Long-term changes in the sensitivity of pre-and postsynaptic dopamine receptors in mouse striatum evidenced by behavioural and biochemical studies

Upregulation of dopamine D3, not D2, receptors correlates with tardive dyskinesia in a primate model

Tardive dyskinesia (TD) is a delayed and potentially irreversible motor complication arising in patients chronically exposed to centrally active dopamine D2 receptor antagonists, including antipsychotic drugs and metoclopramide. The classical dopamine D2 receptor supersensitivity hypothesis in TD, which stemmed from rodent studies, lacks strong support in humans. To investigate the neurochemical basis of TD, we chronically exposed adult capuchin monkeys to haloperidol (median, 18.5 months; n = 11) or clozapine (median, 6 months; n = 6). Six unmedicated animals were used as controls. Five haloperidol-treated animals developed mild TD movements, and no TD was observed in the clozapine group. Using receptor autoradiography, we measured striatal dopamine D1, D2, and D3 receptor levels. We also examined the D3 receptor/preprotachykinin messenger RNA (mRNA) co-expression, and quantified preproenkephalin mRNA levels, in striatal sections. Unlike clozapine, haloperidol strongly induced dopamine D3 receptor binding sites in the anterior caudate-putamen, particularly in TD animals, and binding levels positively correlated with TD intensity. Interestingly, the D3 receptor upregulation was observed in striatonigral neurons. In contrast, D2 receptor binding was comparable to controls, and dopamine D1 receptor binding was reduced in the anterior putamen. Enkephalin mRNA widely increased in all animals, but to a greater extent in TD-free animals. These results suggest for the first time that upregulated striatal D3 receptors correlate with TD in nonhuman primates, adding new insights to the dopamine receptor supersensitivity hypothesis. The D3 receptor could provide a novel target for drug intervention in human TD.

Single dose of a dopamine agonist impairs reinforcement learning in humans: evidence from event-related potentials and computational modeling of striatal-cortical function

Animal findings have highlighted the modulatory role of phasic dopamine (DA) signaling in incentive learning, particularly in the acquisition of reward-related behavior. In humans, these processes remain largely unknown. In a recent study, we demonstrated that a single low dose of a D2/D3 agonist (pramipexole)-assumed to activate DA autoreceptors and thus reduce phasic DA bursts-impaired reward learning in healthy subjects performing a probabilistic reward task. The purpose of this study was to extend these behavioral findings using event-related potentials and computational modeling. Compared with the placebo group, participants receiving pramipexole showed increased feedback-related negativity to probabilistic rewards and decreased activation in dorsal anterior cingulate regions previously implicated in integrating reinforcement history over time. Additionally, findings of blunted reward learning in participants receiving pramipexole were simulated by reduced presynaptic DA signaling in response to reward in a neural network model of striatal-cortical function. These preliminary findings offer important insights on the role of phasic DA signals on reinforcement learning in humans and provide initial evidence regarding the spatiotemporal dynamics of brain mechanisms underlying these processes.

Gender differences in delay-discounting under mild food restriction

Impulsivity, a core symptom of attention-deficit hyperactivity disorder (ADHD), is tested in animal models by delay-discounting tasks. So far, mainly male subjects have been used in this paradigm at severe levels of food restriction. Here we studied the impulsive behaviour of CD-1 adult male and female mice at mild levels of food restriction. Mice maintained at 90 +/- 5% of ad libitum bodyweight, were tested in operant chambers provided with nose-poking holes. Nose poking in one hole resulted in the immediate delivery of one food pellet (small-soon, SS), whereas nose poking in the other hole delivered five food pellets after a delay (large-late, LL), which was increased progressively each day (0-150 s). Two subgroups emerged: individuals that shifted at short delays ("steep") and individuals that did not shift, even at the highest delays ("flat"). Analysis showed that "steep" females shifted at shorter delays than "steep" males, while no difference existed between males and females within the "flat" sub-population. In home-cage circadian activity as well as in a novelty-seeking test, females were more active than males. It can be concluded from these results that female mice are more impulsive than male mice under mild food restriction. This is in contrast with findings in earlier studies with more severe food restriction. Therefore, an alternative explanation is that females are more explorative, and that different features might be tested in delay-discounting paradigms, depending on restriction levels.

Repeated apomorphine administration alters dopamine D1 and D2 receptor densities in pigeon basal telencephalon

When pigeons are repeatedly administered a dose of apomorphine they show an increasing behavioral response, much as rodents do. In birds this expresses itself in an augmented pecking response. This sensitization is assumed to be largely due to a conditioning process. Here we present evidence that sensitization is accompanied by an alteration of the D(1) to D(2) dopamine receptor densities. An experimental group of pigeons was repeatedly injected with apomorphine, and a control group with saline. The basal forebrain tissue, known to be rich in dopamine receptors, was subjected to binding assays using tritiated specific D(1) and D(2) dopamine receptor antagonists. There was a trend towards an increase in D(1) and a significant decrease in D(2) receptor densities in apomorphine-treated birds compared to the saline-treated controls. We conclude that extended apomorphine treatment modifies the D(1) dopamine receptor density in the opposite manner to the D(2) dopamine receptor density.

Kindling and behavioral sensitization: are they relevant to recurrent suicide attempts?

BACKGROUND

Data from a sample of suicidal young adults were used to examine the relevance of the kindling and behavioral sensitization models to suicide attempts. Three predictions derived from the kindling and sensitization models were tested: a higher number of suicide attempts would be associated with (a) lower levels of pre-attempt stress; (b) higher suicidal intent; and (c) greater lethality of the current attempt.

METHODS

Measures of life stress and suicidal intent were collected among 123 young adults who attempted suicide just prior to entering treatment. Data on the total number of suicide attempts and the lethality of the current attempt were also collected.

RESULTS

Number of suicide attempts was significantly and positively associated with pre-suicidal crisis life stress and suicidal ideation, but was not significantly associated with lethality of the most recent attempt.

LIMITATIONS

The young sample drawn from a military medical setting may not accurately represent suicide attempters in the general population. Only total negative life events in the year preceding suicide attempt were examined, not the increase in negative life events immediately prior to suicide attempt.

CONCLUSIONS

The kindling and sensitization models may not accurately describe the progression of recurrent suicide attempts.

Effects of some antioxidative aporphine derivatives on striatal dopaminergic transmission and on MPTP-induced striatal dopamine depletion in B6CBA mice

(S)-(+)-boldine, an aporphine alkaloid displaying antioxidative and dopaminergic properties, and six of its derivatives (glaucine, 3-bromoboldine, 3-iodoboldine, 8-aminoboldine, 8-nitrosoboldine and 2,9-O,O'-dipivaloylboldine) were tested for these properties in comparison with their parent compound. All the tested compounds displayed in vitro antioxidative properties equal to or slightly weaker than those of boldine, and equal to or stronger than (+/-)-6-hydroxy-2,5,7,8,-tetramethylchromane-2-carboxylic acid (Trolox), a water-soluble vitamin E analogue, used as a reference compound. All the aporphine compounds tested displaced [3H]SCH 23390 and [3H]raclopride from their specific binding sites in rat striatum. When tested on dopamine (DA) metabolism in the striatum of B6CBA mice, all the compounds, except 8-aminoboldine, increased striatal levels of DOPAC and HVA, and the HVA/DA ratio, indicating that they cross the blood-brain barrier and that they seem to act as dopamine antagonists in vivo. B6CBA mice were sensitive to the neurotoxic action of MPTP on dopaminergic neurons as indicated by the strongly decreased striatal levels of DA, DOPAC and HVA following administration of MPTP (20 mg/kg, i.p.). Among these aporphine derivatives, only 3-bromoboldine was able to reduce the MPTP-induced decrease of striatal levels of DA and DOPAC, whereas (R)-apomorphine (5 mg/kg, s.c.) and acetylsalicylic acid (100 mg/kg, i.p.), used as reference compounds, were very active. These data suggest that potent in vitro antioxidative properties and the ability to cross the blood-brain barrier are not sufficient criteria to predict the inhibition of neuronal degeneration induced by MPTP.

