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

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

Dissociation between the effects of pre-weaning and/or post-weaning social isolation on prepulse inhibition and latent inhibition in adult Sprague--Dawley rats

Human attentional impairments can be modelled in the rat using the prepulse inhibition (PPI) or the latent inhibition (LI) paradigm. The present study investigated the consequences of a combination of pre-weaning maternal separation (MS) and post-weaning social isolation (SI) on both PPI and LI in male and female Sprague--Dawley rats tested as adults. We report here a double dissociation between the effects of MS (repeated 4 h daily separations) and SI on PPI and LI: MS did not modify PPI, but enhanced LI. In contrast, SI disrupted PPI, the deficits being restricted to male rats, but left LI intact. There were no additive effects of MS and SI on PPI or LI. While MS improved avoidance learning, SI impaired it. Although both PPI and LI assess processes of selective attention, our results support the contention, already stated in the literature, that they involve differing neuro-psychological mechanisms. Furthermore, the fact that only males exhibited PPI deficits following SI has implications for the well-known differential vulnerability of human males to certain psychiatric disorders (e.g. schizophrenia). Finally, the combination of MS and SI could represent a relevant animal model for some aspects of schizophrenia, since both PPI and LI were altered.

Behavioral, neurochemical and endocrinological characterization of the early social isolation syndrome

Rearing rats in isolation has been shown to be a relevant paradigm for studying early life stress and understanding the genesis of depression and related affective disorders. Recent studies from our laboratory point to the relevance of studying the social isolation syndrome as a function of home caging conditions. Accordingly, the present series of experiments assessed the contribution of each condition to the expression of the prepulse inhibition of the acoustic startle, food hoarding and spontaneous locomotor activity. In addition, ex vivo neurochemical changes in the brains of isolated and grouped rats reared either in sawdust-lined or in grid-floor cages were determined by measuring dopamine and serotonin as well as their major metabolites in a "psychosis circuit" that includes mainly the hippocampus and selected hippocampal efferent pathways projecting towards the anterior cingulate and infralimbic cortices, nucleus accumbens, dorsolateral caudate nucleus, amygdala and entorhinal cortex. The results of the present study demonstrate that rearing rats in isolation (i) produces a syndrome of generalized locomotor hyperactivity; (ii) increases the startle response; (iii) impairs prepulse inhibition; (iv) tends to increase food hoarding behavior; (v) increases basal dopamine turnover in the amygdaloid complex; (vi) decreases basal dopamine turnover in the infralimbic part of the medial prefrontal cortex; and (vii) decreases basal turnover of serotonin in the nucleus accumbens. In the entorhinal cortex, dopamine neurotransmission seemed to be more sensitive to the caging conditions since a decreased basal turnover of dopamine was observed in grid-reared animals. Plasma corticosterone levels were also increased in grid-reared animals compared with rats reared in sawdust cages. Finally, isolates reared on grids showed a significant positive correlation between plasma corticosterone levels and dopamine in the left nucleus accumbens.Altogether, these results support the contention that there is a link between social isolation, attention deficit, spontaneous locomotor hyperactivity and reduced dopamine turnover in the medial prefrontal cortex. Furthermore, our data demonstrate that rearing rats in grid-floor cages represents a form of chronic mild stress associated with increased corticosterone levels, decreased basal turnover of entorhinal dopamine and increased dopamine activity in the left nucleus accumbens. Finally, a significant and selective decrease in the basal turnover of serotonin in the nucleus accumbens of isolated rats may be linked to the isolation-induced locomotor hyperactivity.

Anxiolytic profile of ropinirole in the rat, mouse and common marmoset

RATIONALE

Some features of Parkinson's disease are exacerbated by stress and anxiety and it is important to understand the effects of dopamine receptor agonists on measures of anxiety. The aim of this study was to assess the effects of the dopamine D2/D3 receptor agonist ropinirole in models of anxiety and depression in the rat, mouse and marmoset.

RESULTS

In the rat elevated plus-maze test, ropinirole (0.01-1 mg/kg, i.p.) produced an inverted-U dose-response curve in the percentage time spent in the open arms. Compared with vehicle, ropinirole (0.1 mg/kg) had a significant anxiolytic-like effect, which was similar to that observed with 1.5 mg/kg diazepam. This effect was found at doses that did not affect motor behaviour or induce stereotypy. In the mouse black and white box test of anxiety, ropinirole (0.1-10 mg/kg, i.p.) increased both the rearing time and number of line crosses in the white section. This effect reached statistical significance for both measures at a dose of 0.1 mg/kg and suggests an anxiolytic-like action of the compound. By contrast, the dopamine agonist bromocriptine (0.1-10 mg/kg, i.p.) did not produce significant changes in these behaviours. In the marmoset human threat test, ropinirole (0.01-10 microg/kg, s.c.) reduced the number of postures at all doses tested and this reached statistical significance at 10 microg/kg. Ropinirole did not compromise the effect of amitriptyline in the Porsolt test of depression and in itself produced antidepressant-like effects.

CONCLUSIONS

These data demonstrate that systemic administration of ropinirole produces anxiolytic-like effects in three separate models in the mouse, rat and marmoset. This may predict an action of ropinirole in man that would provide a superior profile of action over other presently available anti-parkinsonian agents.

Strain differences in the isolation-induced effects on prepulse inhibition of the acoustic startle response and on locomotor activity

The authors investigated the effects of isolation rearing on acoustic startle response, prepulse inhibition (PPI), its modification by apomorphine, and locomotor activity in 3 rat strains: Wistar (WS), Sprague-Dawley (SD), and Lister hooded (LH). SD and LH, but not WS, showed isolation-induced PPI deficits. In 2 consecutive PPI tests, only SD isolates showed significant PPI deficits. An isolation rearing effect in LH was significant only in the 1st PPI test. Apomorphine dose-dependently (0.0-0.5 mg/kg) disrupted PPI, but sensitivity to the drug differed, with WS and SD rats being more sensitive to lower doses (0.01-0.05 mg/kg) than LH rats (0.5 mg/kg). Isolates, irrespective of strain, did not differ from grouped rats in their response to the apomorphine challenge. Only WS and LH isolates demonstrated significantly increased locomotor activity. Strain differences in the different parameters measured did not predict isolation-induced effects on PPI.

Circadian time does not modify the prepulse inhibition response or its attenuation by apomorphine

The present study investigated the influence of circadian time (experimental testing during the light or dark phase of the light:dark cycle) on the acoustic startle response (ASR), prepulse inhibition (PPI), and apomorphine-induced PPI deficits in Wistar rats housed under a reversed light:dark cycle (lights off at 0700 h and on at 1900 h). There was no significant difference in the startle response amplitude or PPI response of animals tested during the light phase compared with those tested during the dark phase. Similarly, the response to apomorphine (0.01-0.05 mg/kg subcutaneously) was not modulated by circadian time. Thus, under the conditions adopted in the present study, ASR, PPI, and apomorphine-induced PPI deficits remained stable across the circadian cycle. Such findings may be of importance for other investigators using the PPI paradigm to study brain plasticity mechanisms and pharmacological manipulations of apomorphine-induced PPI deficits in rats housed under normal or reversed light:dark cycle conditions.

