Differential neuroendocrine responsiveness to morphine in Lewis, Fischer 344, and ACI inbred rats

Blocking IL-17A prevents oxycodone-induced depression-like effects and elevation of IL-6 levels in the ventral tegmental area and reduces oxycodone-derived physical dependence in rats

Oxycodone is the most prescribed opioid for pain management and has been available in clinics for almost a century, but effects of chronic oxycodone have been studied less than morphine in preclinical and clinical studies. Newly developed depression has been coupled with chronic oxycodone use in a few clinical studies, but no preclinical studies have investigated the pathogenesis of oxycodone-induced depression. Gut microbiome changes following oxycodone use is an understudied area, and interleukin-17A (IL-17A) is linked to both the development of mood disorders and regulation of gut microbiome. The present study investigated effects of chronic oxycodone exposure on mood-related behaviors (depression and anxiety), pain hypersensitivity, physical dependence, immune markers, and the gut microbiome and tested the hypothesis that blocking IL-17A with a systemically administered monoclonal antibody reduces oxycodone-derived effects. Oxycodone (using an incremental dosing regimen) or saline was injected twice a day for 12 days. IL-17A Ab (200 µg/100 µl) or saline was administered every 3rd day during the 12-day interval. Chronic oxycodone induced a depression-like effect, but not anxiogenic- or anxiolytic-like effects; promoted hyperalgesia; increased IL-17A and IL-6 levels in the ventral tegmental area (VTA); and induced physical dependence. IL-17A Ab co-administration with oxycodone prevented the depression-like effect and hyperalgesia, reduced naloxone-precipitated withdrawal signs, and normalized the increase in cytokine levels. Chronic oxycodone exposure did not affect gut microbiome and integrity. Our results identify a role for IL-17A in oxycodone-related behavioral and neuroimmune effects and show that IL-17A Ab has potential therapeutic value in blocking these effects. Given that humanized IL-17A Ab is approved for treatment of psoriasis and psoriatic arthritis, our findings point toward studying it for use in the treatment of oxycodone use disorder.

Differential Hypothalamic-pituitary-adrenal Response to Stress among Rat Strains: Methodological Considerations and Relevance for Neuropsychiatric Research

The hormones of the hypothalamic-pituitary-adrenal (HPA) axis, particularly glucocorticoids (GCs), play a critical role in the behavioral and physiological consequences of exposure to stress. For this reason, numerous studies have described differences in HPA function between different rodent strains/lines obtained by genetic selection of certain characteristics not directly related to the HPA axis. These studies have demonstrated a complex and poorly understood relationship between HPA function and certain relevant behavioral characteristics. The present review first remarks important methodological considerations regarding the evaluation and interpretation of resting and stress levels of HPA hormones. Then, it presents works in which differences in HPA function between Lewis and Fischer rats were explored as a model for how to approach other strain comparisons. After that, differences in the HPA axis between classical strain pairs (e.g. High and Low anxiety rats, Roman high- and low-avoidance, Wistar Kyoto versus Spontaneously Hypertensive or other strains, Flinder Sensitive and Flinder Resistant lines) are described. Finally, after discussing the relationship between HPA differences and relevant behavioral traits (anxiety-like and depression-like behavior and coping style), an example for main methodological and interpretative concerns and how to test strain differences is offered.

Neuroimaging reveals distinct brain glucose metabolism patterns associated with morphine consumption in Lewis and Fischer 344 rat strains

Vulnerability to addiction may be given by the individual's risk of developing an addiction during their lifetime. A challenge in the neurobiology of drug addiction is understanding why some people become addicted to drugs. Here, we used positron emission tomography (PET) and statistical parametric mapping (SPM) to evaluate changes in brain glucose metabolism in response to chronic morphine self-administration (MSA) in two rat strains with different vulnerability to drug abuse, Lewis (LEW) and Fischer 344 (F344). Four groups of animals were trained to self-administer morphine or saline for 15 days. 2-deoxy-2-[¹⁸F]-fluoro-d-glucose (FDG)-PET studies were performed on the last day of MSA (acquisition phase) and after 15 days of withdrawal. PET data were analyzed using SPM12. LEW-animals self-administered more morphine injections per session than F344-animals. We found significant brain metabolic differences between LEW and F344 strains in the cortex, hypothalamus, brainstem, and cerebellum. In addition, the different brain metabolic patterns observed after the MSA study between these rat strains indicate differences in the efficiency of neural substrates to translate the drug effects, which could explain the differences in predisposition to morphine abuse between one individual and another. These findings have important implications for the use of these rat strains in translational morphine and opiate research.