Apomorphine sensitization: evoking conditions, context dependence, effect persistence and conditioned nature

When repeatedly administered a dose of apomorphine (Apo), pigeons, much like rodents, show behavioural sensitization. In birds this sensitization expresses itself as an increasing pecking response to the drug and is found to be partially dependent on the environmental context in which Apo takes effect. In the first experiment we examined what effect different inter-Apo administration intervals have on the development of Apo sensitization and found that, with some smaller variations, intervals between 3 hours and 5 days all yielded comparable courses of sensitization. In the second experiment we examined how long pigeons had to be exposed to the same distinct cage to reveal a maximal context-dependent sensitization. Pigeons were therefore repeatedly injected with Apo and consistently placed in an experimental cage for different lengths of time (5 to 60 min; the overall drug effect lasted for about 1 h) before being returned to their standard home cages. Subsequent tests in the experimental cage and a standard cage showed that 20-min post-injection exposures were sufficient to yield a maximal response in the experimental cage. After training with 20- and 60-min exposures, the pigeons pecked about three times more in the experimental cage than in the standard cage. This confirmed the marked context dependency of the sensitization effect. In the third experiment, groups of pigeons were injected repeatedly with Apo and directly afterwards placed either consistently into the same experimental cage or into different experimental cages. The same-cage group evidenced a significantly much stronger sensitization than the different-cage group. A cage-habituation group served as a control for the possibility that the weaker sensitization of the different-cage group might be due to a cage novelty effect. This cage-habituation group was run under the same conditions as the different-cage group but with additional exposures to the crucial cage while injected with saline. This extra treatment did not augment the pecking response to Apo in that cage. In the fourth experiment we examined how long the sensitization to Apo lasts and found that, even after 2 years of drug abstinence, it only waned to 50% of the original asymptotic response. The overall results support the hypothesis that a very major part of the sensitization to Apo in pigeons is due to a conditioning to the environmental context and to the drug state itself.

Behavioral sensitization in humans

Behavioral sensitization is the process whereby repeated, intermittent stimulant administration produces a progressively greater and enduring behavioral response. For over two decades, behavioral sensitization has been reliably demonstrated in a number of different animal species and has been proposed as a model for the development of stimulant dependence. However, the application of this model to humans is limited since there have been relatively few studies of sensitization in human subjects. Nonetheless, results from these studies suggest that, similar to animal studies, enhanced behavioral responses do occur following repeated stimulant administration that resemble sensitization. Further research is necessary to examine characteristics of sensitization in humans, including the neurobiological systems involved.

The influence of endogenous dopamine levels on the density of [3H]SCH23390-binding sites in the brain of the honey bee, Apis mellifera L

This paper examines the relationship between endogenous dopamine (DA) levels and the density of [3H]SCH23390-binding sites in the brain of the adult worker honey bee. DA levels were reduced pharmacologically using a single 10 microl injection of either alpha-methyl-DL-p-tyrosine (AMT; 250 microg or 500 microg) or alpha-methyl-DL-tryptophan (AMTP; 250 or 500 microg) into the haemolymph of the bee. In all cases, maximum depletion of DA was observed 3 h after treatment, but in bees treated with AMTP (250 or 500 microg) or with 250 microg AMT, DA levels returned to normal within 24 h of treatment. Neither AMT nor AMTP was selective for DA: both drugs also reduced serotonin (5-hydroxytryptamine, 5HT) levels in the brain. However, AMTP was more effective than AMT at depleting 5HT, whereas for DA, the reverse was true. Depletion of DA levels, using 250 microg AMT, led to a dramatic decline in the levels of specific binding of [3H]SCH23390, defined in this study as binding in the presence of 5x10(-6) M cis-(Z)-flupentixol (see Ref. [28] ). In contrast, naturally occurring diel fluctuations in DA levels, identified in the optic lobes of the brain, and changes in brain DA levels resulting from queenlessness, had no significant effect on the density of [3H]SCH23390-binding sites in the brain of the bee. Overall, these results indicate that under normal physiological conditions, there is no direct link in honey bees between changes in endogenous brain DA levels and the density of D(1)-like receptors labelled by [3H]SCH23390.

Changes in histamine H3 receptor responsiveness in mouse brain

Changes in various histamine (HA) H3 receptor-mediated responses and H3 receptor binding in brain were investigated in mice receiving single or repeated administration of ciproxifan, a potent brain-penetrating and selective H3 receptor antagonist. Blockade of the H3 autoreceptor was nearly as effective in enhancing levels of tele-methylhistamine (t-MeHA), a major HA metabolite, in brain areas when ciproxifan was administered once either at 7 a.m. or 8 p.m., in spite of the large differences of basal levels at these two phases of the circadian cycle. Blockade after a single ciproxifan administration was, however, followed by a transient decrease in striatal t-MeHA levels, possibly reflecting rapid development of autoreceptor hypersensitivity. Following a 5-day administration of ciproxifan and a 2-day drug-free period, basal t-MeHA levels were significantly decreased (approximately -20%) in three brain areas, and the ED50 values of the drug to enhance t-MeHA levels were increased by 5-15 times without significant change in maximal response, indicating that H3 autoreceptor hypersensitivity had developed. However, in synaptosomes from the cerebral cortex of these animals, the H3 receptor-mediated inhibition of K+-induced [3H]HA release was not significantly modified. Subchronic administration of ciproxifan for 10 days also resulted in an increased binding of [125I]iodoproxyfan to the H3 receptor of striatal and hypothalamic membranes by 40-54%. Hypersensitivity at H3 somatodendritic autoreceptors and at heteroreceptors attributable to an increased number of HA binding sites could account for the various changes observed in this study.

Biochemical and behavioral effects of boldine and glaucine on dopamine systems

The aporphine alkaloids boldine and glaucine have been reported to show "neuroleptic-like" actions in mice, suggesting that they may act as dopamine antagonists. We have found that in vitro boldine displaces specific striatal [3H]-SCH 23390 binding with IC50 = 0.4 microM and [3H]-raclopride binding with IC50 = 0.5 microM, while the affinities of glaucine at the same sites are an order of magnitude lower. In vivo, however, 40 mg/kg boldine (i.p.) did not modify specific striatal [3H]-raclopride binding and only decreased [3H]-SCH 23390 binding by 25%. On the other hand, 40 mg/kg glaucine (i.p.) displaced both radioligands by about 50%. Behaviors (climbing, sniffing, grooming) elicited in mice by apomorphine (0.75 mg/kg s.c.) were not modified by boldine at doses up to 40 mg/kg (i.p.) but were almost completely abolished by 40 mg/kg glaucine (i.p.). In the apomorphine-induced (0.1 mg/kg s.c.) rat yawning and penile erection model, boldine and glaucine appeared to be similarly effective, inhibiting both behaviors by more than 50% at 40 mg/kg (i.p.). Boldine and glaucine, injected i.p. at doses up to 40 mg/kg, were poor modifiers of dopamine metabolism in mouse and rat striatum. These data suggest that boldine does not display effective central dopaminergic antagonist activities in vivo in spite of its good binding affinity at D1- and D2-like receptors, and that glaucine, although less effective in vitro, does appear to exhibit some antidopaminergic properties in vivo.

