Behavioral sensitization: characterization of enduring changes in rotational behavior produced by intermittent injections of amphetamine in male and female rats

Striatal c-fos Induction by Cocaine or Apomorphine Occurs Preferentially in Output Neurons Projecting to the Substantia Nigra in the Rat

Fluorogold or rhodamine-labelled latex beads were injected in the substantia nigra (SN) or the globus pallidus (GP) in order retrogradely to label striatal output neurons that project to the two target structures. Ten days later, striatal c-fos was induced by systemic administration of cocaine (five normal rats; 25 mg/kg cocaine i.p. 2 h before killing) or apomorphine (five unilaterally dopamine-denervated rats; 0.25 mg/kg apomorphine s. c. 2 h before killing), and detection of the Fos protein in the striatum was achieved by immunofluorescence. Sections through the caudate-putamen that displayed good labelling from both SN and GP were selected for a quantitative analysis: the number of retrogradely labelled cells that exhibited Fos immunoreactivity, as well as the total number of retrogradely labelled cells located within a grid (0.16 mm2 in size) were counted manually at 25 x magnification. Cocaine induced a proportionally higher c-fos expression in striato-nigral compared to striato-pallidal neurons, whereas apomorphine activated Fos almost exclusively in striato-nigral neurons. The present findings are consistent with the idea that striatal c-fos induction by dopaminergic agents is primarily mediated by an interaction with D1-receptors, which are thought to be selectively localized on neurons projecting to SN.

The rate of intravenous cocaine administration determines susceptibility to sensitization

The potential for addiction is thought to be greatest when drugs of abuse reach the brain rapidly, because this produces intense subjective pleasurable effects. However, the ability of drugs to induce forms of cellular plasticity related to behavioral sensitization may also contribute to addiction. Therefore, we studied the influence of rate of intravenous cocaine delivery on its ability to induce psychomotor sensitization. In one experiment, rotational behavior in rats with a unilateral 6-hydroxydopamine lesion was used as an index of psychomotor activation, and in a second experiment, locomotor activity in neurologically intact rats was used. Rapid (5-16 sec) intravenous infusions of cocaine induced robust psychomotor sensitization at all doses tested (0.5-2.0 mg/kg). Treatments given over 25 sec failed to induce sensitization at all doses tested. Treatments given over 50 or 100 sec induced sensitization only at the highest dose tested. Thus, the rate of intravenous cocaine delivery has profound effects on the ability of cocaine to induce psychomotor sensitization. This suggests that the temporal dynamics of drug delivery to the brain is a critical factor in the ability of cocaine to induce forms of neuronal plasticity that may contribute to addiction.

Early social isolation, but not maternal separation, affects behavioral sensitization to amphetamine in male and female adult rats

Early life stressful manipulations, such as maternal separation (MS) or social isolation (SI), can influence the neurobiological development of rats and alter the response of adult animals to drugs of abuse. The present study examined the acute and sensitized behavioral responses (locomotor activity (LMA) and stereotypy) induced by amphetamine after MS or SI in male and female rats. In addition, the hypothesis that the combination of SI and MS could lead to additional effects on the behavioral response to amphetamine was tested. After the repetitive, intermittent administration of 1.5 mg/kg D-amphetamine over five consecutive days, the behavioral expression of sensitization to a challenge injection was assessed following a 2-day withdrawal period. In both sexes, MS and SI did not alter the acute locomotor activating effects of amphetamine as measured in the open-field environment after the first administration of the drug. Whereas SI altered the expression of sensitization to amphetamine in both sexes, MS did not affect it. Finally, in none of the behavioral variables measured did MS and SI interact to further modify the behavioral profile of the animals. The present results suggest that a postweaning manipulation of the environment (SI) is more effective than a preweaning manipulation (MS) in modifying the expression of sensitization to amphetamine.

Individual differences in behavioral responses to novelty and amphetamine self-administration in male and female rats

Previous work has shown that individual differences in locomotor activity in an inescapable novel environment can predict acquisition of amphetamine self-administration. The current study examined whether individual differences in approach to novelty in a free choice test could also predict amphetamine self-administration. Further, the current study examined whether individual differences in either free choice or inescapable novelty tests could predict responding for a nondrug reinforcer (sucrose) in the presence and absence of amphetamine. Male and female rats were first tested for their response to free choice novelty (playground maze and novelty-induced place preference tests) and inescapable novelty. They were then tested for acquisition of sucrose-reinforced responding, amphetamine-induced changes in maintenance of sucrose-reinforced responding, and amphetamine self-administration. Based on the inescapable novelty test, acquisition of sucrose-reinforced responding was more rapid in male high responders (HR) compared to low responders (LR). This effect in males did not generalize to females. None of the novelty tests predicted the ability of amphetamine to decrease sucrose-maintained responding. However, using the inescapable novelty test, both male and female HRs self-administered more amphetamine than LRs within the dose range tested (0.03-0.16 mg/kg/infusion). Neither the playground maze nor the novelty-induced place preference test predicted amphetamine self-administration. These results indicate that responses to free choice novelty and inescapable novelty predict different components of amphetamine-induced behavior.

Gender differences in the behavioral responses to cocaine and amphetamine. Implications for mechanisms mediating gender differences in drug abuse

When ovariectomized female rats receive estrogen, the response to the psychomotor stimulants amphetamine or cocaine is enhanced. Estrous cycle-dependent differences in amphetamine-stimulated behaviors and striatal dopamine release are also noted. Intact female rats exhibit a greater behavioral response to amphetamine on estrus than they do on other days of the cycle. Ovariectomy results in attenuation of amphetamine-induced behavior and the striatal dopamine response to amphetamine. Physiological doses of estrogen given to ovariectomized rats reinstate both of these responses to a level comparable to that in estrous females. Furthermore, a sex difference is noted, in that females tend to exhibit a greater behavioral response to the psychomotor stimulants, and estrogen enhances this sex difference. Repeated treatment with amphetamine or cocaine produces a progressive increase in behavioral responsiveness with subsequent drug administration, a process known as sensitization. In rodents, behavioral sensitization results in increases in both frequency and duration of psychomotor behaviors such as rotational behavior, stereotyped grooming, headbobs, and forelimb movements. Interestingly, females display greater sensitization of behaviors in response to psychomotor stimulants than do males. Previous research results are summarized, and new results are presented, demonstrating that estrogen selectively enhances components of behavior that exhibit sensitization in female rats. Results also indicate gender differences in sensitization independent of gonadal hormones, suggesting that the neural systems that undergo sensitization are sexually dimorphic.

Environmental novelty differentially affects c-fos mRNA expression induced by amphetamine or cocaine in subregions of the bed nucleus of the stria terminalis and amygdala

The environmental context in which amphetamine or cocaine are administered modulates both their acute psychomotor activating effects and their ability to induce sensitization. Here we report that environmental context differentially affects patterns of amphetamine- and cocaine-induced c-fos mRNA expression in the bed nucleus of the stria terminalis (BST) and amygdala of male rats. In the medial amygdala and medial posterior BST, exposure to novelty resulted in a marked increase in c-fos mRNA. Amphetamine given at home did not induce c-fos mRNA, and when given in the novel environment, did not increase levels beyond that observed for novelty alone. In the basolateral and lateral amygdala, amphetamine or cocaine at home or exposure to novelty induced c-fos mRNA. When amphetamine or cocaine was given in a novel environment the c-fos mRNA response was significantly enhanced. In the central nucleus of the amygdala (CEA) and oval subnucleus of the BST (BSTov), amphetamine administration at home produced a robust increase in c-fos mRNA expression, whereas exposure to novelty had little effect. In contrast to other brain regions examined, the c-fos mRNA response to amphetamine in a novel versus home environment was significantly smaller. In both "home" and "novel" amphetamine groups, c-fos mRNA in the BSTov and CEA was predominantly expressed in enkephalin-containing cells; coexpression with corticotropin-releasing hormone was rare. These data suggest that the context in which psychostimulants are given powerfully and differentially alters the response of limbic structures that have been functionally implicated in drug reinforcement and emotional behaviors.