Sex differences in responsiveness to the prescription opioid oxycodone in mice

Over-prescription and increased nonmedical use of oxycodone has become a major concern. Despite its increased use, preclinical data concerning oxycodone's effects are still limited, especially in rodent models. To address this, we examined oxycodone's effects on place preference, locomotor activation, corticosterone levels, and thermal analgesia across a range of doses (between 0.3 and 10mg/kg) in gonadally intact, adult male and female C57BL/6J mice. Males and females showed oxycodone-induced conditioned place preference and did not show significant between-sex differences in their place preference behavior. During both CPP conditioning sessions and open field assay, locomotor activity was increased by 1, 3, and 10mg/kg oxycodone in females and by 3 and 10mg/kg oxycodone in males. Plasma corticosterone levels were higher in females (compared to males) at baseline as well as following acute oxycodone injection and open field testing. The time course of oxycodone-induced analgesia was similar in males and females, however the total antinociceptive effect (AUC0-120min) was larger in males compared to females at the highest dose tested (10mg/kg). Taken together, these data suggest that male and female mice are modestly different in their responses to oxycodone.

Fischer 344 and Lewis Rat Strains as a Model of Genetic Vulnerability to Drug Addiction

Today it is well acknowledged that both nature and nurture play important roles in the genesis of psychopathologies, including drug addiction. Increasing evidence suggests that genetic factors contribute for at least 40–60% of the variation in liability to drug dependence. Human genetic studies suggest that multiple genes of small effect, rather than single genes, contribute to the genesis of behavioral psychopathologies. Therefore, the use of inbred rat strains might provide a valuable tool to identify differences, linked to genotype, important in liability to addiction and related disorders. In this regard, Lewis and Fischer 344 inbred rats have been proposed as a model of genetic vulnerability to drug addiction, given their innate differences in sensitivity to the reinforcing and rewarding effects of drugs of abuse, as well their different responsiveness to stressful stimuli. This review will provide evidence in support of this model for the study of the genetic influence on addiction vulnerability, with particular emphasis on differences in mesolimbic dopamine (DA) transmission, rewarding and emotional function. It will be highlighted that Lewis and Fischer 344 rats differ not only in several indices of DA transmission and adaptive changes following repeated drug exposure, but also in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, influencing not only the ability of the individual to cope with stressful events, but also interfering with rewarding and motivational processes, given the influence of corticosteroids on dopamine neuron functionality. Further differences between the two strains, as impulsivity or anxiousness, might contribute to their different proneness to addiction, and likely these features might be linked to their different DA neurotransmission plasticity. Although differences in other neurotransmitter systems might deserve further investigation, results from the reviewed studies might open new vistas in understanding aberrant deviations in reward and motivational functions.

Stress alters the discriminative stimulus and response rate effects of cocaine differentially in lewis and Fischer inbred rats

Stress enhances the behavioral effects of cocaine, perhaps via hypothalamic-pituitary-adrenal (HPA) axis activity. Yet, compared to Fischer 344 (F344) rats, Lewis rats have hyporesponsive HPA axis function and more readily acquire cocaine self-administration. We hypothesized that stress would differentially affect cocaine behaviors in these strains. The effects of three stressors on the discriminative stimulus and response rate effects of cocaine were investigated. Rats of both strains were trained to discriminate cocaine (10 mg/kg) from saline using a two-lever, food-reinforced (FR10) procedure. Immediately prior to cumulative dose (1, 3, 10 mg/kg cocaine) test sessions, rats were restrained for 15-min, had 15-min of footshock in a distinct context, or were placed in the shock-paired context. Another set of F344 and Lewis rats were tested similarly except they received vehicle injections to test if stress substituted for cocaine. Most vehicle-tested rats failed to respond after stressor exposures. Among cocaine-tested rats, restraint stress enhanced cocaine’s discriminative stimulus effects in F344 rats. Shock and shock-context increased response rates in Lewis rats. Stress-induced increases in corticosterone levels showed strain differences but did not correlate with behavior. These data suggest that the behavioral effects of cocaine can be differentially affected by stress in a strain-selective manner.