Isolation rearing-induced disruption of prepulse inhibition: further evidence for fragility of the response

The present study investigated isolation-induced disruptions of prepulse inhibition (PPI), and effects on locomotor activity as a function of home caging condition (sawdust vs grid-floor) in the Wistar rat. Isolates reared in grid-floor cages did not show a disruption of PPI. However, when isolates were reared in sawdust cages, a PPI deficit was evident. In an open field environment, isolates demonstrated significantly increased spontaneous locomotor activity compared to their group-housed counterparts, irrespective of the caging condition employed. Grouped animals reared in grid-floor cages, however, showed reduced activity compared to grouped animals reared in sawdust cages. Although d-amphetamine treatment appeared to enhance locomotor activity selectively in isolates, particularly in those reared in grid-floor cages, this result could be explained by the existing pre-drug activity levels. With respect to PPI, not only were isolation-induced deficits in the Wistar rat difficult to detect in a variable prepulse intensity PPI procedure, but when apparent, the deficits were of a fragile nature. The findings suggest that caging condition may be a critical methodological factor in experiments investigating isolation-induced PPI deficits. Indeed, our results may indicate that rearing animals in grid-floor cages represents a form of chronic mild stress, which can interfere with normal sensorimotor gating mechanisms, in addition to other behaviours.

Selective effects of 8-OH-DPAT on social competition in the rat

Previous research has demonstrated that dominant-subordinate relationships measured in small groups of rats competing for access to palatable food or fluids can be disrupted by both anxiolytic and anxiogenic drugs, and it has been proposed as a possible animal model of anxiety. The present study investigated the effects of the selective 5-HT1A agonist 8-OH-DPAT on the rank order of triads of rats measured in terms of access to sweetened milk. The effect of 8-OH-DPAT on locomotor activity and intake of sweetened milk was also determined. 8-OH-DPAT (25 and 37.5 micrograms/kg) significantly increased the subordinate animals position in the social hierarchy without effect on the individual intakes of sweetened milk or locomotor activity. The same doses administered to dominant animals had no effect on any of the parameters measured. The 8-OH-DPAT-induced increase in social competition in subordinate rats was dissociable from effects on feeding behavior and locomotor activity. The results from this study provide further evidence that social competition in groups of rats may represent a model that can be used to detect drugs acting via receptor mechanisms believed to be implicated in anxiety.

Acute and chronic arecoline: effects on a scopolamine-induced deficit in complex maze learning

These studies tested the effect of arecoline, a nonselective muscarinic agonist, administered either acutely or by chronic peripheral infusion via osmotic minipumps, on a scopolamine-induced deficit in a Stone (14 unit) T-maze task in rats. Scopolamine alone (0.125-1.0 mg/kg, IP) dose-dependently impaired maze acquisition, increasing maze run-times and to a lesser extent, the number of errors committed. Neither acute administration of arecoline (5.0 and 10.0 mg/kg, IP), when tested against a deficit induced by scopolamine (0.25 mg/kg, IP), nor chronic arecoline administration (30 and 50 mg/kg per 24 h), when tested against a deficit induced by scopolamine (0.5 mg/kg), were able to ameliorate the decrements in maze performance. In fact, the higher dose of arecoline (50 mg/kg per 24 h) infused over 10 days potentiated the scopolamine-induced deficit, with respect to latency. These data indicate that dose selection is of great importance when employing arecoline in tests of learning and memory and that the influence of the method of administration of arecoline on the behavioural outcome warrants further study.

Aged Stone maze-impaired rats exhibit reduced striatal muscarinic receptor densities

Muscarinic receptor densities, measured by saturation radioligand binding using the muscarinic antagonist [3H]QNB, were compared in the striatum, frontal cortex and hippocampus between two populations of young (3 month) and aged (12-20 month) Hooded Lister rats which had previously been tested in a complex maze task. Aged rats were impaired in their performance of a Stone (14-unit T-maze) task and were less spontaneously active than young rats. Muscarinic receptor numbers were significantly decreased in the striatum of aged rats, whilst numbers in the hippocampus and frontal cortex and receptor affinities in all three areas were unaltered. These results indicate that the age-associated depletion of striatal muscarinic receptors may contribute to deficits in cognitive processing and/or motor behaviour which underlie impairments in the performance of complex spatial tasks.

Ondansetron fails to attenuate a scopolamine-induced deficit in a Stone maze task

The 5-HT3 receptor antagonist ondansetron has previously been reported to improve cognition in the mouse, rat and marmoset in a variety of behavioural paradigms. The present study used the Stone maze to test the effect of ondansetron on the deficit caused by scopolamine in the performance of a highly complex spatial memory task in the rat. Ondansetron administered over a large dose range (1.0 ng kg-1-1.0 micrograms kg-1, i.p., b.d.) for a period of 10-15 days failed to attenuate the scopolamine deficit. Indeed at one dose level ondansetron (100 ng kg-1, i.p., b.d.) administered in combination with scopolamine (0.5 mg kg-1, i.p.) significantly potentiated the deficit, compared with the performance of rats receiving scopolamine alone.

Effects of cholecystokinin tetrapeptide and sulfated cholecystokinin octapeptide in rat models of anxiety

The effects of the acutely administered cholecystokinin (CCK) agonists CCK tetrapeptide (BOC-CCK-4) and sulfated CCK octapeptide (CCK-8S) were examined in four animal models of anxiety in rats. In the elevated plus maze, BOC-CCK-4 reduced the time spent in the open arms and the number of entries into the open arms. BOC-CCK-4 but not the anorectic acting CCK-8S increased the suppression of feeding in a conflict paradigm based on novelty suppressed feeding in hungry rats. In the two-compartment black-and-white box, BOC-CCK-4 decreased the time spent and locomotor activity in the white compartment. In the ultrasound vocalization test, using rat pups separated from the mother, BOC-CCK-4 increased the number of distress calls. No evidence was found for inducing anxiety-like behaviour by CCK-8S.

Initiation and maintenance of oral ethanol self-administration in female Sprague-Dawley rats

Group-housed female Sprague-Dawley rats were trained to self-administer 5% ethanol (v/v) in a large self-administration chamber (100 x 40 x 40 cm) following three different initiation methods. The procedures were 1) an ethanol injection procedure, 2) a sucrose substitution procedure, and 3) a prandial drinking technique. Only the prandial drinking method served to maintain responding for ethanol in the absence of water deprivation or sweetening of the alcohol solution. Rats trained using this technique showed a large preference for 5% ethanol over water and a significant increase in locomotor activity while responding for 5% ethanol but not while responding for water. When the concentration of ethanol was increased from 1% to 32%, the amount of ethanol ingested increased up to a maximum of 1.233 +/- 0.3 g/kg of 32% ethanol, and response rates and number of ethanol deliveries followed an inverted U-shaped curve. Appreciable blood ethanol levels were detected immediately following self-administration of 8% ethanol. These results show that, in female Sprague-Dawley rats under the experimental conditions described, the prandial drinking technique was the most effective in inducing stable oral ethanol self-administration and suggest that under these conditions and in these subjects ethanol was acting as a positive reinforcer.

Angiotensin converting enzyme inhibitors as potential cognitive enhancing agents

Anecdotal reports of a mood-elevating effect in patients and improvements in the performance of memory tests in the clinic has led to investigations into the nootropic actions of angiotensin converting enzyme (ACE) inhibitors. A cognitive enhancing action for the ACE inhibitors has been demonstrated in a number of animals models of memory function. Neurochemical studies in animals have shown that angiotensin II acting via an angiotensin II receptor can inhibit the release of 3HAch from entorhinal cortex slices. Thus the ability of ACE inhibitors to facilitate cognitive processes may be related to reduced availability of angiotensin II. Lack of specificity of ACE inhibitors may be a limiting factor in the development of such compounds as cognitive enhancers. However, the recent development of selective antagonists for subtypes of the angiotensin II receptor may represent a novel approach for the treatment of cognitive disorders with an underlying cholinergic disturbance.

