Effect Of Chronic Nicotine Treatment Research Articles

Chronic nicotine-induced changes in gene expression of delta and kappa-opioid receptors and their endogenous ligands in the mesocorticolimbic system of the rat.

Delta and kappa opioid receptors (DOR and KOR, respectively) and their endogenous ligands, proenkephalin (PENK) and prodynorphin (PDYN)-derived opioid peptides are proposed as important mediators of nicotine reward. This study investigated the regulatory effect of chronic nicotine treatment on the gene expression of DOR, KOR, PENK and PDYN in the mesocorticolimbic system. Three groups of rats were injected subcutaneously with nicotine at doses of 0.2, 0.4, or 0.6 mg/kg/day for 6 days. Rats were decapitated 1 hr after the last dose on day six, as this timing coincides with increased dopamine release in the mesocorticolimbic system. mRNA levels in the ventral tegmental area (VTA), lateral hypothalamic area (LHA), amygdala (AMG), dorsal striatum (DST), nucleus accumbens, and medial prefrontal cortex were measured by quantitative real-time PCR. Our results showed that nicotine upregulated DOR mRNA in the VTA at all of the doses employed, in the AMG at the 0.4 and 0.6 mg/kg doses, and in the DST at the 0.4 mg/kg dose. Conversely, PDYN mRNA was reduced in the LHA with 0.6 mg/kg nicotine and in the AMG with 0.4 mg/kg nicotine. KOR mRNA was also decreased in the DST with 0.6 mg/kg nicotine. Nicotine did not regulate PENK mRNA in any brain region studied.

Chronic nicotine treatment enhances vascular smooth muscle relaxation in rats.

To investigate the effect of chronic nicotine treatment on vascular function and to identify the underlying mechanisms. Adult rats were treated with nicotine (3 mg·kg(-1)·d(-1), sc) for 6 weeks. After the rats were sacrificed, aortic rings were prepared for detecting vascular reactivity, and thoracic aorta and periaortic fat samples were collected for histological and molecular biology studies. Chronic nicotine treatment significantly reduced periaortic fat, and specifically enhanced smooth muscle relaxation without altering the aortic adventitial fat and endothelium function. Pretreatment with the soluble guanylyl cyclase inhibitor ODQ (3 μmol/L) or PKG inhibitor Rp-8-Br-PET-cGMP (30 μmol/L) abolished the nicotine-induced enhancement of smooth muscle relaxation, whereas the cGMP analogue 8-Br-cGMP could mimic the nicotine-induced enhancement of smooth muscle relaxation. However, the chronic nicotine treatment did not alter PKG protein expression and activity in aortic media. Chronic nicotine treatment enhances vascular smooth muscle relaxation of rats via activation of PKG pathway.

Role of α7- and α4β2-nAChRs in the neuroprotective effect of nicotine in stress-induced impairment of hippocampus-dependent memory

We have previously shown that nicotine prevents stress-induced memory impairment. In this study, we have investigated the role of α7- and α4β2-nicotinic acetylcholine receptors (nAChRs) in the protective effect of nicotine during chronic stress conditions. Chronic psychosocial stress was induced using a form of rat intruder model. During stress, specific antagonist for either α7-nAChRs [methyllycaconitine (MLA)] or α4β2-nAChRs [dihydro-β-erythroidine (DHβE)] was infused into the hippocampus using a 4-wk osmotic pump at a rate of 82μg/side.d and 41μg/side.d, respectively. Three weeks after the start of infusion, all rats were subjected to a series of cognitive tests in the radial arm water maze (RAWM) for six consecutive days or until the animal reached days to criterion (DTC) in the fourth acquisition trial and in all memory tests. DTC is defined as the number of days the animal takes to make no more than one error in three consecutive days. In the short-term memory test, MLA-infused stressed/nicotine-treated rats made similar errors to those of stress and significantly more errors compared to those of stress/nicotine, nicotine or control groups. This finding was supported by the DTC values for the short memory tests. Thus, MLA treatment blocked the neuroprotective effect of nicotine during chronic stress. In contrast, DHβE infusion did not affect the RAWM performance of stress/nicotine animals. These results strongly suggest the involvement of α7-nAChRs, but not α4β2-nAChRs, in the neuroprotective effect of chronic nicotine treatment during chronic stress conditions.

