Chronic corticosterone administration enhances behavioral sensitization to amphetamine in mice

Neuron-specific cilia loss differentially alters locomotor responses to amphetamine in mice

The neural mechanisms that underlie responses to drugs of abuse are complex, and impacted by a number of neuromodulatory peptides. Within the past 10 years it has been discovered that several of the receptors for neuromodulators are enriched in the primary cilia of neurons. Primary cilia are microtubule-based organelles that project from the surface of nearly all mammalian cells, including neurons. Despite what we know about cilia, our understanding of how cilia regulate neuronal function and behavior is still limited. The primary objective of this study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to amphetamine. To test the consequences of cilia loss on amphetamine-induced locomotor activity we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice. Cilia loss had no effect on baseline locomotion in either mouse strain. In mice lacking cilia on dopaminergic neurons, locomotor activity compared to wild- type mice was reduced in both sexes in response to acute administration of 3.0 mg/kg amphetamine. In contrast, changes in the locomotor response to amphetamine in mice lacking cilia on GAD2-GABAergic neurons were primarily driven by reductions in locomotor activity in males. Following repeated amphetamine administration (1.0 mg kg^-1  day^-1 over 5 days), mice lacking cilia on GAD2-GABAergic neurons exhibited enhanced sensitization of the locomotor stimulant response to the drug, whereas mice lacking cilia on dopaminergic neurons did not differ from wild-type controls. These results indicate that cilia play neuron-specific roles in both acute and neuroplastic responses to psychostimulant drugs of abuse.

Mifepristone treatment affects the response to repeated amphetamine injections, but does not attenuate the expression of sensitization

Rationale Glucocorticoid hormones facilitate sensitization to repeated administration of psychostimulants, an effect that is mediated by glucocorticoid receptors (GRs). It is still unclear, however, at which stage of psychomotor sensitization are stress and GR-mediated effects involved.

OBJECTIVES

In the present study, we have tested the hypothesis that GR-mediated effects during the phase of repeated amphetamine injections play a crucial role in the long-term expression of sensitization. For this purpose, we used DBA/2 mice, an inbred strain commonly used for the study of stress effects on psychostimulant sensitization.

METHODS

Animals were treated with the GR antagonist mifepristone (200 mg/kg) at 2.5 h before each daily injection of amphetamine (2.5 mg/kg) or saline in a 5-day protocol. The amphetamine or saline injections were given in the home or a novel context. This was followed by a 2.5-week withdrawal period, without any drug delivery. Following the withdrawal period, two low-dose amphetamine challenges (1.25 mg/kg) were given subsequently, without additional mifepristone.

RESULTS

The animals receiving amphetamine in the novel context showed a higher expression of sensitization at challenge as compared to those in the home condition. Mifepristone treatment influenced locomotor response to repeated amphetamine injections, but this effect during the initial phase did not affect the expression of sensitization after a withdrawal period.

CONCLUSION

Our results indicate that GR-related processes during the initial phase of sensitization are involved in, but not crucial for, the development of long-term sensitization.

Environmental-induced differences in corticosterone and glucocorticoid receptor blockade of amphetamine self-administration in rats

RATIONALE

Rats raised in isolation self-administer more amphetamine than rats raised in enrichment.

OBJECTIVE

This study examined whether differential rearing alters basal and amphetamine-stimulated corticosterone and whether blocking glucocorticoid receptors alters amphetamine self-administration in differentially reared rats.

METHODS

The rats were raised from 21 to 51 days of age in either an enriched condition (EC), social condition (SC), or isolated condition (IC). Following the repeated collection of basal blood samples, the rats were administered amphetamine (0.5 or 2.0 mg/kg, i.p.) or saline, and blood samples were collected again. In another experiment, EC and IC rats were trained to i.v. self-administer amphetamine (0.003 or 0.03 mg/kg/infusion) and then were pretreated with the glucocorticoid receptor antagonist RU-486 (5, 10, or 20 mg/kg; i.p.) or vehicle prior to the session.

RESULTS

Basal-free corticosterone levels were ~4 times higher in IC rats than in either EC or SC rats with the first blood collection, but not with repeated collections. IC rats showed a more rapid amphetamine-induced increase in corticosterone levels than EC and SC rats. RU-486 pretreatment decreased amphetamine self-administration dose-dependently in both EC and IC rats; however, using an amphetamine unit dose of 0.03 mg/kg/infusion, the effect of RU-486 was blunted in IC rats (maximal decrease of ~40% in IC and ~90% in EC), suggesting an environment-induced difference in the role of glucocorticoid receptors in stimulant reinforcement.

CONCLUSION

The increase in stimulant self-administration produced by social isolation may involve enhanced reactivity of the hypothalamo-pituitary-adrenal stress axis.

Sex, but not repeated maternal separation during the first postnatal week, influences novel object exploration and amphetamine sensitivity

Sensation seeking and early life stress are both risk factors for developing substance use disorders. Neural adaptations resulting from early life stress may mediate individual differences in novelty responsiveness, and, in turn, contribute to drug abuse vulnerability. Animal models also demonstrate associations between novelty responsiveness or early life stress and increased sensitivity to psychostimulants. We investigated whether repeated maternal separation affects responses to novelty during adolescence and to amphetamine during adulthood, and whether maternal separation alters the relationship between these behavioral variables. Rat pups underwent separation (180 min/day) or control procedures (15 min/day) on postnatal days (PND) 2-8. Novel object exploration and amphetamine response were tested at PND 38 and 60, respectively. Adolescent males were less active in a novel environment and approached novel objects more frequently than females, but adult females showed greater amphetamine-induced locomotion. Maternal separation did not affect novelty responsiveness or amphetamine sensitivity. Locomotor activity in an inescapable, novel environment during adolescence predicted amphetamine-induced locomotor activity during adulthood in maternally separated rats, but not in controls. The results of this study suggest that adolescent responses to novelty may be particularly predictive of future substance abuse among survivors of early life trauma. Furthermore, sex differences in novelty and amphetamine responsiveness may complicate the relationship between these behavioral variables.