Role of estrogen in the acquisition of intravenously self-administered cocaine in female rats

Previous work from this laboratory has revealed that female rats acquired cocaine self-administration at a faster rate than male rats and that a greater percentage of females acquired self-administration [Psychopharmacology 144 (1999) 77.]. It has been suggested that sex differences in stimulant self-administration may be related to ovarian hormones, particularly estrogen. To investigate this possibility, we compared four groups (n = 10) of female rats: ovariectomized (OVX) treated with either estradiol benzoate (EB) or vehicle (VEH), and sham-operated intact (SH) females treated with either the antiestrogen tamoxifen (TAM) or VEH. An autoshaping procedure was used to train rats to lever press for intravenous infusions of cocaine (0.2 mg/kg). The criterion for cocaine acquisition was a mean of 100 self-administered infusions over five consecutive 6-h sessions. Results revealed that 70% of the OVX + EB group and 80% of the SH + VEH group acquired self-administration, while only 30% of the OVX + VEH group and 50% of the SH + TAM group met the acquisition criterion. Rats that had estrogen chemically or surgically blocked exhibited significantly less responding for cocaine over the acquisition testing period, and fewer of these rats met the acquisition criterion compared to intact rats and to OVX rats with estrogen (EB) replacement. The percentages for females with estrogen (70% and 80%) vs. those without (OVX, 30%) were similar to those reported for intact females (70%) and males (30%) in the previous study [Psychopharmacology (2000)]. Taken together, these results suggest that estrogen is a key factor influencing drug-seeking behavior in female rats, and it may underlie sex differences in drug-reinforced responding.

Expression of sensitization to amphetamine and dynamics of dopamine neurotransmission in different laminae of the rat medial prefrontal cortex

The present study investigated the effect of acute and repeated administrations of amphetamine (AMPH) on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the two main cytoarchitectonic subterritories of the medial prefrontal cortex (mPFC) (anterior cingulate and dorsocaudal prelimbic cortices vs ventral prelimbic and rostral infralimbic cortices). Both the acute locomotor effects of AMPH and the expression of behavioral sensitization following its repeated administration were also simultaneously assessed. The repeated, intermittent administration of AMPH over five consecutive days led to a significant sensitized locomotor response to a subsequent challenge that occurred following a 48-h withdrawal period. Basal dialysate DA levels were higher in the ventral mPFC compared with its dorsal counterpart in naive animals, that is prior to the acute administration of AMPH. However, the inverse relationship was observed in animals that had developed sensitization: basal dialysate DA levels were significantly lower in the ventral mPFC compared with the dorsal mPFC. In naïve animals, AMPH produced a significant decrease in DA levels in both the ventral and dorsal subregions of the mPFC. However, the inverse relationship was observed in animals that had developed sensitization: dialysate DA levels in response to AMPH remained significantly decreased in the dorsal mPFC, whereas DA went back to baseline levels in the ventral mPFC. Given that a critical concentration of DA is required for normal function of the mPFC, our results suggest that AMPH-induced changes in DA levels in different subregions of the mPFC are critical for both the acute effects of the drug and the expression of behavioral sensitization to its repeated administration by producing either less or more selectivity or sharpening of stimuli to cortico-cortical dendrites and subcortical synaptic afferents to the pyramidal cells located in the dorso-ventral axis of the mPFC.

Differential regulation of calmodulin content and calmodulin messenger RNA levels by acute and repeated, intermittent amphetamine in dopaminergic terminal and midbrain areas

Repeated doses of psychoactive drugs often produce adaptive responses that differ from the initial drug application and additional adaptive processes occur following cessation of the drug. The relationship between alterations in calmodulin protein and messenger RNA produced by an initial versus a repeated dose of amphetamine was examined, as well as changes following drug cessation. Calmodulin protein and messenger RNA of the three individual calmodulin genes were measured in rat dopaminergic cell body and terminal areas following acute or repeated amphetamine. Rats were either injected once with 2.5mg/kg amphetamine or saline and decapitated after 3h, or given 10 injections of amphetamine three to four days apart and decapitated 3h after the final injection. Calmodulin messenger RNA and protein were also measured three and seven days after ceasing drug treatment. Acute amphetamine increased calmodulin 1.7-fold in the striatum and threefold in the ventral mesencephalon, with corresponding elevations in calmodulin messenger RNAs. In response to the 10th dose of amphetamine, however, the degree of increase in calmodulin was diminished in the striatum and ablated in the ventral mesencephalon. Correspondingly, select species of calmodulin messenger RNA were decreased from control levels. In the frontal cortex or nucleus accumbens, calmodulin levels were basically unaltered by the first or 10th doses of amphetamine, but both calmodulin and its messenger RNA were altered with time upon cessation of the drug. Three days later, both calmodulin protein and messenger RNA were decreased in select brain areas. By seven days after the 10th injection, calmodulin content was altered compared to saline controls in all areas, but the change in messenger RNA no longer paralleled the change in protein.Our findings demonstrate that both calmodulin protein and select species of calmodulin messenger RNA are altered by acute amphetamine, but this effect is attenuated after repeated, intermittent amphetamine. There are further time-dependent changes after cessation of repeated amphetamine, which may reflect compensatory neuronal responses. The alterations in calmodulin content and synthesis could contribute to changes in patterns or duration of behaviors that occur upon cessation of repeated amphetamine.

Context determines the type of sensitized behaviour: a brief review and a hypothesis on the role of environment in behavioural sensitization

Behavioural sensitization to psychostimulants may develop context-dependency in certain circumstances. Animals given a stimulant repeatedly in a test cage but not in other environments may show enhanced drug-induced behaviour in the test cage. Conditioning mechanisms have been claimed to be responsible for these phenomena. However, several recent findings are not properly accounted for by conditioning. In addition, growing evidence supports the hypothesis that behavioural sensitization reflects neural changes induced by repeated exposure to psychostimulants (the pharmacological hypothesis). However, the pharmacological hypothesis itself fails to account for environmental influences. In this paper, we propose a hypothesis on the role of environment that is complementary to the pharmacological hypothesis. According to our hypothesis, environment does not have a causal role in the development of sensitization, but it modifies the mode of expression of the sensitized behaviour. Sensitization primarily reflects a neuroadaptive change induced by repeated exposure of the neural system to psychostimulants. However, psychostimulants are known to induce different behaviours in different environments. Therefore, repeated administration of a psychostimulant in different environments would result in augmentation of different behaviours. Our hypothesis potentially accommodates various previous observations. We briefly review the literature and present our hypothesis.

