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

Homer2 regulates alcohol and stress cross-sensitization

An interaction exists between stress and alcohol in the etiology and chronicity of alcohol use disorders, yet a knowledge gap exists regarding the neurobiological underpinnings of this interaction. In this regard, we employed an 11-day unpredictable, chronic, mild stress (UCMS) procedure to examine for stress-alcohol cross-sensitization of motor activity as well as alcohol consumption/preference and intoxication. We also employed immunoblotting to relate the expression of glutamate receptor-related proteins within subregions of the nucleus accumbens (NAC) to the manifestation of behavioral cross-sensitization. UCMS mice exhibited a greater locomotor response to an acute injection of 2 g/kg alcohol than unstressed controls and this cross-sensitization extended to alcohol intake (0-20 percent), as well as to the intoxicating and sedative properties of 3 and 5 g/kg alcohol, respectively. Regardless of prior alcohol injection (2 g/kg), UCMS mice exhibited elevated NAC shell levels of mGlu1α, GluN2b and Homer2, as well as lower phospholipase Cβ within this subregion. GluN2b levels were also lower within the NAC core of UCMS mice. The expression of stress-alcohol locomotor cross-sensitization was associated with lower mGlu1α within the NAC core and lower extracellular signal-regulated kinase activity within both NAC subregions. As Homer2 regulates alcohol sensitization, we assayed also for locomotor cross-sensitization in Homer2 wild-type (WT) and knock-out (KO) mice. WT mice exhibited a very robust cross-sensitization that was absent in KO animals. These results indicate that a history of mild stress renders an animal more sensitive to the psychomotor and rewarding properties of alcohol, which may depend on neuroplasticity within NAC glutamate transmission.

The competitive NMDA receptor antagonist CPP disrupts cocaine-induced conditioned place preference, but spares behavioral sensitization

Recently, the notion that memory and addiction share similar neural substrates has become widely accepted. N-methyl-d-aspartate receptors (NMDAR) are the cornerstones of synaptic models of memory. The present study examined the effect of the competitive NMDAR antagonist CPP on the induction of behavioral sensitization and conditioned place preference to cocaine. Conditioned place preference is an associative memory model of drug seeking, while sensitization is a non-associative model of the transition from casual to compulsive use. There were three principal findings: (1) co-administration of CPP and cocaine altered the acute response to cocaine, suggesting a direct interaction between the two drugs; (2) NMDAR antagonism had no effect on behavioral sensitization; and (3) NMDAR antagonism abolished conditioned place preference. A review of prior evidence supporting a role for NMDARs in sensitization suggests that NMDAR antagonists directly interfere with cocaine's psychostimulant effects, and this interaction could be misinterpreted as a disruption of sensitization. Finally, we suggest that addiction recruits multiple kinds of plasticity, with sensitization recruiting NMDAR-independent mechanisms.

Long-term upregulation of protein kinase A and adenylate cyclase levels in human smokers

Repeated injections of cocaine and morphine in laboratory rats cause a variety of molecular neuroadaptations in the cAMP signaling pathway in nucleus accumbens and ventral tegmental area. Here we report similar neuroadaptations in postmortem tissue from the brains of human smokers and former smokers. Activity levels of two major components of cAMP signaling, cAMP-dependent protein kinase A (PKA) and adenylate cyclase, were abnormally elevated in nucleus accumbens of smokers and in ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of the catalytic subunit of PKA were correspondingly higher in the ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of other candidate neuroadaptations, including glutamate receptor subunits, tyrosine hydroxylase, and other protein kinases, were within normal range. These findings extend our understanding of addiction-related neuroadaptations of cAMP signaling to tobacco smoking in human subjects and suggest that smoking-induced brain neuroadaptations can persist for significant periods in former smokers.

Pleiotrophin gene transcription in the rat nucleus accumbens is stimulated by an acute dose of amphetamine