Adolescent social defeat alters neural, endocrine and behavioral responses to amphetamine in adult male rats

The mesocorticolimbic dopamine system, which governs components of reward and goal-directed behaviors, undergoes final maturation during adolescence. Adolescent social stress contributes to adult behavioral dysfunction and is linked to adult psychiatric and addiction disorders. Here, behavioral, corticosterone and limbic dopamine responses to amphetamine were examined in adult male rats previously exposed to repeated social defeat stress during mid-adolescence. Amphetamine (2.5mg/kg, ip) was administered after a novel environment test, with behavior observed in the same context for 90min thereafter. Adult rats that had been defeated in adolescence showed increased locomotion in the novel environment but reduced amphetamine-induced locomotion relative to non-defeated age matched controls. Monoamine and corticosterone responses to amphetamine were examined following a second amphetamine injection 3 days later. In previously defeated rats, corticosterone and medial prefrontal cortex dopamine responses to amphetamine were blunted while dopamine responses in the nucleus accumbens core were elevated. Our results suggest that experience of social defeat stress during adolescent development can contribute to altered behavioral and endocrine responses to amphetamine in adulthood. Furthermore, these effects are paralleled by changes in amphetamine-induced dopamine responses in corticolimbic systems implicated in addiction disorders.

Corticosterone treatment before puberty sensitizes the effect of oral methylphenidate on locomotor activity in preadolescence and produces differential effects in adulthood

The first objective of this study was to examine the effects of early exposure to methylphenidate and then those of re-exposure in adulthood. The second was to analyze the effect of corticosterone treatment during pre-puberty on oral methylphenidate consumption and, consequently, the effect of this psychostimulant on locomotor activity in preadolescent and adult rats. Experiment 1: from 31 to 39 days of postnatal age (PA), Wistar rats were exposed to either oral methylphenidate or water. Experiment 2: from 24 to 39 days PA, the rats received either corticosterone (2.0 mg/kg/day/subject) or a saline solution. From 31 to 39 days PA, rats were exposed to either methylphenidate or water. During adulthood, all rats in experiments 1 and 2 were exposed to either methylphenidate or water, and subsequently exposed to a free-choice condition of the same two substances.

RESULTS

Experiment 1. Methylphenidate increased locomotor activity (LA) regardless of age. In adulthood, higher methylphenidate consumption was observed in the group that had not been exposed to this substance, compared to the early methylphenidate-exposed group. Experiment 2. Corticosterone did not affect methylphenidate consumption during preadolescence or adulthood; however, the LA induced by methylphenidate was higher in the preadolescents that had been treated with corticosterone+methylphenidate than in the animals treated only with methylphenidate. In adulthood, methylphenidate produced higher LA in the animals previously treated with corticosterone+methylphenidate than in those that had received previous treatment exclusively with corticosterone. These results suggest that preadolescent corticosterone exposure produced a sensitizing effect of methylphenidate on LA in preadolescence. The differential effect on LA in adulthood depended on whether the corticosterone was administered with or without methylphenidate in preadolescence, which would suggest an enduring effect of the early synergic action between these two substances.

Behavioral sensitization to cocaine: cooperation between glucocorticoids and epinephrine

RATIONALE

Stressful life experiences facilitate responsiveness to psychostimulant drugs. While there is ample evidence that adrenal glucocorticoids mediate these effects of stress, the role of the sympatho-adrenal system in the effects of psychostimulants is poorly understood.

OBJECTIVES

The present study investigated the role of the two adrenal stress hormones, corticosterone and epinephrine, in sensitization to the locomotor stimulant effects of cocaine.

MATERIALS AND METHODS

The DBA/2 mouse strain was used, as behavioral sensitization in this strain critically depends on adrenal hormones. Animals were subjected to adrenalectomy ("ADX", surgical removal of the adrenals) or SHAM surgery, and ADX mice were given replacement of epinephrine (5 x 10(-3) mg/kg subcutaneously (s.c.) just prior to each drug administration), corticosterone (20%, s.c., pellet), or both. Mice were subjected to a cocaine sensitization regimen (15.0 mg/kg cocaine on nine consecutive days followed by a 7.5 mg/kg cocaine challenge after a 5-day withdrawal).

RESULTS

In agreement with our previous observations, ADX prevented initiation and expression of cocaine-induced locomotor sensitization. Whereas neither corticosterone nor epinephrine alone were sufficient to reverse the ADX effect, both hormones were necessary to fully restore initiation and retention of sensitization to levels observed in SHAM animals.

CONCLUSIONS

The present findings indicate that corticosterone and epinephrine cooperate to facilitate behavioral responsiveness to cocaine. These data emphasize that in addition to the hypothalamic-pituitary-adrenal axis, the sympathetic nervous system plays a critical role in psychostimulant sensitivity.

Role of prefrontal dopaminergic neurotransmission in glucocorticoid receptor-mediated modulation of methamphetamine-induced hyperactivity

Glucocorticoids are involved in psychostimulant-induced hyperactivity, but the exact mechanism is not known. This study used the selective glucocorticoid receptor antagonist, RU-43044, to determine whether prefrontal neurotransmission is involved in glucocorticoid-mediated modulation of methamphetamine (METH)-induced hyperactivity in mice. Pretreatment with RU-43044 (10-30 mg/kg) attenuated the increased spontaneous locomotor activity induced by METH (1-2 mg/kg). The psychostimulant effect of METH was also attenuated by adrenalectomy. RU-43044 inhibited METH-induced increases in extracellular dopamine (DA), but not serotonin (5-HT), levels in the prefrontal cortex, but did not affect METH-induced increases in extracellular DA levels in the nucleus accumbens shell, although it inhibited increases in extracellular 5-HT levels. Adrenalectomy also attenuated the METH-induced increases in extracellular DA levels in the prefrontal cortex. RU-43044 did not affect METH-induced increases in plasma corticosterone levels. These findings suggest that glucocorticoid receptors are involved in METH-induced hyperactivity, and that prefrontal dopaminergic neurotransmission plays a role in glucocorticoid-mediated modulation of METH-induced behavioral changes.

