Mesolimbic dopamine neurons: effects of 6-hydroxydopamine-induced destruction and receptor blockade on drug-induced rotation of rats

The Amphetamine Induced Rotation Test: A Re-Assessment of Its Use as a Tool to Monitor Motor Impairment and Functional Recovery in Rodent Models of Parkinson's Disease

Rats and mice with unilateral damage to the nigrostriatal dopamine system—induced by neurotoxins, such as 6-hydroxydopamine, overexpression of α-synuclein, or injections of toxic synuclein protofibrils—are widely used as experimental models to mimic the loss of dopamine neurons seen in Parkinson’s disease. The amphetamine rotation test is commonly used to monitor the extent of motor impairment induced by the lesion, and this test has also become the standard tool to demonstrate transplant-induced functional recovery or the efficacy of neuroprotective interventions aimed to preserve or restore DA neuron function. Although the amphetamine-induced rotation test is highly useful for this purpose it has some important pitfalls and the interpretation of the data may not always be straightforward. Unless the test is applied properly and the data are displayed and interpreted appropriately the conclusions may be misleading or simply totally wrong. The purpose of this review is to draw attention to the potential problems and pitfalls involved in the use of drug-induced rotation tests, and to provide recommendations and advice on how to avoid them.

Unilateral nigrostriatal 6-hydroxydopamine lesions in mice I: motor impairments identify extent of dopamine depletion at three different lesion sites

The unilateral 6-hydroxydopamine mouse lesion models of Parkinson's disease have received increasing attention in recent years, but comparison of the different lesion models was largely focused at a histological level. An extensive behavioural comparison between different mouse models on tests of motor function has yet to be carried out, to pin point tests that accurately discriminate between different extents of dopaminergic depletion. In the present study we examine the consequences of injection of the toxin at three sites along the nigrostriatal tract (substantia nigra, medial forebrain bundle, and striatum) on a broad range of simple motor tasks, and on the dopaminergic pathology. All lesion groups demonstrated marked behavioural deficits and displayed distinct profiles of degeneration along the nigrostriatal dopamine pathway. Tests that correlated closely with the level of substantia nigra cell loss included the corridor, cylinder and balance beam tests, the rotarod, inverted cage lid and three types of rotational assessment (spontaneous, amphetamine-induced and apomorphine-induced). Specific tasks are identified which are capable of distinguishing a near-complete lesion, with amphetamine rotation, corridor and cylinder tests showing the highest correlations with levels of nigral cell loss. Performance in the different behavioural tests was associated with distinct profiles of cell loss in the SN and VTA. We provide a comprehensive behavioural assessment of lesion-induced deficits in mouse models of PD, which should facilitate selection of the most appropriate lesion model and most sensitive behavioural tests for use in future studies investigating therapeutic interventions.

Lateralization and gender differences in the dopaminergic response to unpredictable reward in the human ventral striatum

Electrophysiological studies have shown that mesostriatal dopamine (DA) neurons increase activity in response to unpredicted rewards. With respect to other functions of the mesostriatal dopaminergic system, dopamine's actions show prominent laterality effects. Whether changes in DA transmission elicited by rewards also are lateralized, however, has not been investigated. Using [¹¹C]raclopride-PET to assess the striatal DA response to unpredictable monetary rewards, we hypothesized that such rewards would induce an asymmetric reduction in [¹¹C]raclopride binding in the ventral striatum, reflecting lateralization of endogenous dopamine release. In 24 healthy volunteers, differences in the regional D₂/₃ receptor binding potential (ΔBP) between an unpredictable reward condition and a sensorimotor control condition were measured using the bolus-plus-constant-infusion [¹¹C]raclopride method. During the reward condition subjects randomly received monetary awards while performing a 'slot-machine' task. The ΔBP between conditions was assessed in striatal regions-of-interest and compared between left and right sides. We found a significant condition × lateralization interaction in the ventral striatum. A significant reduction in binding potential (BP(ND) ) in the reward condition vs. the control condition was found only in the right ventral striatum, and the ΔBP was greater in the right than the left ventral striatum. Unexpectedly, these laterality effects appeared to be partly accounted for by gender differences, as our data showed a significant bilateral BP(ND) reduction in women while in men the reduction reached significance only in the right ventral striatum. These data suggest that DA release in response to unpredictable reward is lateralized in the human ventral striatum, particularly in males.

