol 5 mg/kg completely abolished the nicotine-induced tremor in control rats. Rats withdrawn from repeated ethanol administration showed a marked hypersensitivity to the tremorogenic action of nicotine so that a single dose of 1 mg/kg of nicotine caused clear tremor. The frequency spectra of this tremor showed peak frequencies at 6, 10 and 12 Hz. Hexamethonium did not change these frequencies. Furthermore, it tended to increase the intensity of nicotine-induced tremor in ethanol-withdrawn rats. Propranolol did not inhibit the tremor although it eliminated the 12 Hz peak frequency. The results suggest that the hypersensitivity to the tremorogenic action of nicotine in ethanol-withdrawn rats is not mediated by a sympathetic beta-adrenergic mechanism.     

Effect of nicotine on the level of extracellular amino acids in the hippocampus of rat

Using microdialysis, we compared intracerebral and subcutaneous administration of nicotine for the effect on the levels of extracellular amino acids in the hippocampus of anesthetized rats. Administration by microdialysis of 10 mM nicotine, resulting in a nicotine concentration of 0.134 μmol/g in the hippocampus, increased the extracellular levels of aspartic acid, glutamic acid, and serine by 26–60%. At 50 mM nicotine the increases in the levels of aspartic acid, glutamic acid, serine, glycine, and glutamine were between 76% and 141%. Subcutaneous administration of nicotine at a dose of 6 μmol/kg caused a 57% increase in the extracellular level of glutamic acid. After a dose of 12 μmol/kg that resulted in a nicotine level of 0.015 μmol/kg in the hippocampus, the extracellular level of glutamic acid was increased by 100%, and that of aspartic acid by 24%. Thus, higher cerebral nicotine levels were needed with intracerebral than with subcutaneous administration to obtain similar amino acid changes. Prior administration of mecamylamine or L-kynurenine prevented the subcutaneous nicotine-induced elevation of the extracellular levels of aspartic acid and glutamic acid. Our results indicate that receptor interactions modulate nicotine effects and that both nicotinic cholinergic and NMDA/glycine glutamatergic receptors participate in the action of nicotine in increasing extracellular amino acid levels.     

Stimulus properties of clonidine in chronically morphine treated rats trained to a morphine-saline discrimination

In an operant behavior procedure of lever pressing on an FR 10 schedule of food reinforcement, morphine dependent and nondependent rats were trained to respond on a lever on one side of the food tray after a morphine (10 mg/kg i.p.) injection and to respond on a lever on the alternate side after a saline injection. Following discrimination training, in both dependent and nondependent rats saline was generalized to various doses of clonidine (10, 30 and 50 micrograms/kg i.p.). A response inhibition of about 65% was obtained with the highest dose. It was concluded that, even if clonidine can suppress signs of narcotic withdrawal, the internal state induced by morphine in an abstinent rat does differ from the one induced by clonidine in the same animal.     

High reinforcing efficacy of nicotine in non-human primates

Although tobacco appears highly addictive in humans, there has been persistent controversy about the ability of its psychoactive ingredient nicotine to induce self-administration behavior in laboratory animals, bringing into question nicotine's role in reinforcing tobacco smoking. Because of ethical difficulties in inducing nicotine dependence in na?ve human subjects, we explored reinforcing effects of nicotine in experimentally-naive non-human primates given access to nicotine for periods of time up to two years. Five squirrel monkeys with no experimental history were allowed to intravenously self-administer nicotine by pressing one of two levers. The number of presses on the active lever needed to obtain each injection was fixed (fixed-ratio schedule) or increased progressively with successive injections during the session (progressive-ratio schedule), allowing evaluation of both reinforcing and motivational effects of nicotine under conditions of increasing response cost. Over time, a progressive shift toward high rates of responding on the active lever, but not the inactive lever, developed. The monkeys' behavior was clearly directed toward nicotine self-administration, rather than presentation of environmental stimuli associated with nicotine injection. Both schedules of reinforcement revealed a high motivation to self-administer nicotine, with monkeys continuing to press the lever when up to 600 lever-presses were needed for each injection of nicotine. Thus, nicotine, by itself, in the absence of behavioral or drug-exposure history, is a robust and highly effective reinforcer of drug-taking behavior in a non-human primate model predictive of human behavior. This supports the use of nicotinic ligands for the treatment of smokers, and this novel preclinical model offers opportunities to test future medications for the treatment of nicotine dependence.     