Pleiotrophin (PTN) is a heparin-binding protein with diverse functions. For example, it stimulates neurite outgrowth, mitogenesis, repair and differentiation, effects that are similar to those of the neurotrophins. The neurotrophins have, in recent years, been implicated as mediators of structural plasticity, suggested to underlie the development of behavioural sensitisation to many drugs of abuse. Since NMDA receptor antagonists inhibit the underlying morphological changes, the mechanisms are thought to be highly dependent on the activation of the NMDA subtype of glutamate receptors. To investigate if PTN has a possible role in structural plasticity, its responsiveness to an acute dose of amphetamine was studied. Amphetamine is a well-characterised inducer of sensitisation. A group of rats was systemically treated with amphetamine (10 mg/kg) and the effect on the PTN gene transcription was studied 4 h later. A separate group of rats was pretreated with the NMDA receptor antagonist MK-801 (0.25 mg/kg) 30 min prior to the administration of amphetamine. Northern blot analysis revealed a significant increase of the PTN transcript after the administration of amphetamine. However, MK-801 pretreatment did not block this effect; in contrast, it further increased PTN mRNA levels. As the response to the two drugs resembles the one earlier reported on the gene expression of brain-derived neurotrophic factor (BDNF), the present results suggest that PTN may be an attractive protein to study further in the field of synaptic plasticity.

Behavioral sensitization due to social defeat stress in mice: antagonism at mGluR5 and NMDA receptors

RATIONALE

Repeated administration of psychostimulants progressively augments the behavioral response to and increases self-administration behavior of these drugs. Experience of repeated intermittent social defeat stress episodes also leads to a sensitized locomotor response following psychostimulant challenge. Both metabotropic and ionotropic glutamate receptors have been shown to be critical in the induction and expression of stimulant sensitization, but their role in sensitization due to social defeat stress remains unclear.

OBJECTIVE

We evaluated the role of mGluR5 and NMDA glutamate receptors in the development of amphetamine-induced and social defeat stress-induced sensitization, using the non-competitive mGluR5 antagonist, MPEP, and the non-competitive NMDA antagonist, dizocilpine (MK-801).

METHODS

In adult, male CFW mice, sensitization was induced by either ten daily injections of D-amphetamine (1 mg/kg) or ten daily brief episodes of social defeat. Mice were pretreated with MPEP (3 mg/kg or 10 mg/kg) or dizocilpine (0.1 mg/kg) prior to amphetamine injections. Mice subjected to social defeat were pretreated with MPEP (10 mg/kg) or dizocilpine (0.1 mg/kg). Ten days after induction, the expression of locomotor sensitization to amphetamine was determined.

RESULTS

The induction of sensitization due to social defeat stress was prevented by MPEP, yet MPEP did not inhibit the development of behavioral sensitization to amphetamine. Confirming and extending earlier results, dizocilpine pretreatment blocked both amphetamine-induced and stress-induced sensitization.

CONCLUSIONS

These data indicate that behavioral sensitization to social defeat stress is dependent on mGluR5 receptors, whereas low-dose amphetamine sensitization may not be.

Augmentation of drug reward by chronic food restriction: behavioral evidence and underlying mechanisms

Chronic food restriction and maintenance of low body weight have long been known to increase the self-administration and motor-activating effects of abused drugs. Using a lateral hypothalamic self-stimulation (LHSS) rate-frequency method, it is shown that chronic food restriction augments the rewarding (i.e., threshold lowering) effect of diverse drugs of abuse. Further, the effect is attributed to increased sensitivity of a neural substrate, rather than a change in drug bioavailability or pharmacokinetics, because it is preserved when drugs are injected directly into the lateral cerebral ventricle (intracerebroventricularly). The food restriction regimen that augments drug reward also increases the induction of c-fos, by intracerebroventricular amphetamine, in limbic forebrain dopamine (DA) terminal areas. The possibility of increased DA receptor function is suggested by findings that rewarding and motor-activating effects of direct DA receptor agonists are augmented by food restriction, and the augmented behavioral effects of amphetamine are reversed by an otherwise subthreshold dose of D-1 antagonist. Initial studies of DA receptor-mediated signal transduction, that are focused on the D-2 receptor, suggest increased functional coupling between receptor and G-protein (i.e., quinpirole-stimulated [(35)S]GTPgammaS binding) in dorsal striatum. Unlike behavioral sensitization induced by intermittent stress or psychostimulant treatment, which persist indefinitely following induction, the augmenting effect of food restriction abates within 1 week of restored ad libitum feeding and weight gain. The possible involvement of endocrine hormones and/or 'feeding-related' neuropeptides, whose levels change dynamically with depletion and repletion of adipose stores, is therefore under investigation. Initial tests have been limited to acute treatments aimed at attenuating the effects of hypoinsulinemia, hypoleptinemia and elevated corticosterone levels in food-restricted rats. None of these treatments has attenuated the behavioral effect of food restriction. While a melanocortin receptor agonist has been found to enhance drug reward, melanocortin receptors do not seem to mediate the augmenting effect of food restriction. Continuing investigations of endocrine adiposity signals, 'feeding-related' neuropeptides and dopaminergic signal transduction may further elucidate the way in which drugs of abuse exploit mechanisms that mediate survival-related behavior, and help explain the high comorbidity of drug abuse and eating disorders.

