Neonatal ethanol exposure produces a hyperalgesia that extends into adolescence, and is associated with increased analgesic and rewarding properties of nicotine in rats

Prevalence and patterns of sensory processing behaviors in a large clinical sample of children with prenatal alcohol exposure

BACKGROUND

Atypical behavioral responses to sensation are reported in a large proportion of children affected by prenatal alcohol exposure (PAE). Systematic examination of symptoms across the fetal alcohol spectrum in a large clinical sample is needed to inform diagnosis and intervention.

AIMS

To describe the prevalence and patterns of atypical sensory processing symptoms in a clinical sample of children with PAE.

METHODS

Retrospective analysis of diagnostic clinical data from the University of Washington Fetal Alcohol Syndrome Diagnostic and Prevention Network (FASDPN). Participants were ages 3 through 11 years, had a diagnosis on the fetal alcohol spectrum, and Short Sensory Profile (SSP) assessment. The proportions of children categorized with definite differences on the SSP across selected clinical and demographic features were examined with chi-square analyses.

OUTCOMES

The sample consisted of 325 children; 73.2 % had SSP total scores in the definite difference range. Atypical sensory processing symptoms were significantly more prevalent among children with higher reported levels of PAE. The prevalence of atypical symptoms was comparably high across age, levels of diagnostic severity, and other prenatal/postnatal risks.

CONCLUSIONS

Results lend support for altered sensory processing as another domain of brain function affected by the teratogenic impact of PAE, guiding clinical work and research.

Prenatal alcohol exposure is a risk factor for adult neuropathic pain via aberrant neuroimmune function

BACKGROUND

Clinical studies show that prenatal alcohol exposure (PAE) results in effects that persist into adulthood. Experimental animal models of moderate PAE demonstrate that young adults with PAE display potentiated sensitivity to light touch, clinically termed allodynia, following sciatic nerve chronic constriction injury (CCI) that coincides with heightened spinal glial, spinal macrophage, and peripheral immune responses. However, basal touch sensitivity and corresponding glial and leukocyte activation are unaltered. Therefore, the current study explored whether the enduring pathological consequences of moderate PAE on sensory processing are unmasked only following secondary neural insult.

METHODS

In middle-aged (1 year) Long Evans rats that underwent either prenatal saccharin exposure (control) or moderate PAE, we modified the well-characterized model of sciatic neuropathy, CCI, to study the effects of PAE on neuro-immune responses in adult offspring. Standard CCI manipulation required 4 chromic gut sutures, while a mild version applied a single suture loosely ligated around one sciatic nerve. Spinal glial immunoreactivity was examined using immunohistochemistry. The characterization and functional responses of leukocyte populations were studied using flow cytometry and cell stimulation assays followed by quantification of the proinflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α). Data were statistically analyzed by ANOVA and unpaired t tests.

RESULTS

The current report demonstrates that mild CCI generates robust allodynia only in PAE rats, while the pathological effects of PAE following the application of a standard CCI are revealed by enhanced allodynia and elevated spinal glial activation. Additionally, mild CCI increases spinal astrocyte activation but not microglia, suggesting astrocytes play a larger role in PAE-induced susceptibility to aberrant sensory processing. Leukocyte populations from PAE are altered under basal conditions (i.e., prior to secondary insult), as the distribution of leukocyte populations in lymphoid organs and other regions are different from those of controls. Lastly, following in vitro leukocyte stimulation, only PAE augments the immune response to antigen stimulation as assessed by heightened production of TNF-α and IL-1β.

CONCLUSIONS

These studies demonstrate PAE may prime spinal astrocytes and peripheral leukocytes that contribute to enduring susceptibility to adult-onset neuropathic pain that is not apparent until a secondary insult later in life.