The Pharmacokinetic Properties of Methamphetamine in Rats with Previous Repeated Exposure to Methamphetamine: The Differences between Long-Evans and Wistar Rats

Repeated treatment with methamphetamine (METH) causes long-term behavioral changes, so-called behavioral sensitization (BS), in humans as well as experimental animals. However, there are no reports as to whether repeated METH treatment can establish BS in stress-sensitive Long-Evans (LE) rats. Thus, we investigated the effect of repeated METH treatment (5 mg/kg x 5 days) on the establishment of BS in LE rats. Wistar (WIS) rats were used as a reference. In LE rats, repeated METH treatment failed to cause BS although it did enhance METH-induced hyperlocomotion in WIS rats. The levels of METH in brain dialysate and the ratio of the area under the concentration-time curve area in plasma to that in brain dialysate was increased in repeated METH-treated WIS rats as reported previously, but not in repeated METH-treated LE rats. METH increases plasma corticosterone (CORT) in both strains. However, the intensity of increment of CORT by repeated METH was lower in LE rats than that in WIS rats. Repeated METH treatment decreased the expression of METH-transposable and CORT-sensitive transporter, organic cation transporter 3 (OCT3), in the brain of WIS rats. However, the intensity of the decrement of OCT3 with repeated METH treatment was similar between both strains. Taken together, these results suggest that the lack of establishment of BS in LE rats might have been caused by the unchanged brain penetration of METH after repeated METH administration, and that the differential CORT response to METH is an important strain difference.

Amphetamine withdrawal leads to behavioral sensitization and reduced HPA axis response following amphetamine challenge

Withdrawal from repeated amphetamine (AMPH) administration leads to behavioral sensitization following a drug or a stress challenge and is commonly used to model anhedonia in rats, a core symptom of depression in humans. It is proposed that corticosteroids are involved in the mediation of sensitization and depression. The aim of the present study was to investigate stress and AMPH- induced release of adrenocorticotropic hormone (ACTH) and corticosterone (CORT) during withdrawal from an escalating dosage schedule of AMPH known to produce depression-like effects in rats. Wistar rats were given 3 injections (i.p.) per day over 3 days, escalating from 1 mg/kg to 9 mg/kg and a final injection of 10 mg/kg AMPH or saline on day 4. On day 2 of withdrawal, the animals were tested in the Porsolt swim test. HPA axis activity in response to restraint stress was tested on withdrawal day 14 and in response to AMPH challenge on withdrawal day 30. We found no effect of AMPH withdrawal in the Porsolt swim test and on the ACTH or CORT response following restraint stress. AMPH withdrawn animals expressed behavioral sensitization in terms of locomotion and reduced ACTH and CORT plasma levels following a 1 mg/kg AMPH challenge in comparison to the controls. We conclude that there is no critical involvement of a sensitized HPA axis stress response in the long-term expression of behavioral sensitization.

Effect of two prednisone exposures on mood and declarative memory

Corticosteroids are essential for life and an integral part of the stress response. However, in excess, corticosteroids can be associated with a variety of effects on the brain including hippocampal atrophy and even neuronal death, mood changes, and declarative memory impairment. The magnitude of mood change in patients receiving prednisone is reportedly associated with previous lifetime corticosteroid exposure, consistent with a sensitization or kindling process whereby greater effects are observed with repeated exposure. To our knowledge, the effect of multiple corticosteroid exposures on mood and memory has not been previously examined prospectively in animals or humans. In this study, 30 human volunteers, with no history of systemic prescription corticosteroid therapy, were given (in random order using a crossover design) two 3-day exposures of prednisone (60 mg/day) and one of identical placebo, with 11-day washouts between each medication exposure. Before and after each 3-day prednisone/placebo exposure, declarative memory was assessed using different versions of the Rey Auditory Verbal Learning Test (RAVLT) to minimize practice or learning effects, while mood was assessed with the 21-item Hamilton Rating Scale for Depression, Young Mania Rating Scale and Internal State Scale. No significant mood changes were found. However, a significant decrease in aspects of RAVLT performance was observed after the first prednisone exposure consistent with a decline in declarative memory performance. The decline in RAVLT performance was significantly smaller after the second prednisone exposure as compared to the initial prednisone exposure. Thus, a second prednisone exposure was associated with an attenuated prednisone-effect on declarative memory. These data suggest tolerance or habituation, rather than sensitization, to prednisone effects on declarative memory during a second exposure. Implications and possible explanations for the findings are discussed.

Hypophysectomy does not block sensitization to the dopamine agonist quinpirole or its modulation by the MAOI clorgyline

Repeated administration of the dopamine agonist quinpirole induces behavioral sensitization in rats that is characterized by a four- to eight-fold increase in the amount of locomotion compared to an acute dose of quinpirole, in the absence of any increases in mouthing behavior. The monoamine oxidase (MAO) inhibitor, clorgyline, switches behavioral sensitization to quinpirole from that of locomotion to self-directed mouthing. The mechanism by which clorgyline produces this switch in behavioral sensitization is unknown, but is independent of the known effects of clorgyline, namely, inhibition of MAO, inhibition of striatal dopamine uptake, or stimulation of sigma and I(2) receptors. Because clorgyline also inhibits hypothalamo-pituitary-adrenal (HPA) axis function, and increased HPA activity facilitates the behavioral effects of psychostimulant drugs, the effects of clorgyline on quinpirole sensitization are possibly due to an inhibition of HPA function. Therefore, the present study examined whether HPA activity is required for sensitization to quinpirole, and whether clorgyline exerts its effects on quinpirole sensitization via inhibition of HPA function. Control and hypophysectomized rats were administered clorgyline (1 mg/kg, s.c.) or vehicle 90 min before each injection of quinpirole (0.5 mg/kg x 8, twice weekly) or saline. To assess the level of sensitization reached by control and hypophysectomized rats, test injections of quinpirole (0.0, 0.07, and 0.2 mg/kg) were administered. Chronic quinpirole administration produced equivalent levels of locomotor sensitization in control and hypophysectomized rats. Clorgyline was equally effective in blocking the development of locomotor sensitization in control and hypophysectomized rats, and in sensitizing self-directed mouthing. The present study suggests that (1). HPA function is not necessary for the development of quinpirole sensitization and, (2). clorgyline does not produce its effects on behavioral sensitization to quinpirole via an inhibition of HPA activity. Moreover, the observation that quinpirole sensitization develops normally in the absence of any pituitary endocrine function suggests that pituitary-gonadal and pituitary-thyroid axes activity are also not necessary for quinpirole sensitization to occur.

