Chronic cocaine enhances defensive behaviour in the laboratory mouse: involvement of D2 dopamine receptors

Nucleus accumbens dopamine/glutamate interaction switches modes to generate desire versus dread: D(1) alone for appetitive eating but D(1) and D(2) together for fear

The medial shell of nucleus accumbens (NAc) and its mesolimbic dopamine inputs mediate forms of fearful as well as of incentive motivation. For example, either appetitive and/or actively fearful behaviors are generated in a keyboard pattern by localized glutamate disruptions in NAc (via microinjection of the AMPA receptor antagonist DNQX) at different anatomical locations along a rostrocaudal gradient within the medial shell of rats. Rostral glutamate disruptions produce intense increases in eating, but more caudally placed disruptions produce increasingly fearful behaviors: distress vocalizations and escape attempts to human touch, and a spontaneous and directed antipredator response called defensive treading/burying. Local endogenous dopamine is required for either intense motivation to be generated by AMPA disruptions. Here we report that only endogenous local signaling at D(1) dopamine receptors is needed for rostral generation of excessive eating, potentially implicating a direct output pathway contribution. In contrast, fear generation at caudal sites requires both D(1) and D(2) signaling simultaneously, potentially implicating an indirect output pathway contribution. Finally, when motivation valence generated by AMPA disruptions at intermediate sites was flipped by manipulating environmental ambience, from mostly appetitive in a comfortable home environment to mostly fearful in a stressful environment, the roles of local D(1) and D(2) signaling in dopamine/glutamate interaction at microinjection sites also switched dynamically to match the motivation valence generated at the moment. Thus, NAc D(1) and D(2) receptors, and their associated neuronal circuits, play different and dynamic roles in enabling desire and dread to be generated by localized NAc glutamate disruptions in medial shell.

Cocaine exposure during adolescence affects anxiety in adult mice

Psychostimulant drugs such as cocaine have profound and long-lasting neurobiological effects, which may affect anxiety or social behaviors. These actions could be greater when cocaine is administered during a developmental period such as adolescence. The present work attempts to further clarify the long-lasting effects of cocaine administration on mice, examining three major variables: age; pattern of drug administration; and housing conditions. Adolescent (postnatal day 26) or early adult mice (postnatal day 46) were exposed to a daily or binge cocaine administration and 15 days later their behavior was evaluated, the mice being housed either in isolation or in groups during this stage. After a period free of drug, the behaviors evaluated were: spontaneous and cocaine-induced motor activity; anxiety, using the elevated plus maze; the social profile, assessed in a social interaction test. Daily cocaine administration increased avoidance and flee in isolated adolescent-treated mice and decreased social contacts in those which were grouped. On the other hand, the binge pattern modified the anxiety of the grouped adolescent-treated mice evidenced by the increase in time spent on the open arms of the plus maze. An increase in spontaneous and cocaine-induced motor activity was shown in animals after a daily pre-treatment. The results are discussed in terms of presenting cocaine-induced behavioral changes within a specific temporal window and depending on the three variables studied.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Role of dopamine and glutamate receptors in cocaine-induced social effects in isolated and grouped male OF1 mice

Cocaine administration in paired male mice decreases social contacts as well as increases avoidance and flee elements. As dopamine (DA) and glutamate seem to be involved in some of cocaine's effects, an attempt was made to assess whether a range of associated receptors influenced the social impacts of this drug of abuse. The NMDA antagonist memantine (10 and 40 mg/kg); the AMPA antagonist CNQX (1 and 20 mg/kg); the DA release inhibitor CGS 10746b (2 and 8 mg/kg): the DA D1 antagonist SCH 23390 (0.05 and 0.5 mg/kg); and the DA D2/D3 antagonist raclopride (0.03 and 0.3 mg/kg) were administered prior to 25 mg/kg of cocaine and behaviour was evaluated during an encounter between an experimental and a standard opponent in a neutral cage for 10 min. Memantine reverts cocaine-induced social withdrawal and the increase in avoidance and flee, CNQX being effective only in these latter actions. On the other hand, SCH 23390 counteracts the social as well as the defensive action of cocaine, raclopride being effective only in blocking the cocaine-induced increase in avoidance and flee behaviours. In conclusion, although both neurotransmitter systems are involved in the effects of cocaine on social behaviour, NMDA and D1DA receptors seem to have an important role.