Intra-cerebral and intra-nasal melanocortin-4 receptor antagonist blocks withdrawal hyperalgesia in alcohol-dependent rats

Humans diagnosed with alcohol use disorder are more sensitive to painful stimuli during withdrawal, which suggests that excessive alcohol drinking worsens pain outcomes. Alcohol-dependent rats exhibit increases in nociceptive sensitivity during withdrawal. Data from animal models suggest that brain melanocortin-4 receptors (MC4Rs) mediate alcohol drinking and nociception. Here we tested: (1) the effect of alcohol dependence on thermal nociception in rats, and (2) the ability of acute alcohol and (3) MC4R antagonists to reverse hyperalgesia during withdrawal in alcohol-dependent rats. Rats were trained to self-administer operant alcohol and were tested for baseline thermal nociception. Half of the rats were made dependent on alcohol, then all rats were cannulated in the lateral ventricle. We tested the effects of acute alcohol drinking, acute fixed-dose alcohol, intra-ventricular agouti-related protein (endogenous MC4R antagonist), intra-ventricular HS014 (synthetic MC4R antagonist) and intra-nasal HS014 on hyperalgesia during withdrawal in alcohol-dependent rats, relative to non-dependent drinkers and alcohol-naïve controls. Alcohol-dependent rats exhibit thermal hyperalgesia that is abolished by alcohol drinking, bolus alcohol and intra-ventricular and intra-nasal MC4R antagonists. These manipulations did not affect thermal nociception in non-dependent drinkers and alcohol-naïve controls, suggesting that alcohol dependence produces neuroadaptations in brain MC4R systems. These results suggest that brain MC4R systems may be an effective therapeutic target for reducing nociception in the alcohol-dependent organism.

Alcohol and nicotine interactions: pre-clinical models of dependence

While the co-morbidity of alcohol (ethanol) and tobacco (nicotine) dependence is well described, the processes that underpin this strong connection are still under debate. With the increasing popularity of electronic cigarettes (e-cigarettes), it is now becoming more important to look to the neurobiological mechanisms involving alcohol and nicotine interactions to effectively treat a new generation of co-dependent individuals. Researchers have already recognized that the neuropathology produced by the combination of nicotine and ethanol is likely to produce an addictive nature very different to that of either one alone, and are employing a mixture of pre-clinical techniques to establish and investigate every stage in the development of both nicotine and ethanol-seeking behaviors. While it is agreed that multiple pathways orchestrate the complex reward profile of alcohol and nicotine co-addiction, several lines of evidence suggest the convergent site of action is within the mesolimbic dopaminergic system, at neuronal nicotinic acetylcholine receptors (nAChRs). A whole host of strategies are currently being employed to discover and unravel previously unknown or ill understood neurobiological processes in the brain, contributing greatly toward the development of novel pharmacotherapies with the aim of improving patient outcomes. This review intends to shed some light on the most influential and most recent pre-clinical work that is leading the charge in modeling this complicated relationship.

Acute alcohol exposure during neurulation: Behavioral and brain structural consequences in adolescent C57BL/6J mice

Prenatal alcohol exposure (PAE) can induce physical malformations and behavioral abnormalities that depend in part on thedevelopmental timing of alcohol exposure. The current studies employed a mouse FASD model to characterize the long-term behavioral and brain structural consequences of a binge-like alcohol exposure during neurulation; a first-trimester stage when women are typically unaware that they are pregnant. Time-mated C57BL/6J female mice were administered two alcohol doses (2.8g/kg, four hours apart) or vehicle starting at gestational day 8.0. Male and female adolescent offspring (postnatal day 28-45) were then examined for motor activity (open field and elevated plus maze), coordination (rotarod), spatial learning and memory (Morris water maze), sensory motor gating (acoustic startle and prepulse inhibition), sociability (three-chambered social test), and nociceptive responses (hot plate). Regional brain volumes and shapes were determined using magnetic resonance imaging. In males, PAE increased activity on the elevated plus maze and reduced social novelty preference, while in females PAE increased exploratory behavior in the open field and transiently impaired rotarod performance. In both males and females, PAE modestly impaired Morris water maze performance and decreased the latency to respond on the hot plate. There were no brain volume differences; however, significant shape differences were found in the cerebellum, hypothalamus, striatum, and corpus callosum. These results demonstrate that alcohol exposure during neurulation can have functional consequences into adolescence, even in the absence of significant brain regional volumetric changes. However, PAE-induced regional shape changes provide evidence for persistent brain alterations and suggest alternative clinical diagnostic markers.