Chronic nicotine exposure selectively activates a carrier-mediated release of endogenous glutamate and aspartate from rat hippocampal synaptosomes

The effect of chronic nicotine treatment on the release of endogenous glutamate (GLU), aspartate (ASP) and GABA evoked in vitro by KCl, 4-aminopyridine (4AP) and nicotinic agonists in synaptosomes of rat hippocampus was investigated. Rats were chronically administered with nicotine bitartrate or saline vehicle each for 14days using osmotic mini-pumps. Hippocampal synaptosomes were stimulated with KCl, 4AP, nicotine or with choline (Ch) and 5-iodo-A-85380 dihydrochloride (5IA85380). The GLU and ASP overflow evoked by Ch, nicotine, KCl and 4AP were increased in treated animals while the nicotine-evoked GABA overflow was reduced and that evoked by Ch, KCl and 4AP was unaffected. The 5IA85380-evoked overflow of the three aminoacids (AAs) was always reduced. The increase of ASP and GLU overflow evoked by KCl, 4AP or Ch was blocked by dl-threo-β-benzyloxyaspartic acid (dl-TBOA), a carrier transporter inhibitor, and by inhibitors of the Na+/Ca2+ exchangers 2-[[4-[(4-nitrophenyl)methoxy]phenyl]methyl]-4-thiazolidinecarboxylic acid ethyl ester (SN-6) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate (KB-R7943). In conclusion long-term nicotine treatment may selectively increase GLU and ASP overflow elicited by KCl, 4AP and Ch through the activation of a carrier-mediated release mechanism and completely abolished the stimulatory effects of α4β2 nAChRs which modulate the release of all the three AA.

Effect of chronic nicotine treatment and withdrawal on rat striatal D1 and D2 dopamine receptors.

The effects on rat striatal dopamine receptors after chronic nicotine administration (3 and 12 mg kg-1 day-1), and after withdrawal from chronic nicotine (12 mg kg-1 day-1), were studied. After 21 days of continuous minipump infusion, the control (saline) and nicotine-treated rats were killed. The nicotine-withdrawal rats were killed on day 28, 7 days after pump removal. Radioligand studies were performed to determine D1 ([3H]SCH23390) and D2 ([3H]spiperone) striatal dopamine receptor affinity (Kd) and maximum binding (Bmax). Dopamine inhibition of antagonist binding at 3 concentrations and the effect of 0.3 mM GTP on binding affinity were examined. No statistically significant differences between control and nicotine treatment or withdrawal groups were noted in either D1 or D2 receptor Kd or Bmax. Although nicotine has been shown to affect nigrostriatal dopamine release, chronic treatment does not appear to alter overall striatal dopaminergic receptor binding parameters.

Nicotine withdrawal increases body weight, neuropeptide Y and Agouti-related protein expression in the hypothalamus and decreases uncoupling protein-3 expression in the brown adipose tissue in high-fat fed mice

Nicotine is known to decrease body weight in normal rodents and human smokers, whereas nicotine withdrawal or smoking cessation can increase body weight. We have found that mice fed a high fat diet do not show the anorectic effect of chronic nicotine treatment, but do increase their body weight following nicotine withdrawal. Nicotine withdrawal is accompanied by increased expression of the orexigenic peptides neuropeptide Y and Agouti-related protein in the hypothalamus, and decreased expression of the metabolic protein uncoupling protein-3 in brown adipose tissue. These data suggest that diet can influence the ability of nicotine to modulate body weight regulation and demonstrate that chronic nicotine exposure results in adaptive changes in central and peripheral molecules which regulate feeding behavior and energy metabolism.

