Chronic corticosterone administration enhances behavioral sensitization to amphetamine in mice

The role of corticosterone (CCS) in regulating sensitization to ampetamine's locomotor activating effects was measured in female DBA/2 mice that had been sham-operated or adrenalectomized and implanted with CCS-containing or cholesterol pellets. Three days following surgery, the mice were injected with saline and circular open field locomotor activity was measured for a 5-min time period starting 15 min after injection. Over the next 4 days, amphetamine (1.0–10.0 mg/kg) was injected and locomotor response measured. Control animals (sham-operated, cholesterol pellet) showed increased locomotor activity following their first injection of 5.0 mg/kg and 10.0 mg/kg amphetamine, while ADX animals showed increased activity only after treatment with the 10 mg/kg dose. Chronic CCS treatment did not significantly alter initial responsiveness to amphetamine in either sham-operated or ADX animals, but it did alter the dose-dependent sensitization to amphetamine. Both sham-operated and ADX animals implanted with cholesterol pellets showed increased locomotor respponse to amphetamine (sensitation) following injection with 2.5, 5.0 and 10.0 mg/kg doses of amphetamine. However, the enhancement of locomotor activity was greater in the sham-operated control animals. CCS-treated sham-operated animals exhibited sensitization to the locomotor-activating effects of amphetamine at the lowest dose used (1.0 mg/kg) and increased stereotypy following treatment with the higher doses. ADX/CCS animals developed sensitization to the locomotor-activating effects of amphetamine following chronic injection with the 2.5 mg/kg dose, and showed sensitization to amphetamine-induced stereotypy at higher doses. These data demonstrate that adrenocortical status modulates the effects of chronic and acute amphetamine administration and suggest that CCS may be an important component of stress-induced alterations in amphetamine sensitivity.     

Effects of antalarmin,a CRF type 1 receptor antagonist,on anxiety-like behavior and motor activation in the rat

Molecular studies point to a role for the type 1 corticotropin-releasing factor receptor (CRF(1)) in anxiogenic-like and activating effects of CRF and stress. However, CP-154,526, a selective CRF(1) antagonist, has yielded mixed results in such tests. Few studies have examined the behavioral effects of other CRF(1) antagonists. Therefore, we examined the effects of antalarmin, a structurally related analog of CP-154,526, on anxiety-like behavior and motor activation. Antalarmin blocked the anxiogenic-like effect of CRF in the elevated plus maze, without affecting anxiety-like behavior in vehicle-treated animals. Antalarmin decreased spontaneous defensive withdrawal behavior in a novel, brightly illuminated open field. Finally, antalarmin blocked the activating effects of CRF, but not D-amphetamine, without producing motor sedation. These findings indicate that the CRF(1) receptor mediates anxiogenic-like effects of novelty stress and the anxiogenic-like and activating effects of CRF and support the hypothesis that CRF(1) antagonists may be useful for the pharmacotherapy of pathological anxiety.     

Central administration of corticotropin releasing factor induces long-term sensitization to d-amphetamine

Corticotropin releasing factor (CRF) has been shown to initiate neuroendocrine and behavioral to stree. As stress and amphetamine (AMPH) show cross-sensitization, we investigated the role of endogenous CRF in behavioral sensitization to d-AMPH. In order to evaluate the participation of the central action and the pituitary-adrenocortical (PA) stimulatory effect of CRF, we compared the effects of repeated intracerebroventricular (i.c.v.) administration of CRF (0, 0.5, 2.5 μg/2 μl), which have central and neuroendocrine consequences, with those of repeated subcutaneous admistration of CRF (0, 0.1, 0.5, 2.5 μg/250 μl), doses which only stimulate the PA axis, on the development of sensitization to AMPH-induced motor activation administered 1 week later. Repeated i.c.v. administration of CRF induced a long-lasting enhancement of the hyperactivity by 0.75 mg/kg peripheral administration of d-AMPH, whereas no sensitization to d-AMPH was observed following repeated subcutaneous administration of CRF. These results favor the hypothesis that a centrally mediated action of CRF is involved in the cross-sensitization of psychostimulants and stress.     