Prenatal cocaine exposure alters spontaneous and cocaine-induced motor and social behaviors

The abuse of cocaine in pregnant women could affect emotional behaviors in their descendents. The aim of this work was to evaluate the effects of prenatal cocaine exposure on spontaneous and cocaine-induced motor and social behaviors in mice. Three kinds of prenatal treatment were used: non-treated animals; mice treated daily with physiological saline during the last week of pregnancy; and finally, those treated with cocaine (25 mg/kg) during the same period. Behavioral studies took place on adult males, which were housed in two different conditions: grouped (non-aggressive), or isolated (aggressive). Cocaine-pretreated animals exhibited slight differences in spontaneous motor activity, but alterations in their social relationship with conspecifics were presented, with decreases in isolated but increases in grouped mice. The cocaine challenge increased aggression specifically in grouped prenatally cocaine-treated mice, but increases in motor activity or avoidance and flee behavior were presented in those animals pretreated with either saline or cocaine. Isolated saline-or cocaine-treated animals exhibited greater concentrations of DA and DOPAC than those grouped. A decrease in 5-HIAA concentrations was presented in pretreated animals, irrespective of their housing conditions. In conclusion, cocaine administration during pregnancy induces long lasting effects on the offspring, for both behavioral abnormalities and cocaine response, which last to adult life.

Social behavioural profile of cocaine in isolated and grouped male mice

Studies concerning the relationship between cocaine and aggression in humans as well as in animals have discrepant outcomes. Increases, decreases, or no changes, have been reported after single or chronic cocaine administration in animal models. To clarify, at least in part, the complex behavioural actions of cocaine, the present study evaluated cocaine effects on social behaviours of mice exposed to different situations (isolated or group housed) using confrontations between two male mice in a neutral area. Different doses of cocaine (6, 25 and 50 mg/kg) were administered in a single or binge pattern (three doses in 24 h) and the behavioural test was performed 20 min after the last injection. No increases in aggression were observed in any situation tested. Instead, cocaine at the two higher doses employed (either in single or binge administration), decreased aggressive behaviours in isolated mice, with no changes being observed in grouped animals. In both types of animals, cocaine increased defensive elements (avoidance/flee) and abolishes social contacts. In conclusion, cocaine presents an anti-aggressive action and may be interpreted as having an anxiogenic-like effect.

Aberrant responses in social interaction of dopamine transporter knockout mice

The dopamine (DA) transporter (DAT) controls the temporal and spatial resolution of dopaminergic neurotransmission. Disruption of the Dat1 gene in mice leads to increased extracellular DA concentrations and reduced expression of D1- and D2-like receptors in striatum. The mutants are hyperactive in the open field and they display deficits in learning and memory. In humans, dopaminergic dysfunction has been associated with a number of different psychiatric disorders and some of these conditions are accompanied by abnormal social responses. To determine whether social responses were also impaired in DAT knockout (KO) mice, behaviors of group- and isolation-housed animals were compared. All group-housed animals readily established hierarchies. However, the social organizations of the mutants were changed over time. Under both group- and isolation-housed conditions, mutants exhibited increased rates of reactivity and aggression following mild social contact. In isolation, exposure to a novel environment exacerbated these abnormal responses. Regardless of housing context, stereotyped and perseverative patterns of social responses were a common feature of the KO repertoire. In fact, many abnormal behaviors were due to the emergence and predominance of these inflexible behaviors. These data suggest that KO mice may serve as a useful animal model for understanding not only how DA dysfunction contributes to social abnormalities, but also how behavioral inflexibility distorts their social responses.

Acute 'binge' cocaine administration elevates dynorphin mRNA in the caudate putamen of C57BL/6J but not 129/J mice

Preprodynorphin mRNA was measured in the nucleus accumbens (NAc) and caudate putamen (CPu) after 3-day 'binge' pattern cocaine administration in C57BL/6J and 129/J mice, strains which differ in behavior and in dopamine increases in the CPu after 'binge' cocaine. In the CPu, there was increased preprodynorphin mRNA in C57BL/6J (P<0.05), but not in 129/J mice, with no differences in the NAc. Thus, 129/J mice are hyporesponsive to the preprodynorphin activating effects of acute 'binge' cocaine in the CPu.