The effect of the 5-HT3 receptor antagonist, RS-42358-197, in animal models of anxiety

The S-isomer of the novel 5-HT3 receptor antagonist RS-42358 ((S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1-H- benzo[de]isoquinolin-1-one, RS-42358-197) disinhibited behaviour in the mouse suppressed by the aversive situation of the light/dark test box. RS-42358-197 was effective at sub-ng/kg dose levels and the efficacy was maintained over a 100 million-fold dose range. In contrast, the R-isomer was ineffective at all doses studied. The S-isomer also disinhibited a suppressed behaviour in social interaction and elevated X-maze tests in the rat and reduced anxiety-related behaviours in a marmoset human threat test. RS-42358-197 prevented the exacerbation of the suppression of behaviour in the mouse light/dark test following withdrawal from treatment with alcohol, nicotine, cocaine and diazepam. Thus, the S-isomer of RS-42358 has a consistent non-sedating anxiolytic profile in rodent and primate models. It is exceptionally potent and a maintained efficacy at high doses distinguishes its actions from many other 5-HT3 receptor antagonists.

Profile of action of a novel 5-hydroxytryptamine1A receptor ligand E-4424 to inhibit aversive behavior in the mouse, rat and marmoset

E-4424 (2-(4-[4-(4-chloro-1-pyrazolyl)butyl]-1-piperazinyl)pyrimidine) was shown to be a 5-hydroxytryptamine1A receptor ligand in radioligand binding assays and in an in vitro guinea pig ileum preparation had both 5-hydroxytryptamine1A antagonist and agonist effects. The antagonist/agonist ratio of E-4424 was greater than in the case of buspirone and ipsapirone. E-4424 was compared to diazepam, buspirone and ipsapirone to inhibit the behavioral response to an aversive situation in the mouse black and white test box, the rat social interaction test and a marmoset human threat test. The acute administration of E-4424 (0.0001-0.5 mg/kg, i.p.) to the mouse decreased aversion to the white area of the test box and was as effective as diazepam (0.125-1.0 mg/kg, i.p.) and much more potent than buspirone (0.25-1.0 mg/kg, i.p.) or ipsapirone (0.5-5.0 mg/kg, i.p.). E-4424 was also effective in enhancing rat social interaction and reducing anxiety-related behaviors in the marmoset and was again more potent than diazepam, buspirone or ipsapirone. Withdrawal from a 14-day administration of diazepam, cocaine, nicotine or alcohol exacerbated the response to the aversive situation in the mouse test. This was not observed after withdrawal from a chronic treatment with E-4424, buspirone or ipsapirone. However, E-4424 administered during drug withdrawal prevented the response caused by withdrawal from cocaine, alcohol, nicotine and diazepam: buspirone was ineffective and ipsapirone only attenuated that syndrome after alcohol withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Ondansetron and arecoline prevent scopolamine-induced cognitive deficits in the marmoset

The cognitive-enhancing potential of the 5-hydroxytryptamine (5-HT) selective 5-HT3 receptor antagonist, ondansetron, was investigated in a model of cognitive impairment induced by the muscarinic receptor antagonist, scopolamine. For this purpose, marmosets were trained in an object discrimination task utilizing the Wisconsin General Test Apparatus. Administration of scopolamine (0.01-0.04 mg/kg, SC) caused a dose-dependent impairment in the acquisition of the object discrimination task in that marmosets required more trials to reach criterion, made more errors, and took longer to choose the objects. Administration of arecoline (0.06-0.1 mg/kg, SC) or 1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol- 1-yl)methyl]-4H-carbazol-4-one,HCl.2H2O (ondansetron) (0.1-1 micrograms/kg, SC) prevented the scopolamine-induced impairment in task acquisition in that the performance of marmosets was indistinguishable from that of saline-treated animals and was significantly better than that following scopolamine/saline. From these studies, we conclude that ondansetron prevents impairment in the cognitive performance of marmosets induced by administration of scopolamine.

Behavioural effects of anxiogenic agents in the common marmoset

The effects of the anxiogenic agents FG7142, caffeine, pentylenetetrazole, and amphetamine were assessed in two anxiety situations in the marmoset, first in an "anxiogenic" test based on the animal's response to a human observer standing in front of the home cage and second in a low-anxiety situation where animals behaviour was videotaped in the absence of the observer. In response to the human observer, the anxiolytic agent diazepam (0.1-2.5 mg/kg, SC) was shown to reduce the intensity of behaviours such as postures, while increasing time spent on the cage front. In this test, with the exception of amphetamine, which only modified responding at stereotypic doses, the anxiogenic agents failed to modify marmoset behaviour. In contrast, in the low-anxiety filming protocol the anxiogenic agents consistently reduced measures of locomotor activity while increasing the amount of time animals spent in the nest box. It is suggested that the low-anxiety protocol may be useful to evaluate drug-induced anxiogenesis and in studies of withdrawal from chronic anxiolytic treatment or drugs of abuse.

The distribution of 5-HT3 recognition sites in the marmoset brain

The distribution of the binding of [3H]GR65630 (0.2 nM) to putative 5-HT3 recognition sites in the brain of the common marmoset was assessed using autoradiography. Specific binding was heterogeneously distributed with the highest densities found in discrete nuclei of the lower medulla (nucleus tractus solitarii, dorsal motor nucleus of the vagus nerve, nucleus of spinal trigeminal nerve tract and the area postrema). In forebrain areas, relatively high binding densities were located in the medial habenula nucleus and the hippocampus (CA3, CA4 and fascia dentata). Low levels of specific binding in the rest of the forebrain hindered quantification.

The effects of umespirone as a potential anxiolytic and antipsychotic agent

Umespirone was compared to buspirone, diazepam and clozapine as a potential anxiolytic and antipsychotic agent. In the mouse black and white test box, umespirone was considerably more potent than diazepam or buspirone to reduce aversive responding, tolerance to its effects was not observed and sedation was absent, a chronic treatment and withdrawal was not associated with an anxiogenic profile, and umespirone prevented the behavioural consequences of withdrawal from diazepam. Umespirone also had an anxiolytic profile of action in the tests of rat social interaction and in the marmoset exposed to a human threat. Both umespirone and clozapine reduced the hyperactivity induced by the infusion of dopamine into the nucleus accumbens of rat. In radioligand binding assays umespirone demonstrated nanomolar affinity for the alpha 1-adrenoceptor and the 5-HT1A and dopamine D2 receptors. It is concluded that umespirone may present as a novel psychotropic agent with anxiolytic and antipsychotic potential.

Anxiolytic effects of CCK-B antagonists

PD134308 and PD135158 are highly selective CCK-B receptor antagonists and were used to investigate the role of CCK-B receptors in aversive responding in rodent and primate models of anxiety. Both PD134308 and PD135158 were as effective as diazepam to antagonise aversive behaviour in the mouse light/dark discrimination test, in the rat social interaction and elevated X-maze tests, and in a marmoset 'human threat' model. However, the CCK-B antagonists were much more potent than diazepam and their effects were recorded over an extensive dose range. Furthermore, even at high doses, sedation or muscle relaxation was not observed and anxiogenesis was absent after withdrawal from a subchronic treatment. In contrast, withdrawal from drugs of abuse, diazepam, alcohol, cocaine and nicotine was associated with a withdrawal anxiogenesis that was completely prevented by PD134308 and PD135158. It is concluded that CCK-B receptors are involved in aversive-anxiety responding and that CCK-B receptor antagonists may provide a novel and improved approach to the treatment of anxiety and withdrawal from drugs of abuse.