Partial reversal of stress-induced behavioral sensitization to amphetamine following metyrapone treatment

Several studies indicate that blockade of stress-induced corticosterone secretion prevents the development of stress-induced sensitization to the behavioral effects of stimulants. The present study examined whether chronic blockade of corticosterone synthesis with metyrapone could reverse stress-induced amphetamine sensitization in rats. Restraint stress in cylindrical chambers, 2 times 30 min/day for 5 days over an 8-day schedule, was used as the stressor. Following completion of the stress protocol, animals were cannulated with microdialysis guide cannulae over the nucleus accumbens, and then treated with either metyrapone (50 mg/kg, i.p.) or vehicle (1 ml/kg) for 7 days. On the seventh day, animals were implanted with microdialysis probes in the nucleus accumbens, and on the following day, all animals were tested for their locomotor, stereotypy, and nucleus accumbens dopamine and DOPAC release responses to an injection of saline followed 60 min later by d-amphetamine (1.5 mg/kg, i.p.). Neither stress or metyrapone treatment had an effect on the behavioral or dopamine release response to saline. However, amphetamine-stimulated locomotion and stereotypy were strongly enhanced, while amphetamine-stimulated dopamine release response was slightly enhanced (significant only by drug×time interaction), in stressed animals. Metyrapone treatment reduced the stress-induced increase in the locomotor, but not stereotypy, response to amphetamine. In contrast, the dopamine release response to amphetamine was enhanced in metyrapone-treated animals, in both stressed and non-stressed groups, while DOPAC levels were unaffected by treatment group. This augmentation was particularly evident in the stressed-metyrapone-treated animals. Furthermore, non-stressed animals showed an increased locomotor and stereotypy response to amphetamine after treatment with metyrapone. These findings indicate that metyrapone treatment can reverse, or inhibit the expression of, stress-induced sensitization to the behavioral effects of amphetamine. However, the ability of metyrapone treatment to enhance the behavioral (in non-stressed animals) and dopamine release (in non-stressed and stressed animals) responses to amphetamine indicate that chronic metyrapone treatment will produce stimulant sensitization when given alone.     

Prolonged exposure to alcohol: effect on CRF mRNA levels, and CRF- and stress-induced ACTH secretion in the rat

We have conducted studies in intact adult male rats, designed to examine the effect of a 3- or 7-day exposure to alcohol (EtOH) on the pituitary's response to corticotropin-releasing hormone (CRF) or stress, and on CRF expression in the hypothalamus. In a first series of experiments, rats exposed to EtOH vapors for 7 days had mean blood alcohol levels (BAL) of 127 ± 12mg%. At the end of the 7-day period, basal plasma ACTH levels were 69 ± 10pg/ml in control rats and 121 ± 23pg/ml in EtOH animals (P 0.01). Resting corticosterone levels were 39 ± 11ng/ml in control animals, and 101 ± 24ng/ml in EtOH rats (P 0.01). In all experiments described here, there was no statistical difference (P > 0.05) between the body weights of controls (animals kept in chambers with normal atmosphere) and EtOH-exposed animals. In control animals, the injection of 0.3–10 μg ovine CRF per rat caused dose-related increases in plasma ACTH levels measured 10 min later. All doses of CRF also significantly (P 0.01) stimulated ACTH secretion by EtOH rats, but there was no clear dose-response curve. Though EtOH-treated animals responded to the lower dose of CRF (0.3 μg) with larger increases in plasma ACTH levels than control rats, the only statistical difference (P 0.01) between control and EtOH rats was observed following administration of 10 μg CRF, which caused a blunted response in EtOH animals. These results suggested that alcohol caused an increased sensitivity and decreased maximum responsiveness of the pituitary to CRF. In a second series of experiments, we examined the effect of a 7-day exposure to EtOH on the hypothalamic-pituitary-adrenal response to mild electroshocks (1.0 mA, 0.5 s duration, 2 shocks/min, 10 min total duration). At the end of the 7-day period, BAL were 158 ± 20mg%, and basal ACTH values were 58 ± 9pg/ml for control rats and 175 ± 52pg/ml for EtOH animals (P 0.01). Resting corticosterone levels for control and EtOH rats were 32 ± 10and115 ± 26ng/ml, respectively (P 0.01). Though stress-induced ACTH levels were statistically equivalent (P > 0.05) in control and EtOH rats (978 ± 220vs768 ± 153pg ACTH/ml), the increase over basal values were significantly (P 0.01) smaller in rats exposed to alcohol. A third series of experiments investigated the effect of a 3- or 7-day exposure to EtOH on CRF expression in the hypothalamus. Both periods of EtOH exposure caused an approximate 2-fold increase (P 0.01) in CRF mRNA levels in the hypothalamus. These results indicate that exposure of male rats to alcohol vapors for 7 days significantly alters the activity of the hypothalamic-pituitary-adrenal axis.     