Repeated cocaine treatment activates flank marking in adolescent female hamsters

Cocaine abuse during adolescence represents a significant health risk due to the potential for both acute and long-term negative physical and psychological sequelae, including increased aggressive behavior. This study examined the effect of adolescent cocaine treatment on flank marking (i.e., a stereotypic motor behavior that is part of the response pattern of offensive aggression) in female and male Syrian hamsters (Mesocricetus auratus). Adolescent cocaine treatment activated flank marking in female hamsters when animals were measured upon return to their home cage immediately following drug treatment. Sex differences were observed in cocaine-induced flank marking, as males failed to flank mark when returned to the home cage. In females, the behavioral response was most marked on Day 11 of cocaine treatment in all doses tested. Yet, animals treated with low-dose cocaine (0.5 mg/kg/day) showed the most significant increase in flank marking on and from Day 11 forward as compared to medium- and high-dose cocaine-treated animals and controls. In addition, the response of cocaine-treated animals was vigorous and nearly immediate, as >75% of the flank marks scored were performed within the first 2 min of the behavioral test in >85% of animals examined. Measures of locomotion showed that cocaine had stimulatory effects on motor activity in adolescent female hamsters at all doses tested. Cocaine-treated animals did not differ in body weight gain from controls, suggesting no dramatic physiological effects of adolescent cocaine exposure on body growth at the doses tested.

Acute and chronic effects of cocaine on isolation-induced aggression in mice

The purpose of the present study was to investigate the effects of acute and chronic cocaine administration on aggressive behaviour in mice. The animals were made more aggressive by individual housing for a period of 6 weeks. Group-housed anosmic conspecifics which were not aggressive were used as intruder controls. In acute studies, cocaine induced no significant change in aggressive behaviour at low doses (0.5-5 mg/kg) but significantly decreased aggressive behaviour after doses of 10 and 20 mg/kg. Cocaine increased the isolation-induced aggressive behaviour in mice when they were injected twice daily for a week with low doses of 0.5 or 1 mg/kg. In particular, the latency to first attack was significantly decreased by the drug and the frequency of attack towards the non-aggressive intruder was dramatically increased. Higher cocaine doses (10 or 20 mg/kg) under the described treatment regimen decreased these agonistic repertories. Tolerance did not develop to the anti-aggressive effects of high doses of cocaine on continued treatment.

Behavioral and mesocorticolimbic dopamine responses to non aggressive social interactions depend on previous social experiences and on the opponent's sex

In these experiments we evaluated the relationship between behavioral and brain dopamine (DA) responses to social interactions. Subjects were group housed male mice confronted with a non aggressive male or female conspecific following either repeated defeat (defeated) or repeated non aggressive experiences (social). Defeated mice showed more defensive/submissive reactions then mice of the social group regardless of the opponent sex. However, mice defeated by females showed reduced social exploration without significant differences in non social exploration whilst the opposite was true for mice defeated by male opponents. Non aggressive social interactions enhanced dopamine metabolism in the prefrontal cortex (pFC) of DEFEATED mice regardless of opponent sex. However, only mice defeated by females showed enhanced dopamine metabolism and release in the nucleus accumbens septi (NAS) and olfactory tubercle (OT) following interaction with the non aggressive opponent. Finally, correlation between central and behavioral responses evidenced that 3,4-dihydroxiphenilacetic acid levels in the pFC were positively correlated with defensive behaviors and negatively correlated with non social exploration in mice confronted with male opponents but not in those confronted with females. The latter, showed a significant positive correlation between 3-methoxytyramine (3-MT) levels in the OT and defensive responses and significant negative correlation between social investigation and 3-MT levels in the OT and in the NAS. These results indicate a strict relationship between mesocorticolimbic dopamine transmission and behavior responses to social cues. Moreover, they strongly support the view that mesocorticolimbic DA modulates social behavior by affecting perceptive processing.

Chronic low-dose cocaine treatment during adolescence facilitates aggression in hamsters

Cocaine abuse during adolescence represents a significant health risk because of the potential for both acute and long-term negative physical and psychological sequelae, including increased aggressive behavior. This study examined the effects of chronic adolescent cocaine exposure on aggression in an animal model. It was hypothesized that chronic cocaine exposure during adolescence predisposes animals to heightened levels of aggressive behavior. To test this hypothesis, adolescent male golden hamsters (Mesocricetus auratus) were administered cocaine hydrochloride during their entire adolescent development (Postnatal Days 27-54) and then tested for offensive aggression using the resident-intruder model. Animals treated with low-dose cocaine during adolescence showed significantly elevated measures of offensive aggression (i.e., increased number of bites, attacks, and decreased latencies to bite), whereas measures of social communication, sexual motivation and motor activity remained constant. Cocaine-treated animals did not differ in body weight gain from controls, suggesting no dramatic physiological effects of adolescent cocaine exposure on body growth at the doses tested.