Similar pharmacological properties of 8-OH-DPAT and alnespirone (S 20499) at dopamine receptors: comparison with buspirone

Alnespirone (S 20499) has previously been described as a potential anxiolytic drug that acts by stimulation of 5-HT1A receptors. Some data suggest that alnespirone might also be a weak dopamine D2 receptor agonist: it displays moderate affinity for dopamine D2 receptors in vitro and it inhibits prolactin release and induces yawning in rats. In order to test for possible interactions of alnespirone with dopamine receptors in vivo, we studied the changes of in vivo striatal [3H]SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zepine) and [3H]raclopride binding following the injection of a tracer dose of either tritiated ligand (4 microCi) in mice treated with increasing doses of alnespirone (5, 10, 20 and 40 mg/kg, i.p.) and, in the same animals, the changes in the levels of dopamine, 5-hydroxytryptamine (5-HT) and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA). These changes were compared with those produced by increasing doses of the reference 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin, 0.25, 1 and 4 mg/kg, i.p.) or buspirone (5 and 20 mg/kg, i.p.). Decreased in vivo striatal [3H]SCH 23390 specific binding was observed in mice treated with 5, 10 and 40 mg/kg alnespirone. In contrast, increased in vivo striatal [3H]raclopride specific binding was observed in mice treated with 5 and 20 mg/kg alnespirone. In these animals, the striatal 5-HIAA/5-HT ratio was decreased by 5 to 40 mg/kg alnespirone, whereas the striatal HVA/DA ratio was unaffected at all tested doses of alnespirone. Similarly, 8-OH-DPAT decreased specific in vivo striatal [3H]SCH 23390 binding at 0.25, 1 and 4 mg/kg, and increased in vivo specific striatal [3H]raclopride binding at 1 and 4 mg/kg. In the same animals, all tested doses of 8-OH-DPAT decreased the striatal 5-HIAA/5-HT ratio but did not modify the striatal HVA/dopamine ratio. Buspirone (5 and 20 mg/kg) completely inhibited in vivo specific striatal [3H]raclopride binding and increased the striatal HVA/DA ratio but did not modify the striatal 5-HIAA/5-HT ratio, whereas apomorphine (3 mg/kg) decreased both in vivo specific striatal [3H]SCH 23390 and [3H]raclopride binding as well as the striatal HVA/DA and 5-HIAA/5-HT ratios. Finally, increasing doses of alnespirone or 8-OH-DPAT weakly increased sniffing induced by apomorphine (0.75 mg/kg, s.c.) in mice and decreased grooming induced by the dopamine D1 receptor agonist SK&F 39393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, 1.87 mg/kg, s.c.), whereas buspirone decreased both apomorphine-induced sniffing and SK&F 39393-induced grooming. These results indicate that alnespirone and 8-OH-DPAT have a similar profile and do not seem to interact directly with dopamine receptors. The results also suggest that the stimulation of 5-HT1A receptors by either alnespirone or 8-OH-DPAT modulates the availability of striatal [3H]SCH 23390 and [3H]raclopride binding sites and possibly the functioning of striatal dopamine D1 and D2 receptors in opposite directions.

Stress promotes major changes in dopamine receptor densities within the mesoaccumbens and nigrostriatal systems

This study investigated the effects of stress on brain dopamine receptor densities in two inbred strains of mice. Analysis of [3H]SCH23390 binding by quantitative autoradiography revealed that repeated restraint stress significantly increases D1-like receptor density in the nucleus accumbens of mice of the DBA/2 strain whist reducing it in the caudate-putamen of C57BL/6 mice. No significant changes in D2-like receptor quantified by [3H](-)-sulpiride binding were observed in caudate, substantia nigra and accumbens of stressed C57BL/6 mice. Instead, in DBA/2 mice, stress significantly increased D2-like receptor density in the nucleus accumbens whilst reducing it in the substantia nigra. Finally, stress significantly increased D2-like receptor density within the ventral tegmental area of C57BL/6 mice whilst significantly reducing it in mice of the DBA/2 strain. These results indicate that stress promotes major changes in mesoaccumbens and nigrostriatal dopamine receptor densities. The direction of these changes depends on receptor subtype, brain area and strain. Moreover, the opposite changes of D2-like receptor densities promoted by stress in the ventral tegmental area of the two inbred strains of mice suggest that mesoaccumbens dopamine autoreceptors density might be controlled by a major genotype x stress interaction.

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.

Parallel strain-dependent susceptibility to environmentally-induced stereotypies and stress-induced behavioral sensitization in mice

Food restriction (9 days) promoted stereotyped behavior in drug-free mice of the DBA/2 (DBA), but not in those of the C57BL/6 (C57), inbred strain. Indeed, behavior presented by food-restricted mice of the DBA strain within the home cage was characterised by a very high response rate within a single response: cage cover climbing. Moreover, enhanced climbing in food-restricted mice of the DBA strain was also observed in a test designed to detect stereotypic effects of drugs in mice. Stereotypic behavior in DBA mice did not depend on nutritional status because: 1. No stereotypies were observed in DBA mice food-deprived for 15 h; 2. no strain-dependent differences in weight loss were observed; and 3. enhanced cage cover climbing was still evident in DBA mice following 24 h of free feeding. Finally, food-restricted DBA mice showed long-lasting sensitization to the locomotor effects of systemic amphetamine, indicating stress-induced behavioral sensitization in this strain of mice. By contrast, no sign of behavioral sensitization was observed in food-restricted mice of the C57 strain. These results indicate that restricted feeding promotes stereotyped behavior, as well as behavioral sensitization to amphetamine, in mice. Moreover, the observed parallelism between strain-dependent susceptibility to environmentally-induced stereotypies and behavioral sensitization supports the hypothesis that these phenomena share common neuro-biological bases.

Behavioural subsensitivity induced by long-term administration of a low dose of haloperidol to rats

This study examines the effects on open-field and stereotyped behaviour of rats of abrupt withdrawal from repeated treatment with a low (0.03 mg kg-1) dose of haloperidol. Single administration of this low dose of haloperidol significantly increased open-field locomotion without modifying apomorphine (0.5 or 2.0 mg kg-1)-induced stereotyped behaviour. Forty-eight hours after abrupt withdrawal from 0.03 mg kg-1 haloperidol (twice daily for 15 days) a significant decrease in locomotion frequency was observed, but no change was observed in apomorphine-induced stereotypy. Our results suggest that dopamine autoreceptor supersensitivity might be evaluated in a behavioural situation of absence of postsynaptic dopamine receptor supersensitivity.