Amphetamine-induced locomotor activation in 5-HT(1B) knockout mice: effects of injection route on acute and sensitized responses

Knockout mice lacking serotonin(1B) (5-hydroxytryptamine(1B); 5-HT(1B)) receptors (1BKO) exhibit increased sensitivity to the stimulant and reinforcing effects of indirect dopamine (DA) agonists and are more reactive to mild stressors such as handling and the injection procedures that commonly occur when using the intraperitoneal (i.p.) route of drug administration. Since the intravenous (i.v.) route of administration allows minimal handling and injection-induced stress, the present study was designed to evaluate the effect of the administration route on amphetamine-induced locomotor activation and behavioural sensitization in 1BKO mice. For this purpose, 1BKO and wild-type (WT) control mice were administered i.p. or i.v. amphetamine in a within-session design, which allows evaluation of a complete dose-response curve within a single session. The locomotor stimulant effects of i.p. (0.5-4.0 mg/kg) and i.v. (0.6-1.2 mg/kg) amphetamine were investigated both acutely and following repeated treatments (four treatments at 48 h intervals). The results showed that acute i.p. amphetamine injection induced a significant higher horizontal activity peak effect in 1BKO mice, while this difference was less profound after acute i.v. injection. However, repeated i.p. or i.v. administration of amphetamine induced significantly higher locomotion in 1BKO mice. We conclude that the stimulant effects of amphetamine can be influenced by the route of administration in a genotype-dependent manner, and that route of drug administration (and associated variables) should be considered an important factor in studies of psychostimulant action in knockout mice.

Nicotine induced behavioral locomotor sensitization

1. Nicotine behavioral sensitization of locomotor activity was investigated in adult female Sprague Dawley rats. Five different experiments were performed with nicotine in various doses of 0.1, 0.32, or 1.0 mg/kg i.p. These included: 1) effects of daily nicotine for 6 days, 2) effects of once per week nicotine for 3 weeks, 3) effects of MK-801 on nicotine-induced locomotor activity, 4) effects of dexamethasone on nicotine-induced locomotor activity, 5) induction of tolerance to nicotine-induced locomotor sensitization and lack of cross tolerance to caffeine. 2. Locomotor activity was measured with a photoelectric computerized system. The first dose of nicotine (0.32 mg/kg) induced marked locomotor depression. Once daily injection of 0.32 mg/kg of nicotine for 6 days produced tolerance to its depressant effects and sensitized the rats to its stimulant effects. Three once weekly doses of 0.32 mg/kg of nicotine also produced tolerance to its depressant effects and some locomotor stimulation. 3. Daily pretreatment for 5 days with a dose of 0.18 mg/kg of MK-801 i.p. partially antagonized the locomotor depressant and stimulant actions of nicotine. 4. Dexamethasone (1 mg/kg i.p.) daily pretreatment barely reduced nicotine locomotor depression and only very slightly enhanced locomotor stimulation. 5. Accumulating doses of 0.32 and 1.0 mg/kg b.i.d. of nicotine produced tolerance to its locomotor stimulant effects in rats previously sensitized to 0.32 mg/kg. There was no cross-tolerance to 32 mg/kg of caffeine citrate in previously sensitized animals tolerant to the stimulant effects of nicotine.

Pharmacological mechanisms mediating phencyclidine-induced apoptosis of striatopallidal neurons: the roles of glutamate, dopamine, acetylcholine and corticosteroids

Phencyclidine (PCP) has recently been shown to induce apoptosis of a subpopulation of striatopallidal neurons which lie in the dorsomedial caudate-putamen. The pharmacological mechanisms underlying this PCP-induced striatal death were investigated in a series of small experiments. Striatal silver-methenamine-stained sections from rats injected acutely with dizocilpine (MK-801; 1.5-5 mg/kg, i.p.) were analysed to determine whether other non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists could induce apoptotic-like changes in striatal cells. The effects of amphetamine (3-12 mg/kg, i.p.) were similarly investigated as PCP can elevate extracellular dopamine levels and dopamine has the potential to be neurotoxic. The potential involvement of dopamine transmission in PCP-induced striatal apoptosis was also tested by determining the effect of co-administering SCH23390 (D1 dopamine receptor antagonist) and quinpirole (D2 dopamine receptor agonist) on PCP (80 mg/kg, s.c.)-induced striatal apoptotic-like cell death. Equivalent experiments were performed using scopolamine (cholinergic antagonist) as this drug blocks PCP-induced damage of the retrosplenial cortex and RU38486 (corticosteroid receptor antagonist) as a similar subpopulation of striatal neurons undergoes apoptosis following dexamethasone administration. Injection of neither MK-801 nor amphetamine induced elevations of apoptotic-like cells in the striatum nor did co-administration of SCH23390 or scopolamine affect the levels of PCP-induced striatal cell death. In contrast, quinpirole elevated the levels of PCP-induced apoptotic-like striatal cell death and RU38486 markedly reduced it. Within the retrosplenial cortex, scopolamine lowered PCP-induced apoptotic-like cell death whereas RU38486 was without effect. These results suggest that PCP-induced striatal apoptosis results from a corticosteroid-dependent mechanism. The results further demonstrate that different pathological mechanisms underlie PCP-induced neuronal damage in the striatum and the retrosplenial cortex.

Coping styles in animals: current status in behavior and stress-physiology

This paper summarizes the current views on coping styles as a useful concept in understanding individual adaptive capacity and vulnerability to stress-related disease. Studies in feral populations indicate the existence of a proactive and a reactive coping style. These coping styles seem to play a role in the population ecology of the species. Despite domestication, genetic selection and inbreeding, the same coping styles can, to some extent, also be observed in laboratory and farm animals. Coping styles are characterized by consistent behavioral and neuroendocrine characteristics, some of which seem to be causally linked to each other. Evidence is accumulating that the two coping styles might explain a differential vulnerability to stress mediated disease due to the differential adaptive value of the two coping styles and the accompanying neuroendocrine differentiation.