The effects of buprenorphine on behaviour in the ACI and BN rat inbred strains

Summary

Buprenorphine is a partial μ, κ agonist that has been shown to influence spontaneous behaviour in animals. Previously, we have demonstrated significant differences in the analgesic response to buprenorphine between the August Copenhagen Irish (ACI)/SegHsd and the Brown Norway (BN)/RijHsd inbred rat strains. The purpose of this study was to determine whether these strains also differed in their behavioural response to buprenorphine in order to provide an additional parameter for the genetic analysis and localization of genes involved in this response. Male and female rats of both strains were used ( n = 6/strain/sex) for this study. Each rat was subjected, respectively, to three treatment regimens at 15:00 h: (A) unchallenged; (B) intravenous saline; (C) intravenous buprenorphine (0.05 mg/kg) according to a crossover design. The relative duration (s/h) of locomotion, grooming, drinking and eating behaviour was subsequently determined from 15:30 to 07:00 h using the automatic registration system, Laboratory Animal Behaviour Registration and Analysis System™. Significant strain differences were observed in unchallenged behaviour between the ACI and the BN rats. ACI rats, but not BN rats, responded to buprenorphine treatment with decreased levels of locomotion, drinking and eating behaviour. The same treatment resulted in an increased grooming behaviour in both strains. Slight but significant sex differences were observed for locomotion and eating in the analysis of variance procedure, but did not reach the level of statistical significance in the multiple comparison procedure. The results of this study emphasize the possibility that strain-specific effects must be taken into account when using behavioural parameters for the assessment of the analgesic effects of buprenorphine in rats.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Strain differences in the acquisition of nicotine-induced conditioned taste aversion

Lewis (LEW) and Fischer (F344) rat strains differ on a variety of physiological and behavioral endpoints, including reactivity to drugs of abuse. Although they differ in drug reactivity, such assessments are generally limited to morphine and cocaine. To determine if these differences generalize to other drugs, the present study examined these strains for their reactivity to the affective properties of nicotine, specifically their sensitivity to nicotine in the conditioned taste aversion preparation. For four or five conditioning cycles given every other day, rats from both strains were allowed access to saccharin and injected with nicotine (0.1, 0.4, 0.8 mg/kg) or vehicle. On intervening days, all rats were given access to water and injected with vehicle. Under this one-bottle training and testing procedure, neither strain displayed aversions at the lowest dose of nicotine (0.1 mg/kg). Aversions were evident for both strains at 0.4 and 0.8 mg/kg, although the F344 rats acquired the aversions at 0.4 mg/kg faster and displayed a significantly greater aversion at 0.8 mg/kg than subjects from the LEW strain. For both strains, aversions were evident at all doses (and in a dose-dependent manner) when subjects were given access to saccharin and water in a two-bottle test. There were, however, no strain differences on this test. Differences between the two strains in their acquisition of nicotine-induced taste aversions were discussed in the context of aversion assessments with other compounds as well as in relation to differences in the self-administration of nicotine in the two strains.

Rat strain differences in sleep after acute mild stressors and short-term sleep loss

Genetic and physiological diversity amongst rodent strains provide the potential for developing models that may give insight into factors that regulate sleep in response to environmental challenges. We examined home cage activity, behavioral performance in the open field and sleep after a number of mild stressors (cage change [CC], open field [OF]) and after 1 and 4h of sleep deprivation (1hSD and 4hSD) in rat strains (Fischer 344 [F344], Lewis [LEW], Wistar [WST] and Sprague-Dawley [Sp-D], n=16 per strain) that differ in behavior and sleep. F344 and WST rats had greater home cage locomotion than LEW and Sp-D rats, but F344 rats exhibited the least relative locomotion in OF. In 24h baseline recordings of sleep, strain rankings were LEW=WST=Sp-D>F344 in rapid eye movement sleep (REM), and LEW=Sp-D>F344 and LEW>WST in non-REM (NREM). Compared to baseline, total sleep was reduced in all four strains after CC, OF and 1hSD, but not after 4hSD, in the first hour after treatment. Afterwards, increases in REM and NREM were seen after all treatments with the amount and time course varying across treatments and strains. CC induced the weakest and 4hSD the largest effects on sleep, whereas OF and 1hSD had intermediate effects. Among strains, the more anxious F344 rats exhibited the greatest sleep increases during the light period after OF, 1hSD and 4hSD. The results are discussed with respect to the relationship between behavioral and sleep responses to stressors, and to potential mechanisms underlying the strain differences.