Nicotine reverses adult‐onset hypothyroidism‐induced impairment of learning and memory: Behavioral and electrophysiological studies

Nicotine alleviates cognitive impairment associated with a variety of health conditions. We examined the effect of chronic nicotine treatment on adult-onset hypothyroidism-induced impairment of learning and memory in rats. Hypothyroidism was induced by surgical removal of thyroid glands (thyroidectomy). One month later, chronic nicotine treatment (1 mg/kg sc, twice/day) was instituted for 4-6 weeks. Test of hippocampus-dependent spatial learning and memory in the radial arm water maze showed that hypothyroidism impaired learning as well as short-term and long-term memory retention. Chronic nicotine treatment reversed the hypothyroidism-induced learning and memory impairment. In normal rats, chronic nicotine treatment had no effect on learning and memory. Extracellular recordings from the CA1 region of anesthetized hypothyroid rats showed severe reduction of both early-phase and late-phase long-term potentiation (LTP) magnitude, which was reversed in nicotine-treated hypothyroid rats. These results show that chronic nicotine treatment prevents hypothyroidism-induced impairment of spatial cognition and LTP.

Chronic nicotine treatment increases GABAergic input to striatal neurons

Recent studies suggest the involvement of the dorsal striatum in the advanced stages of drug addiction as well as motor functions. We investigated the effect of chronic nicotine treatment on GABAergic synaptic transmission in the striatum of mice. Intrastriatal stimulation evoked GABAA receptor-mediated polysynaptic inhibitory postsynaptic currents more frequently in medium-sized spiny projection neurons of mice treated chronically with nicotine (1 mg/kg, twice-daily subcutaneous injections for 10-15 days) than in those of PBS-treated mice. The multiphasic inhibitory postsynaptic currents consisted of monosynaptic early and polysynaptic, nicotinic acetylcholine receptor-mediated late components. Dihydro-beta-erythroidine, an antagonist of the non-alpha7nicotinic acetylcholine receptor, suppressed only the late inhibitory postsynaptic current. These results suggest that chronic nicotine treatment increases GABAergic input to projection neurons in the dorsal striatum.

Effect of chronic nicotine treatment on a1‐adrenoceptor‐mediated contraction and eNOS protein levels in pregnant uterine artery

Cigarette smoking during pregnancy has been associated with high rate of spontaneous abortion, underweight newborns, and sudden infant death syndrome. One of the potential effects of smoking is that nicotine may act on the maternal-fetal unit, including episodic hypoxia-ischemia, and affect the uteroplacental circulation. Uterine artery (UA) is the only source to deliver oxygen and nutrition that ensures normal fetal development. However, little is known about the effect of nicotine on the UA contractility. The present study was designed to test the hypothesis that nicotine enhances α1-adrenoceptor-mediated contraction and decreases in eNOS protein levels of the pregnant UA. UAs were isolated from pregnant ewes (~140 days gestation). Tissues were subjected to nicotine treatment for 48 hrs. After treatment, endothelium cells from the UAs were isolated and eNOS protein levels were determined by Western blot. Phenylepherine (PE) -mediated contractions were measured in tissue bath. PE produced dose-dependent contraction of pregnant UAs. Chronic nicotine treatment significantly potentiated PE-induced contractions. Chronic nicotine treatment also significantly decreased eNOS protein levels compared to control. The results suggest that nicotine has chronic effect on UA contractility through the mechanisms underlying enhancing its contractile sensitivity to α1-adrenoceptor agonist and inhibiting eNOS protein expression. The enhanced UA contractility may cause a reduction in utero-placental blood flow and consequent fetal growth retardation.