Stress- and pharmacologically-induced behavioral sensitization increases vulnerability to acquisition of amphetamine self-administration

Individual vulnerability to drug addiction may be an important factor in the prognosis of this pathological behavior in man. However, experimental investigations have largely neglected the psychobiological substrate of predisposition to addiction. In this study, we show using a self-administration (SA) acquisition paradigm that previous repeated exposure to a stressful experience (tail-pinch) or to amphetamine, increase the locomotor response to this drug (behavioral sensitization) and enhance vulnerability to acquire amphetamine SA. These results show that vulnerability to develop amphetamine SA may be influenced by stressful experiences, and that previous contact with the drug may enhance a predisposition to amphetamine-taking behavior. As tail-pinch and amphetamine sensitization affect both the dopamine (DA) neural system and the propensity to self-administer amphetamine (behavior also modulated by DA activity), stress may influence SA via an action on the DA system.     

Stress-induced sensitization to amphetamine and morphine psychomotor effects depend on stress-induced corticosterone secretion

Repeated exposure to stressful situations has been shown to increase individual reactivity to addictive drugs. However, the biological factors involved in such stress-induced changes are largely unknown. In this study, we investigated the role of corticosterone in the effects of restraint stress on the response to psychostimulants and opioids. The effects of repeated stress on amphetamine- and morphine-induced locomotor activity were compared in: (i) animals with an intact hypothalamo-pituitary-adrenal (HPA) axis; (ii) animals in which stress-induced corticosterone secretion was blocked by adrenalectomy, but who received exogenous corticosterone from a subcutaneous implant. The implanted pellets (50 mg) slowly release corticosterone producing a stable plasma level within the normal physiological range over a period of 20 days. Restraint stress increased the locomotor response to both amphetamine (1.5 mg/kg i.p.) and morphine (2 mg/kg s.c.) in animals with an intact HPA axis, but not in animals in which stress-induced corticosterone secretion was suppressed. These results suggest that corticosterone secretion may be one of the mechanisms by which repeated stress amplifies behavioral responses to amphetamine and morphine. Since an enhanced locomotor reactivity to addictive drugs has been found to be frequently associated with an enhanced vulnerability to drug self-administration, these findings point to a role for glucocorticoids in the susceptibility to drug abuse.     

Inhibition of nitric oxide synthase does not block the development of sensitization to the behavioral activating effects of amphetamine

Pretreatment with the nitric oxide (NO) synthase inhibitor, N^G-nitro-L-arginine methyl ester (L-NAME), a competitive blocker of NO production, did not interfere with the development of sensitization to the behavioral activating effects of amphetamine (AMPH). On five pre-exposure sessions, at 3-day intervals, rats were given two i.p. injections, either 50 mg/kgL-NAME 30 min prior to 1.5 mg/kgD-AMPH sulfate, saline and AMPH,L-NAME and saline, or saline only.L-NAME reduced the levels of activity recorded during the pre-exposure session but had no effect on the degree of sensitization shown to a challenge injection of 0.5 mg/kg AMPH given 10 days later. A separate study using in vivo microdialysis showed that pretreatment withL-NAME did not alter AMPH-stimulated dopamine release in nucleus accumbens.     