Effects of oral administration of nicotine on organ weight, serum testosterone level and testicular histology in adult male rats

This study investigated the effects of oral administration of nicotine on body and reproductive organ weight, serum testosterone level and testicular histology in adult male rats. Forty male rats divided into five groups and treated for a period of 30 days with 0.5mg/kg (low dose) and 1.0 mg/kg (high dose) body weight of nicotine while the control rats received 0.2 ml/kg normal saline. The fourth and fifth groups were gavaged with 0.5mg/kg and 1.0mg/kg body weight of nicotine but were left untreated for another 30 days. These groups served as the recovery groups.  At the end of each experimental period, the animals were scarified and their reproductive organs were removed and weighed immediately. There was no significant change in the body weight. There was a significant decrease (p <0.05) in the testicular and epididymal weight of rats for both treatments while the decrease in the seminal vesicle weight for both treatment groups was not significant. The prostate weight was not significantly increased in both groups. The recovery groups showed appreciable recovery in their organ weight. Serum level of testosterone of both groups was significantly decreased in a dose dependent manner when compared with those of the control rats. The histological section showed testicular degeneration and disorganization in the cytoarchitecture, as the observed changes were pronounced in the high dose group than the low dose group. However, there were both regeneration of the germinal epithelium and restructuring of the interstitum towards normal in the recovery groups. No lesion was observed in the epididymis of the rats. The results suggest that nicotine has deleterious effect on the male reproductive organ of albino rats ameliorated by nicotine cessation.     

Discriminative stimulus properties of L-phenylisopropyl adenosine: blockade by caffeine and generalization to 2-chloroadenosine

Recent neurochemical data on the effects of activation and blockade of adenosine A1 receptors has suggested a direct role of adenosine in neurotransmission. The present research used a drug discrimination procedure to test the hypotheses that A1 adenosine receptor activation could serve as a discriminative stimulus and that caffeine, a drug believed to be an A1 receptor antagonist, could block the adenosine discrimination. Food-deprived rats were trained to press one of two levers on an FR 10 schedule of food-pellet delivery. Responses on one lever were reinforced following i.p. injection of N6 - (L-phenylisopropyl) adenosine (L-PIA); responses on the other lever were reinforced following i.p. injection of saline. L-PIA training dose was increased from 0.064 to 0.08 mg/kg L-PIA in the course of the study. Subjects required an average of 91 sessions to acquire this discrimination. Stimulus control by L-PIA was dose-dependent, with the ED-50 being approximately 0.03 mg/kg. 2-Chloroadenosine (2CA) generalized to L-PIA with a tenth the potency. Caffeine blocked L-PIA-induced lever selection. These results indicate that 1) rats can be trained to discriminate L-PIA from saline in a two-lever food-reinforced task and 2) the discriminative stimuli produced by L-PIA are based on its agonistic action at the adenosine A1 receptor.     

