Cholecystokinin2 receptor-deficient mice display altered function of brain dopaminergic system

RATIONALE: Toluene is a solvent found in many commercial products and is frequently abused by inhalation. Whether previous exposure to toluene alters subsequent responses to other drugs of abuse is not known. OBJECTIVES: This study determined the effects of repeated toluene exposure on the acute motor-stimulant response to cocaine and on cocaine-induced dopamine (DA) concentrations in the nucleus accumbens (NAc). METHODS: One week following bilateral cannulae implantation over the NAc, 27 adult, male Wistar rats began a daily 30-min exposure regimen to either toluene (8,000 ppm) or air for ten sessions. Approximately 24 h or 96 h after their last exposure, animals were injected with either saline or cocaine (15 mg/kg, i.p.) and locomotor activity and DA concentrations in the NAc were measured. RESULTS: Exposure to toluene rendered the rats immobile, and the time required for recovery of normal posture decreased across the ten sessions. In all animals tested, systemic cocaine administration enhanced both locomotor activity and DA concentrations in the NAc. These increases, however, were significantly greater in rats previously exposed to toluene. CONCLUSIONS: Overall, these findings show that repeated toluene exposure enhances behavioral and neurochemical responses to subsequent cocaine administration.     

Patterns of functional activity associated with cocaine self-administration in the rat change over time

Rationale Although imaging studies in human addicts have been valuable for identifying the neural substrates of the effects of abused drugs, few studies have used this approach in animal models where conditions can be carefully controlled.Objective To define the substrates that mediate the effects of cocaine in a rodent model of cocaine self-administration using the 2-[¹⁴C]deoxyglucose method and to assess changes in these patterns over the course of drug exposure.Methods Male Sprague-Dawley rats self-administered cocaine (0.75 mg/kg per injection; FR2; 21 injections/session) and control rats received saline infusions in the same pattern as the cocaine rats for 5 or 30 days. Metabolic mapping was applied immediately after the final session.Results Following 5 days of self-administration, rates of glucose utilization were decreased in the nucleus accumbens, and increased in autonomic brainstem structures and in sensorimotor regions. After 30 days of cocaine exposure, self-administration reduced glucose utilization throughout the dorsal and ventral striatum, central nucleus of the amygdala, medial forebrain bundle, and infralimbic and prelimbic prefrontal cortices. In addition, at this time point glucose utilization was no longer elevated in any autonomic or sensorimotor brain regions.Conclusions These data demonstrate that the distribution of functional activity associated with self-administered cocaine undergoes considerable change over the course of drug exposure. While increases in metabolic rates were largely found in autonomic and sensorimotor structures after short-term cocaine access, decreases were prominent in mesocorticolimbic regions after prolonged exposure. These differences in the patterns of brain activity that develop with long-term cocaine self-administration may play a role in the transition to habitual drug seeking behavior.     

Direct nose-brain transport of benzoylecgonine following intranasal administration in rats.

In our previous research, cocaine applied intranasally in rats diffused or was transported directly from the nasal cavity to the brain. However, the direct nose-brain cocaine transport only contributes to an initial increase in the relative cocaine brain exposure. In this study, we have determined the nose-brain transport of a polar metabolite of cocaine, benzoylecgonine, to help understand factors affecting drug transport via this novel pathway. The nasal cavity of male Sprague-Dawley rats was isolated to prevent drainage of nasally applied dosing solution to non-nasal regions. Benzoylecgonine was then administered, either by intranasal administration or by intravenous (iv) injection. At different times postdose, blood and tissues from different regions of the brain were collected from groups of rats (n = 4 for each collection time) and benzoylecgonine concentrations in these samples were analyzed by high-performance liquid chromatography. Benzoylecgonine concentrations in plasma were at maximal levels immediately after iv dosing and declined as a function of time. Following intranasal administration, benzoylecgonine concentrations in plasma reached maximal levels between 15 and 30 min after dosing and declined as a function of time. To allow comparison of brain benzoylecgonine content after iv and intranasal administration, brain benzoylecgonine contents were normalized by plasma benzoylecgonine concentrations. The ratios of the area under the benzoylecgonine concentration-time curve (AUC) between the olfactory bulb and plasma following intranasal administration were 10-100 times higher than those obtained after iv dosing. The olfactory tract-to-plasma benzoylecgonine AUC ratios after intranasal administration were significantly higher than those after iv dosing up to 120 min following dosing. The brain tissue-to-plasma AUC ratios in cerebellum, brain stem, and cerebral cortex after intranasal administration were significantly higher than the corresponding ratios after iv administration up to 30 min following dosing. We conclude than nasally administered benzoylecgonine was transported directly from the nasal cavity to the brain and that the significant increase in brain levels was sustained for a prolonged period of time. Factors contributing to the observed differences in the nose-brain transport of cocaine and benzoylecgonine are discussed.     