Stressor- or drug-induced sensitization of the corticosterone response is not critically involved in the long-term expression of behavioural sensitization to amphetamine

Repeated exposure to drugs of abuse induces long-lasting behavioural sensitization, which is thought to play a role in the persistence of drug-seeking behaviour. Recently, we showed that repeated exposure of rats to cocaine resulted in a long-lasting (weeks) sensitization of the hypothalamus-pituitary-adrenal axis, i.e. hypersecretion of adrenocorticotropic hormone and of the glucocorticoid corticosterone. Moreover, we found that the administration of a glucocorticoid receptor antagonist abolished the expression of psychostimulant-induced behavioural sensitization. In the present study we tested whether stressor- or drug-induced long-term hypersecretion of corticosterone is associated with the long-term expression of behavioural sensitization to psychostimulant drugs. To that end, groups of male Wistar rats were exposed once to interleukin-1beta or to footshocks, treatments that are known to induce long-term sensitization of the hypothalamus-pituitary-adrenal axis, or were treated with amphetamine or morphine, according to protocols known to induce long-lasting behavioural (locomotor) sensitization. Three weeks later, the groups and their controls were challenged with amphetamine or vehicle. Previous exposure to interleukin-1beta or footshocks enhanced adrenocorticotropic hormone and corticosterone responses, but did not affect the long-term locomotor sensitization to amphetamine. Prior amphetamine treatment enhanced the locomotor response and the adrenocorticotropic hormone and corticosterone responses to amphetamine. Prior morphine treatment resulted in long-term locomotor sensitization, whereas the adrenocorticotropic hormone and corticosterone responses to amphetamine were decreased. From these findings and the absence of within-group correlation between corticosterone and locomotor responses in interleukin-1beta and morphine-pretreated rats, we conclude that there is no correlation between sensitization of the corticosterone response and behavioural sensitization to amphetamine. Apparently, sensitization of the corticosterone response is not a prerequisite for the long-term expression of behavioural sensitization, which suggests that drug-induced long-term behavioural sensitization may involve corticosteroid receptor-dependent (central) mechanisms that occur independent of hypothalamus-pituitary-adrenal axis responsiveness.

Phencyclidine induces D-1 dopamine receptor mediated Fos-like immunoreactivity in discretely localised populations of striatopallidal and striatoentopeduncular neurons in the rat

Phencyclidine (PCP), a non-competitive antagonist of the NMDA subtype of glutamate receptor, which also acts as an indirect dopamine agonist and at sigma sites, can induce a long lasting psychotic state when taken acutely. It is well established that PCP is toxic to specific limbic structures and we have recently demonstrated that it induces apoptosis of a subpopulation of striatal neurons. These neurons lie predominantly in the dorsomedial striatum and project to the globus pallidus. The mechanisms mediating this neuronal death are unclear though manipulations of dopamine transmission can induce striatal c-fos expression and continuous c-fos expression has been implicated in the molecular cascades controlling apoptosis. We accordingly undertook a series of experiments to determine the action of PCP on striatal Fos-like immunoreactivity (FLI). PCP (80 mg/kg, s.c.) elicited FLI in three distinct striatal areas, namely dorsomedial, dorsolateral and the nucleus accumbens. The level of PCP-induced FLI was consistently attenuated by the co-administration of the D-1 antagonist, SCH 23390. Vehicle injections also induced modest levels of FLI in the dorsomedial striatum and the nucleus accumbens which again were attenuated by SCH 23390. The type of striatal neuron in which PCP-induced FLI was determined by the use of a retrograde anatomical tracer. A colloidal gold tracer was thus injected into the major areas of termination of striatal projection neurons prior to the administration of PCP. This procedure demonstrated that the majority of the FLI positive striatal cells were striatopallidal neurons, though some FLI positive striatoentopeduncular neurons were also seen. The potential pharmacological mechanisms underlying the results are discussed. It is argued that the complex pattern of PCP-induced striatal FLI might be accounted for by a differential action upon extracellular dopamine levels whereby they are elevated in some striatal areas and simultaneously reduced in others.

Sympatho-adrenal involvement in methamphetamine-induced hyperthermia through skeletal muscle hypermetabolism

We investigated the involvement of the sympatho-adrenal axis in the hyperthermia induced by methamphetamine by using a biotelemetric system. The intraperitoneal injection of methamphetamine (1 mg/kg) induced hyperthermia preceded by an increase in oxygen consumption in freely moving rats. The hyperthermic effect of methamphetamine was completely blocked by chemical sympathectomy with 6-hydroxydopamine (50 mg/kg, i.p.). Adrenalectomy, but not adrenal demedullation, prevented the hyperthermia. In adrenalectomized rats, dexamethasone supplementation (0.5 mg/kg, s.c.) restored the methamphetamine-induced hyperthermia. Furthermore, dantrolene (1 or 2 mg/kg, i.v.), which blocks Ca2+ release from the sarcoplasmic reticulum in skeletal muscle, attenuated the hyperthermia. These results suggest that methamphetamine stimulates norepinephrine release from sympathetic nerve terminals, which then enhances thermogenesis in skeletal muscle under the permissive action of glucocorticoids.

Defective motor behavior and neural gene expression in RIIbeta-protein kinase A mutant mice

Motor behavior is modulated by dopamine-responsive neurons in the striatum, where dopaminergic signaling uses G-protein-coupled pathways, including those that result in the activation of cAMP-dependent protein kinase (PKA). The RIIbeta isoform of PKA is highly enriched in the striatum, and targeted disruption of the RIIbeta gene in mice leads to a dramatic reduction in total PKA activity in this region. Although the mutant mice show typical locomotor responses after acute administration of dopaminergic drugs, they display abnormalities in two experience-dependent locomotor behaviors: training on the rotarod task and locomotor sensitization to amphetamine. In addition, amphetamine induction of fos is absent, and the basal expression of dynorphin mRNA is reduced in the striatum. These results demonstrate that motor learning and the regulation of neuronal gene expression require RIIbeta PKA, whereas the acute locomotor effects of dopaminergic drugs are relatively unaffected by this PKA deficiency.

Phencyclidine and corticosteroids induce apoptosis of a subpopulation of striatal neurons: a neural substrate for psychosis?