Maternal nicotine exposure increases oxidative stress in the offspring

Fetal and neonatal nicotine exposure causes β-cell apoptosis and loss of β-cell mass, but the underlying mechanisms are unknown. The goal of this study was to determine whether maternally derived nicotine can act via the pancreatic nicotinic acetylcholine receptor (nAChR) during fetal and neonatal development to induce oxidative stress in the pancreas. Female Wistar rats were given saline or nicotine (1 mg/kg/day) via subcutaneous injection for 2 weeks prior to mating until weaning (postnatal day 21). In male offspring, nAChR subunit mRNA expression was characterized in the developing pancreas and various oxidative stress markers were measured at weaning following saline and nicotine exposure. The nAChR subunits 2-4, 6, 7, and β2–β4 were present in the pancreas during development. Fetal and neonatal exposure to nicotine significantly increased pancreatic GPx-1 and MnSOD protein expression, as well as islet ROS production. Furthermore, protein carbonyl formation was higher in nicotine-exposed offspring relative to controls, particularly within the mitochondrial fraction. There was also a nonsignificant trend toward higher serum 8-isoPG levels. These data suggest that β-cell apoptosis in the fetal and neonatal pancreas may be the result of a direct effect of nicotine via its receptor and that this effect may be mediated through increased oxidative stress.     

Selective interaction of drugs with a discriminable stimulus associated with narcotic actions

Rats were trained to bar press on either one of two levers depending on whether they received an injection of morphine (10 mg/kg) or saline. The rats responded on the morphine-correct lever when injected with another narcotic, fentanyl, but responded on the saline-correct lever when injected with a narcotic antagonist or another CNS active, but non-narcotic, drug (e.g., amphetamine, apomorphine). The narcotic antagonist, naloxone, prevented the occurrence of the narcotic discriminable stimulus, but the rats responded on the morphine-correct lever when injected with morphine plus any of a number of potent CNS active, but non-narcotic compounds. These results are discussed with reference to the specificity of this procedure for detecting drugs with narcotic agonist or antagonist properties.     

Repeated stress alters the ability of nicotine to activate the hypothalamic-pituitary-adrenal axis

Acute nicotine administration has been shown to activate the hypothalamic-pituitary-adrenal (HPA) axis and stimulate secretion of adrenocorticotrophic hormone (ACTH), corticosterone/cortisol and beta-endorphin (beta-END) in both rodents and humans, raising the possibility that activation of the HPA axis by nicotine may mediate some of the effects of nicotine. Since stress can increase the risk of drug use and abuse, we hypothesized that repeated stress would increase the ability of nicotine to stimulate the secretion of HPA hormones. To test our hypothesis, mice were exposed to repeated stress (swimming in 15 degrees C water for 3 min/day for 5 days) and killed 15 min after injection of saline or nicotine (0.1 mg/kg, s.c.). Repeated exposure to stress increased the ability of nicotine to stimulate plasma ACTH (p<0.05) and beta-END (p<0.05), but not corticosterone secretion. In contrast, repeated exposure to stress increased the post-saline injection levels of corticosterone (p<0.05), but not ACTH and beta-END. The present results suggest that chronic stress leads to an enhanced sensitivity of some components of the HPA axis to a subsequent nicotine challenge.     

Effects of a selective cannabinoid CB2 agonist and antagonist on intravenous nicotine self administration and reinstatement of nicotine seeking

Over the last decade there have been significant advances in the discovery and understanding of the cannabinoid system along with the development of pharmacologic tools that modulate its function. Characterization of the crosstalk between nicotine addiction and the cannabinoid system may have significant implications on our understanding of the neurobiological mechanisms underlying nicotine dependence. Two types of cannabinoid receptors (CB1 and CB2) have been identified. CB1 receptors are expressed in the brain and modulate drug taking and drug seeking for various drugs of abuse, including nicotine. CB2 receptors have been recently identified in the brain and have been proposed to play a functional role in mental disorders and drug addiction. Our objective was to explore the role of CB2 receptors on intravenous nicotine self administration under two schedules of reinforcement (fixed and progressive ratio) and on nicotine seeking induced by nicotine priming or by nicotine associated cues. For this, we evaluated the effects of various doses of the selective CB2 antagonist AM630 (1.25 to 5 mg/kg) and CB2 agonist AM1241 (1 to 10 mg/kg) on these behavioral responses in rats. Different groups of male Long Evans rats were trained to lever press for nicotine at a unit dose of 30 μg/kg/infusion. Subsequently, animals were randomized using a Latin-square design and injected with either AM1241 or AM630 using a counterbalanced within subject design. Administration of the CB2 ligands did not affect either nicotine-taking nicotine-seeking behavior. Our results do not support the involvement of CB2 receptors in nicotine-taking or nicotine-seeking behavior.     