Prenatal stress induces vulnerability to nicotine addiction and alters D2 receptors' expression in the nucleus accumbens in adult rats

Prenatal stress (PS) can induce several long-lasting behavioral and molecular abnormalities in rats. It can also be considered as a risk factor for many psychiatric diseases like schizophrenia, depression or PTSD and predispose to addiction. In this study, we investigated the effect of prenatal stress on the reinforcing properties of nicotine in the CPP paradigm. Then, we examined the mRNA expression of the D2 dopaminergic receptors using the quantitative real-time PCR technique in the nucleus accumbens (NAcc). We found that prenatally stressed rats exhibited a greater place preference for the nicotine-paired compartment than the control rats. Moreover, we observed an overexpression of the DRD2 gene in adult offspring stressed in utero and a downregulation in the PS NIC group (PS rats treated with nicotine) compared with their control counterparts (C NIC). These data suggest that maternal stress can permanently alter the offspring's addictive behavior and D2 receptors' expression.

A role for matrix metalloproteinases in nicotine-induced conditioned place preference and relapse in adolescent female rats

Reconfiguration of extracellular matrix proteins appears to be necessary for the synaptic plasticity that underlies memory consolidation. The primary candidates involved in controlling this process are a family of endopeptidases called matrix metalloproteinases (MMPs); however, the potential role of MMPs in nicotine addiction-related memories has not been adequately tested. Present results indicate transient changes in hippocampal MMP-2, -3, and -9 expression following context dependent learning of nicotine-induced conditioned place preference (CPP). Members of a CPP procedural control group also indicated similar MMP changes, suggesting that memory activation occurred in these animals as well. However, hippocampal MMP-9 expression was differentially elevated in members of the nicotine-induced CPP group on days 4 and 5 of training. Inhibition of MMPs using a broad spectrum MMP inhibitor (FN439) during nicotine-induced CPP training blocked the acquisition of CPP. Elevations in hippocampal and prefrontal cortex MMP-3 expression-but not MMP-2 and -9-accompanied reactivation of a previously learned drug related memory. Decreases in the actin regulatory cytoskeletal protein cortactin were measured in the HIP and PFC during the initial two days of acquisition of CPP; however, no changes were seen following re-exposure to the drug related environment. These results suggest that MMP-9 may be involved in facilitating the intracellular and extracellular events required for the synaptic plasticity underlying the acquisition of nicotine-induced CPP. Furthermore, MMP-3 appears to be important during re-exposure to the drug associated environment. However, rats introduced into the CPP apparatus and given injections of vehicle rather than nicotine during training also revealed a pattern of MMP expression similar to nicotine-induced CPP animals.

The effects of prenatal alcohol exposure on behavior: rodent and primate studies

The use of alcohol by women during pregnancy is a continuing problem. In this review the behavioral effects of prenatal alcohol from animal models are described and related to studies of children and adults with FASD. Studies with monkeys and rodents show that prenatal alcohol exposure adversely affects neonatal orienting, attention and motor maturity, as well as activity level, executive function, response inhibition, and sensory processing later in life. The primate moderate dose behavioral findings fill an important gap between human correlational data and rodent mechanistic research. These animal findings are directly translatable to human findings. Moreover, primate studies that manipulated prenatal alcohol exposure and prenatal stress independently show that prenatal stress exacerbates prenatal alcohol-induced behavioral impairments, underscoring the need to consider stress-induced effects in fetal alcohol research. Studies in rodents and primates show long-term effects of prenatal and developmental alcohol exposure on dopamine system functioning, which could underpin the behavioral effects.

Moderate prenatal alcohol exposure and serotonin genotype interact to alter CNS serotonin function in rhesus monkey offspring

BACKGROUND

Moderate prenatal alcohol exposure can contribute to neurodevelopmental impairments and disrupt several neurotransmitter systems. We examined the timing of moderate level alcohol exposure, serotonin transporter gene polymorphic region variation (rh5-HTTLPR), and levels of primary serotonin and dopamine (DA) metabolites in cerebrospinal fluid (CSF) in rhesus monkeys.