Amphetamine sensitization of stress-induced turning in animals given unilateral dopamine transplants in infancy

Infant rats given bilateral dopamine-depleting brain lesions and unilateral transplants of embryonic nigral tissue develop turning in response to both amphetamine and stress. However, stress-induced turning did not develop unless animals were previously exposed to amphetamine, and was greatest in animals exposed early to the drug. These findings suggest that amphetamine alters certain properties of the transplanted cells so as to enhance their functional capacity.     

Repeated administration of MK-801 produces sensitization to its own locomotor stimulant effects but blocks sensitization to amphetamine

Repeated amphetamine administration produced behavioral sensitization to subsequent amphetamine challenge. The development of sensitization was blocked by coadministration of the N-methyl-d-aspartate (NMDA) antagonist MK-801. Conditioned locomotion, as revealed by saline challenge, was also blocked by MK-801, suggesting that behavioral sensitization and conditioned locomotion may share a requirement for NMDA receptor stimulation. Repeated MK-801 administration produced behavioral sensitization to MK-801 but not amphetamine challenge, suggesting that MK-801 itself produces sensitization through a different mechanism than amphetamine.     

MK-801 prevents the development of behavioral sensitization during repeated morphine administration

Acute administration of morphine (10 mg/kg) to rats elicited an increase in locomotion that became sensitized upon repeated treatment over 14 days. Administration of the noncompetitive N-methyl-D-aspartate receptor (NMDA) antagonist MK-801 (0.1 or 0.25 mg/kg) prior to each morphine injection prevented the development of behavioral sensitization to morphine, an effect that persisted even after a 7-day withdrawal from repeated treatment. Sensitization was also prevented by coadministration of the competitive NMDA receptor antagonist CGS 19755 (10 mg/kg). In contrast, acute pretreatment with MK-801 did not alter the response of sensitized rats to morphine challenge, indicating that MK-801 does not prevent the expression of sensitization. When administered alone, MK-801 produced stereotyped movements at moderate doses (0.25 rng/kg) and horizontal locomotion at higher- doses, (0.5 mg/kg). Repeated administration of 0.25 mg/kg MK-801 elicited sensitization to its own locomotor stimulatory effects, such that this dose became capable of eliciting horizontal locomotion. Sensitization was not seen during repeated administration of 0.1 mg/kg MK-801 or 10 mg/kg CGS 19755, although both of these pretreatments did produce a sensitized response to subsequent challenge with 0.25 mg/kg MK-801. This effect was enhanced by coadministration of morphine, even though repeated administration of morphine alone failed to sensitize rats to MK-801 challenge. These results suggest a complex interplay between NMDA and opioid receptors, such that NMDA antagonists prevent morphine sensitization while morphine enhances the ability of NMDA antagonists to elicit sensitization to their own locomotor stimulatory effects. © 1994 Wiley-Liss, Inc.     

Corticotropin-releasing factor (CRF) or CRF binding-protein ligand inhibitor administration suppresses food intake in mice and elevates body temperature in rats

Corticotropin-releasing factor (CRF) receptor agonist and CRF binding-protein (CRF-BP) ligand inhibitor peptides both activate CRF systems but exert very distinct functional profiles in animal models of arousal, energy balance and emotionality. The present studies were designed to extend the dissimilar efficacy profiles of central administration of a CRF agonist, r/h CRF(1–41), versus a CRF-BP ligand inhibitor, r/h CRF(6–33), into mouse and rat models of energy balance in order to further explore in vivo efficacy of these ligands in two separate animal species. In CD-1 mice, food intake was significantly attenuated 3 h after acute administration of CRF(1–41) (0.007–0.2 nmol), but not CRF(6–33). In obese Ob/Ob mice, both CRF(1–41) (0.007–0.2 nmol) and CRF(6–33) (0.02–2.3 nmol) significantly attenuated basal feeding over 3 h following acute peptide administration. In rats, CRF(1–41) (1 nmol) and CRF(6–33) (1.5–7.7 nmol) infusion significantly increased rectal temperature. In studies employing a telemetry apparatus, core temperature was also increased by CRF(1–41) (1 nmol) and CRF(6–33) (1.5 nmol), whereas only CRF(1–41) increased locomotor activity and heart rate. These results suggest that CRF receptor agonist administration is capable of producing a global profile of negative energy balance by reducing food intake in mice and increasing energy expenditure in rats. In contrast, CRF-BP ligand inhibitor administration appears to suppress food intake in a mouse strain selective manner and to elevate rectal and core temperature in rats without accompanying cardiovascular activation.     