Gender differences in dopaminergic function in striatum and nucleus accumbens

In female rats the gonadal hormones estrogen and progesterone modulate dopamine (DA) activity in the striatum and nucleus accumbens. For example, there is estrous cycle-dependent variation in basal extracellular concentration of striatal DA, in amphetamine (AMPH)-stimulated DA release, and in striatal DA-mediated behaviors. Ovariectomy attenuates basal extracellular DA, AMPH-induced striatal DA release, and behaviors mediated by the striatal DA system. Estrogen rapidly and directly acts on the striatum and accumbens, via a G-protein-coupled external membrane receptor, to enhance DA release and DA-mediated behaviors. In male rats, estrogen does not affect striatal DA release, and removal of testicular hormones is without effect. These effects of estrogen also result in gender differences in sensitization to psychomotor stimulants. The effects of the gonadal hormones on the striatum and ascending DA systems projecting to the striatum and nucleus accumbens are hypothesized to occur as follows: estrogen induces a rapid change in neuronal excitability by acting on membrane receptors located in intrinsic striatal GABAergic neurons and on DA terminals. The effect of these two actions results in enhanced stimulated DA release through modulation of terminal excitability. These effects of gonadal hormones are postulated to have important implications for gender differences in susceptibility to addiction to the psychomotor stimulants. It is suggested that hormonal modulation of the striatum may have evolved to facilitate reproductive success in female rats by enhancing pacing behavior.

A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

Repeated treatment with psychostimulant drugs causes long-lasting behavioral sensitization and associated neuroadaptations. Although sensitization induced by a single psychostimulant exposure has also been reported, information on the behavioral and neurochemical consequences of a single psychostimulant exposure is sparse. Therefore, to evaluate whether behavioral sensitization evoked by single and repeated psychostimulant pretreatment regimens represent the same neurobiological phenomenon, the time-dependent expression of behavioral, neurochemical, and neuroendocrine sensitization after a single exposure to amphetamine was investigated in rats. A single exposure to amphetamine (5 mg/kg, i.p.) caused context-independent sensitization of the locomotor effects of amphetamine, which intensified over time. Thus, sensitization to amphetamine was marginal at 3 d after treatment and more evident after 1 week, whereas 3 weeks after treatment, profound sensitization, as well as cross-sensitization, to cocaine was observed. Amphetamine pretreatment caused an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens, caudate putamen, and medial prefrontal cortex slices and of [(14)C]acetylcholine from accumbens and caudate slices. The hyperreactivity of dopaminergic nerve terminals appeared to parallel the development of locomotor sensitization, i.e., whereas hyperreactivity of accumbens dopaminergic terminals increased between 3 d and 3 weeks after treatment, the hyperreactivity of medial prefrontal dopaminergic terminals decreased. Pre-exposure to amphetamine also sensitized the hypothalamus-pituitary-adrenal axis response to amphetamine at 1 and 3 weeks, but not at 3 d after treatment. Because these data closely resemble those reported previously for repeated amphetamine pretreatment, it is concluded that a single exposure to amphetamine is sufficient to induce long-term behavioral, neurochemical, and neuroendocrine sensitization in rats.

Buprenorphine potentiates L-DOPA-induced contralateral rotation in 6-hydroxydopamine-treated rats

Animals are commonly given opioid analgetics such as buprenorphine for post-operative pain management. In this study, the effect of the analgetic buprenorphine, a partial mu receptor agonist and kappa receptor antagonist, on L-DOPA-induced contralateral rotation was measured in 6-hydroxydopamine (6-OHDA) treated rats. Male Sprague-Dawley rats received dopamine-depleting brain lesions by infusion of 6-OHDA into the medial forebrain bundle. After the procedure, buprenorphine was administered (430 microg/kg, s.c.) to 17 of 54 animals. Three weeks after 6-OHDA treatment, animals were given benserazide HCI (25 mg/kg, i.p.) and L-DOPA (4 mg/kg, i.p.). Contralateral rotations were monitored for 2 h. Animals receiving buprenorphine had significantly higher rates of rotation as compared with non-buprenorphine-treated animals (P = 0.023). The results suggest that buprenorphine sensitizes animals to the effects of L-DOPA.

Strain and sex differences in the locomotor response and behavioral sensitization to cocaine in hyperactive rats

Individual variability in the behavioral responsiveness to psychostimulant drugs is due, in part, to genetic factors. The present study investigated the effects of acute and repeated administrations of cocaine (0, 5, 10 or 20 mg/kg, i.p.) on locomotor activity in male and female rats from genetically distinct strains often used as a model of human childhood hyperactivity/attentional deficit disorder: Wistar Kyoto Hyperactive (WKHA) rats, Spontaneous Hypertensive rats (SHR) and their control Wistar Kyoto (WKY). The results, expressed as percent change in locomotor activity relative to respective control groups, showed that cocaine elicits a dose-dependent hyperactivity in all strains and revealed neither strain nor sex differences in acute sensitivity to moderate doses of the drug. Nevertheless, across repeated administrations, strain and sex differences appeared: WKHA rats displayed a moderate extent of sensitization to psychomotor stimulant effects of cocaine and female rats showed more robust sensitization than males, whatever the strain. These findings support the genotype-dependence in the development of behavioral sensitization to cocaine and confirm the robustness of the sexual dimorphism across different inbred rat strains. Interestingly, the present results demonstrate that sensitization to psychostimulant drugs occurs in genetically hyperactive strains as well as in their normoactive control strain.

Alterations in the morphology of dendrites and dendritic spines in the nucleus accumbens and prefrontal cortex following repeated treatment with amphetamine or cocaine

Repeated treatment with psychostimulant drugs produces changes in brain and behaviour that far outlast their initial neuropharmacological actions. The nature of persistent drug-induced neurobehavioural adaptations is of interest because they are thought to contribute to the development of dependence and addiction, and other forms of psychopathology, e.g. amphetamine psychosis. There are many reports that psychostimulants produce biochemical adaptations in brain monoamine systems, especially dopamine systems. The purpose of the present study was to determine if they might also alter the morphology of neurons in brain regions that receive monoaminergic innervation. Rats were given repeated injections of either amphetamine or cocaine, or, to control for general motor activity, allowed access to a running wheel. They were then left undisturbed for 24-25 days before their brains were processed for Golgi-Cox staining. Treatment with either amphetamine or cocaine (but not wheel running experience) increased the number of dendritic branches and the density of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens, and on apical dendrites of layer V pyramidal cells in the prefrontal cortex. Cocaine also increased dendritic branching and spine density on the basilar dendrites of pyramidal cells. In addition, both drugs doubled the incidence of branched spines on medium spiny neurons. It is suggested that some of the persistent neurobehavioural consequences of repeated exposure to psychostimulant drugs may be due to their ability to reorganize patterns of synaptic connectivity in the nucleus accumbens and prefrontal cortex.

Amphetamine-induced behavior, dopamine release, and c-fos mRNA expression: modulation by environmental novelty

We have shown recently that the psychomotor activating effects of amphetamine in the rat are much greater when this drug is administered in association with environmental novelty than when it is given in a home environment. The main purpose of the present study was to explore the neural basis of this phenomenon. We found, using in situ hybridization of c-fos mRNA, that the pattern of neuronal activation in the cortex, in the caudate, in the shell and core of the nucleus accumbens, and in other subcortical structures was markedly different when amphetamine (2.0 mg/kg, i.p.) was given in association with exposure to environmental novelty relative to when it was given at home. In most brain regions the magnitude of c-fos expression was over two times greater in rats given amphetamine plus novelty than in rats given amphetamine alone. In contrast, an in vivo microdialysis study indicated that environmental novelty did not affect amphetamine-induced dopamine release in either caudate or nucleus accumbens. Furthermore, a unilateral 6-hydroxydopamine lesion of the mesostriatal dopamine system reduced amphetamine- but not novelty-induced c-fos expression. Finally, we found no differences in the amount of corticosterone secreted after exposure to novelty, amphetamine, or both, suggesting that corticosterone does not play a critical role in the ability of novelty to modulate amphetamine-induced psychomotor activation. In conclusion, it seems that environmental novelty alters the neurobiological effects of amphetamine independently of the primary neuropharmacological actions of this drug in the striatum.