D-cycloserine Deters Reacquisition of Cocaine Self-Administration by Augmenting Extinction Learning

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may offer an effective strategy to combat cocaine relapse. To investigate this possibility at the preclinical level, rats and squirrel monkeys were trained to self-administer cocaine paired with a brief visual cue. Lever pressing was subsequently extinguished by withholding cocaine injections while maintaining response-contingent presentations of the cue. The glycine partial agonist D-cycloserine (DCS; 15 and 30 mg/kg in rats, 3 and 10 mg/kg in monkeys) was evaluated for its effects on the rate of extinction and subsequent reacquisition of cocaine self-administration. Compared with vehicle, pretreatment with 30 mg/kg DCS 0.5 h before extinction training reduced the number of responses and latency to reach the extinction criterion in rats, but neither dose of DCS altered these measures in monkeys. In both species, pretreatment with the higher dose of DCS before extinction training significantly attenuated reacquisition of cocaine self-administration compared with either extinction training in the absence of DCS or DCS in the absence of explicit extinction. Furthermore, treatment with 30 mg/kg DCS accompanied by brief handling (a stress induction) immediately after but not 6 h after extinction training attenuated reacquisition of cocaine self-administration in rats. No adverse effects of 10 mg/kg DCS were evident in quantitative observational studies in monkeys. The results suggest that DCS augmented consolidation of extinction learning to deter reacquisition of cocaine self-administration in rats and monkeys. The results suggest that DCS combined with exposure therapy may constitute a rational strategy for the clinical management of cocaine relapse.     

Differential neurochemical and behavioral adaptation to cocaine after response contingent and noncontingent exposure in the rat

Rationale In naive rats, passive administration of drugs of abuse preferentially increases extracellular dopamine (DA) in the nucleus accumbens (NAc) shell as compared to the core. Repeated exposure to the same drugs results in behavioral and biochemical sensitization characterized by stereotyped activity and reduction of the shell/core DA response ratio. Objectives The aim of this work is to study the neurochemical and behavioral effects of response-contingent vs response-noncontingent drug administration in rats, who were bilaterally implanted with chronic intracerebral guide cannulae and trained to self-administer cocaine by nose poking in daily 1-h sessions for 3 weeks (5 days/week). Nose poking in the active hole by master rats resulted in intravenous injection of cocaine (0.25 mg/kg) in master rats and in rats yoked to them. Dialysate DA was monitored before, during, and for 30 min after cocaine availability on alternate days by inserting the probe into the NAc shell and core. Stereotyped and non-stereotyped behavior was recorded during the sessions. Results In master rats, dialysate DA increased preferentially in the NAc shell during cocaine self-administration throughout the 3 weeks of cocaine exposure. In yoked rats, DA increased preferentially in the shell but to a lesser extent than in master rats. With continued exposure to cocaine, the shell/core ratio of DA changes decreased progressively and, on the third week, was reversed so that DA increased more in the core than in the shell. Yoked rats showed a progressive and faster increase in stereotyped behaviors than master rats. Conclusions Response-noncontingent cocaine administration is particularly prone, compared to response-contingent administration, to induce behavioral and biochemical sensitization.     