METHODS

Thirty-two 30-month old rhesus monkeys (Macaca mulatta) from 4 groups of females were assessed: (i) early alcohol-exposed group (n = 9), in which mothers voluntarily consumed 0.6 g/kg/d alcohol solution on gestational days 0 to 50; (ii) middle-to-late gestation alcohol-exposed group (n = 6), mothers consumed 0.6 g/kg/d alcohol solution on gestational days 50 to 135; (iii) a continuous-exposure group (n = 8), mothers consumed 0.6 g/kg/d alcohol solution on gestational days 0 to 135; and (iv) controls (n = 9), mothers consumed an isocaloric control solution on gestational days 0 to 50, 50 to 135, or 0 to 135. Serotonin transporter promoter region allelic variants (homozygous s/s or heterozygous s/l vs. homozygous l/l) were determined. We examined CSF concentrations of the 5-HT and DA metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), respectively, at baseline and 50 hours after separation from cage-mates, when the monkeys were 30 months old.

RESULTS

Early- and middle-to-late gestation-alcohol exposed monkeys carrying the short allele had lower concentrations of 5-HIAA in CSF relative to other groups. Concentrations of 5-HIAA in CSF were lower for s allele carriers and increased from baseline relative to pre-separation values, whereas 5-HIAA levels in l/l allele carriers were not affected by separation. Monkeys carrying the short allele had lower basal concentrations of HVA in CSF compared with monkeys homozygous for the long allele.

CONCLUSION

Carrying the s allele of the 5-HT transporter increased the probability of reduced 5-HIAA in early- and middle-to-late gestation alcohol-exposed monkeys and reduced HVA at baseline. These findings that prenatal alcohol exposure altered central 5-HT activity in genetically sensitive monkeys raise questions about whether abnormal serotonin biological pathways could underlie some of the psychiatric disorders reported in fetal alcohol spectrum disorder.

Nicotine: alcohol reward interactions

It is well established that the continued intake of drugs of abuse is reinforcing-that is repeated consumption increases preference. This has been shown in some studies to extend to other drugs of abuse; use of one increases preference for another. In particular, the present review deals with the interaction of nicotine and alcohol as it has been shown that smoking is a risk factor for alcoholism and alcohol use is a risk factor to become a smoker. The review discusses changes in the brain caused by chronic nicotine and chronic alcohol intake to approach the possible mechanisms by which one drug increases the preference for another. Chronic nicotine administration was shown to affect nicotine receptors in the brain, affecting not only receptor levels and distribution, but also receptor subunit composition, thus affecting affinity to nicotine. Other receptor systems are also affected among others catecholamine, glutamate, GABA levels and opiate and cannabinoid receptors. In addition to receptor systems and transmitters, there are endocrine, metabolic and neuropeptide changes as well induced by nicotine. Similarly chronic alcohol intake results in changes in the brain, in multiple receptors, transmitters and peptides as discussed in this overview and also illustrated in the tables. The changes are sex and age-dependent-some changes in males are different from those in females and in general adolescents are more sensitive to drug effects than adults. Although nicotine and alcohol interact-not all the changes induced by the combined intake of both are additive-some are opposing. These opposing effects include those on locomotion, acetylcholine metabolism, nicotine binding, opiate peptides, glutamate transporters and endocannabinoid content among others. The two compounds lower the negative withdrawal symptoms of each other which may contribute to the increase in preference, but the mechanism by which preference increases-most likely consists of multiple components that are not clear at the present time. As the details of induced changes of nicotine and alcohol differ, it is likely that the mechanisms of increasing nicotine preference may not be identical to that of increasing alcohol preference. Stimulation of preference of yet other drugs may again be different -representing one aspect of drug specificity of reward mechanisms.

Sensory processing disorder in a primate model: evidence from a longitudinal study of prenatal alcohol and prenatal stress effects

Disrupted sensory processing, characterized by over- or underresponsiveness to environmental stimuli, has been reported in children with a variety of developmental disabilities. This study examined the effects of prenatal stress and moderate-level prenatal alcohol exposure on tactile sensitivity and its relationship to striatal dopamine system function in thirty-eight 5- to 7-year-old rhesus monkeys. The monkeys were from four experimental conditions: (a) prenatal alcohol exposed, (b) prenatal stress, (c) prenatal alcohol exposed + prenatal stress, and (d) sucrose controls. Increased D(2) receptor binding in the striatum, evaluated using positron emission tomography neuroimaging, was related to increased withdrawal (aversion) responses to repetitive tactile stimuli and reduced habituation across trials. Moreover, prenatal stress significantly increased overall withdrawal responses to repetitive tactile stimulation compared to no prenatal stress.

