Rats exhibiting acute behavioural tolerance to nicotine have more [125I]alpha-bungarotoxin binding sites in brain than rats not exhibiting tolerance

Reduction of nicotine and ethanol intake in alcohol-preferring (UChB) female rats by the α4β2 nicotinic acetylcholine receptor partial agonists 5-bromocytisine and cytisine

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of nicotine and ethanol. Previous studies have shown that cytisine and its 5-bromo derivative are partial agonists at the α4β2 nAChRs and that the parent molecule cytisine is effective in reducing both nicotine- and ethanol-self-administration in rats. However, whether 5-bromocytisine affects nicotine or ethanol self-administration was unknown.

OBJECTIVES

The present study compared the effects of 5-bromocytisine and cytisine on nicotine self-administration and further assessed the effect of daily drug injection on voluntary ethanol consumption in alcohol-preferring female rats. Animals were administered a 1.5mg/kg i.p. dose of 5-bromocytisine or cytisine every day for 15-16 days.

RESULTS

The initial efficacy of 5-bromocytisine and cytisine in reducing nicotine intake was similar (-80%) while for voluntary ethanol intake 5-bromocytisine was a superior inhibitor over cytisine (-78% and -40% respectively). The efficacy of cytisine began to diminish after 10 days of daily administration, which was attributed to tolerance development to its inhibitory effects both on nicotine and ethanol self-administration. Tolerance did not develop for 5-bromocytisine.

CONCLUSION

5-Bromocytisine, a weaker α4β2 nAChR partial agonist than cytisine, also produces a sustained inhibition of both nicotine and ethanol self-administration, and unlike cytisine, it does not develop tolerance.

Rapid nicotine tolerance and cross-tolerance to varenicline in rhesus monkeys: Drug discrimination

Acute tolerance to effects of nicotine plays an important role in nicotine dependence, but the mechanism underlying these effects is unclear. Drug discrimination was used in the current study to examine the impact of nicotine pretreatment on sensitivity to the discriminative stimulus effects of nicotine and the FDA-approved smoking cessation pharmacotherapy varenicline. Rhesus monkeys (n = 4) discriminated 0.032 mg/kg nicotine base iv from saline under an FR5 schedule of stimulus-shock termination. Both nicotine and varenicline increased drug-appropriate responding; ED50 values (95% confidence limits) were 0.0087 [0.0025, 0.030] and 0.028 [0.0096, 0.082] mg/kg, respectively. Additional pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced tolerance to its discriminative stimulus effects and the magnitude of this effect was related to the number of pretreatment injections administered. Two pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced a 5.4-fold rightward shift in the nicotine dose-response function and a sevenfold rightward shift in the varenicline dose-response function. The duration of tolerance under these conditions was less than 60 min. These results demonstrate that tolerance to the discriminative stimulus effects of nicotine can be produced by acute nicotine exposure. Acute cross-tolerance from nicotine to varenicline is consistent with similar actions at nAChRs, and suggests that conditions resulting in acute nicotine tolerance could impact sensitivity to other nAChR agonists. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

³H-deprenyl and ³H-PIB autoradiography show different laminar distributions of astroglia and fibrillar β-amyloid in Alzheimer brain

BACKGROUND

The pathological features in Alzheimer's disease (AD) brain include the accumulation and deposition of β-amyloid (Aβ), activation of astrocytes and microglia and disruption of cholinergic neurotransmission. Since the topographical characteristics of these different pathological processes in AD brain and how these relate to each other is not clear, this motivated further exploration using binding studies in postmortem brain with molecular imaging tracers. This information could aid the development of specific biomarkers to accurately chart disease progression.