Methamphetamine decreases calcium-calmodulin dependent protein kinase II activity in discrete rat brain regions

A Ca(2+)/calmodulin-dependent signaling cascade has been implicated in the regulation of dopaminergic neurotransmission after chronic administration of amphetamine and methamphetamine (METH). We found a decrease in Ca(2+)/calmodulin-dependent protein kinase II (CaM-kinase II) activity in five regions of the rat brain (parietal cortex; frontal cortex; hippocampus; striatum; and nucleus accumbens) after a single injection of METH. Pretreatment with the selective dopamine D1 receptor antagonist SCH 23390 prevented the acute METH-induced decrease in CaM-kinase II activity in the parietal cortex, striatum, nucleus accumbens, and substantia nigra/ventral tegmental area (SN/VTA). Pretreatment with the N-methyl-D-aspartate receptor antagonist MK-801 significantly restored the acute METH-induced decrease in CaM-kinase II activity in the parietal cortex, nucleus accumbens, and SN/VTA. Striatal CaM-kinase II activity was still significantly lower than that of the chronic saline-treated controls after a 1-week, but not a 4-week, abstinence from chronic administration of METH. A METH challenge after a 4-week abstinence period induced a more pronounced decrease in CaM-kinase II activity in rats chronically injected with METH than in rats chronically injected with saline. Western blot analysis revealed that the amount of CaM-kinase II protein was not altered after a single METH injection or after chronic METH injections, compared with saline-treated controls. However, amounts of phosphorylated (Thr(286)) CaM-kinase II in the parietal cortex, striatum and SN/VTA were significantly decreased at 3 h after an acute METH injection compared with saline-treated controls. These results suggest that dephosphorylation of CaM-kinase II may contribute to the decreased enzyme activities induced by acute METH administration, and that chronic treatment with METH leads to an enhanced capacity of METH to decrease CaM-kinase II activity after an extended withdrawal period.

Behavioural and hormonal differences between two Lewis rat lines

Lewis (LEW) is an inbred strain of rats frequently used as an animal model of autoimmune diseases. However, there is evidence that some lines of LEW rats develop autoimmune diseases more readily than do other LEW rat lines. Because the hypothalamus-pituitary-adrenal system is involved in the pathophysiology of these diseases, we compared two LEW lines (SsNHsd and HANRijHsd) in their behavioural and neuroendocrine response to stress. In addition, we studied the psychostimulant effects of acute and repeated amphetamine in these two LEW rat lines. HAN rats were less active in the open field test and showed faster habituation of novelty-induced locomotion. The acoustic startle response was lower in HAN than in SSN rats, whereas prepulse inhibition of the startle response was greater in the HAN than in the SSN LEW subline. Moreover, HAN rats showed impaired acquisition of the two-way active avoidance response relative to SSN rats. The psychostimulant effects of acute amphetamine were smaller in HAN rats. Following repeated injections of amphetamine, behavioural sensitization to the psychostimulant effects of amphetamine was more pronounced in HAN than in SSN rats. Basal concentrations of serum corticosterone did not differ between the two rat lines. Following stress, however, HAN rats showed slightly higher corticosterone secretion than SSN rats. Our results show that two sublines of the LEW inbred strain of rats show profound behavioural differences which are only marginally paralleled by differences at the level of the HPA system.

An integrated view of pathophysiological models of schizophrenia

Pathophysiological processes that underlie the profound neuropsychiatric disturbances in schizophrenia are poorly understood. However, the clinical course of the disease, and a number of clinical and basic science observations, provide direction for formulating pathophysiological models that could be empirically tested. For example, repeated psychostimulant administration to healthy subjects can induce psychotic symptoms, and acute stimulant challenge in schizophrenia patients can precipitate psychosis. Also, NMDA antagonists induce positive, negative, and cognitive schizophrenic-like symptoms in healthy volunteers and precipitate thought disorder and delusions in schizophrenia patients. These human studies provide support for the dopamine and NMDA receptor hypofunction hypotheses of schizophrenia. Well-documented effects of NMDA antagonists on dopamine systems provide a basis to integrate the dopamine and NMDA receptor hypofunction hypotheses. Furthermore, it has become apparent that prominent actions of antipsychotic drugs, especially those with 'atypical' properties, involve antagonism of behavioral, electrophysiological and brain metabolic effects produced by administration of NMDA receptor antagonists. A confluence of clinical and basic science data suggests that an early developmental insult, potentially involving reduced NMDA receptor function, could facilitate sensitization of dopamine systems, leading to the formal onset of schizophrenia in late adolescence and early adulthood. Although clearly speculative, this conceptual model is consistent with existing evidence and suggests lines of future experimental investigation.