Phencyclidine, a non-competitive N-methyl-D-aspartate receptor antagonist and indirect dopamine agonist, has neuroprotective properties. Phencyclidine, however, can also exert toxic effects and causes degeneration of neurons in the retrosplenial cortex. In this paper we demonstrate that acute administration of a high dose of phencyclidine to rats, (80 mg/kg), also causes death of a subpopulation of striatal neurons. The dying cells exhibited many of the morphological and biochemical features of cells undergoing apoptosis as revealed by a silver methenamine stain, propidium iodide fluorescence histochemistry and a TUNEL procedure. The majority of the dying cells tended to be clustered within the dorsomedial aspect of the striatum. The type of striatal cell undergoing apoptosis was determined by stereotaxically injecting a colloidal gold retrograde anatomical tracer into the major areas of striatal termination prior to the administration of phencyclidine. This procedure demonstrated that phencyclidine induced striatal apoptosis is almost exclusively limited to striatopallidal neurons. A similar series of experiments was conducted to determine whether the synthetic corticosteroid, dexamethasone, also induces apoptosis of striatal neurons. Corticosteroids are known to be toxic to hippocampal neurons and interact with striatal dopamine transmission. Acute administration of dexamethasone, (20 mg/kg), induced apoptosis of a subpopulation of striatal cells. As was the case with phencyclidine, most of the dexamethasone-induced apoptotic striatal cells were striatopallidal neurons located within the dorsomedial striatum. The pathology during the early stages of Huntington's disease is restricted to an equivalent subpopulation of striatal neurons. Many Huntington's patients are extremely psychotic during this stage in the progression of the disease. Psychosis is also associated with the acute administration of both phencyclidine and dexamethasone to humans. We accordingly speculate that the selective loss of striatopallidal neurons in the dorsomedial striatum may represent the neural substrate of many forms of psychosis.

Alterations in excitatory amino acid-mediated regulation of midbrain dopaminergic neurones induced by chronic psychostimulant administration and stress: relevance to behavioural sensitization and drug addiction

Repeated, intermittent administration of the psychostimulants d-amphetamine and cocaine, as well as other drugs of abuse, leads to an enduring augmentation of certain behavioural responses (e.g. locomotor activity) produced by these drugs. This behavioural sensitization has been the subject of considerable interest due to its potential relevance to drug addiction. Repeated administration of d-amphetamine also leads to an enhancement in the ability of electrical stimulation of the prefrontal cortex to induce burst firing in midbrain dopaminergic (DA) neurones. This hyper-responsiveness probably reflects a potentiation of transmission at excitatory amino acid (EAA)ergic synapses on DA neurones. In addition, we have previously reported that selective activation of mineralocorticoid receptors (MRs) by corticosterone leads to a potentiation of EAA-induced burst firing in midbrain DA neurones, an effect antagonized by glucocorticoid receptor (GR) activation. In this review article, we propose a model describing how drugs of abuse and stress alter EAA function at the level of DA cells in the ventral tegmental area (VTA), which can result in a long-lasting impact on behaviour. D-amphetamine produces a transitory increase in EAA-mediated transmission at the level of DA cells in the VTA, which triggers a more long-lasting change in EAAergic function resembling hippocampal long-term potentiation. Dopaminergic burst events are likely to be a critical link between enhanced EAAergic activity in afferents synapsing on DA neurones and plasticity at these synapses, by increasing calcium transport into the cell, which is known to be an important factor in synaptic plasticity. Selective MR occupation by corticosterone in the VTA facilitates the development of this plasticity. However, we hypothesize that during stress, GR-occupation also activates EAAergic afferents to DA neurones in a manner similar to that following psychostimulants. Under these circumstances, GR-occupation acts via circuitry external to the VTA, which may include the hippocampus. Thus, potentiation of EAAergic synapses on DA neurones in the VTA may represent a final common pathway by which two divserse means (psychostimulants and stress) achieve the same end (sensitization). Alterations in EAA-mediated transmission at the level of DA cells not only plays a critical role in the induction of behavioural sensitization, but probably continues to produce abnormal DA cell responses in the drug-free situation.

The role of corticosteroids in nicotine's physiological and behavioral effects

This paper reviews evidence indicating that adrenal corticosteroids modulate the responsiveness of mice and rats to nicotine. Adrenalectomy increases, and both acute and chronic corticosteroid administration decrease, some of the physiological and behavioral effects of nicotine. One function of adrenal steroids may be to regulate stress-induced changes in nicotine sensitivity. Another is to mediate the development of chronic tolerance when nicotine is given intermittently, and when the resulting tolerance has a learned component. A role of glucocorticoids in the development of tolerance to nicotine is suggested by the findings that a conditioned elevation of plasma corticosterone, which anticipates nicotine delivery, accompanies the development of chronic tolerance and that environmental cues evoke a conditioned corticosterone response, but only after they have become associated with nicotine delivery. The mechanisms by which adrenal steroids modulate nicotine sensitivity are not known, although recent in vitro evidence suggests that steroids can rapidly and reversibly reduce nicotinic receptor function. While most of the data are consistent with the hypothesis that corticosteroids reduce nicotine responsiveness, and thus promote a learned form of tolerance, there are new findings that corticosteroids increase the development of sensitization to the locomotor-activating effects of nicotine. These data suggest that formulations postulating a unidirectional effect of corticosteroids on nicotine's actions (e.g. decreased sensitivity) must be revised to take into account interacting variables such as the specific nicotine effect being studied and whether that effect normally exhibits tolerance or sensitization. Finally, research is presented which indicates that the corticosterone-elevating effects of nicotine, previously reported for experimenter-administered drug, are also produced when nicotine administration is contingent on an operant response, and at a dose which sustains the development of nicotine self-administration in rats. These findings highlight the feasibility of using self-administration models in future explorations of the relationship between adrenal steroids and nicotine function.

Effects of repeated amphetamine treatment on the locomotor activity of the dopamine D1A-deficient mouse

The role of dopamine D1A receptors in mediating amphetamine-induced sensitization was investigated using the D1A-deficient mouse. During the drug pre-exposure phase, D1A-deficient and control mice were injected for five consecutive days with saline or amphetamine (2 mg/kg, i.p.). Locomotor activity was measured on the first and fifth pre-exposure day. After three abstinence days, mice were given either amphetamine or saline and locomotor activity was again assessed. Mice were then sacrificed and protein kinase A (PKA) activity was measured. In contrast to control mice, D1A-deficient mice did not show a progressive increase in locomotor activity across days. Importantly, both control and mutant mice did exhibit behavioral sensitization, because mice pre-exposed and tested with amphetamine were more active than mice acutely tested with the drug. Even so, the amphetamine-induced locomotor activity of the mutant mice was significantly reduced when compared with similarly treated control mice, indicating that the sensitized response was less pronounced in the D1A-deficient mouse. PKA activity also varied depending on genotype, since amphetamine decreased PKA activity in control but not D1A-deficient mice.