Voluntary running distance is negatively correlated with striatal dopamine release in untrained rats

This study examined the relationship between voluntary running distance and glutamate- and K+-stimulated dopamine release in the striatum (nucleus accumbens and caudate-putamen) of male Long-Evans rats. Twenty-one rats were housed individually in cages with attached running-wheels for 1 week. There was a 19-fold variability between rats in voluntary running distances over this period (range = 2.3-44.6 km). The average distance completed during the week was 16 +/- 2.8 km. There was a strong positive correlation between the running distances completed during the first 24 h (day 1) and the last 24 h. Certain rats were therefore inclined to run from the start. The average daily running distance (2.4 +/- 0.4 km per day) was negatively correlated with the weight of the rat (r = -0.82). Glutamate-stimulated release of dopamine was not a significant predictor of voluntary running distance. However, the average daily running distance was negatively correlated with K+-stimulated dopamine release in the nucleus accumbens core and caudate-putamen but not the nucleus accumbens shell. The present findings suggest that decreased depolarization-induced release of striatal dopamine may be a predictor of hyperactivity. The results show, in a normal population of Long-Evans rats, that there are, at the end of the continuum, rats that display some of the neurochemical and behavioral characteristics of a rat model for attention-deficit hyperactivity disorder.

Drug-induced rotation intensity in unilateral dopamine-depleted rats is not correlated with end point or qualitative measures of forelimb or hindlimb motor performance

The pharmacological induction of rotational (circling) behavior is widely used to assess the effects of lesions to the dopaminergic system and the success of treatment strategies in rat models of Parkinson's disease. While the number of rotations under apomorphine, L-DOPA and amphetamine is related to the extent of dopamine depletion after unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine system, the relationship of the intensity of rotational behavior to the degree of impairment in motor behavior is unclear. The present study examined this question by correlating rotational behavior and motor abilities in a rat analogue for Parkinson's disease produced by unilateral nigrostriatal bundle lesion with 6-hydroxydopamine. Ipsiversive and contraversive rotation was measured in the rats following systemic administration of low and high doses of apomorphine, the dopamine precursor L-DOPA, and amphetamine. The motor assessment included end point and qualitative measures of fore- and hindlimbs assessed in a skilled reaching task and a skilled ladder rung walking task. The intensity of drug-induced rotation did not correlate with the measures of motor performance. We conclude that independence of rotational behavior and motor performance argues that both the assessment of 6-hydroxydopamine behavioral deficits and potential treatments for the functional deficits require comprehensive assessment, including both measures of rotation and motor behavior.

Parkinsonian motor deficits are reflected by proportional A9/A10 dopamine neuron degeneration in the rat

In a model of Parkinson's disease (PD), amphetamine, a dopamine (DA)-releasing drug, fails to induce ipsilateral drug rotations in a proportion of rats with complete unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle and DA neurons of the substantia nigra. To investigate this phenomenon, individual 6-OHDA lesions (measured by tyrosine hydroxylase immunohistochemistry) in the substantia nigra pars compacta (A9), ventral tegmental area (A10), and striatum were examined in conjunction with outcomes of four behavioral tests. The behavioral tests were skilled paw reaching, a head-turning test, and apomorphine (0.05 mg/kg) and amphetamine (4 mg/kg) drug-induced rotations. Four weeks postlesion, ipsilateral side bias measured by the head-turning test correlated strongly with extent of A9 DA neuronal lesion. Additional A10 neuronal DA lesions did not substantially improve the model fit, indicating that the head-turning bias was primarily A9 dependent. In contrast, total head-turning activity increased monotonically with lesions of A10 striatal DA fibers. Skilled paw-reaching accuracy decreased with increased lesion of both A9 and A10 DA neuronal systems. Associating amphetamine-induced rotations with extent of A9 DA lesion generated a second-order polynomial model, y = -11.1x + 0.20 x(2) + 208.7 (R(2) = 0.73), with an overall F ratio (df = 2,21) of 28.4 (P < 0.0001). This model predicts that an A9 DA lesion of about 50% is required to induce an ipsilateral turning bias, after which rotations increase with the degree of A9 DA neuronal lesion. No further change in rotational behavior was seen until an additional A10 DA lesion reached 60%, after which the rotational response decreased. This analysis provides tests that differentiate between A9 DA degeneration and combined A9/A10 lesions in animal models and in addition allows predictive testing of PD therapeutic intervention at a preclinical level.