Sex Differences in Opioid-Induced Enhancement of Contact Hypersensitivity

Previous research has demonstrated that, in male rats, the magnitude of contact hypersensitivity (CHS) can be enhanced by morphine treatment. The present experiments test the hypothesis that the mu-opioids morphine, etorphine, and buprenorphine would produce significant sex differences in the magnitude of 2,4-dinitrofluorobenzene-induced CHS. During tests conducted over a 192-h period, morphine, etorphine, and buprenorphine administered before elicitation of CHS on the external surface of the ear (pinna) potentiated the CHS response, and the magnitude of this enhancement was significantly greater in females than males. By contrast, morphine had no effect on croton oil-induced irritant contact dermatitis, indicating that morphine's effects on CHS do not generalize to immunologically nonspecific forms of contact dermatitis. Activation of brain mu-opioid receptors is responsible for the effects of morphine on CHS, because intracerebroventricular treatment with the mu-opioid receptor antagonist beta-funaltrexamine blocked morphine potentiation of CHS in females and males. The sex differences in morphine potentiation of CHS appear to be a result of the gonadal hormonal milieu, because castration enhanced the CHS response following vehicle and morphine treatment, whereas ovariectomy significantly attenuated the enhancement of CHS by morphine. Because ovariectomy had no effect on the CHS response following vehicle treatment, the presence of female gonadal hormones may underlie the sex differences in morphine potentiation of CHS in gonadally intact animals. Overall, these results support an increased sensitivity to the modulatory effects of opioids on the CHS response in females that depends on the interaction between gonadal hormones and the central mu-opioid system.

Differential impact of audiogenic stressors on Lewis and Fischer rats: behavioral, neurochemical, and endocrine variations

Exposure to intense noise can trigger a cascade of neuroendocrine events reminiscent of a stress response, including activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Using male Fischer and Lewis rats, which exhibit differences in their corticosterone response to stressors, this investigation assessed effects of acute noise exposure on neurochemical and neuroendocrine responses. In response to the noise exposure, Fischer rats displayed greater plasma adrenocorticotropin-releasing hormone (ACTH) and corticosterone responses than their Lewis counterparts. However, both strains responded with similar increases of plasma prolactin, suggesting that strain differences in the HPA response were not likely because of differences in noise perception. Post-mortem analyses revealed that noise exposure induced strain-dependent variations of corticotropin-releasing hormone (CRH) across several brain regions. These effects were evident irrespective of whether the rats were noise exposed in a familiar (home cage) or unfamiliar environment. In vivo, dynamic assessment of immunoreactive (ir)-CRH at the pituitary gland revealed that noise exposure elicited an immediate rise in ir-CRH among Fischer rats, relative to the delayed response in Lewis rats. Similarly, the rise in local interstitial corticosterone was more rapid and pronounced in Fischer rats. In contrast to these differences, ir-CRH released at the central nucleus of the amygdala (CeA) was gradual and protracted following noise exposure in both strains. Behaviorally, the Fischer rats displayed an active stress response, whereas the Lewis strain adopted freezing as a defensive style. The role of CRH in the genesis of the overall strain-dependent response to stressors is discussed.

Strain differences in response to escapable and inescapable novel environments and their ability to predict amphetamine-induced locomotor activity

RATIONALE

Locomotor response to novelty predicts locomotor and reinforcing effects of psychostimulant drugs in outbred rats. Among Lewis and Fischer 344 (F344) inbred rats this association is less clear, perhaps due to strain-selective differences in responses to novelty.

OBJECTIVE

We examined responses to novel inescapable and escapable environments and to novel objects in these strains.

METHODS

Experiment 1 utilized a place conditioning procedure. Rats were confined to one side for 8 days and then allowed access to both this (familiar) and the novel sides. Experiment 2 assessed locomotor response within an inescapable environment. On another occasion, contacts with novel objects within a novel environment were tabulated. Corticosterone levels and fecal boli were measured. Whether these responses predicted amphetamine-induced locomotor activity was determined. To further assess genetic contributions to this association, experiment 3 assessed novelty responses in F1 hybrid Lewis-F344 rats.

RESULTS

Lewis rats showed greater novelty-seeking behavior in the escapable environment but lower locomotor activity in the inescapable environment compared to F344 rats. There were no strain differences in novel object contacts, corticosterone, or fecal boli responses. Baseline corticosterone levels and activity levels in the novel environment were positively correlated with amphetamine activity based on data from all rats. However, novelty and amphetamine-induced activity showed non-significant negative correlations in F344 and Lewis rats. Yet, F1 rats showed a significant positive correlation between these variables, even though some of their other responses were Lewis-like or F344-like.