Enhanced behavioral response to repeated d-amphetamine and personality traits in humans

BACKGROUND

This study examined whether the magnitude of the behavioral response to repeated d-amphetamine administration previously demonstrated in a double-blind study in humans was associated with certain personality characteristics.

METHODS

Eleven normal volunteers completed the Tridimensional Personality Questionnaire (TPQ) prior to being administered three doses of d-amphetamine (0.25 mg/kg) alternating with three doses of placebo over 6 consecutive days. Behavioral measures included eye-blink rates as well as ratings of elevated mood, increased motor activity/energy, and increased speech. These variables were assessed once prior to dosing, then hourly for 5 hours.

RESULTS

A greater magnitude of change in elevated mood over the three amphetamine doses significantly correlated with ratings of the Novelty Seeking on the TPQ.

CONCLUSIONS

These results suggest that similar to findings in animal models, there are certain intrinsic behavioral characteristics that are associated with amphetamine sensitivity in humans.

Growth/differentiation factor 5 and glial cell line-derived neurotrophic factor enhance survival and function of dopaminergic grafts in a rat model of Parkinson's disease

Growth/differentiation factor 5 is a member of the transforming growth factor beta superfamily, which has neurotrophic and neuroprotective effects on dopaminergic neurons both in vitro and in vivo. Here we investigate the effects of growth/differentiation factor 5 on foetal mesencephalic grafts transplanted into a rat model of Parkinson's disease, and compare them with those of glial cell line-derived neurotrophic factor. Mesencephalic tissue was suspended in solutions containing either growth/differentiation factor 5 or glial cell line-derived neurotrophic factor prior to transplantation into the left striatum of rats with 6-hydroxydopamine lesions of the left medial forebrain bundle. Both proteins enhanced graft-induced compensation of amphetamine-stimulated rotations. Positron emission tomography studies showed that both neurotrophins increased graft-induced recovery of striatal binding of [11C]RTI-121, a marker for dopaminergic nerve terminals. Post mortem analysis at 8 weeks after transplantation showed that both neurotrophins significantly increased the survival of grafted dopaminergic neurons. This study shows that growth/differentiation factor 5 is at least as effective as glial cell line-derived neurotrophic factor in enhancing the survival and functional activity of mesencephalic grafts, and thus is an important candidate for use in the treatment of Parkinson's disease.

Alterations in calmodulin mRNA expression and calmodulin content in rat brain after repeated, intermittent amphetamine

To assess whether calmodulin (CaM) gene expression could have a role in behavioral sensitization induced by repeated, intermittent amphetamine, CaM protein and mRNA of the three separate CaM genes were measured in several different brain areas from rats repeatedly administered saline or amphetamine. Rats were injected twice weekly for five weeks, followed by one week of withdrawal. CaM protein and mRNA were measured in dorsal striatum, limbic forebrain, prefrontal cortex, ventral mesencephalon and piriform cortex. There were increases of CaM protein content and decreases of CaM I mRNA in the dorsal striatum and prefrontal cortex. CaM II mRNA was also decreased in the dorsal striatum. A decrease of CaM protein and an increase of CaM I mRNA were found in the ventral mesencephalon. There was no change of CaM protein in the limbic forebrain, although a decrease of CaM I mRNA was detected. CaM protein and mRNA were not altered in the piriform cortex. Our findings demonstrate that both CaM content and mRNA are altered after an amphetamine regimen leading to sensitization. The fact that the changes in CaM content and mRNA are in dopaminergic brain areas associated with sensitization suggests that CaM could contribute to neurochemical events underlying behavioral sensitization to amphetamine.

Neuropsychopharmacological mechanisms of stimulant drug action in attention-deficit hyperactivity disorder: a review and integration

The psychostimulants, D-amphetamine (D-AMP) and methylphenidate (MPH), are widely used to treat attention-deficit hyperactivity disorder (ADHD) in both children and adults. The purpose of this paper is to integrate results of basic and clinical research with stimulants in order to enhance understanding of the neuropharmacological mechanisms of therapeutic action of these drugs. Neurochemical, neurophysiological and neuroimaging studies in animals reveal that the facilitative effects of stimulants on locomotor activity, reinforcement processes, and rate-dependency are mediated by dopaminergic effects at the nucleus accumbens, whereas effects on delayed responding and working memory are mediated by noradrenergic afferents from the locus coeruleus (LC) to prefrontal cortex (PFC). Enhancing effects of the stimulants on attention and stimulus control of behavior are mediated by both dopaminergic and noradrenergic systems. In humans, stimulants appear to exert rate-dependent effects on activity levels, and primarily enhance the motor output, rather than stimulus evaluation stages of information-processing. Similarity of response of individuals with and without ADHD suggests that the stimulants do not target a specific neurobiological deficit in ADHD, but rather exert compensatory effects. Integration of evidence from pre-clinical and clinical research suggests that these effects may involve stimulation of pre-synaptic inhibitory autoreceptors, resulting in reduced activity in dopaminergic and noradrenergic pathways. The implications of these and other hypotheses for further pre-clinical and clinical research are discussed.

Drug conditioning induced by intrastriatal apomorphine administration

The present study examined (1) whether the neostriatum is involved in a drug-induced conditioned locomotor response and; (2) whether this structure participates in the development of behavioral sensitization. Moreover, the present study addressed the question whether the development of behavioral sensitization is necessary for the induction of conditioning. Rats received injections of either apomorphine (2 microg) or vehicle (solution of 0.1% ascorbate/saline) into the dorsal neostriatum daily for 7 days. These treatments were performed immediately prior to (apomorphine-paired group and vehicle group) or 30 min following (apomorphine-unpaired group) 10-min placement in an open field which served as the test environment. After a 3-day drug withdrawal period, the animals were given a 10-min non-drug vehicle test trial in the test environment. Three days later, a drug test with apomorphine was administered to the animals of the paired and unpaired treatment groups; the vehicle group again received an injection of vehicle. The analysis of locomotor activity in the open field (measured as the distance traversed) revealed that locomotor activity in the apomorphine-paired group was higher than in the other groups. There were no indications for behavioral sensitization to intrastriatal apomorphine, since the locomotor response in the apomorphine-paired group did not increase, but rather decreased with daily repeated injections of apomorphine. Furthermore, only the apomorphine-paired animals showed a higher locomotor response when tested after an intrastriatal injection of vehicle in the previously apomorphine-paired environment, which is indicative of a conditioned drug effect. These results suggest that the neostriatum is directly involved in the development of drug-induced conditioning of locomotor behavior but not in the establishment of behavioral sensitization.