The nucleus accumbens motor-limbic interface of the spontaneously hypertensive rat as studied in vitro by the superfusion slice technique

The behavioral disturbances of attention-deficit hyperactivity disorder (ADHD) have been attributed to dysfunction of the mesolimbic dopaminergic (DA) projection from the ventral tegmental area of the midbrain. DA released from terminals in the nucleus accumbens (interface between limbic and motor areas of the brain) draws attention to unexpected, behaviorally significant events and provides the motivational drive for reward-related behavior. An in vitro superfusion technique was used to show that depolarization (25 mM K+)-induced release of DA from nucleus accumbens slices of spontaneously hypertensive rats (SHR, animal model for ADHD) was significantly lower than that of Wistar-Kyoto controls (WKY). Evidence also suggested that DA autoreceptor efficacy was increased at low endogenous agonist concentrations. D2 receptor blockade by the antagonist, sulpiride, caused a significantly greater increase in the electrically stimulated release of DA from nucleus accumbens slices of SHR compared to WKY. This suggested that presynaptic regulation of DA release had been altered in SHR to cause down-regulation of the DA system. This could have occurred at an early stage of development in an attempt to compensate for abnormally high DA concentrations. The reduction in DA transmission could have left the adult SHR with impaired DA reward/reinforcement mechanisms, resulting in the behavioral disturbances characteristic of ADHD.

Role of the nucleus accumbens and the striatum in the production of turning behaviour in intact rats

Recent knowledge of the mechanisms underlying turning or circling behaviour in intact rats is reviewed. Most interest has been directed towards the striatum because of the classical hypothesis that turning behaviour results from lateral differences in the activity of the bilateral nigrostriatal pathway. However, the assumption that asymmetrical activation of the striatum is a necessary condition for dopamine-dependent turning behaviour has been questioned by several studies showing that unilateral injection of amphetamine or dopamine receptor agonists into the nucleus accumbens, a target of the mesolimbic dopaminergic system, also produces reliable circling away from the side of injection. Apart from discussing differences in stepping patterns of turning and discussing the role of the dopamine D1/D2 receptor interaction, the present survey focuses attention upon the two-component hypothesis, especially in relation to our recent studies in which activities of dopamine D1 and D2 receptors in the striatum and the nucleus accumbens have been manipulated separately in intact rats. It is hypothesized that turning behaviour is produced by asymmetry within nucleus accumbens circuits which involve neuronal connections from the nucleus accumbens to the A9 cell area, which in turn projects to the ventrolateral striatum that determines the direction of turning.

Amphetamine-induced rotation reveals post 6-OHDA lesion neurochemical reorganization

Early postlesion amphetamine-induced contralateral rotation has been linked to intraneuronal dopamine (DA) accumulation and transmitter release associated with axonal degeneration. Animals in the present study sustained severe unilateral depletion of striatal DA with or without a near-complete loss of ipsilateral mesolimbic DA. Contralateral rotation to 1.0 mg/kg (+)-amphetamine was observed on days 1 and 4 postlesion, and was greatly enhanced in mesolimbic-lesioned animals on day 4. On days 7 and 14, very little contralateral turning was observed and there was an emergence of low-rate ipsilateral rotation. These changes in direction and magnitude of rotational response suggest neurochemical adaptations which continue beyond initial periods of intraneuronal accumulation and degeneration-induced release.