RESULTS

In vitro binding assays demonstrated increased [³H]-PIB (fibrillar Aβ) and [³H]-PK11195 (activated microglia) binding in the frontal cortex (FC) and hippocampus (HIP), as well as increased binding of [³H]-L-deprenyl (activated astrocytes) in the HIP, but a decreased [³H]-nicotine (α4β2 nicotinic acetylcholine receptor (nAChR)) binding in the FC of AD cases compared to age-matched controls. Quantitative autoradiography binding studies were also performed to investigate the regional laminar distributions of [³H]-L-deprenyl, [³H]-PIB as well as [¹²⁵I]-α-bungarotoxin (α7 nAChRs) and [³H]-nicotine in hemisphere brain of a typical AD case. A clear lamination pattern was observed with high [³H]-PIB binding in all layers and [³H]-deprenyl in superficial layers of the FC. In contrast, [³H]-PIB showed low binding to fibrillar Aβ, but [³H]-deprenyl high binding to activated astrocytes throughout the HIP. The [³H]-PIB binding was also low and the [³H]-deprenyl binding high in all layers of the medial temporal gyrus and insular cortex in comparison to the frontal cortex. Low [³H]-nicotine binding was observed in all layers of the frontal cortex in comparison to layers in the medial temporal gyrus, insular cortex and hippocampus. Immunohistochemical detection in the AD case revealed abundant glial fibrillary acidic protein positive (GFAP+) reactive astrocytes and α7 nAChR expressing GFAP+ astrocytes both in the vicinity and surrounding Aβ neuritic plaques in the FC and HIP. Although fewer Aβ plaques were observed in the HIP, some hippocampal GFAP+ astrocytes contained Aβ-positive (6 F/3D) granules within their somata.

CONCLUSIONS

Astrocytosis shows a distinct regional pattern in AD brain compared to fibrillar Aβ, suggesting that different types of astrocytes may be associated with the pathophysiological processes in AD.

Acute nicotine reduces and repeated nicotine increases spontaneous activity in male and female Lewis rats

The Lewis (LEW) strain of rat appears more sensitive to nicotine than other strains in self-administration, conditioned place preference, and drug discrimination behavioral studies. The present study sought to further evaluate the behavioral effects of chronic nicotine treatment in the LEW strain by assessing spontaneous activity, which has consistently revealed sensitization to chronic nicotine administration in Sprague Dawley (SD) rats. High active and low active male and female LEW rats (N=8 per group) were treated twice daily with either nicotine (0.4 mg/kg, sc) or vehicle for 14 consecutive days. Regardless of baseline activity level or sex, spontaneous activity was significantly decreased, compared to saline-treated rats, after a single nicotine injection. However, spontaneous activity increased in both low- and high-activity rats (both sexes) over the two weeks of nicotine administration to levels that were significantly higher than saline-treated rats. Based on these findings, acute and chronic nicotine administration had greater suppressive and enhancing effects on spontaneous activity in LEW rats compared to other strains of rats previously studied. These results further clarify the behavioral sensitivity of the LEW strain of rat to nicotine exposure and lend credence to the role of genetics in the individual susceptibility to nicotine dependence.

Cellular nicotinic receptor desensitization correlates with nicotine-induced acute behavioral tolerance in rats

RATIONALE

Individuals vary in their susceptibility to nicotine addiction. However, there is little evidence that behavioral sensitivity to nicotine is dependent upon the functional state of nicotinic cholinergic receptors (nAChRs).

OBJECTIVE

This study aims to determine the relationship between in vivo behavioral desensitization and in vitro desensitization of nAChR function.

METHODS

Male Sprague-Dawley rats trained to discriminate nicotine were tested for development of acute behavioral tolerance. The rats were injected with nicotine (0.4 mg/kg free base, s.c.), tested for nicotine discrimination for 2 min, then injected with the same dose of nicotine 90, 180, and 270 min after the first injection and tested for nicotine discrimination after each injection. Susceptibility of nAChRs of individual rats to desensitization was assessed by use of the (86)Rb(+) efflux assay using synaptosomes prepared from the "thalamus," which included the hypothalamus and midbrain as well as the thalamic nuclei. To desensitize nAChRs, synaptsosomes were superfused with low concentrations of nicotine (5, 10, 20, and 30 nM) before stimulation of (86)Rb(+) efflux with nicotine (10 muM).

RESULTS

The slopes of the behavioral desensitization were plotted as a function of the decline of nicotine-stimulated (86)Rb(+) efflux after in vitro desensitization. A significant correlation was observed between the in vitro desensitization of thalamic (86)Rb(+) efflux and the extent of behavioral desensitization of individual rats.

CONCLUSIONS

These findings are consistent with the idea that production of acute behavioral tolerance by nicotine is related to its ability to induce nAChR desensitization at the cellular level.

Evidence of cellular nicotinic receptor desensitization in rats exhibiting nicotine-induced acute tolerance

RATIONALE

Individuals vary in their susceptibility to nicotine addiction. However, there is little evidence that behavioral sensitivity to nicotine is dependent upon the functional state of nicotinic cholinergic receptors (nAChRs).