Intravenous nicotine conditions a place preference in rats using an unbiased design

The rewarding effects of nicotine contribute to the chronic use of tobacco products. The place conditioning task, a widely used pre-clinical model to study drug reward, has lead to mixed results in rats when nicotine was administered subcutaneously or intraperitoneally; intravenously administered nicotine has not been examined. Further, much of the research demonstrating a nicotine-conditioned place preference in rats has used a biased design making these results susceptible to non-reward interpretations. The present study assessed whether intravenous (IV) nicotine would condition a place preference in an unbiased design and evaluated important behavioral parameters: nicotine dose, number of conditioning trials, and infusion-to-placement interval. In adult male Sprague Dawley rats, IV nicotine (0.03 mg/kg) conditioned a place preference after 8 conditioning trials. This conditioned preference was observed whether nicotine was infused 10 min before or immediately after placement in the paired environment for 10 min; infusing nicotine immediately after removal from the paired environment did not condition a preference after 4 or 8 conditioning trials. Four conditioning trials were not sufficient to condition a preference regardless of the temporal relation between the paired environment and 0.03 mg/kg nicotine. A 0.01 mg/kg dose of nicotine did not condition a place preference after 4 or 8 trials when infused immediately upon placement in the paired environment. Intravenous nicotine (0.03 mg/kg) has rewarding effects in an unbiased design suggesting that the place conditioning protocol used in the present study might be an especially useful model for studying the processes underlying the conditioned rewarding effects of nicotine.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Combined exposure to nicotine and ethanol throughout full gestation results in enhanced acquisition of nicotine self-administration in young adult rat offspring

RATIONALE

Epidemiological evidence shows positive correlation between either maternal cigarette smoking or alcohol consumption on subsequent drug-taking behavior in offspring. However, the consequences of full gestational exposure to both drugs have not been studied experimentally despite concurrent use frequently reported among women of childbearing age. Such comorbid gestational drug exposure may increase susceptibility to acquiring cigarette smoking (i.e., nicotine self-administration), a major gateway drug.

OBJECTIVES

We developed a noninvasive rat model for exposure to both nicotine (2-6 mg kg(-1) day(-1)) and EtOH (4 g/kg gavage) that continued throughout pregnancy and postnatal (P) days 2-12, the rodent equivalent of the human third trimester, a critical brain developmental period. Offspring with this full gestational exposure to both drugs (Nic+EtOH) were compared to controls: nicotine alone, EtOH alone, pair-fed (comparable nutrition and handling), and ad libitum chow-fed. At P60-90, offspring had unlimited chronic access to acquire i.v. nicotine self-administration.

RESULTS

There were no differences in gender ratio, stillbirths, birth weights, righting reflex, eye opening age, or weight gain. However, Nic+EtOH offspring of both genders acquired nicotine self-administration (15 or 30 microg kg(-1) injection(-1)) more rapidly, at a higher percentage, and at a higher level than offspring in the other cohorts.

CONCLUSION

Full gestational Nic+EtOH exposure produced no overt alterations in standard postnatal measures but resulted in an enhanced acquisition of nicotine self-administration in young adult offspring.