The influence of dopaminergic A10 neurons on the motor pattern evoked by substantia nigra (pars compacta) stimulation

The influence of the mesolimbic-mesocortical dopaminergic (DA) system on the motor pattern evoked by substantia nigra pars compacta (SNpc) stimulation was studied. Electrical stimulation of the A10 group of neurons caused an inhibitory effect preferentially directed towards the orientation movement. Sulpiride administration at low dosages (50 mg/kg i.p.) did not modify this movement at the basal condition, but abolished the increase of its duration induced by ventral tegmental area (VTA) co-stimulation. Mesolimbic activation opposes the effects of SNpc stimulation, restraining the animal in its antero-posterior axis by means of orientation movement inhibition. The results suggest a role of the DA mesolimbic-mesocortical system in the maintenance of focused attention.

Comparison of the effect of prolactin on dopamine release from the rat dorsal and ventral striatum and from the mediobasal hypothalamus superfused in vitro

In this paper, by means of a superfusion technique, we have examined the effect of ovine prolactin on dopaminergic neurons innervating the dorsal striatum, ventral striatum and the mediobasal hypothalamus of male rats. Fragments from dorsal striatum were superfused with ovine prolactin dissolved in normal medium (Krebs-Ringer phosphate) or medium containing tetrodotoxin (TTX, 10(-6) M). Ovine prolactin stimulated the in vitro release of dopamine from dorsal and ventral striatal fragments. In dorsal striatal fragments a linear dose-dependent dopamine release was observed only when fragments were superfused with Krebs-Ringer phosphate-TTX medium. In addition, [Leu5]enkephalin (10(-6) and 10(-5) M) decreased the prolactin-induced in vitro dopamine release from dorsal striatal fragments superfused with Krebs-Ringer phosphate-TTX medium. Ovine prolactin (10(-9)-10(-5) M) did not elicit changes in dopamine, 3,4-dihydroxyphenylacetic acid or 5-hydroxyindoleacetic acid outputs from mediobasal hypothalamic fragments superfused with Krebs-Ringer phosphate medium containing TTX. The possible regulatory mechanisms of ovine prolactin on dopaminergic neurons are discussed.

Synergistic interaction between dopamine D-1 and D-2 receptor agonists: circling behaviour of rats with hemitransection

Circling behaviour induced by dopamine (DA) agonists with different D-1/D-2 receptor selectivity was studied in rats with hemitransection at a level caudal to the striatum. The mixed D-1/D-2 agonist apomorphine induced ipsilateral circling behaviour after administration of doses similar to those that induced stereotyped behaviour in unlesioned rats. The effect of apomorphine was not influenced by co-treatment with SK & F 38393 or quinpirole, indicating that apomorphine induces a comparable D-1 and D-2 receptor stimulation in vivo also. Three selective D-1 agonists, SK & F 38393, SK & F 75670 and Lu 24-040 had no effects alone, while the preferential D-2 agonists quinpirole, pergolide and (-)-N-propylnorapomorphine induced ipsilateral circling of weaker intensity than did apomorphine. After co-treatment with SK & F 38393 the effects of these compounds were markedly increased. Combination of SK & F 38393, SK & F 75670 or Lu 24-040 with quinpirole induced circling with intensities similar to those seen after apomorphine. Pretreatment with the D-1 antagonist SCH 23390 or the D-2 antagonist YM 09151-2 completely antagonized the ipsilateral circling induced by either apomorphine or quinpirole + SK & F 38393. A range of partial (autoreceptor) D-2 agonists, i.e. (-)-3-PPP, (+)-3-phenethyl-PP, terguride, EMD 23448 and B-HT 920 were all ineffective as was the alpha 2-adrenoceptor agonist clonidine. However, B-HT 920 induced strong ipsilateral circling after combination with SK & F 38393, whereas (-)-3-PPP was ineffective.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of brain catecholamines in the exhibition of muricide induced by nucleus accumbens lesions and the effect of antidepressants in rats