The behavioral and neurochemical effects of phencyclidine in humans and animals: some implications for modeling psychosis

Phencyclidine (PCP) produces a psychotic reaction in humans which closely resembles an acute episode of schizophrenia and has therefore been given an increasing amount of attention as a model for schizophrenia. The present article reviews the behavioral and neurochemical effects of PCP in both humans and animals. Where possible, comparisons are made between the effects of PCP and amphetamine. The merits of the dopamine versus NMDA/PCP receptor mediated expression of PCP-induced psychosis are discussed, as well as the importance of selecting behavioral models which are best suited to model the expression of psychosis, rather than the motor effects of psychotomimetics.

A comparison of the behavioral effects of minaprine, amphetamine and stress

Different types of clinically effective antidepressants prevent the behavioral effects of experimental stress, and some of these treatments affect mesolimbic dopamine (DA) functioning. Animal studies have demonstrated that repeated psychostimulant administration and repeated or chronic stressful experiences also affect mesolimbic DA functioning. These results could suggest homologies among stress, psychostimulants and antidepressants. The present experiments show that either repeated stress (120 min restraint daily for 10 consecutive days) or subchronic treatment with the antidepressant minaprine (5 mg/kg daily for 10 consecutive days) significantly reduced the inhibitory effect of 120 min of restraint on climbing, a behavioral response dependent on mesolimbic DA functioning. However, the antidepressant did not induce the altered sensitivity of presynaptic DA receptors promoted by repeated stress. Chronic stressful experience (13 days of food restriction) and repeated amphetamine (2.5 mg/kg daily for 10 consecutive days) were as effective as subchronic minaprine in reducing immobility in the Porsolt's swimming test. However, whilst both stress and amphetamine enhanced struggling, minaprine promoted swimming. Finally, chronically stressed mice and mice pretreated with amphetamine showed enhanced sensitivity to amphetamine-induced locomotion, whilst this effect was absent in animals pretreated with the antidepressant. These results indicate that although chronic and repeated stress as well as amphetamine have some antidepressant-like behavioral effects, their mode of action could be different from that of clinically active substances.

Risperidone prevents the development of supersensitivity, but not tolerance, to phencyclidine in rats treated with subacute phencyclidine

We investigated whether risperidone, a 5-HT2/dopamine-D2 receptor antagonist, inhibits the development of tolerance and supersensitivity to PCP and whether subacute administration of PCP with risperidone affects the [3H]MK-801 binding in rat brain, in comparison with dopamine-D2 receptor antagonist haloperidol and 5-HT2 receptor antagonist ritanserin. In rats treated with PCP (10 mg/kg, i.p.) for 14 days, PCP (10 mg/kg, i.p.)-induced hyperlocomotion, rearing and sniffing were potentiated (supersensitivity), and head-weaving, head-twitch, backpedalling and turning were diminished (tolerance). The development of supersensitivity to PCP was blocked by oral co-administration of risperidone (2.4 mg/kg, p.o.) and haloperidol (1.0 mg/kg, p.o.) for 14 days, but not ritanserin (10 mg/kg, p.o.) and risperidone (0.8 mg/kg, p.o.), while no drugs prevented the development of tolerance to PCP. Both risperidone (2.4 mg/kg, p.o.) and haloperidol (1.0 mg/kg, p.o.) also inhibited the cross-supersensitivity to methamphetamine (MAP; 2.5 mg/kg, i.p.)-induced rearing in rats treated with PCP for 14 days. The profiles of [3H]MK-801 binding in discrete brain areas did not change after subacute administration of PCP alone or in combination with risperidone, haloperidol or ritanserin for 14 days. Therefore, it is suggested that subacute administration of PCP may cause functional changes in the dopaminergic neuronal transmission under conditions where the binding of [3H]MK-801 in discrete brain areas is unchanged, and that co-administration of risperidone may block these PCP-induced changes in neuronal function.

Loss of D1/D2 dopamine receptor synergisms following repeated administration of D1 or D2 receptor selective antagonists: electrophysiological and behavioral studies

Many effects resulting from D2 dopamine (DA) receptor stimulation are manifest only when D1 DA receptors are stimulated by endogenous DA. When D1 receptor stimulation is enhanced by administration of selective D1 receptor agonists, the functional effects of selective D2 agonists are markedly increased. These qualitative and quantitative forms of D1/D2 DA receptor synergism are abolished by chronic DA depletion when both D1 and D2 DA receptors are supersensitive. Using both electrophysiological and behavioral methods, the present study examined the effects of selective D1 and D2 receptor supersensitivity, induced by repeated administration of selective D1 or D2 receptor antagonists, on the synergistic relationships between D1 and D2 receptors. Daily administration of the selective D2 antagonist eticlopride (0.5 mg/kg, s.c.) for 3 weeks produced a selective supersensitivity of both dorsal (caudate-putamen) and ventral (nucleus accumbens) striatal neurons to the inhibitory effects of the D2 agonist quinpirole (applied by microiontophoresis). This treatment also abolished the normal ability of the D1 agonist SKF 38393 to potentiate quinpirole-induced inhibition, and relieved D2 receptors from the necessity of D1 receptor stimulation by endogenous DA (enabling), as indicated by significant electrophysiological and behavioral (stereotypy) effects of quinpirole in eticlopride-pretreated, but not saline-pretreated, rats that were also acutely depleted of DA. Daily administration of the selective D1 receptor antagonist SCH 23390 (0.5 mg/kg, s.c.) caused supersensitivity of striatal neurons to the inhibitory effects of SKF 38393 and also abolished both the ability of SKF 38393 to potentiate quinpirole-induced inhibition and the necessity of D1 receptor stimulation for such inhibition. However, both quinpirole-induced inhibition of striatal cells and stereotyped responses were also somewhat enhanced in SCH 23390-pretreated rats. When such D1-sensitized rats were acutely depleted of DA, the behavioral effects of quinpirole were intermediate between saline-pretreated rats with acute DA depletion and SCH 23390-pretreated rats without acute DA depletion. Based upon these and related results, it is argued that the enhanced effects of quinpirole in D1-sensitized rats are due to a heterologous sensitization of D2 receptors rather than to enhanced enabling resulting from supersensitive D1 receptors. It is suggested that supersensitivity of either D1 or D2 receptors can lead to an uncoupling of normal qualitative and quantitative D1/D2 synergisms and that the heterologous regulation of D2 receptor sensitivity by D1 receptors may be related to uncoupling of functional D1/D2 synergisms.

Lasting effects of dopamine receptor agonists upon striatal dopamine release in free-moving rats: an in vivo voltammetric study

It is now well known that dopamine (DA) receptors agonists can reduce striatal DA release. These compounds are generally thought to produce short-term effects. However, in a recent in vivo study we have reported that the D1/D2 receptor agonist apomorphine might induce decrements in striatal DA release that lasted several hours. In order to establish whether the effect of apomorphine was idiosyncratic or extended to other DA receptor agonists, we have investigated the effects of the selective D1 receptor agonist SKF 38393 and of the selective D2 receptor agonist LY 171555 upon striatal DA release using differential pulse voltammetry and multi-fibre carbon electrodes selective for DA. Results support that these DA receptor agonists can reduce DA release for several hours. The effects of SKF 38393 and of LY 171555 would be DA receptor-mediated since they can be blocked by the selective D1 receptor antagonist SCH 23390 and the selective D2 receptor antagonist sulpiride respectively. These findings are discussed at the light of current literature including methodological and biological data.