Role of ventral medial prefrontal cortex in incubation of cocaine craving

Cue-induced drug-seeking in rodents progressively increases after withdrawal from cocaine, suggesting that cue-induced cocaine craving incubates over time. Here, we explored the role of the medial prefrontal cortex (mPFC, a brain area previously implicated in cue-induced cocaine seeking) in this incubation. We trained rats to self-administer cocaine for 10days (6h/day, infusions were paired with a tone-light cue), and then assessed after 1 or 30 withdrawal days the effect of exposure to cocaine cues on lever presses in extinction tests. We found that cue-induced cocaine-seeking in the extinction tests was higher after 30 withdrawal days than after 1day. The time-dependent increases in extinction responding were associated with large (ventral mPFC) or modest (dorsal mPFC) increases in ERK phosphorylation (a measure of ERK activity and an index of neuronal activation). After 30 withdrawal days, ventral but not dorsal injections of muscimol+baclofen (GABAa+GABAb receptor agonists that inhibit neuronal activity) decreased extinction responding. After 1 withdrawal day, ventral but not dorsal mPFC injections of bicuculline+saclofen (GABAa+GABAb receptor antagonists that increase neuronal activity) strongly increased extinction responding. Finally, muscimol+baclofen had minimal effect on extinction responding after 1day, and in cocaine-experienced rats, ventral mPFC injections of muscimol+baclofen or bicuculline+saclofen had no effect on lever presses for an oral sucrose solution. The present results indicate that ventral mPFC neuronal activity plays an important role in the incubation of cocaine craving.     

5-HT(2C) receptor activation is a common mechanism on proerectile effects of apomorphine, oxytocin and melanotan-II in rats

This study was conducted to determine the phosphorylation state of N-methyl-d-aspartate (NMDA) NR1 subunit on serine residues 896 (Ser896) and 897 (Ser897), the extracellular signal-regulated kinase 1/2 (ERK1/2), and the cyclic AMP response element-binding protein (CREB) after repeated exposure to cocaine (20 mg/kg, once daily for 9 days) in the dorsal striatum of rats. The real-time changes of glutamate concentration evoked by repeated cocaine injections were examined using a glutamate biosensor in order to evaluate the correlation between glutamate concentration and the change in these phosphoproteins. The results of this study showed that the immunoreactivity of phosphorylated (p)NMDA NR1 subunit at Ser896 and Ser897 as well as pERK1/2, but not pCREB, in the dorsal striatum was increased at 30 min and then returned to basal levels 4 h after repeated cocaine injections. Similarly, glutamate responses evoked by repeated cocaine injections were also increased 30 min after repeated cocaine injections for 3 days and were prolonged by the 9th day of treatment. However, the glutamate responses were not detected at 4 h after repeated cocaine injections for 5 days. In addition, the elevated immunoreactivity of the phosphoproteins 2 h after repeated cocaine injections was attenuated by the blockade of dopamine D1 receptors and NMDA receptors with the SCH23390 or MK801 antagonists, respectively. These findings suggest that glutamate release and dopamine D1 and NMDA receptor stimulation after repeated exposure to cocaine are associated with NMDA NR1 subunit, ERK1/2 and CREB phosphorylation in the dorsal striatum.     

Enhanced susceptibility to cocaine- and pentylenetetrazol-induced seizures in prenatally cocaine-treated rats

We previously reported that prenatal cocaine exposure increased susceptibility to cocaine-induced seizures later in life. Here we examine whether this enhanced susceptibility to seizures generalizes to other chemoconvulsants, and whether postnatal cocaine treatment similarly increases susceptibility. Following prenatal cocaine treatment (40 mg/kg; E10-20), both male and female rats were more likely to seize to a dose of 30 mg/kg pentylenetetrazol (PTZ) at 2 months of age, although the severity of the seizures observed was increased only in females. Daily cocaine injections (10-20 mg/kg SC) during the first 10 days after birth also produced effects that were dependent on the sex of the animal. Postnatally cocaine-treated female rats showed no greater incidence of seizures in response to an acute high dose of cocaine, but did exhibit an increased susceptibility to cocaine-kindled seizures. Male, but not female, postnatally cocaine-treated rats were more susceptible to PTZ-induced seizures. The increased susceptibility to seizures induced by two different chemoconvulsants after prenatal cocaine treatment suggests that developmental cocaine exposure, particularly during the second trimester equivalent, alters the balance between excitation and inhibition in the brain.     

Enhanced reactivity and vulnerability to cocaine following methylphenidate treatment in adolescent rats.

Treatment of attention deficit hyperactivity disorder with the psychostimulant drug methylphenidate (MP) has increased dramatically among schoolchildren. We tested whether repeated exposure to moderate doses of MP (5 and 10 mg/kg IP for 5 or 7 days) in adolescent rats increased reactivity to cocaine measured by motor responses (ambulations and rearing) to a cocaine challenge in adulthood. We later tested whether repeated exposure to a low dose of MP (2 mg/kg IP for 7 days) enhanced the psychomotor effects of cocaine, measured by different challenge doses (0-30 mg/kg) as well as to the reinforcing effects of cocaine, measured by self-administration of low-dose infusion (75 microg/kg, IV). We found that exposure to moderate doses of MP enhanced psychomotor responses to cocaine but exposure to a low dose only increased cocaine self-administration. These results suggest that adolescent exposure to low doses of MP in rats may increase the incentive value of low reinforcers, thereby rendering adult rats more susceptible to cocaine self-administration.     