Changes in brain catecholamine content after lesioning the nucleus accumbens (ACC) and the effects of antidepressants were investigated using HPLC-ECD. ACC lesion reduced dopamine (DA) in the rostral caudate-putamen (r-CP), lateral hypothalamus (LH) and central amygdala (ACE). Imipramine (IMP) and nomifensine (NOM) increased DA in r-CP, caudal (c)-CP and basolateral amygdala. Mianserin (MIAN) and zimelidine (ZIM) increased DA only in c-CP. ACC lesion did not change DOPAC. Only IMP (in c-CP) and NOM (in r-CP and c-CP) increased DOPAC. Noradrenaline (NA) was decreased in c-CP and ACE after ACC lesion. IMP and ZIM displayed no effect on NA, while NOM increased NA in LH and frontal cortex (FC) and MIAN only in FC. These results suggest an important role for DA but not NA in the exhibition of muricide after ACC lesion, and in the antimuricide effect of antidepressants.

The dopaminergic innervation of the ventral striatum in the rat: a light- and electron-microscopical study with antibodies against dopamine

Dopamine (DA) in the dorsal and ventral striatum is associated with different aspects of locomotor activity control. The ventral striatum may form an interface between the limbic system and the extrapyramidal motor system. The distribution of dopaminergic fibers in this interface position was studied in detail with a method applying antibodies against DA. Furthermore, the ultrastructural morphology of the DA fibers was examined by means of immuno-electron microscopy. The results show that DA immunoreactivity is distributed over the ventral striatum in a highly compartmentalized fashion. In the dorsal striatum few compartments were found. The DA fibers in the ventral striatum establish mainly symmetric synaptic contacts, preferably with dendritic shafts and spines. The results are discussed in relation to previous data concerning the light and electron-microscopic identification of catecholaminergic fibers in the ventral and dorsal striatum.

Dopaminergic and serotonergic correlates of stimulation-induced circling

Rotation induced by electrical stimulation of the medial forebrain bundle at the level of the lateral hypothalamus was associated with increases in dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum ipsilateral to the site of stimulation (i.e. contralateral to direction of turning). The concentrations of DA, DOPAC and HVA within the nucleus accumbens (NAS) were not altered. In the olfactory tubercle (OT), concentrations of DA and both metabolites were, in general, elevated ipsilateral to the electrode. However, relative to non-stimulated controls, HVA concentrations were increased bilaterally in rats exhibiting circling. Stimulation-induced circling also resulted in a bilateral enhancement of striatal serotonin (5-HT) metabolism as indicated by elevated 5-hydroxyindoleacetic acid: 5-HT ratios. No changes in 5-HT metabolism were observed in the NAS. The utilization of 5-HT was elevated in the OT ipsilateral to the electrode in rats that exhibited stimulation-induced rotation. While most subjects that exhibited contraversive rotation in response to the stimulation demonstrated enhanced DA activity, the neurochemical changes were not observed in all subjects. As such, it is concluded that while stimulation of the mesotelencephalic DA system can be associated with stimulation-induced rotation it is not necessary for its elicitation.

Effects of estrogen on the basal ganglia

Recent research suggests that estrogen regulates the activity of dopamine-containing fibers originating in the midbrain and terminating in the basal ganglia, and/or dopamine-sensitive cells in the basal ganglia. The mechanism by which estrogen acts is not clear, since cells in neither of these regions concentrate estrogens. Nevertheless, estrogens clearly affect behaviors mediated by the basal ganglia, as illustrated in human patients suffering from extrapyramidal disorders. Both biochemical and behavioral research in animals has confirmed that estrogen modulates basal ganglia function, but there has not been agreement concerning either the locus, the direction, or the mechanism of its action. These topics are the focus of this review. The effects of estrogen on behaviors mediated by DA in the basal ganglia depend on the dose of estrogen administered, the time interval between estrogen treatment and testing, the behavior measured, and the part of the basal ganglia from which the behavior is elicited. A high dose of estrogen results in an initial suppression and later enhancement of DA-related behaviors elicited from the striatum. However, no later enhancement of these behaviors occurs if a low dose of estrogen is given. Even after low doses of estrogen, the latency to behavioral suppression varies depending upon the behavior measured. These varying latencies suggest that more than one mechanism is involved in the effects of estrogen on basal ganglia output. In addition, estrogen may also act on some regions in the mesolimbic DA system. While estrogen may act indirectly via the catechol estrogens and prolactin, it has been demonstrated that estrogen can act directly on the striatum. These findings are related to the effects of estrogen on human extrapyramidal disorders.