CONCLUSIONS

These data suggest that responses to different novelty situations are strain-dependent.

Strain-dependent differences in schedule-induced polydipsia: an assessment in Lewis and Fischer rats

Strain-dependent differences have been used to highlight unknown genetic contributions to important behavioral and physiological end points. In this regard, the Fischer (F344) and Lewis (LEW) rat strains have often been studied because they exhibit a myriad of behavioral and physiological differences. Recently, schedule-induced polydipsia (SIP), a potential model of stress and drug abuse, has been reported to differ between the two strains (see [Pharmacol. Biochem. Behav. 67 (2002) 809]) with F344 rats displaying greater levels of consumption than LEW rats. Given the importance of SIP as a behavioral model of stress and of drug abuse, the present study further explored SIP in F344 and LEW strains by assessing the acquisition and steady-state performance of SIP (under a fixed-time 30 schedule of food delivery; FT30), its characteristic postprandial temporal licking pattern and its modulation by variations in the food delivery schedule (FT15, FT30 and FT60). F344 rats acquired SIP at a faster rate and drank at a higher asymptotic level than LEW rats. Both strains displayed the typical inverted U-shaped post-pellet pattern of drinking and changes in levels of consumption (and displacement of the initiation of post-pellet drinking) with changes in the FT value, supporting the position that the increased drinking seen in both groups was schedule induced. These strain differences in SIP are consistent with the fact that the F344 and LEW strains differ on other behavioral and physiological indices of stress and raise the issue of the use of this model in the assessment of differential drug intake between the two strains.

Preclinical evaluation of GBR12909 decanoate as a long-acting medication for methamphetamine dependence

Methamphetamine (METH) abuse is a growing health problem, and no treatments for METH dependence have been identified. The powerful addictive properties of METH are mediated by release of dopamine (DA) from nerve terminals in mesolimbic reward pathways. METH stimulates DA release by acting as a substrate for DA transporter (DAT) proteins, thereby triggering efflux of DA from cells into the synapse. We have shown that blocking DAT activity with high-affinity DA uptake inhibitors, like GBR12909, can substantially reduce METH-evoked DA release in vitro, suggesting GBR12909 may have potential as a pharmacotherapy for METH dependence. The purpose of the present study was to examine the neurobiological effects of a long-acting oil-soluble preparation of GBR12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-hydroxy-3-phenylpropyl) piperazinyl decanoate, or GBR-decanoate). Male rats received GBR-decanoate (480 mg/kg, i.m.) or its oil vehicle, and were tested using a variety of methods one and two weeks later. Ex vivo autoradiography showed that GBR-decanoate decreases DAT binding in DA-rich brain regions. In vivo microdialysis in the nucleus accumbens revealed that GBR-decanoate elevates baseline levels of extracellular DA and antagonizes the ability of METH to evoke DA release. The dopaminergic effects of GBR-decanoate were sustained, lasting for at least two weeks. Rats pretreated with GBR-decanoate displayed enhanced locomotor responses to novelty at one week, but not two weeks, postinjection. Administration of the D(2)/D(3) receptor agonist quinpirole (10 and 100 microg/kg, s.c.) decreased locomotor activity and suppressed plasma prolactin levels; quinpirole-induced responses were not altered by GBR-decanoate. Thus, GBR-decanoate is able to elevate basal synaptic DA levels and block METH-evoked DA release in a persistent manner, without significant perturbation of DA receptor function. The findings suggest that GBR-decanoate, or similar long-acting agents, should be evaluated further as potential treatment adjuncts in the management of METH addiction in humans.