Effect of chronic nicotine treatment on localization of neuronal nicotinic acetylcholine receptors at cellular level

Chronic nicotine treatment increases the number of neuronal nicotinic acetylcholine receptors (nAChRs). Localization of nAChRs at a cellular level determines their functional role. However, changes in the localization of nAChRs caused by chronic nicotine treatment are not well known. In this study, we have examined the effects of chronic nicotine treatment on alpha7 and beta2 nAChR subunits in vitro in cell lines and in vivo in mouse striatum. In vitro, two different cell lines were used, SH-SY5Y cells endogenously expressing several nAChR subtypes and SH-EP1-halpha7 cells, transfected with the human alpha7 nAChR subunit gene. Effects of chronic nicotine treatment (10 microM, 3 days) were studied in vitro by using confocal and electron microscopy and calcium fluorometry. In vitro in SH-SY5Y cells, alpha7 and beta2 subunits formed groups, unlike alpha7 subunits in SH-EP1-halpha7 cells, which were partially localized on endoplastic reticulum. Chronic nicotine treatment did not change the localization of nAChRs in endosomes, but caused clustering of alpha7 subunits in SH-EP1-halpha7 cells. In vivo, nicotine was given to mice in their drinking water for 7 weeks. Results showed that alpha7 and beta2 subunits formed groups, and that chronic nicotine treatment increased the size of the clusters. As a conclusion, our data show that there are large intracellular pools of nAChR subunits, which are partially localized on endoplastic reticulum. Chronic nicotine treatment does not change endocytotic trafficking of nAChRs. Chronic nicotine treatment increased clustering of nAChRs, which could have a role in the release of dopamine (DA) evoked by nicotine.

CYP2B6 is expressed in African Green monkey brain and is induced by chronic nicotine treatment

CYP2B6 is a drug-metabolizing enzyme expressed in human tissues that can activate bupropion (a smoking cessation drug) and tobacco smoke nitrosamines and can inactivate drugs such as nicotine. Smokers have higher brain CYP2B6 protein levels compared to non-smokers but the cause of this elevation is unknown. We investigated the basal expression and the effect of chronic nicotine treatment on CYP2B6 protein in African Green monkey ( Cercopithecus aethiops) brain. Basal expression of brain CYP2B6 was strong in specific cells such as the frontal cortical pyramidal cells, the cerebellar Purkinje cells and the neurons in the substantia nigra. Basal CYP2B6 protein levels varied 2.7-fold (non-significant) among 12 brain regions. All monkeys were given a subcutaneous 0.1 mg/kg nicotine test dose prior to treatment and the maximum plasma concentration achieved was 87 ± 69 ng/ml and the half-life was 2.6 ± 1.5 h. Monkeys were treated subcutaneously twice daily with nicotine at 0.05 mg/kg for 2 days, 0.15 mg/kg for 2 days followed by 0.3 mg/kg for 18 days ( n = 6) or saline ( n = 6). Chronic nicotine treatment induced CYP2B6 expression in specific cells such as astrocytes and neurons in the frontal cortex, caudate, thalamus and hippocampus. CYP2B6 protein levels were induced 1.5-fold in the frontal cortex ( p < 0.01). Hepatic CYP2B6 expression was not altered by nicotine. In conclusion, CYP2B6 protein is expressed in specific cells in monkey brain and is induced by chronic nicotine treatment which may impact central metabolism of CYP2B6 substrates such as bupropion and nicotine.

Nicotine prevents stress‐induced enhancement of long‐term depression in hippocampal area CA1: Electrophysiological and molecular studies