Ibogaine reduces preference for cocaine consumption in C57BL/6By mice

After a period of forced exposure to 300 mg/l cocaine HCl in drinking water for a period of one week, followed by forced exposure to 200 mg/l cocaine for an additional week, male C57BL/6By mice developed a preference for cocaine when given a choice of drinking either water or a solution containing cocaine (200 mg/l). The mean daily intake of cocaine during the choice period was 26 +/- 1 mg/kg or, when expressed as the ratio of cocaine over total fluid intake, represented a cocaine preference of 71 +/- 2%. Administration of ibogaine HCl (40 mg/kg, two injections 6 h apart) two weeks after the beginning of the choice period reduced the cocaine preference for at least five days; the mean daily intake of cocaine was reduced by 38% (to 16 +/- 1 mg/kg per day; p < 0.05) and cocaine preference was reduced to 41 +/- 2% (cocaine fluid consumption/total fluid intake). An acute challenge injection of cocaine (25 mg/kg SC) produced a significant increase in cocaine-induced locomotor activity and stereotypy in mice previously exposed to cocaine in their drinking water (cocaine choice group). Five days after ibogaine administration, locomotor and stereotypy activity were significantly lower after a challenge injection of cocaine (25 mg/kg SC). Brain levels of cocaine 35 min after the challenge injection of cocaine were approximately 25% higher in ibogaine-treated mice (7.2 +/- 0.5 and 9.3 +/- 0.8 micrograms/g wet wt for water vs. mice treated with water plus ibogaine and 9.3 +/- 0.2 and 11.8 +/- 0.7 micrograms/g wet wt for cocaine drinking vs. cocaine drinking plus ibogaine treatment).(ABSTRACT TRUNCATED AT 250 WORDS)

Post-training minaprine enhances memory storage in mice: involvement of D1 and D2 dopamine receptors

Post-training administration of minaprine (2.5, 5 and 10 mg/kg) dose-dependently improved retention of an inhibitory avoidance response in mice. Animals receiving nine daily injections of 5 mg/kg and administered a challenge dose post-training showed an improvement in memory consolidation similar to that produced by acute injection of 10 mg/kg. The effects on retention performance induced by the drug appear to be due to an effect on memory consolidation. They were observed when drugs were given at short, but not long, periods of time after training, i.e. when the memory trace was 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 training were not affected by post-training drug administration. The effects of an acutely injected dose (10 mg/kg) of minaprine as well as those of a challenge dose (5 mg/kg) of the drug administered to repeatedly treated animals were reversed by pretreatment with either selective D1 or D2 dopamine receptor antagonists 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 the effects of minaprine on memory consolidation. These results show that minaprine improves memory consolidation and that repeated drug administration leads to potentiation of this effect. Moreover, the effects of minaprine on memory consolidation are related to its dopaminergic action.

Long-term sensitization of apomorphine-induced rotation behavior in rats with dopamine deafferentation or excitotoxin lesions of the striatum

Following unilateral 6-hydroxydopamine (6-OHDA)-induced deafferentation or unilateral kainic acid (KA) lesions of the striatum, rats displayed rotation behavior in response to apomorphine (0.25 or 1 mg/kg, SC, for 6-OHDA- and KA-lesioned rats, respectively). Three days following the initial apomorphine injection, rats were challenged under identical conditions with the same dose of apomorphine received previously. A third trial with apomorphine was again repeated after 3 days. Two more sets of behavioral data, each consisting of three trials, were collected under the same conditions as the first. Each set was separated by a period of 5-6 weeks. Following the second trial of the first set, rats showed a significant increase in the maximal number of rotations, demonstrating behavioral sensitization. Following the two 5-week intervals, rats were still sensitized to apomorphine, showing behavioral responses similar to the sensitized. Following the two 5-week intervals, rats were still sensitized to apomorphine, showing behavioral responses similar to the sensitized responses observed after the initial trials. Thus, the postsynaptically mediated sensitization of apomorphine-induced rotation behavior in 6-OHDA- or KA-lesioned rats is a long-lasting phenomenon. That lesions producing postsynaptic dopaminergic hypersensitivity and hyposensitivity can both show long-lasting sensitization may indicate multiple mechanisms underlying the sensitization.

Effects of postnatal stress on dopamine mesolimbic system responses to aversive experiences in adult life

The effects of postnatal stress on mesolimbic dopamine (DA) functioning in 90-day-old mice were investigated. Postnatal stress consisted of 15 min daily exposure to clean bedding (CB) in the absence of the mother for the first two weeks of life. Controls were daily exposed to home cage bedding (HCB) in the absence of the mother. A single brief (5-10 min) exposure to restraint produced a clear-cut increase in DA metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT)) in the nucleus accumbens septi (NAS) of adult HCB but not CB mice. Moreover, when tested in an elevated plus maze, CB mice showed more exploration and reduced fearfulness in comparison with HCB mice. Taken together, these results indicate reduced emotional reactivity in adult mice repeatedly stressed during postnatal development. Moreover, HCB mice but not CB mice showed altered behavioral responsiveness to apomorphine following repeated restraint stress (10 daily 120 min) in adult life, although no difference in the behavioral response to either a low or a high dose of apomorphine was observed in adult unstressed mice of the CB and HCB groups. These results indicate that the effects of early experiences on brain DA functioning may not be evident in basal conditions and be revealed only under environmental pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitization of c-fos expression in rat striatum following multiple challenges with D-amphetamine

D-Amphetamine transiently stimulates the expression of the immediate-early response gene, c-fos, in rat striatal cell nuclei. D-Amphetamine (2.5 mg/kg i.p.) induced a significantly greater expression of Fos-like immunoreactivity in striatum of rats treated three days previously with D-amphetamine compared to rats treated three days previously with saline. This increase in the expression of Fos-like immunoreactivity in rat striatum was characterized by a significantly greater number of immunoreactive nuclei and a significant increase in the intensity of the immunoreactivity. This sensitization of c-fos expression following a repeated administration of D-amphetamine indicates an increased activation of post-synaptic elements in rat striatum.

Sensitization, response fluctuation and long-term effect of SKF-82958 and bromocriptine in the hemi-parkinsonian rat

Rats with a unilateral 6-hydroxydopamine lesion of substantia nigra were treated with the dopamine agonists SKF-82958 (D1 receptor selective) or bromocriptine (D2 receptor-selective) and their circling response recorded. Both of the compounds induced an acute episode of rotation directed away from the lesioned side. Consecutive daily treatments with either compound usually resulted in a significantly increased average response (sensitization) over a 3- to 6-day treatment period. But nearly all animals treated with low doses of either SKF-82958 or bromocriptine exhibited one or more days when they were totally unresponsive to drug treatment. Response fluctuations thus were not exclusively associated with D1 or D2 receptor agonist treatment. When subsequently tested, undrugged, in the drug-associated environment, 2, 4 and 10 weeks after their last drug treatment, rats that had previously been treated with SKF-82958 exhibited rapid contralateral rotation while rats that had previously been treated with bromocriptine showed no such undrugged rotation. This result is consistent with previous findings that the D1 receptor agonist, SKF-38393, but not the D2 receptor agonist, quinpirole, had long-term behavioral effect in nigral rats, and suggests that persistent motor consequences of limited treatment with dopamine receptor agonists are D1 receptor-related.