Acute and chronic effects of nicotine on synthesis and storage of catecholamines in the rat adrenal medulla.

Nicotine (1 mg/kg or 10 mg/kg) was administered twice daily to rats and the adrenals were analyzed for catecholamines (CA), tyrosine hydroxylase (TH), dopamine β-hydroxylase (DBH), and for the ability of isolated storage vesicles to incorporate ³H-epinephrine. Four hours after the first dose, there were few alterations at 1 mg/kg, but at 10 mg/kg, there was 30% depletion of CA, accompanied by a slight reduction in DBH and a decline in the number of functional vesicles (determined by uptake). Chronic administration of either dose produced elevations in both TH and DBH; at 10 mg/kg, CA levels and functional vesicles continued to decline for at least 4 days, but were at or above normal after 2 weeks of treatment. At 1 mg/kg, CA levels were never significantly below normal, and were elevated after 2 weeks. After 2 weeks at either dose level, a defect appeared in the abilities of vesicles to incorporate exogenous epinephrine relative to endogeneous CA content; this alteration also appeared if nicotine was discontinued. These data suggest that chronic nicotine administration can produce long-term alterations in CA release, synthesis and storage, and that these alterations can occur even at a dose which has little or no acute effect.     

Effects of nicotine on bone and calciotropic hormones in aged ovariectomized rats

The objective of this investigation was to assess the effects of chronic nicotine administration on bone status and serum calcium and calciotropic hormone levels in aged, estrogen-replete (intact, sham-operated) and estrogen-deplete (ovariectomized) female rats. Eight-month-old sham-operated (sham) and ovariectomized (ovx) retired breeder rats were maintained untreated for 3 months to allow for the development of osteopenia in the ovx group. The animals were then administered either saline, low dose nicotine (6.0 mg/kg/day), or high dose nicotine (9.0 mg/kg/day) via osmotic minipumps for 3 months. Blood was drawn at necropsy for determination of serum nicotine, cotinine, Ca, PTH, 25(OH)D, and 1,25(OH)(2)D. Right tibiae were collected and processed undecalcified for cancellous and cortical bone histomorphometry. Histomorphometric endpoints evaluated at the proximal tibial metaphysis included cancellous bone volume (BV/TV), osteoclast surface (Oc.S), osteoid surface (OS), mineralizing surface (MS), mineral apposition rate (MAR), and bone formation rate (BFR). Histomorphometric endpoints evaluated at the tibial diaphysis included cortical area (Ct.Ar), marrow area (Ma.Ar), and periosteal and endocortical MS, MAR, and BFR. Ovariectomy resulted in lower cancellous BV/TV and Ct.Ar and higher cancellous, endocortical, and periosteal MS and BFR. The presence of nicotine in serum confirmed successful delivery of the drug via osmotic minipumps. Administration of nicotine at the high dose resulted in lower serum 25(OH)D levels but differences in serum Ca or PTH were not detected with either nicotine treatment. Differences with nicotine treatment were also not detected for Oc.S at the proximal tibia. While treatment with nicotine at the high dose resulted in higher MS and BFR, in both sham and ovx rats, there were no differences due to nicotine treatment in cancellous BV/TV. Marrow area was greater in rats treated with nicotine than in rats treated with vehicle. However, differences with nicotine treatment were not detected in Ct.Ar in either intact or ovx rats. Overall, these findings indicate that steady state nicotine exposure does not alter bone mass in intact or ovx rats but may have detrimental effects on body storage of vitamin D.     