Previous exposure to cocaine enhances cocaine self-administration in an alpha 1-adrenergic receptor dependent manner.

Noradrenergic transmission is implicated in the biochemical and behavioral effects of cocaine. Recently, we demonstrated that the alpha 1-adrenergic receptor antagonist prazosin attenuates cocaine-induced reinstatement of drug-seeking behavior. We now assessed whether prazosin could counter the effect of previous exposure to cocaine to enhance subsequent self-administration behavior. Rats were pre-exposed to systemic injections of either saline, prazosin (0.3 mg/kg), saline+cocaine (10 mg/kg), or prazosin+cocaine for 5 days. Starting 15-18 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.5 mg/kg/infusion) under a fixed ratio 3 (FR3) schedule of reinforcement. Several tests were conducted. First, responding for cocaine under an FR3 schedule was assessed across several doses (0.125-1.0 mg/kg/infusion). Second, responding for cocaine (0.5 mg/kg/infusion) under a progressive-ratio (PR) schedule was examined for 6 consecutive days. Finally, responding for cocaine (0, 0.5, and 1.0 mg/kg/infusion) was determined under the PR schedule of reinforcement. Results showed that cocaine pre-exposed rats self-administer more cocaine compared to saline pre-exposed rats when tested under both the FR and PR schedules. Rats pre-exposed to cocaine plus prazosin did not show enhanced cocaine self-administration. These rats, as well those pre-exposed to prazosin alone, showed levels of cocaine self-administration similar to saline pre-exposed rats. Thus, previous exposure to cocaine enhanced cocaine self-administration, an effect that appears to involve activation of alpha 1-adrenergic receptors. These data, along with several recent studies, show further support for the contribution of noradrenergic transmission in the behavioral effects of cocaine.     

mTOR信号通路在大鼠可卡因自身给药复燃中的作用

背景:哺乳动物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)是一种丝氨酸/苏氨酸激酶,其作用可被雷帕霉素所阻断。mTOR活化后参与调节基因转录、蛋白质翻译起始、核糖体生物合成、细胞凋亡等多个生物学过程。mTOR信号转导通路在学习记忆等神经活动的长期可塑性过程起着重要的作用。药物成瘾的过程是药物作用于中脑多巴胺奖赏系统发挥奖赏效应并使之发生长期神经可塑性改变的过程。长期使用成瘾性药物能产生强烈的心理渴求,使得药物成瘾者在戒断后再次暴露于药物、相关环境或压力时极易发生复吸。目前关于mTOR在药物成瘾中的作用研究较少,已有研究证实mTOR信号转导通路参与了药物作用于中脑边缘多巴胺系统引起的奖赏效应和长期神经可塑性,但mTOR信号通路在药物成瘾后发生复吸中的作用尚不清楚。目的:本研究旨在通过大鼠可卡因自身给药戒断后的复燃模型探讨mTOR信号通路在药物成瘾戒断后发生复吸行为中的作用,从而为成瘾神经生物学机制的研究和临床戒毒治疗提供新的实验依据。方法:(1)建立大鼠可卡因自身给药模型(0.75mg·kg-1 infusion,3h·d-1),经过消退后给予药物相关线索暴露诱导复燃行为,比较经过暴露和未经过暴露大鼠伏隔核中mTOR下游靶蛋白p70s6k磷酸化水平变化;(2)建立大鼠可卡因自我给药模型,经过消退后给予药物相关线索暴露诱导复燃行为。暴露前30min在大鼠伏隔核core/shell部分别微注射mTOR抑制剂雷帕霉素或溶媒,观察其对大鼠的复燃行为的影响,并检测暴露后大鼠伏隔核不同脑区中p70s6k磷酸化水平变化;(3)建立大鼠可卡因自身给药模型,经过消退后给予10mg可卡因点燃诱导复燃行为。可卡因点燃30min前在大鼠伏隔核core/shell部分别微注射雷帕霉素或溶媒,观察其对大鼠的复燃行为的影响;(4)建立大鼠可卡因自身给药模型,当大鼠获得稳定的自身给药行为后,在大鼠伏隔核core/shell部分别微注射雷帕霉素或溶媒,d2继续进行可卡因自身给药训练,观察雷帕霉素对可卡因强化效应的影响。结果:药物线索暴露后伏隔核core部的磷酸化p70s6k水平与未暴露组相比明显上升,说明药物线索暴露诱导的复燃能激活伏隔核的mTOR信号通路。在大鼠伏隔核core部给予雷帕霉素微注射能够抑制药物相关线索诱导的复燃行为,而伏隔核shell部给药无效。大鼠伏隔核core部注射雷帕霉素导致伏隔核core部磷酸化p70s6k水平下降,shell部磷酸化p70s6k水平不变。两个脑区的总p70s6k表达水平均无变化。在大鼠伏隔核shell部给予mTOR抑制剂雷帕霉素微注射能够抑制可卡因点燃诱导的复燃行为,而伏隔核core部给药无效。说明抑制伏隔核mTOR信号通路能够抑制药物相关线索或可卡因诱导的复燃行为,且此作用具有脑区特异性。行为学结果还表明,大鼠获得稳定的可卡因自身给药行为后,在大鼠伏隔核core/shell部微注射雷帕霉素均不能够改变已获得的自身给药行为。说明抑制伏隔核mTOR信号通路不影响可卡因的强化效应,雷帕霉素对大鼠戒断后复燃行为的抑制效应并不是由于其降低了可卡因的强化效应。结论:本研究通过一系列实验证实了在伏隔核shell和core部分别给予雷帕霉素能有效抑制大鼠自身给药戒断后由药物点燃和药物相关线索诱导的复燃行为,这一效应是通过抑制伏隔核的mTOR信号通路实现的。mTOR信号通路脑区特异性的参与了自身给药戒断后的复燃行为。本研究结果为成瘾的神经生物学机制研究和临床开发防复吸药物提供了新的依据。     