Nonhuman behavioral models in the genetics of disturbed behavior

The development of the association method in which genetic markers match quantitative traits had led to quantitative trait loci (QTL) interval mapping. The association method has been extensively used in animal behavior genetics. Animal research allows more suitable linkage studies and detailed assessment of cellular and subcellular components of the central nervous system that may play a crucial role in the development susceptibility to behavioral disorders. Moreover, experimental designs in the laboratory setting allow genotype x environment interactions to be controlled, thus possibly providing more information on the role of nongenetic factors in gene expression. Experimental results are discussed which indicate that animal studies will provide a sort of test for hypotheses arising in clinical settings, allowing gene-product and product-behavior pathways to be examined at molecular levels when the gene accounts for a very small amount of genetic variance. In such a perspective, new molecular biology approaches and behavior genetics in nonhuman species could provide useful tools in the assessment of the genetic as well as nongenetic factors that lead to psychopathology.

Effects of dopaminergic transmission interruption on the D2 receptor isoforms in various cerebral tissues

We examined the effects of an interruption of dopamine neurotransmission, by either dopamine receptor blockade or degeneration of dopamine neurons by 6-hydroxydopamine, on the levels of D2 receptor mRNAs. In addition, we evaluated by the polymerase chain reaction (PCR) the relative abundance of the two D2 receptor isoform mRNAs generated by alternative splicing. Daily injections of 4 mg/kg of haloperidol to rats elicited in striatum a rapid and progressive increase in D2 receptor mRNA levels, which reached 70% after a 15-day treatment. By contrast, there was no apparent change in D2 receptor mRNA levels in cerebral cortex and pons-medulla, in spite of an increased density of D2 receptor in the former tissue. Using the PCR with primers flanking the alternative exon, we observed that the relative proportion of the shorter receptor isoform (D2S) mRNA was slightly but significantly enhanced in cerebral cortex (17%) and pons-medulla (18%) after a 15-day haloperidol treatment. Unilateral degeneration of dopamine neurons induced by local injection of 6-hydroxydopamine resulted in a marked decrease in levels of total D2 receptor mRNAs in substantia nigra (-79%) and ventral tegmental (-63%) area, two cell body areas. In the substantia nigra, the longer isoform (D2L) mRNA was significantly more decreased in content than the D2S isoform mRNA, so that there was a large enhancement in the relative abundance of the latter (81%).(ABSTRACT TRUNCATED AT 250 WORDS)

Possible involvement of dopamine D-1 and D-2 receptors in diazepam-induced hyperphagia in rats

Possible involvement of dopamine receptors in diazepam-induced (1 mg/kg, subcutaneous (sc] hyperphagia was studied in nondeprived rats. Pretreatment with the selective D-1 antagonist, SCH23390 (0.03 mg/kg, sc) inhibited diazepam-induced hyperphagia. In addition, pretreatment with the preferential D-2 antagonists, haloperidol (0.1 to 0.3 mg/kg, sc) and clebopride (0.1 to 0.3 mg/kg, sc) inhibited diazepam-induced hyperphagia in a dose-dependent manner. Pretreatment with co-administration of SCH23390 (0.1 mg/kg, sc) and clebopride (0.03 mg/kg, sc) completely inhibited this hyperphagia. The selective D-2 antagonist, sulpiride (40 mg/kg, sc) and the peripheral D-2 antagonist, domperidone (10 mg/kg, sc) did not affect diazepam-induced hyperphagia. However, sulpiride (10 micrograms, icv) or domperidone (2 micrograms, icv) administered centrally inhibited this hyperphagia. The highest dose of haloperidol (0.3 mg/kg, sc) or clebopride (0.3 mg/kg, sc) and higher doses of SCH23390 (0.01 and 0.03 mg/kg, sc) or SCH23390/clebopride (0.01/0.03 and 0.01/0.1 mg/kg, sc) tended to decrease spontaneous feeding in non-deprived rats. In addition, the highest dose of haloperidol, clebopride or SCH23390/clebopride inhibited spontaneous feeding in deprived rats. Interestingly, diazepam-induced hyperphagia was inhibited significantly by doses of haloperidol (0.1 mg/kg, sc), clebopride (0.1 mg/kg, sc) and SCH23390/clebopride (0.003/0.03 and 0.003/0.1 mg/kg, sc) which did not affect spontaneous feeding in non-deprived or deprived rats. Pretreatment with alpha-methyl-p-tyrosine (40 mg/kg, IP x 2, 6 and 2 h prior to diazepam administration) failed to inhibit this hyperphagia. Furthermore, pretreatment with a large dose of haloperidol (5 mg/kg, sc, 4 days before diazepam administration) augmented the sub-hyperphagic effect to diazepam (0.5 mg/kg, sc). Thus, these findings suggest that hyperphagia to diazepam is mediated in part by both dopamine D-1 and D-2 receptors in non-deprived rats.

Behavioural and biochemical effects of subchronic treatment with raclopride in the rat: tolerance and brain monoamine receptor sensitivity

Male Sprague-Dawley rats were treated with the dopamine (DA) D2 receptor blocking agent raclopride 0.5 or 8.0 mg kg-1 subcutaneously (1.0 and 16.0 mumol kg-1, respectively), twice daily for 21 days. The animals treated with raclopride gained weight at the same rate as saline controls, and gross observation did not indicate any behavioural abnormalities due to the subchronic raclopride treatment. Possible changes in brain DA receptor sensitivity due to prolonged blockade of DA receptors were evaluated in behavioural and biochemical models. There were no effects on locomotor activity, as observed by means of photobeam-equipped activity cages, 24 hr or 72 hr after withdrawal of 0.5 or 8.0 mg kg-1 subchronic raclopride treatment. Twenty-four hr after withdrawal of the raclopride treatment there was an increased post-synaptic DA receptor sensitivity as evidenced by increased behavioural and biochemical responses to apomorphine, and by an attenuated response to acute raclopride treatment, 0.1 mg kg-1. Thus, there was an increase in locomotor activity by the apomorphine treatment in animals pretreated with the 8 mg kg-1 dose, as compared to the response obtained in saline controls. Furthermore, the suppression of locomotor activity in saline controls produced by acute raclopride treatment was dose-dependently antagonized by the raclopride pretreatment and this also applied to the increase in striatal DOPAC levels produced by acute raclopride treatment. Finally, there was an increased DA receptor sensitivity presynaptically as evidenced by an enhanced effect on striatal DOPA levels by apomorphine in rats treated with NSD 1015 and reserpine.(ABSTRACT TRUNCATED AT 250 WORDS)

Genotype-dependent effects of chronic stress on apomorphine-induced alterations of striatal and mesolimbic dopamine metabolism

After 10 daily consecutive restraint experiences, DBA/2 (DBA) mice showed an increase of climbing behavior after injection of 0.25 mg/kg of the dopamine (DA) agonist apomorphine (APO), while no changes were observed following vehicle or 1 mg/kg of APO. By contrast, chronically stressed C57BL/6 (C57) mice showed a clear-cut decrease of climbing behavior at the dose of 0.25 mg/kg of APO and a similar, although less pronounced, effect of stress on the behavior of mice injected either with vehicle or with 1 mg/kg APO. The DA agonist at these same doses decreased 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) concentrations in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of both strains. Higher DOPAC, HVA and 3-MT concentrations were evident in stressed DBA mice receiving 0.25 mg/kg but not 1 mg/kg of APO, in both CP and NAS. Concerning C57 mice, lower concentrations of the 3 metabolites were present at both doses of APO in the NAS of stressed mice in comparison with non-stressed animals, while no significant stress-related effects were evident in the CP. Non-significant differences between control and stressed mice of both strains were evident as regards DA concentrations in CP and NAS. These results suggest that repeated stressful experiences lead to a hyposensitivity of DA presynaptic receptors in DBA mice while they produce a sensitization of mesolimbic DA presynaptic receptors possibly accompanied by down-regulation of postsynaptic DA receptors in the C57 strain.