Importance of initial environments in the development of ambulatory sensitization to methamphetamine and cocaine in mice

Repeated administration of CNS stimulants such as amphetamines and cocaine induces behavioural sensitization which can be influenced by the animal's environment. This study has evaluated the effect of restraint on the development and maintenance of ambulatory sensitization to methamphetamine and cocaine in mice. Subcutaneous administration of the CNS stimulants methamphetamine (2 mg kg(-1)) and cocaine (20 mg kg(-1)) seven times at three-day intervals resulted in ambulatory sensitization when the mice were placed in 20-cm diameter activity cages after each dose of the drug. However, if methamphetamine or cocaine was administered when the mice were in small jars (6-cm diameter) in which expression of ambulation, but not of circling and rearing, was completely restricted, the development of ambulatory sensitization was retarded or inhibited, with circling behaviour concurrently increased, when subsequent repeated doses of the drug were administered in the activity cage. Subsequent repeated treatment of ambulatory-sensitized mice with the drug or saline when the mice were in the jars did not change the levels of the ambulatory sensitization or the circling behaviour. These results suggest that the mice are sensitized to the behavioural effect of CNS stimulants which can be expressed in the environment in which the drug is administered. It is also considered that the established sensitization is strongly retained and is responsible for retardation or suppression of the development of sensitization to other behavioural stimulant effects.

Enhanced response to repeated d-amphetamine challenge: evidence for behavioral sensitization in humans

Behavioral sensitization is the process whereby intermittent stimulant exposure produces a time-dependent, enduring, and progressively more robust behavioral response. This process serves as an important model of neural plasticity and has also been proposed as a model for a variety of psychiatric syndromes; however, there are no published controlled studies of behavioral sensitization in human subjects. The authors report results from a double-blind, placebo-controlled study of repeated d-amphetamine challenges in a sample of normal human volunteers. Eleven consecutively recruited normal volunteers participated in this 4-day protocol. Each subject received two daily doses of d-amphetamine (0.25 mg/kg) separated by 48 hours that alternated with two daily doses of matched placebo. Symptoms (activity/energy level, mood, rate, and amount of speech) and eye-blink rates were measured hourly for 5 hours following drug administration. All four measures demonstrated significantly enhanced increases following the second amphetamine dose as compared to the first amphetamine dose and both placebo conditions. These findings suggest that behavioral sensitization is measurable in human subjects.

Signalled versus unsignalled intravenous amphetamine: large differences in the acute psychomotor response and sensitization

Both the acute psychomotor response and the development of sensitization to amphetamine are attenuated if i.p. injections are given in the cage where animals live (HOME), relative to when injections are given in a novel (NOVEL), but otherwise physically identical cage. It was suggested that this effect of environment on sensitization may be due to the relative availability of cues predictive of drug administration in the two conditions. It was hypothesized, therefore, that removal of all environmental cues predictive of drug administration would attenuate the development of sensitization even further. This was accomplished by comparing the psychomotor activating effects (rotational behavior) of repeated unsignalled intravenous (i.v.) infusions of 1.0 mg/kg amphetamine given in a HOME environment with those of signalled i.v. infusions given in a NOVEL environment. It was found that signalled i.v. amphetamine administration (NOVEL) produced a large acute psychomotor response, and repeated administrations resulted in a significant increase in psychomotor response (i.e., sensitization). In contrast, the same treatment in the HOME condition produced only a very small acute response and no sensitization. Indeed, the magnitude of the psychomotor response to an amphetamine challenge varied approximately 23-fold as a function of past drug history and environmental condition. It is suggested that this paradigm provides a powerful new model to study how environmental factors modulate responsiveness to psychoactive drugs.

Effect of age and sex on N-methyl-D-aspartate antagonist-induced neuronal necrosis in rats

BACKGROUND AND PURPOSE

Although N-methyl-D-aspartate (NMDA) antagonism may be a useful therapeutic approach in stroke treatment, it has been found that these pharmacological agents cause neuronal necrosis in restricted cortical regions of the rodent brain.

METHODS

To test the hypothesis that age and sex influence NMDA antagonist-induced neuronal necrosis, male and female rats were studied at 2 months (young), 12 months (middle-aged), and 24 months (old) of age. A dose of 5 mg/kg MK-801 was administered, followed by quantitation of neuronal necrosis at nine coronal levels in the cingulate and retrosplenial cortex at 1 week of survival.

RESULTS

Mortality was dependent on age but not sex and was higher in the old rats (P<.01). The number of necrotic neurons per hemisphere was greater in female than in male rats at all ages (P<.0001). Female rats also showed increasing neuronal necrosis with age (P<.05).

CONCLUSIONS

The results indicate a major sex difference in neuronal cytotoxicity caused by NMDA antagonists and a minor increase in susceptibility with increasing age in females. The findings may be relevant to development of drugs with NMDA antagonist properties for use in human stroke.

Comparative behavioral sensitization to stereotypy by direct and indirect dopamine agonists in CF-1 mice

The present experiments were designed to compare the properties of behavioral sensitization induced by the indirect agonists, amphetamine and cocaine, to that induced by the direct dopamine agonists, apomorphine and PPHT. Both classes of agonist produced sensitization when administered either in relatively low daily doses or in a single high dose. Mice sensitized to the indirect agonists were cross-sensitized to the direct agonists and vice versa. A pharmacological evaluation of the sensitization induced by the two types of agonist demonstrated both similarities and dissimilarities. Induction to the indirect agonists is blocked by CPP, DNQX and diltiazem, whereas only CPP and diltiazem blocked induction to the direct agonists. Furthermore, although none of these antagonists block the expression of sensitization by the direct agonists, all three were previously shown to block the amphetamine expression of sensitization. Striking differences were also observed in the persistence of the sensitization induced by the two types of agonists. While the indirect agonist-induced sensitization is long lasting, the direct agonist-induced sensitization is relatively short-lived. Furthermore, cross-sensitization of PPHT in amphetamine-sensitized animals was also short-lived, as was amphetamine cross-sensitization in PPHT-sensitized animals. The data suggest that the induction of sensitization consists of two separable mechanisms, one for induction per se, the other for persistence.

Administration of amphetamine does not increase the functional efficacy of dopaminergic grafts made in infancy

Previous reports have evoked the possibility that a priming stimulation of grafted dopaminergic (DA) neurones by amphetamine enhances their efficacy in behavioural tests performed several days later. The present study was designed to test this hypothesis. Five days after the unilateral destruction of the DA mesotelencephalic system of 3-day-old rat pups, DA grafts were implanted into the denervated neostriatum of half of the lesioned pups. At adulthood, lesion and graft groups were subdivided into 4 subgroups which received one of the following treatments: saline or amphetamine injection in an environment where the behavioural test was subsequently conducted (paired environment) or in an unrelated environment (unpaired environment). Five days later, rotational response to a tail-pinch stress was tested in the paired environment. In these conditions, we found no evidence for a priming effect of amphetamine. Animals that received amphetamine or saline in the unpaired environment displayed the same rotational response to the tail-pinch stress. On the other hand, a conditioning influence of the environment was detected. Thus, the effect previously described might have been caused by a conditioning effect and/or might be due to differences in the experimental conditions. This suggests that 'priming' the graft with amphetamine does not provide a general strategy to enhance the functional efficacy of DA grafts.