Tolerance to the antinociceptive effects of ethanol during ethanol withdrawal

Prior research has indicated that tolerance develops to the antinociceptive effects of ethanol and continues even during withdrawal. Three potential pharmacological mechanisms for this tolerance are examined, using nitrendipine (L-type calcium channel blocker), theophylline (adenosine A1/A2 antagonist) and flumazenil (benzodiazepine antagonist). Rats received 10 days of exposure to an ethanol-containing liquid diet (6.5% w/v). A radiant heat tail-flick assay was used to assess hyperalgesia at 12 h after removal of the liquid diet, as well as tolerance to the effects of cumulative doses of ethanol (0.5-2 g/kg). Co-administration of flumazenil (10 mg/kg, i.p., b.i.d.), nitrendipine (5 mg/kg, i.p., b.i.d.) or theophylline (1 mg/kg, i.p., b.i.d.) with chronic ethanol prevented development of the hyperalgesia produced by ethanol withdrawal, but only theophylline reduced tolerance to the antinociceptive effects of ethanol administered during ethanol withdrawal. In contrast, when administered during ethanol withdrawal, theophylline (1-10 mg/kg) blocked the anti-hyperalgesic effects of ethanol during ethanol withdrawal, whereas nitrendipine (5-25 mg/kg) enabled ethanol to produce levels of antinociception comparable to non-dependent rats. These findings indicate that L-type calcium channels and adenosine receptors play important, but differing roles in the development of hyperalgesia during withdrawal, and to tolerance to the antinociceptive effects of ethanol.

Nicotine place preference in the mouse: influences of prior handling, dose and strain and attenuation by nicotinic receptor antagonists

RATIONALE

Although conditioned place preferences (CPPs) are seen with most abused drugs, nicotine does not always produce a preference in this design.

OBJECTIVES

The goals of the present experiment were to (1) examine various factors that could contribute to these inconsistent results and (2) begin to evaluate the specific nicotinic receptors involved in the nicotine CPP.

METHODS

The influences of prior handling, environmental habituation, and injection habituation on a nicotine CPP were first evaluated in ICR mice. Subsequently, various nicotine doses were assessed for their abilities to produce a CPP, and the effectiveness of nicotinic receptor antagonists in attenuating this preference was examined. Finally, nicotine CPPs were assessed in C57BL/6J and DBA/2J mice to examine the influence of strain in this design.

RESULTS

Nicotine CPPs were seen in handled/environmentally habituated, but not in unhandled, ICR mice. Habituation to the injection techniques failed to strengthen the preference. In ICR mice, a CPP was seen with one intermediate dose of nicotine. This CPP was attenuated by mecamylamine and dihydro-beta-erythroidine (DHbetaE). A nicotine CPP was also seen in C57BL/6J, but not in DBA/2J, mice.

CONCLUSION

Earlier handling experience and strain are important factors when evaluating a nicotine CPP in the mouse. In addition, certain nicotinic receptors underlie the nicotine CPP, indicating that this model can elucidate underlying mediators of nicotine reward.

Ethanol withdrawal-associated allodynia and hyperalgesia: age-dependent regulation by protein kinase C epsilon and gamma isoenzymes

Ethanol (EtOH) withdrawal increases sensitivity to painful stimuli in adult rats. In this study, withdrawal from a single, acute administration of EtOH dose-dependently produced mechanical allodynia and thermal hyperalgesia in postnatal day 7 (P7) rats. In contrast, P21 rats exhibited earlier and more prolonged mechanical allodynia but not thermal hyperalgesia. For both P7 and P21 rats, blood and spinal cord EtOH levels peaked at 30 minutes after administration, with P7 rats achieving overall higher spinal cord concentrations. Protein kinase C (PKC) has been implicated in mediating pain responses. Inhibitory PKC- and gamma-specific peptides attenuated mechanical allodynia and thermal hyperalgesia in P7 rats, whereas only the PKCgamma inhibitor prevented mechanical allodynia in P21 rats. Immunoreactive PKC in dorsal root ganglion and PKCgamma in lumbar spinal cord increased at 6 hours after EtOH administration in P7 rats. In P21 rats, the density of PKC immunoreactivity remained unchanged, whereas the density of PKCgamma immunoreactivity increased and translocation occurred. These studies demonstrate developmental differences in neonatal nociceptive responses after withdrawal from acute EtOH and implicate a role for specific PKC isozymes in EtOH withdrawal-associated allodynia and hyperalgesia.

PERSPECTIVE

This study examines age-specific nociceptive responses after ethanol exposure by using 2 different ages of rats. The results suggest that ethanol age-dependently alters sensitivity to mechanical and thermal stimuli via specific protein kinase C isozymes. These results begin to ascertain the mechanisms that produce abnormal pain after alcohol exposure.