Preclinical pharmacology of ropinirole (SK&F 101468-A) a novel dopamine D2 agonist

These studies characterise the pharmacology of ropinirole, a selective D-2 agonist. High-affinity human caudate binding revealed a Ki for D2 receptors of 2.9 x 10(-8) M with no affinity for D1 at 10(-4) M in the rat. Ropinirole was weakly active at alpha 2-adrenoceptors and 5-HT2 receptors but inactive at 5-HT1, benzodiazepine and gamma-aminobutyric acid receptors or alpha 1 and beta-adrenoceptors. In rodents, ropinirole, like apomorphine, caused biphasic spontaneous locomotor activity and contralateral circling in 6-OHDA-lesioned mice with no tolerance to the latter after 14 days treatment. Amphetamine caused ipsilateral responses in the lesioned mice. Ropinirole did not cause marked stereotypies. In marmosets ropinirole (0.05-1.0 mg/kg SC or 0.1 mg/kg PO) reversed all motor and behavioural deficits induced by MPTP. This response started 10-20 minutes after dosing, and exceeded 2 hours. No tolerance was seen following chronic b.i.d. treatment. Similar results were obtained with 1-dopa plus benserazide; however, 1-dopa always caused emesis, whereas beneficial effects were shown with ropinirole in the absence of this side effect. These results support the continued clinical assessment of ropinirole for the treatment of Parkinson's disease.

The effect of ondansetron on cognitive performance in the marmoset

The 5-HT3 receptor antagonist, ondansetron, was administered to marmosets to determine its effect on their performance in a Wisconsin General Test Apparatus using an object discrimination reversal learning task. Briefly, this comprised a test situation in which marmosets were required to select a food rewarded object to reach criterion in performance (this was termed the initial discrimination task); the rewarded object was then changed (in the same test session) and the marmoset was required to abandon its recently learned strategy to gain reward by selection of the second object (this was termed the reversal task). At doses of 1-10 ng/kg SC b.i.d. ondansetron improved performance in both the initial discrimination and reversal tasks. This was indicated as a reduction in the number of trials required to reach criterion, a reduction in choice latency time and a reduction in the number of errors made in each test session. Higher doses of ondansetron impaired performance as measured by several criteria. The major conclusion of this study is, therefore, that ondansetron at low doses is able to improve the performance of marmosets in a cognitive task. This would support the concept that a 5-HT3 receptor antagonist can act as a cognitive enhancer.

Neurochemical consequences following injection of the substance P analogue, DiMe-C7, into the rat ventral tegmental area

The effect on forebrain catecholamine- and indoleamine-related neurochemical levels was investigated following stimulation of the rat ventral tegmental area with the substance P analogue, DiMe-C7. DiMe-C7 (6.0 micrograms) induced a marked hyperactivity in rats with maximal response between 15 and 30 min following the injection. Fifteen min following the DiMe-C7 injection levels of dopamine and/or its metabolites (3,4-dihydroxyphenylacetic acid, homovanillic acid) were significantly increased in the nucleus accumbens, amygdala, entorhinal cortex and striatum relative to vehicle-injected animals. Although the increase in dopamine metabolism in the nucleus accumbens is consistent with the behavioural hyperactivity, it is concluded that other forebrain nuclei may also be involved in the mediation of the hyperactivity response.

The differential activities of R (+)- and S(-)-zacopride as 5-HT3 receptor antagonists

R(+)- and S(-)-zacopride were assessed as potential 5-HT3 receptor antagonists in behavioural and biochemical tests. The S(-)isomer was more potent than the R(+)isomer to antagonise the hyperactivity induced by the injection of amphetamine or the infusion of dopamine into the nucleus accumbens in the rat. In contrast, the R(+)isomer was more potent to reduce the aversive behaviour of mice to a brightly illuminated environment and in a marmoset human threat test, to facilitate social interaction in rats, to increase performance in a mouse habituation test and prevent a scopolamine-induced impairment, and to antagonise the inhibitory effect of 2-methyl-5-hydroxytryptamine to reduce [3H]acetylcholine release in slices of the rat entorhinal cortex. In binding assays, [3H]S(-)-zacopride and [3H]R(+)-zacopride labelled homogenous populations of high-affinity binding sites in the rat entorhinal cortex, R(+)-zacopride compete for a further 10 to 20% of the binding of [3H]R(+)/S(-)-zacopride or [3H]R(+)-zacopride in excess of that competed for by (S)(-)-zacopride. It is concluded that both isomers of zacopride have potent but different pharmacological activities, with the possibility of different recognition sites to mediate their effects.

5-HT3 receptor antagonists attenuate dopamine-induced hyperactivity in the rat

The 5-HT3 receptor antagonists, ICS205-930, granisetron and zacopride, at low doses, inhibited the hyperactivity caused by a 13 day mesolimbic dopamine infusion in the rat. The antagonism decreased with the use of higher doses of the 5-HT3 receptor antagonists. The ability of low doses of the 5-HT3 receptor antagonists to inhibit dopamine-induced behavioural changes is similar to the inhibitory profile of known antipsychotic agents. It is suggested that 5-HT3 receptor antagonists may represent a new class of atypical antipsychotic agents.

Fluphenazine, ICS 205-930 and dl-fenfluramine differentially antagonise drug-induced emesis in the ferret

The intravenous injection of cisplatin (10 mg/kg), the subcutaneous injection of apomorphine (0.125-1 mg/kg) and lisuride (0.001-0.1 mg/kg), the oral administration of ipecacuanha (0.3-2.4 mg/kg) and the intragastric administration of copper sulphate (25-100 mg/kg), induced a vomiting and retching response in the ferret. Pretreatment with dl-fenfluramine (5 mg/kg i.p.) prevented or reduced the emesis induced by cisplatin, apomorphine, ipecacuanha and lisuride but failed to significantly antagonise copper sulphate-induced emesis. The 5-HT3 receptor antagonist ICS 205-930 (0.1 mg/kg i.p.) prevented emesis induced by cisplatin and ipecacuanha but failed to prevent or significantly reduce the emesis induced by apomorphine, lisuride or copper sulphate. Dopamine receptor antagonists, including fluphenazine (0.1-1.0 mg/kg i.p.) prevented apomorphine- and lisuride-induced emesis but were less potent or had inconsistent actions to antagonise cisplatin- or ipecacuanha-induced emesis and failed to inhibit the emesis induced by copper sulphate. The data indicate that dopamine and/or 5-HT3 receptor systems are involved in drug-induced emesis but that emesis caused by gastric irritation induced by copper sulphate is mediated by different receptor mechanisms.

Effects of captopril and SQ29,852 on anxiety-related behaviours in rodent and marmoset

The abilities of the ACE inhibitors captopril and SQ29,852 to modify aversive behaviour was compared to the effects of diazepam in the light/dark exploration test in the mouse, the elevated plus maze and social interaction test in the rat, and in anxiety-related behaviours induced by human threat in the marmoset. In the four tests the acute administration of captopril, SQ29,852 and diazepam had the same profiles of action to reduce aversive responding. This was also observed during chronic administration with the three agents in the mouse. However, withdrawal from a chronic treatment with diazepam precipitated a syndrome of increased aversion, whereas withdrawal from treatment with captopril and SQ29,852 was uneventful, values waning to control levels. Withdrawal from treatment with ethanol, nicotine and cocaine also enhanced aversive responding. Treatment with captopril and SQ29,852 antagonised the behavioural consequences of withdrawal from treatment with diazepam and nicotine and SQ29,852 also blocked the consequences of withdrawal from ethanol and cocaine. It is concluded that captopril and SQ29,852 have an anxiolytic profile of action in 3 species, that cessation of treatment is not associated with a withdrawal syndrome, that the ACE inhibitors cross tolerate with diazepam and can antagonise the behavioural consequences of withdrawal from treatment with drugs of abuse.