OBJECTIVE

To determine the relationship between in vivo pharmacological desensitization (in other words, acute tolerance) and brain regional nAChR function.

METHODS

Male Sprague-Dawley rats, trained to discriminate nicotine (0.4 mg/kg free base) from saline in a two-lever drug discrimination task, were tested for the development of acute tolerance. Rats were injected with 0.4 mg/kg nicotine, tested for nicotine discrimination for 2 min, then injected with the same dose of nicotine 90 min, 180 min, and 270 min after the first injection and tested for nicotine discrimination after each injection. These subjects were separated into two groups, desensitizers (DZ) and nondesensitizers (NDZ), based upon performance in the repetitive dosing drug discrimination paradigm. The sensitivity of nAChRs in specific brain regions of these two groups was assessed by the use of an 86Rb+ efflux assay using synaptosomes prepared from the frontal cortex, hippocampus, striatum, and "thalamus," which included the midbrain and hypothalamus as well as the thalamus.

RESULTS

The nicotine-induced increase in 86Rb+ efflux was significantly greater in NDZ as compared to DZ in the "thalamus." There was no statistically significant difference in the effects of nicotine in the frontal cortex, hippocampus, and striatum of these two groups. A significant correlation was observed between thalamic 86Rb+ efflux and the rate of behavioral desensitization of individual rats.

CONCLUSION

These findings are consistent with the concept that the production of acute tolerance by nicotine in vivo correlates directly with its ability to induce nAChR desensitization at the cellular level.

Selective nicotinic receptor consequences in APP(SWE) transgenic mice

The nicotinic (nAChRs) and muscarinic (mAChRs) acetylcholine receptors and acetylcholinesterase (AChE) activity were studied in the brains of APP(SWE) transgenic mice (Tg+) and age-matched nontransgenic controls (Tg-) that were between 4 and 19 months of age. A significant increase in the binding of 125I-labeled alpha-bungarotoxin (alpha7 nAChRs) was observed in most brain regions analyzed in 4-month-old Tg+ mice, preceding learning and memory impairments and amyloid-beta (Abeta) pathology. The enhanced alpha7 receptor binding was still detectable at 17-19 months of age. Increase in [3H]cytisine binding (alpha4beta2 nAChRs) was measured at 17-19 months of age in Tg+ mice, at the same age when the animals showed heavy Abeta pathology. No significant changes in [3H]pirenzepine (M1 mAChRs) or [3H]AFDX 384 (M2 mAChRs) binding sites were found at any age studied. The upregulation of the nAChRs probably reflects compensatory mechanisms in response to Abeta burden in the brains of Tg+ mice.

The discriminative stimulus and reinforcing effects of nicotine in humans following nicotine pretreatment

Smokers often report that the first cigarette of the day is the most rewarding, and subsequent smoking is less rewarding. Reduction in smoking enjoyment later in the day may be related to acute tolerance to the discriminative stimulus effects of nicotine. We examined changes in nicotine discrimination behaviour in humans as a function of acute nicotine pretreatment. Male and female dependent smokers (n = 15) were initially trained to discriminate 20 microg/kg nicotine by nasal spray from placebo (0 microg/kg) without nicotine pretreatment. They then were tested on generalization of discrimination across a range of spray doses from 0-20 microg/kg following pretreatment with placebo, moderate dose (14-21 mg) or high dose (28-42 mg) transdermal nicotine. Generalization testing involved both two- and three-response ('novel' option) quantitative procedures. Subjects also engaged in a self-administration phase at the end of each session, involving choices between nicotine (20 microg/kg) and placebo spray. Nicotine pretreatment significantly attenuated nicotine-appropriate responding at higher nicotine spray doses, suggesting acute tolerance, but only in women. Similar results were seen for subjective 'head rush', suggesting this effect may be related to discrimination behaviour in women. However, nicotine pretreatment also increased novel-appropriate responding, especially in men, following intermediate generalization doses, suggesting qualitatively different stimulus effects. Although differences were not significant, nicotine self-administration tended to be inversely associated with nicotine pretreatment dose in men but not in women. These results only modestly support the notion of acute tolerance to the discriminative stimulus effects of nicotine, and even then only in women and not in men.