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.     

Behavioral sensitization to amphetamine is dependent on corticosteroid receptor activation

Thirty rats received 3 amphetamine injections (1.5 mg/kg, s.c.) 6 days apart and the locomotor response was measured. One day before the second injection they were adrenalectomized or sham operated. Corticosteroid replacement treatments (500 μg/kg, s.c.) were given every evening. Sham adrenalectomized animals exhibited behavioral sensitization to successive injections of amphetamine, which was prevented by adrenalectomy. Treatment with corticosterone or deoxycorticosterone did not reverse the effect of adrenalectomy, whereas dexamethasone completely restored and even potentiated sensitization to amphetamine. These results demonstrate that corticosteroids are necessary for sensitization of the dopaminergic system to occur and that they most probably act through the type II (or glucocorticoid) receptor subtype.     

Age-related alterations in emotional behaviors and amygdalar corticotropin-releasing factor (CRF) and CRF-binding protein expression in aged Fischer 344 rats

Corticotropin-releasing factor (CRF) coordinates the mammalian response to stress. In the amygdala, the CRF system appears to be responsible, at least in part, for the behavioral responses resulting from stress. Associated with amygdalar CRF is a 37 kDa binding protein (CRF-BP) which may also play a role in regulating stressful stimuli. Aging has been shown to be associated with abnormal neuroendocrine stress systems and little is known with regards to how amygdalar stress systems change with aging. In our study, we have assessed levels of amygdalar CRF and CRF-BP mRNA in Fischer 344 rats of 4, 12 or 24 months of age following 14 days of hourly restraint. Prior to sacrifice, rats were also tested for anxiety-like behaviors on the elevated plus maze. After behavioral testing, rats were perfused with 4% paraformaldehyde and the brains were processed for in situ hybridization. Twenty micron sections were hybridized with a CRF as well as a CRF-BP riboprobe. Following hybridization, tissue sections were oppossed to X-ray film and relative amounts of mRNA in the amygdala were quantitated. Levels of CRF mRNA in the amygdala of 12 and 24 month-old rats following chronic restraint were significantly lower relative to rats which were handled for 14 days. There were no significant differences in amygdalar CRF gene expression between stressed and handled 4 month-old rats. At 12 and 24 months of age but not 4 months, there were also significant effects of restraint associated with decreases in amygdalar CRF-BP gene expression. Furthermore, there were reciprocal decreases in anxiety-like behaviors in the 12 and 24 month-old rats which were significant; the changes in anxiety-like behaviors between restrained vs. handled 4 month-old rats were not significantly different. The decreased gene expression of CRF in the amygdala in concert with decreased anxiety-like behaviors following restraint is consistent with the known behavioral effects of exogenously applied intra-amygdalar CRF. The changes in amygdalar CRF-BP observed may be secondary to the known regulatory effects that CRF exhibits on its binding-protein. These studies have relevance to better understanding the molecular basis of aging related changes in neuroendocrine stress systems.     