The effects of ondansetron, a 5-HT3 receptor antagonist, on cognition in rodents and primates

The selective 5-HT3 receptor antagonist, onansetron, has been assessed in three tests of cognition in the mouse, rat and marmoset. In a habituation test in the mouse, ondansetron facilitated performance in young adult and aged animals, and inhibited an impairment in habituation induced by scopolamine, electrolesions or ibotenic acid lesions of the nucleus basalis magnocellularis. Arecoline failed to improve basal performance in young adult mice but inhibited the impairment caused by scopolamine and lesions of the nucleus basalis magnocellularis. In the T-maze reinforced alternation task in rats, ondansetron and arecoline antagonised a scopolamine-induced impairment. In an object discrimination and reversal learning task in the marmoset, assessed using a Wisconsin General Test Apparatus, ondansetron improved performance in a reversal learning task. We conclude that ondansetron potently improves basal performance in rodent and primate tests of cognition and inhibits the impairments in performance caused by cholinergic deficits.

Actions of ORG 5222 as a novel psychotropic agent

ORG 5222 is a tetracyclic compound with high affinity for dopamine and 5-HT2 receptors. ORG 5222 was compared to fluphenazine in behavioural tests and was shown to be less potent to cause catalepsy on peripheral administration or to induce asymmetric body posturing following intrastriatal injection. On injection into the nucleus accumbens, ORG 5222 antagonised spontaneous and amphetamine-induced hyperactivity. The peripheral administration of ORG 5222 antagonised the hyperactivity induced by infusion of dopamine into the nucleus accumbens of rat or ventral striatum of the marmoset and, unlike the use of fluphenazine, there was no evidence of a 'rebound' hyperactivity after discontinuation of treatment. Furthermore, ORG 5222 prevented changes in responsiveness to dopamine agonist challenge following dopamine infusion. In a mouse black and white test box and the rat elevated plus maze ORG 5222 released exploratory behaviour suppressed by the aversive white or elevated environments. It is concluded that ORG 5222 is effective to antagonise mesolimbic dopamine function in the rodent and primate and an aversive behaviour in rodent tests. Such effects reveal a novel profile of action of ORG 5222 in behavioural paradigms predictive of antipsychotic and anxiolytic potential and may relate to a dopamine and 5-HT receptor antagonism.

Desenkephalin-gamma-endorphin is an antagonist of the hyperactivity response induced by infusion of dopamine into the nucleus accumbens of rat and ventral striatum of marmoset

The ability of desenkephalin-gamma-endorphin (DE gamma E; ORG5878) to antagonise a raised limbic dopamine function was investigated in the rat and common marmoset. Dopamine was infused for 13 days directly into the nucleus accumbens of the rat and ventral striatum of the marmoset and increased locomotor activity. Such increases in both the rat and marmoset were antagonised by the subcutaneous injection of DE gamma E, administered in a range 10-500 micrograms/kg (t.i.d.), during the 13 day period of infusion of dopamine. Treatment with dopamine alone or in combination with DE gamma E failed to influence the level of spontaneous locomotor activity after discontinuing treatment. In contrast, in experiments performed in the rat, the level of spontaneous locomotor activity was increased 2- to 3-fold after cessation of a regimen of infusion of dopamine and haloperidol. The increases in activity were antagonised by DE gamma E (50 and 100 micrograms/kg t.i.d., s.c., for 2 days). In additional experiments in the marmoset, using animals initially selected as "high activity" responders to challenge with (-)N-n-propylnorapomorphine, the infusion of dopamine caused a reversal in responsiveness to the stimulant effects of (-)N-n-propylnorapomorphine on locomotor activity some 2-4 weeks after discontinuing the infusion of dopamine. The administration of fluphenazine (0.01-2.5 mg/kg b.d.), during the infusion of dopamine, failed to prevent the subsequent change in responsiveness to (-)N-n-propylnorapomorphine, whereas a regimen of dopamine and DE gamma-E (25-100 micrograms/kg t.i.d.) prevented such changes.(ABSTRACT TRUNCATED AT 250 WORDS)

The histological effects of intracerebral injection or infusion of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP+ (1-methyl-4-phenylpyridinium) in rat and mouse

Bilateral infusion of MPTP into the substantia nigra, ventral tegmental area or putamen of the rat brain provoked accumulation of leucocytes, but the periphery of nigral lesions showed no significant nerve cell loss. Tyrosine hydroxylase labelling of dopaminergic perikarya showed a normal staining pattern. In contrast, MPP+ was more destructive. After one day there was local degeneration of neuronal and glial elements. After one month there was tissue necrosis and central cavitation, but, like MPTP, there was no evidence of selective nigral cell loss. Striatal injection of MPP+ also failed to produce retrograde nigral damage. It is concluded that the toxicity of MPP+ applied directly to the dopamine system is a consequence of a severe non-selective necrotic lesion.

Zacopride: anxiolytic profile in rodent and primate models of anxiety

Zacopride, a substituted benzamide derivative, was compared with diazepam in three models of experimental or provoked anxiety. The drug's action (i) in reducing aversion to a brightly lit environment was assessed in mice using a two compartment black and white test box system, (ii) in disinhibiting a suppressed behaviour was measured in the rat social interaction test under high light/unfamiliar conditions and (iii) in antagonizing a defensive response in the marmoset was assessed using the threat of a human presence. Both zacopride and diazepam enhanced exploratory behaviour and social interaction in the mouse and rat models and antagonized the defensive response in the marmoset, zacopride being 100 times more potent than diazepam. It is concluded that the 5-HT3 receptor antagonist, zacopride, alters rodent and primate behaviour in a manner consistent with that of an anxiolytic agent.

The potential anxiolytic activity of GR38032F, a 5-HT3-receptor antagonist

1. The highly selective 5-HT3-receptor antagonist, GR38032F, has been tested in five animal models predictive for anxiolytic activity. 2. In the social interaction test in the rat and in a light/dark exploration test in the mouse, GR38032F dose-dependently released suppressed behaviour without modifying locomotor activity. 3. In the cynomolgus monkey and the marmoset, GR38032F reduced anxiety-related symptoms without causing sedation. In the marmoset, the effects were clearly dose-related. 4. GR38032F did not have any detectable activity in the water-lick conflict test in the rat. 5. We conclude that GR38032F is potentially a very potent anxiolytic agent without sedative, anticonvulsant or hypnotic activity.