Effect of repeated methamphetamine administrations on dopamine and glutamate efflux in rat prefrontal cortex

Pretreatment with psychostimulants such as methamphetamine (METH) results in augmented mesostriatal dopamine transmission upon a challenge administration of the drug. This effect can be blocked by dopamine antagonists and excitatory amino acid antagonists. However, no direct comparisons have been made with respect to the effects of a low-dose pretreatment regimen of METH on impulse and transporter-mediated dopamine release or to what extent glutamate release is altered by a pretreatment regimen with METH. The purpose of this study was to examine dopamine and glutamate efflux in the prefrontal cortex and striatum in rats pretreated with METH following either high potassium (80 microM) infusion or after a systemic injection of a low dose of METH. Extracellular dopamine and glutamate concentrations in the prefrontal cortex and striatum were measured in vivo by microdialysis. Potassium infusion increased extracellular dopamine and glutamate concentrations to a greater extent in the prefrontal cortex than in the striatum of METH-pretreated rats compared to saline-pretreated controls. A low dose METH challenge significantly increased extracellular dopamine but not glutamate concentrations in both prefrontal cortex and striatum of all animals. Moreover, the acute METH-induced increased in cortical dopamine efflux was significantly greater in rats pretreated with METH. Overall, these data are the first evidence that repeated METH administrations can enhance cortical glutamate efflux and indicate that a low dose pretreatment regimen of METH enhances dopamine transmission in the prefrontal cortex through both transporter and depolarization-induced mechanisms.

Effects of acute and repeated ethanol exposures on the locomotor activity of BXD recombinant inbred mice

Investigations of ethanol's (EtOH's) complex response profile, including locomotor and other effects, are likely to lead to a more in-depth understanding of the constituents of alcohol addiction. Locomotor activity responses to acute and repeated EtOH (2 g/kg, ip) exposures were measured in BXD recombinant inbred (RI) mice and their C57BL/6J (B6) and DBA/2J (D2) progenitors. Both the acute response and the change in initial EtOH response with repeated treatments were strain-dependent. The coefficient of genetic determination was 0.38-0.49 for initial locomotor response to EtOH, and 0.29 for change in response. Changes in response were largely attributable to sensitization of locomotor stimulation. Quantitative trait loci (QTL) analyses identified significant marker associations with basal activity, acute locomotor response, and change in response. Markers were for QTL on several chromosomes, and there was only one case of overlap in marker associations among phenotypes. Acute locomotor response and locomotor sensitization were negatively correlated with 3% EtOH preference drinking data collected in BXD RI strains. Overall, these results demonstrate locomotor sensitization induced by EtOH, suggest independence of genetic determination of locomotor responses to acute and repeated EtOH exposure, and partially support a relationship between reduced sensitivity to the locomotor stimulant/sensitizing effects of EtOH and EtOH consumption.

Influence of novel versus home environments on sensitization to the psychomotor stimulant effects of cocaine and amphetamine

The acute psychomotor response (rotational behavior in rats with a unilateral 6-OHDA lesion), and the development of sensitization, were studied in rats that received seven consecutive daily injections of amphetamine (Experiment 1) or cocaine (Experiment 2) either at home or in a 'novel' test environment. The home (HOME) and novel (NOVEL) cages were physically identical, but one group lived and was tested in these cages, whereas the rats in the other group were transported from the stainless steel hanging cages where they lived, to those NOVEl test cages, for each test session. In Exp. 1, the acute psychomotor response to 3.0 mg/kg of amphetamine i.p. and the development of sensitization (increase in the rotational response between the first and the the seventh test session) were greater in the NOVEL than in the HOME environment. In Expt. 2, there were no significant group differences in the acute response to 20 mg/kg of cocaine i.p., but the animals tested in the NOVEL environment showed greater sensitization than animals tested in the HOME environment. In addition, the animals pretreated with cocaine in the NOVEL environment, but not those pretreated with cocaine in the HOME environment, showed conditioned rotational behavior in response to an injection of saline. These data indicate that: (i) sensitization to the psychomotor activating effects of both amphetamine and cocaine is enhanced in a NOVEL environment; (ii) this phenomenon appears to the independent of the effects of the NOVEL environment on the acute response to these drugs; (iii) a robust conditioned psychomotor response to contextual cues develops only when cocaine treatments are given in the NOVEL test environment.

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.

Collagenase-induced intrastriatal hemorrhage in rats results in long-term locomotor deficits

BACKGROUND AND PURPOSE. Previous studies have shown that injection of the metalloproteinase collagenase directly into the caudate nucleus of rats causes an intracerebral hemorrhage. The purpose of the present study is to determine functional deficits associated with a collagenase-induced hemorrhagic lesion of the striatum.

METHODS

Twelve adult rats received a 2-microL infusion of bacterial collagenase (0.5 U in saline) into the right striatum. The rotational response to apomorphine (1 mg/kg SC) administration was then examined at 1, 4, 7, 21, 35, and 70 days after the surgery. In addition to the rotational asymmetry studies, the initiation of stepping movements in each forelimb was determined 8 weeks after the collagenase injections. In the assessment of rotational asymmetry and stepping ability, an additional six control animals received unilateral injections of saline alone. After behavioral testing, brains were processed for neuropathological evaluation.

RESULTS

A net ipsilateral rotation was noted at all posthemorrhage time periods. The average rotational asymmetries on these days were 14.57 +/- 2.9, 20.33 +/- 2.7, 19.99 +/- 4.4, 18.95 +/- 4.9, 17.03 +/- 4.9, and 14.4 +/- 4.7, respectively (data expressed as mean clockwise rotations per 5 minutes +/- SEM). The average number of steps initiated by the forelimb ipsilateral and contralateral to the lesion was 28.3 +/- 2.1 steps per minute and 13.6 +/- 1.5 steps per minute, respectively. This difference between left and right forelimb stepping was stable and reproducible for 3 consecutive days. Histological studies revealed a long-lasting hematoma cavity surrounded by dense reactive gliosis in the striatum.

CONCLUSIONS

We conclude that collagenase-induced intrastriatal hemorrhage results in long-term locomotor deficits and is therefore a useful model for developing and assessing therapeutic approaches for the restoration of neurological function after intracerebral hemorrhage.

Amphetamine-induced time-dependent sensitization of dopamine neurotransmission in the dorsal and ventral striatum: a microdialysis study in behaving rats

The purpose of this study was to compare the effects of amphetamine exposure on subsequent amphetamine-induced changes in behavior and dopamine (DA) release in the dorsal and ventral striatum, as a function of time following the discontinuation of repeated amphetamine treatment. Rats were pretreated with either saline or an escalating-dose amphetamine regimen, and then received a 0.5 mg/kg amphetamine "challenge" after either 3, 7, or 28 days of withdrawal. Animals tested after 28 days of withdrawal were hypersensitive (sensitized) to the locomotor-activating effects of amphetamine, and relative to control animals showed a significant enhancement in amphetamine-stimulated DA release in both the dorsal and ventral striatum, as revealed by in vivo microdialysis. Animals tested after only 3 or 7 days of withdrawal showed neither behavioral sensitization nor enhanced amphetamine-stimulated DA release. These results establish that time-dependent changes in behavioral sensitization to amphetamine are associated with time-dependent changes in amphetamine-stimulated DA release, and support the hypothesis that persistent sensitization-related changes in striatal DA neurotransmission contribute to the expression of behavioral sensitization.