Cannabinoid CB1 receptors are involved in motivational effects of nicotine in rats

RATIONALE

The endocannabinoid system plays a role in mediating the appetitive value of a variety of reinforcing compounds, either natural rewards or drugs of abuse, but little is known about its involvement in the incentive properties of nicotine.

OBJECTIVES

The objective of the study is to evaluate whether activation of CB1 cannabinoid receptors is necessary for the establishment and the short- and long-term expression of nicotine-induced conditioned place preference (CPP). This was studied in rats subjected to an unbiased, one-compartment place conditioning procedure, using the selective CB1 receptor antagonist, rimonabant, as a pharmacological tool.

METHODS

Wistar rats, given previous experience with nicotine in their home cage, were subjected to eight alternating nicotine (0.006-0.6 mg/kg s.c.) and saline pairings with distinct floor textures in an open field and given a test session, with no nicotine injection, in the open field whose floor was covered by two quadrants of the saline-paired texture and two quadrants of the nicotine-paired texture. Rimonabant (0.3-3 mg/kg i.p.) was administered 30 min before each nicotine (0.06 mg/kg) pairing to assess its effect on the establishment of nicotine-CPP. To study the effects of CB1 receptor blockade on short- and long-term expression of nicotine-CPP, rimonabant was administered as a single injection 30 min before the test session, conducted either 24 h, 3 weeks or 12 weeks after the last conditioning session.

RESULTS

Rats developed reliable and robust CPP to the 0.06- and 0.125-mg/kg doses of nicotine. Once established, CPP persisted for at least 12 weeks without additional exposure to nicotine and the test apparatus. Pre-pairing injections of rimonabant (3 mg/kg, but not lower doses) prevented the acquisition of nicotine-CPP, and a single pretest administration of rimonabant (3 mg/kg) abolished the expression of nicotine-CPP when the test session took place 24 h after the last conditioning session. However, rimonabant (3 mg/kg) did not antagonize the expression of nicotine-CPP when the test session was conducted 3 or 12 weeks after the acquisition phase.

CONCLUSIONS

The endocannabinoids are a necessary component in both the perception by rats of the motivational value of nicotine and the short-term capacity of nicotine-paired conditioned stimuli to elicit approach behaviour. In contrast, the acute blockade of CB1 receptors no longer impairs the long-term control of behaviour by nicotine-associated environmental cues. These data provide support to the notion that the blockade of CB1 receptors can oppose tobacco dependence, withdrawal and even relapse, though the time window of efficacy and/or the schedule of administration remain to be established.

Nicotine induces conditioned place preferences over a large range of doses in rats

RATIONALE

Conditioned place preference (CPP) procedures provide one measure of potential rewarding effects of abused drugs. Many attempts to induce CPP with nicotine have been unsuccessful.

OBJECTIVES

To assess the influence of nicotine dose and stimulus assignment procedure on development of nicotine-induced CPP.

METHODS

Initial preferences for one side of a two-compartment apparatus were first determined in Sprague-Dawley rats. In subsequent conditioning trials, the compartment paired with nicotine was the initially preferred side for half of the rats, and the initially non-preferred side for the other half. Rats received either an injection of nicotine (0.01-2 mg/kg SC) before being placed in one compartment (three trials) or saline before being placed in the other compartment (three trials). Control rats had saline injections associated with both compartments. A final test trial with no injection assessed final place preference.

RESULTS

Significant CPP were induced by 0.1-1.4 mg/kg doses of nicotine. Nicotine-induced CPP were only apparent when nicotine was paired with the initially non-preferred side. Moreover, a very high dose of nicotine (2 mg/kg) induced conditioned place aversion when paired with the initially preferred side of the apparatus.

CONCLUSIONS

Nicotine induced significant CPP across a wide range of doses, in accordance with its role as the primary addictive component of tobacco. Small preferences for one side of the apparatus played a major role in the development of nicotine-induced CPP. These findings suggest that biased procedures may be more suitable than unbiased procedures for evaluation of rewarding effects of nicotine using CPP paradigms.