Effects of the 5-HT3 receptor antagonist, GR38032F, on raised dopaminergic activity in the mesolimbic system of the rat and marmoset brain

1 The ability of the selective 5-HT3 receptor antagonist GR38032F to reduce raised mesolimbic dopaminergic activity was studied in behavioural experiments in the rat and marmoset. 2 GR38032F injected into the nucleus accumbens (0.01-1 ng) or peripherally (0.01-1 mg kg-1 i.p.) inhibited the locomotor hyperactivity caused by the acute intra-accumbens injection of amphetamine (10 micrograms) in the rat. Similar treatments with sulpiride and fluphenazine also inhibited the amphetamine-induced hyperactivity. 3 The peripheral administration of GR38032F (0.001-0.1 mg kg-1 i.p., b.d.) during a 13 day period of dopamine infusion (25 micrograms 24 h-1) into the nucleus accumbens of the rat reduced the dopamine-induced hyperactivity response to control (vehicle infused) levels. Locomotor activity remained at control levels after discontinuing the dopamine/GR38032F treatment regimen. 4 The hyperactivity caused by the infusion of dopamine into the rat nucleus accumbens was also inhibited by fluphenazine (0.01-0.05 mg kg-1 i.p., b.d.), but locomotor activity was suppressed to levels below control values and a rebound hyperactivity occurred after discontinuation of the dopamine/fluphenazine treatment regimen. 5 The discontinuation of a concomitant 13 day intra-accumbens infusion of dopamine with haloperidol, 0.01 mg kg-1 i.p.t.d.s., caused a rebound hyperactivity. This hyperactivity was suppressed by GR38032F (0.001-0.1 mg kg-1 i.p.). 6 The unilateral infusion of dopamine (25 micrograms 24 h-1, 13 days) into the left amygdala of rats having right hemispheric dominance (as measured in a turn preference test) caused locomotor hyperactivity. Intraperitoneal administration of GR38032F (0.1-100 micrograms kg-1) or fluphenazine (0.025-0.1 mg kg-1), and the intra-amygdaloid injection of GR38032F (0.1-100 ng) or fluphenazine (25-500 pg), either into the infused or non-infused side, inhibited the dopamine-induced locomotor hyperactivity. 7 Marmosets receiving bilaterial infusions of dopamine (25 micrograms 24 h-1 for 13 days) into the nucleus accumbens also exhibited increased locomotor activity, GR38032F (0.1-1.0 micrograms kg-1 t.d.s.), reduced the hyperactivity to control levels with no rebound hyperactivity following the discontinuation of the dopamine/GR38032F treatment regimen. Fluphenazine (0.01-2.5 mg kg-1 i.p., t.d.s.) also inhibited the hyperactivity, but locomotor activity was reduced to values below control levels and a rebound hyperactivity followed the discontinuation of the dopamine/fluphenazine treatment. 8. It is concluded that the 5-HT3 receptor antagonist GR38032F, and the neuroleptic agents fluphenazine, sulpiride and haloperidol, can reduce raised mesolimbic dopaminergic activity in the rat and marmoset. GR38032F is distinguished from the dopamine receptor antagonists by, firstly, its ability to return the hyperactivity response to control values, without excessive suppression of locomotion even on enhanced dosage regimes and, secondly, by the lack of rebound hyperactivity following abrupt discontinuation of its treatment.

Emesis induced by cisplatin in the ferret as a model for the detection of anti-emetic drugs

The intravenous injection of cisplatin in the ferret caused a consistent emetic (vomiting/retching) response. Emesis induced by cisplatin was abolished by the 5-hydroxytryptamine (5-HT) M-receptor antagonists ICS205-930, zacopride, dazopride and metoclopramide. The neuroleptic agents haloperidol, fluphenazine, domperidone, sulpiride and tiapride also antagonized emesis induced by cisplatin but only a proportion of the animals were completely protected and diazepam and prednisolone only reduced the intensity of the response. It is concluded that compounds used in the clinic to antagonise emesis induced by chemotherapy are effective in the ferret model. Antagonism of emesis induced by cisplatin in the ferret was most potently achieved by the use of the 5-HT M-receptor antagonists ICS205-930 and zacopride. However, an antagonism of dopamine receptors would appear relevant to the anti-emetic effects of the neuroleptic agents and may contribute to the anti-emetic effects of metoclopramide. Diazepam and prednisolone exert their modest antagonism by unknown mechanisms. The use of the 5-HT M-receptor antagonists is revealed as a novel approach to the treatment of emesis induced by cisplatin.

Behavioural consequences of the infusion of dopamine into the nucleus accumbens of the common marmoset (Callithrix jacchus)

Marmosets, shown to have comparable levels of spontaneous locomotor activity, assessed in cages equipped with infra-red photocell units, could be separated into "high", "moderate" and "low activity" responders on the basis of their locomotor hyperactivity response to peripherally administered (-)N-n-propylnorapomorphine [(-)NPA]. Animals selected as "low" and "high activity" responders to (-)NPA were subjected to chronic infusion of dopamine, or its solvent, bilaterally into the nucleus accumbens for 13 days through Alzet osmotic minipumps. Both "low" and "high activity" responders exhibited an increased locomotor activity which peaked on days 6-7 of the infusion. This hyperactivity, caused by infusion of dopamine was antagonised by small doses of sulpiride and fluphenazine. After the infusion, the level of spontaneous locomotor activity of the marmosets was unchanged from pre-infusion values. However, 2-3 weeks after discontinuing the infusion, the animals initially classified as "low activity" responders showed markedly enhanced activity when challenged with (-)NPA, and conversely, animals initially classified as "high activity" responders showed a reduced responsiveness to (-)NPA. It is concluded that the consequences of a persistent increase in the activity of dopamine in the nucleus accumbens of the brain of the marmoset are to (a) enhance locomotor activity during infusion and (b) after discontinuing infusion, to modify the locomotor responsiveness to challenge with a dopamine agonist, with the direction of the change dependent on the initial basal locomotor responsiveness to (-)NPA.

The action of dazopride to enhance gastric emptying and block emesis

The substituted benzamide derivatives, dazopride and metoclopramide, enhanced field stimulation-induced contractions of guinea-pig stomach strips and gastric emptying in the guinea-pig after peripheral, intracerebroventricular and intrahypothalamic injection. In the isolated vagal nerve preparation from the rabbit, both compounds were shown to be 5-hydroxytryptamine M-receptor antagonists. Dazopride and metoclopramide were equipotent in antagonising cisplatin-induced emesis in the ferret, whereas metoclopramide was approximately 200 times more potent than dazopride in antagonising the emesis caused by the dopamine agonist 2-di-n-propylamino-5,6-dihydroxytetralin in the marmoset. In behavioural tests which indicate dopamine receptor antagonism in the rat, metoclopramide induced catalepsy, antagonised amphetamine-induced stereotypy and the hyperactivity induced by the intrastriatal injection of dopamine, caused body asymmetry on unilateral injection into the striatum and also antagonised apomorphine-induced climbing and circling behaviour in the mouse. In contrast, dazopride had little or no action in these tests and failed to displace [3H]spiperone in radioligand binding assays. The use of dazopride provides evidence to dissociate a dopamine receptor blockade from an ability to facilitate gastric emptying and to antagonise cisplatin-emesis, and indicates that antagonism of 5-hydroxytryptamine M-receptors is the essential basis of action for dazopride and plays an important role in the actions of metoclopramide.

Ketotifen can antagonise changes in sensitivity of cerebral dopamine receptors: behavioural correlates in rodent and primate

Rats preselected as "low responders" to the hyperactivity-inducing action of (-)N-n-propylnorapomorphine [(-)NPA] responded to an infusion of dopamine into the nucleus accumbens (25 micrograms/24 hr for 13 days) with hyperactivity during the infusion and long-term increased sensitivity to (-)NPA administered after the infusion. Both effects were antagonised by ketotifen (intraperitoneal infusion, 1 mg/kg/day), administered during the period of infusion of dopamine. Animals subject to infusion of dopamine also showed enhanced hyperactivity to L-DOPA (plus benserazide): this enhanced response was also antagonised by ketotifen, given acutely as a single dose (1 mg/kg i.p.). When haloperidol was given concurrently with the infusion of dopamine, spontaneous locomotor activity was markedly increased after the infusion for a period of at least 7 weeks: this long-term change was antagonised by ketotifen (1 mg/kg/24 hr), given during the period of treatment with dopamine/haloperidol or by a single acute injection of ketotifen (0.1-1.0 mg/kg i.p.) on an established response. Ketotifen, in doses up to 20 mg/kg (i.p.) did not reduce spontaneous locomotor activity, cause catalepsy or antagonise amphetamine-induced stereotypy in normal rats. In the marmoset, an enhanced sensitivity to the locomotor-stimulant effects of L-DOPA was induced 5 to 8 weeks after a treatment for 4 days with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which led to the development of akinesia and severe depletion of dopamine in the striatum. Treatment with a single dose of ketotifen (1 mg/kg i.p.) 60 min before L-DOPA antagonised the enhanced locomotor responsiveness to the L-DOPA/benserazide regimen.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine M-receptor antagonism to prevent cisplatin-induced emesis