Nicotine treatment prevents chronic psychosocial stress-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). In this study, we investigated the effect of chronic nicotine treatment on stress-induced enhancement of long-term depression (LTD). After paired-pulse stimulation, LTD was evoked in area CA1 of anesthetized control, stressed, nicotine-treated, and nicotine-treated stressed rats. In stressed rats, a significantly greater LTD magnitude was seen than in control rats. Stress also facilitated the induction of LTD. Nicotine treatment of stressed rats prevented stress-induced enhancement and facilitation of LTD. For chronically stressed rats, we previously reported marked decreases in the basal levels of brain-derived neurotrophic factor (BDNF), CaMKII, P-CaMKII, and calmodulin as well as a significant increase in calcineurin basal levels. Herein, Western blot analysis conducted 1 hr after induction of LTD by paired-pulse stimulation showed that the levels of calcineurin and P-CaMKII were increased in the stressed group compared with the other groups and were normalized by chronic nicotine treatment. Additionally, after paired-pulse stimulation, the levels of total CaMKII were increased in all groups with no change in the levels of BDNF and calmodulin. Therefore, the increase in the levels of calcineurin and P-CaMKII during expression of LTD in area CA1 may explain the enhanced magnitude of LTD in chronically stressed rats.

Nicotine blocks stress-induced impairment of spatial memory and long-term potentiation of the hippocampal CA1 region

The effect of chronic nicotine treatment on chronic psychosocial stress-induced impairment of short-term memory and long-term potentiation (LTP) was determined. An "intruder" stress model was used to induce psychosocial stress for 4-6 wk, during which rats were injected with saline or nicotine (1 mg/kg s.c.) twice a day. The radial arm water maze memory task was used to test hippocampus-dependent spatial memory. Chronic psychosocial stress impaired short-term memory without affecting the learning phase or long-term memory. Concurrent chronic nicotine treatment prevented stress-induced short-term memory impairment. In normal rats chronic nicotine treatment had no effect on learning and memory. Extracellular recordings from the CA1 region of anaesthetized rats showed severe reduction of LTP magnitude in stressed rats, which was normalized in nicotine-treated stressed rats. Nicotine had no effect on LTP in control animals. These results showed that chronic nicotine treatment improved hippocampus-dependent spatial memory and LTP only when impaired by stress.

Effects of chronic nicotine treatment on the expression of phospholipase C isozymes and the α subunit of G q/11 protein in the rat brain

The effect of chronic nicotine treatment on postreceptor sites of the phosphoinositide (PI) signaling system (i.e. phospholipase C (PLC) isozymes, G q/11 protein) in the rat cortex was investigated. The expression of PLC isozymes ( β 1, γ 1, and δ 1) and the α subunit of G q/11 protein in the cortices of rats treated with saline or nicotine was determined by the Western blot technique using specific monoclonal antibodies. It was observed that chronic nicotine treatment (10 days) significantly increased the immunolabeling of the total PLC- β 1, which was due to an increase in the immunolabeling of both the membranal and the cytosolic PLC- β 1 isozyme in the ra cortex. On the other hand, the immunolabeling of total, membranal, and cytosolic PLC- δ 1 and - γ 1 isozymes in the rat cortex was not significantly altered by chronic nicotine treatment. In addition, Western blot analysis also demonstrated that chronic nicotine treatment has no significant effect on the immunolabeling of the a subunit of G q/11 protein in the rat cortex. Thus, these findings suggest that an increase in the expression of the PLC- β 1 isozyme may be involved in the neuroadaptational mechanisms associated with tobacco addiction.

Dopamine efflux from striatum after chronic nicotine: evidence for autoreceptor desensitization.