MK-801 blocks the development of sensitization to the locomotor effects of methylphenidate

Male Sprague-Dawley rats were divided to three groups (each n = 8) and were housed in test cages where motor activity was recorded continuously for 16 days using a computerized motor activity monitoring system to determine whether repeated administration of MK-801 could block the development and/or the expression of sensitization to the locomotor effects of methylphenidate (MPD). One group of rats received six daily injections (days 4-9) of 0.30 mg/kg MK-801, followed by 5 days without injection (days 10-14) and re-challenged (day 15) with 0.30 mg/kg MK-801. The second group received a challenge dose of 2.5 mg/kg MPD (day 4) followed by 5 days of co-treatment with MK-801 (0.30 mg/kg) given 1 h prior to MPD (days 5-9). This group was then re-challenged with MPD (2.5 mg/kg) on day 15. The last group received six daily injections of 2.5 mg/kg MPD (days 4-9). They were then split into two subgroups of rats which received either no treatment (control) or five daily injections of 0.30 mg/kg MK-801 (days 10-14) before being re-challenged on day 15 with 2.5 mg/kg MPD. MK-801 sensitized to its own locomotor effects. MK-801 given after sensitization had developed (i.e., days 10-14) was able to mask the expression of a sensitized response on day 15, but the effect was only transient since the sensitized response was present 3 weeks later. Moreover, MK-801, when coadministered during the repeated treatment phase was able to block the development of a sensitized response, which suggest that NMDA receptors involved in the process of MPD sensitization.     

CRF antagonist partially reverses CRF- and stress-induced effects on feeding

Exogenous corticotropin releasing factor (CRF) causes centrally mediated behavioral changes including decreased feeding and increased grooming. These behavioral changes are also seen in response to some stressors. However, the role of endogenous CRF in the behavioral response to stressors has not been investigated fully. We report below our findings on the behavioral effects of alpha-helical CRF (9-41), a recently discovered competitive antagonist of CRF-induced ACTH release. Alpha-helical CRF (9-41) partially reversed the decrement in feeding induced by CRF. Furthermore, the reduction in food intake due to restraint stress was partially reversed by alpha-helical CRF (9-41). These results indicate that changes in endogenous CRF release induced by the restraint stressor may play a role in stress-induced anorexia.     

Urocortin interaction with corticotropin-releasing factor (CRF) binding protein (CRF-BP) : a novel mechanism for elevating ‘free’ CRF levels in human brain

Here we demonstrate that urocortin, a new mammalian member of the corticotropin-releasing factor (CRF) neuropeptide family has high affinity for both the recombinant human CRF binding protein (CRF-BP) and for a membrane-associated form of the protein solubilized from postmortem human cerebrocortical brain tissue. The rank order of binding potency for both the human recombinant CRF-BP and for the solubilized human brain CRF-BP is: urotensin > hCRF > urocortin > sauvagine. The bound hCRF/hCRF-BP complex was detected in the postmortem human brain tissue using an ELISA assay specific for the hCRF/hCRF-BP complex. A large proportion (65%) of the endogenous hCRF was found to be complexed to the CRF-BP and thus unavailable for CRF receptor activation. Incubation of human brain postmortem tissue extracts with urocortin and urotensin resulted in a dramatic decrease in hCRF/hCRF-BP levels and a concomitant increase in ‘free’ hCRF levels. Thus, urocortin and other putative CRF-related peptides may elevate endogenous levels of ‘free’ hCRF in brain by displacing hCRF from the binding protein. These data define an indirect endogenous mechanism for activation of CRF receptors by new mammalian members of the CRF family of neuropeptides.     

Amphetamine sensitization augments amphetamine-induced Fos expression in the lateral habenula

Repeated amphetamine (AMPH) administration results in behavioral sensitization. To investigate the neuroanatomical basis of this phenomenon, we examined the effects of AMPH sensitization on AMPH-induced Fos expression in 24 regions of the rat brain. Rats received repeated injections of AMPH (4 mg/kg, intraperitoneally, once every other day, eight times in total) or saline (same schedule as for AMPH). After a 14-day drug abstinence period, rats were challenged with 2 mg/kg AMPH intraperitoneally. As measured by Fos immunohistochemistry, the AMPH sensitization procedure enhanced subsequent AMPH-induced Fos expression in only one structure, the medial part of the lateral habenula. These results indicate that AMPH-induced behavioral sensitization is not accompanied by widespread increases in the ability of AMPH to increase regional Fos expression in the forebrain. The lateral habenula appears to be involved in the possible neural framework that is responsible for the expression of behavioral sensitization.