HPA axis function and drug addictive behaviors: insights from studies with Lewis and Fischer 344 inbred rats

Much research supports a link between stress and its concomitant hypothalamic-pituitary-adrenal (HPA) axis responses with behavioral sensitivity to psychoactive drugs. Our research demonstrates that Lewis inbred rats more readily acquire drug self-administration than Fischer 344 (F344) inbred rats and, compared to this strain, Lewis rats have hyporesponsive HPA axis responses to stress exposure. This association appears to conflict with investigations using outbred rats and suggests that the relationship between drug sensitivity and HPA axis responsiveness is more complicated than originally thought. It is essential to better understand this relationship because of its relevance to vulnerability and relapse to drug abuse. Thus, this paper reviews the literature in which these two inbred strains have been compared. We discuss strain differences in HPA axis function, in characteristics of the mesolimbic dopamine system, and in behaviors thought to reflect emotionality. Strain differences in unconditioned and conditioned effects of psychoactive drugs are then reviewed. Next, we discuss the possible role of sex and gonadal hormones on responsiveness to psychoactive drugs in these strains. Finally, a comparison of results obtained from these strains to three other comparator groups (e.g., high and low responders) suggests that a non-monotonic relationship between behavioral sensitivity to drugs and HPA axis responsiveness can explain much of the discrepancies in the literature.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Deficiency of growth hormone-releasing hormone signaling is associated with sleep alterations in the dwarf rat

The somatotropic axis, and particularly growth hormone-releasing hormone (GHRH), is implicated in the regulation of sleep-wake activity. To evaluate sleep in chronic somatotropic deficiency, sleep-wake activity was studied in dwarf (dw/dw) rats that are known to have a defective GHRH signaling mechanism in the pituitary and in normal Lewis rats, the parental strain of the dw/dw rats. In addition, expression of GHRH receptor (GHRH-R) mRNA in the hypothalamus/preoptic region and in the pituitary was also determined by means of reverse transcription-PCR, and GHRH content of the hypothalamus was measured. Hypothalamic/preoptic and pituitary GHRH-R mRNA levels were decreased in the dw/dw rats, indicating deficits in the central GHRHergic transmission. Hypothalamic GHRH content in dw/dw rats was also less than that found in Lewis rats. The dw/dw rats had less spontaneous nonrapid eye movement sleep (NREMS) (light and dark period) and rapid eye movement sleep (REMS) (light period) than did the control Lewis rats. After 4 hr of sleep deprivation, rebound increases in NREMS and REMS were normal in the dw/dw rat. As determined by fast Fourier analysis of the electroencephalogram (EEG), the sleep deprivation-induced enhancements in EEG slow-wave activity in the dw/dw rats were only one-half of the response in the Lewis rats. The results are compared with sleep findings previously obtained in GHRH-deficient transgenic mice. The alterations in NREMS are attributed to the defect in GHRH signaling, whereas the decreases in REMS might result from the growth hormone deficiency in the dw/dw rat.

Differences in nicotine-induced dopamine release and nicotine pharmacokinetics between Lewis and Fischer 344 rats

Studies have shown a greater preference for the self-administration of drugs such as nicotine and cocaine in the Lewis rat strain than in the Fischer 344 strain. We examined some factors that could contribute to such a difference. The baseline level of extracellular dopamine in nucleus accumbens shell was about 3-times higher in Fischer rats than in Lewis rats (3.18 +/- 0.26 vs. 1.09 +/- 0.14 pg/ sample). Nicotine (50-100 microg/kg)-induced release of dopamine, expressed in absolute terms, was similar in the two strains. Dopamine release expressed in relative terms (as percent of baseline), however, was significantly greater in Lewis rats than in Fischer rats at 30 min after the first nicotine injection. We suggest that the relative increase is of more influence than the absolute level for determining preference; a lower physiological extracellular dopamine level thus represent a risk factor for increased preference. Amphetamine-induced dopamine release expressed in relative terms was not greater in the Lewis strain. In the initial time period of the microdialysis experiments, a sharper peak in nicotine-induced accumbal dopamine release in Lewis and a less but more sustained release in Fischer rats was observed. This release pattern paralleled the faster clearance of nicotine from blood of Lewis compared to Fischer rats. In tissue slices the electrically induced dopamine release was highest in the nucleus accumbens and lowest in the ventral tegmentum. A significant effect of nicotine was lowering the electrically induced release of dopamine in frontal cortex slices from Fischer brain and increasing this dopamine release in the ventral tegmentum of Lewis brain slices indicating that the ventral tegmentum, an area controlling dopamine release in the accumbens, is more responsive to nicotine in the Lewis rat. Nicotine levels tended to be more sustained in Fischer rats in different brain regions, although the difference in nicotine levels between the strains was not significant at any time period. Several factors contribute to nicotine preference, including the endogenous dopamine level, and the sensitivity of ventral tegmentum neurons to nicotine-induced dopamine release. Strain differences in pharmacokinetics of nicotine may also play a role.