Prenatal and postnatal cocaine exposure enhances the induction and expression of locomotor sensitization to cocaine in rats

Prenatal and postnatal exposure to cocaine can affect the development and function of the central nervous system in offspring. It also produces changes in cocaine-induced dopamine release and increases cocaine self-administration and cocaine-induced conditioned place preference. Further, prenatal cocaine exposure involves greater risk for development of a substance use disorder in adolescents. Therefore, the objective of this study was to determine the effect of prenatal and postnatal cocaine exposure on locomotor sensitization in rats. A group of pregnant female Wistar rats were administered daily from day GD0 to GD21 with cocaine (cocaine pre-exposure group) and another group pregnant female rats were administered daily with saline (saline pre-exposure group). During lactation (PND0 to PND21) pregnant rats also received cocaine administration or saline, respectively. Of the litters resulting of the cocaine pre-exposed and saline pre-exposed pregnant female groups, only the male rats were used for the recording of the locomotor activity induced by different doses of cocaine (1, 5, 10, 20 and 40 mg/Kg/day) during the induction and expression of locomotor sensitization at different postnatal ages (30, 60, 90 and 120 days), representative of adolescence and adult ages. The study found that prenatal and postnatal cocaine exposure enhanced locomotor activity and locomotor sensitization, and such increase was dose- and age-dependent. This suggests that prenatal and postnatal cocaine exposure can result in increased vulnerability to cocaine abuse in young and adult humans.     

Chronic Inhalation Toxicity and Carcinogenicity Study on Potassium Octatitanate Fibers (TISMO) in Rats