Chronic cocaine treatment does not alter rat striatal D2 autoreceptor sensitivity to pergolide

Rat superfused striatal slices, preloaded with [3H]dopamine, were electrically stimulated and the stimulation-induced outflow of radioactivity was taken as an index of dopamine release. In the presence of 10 microM nomifensine, exposure of striatal slices to unlabelled dopamine (0.3 microM) for 6 min prior to stimulation, significantly reduced stimulation-induced outflow. In contrast, a 21-min exposure to dopamine did not significantly alter stimulation-induced outflow. These results suggest that D2 receptors modulating dopamine release in the rat striatum may be rapidly desensitized in vitro. Rats were pretreated for 14 days with cocaine HCl (10 mg/kg/day i.p.) or saline. A progressive enhancement of locomotor activity in cocaine-treated rats over the pretreatment period compared to that in saline-treated rats indicated a behavioural sensitization to cocaine. The inhibitory effect of pergolide (1, 10 and 100 nM) on stimulation-induced outflow from striatal slices obtained from cocaine-pretreated rats was not different from that in slices obtained from saline-pretreated rats. Therefore no evidence was obtained for either a desensitization or a supersensitivity of striatal D2 autoreceptors by chronic cocaine administration.

The enhancement of the hypomotility induced by small doses of haloperidol in the phase of dopaminergic supersensitivity in mice

Dopaminergic supersensitivity in mice was induced by pretreatment with a single injection of haloperidol (4.8 mg/kg). After the pretreatment, further treatment with haloperidol (0.6 or 0.01 mg/kg) was made at varying intervals, and catalepsy, locomotor activity and homovanillic acid (HVA) were measured. The intensity of the supersensitivity was evaluated by enhanced apomorphine (1 mg/kg)-induced climbing behavior. Supersensitivity was displayed on the 2nd and the 4th day. The cataleptogenic effect of haloperidol (0.6 mg/kg) was significantly weakened on the 1st, 2nd and 4th days. The motor inhibitory effect of haloperidol (0.01 mg/kg) increased on the 1st, 2nd and 4th days. Homovanillic acid was measured in the striatum and the prefrontal cortex on the 2nd day. Haloperidol (0.6 mg/kg) increased the concentrations of HVA in both regions of the brain. The increase in the concentrations of HVA in the striatum was blunted after the pretreatment, but such tolerance did not develop in the prefrontal cortex. Haloperidol (0.01 mg/kg) did not influence the concentration of HVA in both regions. These results suggest that the behavioral effect of a small dose of haloperidol may be enhanced, rather than reduced, in the phase of supersensitivity.

Sensitization of rotation behavior in rats with unilateral 6-hydroxydopamine or kainic acid-induced striatal lesions

Following unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra or unilateral kainic acid (KA) lesions of the striatum rats displayed rotation behavior in response to apomorphine (0.25 mg/kg SC or 1 mg/kg SC for the 6-OHDA- and KA-lesioned rats respectively). Three to five days following the initial apomorphine challenge rats were challenged under identical conditions with the same dose of apomorphine received previously. Both 6-OHDA- and KA-lesioned rats demonstrated a significant increase in the total number of rotations. Following a subsequent challenge with apomorphine, rats showed further increases in the total number of rotations. With the second and the subsequent apomorphine challenges there were significant increases in the maximal number of rotations, a significant decrease in the time of onset of rotation behavior and in some cases an increase in the duration of the rotation behavior. These increases in rotation behavior following repeated challenges with apomorphine indicate a supersensitivity to dopamine receptor agonists distinct from that elicited by lesions and chronic antagonist treatments. Furthermore, the utility of the rotation behavior model for testing the efficacy of dopaminergic agonists might be compromised if repeated challenges in individual animals are employed.

Role of genotype in the adaptation of the brain dopamine system to stress

Behavioral and biochemical analysis of the effects of stress on brain dopamine (DA) functioning in two inbred strains of mice reveals opposite patterns of adaptation to chronic stress. Chronically stressed mice of the C57BL/6 (C57) strain are characterized by hypersensitive mesolimbic DA autoreceptors and by a dramatic increase of D1/D2 DA receptor ratio (possibly postsynaptic) in the nucleus accumbens septi (NAS) as revealed by in vivo binding of 3H-spiperone and 3H-SCH23390. Chronically stressed DBA/2 (DBA) mice present, on the contrary, hyposensitive DA autoreceptors and no changes in the D1/D2 DA receptors ratio in this brain area. The analysis of the behavioral responses of chronically stressed mice of the C57 strain to the mixed D1/D2 receptor agonist apomorphine, to the selective D2 agonist LY171555 and to the selective D1 agonist SKF 38393 suggest a close relationship between the behavioral alterations produced by chronic stress and the alterations of sensitivity of D2 pre- and postsynaptic receptors in the mesolimbic system. Furthermore, chronically stressed C57 mice present a marked decrease of spontaneous-climbing behavior which is not observed in the mice of the DBA strain and is dependent on the alteration of the biphasic evolution of this behavior during exposure to the test situation which, for these mice, represents a novel environment. Acute exposure to aversive environmental conditions induces a biphasic alteration of DA transmission (initial increase of DA release followed by a decrease under control levels) in the NAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular concentrations of cocaine and dopamine are enhanced during chronic cocaine administration

Chronic cocaine administration produces significant increases in cocaine-induced locomotor activity and stereotypy. In vivo microdialysis procedures were used to monitor extracellular dopamine (DA) and cocaine concentrations in the nucleus accumbens (N ACC) and cocaine concentrations in plasma of animals that received chronic or acute cocaine treatments. Following a cocaine challenge injection, concentrations of both cocaine and DA increased to significantly higher levels over time in animals that had received daily cocaine injections for 10 or 30 days than in control animals that received daily injections of saline. Concentrations of cocaine and DA in the N ACC reached maximum levels in the first 30 min following a challenge injection of cocaine. The maximum cocaine concentrations of 10- and 30-day chronic animals were, respectively, 186% and 156%, whereas the maximum DA concentrations were 264% and 216% above the maximum values observed in acute control animals. The results indicate that reverse tolerance effects observed following chronic cocaine administration may in part be accounted for by increased cocaine concentrations. Furthermore, chronic cocaine administration (over a 10- or 30-day period) increased the concentration of cocaine detected in plasma above control levels following a challenge injection. The increase in brain concentrations of cocaine in chronic animals is apparently due to increased concentrations of cocaine in plasma. A physiological change occurs in the periphery as a result of chronic cocaine administration that increases cocaine concentrations in plasma, increases extracellular cocaine levels in the brain, and increases the extracellular concentration of DA in the N ACC.

Selective adenylate cyclase increase in the limbic area of long-term imipramine-treated rats

Long-term administration of imipramine to rats produced an increase in the Vmax of forskolin- or guanylylimidodiphosphate (Gpp(NH))p-activated adenylate cyclase only in the limbic area. This effect was prevented by the daily administration of alpha-methyl-p-tyrosine (alpha-MPT), given together with imipramine, at a dose (50 mg/kg) which had no effect on adenylate cyclase activity per se. The time course of the effects of chronic imipramine on dopaminergic transmission in the limbic area showed that the decrease in both D-1 receptor number and adenylate cyclase stimulation by dopamine (DA) reached significance on day 8 of treatment and were maximal on day 15. The Vmax of the enzyme started to increase on day 15 and was further increased on day 21. Possible mechanisms underlying these effects are discussed.