Behavioral sensitization to ethanol: genetics and the effects of stress

Some aspects of drug abuse syndromes may be influenced by sensitization to some drug effects. This enhancement of drug effect has been associated with prior drug exposure and with exposure to stressful stimuli. It has been postulated that sensitization to psychomotor stimulant drug effects influences sensitivity to drug reward. The drugs of abuse best characterized for sensitization phenomena include cocaine, amphetamine, and morphine. In general, ethanol's molecular mechanisms of action have been difficult to define relative to drugs with known receptor or transporter binding sites and, likewise, ethanol sensitization has been less thoroughly examined. Evidence supporting the existence of behavioral sensitization to ethanol, for genetic differences in the occurrence of ethanol sensitization, and for the influence of corticosterone on the development of ethanol sensitization is reviewed herein. There appear to be different genetic determinants of acute drug sensitivity and sensitization. Cross-sensitization between stress and ethanol suggest a potential role for hypothalamic-pituitary-adrenal (HPA) axis associated changes in ethanol sensitization, consistent with mechanisms likely contributing to sensitization to other abused drugs. Furthermore, glucocorticoid receptors appear to mediate both ethanol- and stress-induced sensitization to ethanol. A biological link between drug reward and drug sensitization involving HPA axis hormones may exist and, thus, study of the sensitization process may elucidate mechanisms relevant to drug abuse.

Neurotoxicity of d-amphetamine in the C57BL/6J and CD-1 mouse. Interactions with stress and the adrenal system

Substantial evidence suggests that stress can alter the general toxicological properties of the substituted amphetamines (AMPs) as well as their psychostimulant properties. Research concerning the interactions between stress and the neurotoxicity associated with the AMPs is, however, limited. Our previous work demonstrated that a variety of AMPs, including d-METH, d-MDA, d-MDMA but not d-FEN are able to damage dopaminergic elements of the striatum as shown by decreases in dopamine and tyrosine hydroxylase. The neurotoxic capabilities of these AMPs appear linked to their hyperpyrexic actions as diverse manipulations able to block AMP-induced hyperthermia are also neuroprotective. Surprising, since stress usually potentiates the actions of the AMPs, it is our finding that restraint, a commonly used stressor, is protective against the injurious actions of all neurotoxic AMPs evaluated to date. In the mouse restraint acts to elevate blood levels of corticosterone (CORT) by activating the hypothalamic-pituitary-adrenal (HPA) axis as well as inducing a profound hypothermia. The role CORT may play in the neuroprotective actions of restraint, if any, is unknown. Here, data is presented showing the impact of several HPA axis manipulations, including restraint, supplementation with CORT in the drinking water and removal of CORT by adrenalectomy (ADX) on the striatal dopaminergic neurotoxicity of d-AMP. As strain is known to be a powerful determinant of the actions of stress an essential element of these experiments was the evaluation of both an inbred, C57BL/6J and outbred, CD-1, mouse strain. Exposure to d-AMP caused hyperthermia and substantial striatal dopaminergic neurotoxicity in both strains suggesting that an elevation in body temperature is as important a component of the neurotoxicity of d-AMP, as it is of the other neurotoxic AMPs. Restraint was equally effective in both strains and completely blocked the hyperthermia and striatal neurotoxicity induced by d-AMP. CORT supplementation, evaluated in only the C57BL/6J mouse at dosages not capable of involuting either the thymus or the spleen, did not alter d-AMP-induced neurotoxicity. Although the immune system organs of the two strains responded differentially to the removal of CORT, ADX provided equivalent partial protection against the loss of dopaminergic elements in striatum for both strains. Adrenal status clearly affects d-AMP neurotoxicity but the interaction is complex. Future work should examine the roles of the cortical and medullary components of the adrenal gland in the neuroprotective actions of ADX. A precise assessment of the role of circulating CORT In the neurotoxicity of the AMPs will require additional work in which a wider range of CORT dosages, including those capable of involuting thymus and spleen, are evaluated.

Evidence for dissociable mechanisms of amphetamine- and stress-induced behavioral sensitization: effects of MK-801 and haloperidol pretreatment

The present study examined the ability of pretreatment with MK-801 or haloperidol to block the induction of behavioral sensitization to amphetamine challenge by repeated immobilization stress in male Sprague-Dawley rats. Fifteen minutes before each of ten 30-min restraint sessions, rats were administered saline, MK-801 (0.01, 0.10 or 0.25 mg/kg i.p.) or haloperidol (0.10 or 0.25 mg/kg i.p.). Control rats received the same injection regimen without restraint. An additional experiment examined the ability of MK-801 to block the induction of sensitization by repeated d-amphetamine. In this experiment, rats were administered saline or MK-801 (0.25 mg/kg i.p.) 15 min before each of ten amphetamine injections (1.0 mg/kg i.p., administered under the same regimen as immobilization stress). Four days after the final immobilization or amphetamine injection, rats were tested for locomotor response to novelty, saline and d-amphetamine (1.5 mg/kg i.p.). Exposure to repeated immobilization stress significantly enhanced the locomotor response to amphetamine challenge but not to saline challenge whether rats were pretreated with saline, MK-801 or haloperidol. Secondary analysis of dose effects in each pretreatment group revealed that at 0.25 mg/kg, repeated MK-801 in itself induced sensitization to the response to amphetamine in control rats and potentiated stress-induced sensitization in restrained rats. In contrast, the sensitization induced by repeated amphetamine was attenuated by MK-801 pretreatment. Neither dose of haloperidol affected the locomotor response to saline or amphetamine in control or stressed rats. These results indicate that the effects of MK-801 on the induction of sensitization are complex and suggest that amphetamine- and stress-induced behavioral sensitization may arise through different mechanisms.