We examined the effect of chronic nicotine treatment on dopaminergic activity by measuring the effects of D1 and D2 dopamine (DA) receptor agonists and antagonists on tritium release from mouse striatum preloaded with [3H]DA. The radioactivity released during superfusion was separated on alumina columns and the distribution and efflux of [3H]DA and its main 3H-labeled metabolites were quantified. After preloading by incubation with [3H]DA, the electrical stimulation-evoked tritium overflow was higher in striatum prepared from nicotine-treated mice, whereas in vitro addition of nicotine caused a similar increase in tritium release from striatum of untreated and chronic nicotine-treated mice. The overflow of [3H]DA and its 3H-metabolites exhibited similar distribution patterns in [3H]DA-preloaded striatum dissected from untreated and chronic nicotine-pretreated mice, indicating that repeated injections with nicotine did not alter the metabolism of [3H]DA taken up by the tissue. (-)-Quinpirole, a selective agonist for D2 DA receptors, and apomorphine, a nonselective D1/D2 agonist, inhibited the electrical stimulation-induced tritium efflux from striatum of untreated mice, whereas (+/-)-sulpiride, a D2 DA receptor antagonist, enhanced the evoked release of tritium. These changes in tritium efflux effected by (-)-quinpirole and (+/-)-sulpiride reflected changes in [3H]DA release and not in DA metabolism, as shown by separation of the released radioactivity on alumina columns. The D1 receptor agonist (+/-)-SKF-38393 did not affect the tritium overflow, whereas the D1 receptor antagonist (+)-SCH-23390 exerted a stimulatory action but only at a high concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Chronic Nicotine Treatment and its Withdrawal on the Vasopressinergic System in Rats

Abstract We have studied the effect of chronic nicotine treatment and its withdrawal on the hypothalamo-neurohypophyseal Vasopressinergic system in male Sprague-Dawley rats. They were subcutaneously infused with low and high doses of nicotine, free base (0.6 and 6.0 mg/kg/day, respectively) for 28 days, via Alzet osmotic pumps. The studies were carried out immediately after the period of infusion and 1, 7, 14 and 28 (the latter in high dose only) days later. Basal, high K(+)-stimulated and total vasopressin release from the superfused neural lobes, the residual vasopressin content in the neural lobes, and hypothalamus and plasma vasopressin concentration were measured by radioimmunoassay. Treatment with the high dose of chronic nicotine alone decreased vasopressin content in and release from the neural lobe and its plasma concentration, but it did not change significantly the vasopressin content in the hypothalamus. A similar pattern of changes in vasopressin release and plasma concentration, though less pronounced, was observed in the rats infused with the low dose of nicotine. The withdrawal of the high dose of chronic nicotine gradually returned the decreased plasma vasopressin concentration, and its content in and release from the neural lobe to control values within 2 weeks. However, vasopressin content in the hypothalamus started to decline 1 week after nicotine withdrawal and persisted to decline for at least 3 subsequent weeks. The withdrawal of the high dose was associated with a marked suppression of high K(+)-stimulated vasopressin release from the neural lobe for a period of 4 weeks. The withdrawal of the low dose of nicotine exhibited a significant decline in plasma vasopressin concentration for up to 2 weeks only following chronic nicotine withdrawal, tending to return subsequently to control levels.

Chapter 18 Nicotine and opioid peptides

This chapter discusses the relationship between nicotinic cholinergic receptors and opioid peptides. Nicotine appears to exert its stimulatory effect most likely at the level of the hypothalamus by causing a release of corticotropin releasing factor. In contrast to the acute treatment, normal levels of ACTH and/or glucocorticoids have been observed after chronic administration of nicotineindicating that tolerance to the stimulatory effect of nicotine on the hypothalamo-pituitary-adrenal axis can develop. Moreover, chronic administration of nicotine into mice did not change the levels of immunoreactive β -endorphin in the pituitary. The chapter investigated the effect of chronic nicotine treatment on the levels of β -endorphin, Met-enkephalin and other neuropeptides in various areas of the mouse brain. With the exception of the levels of β -endorphin in the hypothalamus, no significant changes in the brain concentrations of the peptides have been found following chronic nicotine administration. Peptide levels, however, reflect the balance between biosynthesis and release of peptides in a tissue. Therefore, an altered activity of a peptidergic neuron might not be detectable, if a change in release is associated by a compensatory change in the biosynthesis of the peptide. The biosynthetic activity of a peptidergic neuron is reflected by the level of mRNA coding for the respective peptide precursor. Using RNA-hybridization techniques the effect of chronic nicotine treatment of rats on the tissue levels of mRNAs coding for the three opioid peptide precursors proopiomelanocortin (POMC), proenkephalin (PENK) and prodynorphin (PDYN)) is investigated in the chapter.