A chronic inhalation toxicity/carcinogenicity study of potassium octatitanate fibers (TISMO) was conducted in male Fischer 344 rats. Groups of 135 rats were exposed via whole-body inhalation to 0, 20, 60, or 200 WHO fibers/cc of TISMO, 6 h/day, 5 days/w for 24 mo. Six of 30 subgroup rats were killed after 3, 6, 12, 18, and 24 mo of exposure for lung burden evaluations. Another 30 subgroup rats were removed from the exposure chambers after 6 mo of exposure, placed in clean air, and from this group 6 rats were killed at 3, 6, 9, 12, and 18 mo later to study lung clearance. The remaining 75 rats in each group were subjected to 24 mo of exposure for chronic toxicity and carcinogenicity study. Rats exposed to HEPA-filtered air (chamber control) were used as a negative control in each study. The lung burden results indicated that a time point of equilibrium between lung burden and lung clearance at 20 WHO fibers/cc exposure was attained after approximately 18 mo of exposure. There was no difference in the number of WHO fiber from the lungs between 18 and 24 mo at 20 WHO fibers/cc exposure. But disproportional rapid increase in lung burden at 200 WHO fibers/cc exposure appeared to be saturation of lung clearance mechanism resulting from lung overloading. At 200 WHO fibers/cc exposure, approximately 22.9 and 70.5 million WHO fibers were retained in the lung after 3 and 6 mo of exposure, respectively, but lungs revealed normal in appearance. However, alveolar walls enclosing aggregated TISMO-laden alveolar macrophages (AMs) showed fibrotic thickening and approximately 197.3 million WHO fibers were retained in the lungs after 18 mo of exposure. Inhaled fibers were rapidly cleared during 3- and 6-mo recovery periods, and thereafter gradually progressive fiber reduction was observed throughout 18 mo of recovery. The number of WHO fibers decreased by approximately 72%, 74%, and 79% in the 200, 60, and 20 WHO fibers/cc groups, respectively, at the end of the 18-mo recovery period following 6 mo of exposure. Although inhaled TISMO fibers in the 20 WHO fibers/cc exposure group were phagocytized by alveolar macrophages (AMs) the lung morphology appeared normal throughout 24 mo of exposure. At 60 WHO fibers/cc exposure, a slight dose- and time-dependent increase in TISMO-laden AMs was observed throughout 3, 6, and 12 mo of exposure and some alveoli containing aggregated TISMO-laden AMs showed alveolar wall thickening at 18 mo of exposure and minimal alveolar fibrosis at 24 mo of exposure. The exposure concentration is interpreted as a borderline effect level. At 200 WHO fibers/cc exposure, lungs preserved normal architecture at 3 and 6 mo of exposure. Some alveolar walls enclosing aggregates of TISMO-laden AMs were slightly thickened after 12 mo of exposure and revealed slight alveolar fibrosis after 18 and 24 mo of exposure. Neither exposure related-pulmonary neoplasm nor mesothelioma was observed in 24 mo of exposure. The 20 WHO fibers/cc exposure concentration is considered to be a no-observable-adverse-effect level (NOAEL). TISMO exposure limits of 1 WHO fiber/cc would not impose a significant health hazard to humans in the workplace based on the animal experiments and medical surveys on workers.     

Chronic inhalation toxicity and carcinogenicity study on potassium octatitanate fibers (TISMO) in rats

A chronic inhalation toxicity/carcinogenicity study of potassium octatitanate fibers (TISMO) was conducted in male Fischer 344 rats. Groups of 135 rats were exposed via whole-body inhalation to 0, 20, 60, or 200 WHO fibers/cc of TISMO, 6 h/day, 5 days/w for 24 mo. Six of 30 subgroup rats were killed after 3, 6, 12, 18, and 24 mo of exposure for lung burden evaluations. Another 30 subgroup rats were removed from the exposure chambers after 6 mo of exposure, placed in clean air, and from this group 6 rats were killed at 3, 6, 9, 12, and 18 mo later to study lung clearance. The remaining 75 rats in each group were subjected to 24 mo of exposure for chronic toxicity and carcinogenicity study. Rats exposed to HEPA-filtered air (chamber control) were used as a negative control in each study. The lung burden results indicated that a time point of equilibrium between lung burden and lung clearance at 20 WHO fibers/cc exposure was attained after approximately 18 mo of exposure. There was no difference in the number of WHO fiber from the lungs between 18 and 24 mo at 20 WHO fibers/cc exposure. But disproportional rapid increase in lung burden at 200 WHO fibers/cc exposure appeared to be saturation of lung clearance mechanism resulting from lung overloading. At 200 WHO fibers/cc exposure, approximately 22.9 and 70.5 million WHO fibers were retained in the lung after 3 and 6 mo of exposure, respectively, but lungs revealed normal in appearance. However, alveolar walls enclosing aggregated TISMO-laden alveolar macrophages (AMs) showed fibrotic thickening and approximately 197.3 million WHO fibers were retained in the lungs after 18 mo of exposure. Inhaled fibers were rapidly cleared during 3- and 6-mo recovery periods, and thereafter gradually progressive fiber reduction was observed throughout 18 mo of recovery. The number of WHO fibers decreased by approximately 72%, 74%, and 79% in the 200, 60, and 20 WHO fibers/cc groups, respectively, at the end of the 18-mo recovery period following 6 mo of exposure. Although inhaled TISMO fibers in the 20 WHO fibers/cc exposure group were phagocytized by alveolar macrophages (AMs) the lung morphology appeared normal throughout 24 mo of exposure. At 60 WHO fibers/cc exposure, a slight dose- and time-dependent increase in TISMO-laden AMs was observed throughout 3, 6, and 12 mo of exposure and some alveoli containing aggregated TISMO-laden AMs showed alveolar wall thickening at 18 mo of exposure and minimal alveolar fibrosis at 24 mo of exposure. The exposure concentration is interpreted as a borderline effect level. At 200 WHO fibers/cc exposure, lungs preserved normal architecture at 3 and 6 mo of exposure. Some alveolar walls enclosing aggregates of TISMO-laden AMs were slightly thickened after 12 mo of exposure and revealed slight alveolar fibrosis after 18 and 24 mo of exposure. Neither exposure related-pulmonary neoplasm nor mesothelioma was observed in 24 mo of exposure. The 20 WHO fibers/cc exposure concentration is considered to be a no-observable-adverse-effect level (NOAEL). TISMO exposure limits of 1 WHO fiber/cc would not impose a significant health hazard to humans in the workplace based on the animal experiments and medical surveys on workers.     