Psychopharmacology of dopamine: the contribution of comparative studies in inbred strains of mice

Comparative studies of behavioral responses to centrally acting drugs in inbred strains of mice which show differences in brain neurotransmitter activity represent a major strategy in the investigation of the neurochemical bases underlying behavioural expression. Moreover, these studies represent a preliminary stage in behavioral genetic research since they allow quantitative scales to be established and suggest correlations to be tested in recombinant inbred strains. The present review evaluates results obtained in mice of the C57BL/6 (C57) and DBA/2 (DBA) inbred strains which have been used for studies of the behavioral pharmacology of dopamine (DA) and investigated for the functional and anatomical characteristics of their brain DA systems. Differences between C57 and DBA strain involve susceptibility and sensitivity as well as qualitative differences in the type or direction of the behavioral effects of DA agonists. Moreover, data on strain-dependent differences for DA metabolism, release and receptor densities and distribution provide important indications about the relationship between behavioral and central effects of DA agonists and, more generally, about the involvement of brain DA in behavior. Comparative studies in C57 and DBA mice have also revealed differences in susceptibility to context-dependent, context-independent and stress-induced behavioral sensitization to psychostimulants. Consequently, they support the view that the term "behavioral sensitization" may define different phenomena in which different, independent genotype-related factors play a major role. Finally, studies on the behavioral and central effects of stressful experiences in C57 and DBA mice together with psychopharmacogenetic analyses, indicate that different symptomatological profiles may derive from genotype-dependent adaptation of brain DA receptors to environmental pressure.

Strain-dependent effects of post-training cocaine or nomifensine on memory storage involve both D1 and D2 dopamine receptors

Post-training administration of cocaine (1-10 mg/kg) or nomifensine (1-10 mg/kg) dose-dependently improves retention of an inhibitory avoidance response in C57BL16 mice, while impairign it in the DBA/2 strain. The effects of retention performance induced by the psychostimulant and the dopamine (DA) reuptake blocker in C57BL/6 and DBA/2 mice appear to be due to an effect on memory consolidation. In fact, they were observed when drugs were given at short, but not long, periods of time after training, i.e. when the memory trace is susceptible to modulation. Moreover, these effects are not to be ascribed to an aversive or a rewarding or non-specific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during the training were not affected by the post-training drug administration. The strain-dependent effects of an intermediate dose (5 mg/kg) of both cocaine and nomifensine were reversed by pretreatment with either selective D1 or D2 DA receptor antagonist SCH 23390 and (-)-sulpiride administered at per se non-effective doses (0.025 and 6 mg/kg, respectively), thus suggesting that D1 and D2 receptor types are similarly involved in modulating memory processes. These results show that the effects of cocaine on memory consolidation are related to to its dopaminergic action, since they are similar to those produced by nomifensine and, what is more important, are antagonized by pretreatment with DA receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of dopamine transporter and locomotor effects of cocaine, GBR 12909, epidepride, and SCH 23390 in C57BL and DBA mice

C57BL/6 and DBA/2 mice were used to examine genetic differences in locomotor activating effects of acute cocaine administration and to determine whether differences were mediated by dopaminergic systems. C57BL/6 mice were less activated than DBA/2 mice at 5 and 10 min after 10 and 15 mg/kg cocaine. HPLC analysis showed equivalent brain cocaine concentrations in the two strains at 5 and 10 min after 10, 15, or 20 mg/kg doses. The selective dopamine uptake inhibitor, GBR 12909, at 5 and 7.5 mg/kg, produced greater locomotor activation in DBA/2 mice than in C57BL/6 mice. However, binding studies with the selective dopamine uptake ligand [3H]GBR 12935, revealed no between-strain difference in Kd or Bmax in caudate putamen (CP) or nucleus accumbens (NA) membranes. Competition assays using unlabeled dopamine to compete for [3H]GBR 12935 binding in CP or NA membranes showed no between-strain difference by brain region. The specific D1 or D2 antagonists, SCH 23390 or epidepride, respectively, produced dose-dependent decreases in locomotor activity but there were no between-strain differences. However, epidepride, at a dose of 0.003 mg/kg, completely reversed cocaine-induced (15 mg/kg) activation in both strains. These findings show that C57BL/6 and DBA/2 mice differ in dopamine-related behaviors and suggest that dopaminergic processes may mediate genetic differences in cocaine sensitivity.