Role of mesencephalic glycine in locomotor activity

Microinjection of glycine (20 micrograms) into the ventral mesencephalon of rats caused a stimulation of locomotor activity. Microinjection of haloperidol (5 micrograms) into the nucleus accumbens immediately before the glycine injection did not reduce this locomotor stimulation. In rats with 6-hydroxydopamine-induced destruction of mesolimbic dopamine (DA) terminals the ability of apomorphine (0.1 mg/kg, s.c.) to stimulate locomotor activity was reduced by microinjection of the glycine antagonist strychnine into the ventral mesencephalon. Increased glycinergic activity in the ventral mesencephalon therefore appears to stimulate locomotor activity by a mechanism other than the activation of mesolimbic DA neurons.

Rotational behaviour elicited by intracerebral injections of apomorphine and pergolide in 6-hydroxy-dopamine-lesioned rats. II: The striatum of the rat is heterogeneously organized for rotational behaviour

The present study compares the role of the nucleus accumbens and the striatum, as well as various regions of the striatum, in the ability of intracerebral injections of the dopamine agonists apomorphine and pergolide to elicit rotational behaviour in 6-hydroxy-dopamine-lesioned rats. We found that apomorphine and pergolide elicit rotational behaviour when injected into the denervated striatum, but not when injected into the ipsilateral nucleus accumbens. The striatum seems heterogeneously organized as regards rotational behaviour since maximal-apomorphine rotation was elicited from the corpus of the striatum as compared to the effects produced by injections into the head and the tail of the striatum. This topographical distribution is similar to the distribution of dopamine-stimulated cyclic AMP. The pergolide response is more evenly distributed in the striatum. It is suggested that the difference in the topographical distribution of the ability of apomorphine and pergolide to elicit rotational behaviour reflects a regional distribution of dopamine receptors in the striatum of the rat.

Rotational behaviour elicited by intracerebral injections of apomorphine and pergolide in 6-hydroxy-dopamine-lesioned rats. I: Comparison between systemic and intrastriatal injections

Rotational behaviour in unilaterally 6-hydroxy-dopamine-denervated rats has been attributed to stimulation of dopamine receptors on striatal as well as limbic areas. In the present study the rotational behaviour elicited by local intrastriatal injections of apomorphine or pergolide was compared to the rotation elicited by systemic injections of the drugs. We found that intrastriatal injections induced rotational behaviour almost identical to the behaviour occurring after systemic treatment. Furthermore, studies of the spread of [3H]apomorphine in brain tissue showed that at the peak of rotation the radioactivity was confined within the limits of the striatum. Non-significant amounts of radioactivity was found in the nucleus accumbens. On the basis of these data we conclude that rotational behaviour elicited by systemic injections of apomorphine or pergolide originates from stimulation of striatal sites. The difference in rotational patterns elicited by these drugs is more likely to relate to differences in receptor stimulation within the striatum than differences in, for example, relative distribution between limbic and striatal areas.

Dopamine-dependent contralateral circling induced by neurotensin applied unilaterally to the ventral tegmental area in rats

Unilateral application of neurotensin (5, 10, and 15 micrograms) to the ventral tegmental area in rats caused contralateral circling. Pretreatment with pimozide (0.5 mg/kg) blocked the behavior. As is the case with circling elicited by unilateral ventral tegmental morphine, the diameter of the circles made by the animals was determined by environmental factors rather than by internal motor programming.