Self-administration of cocaine increases the release of acetylcholine to a greater extent than response-independent cocaine in the nucleus accumbens of rats

  Rationale: The neurochemical effects of psychostimulant exposure may depend on how these drugs are encountered. A useful method for examining this issue is to compare neurotransmitter release following response-dependent, or self-administered, drug exposure and response-independent exposure. Objectives: This experiment examined the effect of active and passive cocaine administration on acetylcholine (ACh) efflux in the shell region of the nucleus accumbens (NAc) in rats. Methods: One group of rats (CSA: cocaine self-administration) was trained to lever-press for intravenous infusions of cocaine (0.42 mg/kg per infusion) on a fixed-ratio-1 schedule of reinforcement. Cocaine infusions were accompanied by the onset of a stimulus light that signaled a 20-s time-out period. Control rats received intravenous cocaine (cocaine non-contingent: CNC) or saline (SAL) in a manner that was not contingent upon their behavior. Drug infusions in these groups were determined by the lever-press behavior of the animals in the CSA group, i.e. they were yoked to rats in the self-administration group such that CNC animals received equal amounts of cocaine as CSA rats. Animals received cocaine or saline in 3-h sessions for 13 consecutive days before testing. On day 14, extracellular ACh was measured in 15-min intervals before, during and after a 3-h session of cocaine exposure using unilateral microdialysis probes located in the NAc shell coupled with HPLC. Results: ACh efflux was significantly increased above baseline in both groups of rats that received cocaine but CSA rats had significantly higher ACh levels during the self-administration period compared to their yoked counterparts. In addition, ACh efflux remained elevated longer in CSA animals relative to CNC rats following cessation of cocaine exposure. Conclusions: These results demonstrate that ACh interneurons in the NAc shell are responsive to cocaine exposure. In addition, these findings suggest that the manner in which the drug is administered (i.e. either by active self-administration or passive exposure) may be relevant to the magnitude of the neural response. Received: 28 April 1998 / Final version: 4 November 1998     

Influence of cocaine self-administration on learning related to prefrontal cortex or hippocampus functioning in rats

Rationale Individuals who abuse cocaine have cognitive deficits, particularly in functions associated with the orbitofrontal cortex. It is not clear to what extent the impact of cocaine on cognitive functioning is related to its role as a behavioral reinforcer. A preclinical means to investigate this issue is to use a yoked-triad procedure in which sets of three animals either contingently self-administer cocaine or receive passive administration of cocaine or saline in a noncontingent manner. Objective Using this procedure, we assessed cocaine’s effect on learning that requires a functionally intact prefrontal cortex (prelimbic or insular/orbital subregions) or hippocampus. Methods Rats self-administering 1-mg/kg unit doses of cocaine responded under a fixed-ratio 5, time-out 20-s schedule of drug delivery. Testing took place in a radial-arm maze within the first 30 min after 2-hr drug sessions ended, beginning after 2.5 months of cocaine or saline exposure. Results Rats self-administering cocaine earned 14–18 infusions on average throughout different phases of the study. In groupwise comparisons, learning in the visually guided delayed win-shift (prelimbic prefrontal cortex-related) and win-shift (hippocampus-related) tasks was not influenced by contingent or noncontingent cocaine exposure. Session latency, though, was shorter in both cocaine-exposed groups during the win-shift task. During the odor-guided delayed win-shift task (insular/orbital prefrontal cortex-related), learning was disrupted in rats self-administering cocaine, with no influence of noncontingent cocaine exposure. Conclusions Based on these and previous findings, learning related to functioning of the insular/orbital prefrontal cortex and amygdala is the most consistently disrupted in cocaine-intoxicated rats after long-term drug exposure.     