Differential Effect of Chronic Nicotine Treatment on Nicotinic Receptor Numbers in Various Brain Regions of Mice

Studies of smoking behavior in human twin pairs indicate that genetic factors may influence whether people do, or do not, smoke. Because the ability to develop tolerance to behavioral and physiological effects of nicotine may regulate nicotine intake, studies of potential genetic influences on tolerance development were carried out using inbred mouse strains that differ in initial (first dose) sensitivity to nicotine. Nicotine-sensitive strains such as the C57BL and DBA strains develop tolerance to nicotine following chronic intravenous infusion of lower doses of nicotine than are required to elicit tolerance to nicotine in the nicotine-resistant strains such as C3H and BUB. Chronic nicotine infusion results in a dose-related increase in binding of [3H]-nicotine to brain membranes in all strains. Differences in this effect were seen among brain regions as well as between the strains. A quantitative autoradiographic analysis of the effects of chronic nicotine infusion on DBA/2 mice, which develop marked tolerance to nicotine, clearly indicates that nicotine binding is affected to various degrees in various brain nuclei. These results are discussed in terms of genetic influences on tolerance development.

The effect of chronic nicotine treatment on nicotine-induced seizures.

Tolerance to nicotine occurs in mice after its chronic administration. This tolerance is accompanied by an up-regulation of nicotinic receptors as assessed by the binding of (3H)-nicotine and alpha-(125I)-bungarotoxin (BTX). Past studies (Marks et al. 1983, 1986) have shown that the increase in BTX binding sites is most evident in the hippocampus. Mice that have a greater concentration of BTX binding sites in the hippocampus are more sensitive to the convulsant effects of nicotine (Miner et al. 1984, 1985, 1986). In the study reported here, mice from the DBA/2Ibg strain, which are relatively resistant to nicotine-induced seizures and have a relatively low concentration of hippocampal BTX binding sites (Miner et al. 1984), were infused with nicotine for 10 days. At various time points after cessation of nicotine administration, sensitivity to the convulsant effects of nicotine was assessed. Mice were then sacrificed and BTX binding was determined in three regions: cortex, midbrain, and hippocampus. As expected, chronic treatment with nicotine resulted in a significant up-regulation of BTX binding sites in the hippocampus. Chronic nicotine treatment also produced significant tolerance to nicotine-induced seizures. Hippocampal BTX binding sites returned to control levels within 2 days after stopping nicotine infusion, whereas tolerance was not lost until 5 days. These results suggest that factors other than the number of hippocampal BTX binding sites affect nicotine-induced seizure sensitivity, at least following chronic nicotine treatment.

Effect of chronic nicotine treatment against repeated immobilization stress

Alpha 2 and beta adrenoceptors, and muscarinic cholinoceptors in 2 brain regions (cerebral cortex and hippocampus) were measured in rats which received either tap water or nicotine added to the drinking water (5–8 mg/kg/day) for 4 weeks, and immobilization stress (daily 2 hr) for the last 5 days. The repeated stress induced a reduction in the maximum number of binding sites (Bmax) for ( 3H)dihydroalprenolol (DHA) in the cerebral cortex of rats with tap water, without affecting ( 3H)clonidine binding. Nicotine-treatment also caused a decrease in the Bmax of cortical ( 3H)DHA binding comparable to the case of stress, and increased the ( 3H)clonidine binding. However, the combination of nicotine- and stress-treatments failed to induce further no changes in the 2 radioligands binding. The binding of ( 3H)quinuclidinyl benzilate in the cerebral cortex and of the 3 radioligands in the hippocampus was unaltered by nicotine- and/or stress-treatments. These results indicate that long-term administration of nicotine induces down-regulation of cortical beta adrenoceptors and seemingly attenuates the receptor alteration by repeated stress.