Effects of serotonergic activity in nucleus accumbens septi on drug-induced circling

The effects of injections of 5-hydroxytryptamine (5-HT) into the nucleus accumbens and lesions of the nucleus accumbens induced by 5,7-dihydroxytryptamine (5,7-DHT) on drug-induced circling were investigated in rats with unilateral nigrostriatal lesions induced by 6-hydroxydopamine (6-OHDA). Injections of 5-HT (60-120 micrograms in 1 microliter; 1 microliter/min) into the nucleus accumbens caused a significant decrease in the circling response to 5.0 mg/kg of d-amphetamine (s.c.). The distribution of radioactivity after intracerebral injections of [3H]5-HT using these parameters showed that although much of the injected material was retained in the nucleus accumbens there was also considerable spread to the frontal cortex. However, in further behavioural experiments, using an injection procedure (0.5 microliter; 0.11 microliter/min) which caused much greater retention of injected material in the nucleus accumbens, with minimal spread to the frontal cortex, the ability of 5-HT injected into the accumbens to block amphetamine-induced circling was not diminished. Moreover, injections of 5-HT into the frontal cortex did not have any effect on amphetamine-induced circling. Lesions of the nucleus accumbens induced by 5,7-DHT caused a significant enhancement of the contralateral circling response to 1.0 mg/kg of apomorphine and a similar but non-significant tendency to increase the circling responses to several other doses of apomorphine and amphetamine. The results provide evidence that serotonergic mechanisms in the nucleus accumbens inhibit circling behaviour generated by unilateral activation of nigrostriatal dopaminergic mechanisms.

Locomotor response of nialamide pretreated old rats to intraaccumbens dopamine

The objective of this study was to determine the locomotor activity response of young (6 month), mature (15 month), and old (26 month) rats to bilateral intraaccumbens injections of dopamine after pretreatment with nialamide. Young and mature rats responded to dopamine with high rates of activity, while old rats either did not respond at all or responded with a lower intensity of activity. In contrast, the response of old rats to dopamine or ergometrine alone or to dopamine after pargyline pretreatment was not less than that of mature and young rats. These results suggest that the attenuated response of old rats to dopamine after nialamide pretreatment is not due to a decrease in dopamine receptor activity, but appears to be due to some unique property of nialamide in these animals. However, the reduced response of old rats to dopamine was not due to the inability of nialamide to inhibit monoamine oxidase, since nialamide completely inhibited the activity of this enzyme in the nucleus accumbens of old rats.

Unilateral striatal dopamine denervation: reduced motor inhibitory effects of dopamine antagonists revealed in models of asymmetric and circling behaviour

Circling and asymmetric behaviours to apomorphine (dopamine agonist/antagonist) challenge were studied in rats with unilateral striatal electrolesions or 6-hydroxydopamine (6-OHDA) lesions, each induced by combined lesions at 3 striatal locations, to allow an assessment of drug action on 'normal' receptors in the intact striatum or 'supersensitive' receptors in the lesioned striatum respectively. The minimally effective dose of 6-OHDA (given in the presence of DMI and tranylcypromine) to cause functional change was 3 X 8 micrograms, with 3 X 32 micrograms providing maximal change. Electrolesions were shown histologically to be confined to striatal tissue, and dopamine depletions caused by 6-OHDA were selective for the striatum. Temporal differences were recorded for onset of asymmetry and circling behaviour, both between behaviours and between lesions. Thus, asymmetry developed during the 2nd-4th days after 6-OHDA lesion but circling developed more abruptly on postoperative days 10-12. In contrast, both asymmetry and circling behaviours were apparent from the first day following electrolesion. The dose-dependent effects of apomorphine were apparent at much lower doses in 6-OHDA lesioned than electrolesioned rats. This potency difference was also demonstrated for two further dopamine agonists, 2-di-n-propylamino-5,6-dihydroxytetralin and SK & F 38393. In contrast, the agonist-induced asymmetric and circling behaviours of electrolesioned rats were some 9-44 times more sensitive than those of 6-OHDA lesioned rats to antagonism by the neuroleptic agents haloperidol, alpha-flupenthixol and oxiperomide, although tiapride antagonism was very similar in both the electrolesioned and 6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of denervation and chronic haloperidol treatment on neostriatal dopamine receptor density are not additive in the rat

The effects of combined denervation and chronic haloperidol treatment on neostriatal D-2 receptor density were examined, and a lack of additivity was found. In the first experiment, haloperidol treatment for 14 days initiated on day 4 after the unilateral intracerebral injection of 6-hydroxydopamine (6-OHDA) abolished the lesion-induced asymmetry of [3H]spiroperidol binding by elevating the density of sites in the intact striatum. These results were replicated in the second experiment, in which the drug treatment was begun 4-5 weeks after 6-OHDA. In both experiments the apomorphine-induced rotation did not differ significantly between lesioned rats treated with haloperidol or its vehicle.