The administration to the ferret of cisplatin, 10mg/kg (i.v.), caused an intense emetic response that was prevented by ICS 205-930 (0.1 and 1.0 mg/kg i.v.) and metoclopramide (4.0 mg/kg i.v.). Smaller doses of ICS 205-930 (0.01 mg/kg i.v.) and metoclopramide (2.0 mg/kg i.v.) attenuated the emetic response to cisplatin. It is concluded that the potent action of ICS 205-930 against cisplatin-induced emesis is the consequence of a 5-hydroxytryptamine M-receptor antagonism which may also contribute to the antiemetic action of metoclopramide.

The neurotoxic actions of 6-hydroxydopamine infused into the rat substantia nigra

6-Hydroxydopamine (6-OHDA) 10 and 40 micrograms/24 h infused bilaterally for 4 days into the rat substantia nigra (SN) caused a 'freezing' akinetic response which was apparent within 24 h and which persisted throughout the period of infusion. The infusion of 10 or 40 micrograms/24 h 6-OHDA into the SN failed to cause any change in motor coordination, or induce limb and body rigidity. The infusion of 40 micrograms/24 h 6-OHDA led to significant reductions in the striatal levels of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid and noradrenaline; no changes were observed following the 4-day infusion of 10 micrograms 6-OHDA. The consequences of infusing 6-OHDA into the SN are discussed in relation to those induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) where the spectrum of behavioural and biochemical change caused by 6-OHDA is shown not to mimic the actions of either MPTP or MPP+.

Biochemical changes caused by the infusion into the substantia nigra of the rat of MPTP and related compounds which antagonise dihydropteridine reductase

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium bromide (MPP+), 1-methyl-4-(3', 4'-dihydroxyphenyl)pyridinium bromide, 4-(3',4'-dihydroxyphenyl)pyridine, 4-phenyl-1,2,3,6-tetrahydropyridine and 4-(3',4'-dimethoxyphenyl)1,2,3,6-tetrahydropyridine were infused bilaterally into the substantia nigra of the rat (10 micrograms/24 hr for 4 days). The ability to inhibit spontaneous locomotor activity and to reduce levels of neurotransmitters and metabolites in the nigrostriatal system (striatum, substantia nigra) was compared with activity to inhibit dihydropteridine reductase (DHPR) in vitro. The compound MPP+ was most effective to reduce motor responding and to decrease levels of dopamine, DOPAC and HVA (50-56%) in the striatum in addition to reducing levels of dopamine, DOPAC, noradrenaline, serotonin and 5-HIAA (42-86%) in the substantia nigra, yet MPP+ has been shown to have very weak ability to inhibit DHPR. In contrast, 4-(3',4'-dihydroxyphenyl)pyridine and 1-methyl-4-(3',4'-dihydroxyphenyl)pyridinium bromide were in the order of 10(4) and 2 X 10(5) times, respectively, more potent than MPP+ to inhibit DHPR in vitro, but these compounds failed to modify dopamine neuronal function when assessed in vivo. Therefore, there would not appear to be any correlation between the ability to modify dopamine neuronal function, as assessed behaviourally or biochemically, and ability to inhibit DHPR in synaptosomes from the striatum of the rat in vitro.

1-methyl-4-phenylpyridine is neurotoxic to the nigrostriatal dopamine pathway

Systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is neurotoxic to cerebral dopaminergic neurones in several animal species, and can cause parkinsonism in man. The mechanism of this action may be indirect. MPTP is oxidized in the brain to a pyridinium species, 1-methyl-4-phenylpyridine (MPP+)6. This oxidation is greatly decreased by inhibition of monoamine oxidase B6, as are the biochemical effects of MPTP in the mouse and its neurotoxicity in the monkey. We now show that MPP+ exerts a powerful neurotoxic action on the nigrostriatal dopamine system of the rodent.

Lithium and bupropion antagonise the phasic changes in locomotor activity caused by dopamine infused into the rat nucleus accumbens

Dopamine infused persistently (25 micrograms/24 h for 13 days) into the nucleus accumbens of rat brain caused phasic increases in spontaneous locomotor activity during the period of infusion. This phasic responding was prevented by lithium administered throughout the infusion period in divided doses (3 X daily administrations of 2.5 mg/kg IP) or as a continuous IP infusion (7.5 mg/kg/24 h), and by bupropion treatment (5-20 mg/kg 3 X daily). In contrast, imipramine, amitriptyline and nomifensine failed to prevent the phasic locomotor response to dopamine at doses which did not by themselves cause marked motor changes. Locomotor activity was measured using individual photocell cages, and rats preselected to (-)NPA were those initially showing a modest locomotor activity. Fourteen to twenty-eight days after discontinuing the dopamine infusion rats showed increased responsiveness to (-)NPA which persisted throughout the remainder of the 70-day withdrawal period. This long-term change was prevented when lithium was given continuously throughout the period of dopamine infusion, but not when lithium was given in divided doses, showing the importance of the mode of drug delivery. The long-term change caused by the dopamine infusion could also be prevented by bupropion but not by imipramine, amitriptyline or nomifensine to show again that the actions of classical antidepressant drugs may be differentiated from those of lithium and bupropion. Therefore, it is suggested that the model of phasic hyperactivity described may provide a means for more closely analysing, both behaviourally and biochemically, the site and mechanism of action of lithium (and bupropion) in the control of the short- and long-term consequences of an enhanced mesolimbic dopamine activity.

Laterality of dopamine function and neuroleptic action in the amygdala in the rat

Rats selected according to turn preference in an open field were categorised as showing left or right hemispheric dominance (turning to right or left respectively). Dopamine was persistently infused into the central area of the left or right amygdala of animals showing left or right hemispheric dominance. Infusion was effected for 9 days by Alzet osmotic minipumps, implanted subcutaneously, which delivered 25 micrograms/24 hr of dopamine via chronically-indwelling, stereotaxically-located injection units. Dopamine caused marked and consistent hyperactivity only when infused into the left amygdala when the right hemisphere was dominant. This hyperactivity developed during the first day of infusion and persisted throughout the 9-day infusion period. Both (-)sulpiride and fluphenazine, injected unilaterally into the amygdala, antagonised the hyperactivity caused by infusion of dopamine into the left amygdala in animals with right hemispheric dominance. This antagonism could be effected both from the infused amygdala [5-50 pg (-)sulpiride, 25-100 pg fluphenazine] and from the contralateral amygdala [100-250 pg (-)sulpiride, 25-100 pg fluphenazine]. Thus, a laterality was shown for the action of dopamine in the amygdala of the rat. In contrast, neuroleptic agents failed to show an exclusive laterality of action, but were able to act in either hemisphere to antagonise the effects of dopamine injected into the left amygdala. Interhemispheric biochemical differences "within-animals" could not be shown, although differences were seen between rats having right or left hemispheric dominance.(ABSTRACT TRUNCATED AT 250 WORDS)

The toxic actions of MPTP and its metabolite MPP+ are not mimicked by analogues of MPTP lacking an N-methyl moiety