Does the noncompetitive NMDA receptor antagonist dizocilpine (MK801) really block behavioural sensitization associated with repeated drug administration?

The term 'behavioural sensitization' refers to the progressively augmented behavioural response that is produced by many drugs of abuse upon their repeated administration. From most of the available data, it has been concluded that co-administration of an N-methyl-D-aspartate (NMDA) receptor antagonist [such as dizocilpine (MK801)] together with the sensitizing drug can block the development of behavioural sensitization. However, the picture might not be that simple. Recent experimental evidence suggests that instead of blocking sensitization, co-administration of dizocilpine enhances the effect of the the sensitizing drug or has more complex effects on the development of sensitization. In this article, Thomas Tzschentke and Werner Schmidt present these two different views and emphasize that the conclusions that can be drawn from sensitization experiments about the effects of dizocilpine and related drugs on behavioural plasticity crucially depend on how, when and under what conditions a test for sensitization is conducted.

The role of excitatory amino acids in behavioral sensitization to psychomotor stimulants

Behavioral sensitization refers to the progressive augmentation of behavioral responses to psychomotor stimulants that develops during their repeated administration and persists even after long periods of withdrawal. It provides an animal model for the intensification of drug craving believed to underlie addiction in humans. Mechanistic similarities between sensitization and other forms of neuronal plasticity were first suggested on the basis of the ability of N-methyl-D-aspartate (NMDA) receptor antagonists to prevent the development of sensitization [Karler, R., Calder, L. D., Chaudhry, I. A. and Turkanis, S. A. (1989) Blockade of "reverse tolerance" to cocaine and amphetamine by MK-801. Life Sci., 45, 599-606]. This article will review the large number of subsequent studies addressing: (1) the roles of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors in the development and expression of behavioral sensitization, (2) excitatory amino acids (EAAs) and the role of conditioning in sensitization, (3) controversies regarding EAA involvement in behavioral sensitization based on studies with MK-801, (4) the effects of acute and repeated stimulant administration on EAA neurochemistry and EAA receptor expression, and (5) the neuroanatomy of EAA involvement in sensitization. To summarize, NMDA, AMPA metabotropic glutamate receptors all participate in the development of sensitization, while maintenance of the sensitized state involves alterations in neurochemical measures of EAA transmission as well as in the expression and sensitivity of AMPA and NMDA receptors. While behavioral sensitization likely involves complex neuronal circuits, with EAAs participating at several points within this circuitry, EAA projections originating in prefrontal cortex may play a particularly important role in the development of sensitization, perhaps via their regulatory effects on midbrain dopamine neurons. The review concludes by critically evaluating various hypotheses to account for EAA involvement in the development of behavioral sensitization, and considering the question of whether EAA receptors are involved in mediating the rewarding effects of psychomotor stimulants and sensitization of such rewarding effects.

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 x 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.

Does dizocilpine (MK-801) inhibit the development of morphine-induced behavioural sensitization in rats?

Intermittent morphine pretreatment (10 mg/kg/day for 14 days) induced long-lasting (one month post-treatment) sensitization to the locomotor effects of morphine and amphetamine in rats. Co-administration of the non-competitive NMDA-receptor antagonist dizocilpine (MK-801) (0.1 mg/kg) with morphine did not prevent the development of long-term behavioural sensitization. However, this dose of MK-801 did cause long-term sensitization to its own locomotor effects. Co-administration of 0.25 mg/kg MK-801 with morphine caused death in 60% of the animals. In the animals that survived MK-801 plus morphine pretreatment, neither short-term (3 days) nor long-term morphine-induced sensitization was observed. MK-801 alone (0.25 mg/kg/day for 14 days) induced short-term cross-sensitization to morphine. Thus, the development of long-term morphine-induced locomotor sensitization could only be prevented by a dose of MK-801 that yields a lethal combination with morphine. In addition, MK-801 induced sensitization to its own locomotor effects and cross-sensitization to morphine. These findings seriously question whether MK-801 can be used to study the development of morphine-induced behavioural sensitization.