Social deprivation enhances the vulnerability of male Wistar rats to stressor- and amphetamine-induced behavioral sensitization

The aim of the present study was to assess the influence of experiential factors on the vulnerability of rats to develop amphetamine (AMPH)- and stressor-induced behavioral sensitization. Young male Wistar rats with previous social experience were isolated from their peers for 2 weeks. 1) The effect of this short-lasting social deprivation were: a) a reduced tendency to explore a fearful environment; b) a prolonged exploratory activity in response to a novel but little fearful environment; and c) a dose-dependent increase in the psychomotor stimulation induced by systemic AMPH injection. 2) After repeated AMPH injections (injection every other day for 10 days), isolated rats exhibited behavioral sensitization at lower doses (0.5 and 0.75 mg/kg) than those required for group-housed rats (1 mg/kg). 3) After being submitted to a repeated stressor (3, 7 or 14 footshock sessions, with 2 days between sessions), the isolated rats exhibited a greater increase in the behavioral responsivity to a subsequent AMPH challenge (1 mg/kg) than did the group-housed rats regardless of the number of stress sessions. In conclusion, these results suggest that experiential factors such as privation of contact with peers (social isolation) may make rats more vulnerable to the long-term repercussions of chronic environmental and pharmacological challenges.

NMDA receptor-mediated expression of Fos protein in the rat striatum following methamphetamine administration: relation to behavioral sensitization

In order to clarify the possible involvement of N-methyl-D-aspartate (NMDA) receptors in mediating striatal Fos protein induction and behavioral sensitization after methamphetamine administration, we examined the effects of non-competitive NMDA receptor antagonist MK-801 on these phenomena in rats. A single administration of 1.0 and 5.0 mg/kg methamphetamine resulted in a dose-dependent increase in Fos-immunoreactive cells in the medial striatum. Prior exposure to 5.0 mg/kg methamphetamine enhanced ipsilateral rotational behavior in response to subsequent methamphetamine administration in unilateral nigral-lesioned rats (sensitization). Pretreatment with 1.0 mg/kg MK-801 completely prevented both the expression of striatal Fos protein and the development of acute behavioral sensitization following a single injection of 5.0 mg/kg methamphetamine. These results suggest that NMDA receptor-mediated mechanisms contribute to the expression of striatal Fos protein associated with behavioral sensitization that follows exposure to methamphetamine.

Metyrapone-induced suppression of corticosterone synthesis reduces ethanol consumption in high-preferring rats

The fluid intake of male Wistar rats with simultaneous access to water and 6% ethanol was determined between 0900 and 1500 h. In high-preferring males (normally covering > 60% of their daily fluid consumption in the form of ethanol), two injections with the corticosterone synthesis inhibitor metyrapone (50 mg/kg) at 0900 h and 1200 h for 4 consecutive days significantly reduced ethanol preference such that they preferred water over alcohol. Treatment with corticosterone (0.6 mg/kg) 2 h before each metyrapone injection partially cancelled this effect of the synthesis inhibitor. By contrast, there was no significant effect of metyrapone treatment on the drinking of ethanol in low-preferring rats (normally covering < 30% of their daily fluid consumption in the form of ethanol). These results suggest that the adrenal secretion of corticosterone directly or indirectly modulates the intake of alcohol in high-preferring rats.

Effects of dose and interdose interval on locomotor sensitization to the dopamine agonist quinpirole

To assess whether the interval between injections affects the course of locomotor sensitization to quinpirole, groups of rats were injected every 2, 4, or 8 days with quinpirole (0, 0.025, 0.25, 0.5, and 2.5 mg/kg; n = 222) and their locomotor activity monitored after each injection for a total of 10 tests. Results indicate that the number of drug injections, rather than the interval between them, predominantly controls the development of locomotor sensitization to quinpirole. It is suggested that this may reflect a rapid induction but slow decay time for a response-enhancing factor stimulated by each injection of quinpirole, and that the effects of this putative factor are cumulative but saturable.

Dynamics of behavioral sensitization induced by the dopamine agonist quinpirole and a proposed central energy control mechanism

The study characterizes the process of sensitization induced by intermittent administrations of quinpirole (0.5 mg/kg) in rats in a large open field. Sensitization was found to be self-limiting, with all measures of behavior reaching a plateau after the tenth twice-weekly injection. Kinetics of sensitization were a simple hyperbolic function of the number of drug injections for some measures (speed of locomotion, length of locomotor bouts) but showed positive co-operativity for others (distance travelled, duration of locomotion, frequency of stops, route stereotypy), suggesting potentiation of the effect by preceding injections. The pace of sensitization varied for different behaviors: locomotor speed changed fastest in the early portion of chronic treatment; stereotypy of route changed primarily during the late phase; mouthing did not sensitize. Sensitization evolved by a cascade of changes that included: advancing the onset of locomotor activation; prolonging the duration of locomotion; establishing new maxima of observable responses; altering the mode of locomotion; raising speed, rate and length of locomotor bouts; and increasing stereotypy of travel. These observations do not substantiate the prediction that development of behavioral sensitization is associated with emergence of disorganized activity and/or fractionation of response chains. Instead, it is proposed that development of sensitization may represent a build-up and strengthening of performance, reflecting enhanced central control of energy expenditure stimulated by repeated injections of quinpirole. Furthermore, it is suggested that for at least one response, the maximum observable amount of locomotion, development of sensitization requires only D2 stimulation, independent of D1 tone.

Six differences in the locomotor-activating effects of amphetamine: role of circulating testosterone in adulthood

The effects of circulating testosterone (T) on sex differences in locomotor activity elicited by both acute and repeated amphetamine (AMPH) administration were evaluated in adult rats. Male and female rats were gonadectomized in adulthood and implanted with Silastic capsules containing either T or cholesterol (CHOL). In the preexposure period, locomotor activity in response to IP injections of either AMPH (1.5 or 1.3 mg/kg) or saline (1.0 ml/kg) was measured for 2 h, every third day on five occasions. In a subsequent test for sensitization, all animals received AMPH (0.75 or 0.65 mg/kg). Results indicate that regardless of the presence of circulating T, females showed higher levels of activity in response to AMPH than males. In male animals, T suppressed AMPH-induced activity on the first day of the preexposure period, but this effect was lost with repeated testing. In female animals, T enhanced AMPH-induced activity during the first hour of testing. The presence of circulating T did not influence the degree of sensitization in either sex as determined by the difference between AMPH preexposed and SAL preexposed animals on the test day for sensitization.

Phosphorylation of neuromodulin in rat striatum after acute and repeated, intermittent amphetamine