Differential blockade of chronic versus acute effects of intravenous cocaine by dopamine receptor antagonists

The objectives of this study were to investigate behavioral sensitization to repeated once daily IV injections of cocaine, and to determine whether dopamine receptor antagonists differentially block chronic versus acute cocaine effects. Acute cocaine (0.3-3.0 mg/kg) produced a dose-dependent increase in both horizontal and stereotypic movements in male Sprague-Dawley rats. Repeated once daily injections of 1 or 3 mg/kg of cocaine augmented these effects. Pretreatment with either the D2 dopamine receptor antagonist haloperidol (0.03-0.3 mg/kg) or the D1 dopamine receptor antagonist R(+)-SCH-23390 (0.003-0.1 mg/kg) dose dependently attenuated cocaine's behavioral effects in both sensitized and cocaine-naive animals. There was a rightward shift in the dose-effect relationship of these antagonists in blocking the expression of behavioral sensitization as compared to their ability to block the acute behavioral effects of cocaine. These results indicate that repeated once daily IV injections of cocaine produced behavioral sensitization and both D1 and D2 dopamine receptor antagonists attenuated the expression of this sensitization. The data also suggest that dopamine receptor antagonists were more potent in blocking cocaine's effects in cocaine-naive animals than in cocaine-sensitized animals.

Post-training dopamine receptor agonists and antagonists affect memory storage in mice irrespective of their selectivity for D1 or D2 receptors

Post-training administration of the selective D1 and D2 agonists SKF 38393 and LY 171555 dose-dependently facilitated retention of an inhibitory avoidance response in mice, while the selective D1 or D2 antagonists SCH 23390 and (-)sulpiride produced an impairment of retention. These effects are not to be ascribed to a nonspecific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during the training were not increased by the post-training drug administration. The effects on retention performance induced by DA agonists and antagonists seem to be due to an effect on memory consolidation, since they have been observed when drugs were given at short, but not at long, periods of time after training. These results showing a similar role of D1 and D2 receptor types on memory storage appear not to be consistent with a body of neuropharmacological, neurophysiological, and behavioral evidence pointing to a different functional role of these types of DA receptors. This discrepancy is discussed in terms of possible involvement of different brain systems, peripheral systems, or possible second messenger processes activated by the two receptor types and leading to similar effects on memory processes.

D1 and D2 receptor antagonists differently affect cocaine-induced locomotor hyperactivity in the mouse

Pretreatment with small, per se ineffective doses of the selective D1 antagonist SCH 23390 inhibited hyperactivity induced by cocaine. On the other hand, the classic neuroleptic haloperidol and the selective D2 antagonist metoclopramide prevented the stimulatory effects of cocaine on locomotion only at hypokinetic doses, while the atypical neuroleptic (-)-sulpiride, a selective D2 antagonist, did not produce significant effects when administered at the hypokinetic dose of 12 mg/kg. Finally, at low doses (-)-sulpiride dose-dependently potentiated the locomotor-stimulating effects of cocaine, an effect that is not shared either with haloperidol or with metoclopramide. These results are discussed in terms of different roles of DA receptor subtypes in the modulation of the stimulant effects of cocaine on locomotion.

Behavioral effects of cocaine alone and in combination with selective dopamine antagonists in the squirrel monkey

Effects of cocaine, alone and in combination with the dopaminergic antagonists, SCH 23390 and haloperidol were studied in squirrel monkeys trained to respond under fixed-interval schedules of electric-shock presentation. Cocaine at intermediate doses (0.1 and 0.3 mg/kg) increased rates of responding under the fixed-interval schedule and during 1-min timeout periods that separated each fixed-interval component. Higher doses (1.0 - 3.0 mg/kg) decreased response rates. Cocaine also dose-dependently altered the temporal pattern of responding characteristic of behavior under fixed-interval schedules. Haloperidol (0.003-0.1 mg/kg) and SCH 23390 (0.001-0.03 mg/kg) dose-dependently decreased rates of responding. A low dose (0.001 mg/kg) of the selective D1 antagonist, SCH 23390, did not appreciably alter the effects of cocaine. Higher doses (0.003-0.01 mg/kg), which when given alone decreased rates of responding, attenuated the increases in response rates produced by cocaine. In addition, the decreases in response rates produced by the higher dose of cocaine were attenuated by 0.01 mg/kg SCH 23390. The alterations in temporal patterning of responding under the fixed-interval schedule were not antagonized by any dose of SCH 23390. Haloperidol (0.003 mg/kg) did not appreciably alter the effects of cocaine; higher doses (0.01-0.03 mg/kg), which when given alone decreased rates of responding, attenuated the increases in response rates produced by 0.1 mg/kg cocaine. The decreases in response rates under the fixed-interval schedule that were produced by higher doses, or the changes in temporal patterning of responding at any dose of cocaine, were not antagonized by any dose of haloperidol that was studied.(ABSTRACT TRUNCATED AT 250 WORDS)