Effect of adrenalectomy on the initiation and expression of cocaine-induced sensitization

The role of corticosterone in the initiation and expression phases of sensitization to cocaine was examined. To determine the effect of corticosterone on the initiation of sensitization, male Sprague-Dawley rats were given a sham adrenalectomy (ADX), or ADX surgery. Approximately 1 week later, rats were given a cocaine (15 mg/kg, i.p.) injection on day 1. On days 2-6, rats were given cocaine (30 mg/kg, i.p.), and the next day, a cocaine challenge (15 mg/kg) was administered (= early withdrawal). Four days later, 50% of the ADX rats were given corticosterone pellets and corticosterone in the evening drinking water to mimic the circadian variation in corticosterone levels. After 1 week, rats were given a final saline challenge followed by a cocaine challenge (15 mg/kg) the next day (= late withdrawal). Locomotor activity was monitored after cocaine treatment on day 1 and after challenge at early and late withdrawal. Sham controls demonstrated a sensitized locomotor response to the cocaine challenge at early withdrawal, with a slight increase in this behavioral sensitization at the late withdrawal time. In contrast, sensitization was not observed in ADX rats after early withdrawal from cocaine, but this attenuation was not permanent, since ADX animals demonstrated control levels of sensitization by the late withdrawal time 12 days later. Animals given corticosterone replacement 1 week prior to the late cocaine challenge also demonstrated a sensitized response similar to control levels. The effect of corticosterone on the expression of sensitization was examined by administering daily cocaine as before followed by surgery a few days later. The treatment groups were sham, ADX and ADX+corticosterone replacement as described. Fourteen days later, a saline injection was given followed by a cocaine challenge the next day. Behavioral sensitization to a cocaine challenge was found in all three treatment groups. These data suggest that adrenal hormones are necessary during the initiation phase of sensitization when observed after early withdrawal (1 day), but not when sensitization is examined at a later withdrawal time (12 days). In addition, corticosterone levels do not significantly affect the expression phase of behavioral sensitization to cocaine.

Adrenocortical suppression blocks the memory-enhancing effects of amphetamine and epinephrine

This study examined glucocorticoid-adrenergic interactions in modulating acquisition and memory storage for inhibitory avoidance training. Systemically (s.c.) administered amphetamine (1 mg/kg), but not epinephrine (0.1 mg/kg) or the peripherally acting amphetamine derivative 4-OH amphetamine (2 mg/kg), given to rats shortly before training facilitated acquisition performance in a continuous multiple-trial inhibitory avoidance (CMIA) task. Adrenocortical suppression with the 11beta-hydroxylase inhibitor metyrapone (50 mg/kg; s.c.), given to rats 90 min before training, did not block the effect of amphetamine and did not affect acquisition performance of otherwise untreated animals. Retention of CMIA and one-trial inhibitory avoidance was enhanced by either pre- or posttraining injections of amphetamine as well as 4-OH amphetamine and epinephrine. The finding that injections of amphetamine and epinephrine have comparable effects on memory is consistent with the view that amphetamine may modulate memory storage, at least in part, by inducing the release of epinephrine from the adrenal medulla. Metyrapone pretreatment blocked the memory-enhancing effects of amphetamine, 4-OH amphetamine, and epinephrine but did not affect retention performance of otherwise untreated animals. Posttraining injections of different doses of epinephrine (ranging from 0.0001 to 1.0 mg/kg) produced a dose-dependent memory enhancement for inhibitory avoidance training and metyrapone blocked the memory-enhancing effects of all these doses. These findings provide further evidence that the sympathoadrenal and adrenocortical systems are intimately coupled during processes of memory storage.

Dexamethasone pretreatment reduces the psychomotor stimulant effects induced by cocaine and amphetamine in mice

1. The present study examined a comparison of the effect of DEX on psychomotor stimulant effects of cocaine and amphetamine in mice by using the locomotor activity test. 2. Cocaine (10 mg/kg/i.p.) and amphetamine (5 mg/kg/i.p.) increased markedly locomotor activity of mice whereas DEX per se (0.1-1.0-10 mg/kg/i.p.) did not modify the activity of control mice. 3. DEX pretreatment decreased the stimulating effects induced both by cocaine and amphetamine but no consistent dose-related effects were observed. 4. The results suggest that DEX may play an important role on the stimulating effects of cocaine and amphetamine and that it may be of some utility in the clinical management of psychostimulants abuse.

Circulating adrenal hormones are not necessary for the development of sensitization to the psychomotor activating effects of amphetamine

We reported previously that when amphetamine is given in NOVEL test cages both its acute psychomotor activating effects (rotational behaviour and locomotor activity) and the degree of sensitization are greater than when amphetamine is given in HOME cages that are physically identical to the NOVEL test cages. Since exposure to the NOVEL environment increases plasma corticosterone levels (Experiment 1) it is possible that the enhancement in the effects of amphetamine in the NOVEL condition is mediated by corticosterone. If this hypothesis is correct adrenalectomy (ADX) should abolish the difference between the HOME and NOVEL groups. This was tested in three independent experiments, in which the response (rotational behavior in Experiments 2 and 3; locomotor activity and rearing behavior in Experiment 4) to repeated injections of amphetamine was assessed in rats that underwent adrenalectomy (ADX) or a sham operation (SHAM). ADX animals received either no corticosterone replacement or one of two corticosterone replacement treatments. Adrenalectomy, with or without corticosterone replacement treatment, had no significant effect on the development of amphetamine sensitization, either in the HOME or the NOVEL environment. By contrast, the effects of adrenalectomy on the acute response to amphetamine varied depending on the behavioral measure and possibly on the dose of amphetamine (2.0 mg/kg, 3.0 mg/kg and 1.5 mg/kg IP, in Experiments 2, 3 and 4, respectively). We conclude that: (i) a stress-induced secretion of adrenal hormones is not responsible for the enhancement in sensitization to amphetamine seen in animals tested in a NOVEL environment; (ii) circulating adrenal hormones are not necessary for development of sensitization to the psychomotor activating effects of amphetamine.

The development of sensitization to the psychomotor stimulant effects of amphetamine is enhanced in a novel environment

Two experiments were designed to assess the effect of a "novel" environment on the development of sensitization to the psychomotor activating effects of d-amphetamine. In the first experiment, rats with a unilateral 6-hydroxydopamine lesion of the mesostriatal dopamine system received ten daily injections of amphetamine (2 mg/kg), either in their home cages or in novel test cages. The home and novel cages were physically identical (cylindrical transparent buckets), but one group lived and were tested in these cages, whereas the other group was transported from the stainless steel hanging cages where they lived to these novel test cages, for each test session. The first injection of amphetamine produced significantly more rotational behavior in animals tested in a novel environment than in animals tested at home. In addition, animals tested in a novel environment showed greater sensitization than animals tested at home, so the difference between the two groups was even more pronounced following the last injection. In a second experiment, locomotor activity was quantified in rats that received ten injections of either saline or 1.5 mg/kg amphetamine, in their home cages or in a physically identical novel environment. Again, there was a significantly greater locomotor response to the first injection of amphetamine, and greater sensitization, in animals tested in a novel environment than in animals tested at home. These data indicate that environmental factors can exert a large effect on the susceptibility to sensitization, and mechanisms by which this may occur are discussed.