Acute reduction of brain substance P induced by nicotine

Ten minutes after a single injection of 0.8 mg/kg nicotine SC (free base) the level of substance P-like immunoreactivity (SPLI) was reduced by 61–73% in rat caudate-putamen, nucleus accumbens, and olfactory tubercle, with smaller and not significant reductions in the frontal cortex, substantia nigra, and ventral tegmental area. The nicotinic receptor antagonist mecamylamine (1.0 mg/kg IP) prevented the reductions in SPLI. The rapidity and the degree of the changes in SPLI after nicotine exceed those previously reported for other agents and implicate substance P neurotransmission as a major component of nicotinic action.Preliminary data were presented at the 17th annual meeting of the American Society for Neurochemistry, Montreal, 1986 (1).     

Effects of nicotine on oviducal blood flow and embryo development in the rat

Nicotine (5.0 mg/kg) was injected (s.c.) twice daily on Day 1 or Days 1-4 or 1-5 of pregnancy. Cumulative doses of nicotine retarded embryo cell cleavage and substantially reduced embryo cell number (saline vs nicotine: 42.5 +/- 1.7 vs 22.1 +/- 1.9 nuclei/embryo, at 12:00 h on Day 5; P less than 0.05). However, treatment for even 1 day (Day 1) significantly reduced cell number (saline vs nicotine: 42.5 +/- 1.7 vs 30.5 +/- 0.9, at 12:00 h day on Day 5; P less than 0.01). Nicotine injection also resulted in a marked and prolonged reduction in oviduct blood flow (pretreatment vs 90 min after nicotine: 0.61 +/- 0.06 vs 0.37 +/- 0.10 ml/min . g-1; P less than 0.005). The results indicate that, in the rat, even a brief exposure to nicotine, the chief alkaloid of tobacco, reduces oviducal blood flow and the rate of embryo cell proliferation. The embryo is therefore susceptible to the effects of nicotine before implantation.     

Heterogeneity of hepatic nicotine oxidase

When rats were pretreated with 3-methylcholanthrene or β-naphthoflavone, hepatic nicotine oxidase activity per cytochrome P-448 molecule decreased, but the specific activity of the enzyme remained unchanged. After phenobarbital pretreatment, the specific activity of nicotine oxidase increased while the activity of the enzyme per cytochrome P-450 molecule decreased. α-Naphthoflavone selectively inhibited the activities of phenobarbital-induced nicotine oxidase and constitutive form(s) of the enzyme. These results show that phenobarbital-induced cytochrome P-450 and constitutive forms(s) of the enzyme may be active in hepatic nicotine oxidation.     

Dose-finding study with nicotine as a proconvulsant agent in PTZ-induced seizure model in mice

The present study was conducted to investigate the possible interaction between low doses of nicotine and pentylenetetrazole (PTZ) in vivo and also to evaluate the influence of nicotine on the antiseizure efficacy of topiramate and sodium valproate in the PTZ-induced seizure model in mice. Graded dose–response study with nicotine showed the CD50 value for nicotine at 6.76 mg/kg. i.p. Subtheshold dose of nicotine (4 mg/kg, i.p.) pretreatment significantly decreased the CD50 value for PTZ from 47.86 mg/kg, i.p. (of PTZ per se) to 31.62 mg/kg, i.p. Sodium valproate but not topiramate, significantly inhibited PTZ-induced seizures in mice with an ED50 value of 177.83 mg/kg, i.p. Nonconvulsive dose of nicotine (1 mg/kg, i.p.) significantly antagonized the protective efficacy of sodium valproate against PTZ-induced seizures and increased the ED50 value to 338.84 mg/kg, i.p. PTZ-induced seizures significantly increased the mouse brain levels of MDA and reduced the level of GSH while sodium valproate reversed such changes. Nicotine pretreatment reversed the anti-lipid peroxidative action of sodium valproate in the PTZ-induced seizure model in mice. The study highlighted the convulsant as well as proconvulsant role of nicotine and established dose discrimination for nicotine as a proconvulsant agent and an anti-antiseizure agent. The study bears significant clinical relevance particularly amongst epileptic smokers who may show failure of efficacy of antiepileptic agents and present with breakthrough seizure attacks on exposure to nicotine.