Behavioral and neurochemical differences in an inbred strain of rats

The spontaneous motility of 75 Sprague-Dawley rats obtained from an inbred strain was found to vary widely. To test the hypothesis that heterogeneity in motility is accompanied by heterogeneity in neurochemical and vegetative function variables, the median motility score was used to divide the animals into high and low motility groups. Subgroups of high and low motile animals were compared on fecal pellet scores and on dopamine, dihydroxyphenyl acetic acid and homovanillic acid content of the corpus striatum. Significant differences were obtained between the two groups, indicating that even in an inbred strain considerable heterogeneity (possibly representing further subtypes) may exist. The theoretical and experimental implications of these findings are discussed.

Heterocyclic antidepressant, monoamine oxidase inhibitor and neuroleptic withdrawal phenomena

1. The authors review the literature describing acute symptomatology produced by the gradual or abrupt withdrawal of heterocyclic antidepressants, monoamine oxidase inhibitors (MAOI) and neuroleptics. 2. Withdrawal of heterocyclic antidepressants and antipsychotic agents causes similar symptomatology. Symptoms produced by the discontinuation of these drugs include nausea, emesis, anorexia, diarrhea, rhinorrhea, diaphoresis, myalgias, paresthesias, anxiety, agitation, restlessness, and insomnia. 3. Psychotic relapse is often presaged by anxiety, agitation, restlessness, and insomnia. Prodromal symptoms are distinguished from the effects of neuroleptic withdrawal by a temporal relationship of the latter to reductions in the dosage or discontinuation of antipsychotic agents. 4. Withdrawal of MAOIs can result in severe anxiety, agitation, pressured speech, sleeplessness or drowsiness, hallucinations, delirium, and paranoid psychosis. 5. MAOI withdrawal phenomena resemble the symptoms produced by the discontinuation of chronically administered psychostimulants. 6. The capacity of MAOIs to exert amphetamine-like effects presynaptically and the propensity of somatic treatments for depression to subsensitize presynaptic receptors regulating the release of catecholamines provide a basis for the development of psychotic symptoms upon the withdrawal of MAOI. Evidence for this hypothesis is reviewed.

Yawning and suppression of exploration induced by dopamine agonists: no relation to extracellular striatal levels of dopamine

The present study was aimed at testing the hypothesis that yawning and suppression of exploration induced by low doses of dopamine agonists in the rat are caused by a reduction of synaptic dopamine levels. The decrease in extracellular levels of dopamine in the corpus striatum induced by alpha-methyl-p-tyrosine (alpha MPT, 50-200 mg/kg IP), reserpine (2-5 mg/kg SC) and apomorphine (APO, 0.05 mg/kg SC) was measured in microdialysis experiments. Reserpine and alpha MPT reduced the dopamine levels to the same extent as APO. Exploratory behaviour was suppressed by APO, but not by alpha MPT (50 and 100 mg/kg) when tested in a separate experiment. Reserpine (2 mg/kg) suppressed exploration after 4 hr, but not after 3 hr. Changes in extracellular levels of dopamine were tested simultaneously with changes in yawning in another group of rats implanted with guide cannulae for microdialysis probes. There was a discrepancy in the time-course for the induction of yawning as compared to the changes in extracellular dopamine levels after APO (0.05 mg/kg) as well as after pergolide (0.02 mg/kg SC). Yawning appeared before and lasted shorter than the decrease in dopamine. The time-courses for APO-induced suppression of exploration and yawning were similar. The dose-response curve for APO-induced yawning was not changed by alpha MPT (200 mg/kg), while the suppression of exploration induced by APO, but not by pergolide, was enhanced by pretreatment with alpha MPT. The results show that yawning and suppression of exploration induced by dopamine agonists are not related to changes in extracellular levels of dopamine. It is proposed that these behaviours may be mediated by postsynaptic receptors.

Dopamine D1 and D2 receptors mediate opposite effects of apomorphine on the body temperature of reserpinized mice

In mice, rendered poikilothermic by a prior (18 hr) subcutaneous administration of reserpine (3 mg/kg), the subcutaneous administration of apomorphine increased dose-dependently the body temperature. This effect was potentiated by the specific D2 dopamine antagonist sulpiride. On the contrary, it was reduced by the specific D1 dopamine antagonist SCH 23390. A desensitization of D2 receptors was produced by the repeated administration of the specific D2 agonist RU 24926. This pretreatment led to an increased efficacy of apomorphine in antagonizing reserpine-induced hypothermia. Similarly, a desensitization of D1 receptors was created by the repeated administration of the specific D1 agonist CY 208-243. This pretreatment significantly diminished the efficacy of apomorphine in antagonizing reserpine-induced hypothermia. The repeated administration of the D1 agonist CY 208-243, in non-reserpinized mice, significantly increased the hypothermic effect of apomorphine (1 mg/kg). Thus, it appears that, in normal mice, but especially in reserpinized mice, the stimulation of D1 receptors by apomorphine induces an increase in body temperature that is masked, especially in normal mice, by the hypothermic effect, resulting from the simultaneous stimulation of D2 receptors.

Potentiation of apomorphine-induced stereotyped behaviour by acute treatment with dopamine depleting agents: a potential role for an increased stimulation of D1 dopamine receptors

Treatment of mice with reserpine (10 mg/kg, s.c.) and alpha-methylparatyrosine (400 mg/kg) led to the potentiation of stereotyped behaviour, induced by apomorphine (0.37-1.5 mg/kg, s.c.), i.e. to the appearance of licking and gnawing in addition to climbing and sniffing occurring in control mice. Similar results were obtained by combined treatment with SK&F 38393 (30 mg/kg, s.c.) and RU 24926 (15 mg/kg, i.p.). In mice treated with dopamine depleting agents, SCH 23390 (1.25-20 micrograms/kg, s.c.) and metoclopramide (0.62-20 mg/kg, i.p.) antagonized gnawing induced by 0.75 mg/kg (s.c.) apomorphine, at doses significantly larger than those required for the antagonism of climbing and sniffing. The same treatment with reserpine and alpha-methylparatyrosine produced an increased formation of cyclic AMP, induced by SK&F 38393 (10(-8)-10(-4) M), from homogenates of the striatum of the rat. Potentiation of apomorphine-induced stereotyped behaviour and increased SK&F 38393-induced formation of cyclic AMP had similar time-courses with a maximum 18 hr after treatment. These data suggest that the potentiation of apomorphine-induced stereotyped behaviour produced by acute treatment with dopamine depleting agents is at least partly due to an increased activity of the adenylate cyclase linked to D1 dopamine receptors. Finally, a small dose of amisulpride (a discriminant benzamide derivative) potentiated the stereotyped behaviour induced by the combined treatment with SK&F 38393 and RU 24926 in naive mice and, in a more marked manner, in mice treated with dopamine depleting agents; amisulpride did not produce stereotyped behaviour when combined with SK&F 38393 or RU 24926 administered alone.(ABSTRACT TRUNCATED AT 250 WORDS)