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), its metabolite 1-methyl-4-phenylpyridine (MPP+) and three analogues of MPTP, lacking an N-methyl moiety, namely, 4-phenylpiperidine (I), 4-phenyl-1,2,3,6-tetrahydropyridine (II) and 4-phenylpyridine (III), were infused continuously for a period of 4 days into the rat substantia nigra. Within 12 h of commencing the bilateral infusion of MPTP or MPP+, rats showed marked motor deficits with reduction in locomotor activity, loss of ability to move the forelimbs and grip with forepaws and, following MPP+ infusions, similar loss of movement in the hindlimbs associated with the development of limb and body rigidity. These motor deficits were not induced by the 3 analogues of MPTP on infusion into the substantia nigra. After 4 days of infusion, the motor deficits caused by MPTP and, in particular, MPP+, were still marked, and for MPP+ these correlated with marked loss of striatal dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid. 4-Phenyl-1,2,3,6-tetrahydropyridine caused a small loss in striatal DA and DOPAC, but the other analogues failed to modify the striatal content of DA or its metabolites. Small alterations of chemical structures related to MPTP and its metabolite can critically alter ability to induce behavioural and neurochemical changes reflecting toxicity on the nigrostriatal DA system.

The continuity of dopamine receptor antagonism can dictate the long-term behavioural consequences of a mesolimbic infusion of dopamine

An infusion of dopamine for 13 days into the nucleus accumbens of rat caused biphasic peaks of hyperactivity responding during infusion and an enhanced locomotor responsiveness to the dopamine agonist (-)N-n-propylnorapomorphine [(-)NPA] after the infusion when rats where initially preselected as low activity responders to (-)NPA. Antagonism of the response to dopamine during the infusion by sulpiride, given every 8 hr (three daily doses provided 30 mg/kg, i.p., daily), could both facilitate spontaneous locomotor activity after the infusion, and potentiate the consequence of enhanced hyperactivity responding to (-)NPA, for at least 11 weeks. In contrast, when sulpiride was administered in a daily dose of 30 mg/kg but by continuous intraperitoneal infusion, not only were the events during the infusion prevented, without subsequent change in spontaneous locomotion after the infusion, but also the long-term consequences for responding to (-)NPA were prevented and the rats remained at their preselected low activity response levels. The repeated treatment with (-)NPA or the repeated/continuous treatment with sulpiride alone were not responsible for the changes observed. It is concluded that the consequence of intervention with sulpiride during a period of infusion of dopamine into the mesolimbic region depends on the degree and/or continuity of antagonism of dopamine receptors such that fluctuating antagonism (daily injections) can exacerbate, whilst a continuous and constant receptor antagonism (effected by infusion), can prevent the long-term consequences of increased sensitivity to challenge with a dopamine agonist.

Long-term consequences of antagonism by neuroleptics of behavioural events occurring during mesolimbic dopamine infusion

Rats selected as low-activity responders to peripherally administered (-)N-n-propylnorapomorphine [(-)NPA] were subjected to a 13-day continuous infusion of dopamine bilaterally into the nucleus accumbens (0.48 microliter/hr, 25 micrograms in 24 hr). This caused biphasic increases in spontaneous levels of hyperactivity with peaks occurring between days 2-5 and 8-12 of infusion. Two weeks after the infusion was withdrawn the low-activity status of the animals, to (-)NPA challenge, converted to high-activity and remained at this changed level for 18 to 20 weeks. The effects of sulpiride (2.5 mg/kg, i.p. daily) or haloperidol (0.025 mg/kg, i.p. daily) on the behavioural changes during infusion and after its withdrawal were assessed by administration on days 1-4 to inhibit the first peak of the enhanced spontaneous locomotion, on days 8-11 to inhibit the second peak, on days 1-4 and 8-11 to inhibit both peaks, on days 6-9 to influence the "trough" of behavioural responding between the peaks of hyperactivity, and on days 1-11 to modify all components of hyperactivity responding to infusion of dopamine. The usual consequence of infusion, the conversion of low-activity to high-activity responders to (-)NPA, was not prevented by any treatment with neuroleptic. It is concluded that the long-term consequences (up to 1 year) of a brief period (13 days) of overactivity induced by mesolimbic infusion of dopamine cannot be prevented by daily treatment with haloperidol or sulpiride in doses adequate to prevent the behavioural expression of the effects of the dopamine stimulation during its infusion.

Locomotor hyperactivity caused by dopamine infusion into the nucleus accumbens of rat brain: specificity of action

Rats selected as high-activity and low-activity responders to the hyperactivity-inducing action of peripherally administered (-)N-n-propylnorapomorphine [(-)NPA] were subject to intra-accumbens infusion of dopamine, noradrenaline, serotonin, acetylcholine and GABA (0.48 microliters/h, 25 micrograms/24 h, 13 days). Locomotor activity was measured during infusion and for a minimum of 35 days thereafter. After discontinuation of infusion the animals' responsiveness to (-)NPA was also assessed and, on the 2nd day of withdrawal, sensitivity to the hyperactivity-inducing action of acute intra-accumbens dopamine was determined. Dopamine caused a biphasic pattern of hyperactivity during infusion with peaks of responding between days 2-5 and 8-12: normal values returned after withdrawal of infusion. However, 2-3 weeks after withdrawal of intra-accumbens dopamine infusion animals showed reversed responding to (-)NPA challenge, the initial low-active animals giving a high-active response and high-active animals giving low-activity. Infusions of noradrenaline, serotonin, GABA and acetylcholine produced some increase in locomotor activity towards the termination of infusion, but no treatment could replicate the first hyperactivity peak and no treatment, after withdrawal, could reverse the responsiveness to (-)NPA of high- and low-active animals. Acute injections of dopamine into the nucleus accumbens showed that the infusion of the different neurotransmitter substances caused change within that nucleus. Nevertheless, changes in locomotor behaviour following the infusion of dopamine into the nucleus accumbens are specific for dopamine.

A comparison of the behavioural consequences of chronic stimulation of dopamine receptors in the nucleus accumbens of rat brain effected by a continuous infusion or by single daily injections

Mesolimbic dopamine (DA) receptors of the rat were subject to a chronic (13 day) stimulation effected either by single, repeated daily injections or by infusion from osmotic minipumps of 1.56-50 micrograms DA bilaterally into the nucleus accumbens. Rats were preselected according to their responsiveness to the hyperactivity inducing effect of the DA agonist (--)N-n-propylnorapomorphine [(--)NPA], and the two groups categorised as 'high' and 'low' activity responders were used in the present studies. Whilst the stimulation of mesolimbic DA receptors by a single acute injection or by a 24 h infusion was ineffective to modify locomotor responding, the repetition of the daily injections or the continuation of the infusion caused dose-related increases in locomotor activity of both groups of rats. The spectrum of enhanced activity responding depended on the mechanism of stimulation, a 'biphasic' or 'monophasic' pattern resulting from the infusion or daily injections respectively. It was an important observation that a repetitive biting behaviour developed concomitant to the hyperactivity during daily DA injections, a response not observed during infusion, indicating that the nature of the receptor stimulation, continuous or pulsatile, can dictate the development of a motor response. The effects of discontinuing the chronic intra-accumbens stimulation on responsiveness to (--)NPA was the same whether the chronic stimulation was achieved by repeated injection or infusion. However, the consequences of DA withdrawal critically depended on the initial classification of rat activity according to (--)NPA. Thus, initially 'high' response animals developed a 'low' sensitivity whilst initially 'low' sensitive animals reversed to 'high' responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)