Repeated, intermittent treatment of rats with amphetamine results in a sensitization of locomotor and stereotyped behaviors that is accompanied by an enhancement in stimulus-induced dopamine release. Increased phosphorylation of the neural specific calmodulin-binding protein, neuromodulin (GAP-43, B-50, F1) has been demonstrated in other forms of synaptic plasticity and plays a role in neurotransmitter release. To determine whether neuromodulin phosphorylation was altered during amphetamine sensitization, the in vivo phosphorylated state of neuromodulin was examined in rat striatum in a post hoc phosphorylation assay. Female, Holtzman rats received saline or 2.5 mg/kg amphetamine twice weekly for 5 weeks. One week after the last dose of amphetamine, rats were challenged with either 1 mg/kg or 2.5 mg/kg amphetamine or saline and the rats were sacrificed 30 min later. Purified synaptic plasma membranes were prepared in the presence of EGTA and okadaic acid to inhibit dephosphorylation, and were subsequently phosphorylated in the presence of purified protein kinase C and [gamma-32P]ATP. The protein kinase C-mediated post hoc phosphorylation of neuromodulin was significantly reduced in groups that received either acute or repeated amphetamine suggesting that neuromodulin in those groups contained more endogenous phosphate. The acute, challenge dose of amphetamine increased neuromodulin phosphorylation in the saline-treated controls but not in the repeated amphetamine-pretreated group. Anti-neuromodulin immunoblots showed no change in neuromodulin levels in any group. There was no significant change in protein kinase C activity in any treatment group. To further investigate the effect of acute amphetamine, the ability of amphetamine to alter neuromodulin phosphorylation in 32Pi-preincubated Percoll-purified rat striatal synaptosomes was examined. Amphetamine (10 microM) significantly increased phosphorylation of a 53 kDa band that migrated with authentic neuromodulin in the synaptosomes by 22% while 500 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) increased neuromodulin phosphorylation by 45%. These data suggest that one injection of amphetamine can increase neuromodulin phosphorylation in rat striatum and that this increase is maintained for at least 1 week following a repeated, sensitizing regimen of amphetamine. Since sensitization can be induced with one dose of amphetamine, it is possible that enhanced neuromodulin phosphorylation could contribute to neurochemical events leading to enhanced release of dopamine and/or behavioral sensitization.

Sex differences in amphetamine-induced locomotor activity in adult rats: role of testosterone exposure in the neonatal period

The present studies assessed the extent to which adult sex differences in responsiveness to both acute and repeated amphetamine (AMPH) treatment can be attributed to differential exposure to testosterone (T) during the early critical period for sexual differentiation. At birth, male pups were sham-operated or gonadectomized, whereas female pups were given T or an oil injection. In adulthood, all animals were gonadectomized or sham-operated. Locomotor activity in response to either 1.5 mg/kg AMPH (IP) or the saline vehicle was measured for 2 h every third day, on five occasions. On the sixth occasion, all animals received 0.75 mg/kg AMPH (IP) in a test for sensitization. In Experiment 1, animals were tested in the absence of circulating gonadal hormones, whereas in Experiment 2, all animals received 5.0 micrograms estradiol benzoate (SC), 30-35 min prior to each behavioral test. Results indicate that neonatal exposure to T suppresses responsiveness to AMPH in adulthood. The differences between neonatal T-exposure groups were magnified in the presence of circulating estradiol. The fact that female animals were more responsive to AMPH regardless of neonatal T exposure suggests that lifetime exposure to estradiol alters responsiveness to this hormone, and to AMPH, in adult animals and/or that exposure to T both pre- and postnatally is necessary for the full suppression of responsiveness seen in adult male animals.

Correlation of apomorphine- and amphetamine-induced turning with nigrostriatal dopamine content in unilateral 6-hydroxydopamine lesioned rats

In the unilateral 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease, controversy exists concerning the use of apomorphine- or D-amphetamine-induced rotations as reliable indicators of nigrostriatal dopamine depletion. Our objective was to evaluate which, if either, drug-induced behavior is more predictive of the extent of nigrostriatal dopamine depletion. Fischer 344 and Sprague-Dawley rats were unilaterally injected with 9 micrograms/4 microliters/4 min 6-hydroxydopamine into the medial forebrain bundle. The animals were behaviorally tested with apomorphine (0.05 mg/kg, s.c.) and D-amphetamine (5.0 mg/kg, s.c.). Following testing, the brains were removed and the right and left striata, substantia nigra and ventral tegmental area were dissected free and quickly frozen at -70 degrees C for analysis of catecholamine content by high performance liquid chromatography coupled with electrochemical detection. Our results indicate that an animal which has greater than a 90% depletion of dopamine in the striatum might not rotate substantially on apomorphine, without a concomitant depletion of > 50% of the DA content in the corresponding substantia nigra. No correlations were seen involving depletions of the ventral tegmental area and the extent of the lesions to the striatum. Submaximally lesioned (75-90% depleted) rats were found to rotate on D-amphetamine but not on apomorphine. In addition, control rats that did not receive lesions were often seen to rotate extensively on D-amphetamine. We therefore conclude that maximal lesions of the striatum and substantia nigra are required to generate rotations demonstrable with low dose apomorphine but not with D-amphetamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine transmission increases in the nucleus accumbens of male rats during their first exposure to sexually receptive female rats

In vivo microdialysis was used to monitor extracellular concentrations of dopamine (DA), and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the nucleus accumbens of sexually naive male rats during their first exposure to sexually receptive or nonreceptive females. DA, DOPAC, and HVA increased progressively and significantly in males that copulated to ejaculation with receptive females. In contrast, DA, DOPAC, and HVA did not increase significantly in males exposed to non-receptive females, despite several attempts by these males to mount the non-receptive females. These results indicate that DA is released unconditionally in the nucleus accumbens of male rats by exposure to sexually receptive female rats, and that copulation with intromission, but not mounting alone, leads to further increases in DA release.

Antiaggressive and motor effects of haloperidol show different temporal patterns in the development of tolerance

The study of the temporal course of tolerance development was used as a means to separate different aspects of the action of haloperidol on social behavior. Agonistic behavior was studied in isolated male mice that confronted standard opponents (anosmic and grouped conspecifics) in a neutral area. The aggressive and motor behaviors of the experimental animals were evaluated 30 min or 24 h either after a single injection of haloperidol (0.4 mg/kg) or following the last of a series of 15 or 30 injections. When animals were evaluated 30 min after the haloperidol injection, no tolerance to the antiaggressive effects was evident. The action on immobility, on the contrary, showed a clear tolerance development with repeated drug administration, both with 15 and 30 injections. When evaluated 24 h after the last injection, tolerance to the antiaggressive effects developed with repeated injections. Increased immobility was never found in the tests carried out after 24 h, not even in the single injection group. The clear divergence found in the temporal courses of tolerance to haloperidol in its antiaggressive and motor effects suggests that these actions are mediated through different neurophysiological mechanisms. A parallel with extrapyramidal and therapeutic effects is discussed.

Paw-reaching, sensorimotor, and rotational behavior after brain infarction in rats

BACKGROUND AND PURPOSE

Functional tests that are stable and consistent over time are an advantage for long-term evaluation of treatment in experimental stroke research. Because little information on this subject is available in rodents with focal cerebral ischemia, we investigated the outcome of three behavioral tests for a period of 3 months after the insult.

METHODS

Spontaneously hypertensive rats were sham-operated (n = 27) or underwent an occlusion (n = 36) of the right middle cerebral artery. Before surgery all rats were tested for amphetamine-induced rotational behavior, and half of the rats were trained in a paw-reaching task. One, 2, and 3 months after surgery the tests were repeated, together with a test for sensorimotor function. Infarct size was measured morphometrically.

RESULTS

In the lesion group, total hemisphere area was reduced by 22%, caudate putamen by 47%, and the thalamus by 24%. Contralateral to the lesion, paw-reaching was highly impaired, regardless of whether or not the rats had been pretrained, and lesion size correlated significantly to paw-reach performance. Ipsilateral rotation increased and sensorimotor function recovered with time in infarcted rats.

CONCLUSIONS

In contrast to amphetamine-induced rotation and sensorimotor behavior, the paw-reaching test provides a stable behavioral parameter after a middle cerebral artery occlusion. Moreover, the lesion-induced deficit in paw-reaching is highly correlated to the extent of the infarct, suggesting that this test is useful in evaluating treatment effects for a longer period of time.