LY 171555-induced hyperdefensiveness in the mouse does not implicate benzodiazepine receptors

In naive mice the selective D2 agonist LY171555 dose-dependently (0.5-5 mg/kg) induces defensive responses toward non-aggressive conspecifics. In order to investigate possible anxiogenic properties of the D2 agonist, its behavioural effects were compared with those produced by the benzodiazepine receptor inverse agonist methyl-beta-carboline-3-carboxylate(beta-CCM) in the elevated plus maze and in social interactions with non-aggressive opponents. When tested in the elevated plus maze, mice injected with LY 171555 (0.005-1 mg/kg) showed no decrease either of the number of entries or of the time spent in the open arms. At 5 mg/kg an actual increase of these two measures was observed. By contrast, beta-CCM (1-3 mg/kg) dose-dependently decreased both the number of entries and the time spent in the open arms without altering locomotion. The effects of beta-CCM were antagonized by the benzodiazepine receptor antagonist RO 15-1788 (3 mg/kg) showing a selective involvement of benzodiazepine receptors in their modulation. On the other hand, beta-CCM, (1-3 mg/kg) did not produce significant effects on defensive behaviour of mice interacting with non-aggressive opponents and the defensive responses of mice treated with 1 mg/kg LY 171555 were not prevented by 5 mg/kg chlordiazepoxide. These results show that DA D2-mediated hyperdefensiveness and anxiety modulated by benzodiazepine receptors are unrelated phenomena and suggest that this behavioural response may represent a model of those forms of fear-related reaction that do not respond to benzodiazepine treatment.

Effects of repeated injections of cocaine on D1 and D2 dopamine receptors in rat brain

In order to determine if chronic administration of cocaine produced long-lasting alterations in dopamine receptor binding, rats were treated with single daily injections of cocaine (0, 10, or 20 mg/kg) for 15 consecutive days and killed either 20 min or 2 weeks after the last injection. The density of D1 binding sites in frontal cortex was either unchanged (10 mg/kg) or slightly increased (20 mg/kg) 20 min after the last daily injection, but was decreased 2 weeks later. D1 sites in striatum were decreased both immediately and 2 weeks after the injection regimen. Decreases in D1 binding site density in nucleus accumbens were observed only immediately after the last injection. In contrast to these effects on D1 binding sites, D2 binding sites were decreased in striatum and frontal cortex and increased in the nucleus accumbens 20 min after repeated cocaine, but were unaffected 2 weeks after repeated cocaine. Computer-assisted analysis of the saturation isotherms revealed that chronic administration of cocaine did not affect the affinity (Kd) of the radioligands used to label D1 or D2 sites. These findings suggest that repeated administration of cocaine results in long-term decreases in D1 binding sites in striatum and frontal cortex and transient decreases in D2 binding sites. Furthermore, cocaine caused opposite, transient effects on D1 and D2 site density in nucleus accumbens.

LY 171555-induced catalepsy and defensive behavior in four strains of mice suggest the involvement of different D2 dopamine receptor systems

The D2 dopamine receptor agonist LY 171555 (0.5 to 5 mg/kg) induces dose-dependent catalepsy in C57BL/6, DBA/2 and BALB/c inbred strains of mice. This effect shows marked strain-dependent differences, since the response of C57BL/6 is significantly lower than those presented by the other two inbred strains at all doses tested. In previous studies we have shown that the D2 agonist at doses ranging from 0.5 to 5 mg/kg induces hyperdefensive responses toward nonaggressive opponents in mice of the C57BL/6 and BALB/c but not of the DBA/2 strain. Here we report that the outbred CD1 mice present both cataleptic and hyperdefensive responses when challenged with LY 171555. Forty-five percent of individuals presenting high defensive response and 11% high cataleptic scores. No correlation was found between catalepsy and hyperdefensiveness in CD1 mice following administration of 1 mg/kg of the D2 agonist. These results suggest that D2 receptor stimulation results in different behavioral responses, possibly mediated by different dopaminergic systems, depending on the genetic make up.