Cocaine-seeking behavior after extended cocaine-free periods in rats: role of conditioned stimuli

Rationale. Cocaine abstinence symptoms and conditioned stimuli (CSs) previously associated with cocaine administration are postulated to contribute to relapse to drug taking in humans. Objective. The present study assessed the role of both non-contingent CS presentation and experimenter-imposed extended cocaine-free periods on cocaine-seeking behavior in rats. Methods. A fixed interval (FI) second-order schedule of intravenous cocaine (0.5 mg/infusion) reinforcement of the type FI 15 min (fixed ratio 8:S) was used. Results. Non-contingent CS presentation before exposure to a cocaine binge had no effect on responding under the second-order schedule of reinforcement for cocaine after 23 h of no access to cocaine. By contrast, six non-contingent presentations of the CS during a 1-min period before the test session increased the number of responses in both no-binge (daily 2-h sessions, five infusions) and binge (two 12-h overnight sessions; maximum 48 infusions) exposed rats on day 7 of the cocaine-free period compared to no-binge- and binge-exposed rats that were not presented with the CSs. On day 30 of the cocaine-free period, only binge-exposed rats presented with the CSs exhibited a tendency for increased level of responding. Conclusions. The results indicated that non-contingent CS presentation had no effect after 23 h of no access to cocaine, increased drug-seeking behavior on day 7 of the cocaine-free period independent of binge exposure, and a strong tendency to increase drug-seeking behavior only in binge-exposed rats, on day 30 of the cocaine-free period, illustrating the interactive effects of conditioned stimuli with the extended cocaine-free period.     

Developmental lead exposure alters the stimulatory properties of cocaine at PND 30 and PND 90 in the rat.

The aim of this study was to examine the effects of perinatal lead exposure on locomotor responding following acute and repeated cocaine challenges (sensitization). Adult female rats were gavaged daily with 0, 8, or 16 mg lead acetate for 30 days prior to breeding. This exposure regimen was maintained throughout gestation and lactation (perinatal exposure). On Day 21, male pups were weaned and lead exposure was discontinued for the remainder of the study. Beginning on postnatal day (PND) 30 or PND 90, and continuing for 14 successive days, separate groups of perinatally-exposed animals were presented with challenges of 10 mg/kg cocaine HCl (i.p.), and tested for locomotor responding. Following this testing period, dose-effect profiles were determined, with animals receiving daily injections of 0, 10, 20, and 40 mg/kg cocaine. The results indicated that both at PND 30 and PND 90 lead-exposed animals were less responsive to the initial administration of cocaine, but exhibited a supersensitivity to the stimulatory effects associated with repeated administration of cocaine, i.e., behavioral sensitization to cocaine was augmented by perinatal lead exposure. Analyses of blood lead levels following the completion of testing revealed that lead levels were below detectable limits for all animals (< 1 microg/dl). Collectively, these findings show that developmental lead contamination produces changes in cocaine sensitivity long after exposure has been discontinued and the toxicant has gained clearance from blood.     

Reduced Forebrain Serotonin Transmission is Causally Involved in the Development of Compulsive Cocaine Seeking in Rats

Whereas the majority of cocaine users quit as they experience the negative consequences of drug use, some lose control over their drug taking and compulsively seek drugs. We report that 20% of rats compulsively seek cocaine despite intermittent negative outcomes after escalating their cocaine self-administration. This compulsive subgroup showed marked reductions in forebrain serotonin utilization; increasing serotonin transmission reduced their compulsive cocaine seeking. Depleting forebrain serotonin induced compulsive cocaine seeking in rats with a limited cocaine taking history; this was reversed by systemic treatment with a 5-hydroxytryptamine (5-HT2C) receptor agonist and mimicked by systemic treatment with a 5-HT2C receptor antagonist in intact animals. These results indicate the causal involvement of reduced serotoninergic transmission in the emergence of compulsive drug seeking after a long cocaine-taking history.