Neurochemical asymmetries in the albino rat's cortex, striatum, and nucleus accumbens

The concentrations of dopamine (DA), norepinephrine (NE), serotonin (5-HT), dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) were measured in the right and left cortex, striatum, and nucleus accumbens of adult Purdue-Wistar rats. There was more DA in the right cortex and accumbens and a greater concentration of NE in the left striatum. There is more 5-HT in the left striatum and right accumbens, more 5-HIAA in the left cortex, as well as a greater 5-HT turnover in the left accumbens. These results are considered in the light of previous findings concerning the relationship of neurochemical asymmetries and behavioral lateralization.

Nigral dopaminergic mechanisms in drug-induced circling

Unilateral injections of dopamine into the substantia nigra pars reticulata of pargyline-pretreated rats caused a prolonged, contralateral circling, similar in magnitude to that elicited by the injection of the same amount of dopamine intrastriatally. Contralateral circling was also elicited by the unilateral intranigral injection of amphetamine (after pargyline pretreatment), or by the dopamine agonists ergometrine and SKF 38393. In contrast, bilateral intranigral injection of the dopamine antagonist haloperidol greatly reduced the amphetamine-induced circling of rats with unilateral 6-hydroxydopamine-induced nigrostriatal lesions. These results support the hypothesis that dopaminergic mechanisms in the substantia nigra are involved in motor behavior.

Inhibition of drug-induced circling by GABA-ergic activity in the nucleus accumbens

Injections of GABA (125-500 micrograms in 1 microliter) or muscimol (40-200 ng in 1 microliter) into the nucleus accumbens markedly reduced the amphetamine-induced circling of rats with unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of nigrostriatal dopamine neurons. The potency of muscimol was approximately 10(4) times that of GABA. Muscimol exerted a similar effect when injected in a smaller volume (0.5 microliter) at a slower rate (0.11 microliter/min), a procedure which was shown to reduce the spread of injected [3H]muscimol. Intra-accumbens injections of subconvulsive doses of picrotoxin had no effect on amphetamine-induced circling. Intra-accumbens muscimol (40 ng, 0.5 microliter, 0.11 microliter/min) also reduced the contralateral circling evoked by apomorphine in rats with unilateral 6-OHDA-induced lesions of the nigrostriatal pathway and bilateral 6-OHDA-induced lesions of the mesolimbic dopamine terminals. These results suggest that GABA-ergic activity in the nucleus accumbens exerts an inhibitory influence on drug-induced circling. At least part of this action is at a step beyond the release of dopamine from mesolimbic terminals.

Circling from intracranial morphine applied to the ventral tegmental area in rats

Crystalline morphine applied unilaterally to the ventral tegmental area in rats caused circling away from the side of application. This circling was reversed by naloxone (3 mg/kg) and blocked by pretreatment with pimozide (0.5 mg/kg). When tested in an open field the animals followed the perimeter of the enclosure; thus the radius of the circles described was determined by environmental rather than central factors. Morphine induced forward locomotion in all four limbs; there were no major signs of postural asymmetry noted in the longitudinal axis of the animal's body. This study suggests that morphine activates a population of A-10 dopamine cells known to be involved in locomotion.

Behavioral activity of some novel aporphines in rats with 6-hydroxydopamine lesions of caudate or nucleus accumbens

The behavioral actions of some novel aporphines have been examined in rats with selective unilateral 6-hydroxydopamine (6OHDA)-induced destruction of nigrostriatal dopaminergic neurons, and in rats with bilateral 6OHDA-induced destruction of mesolimbic dopaminergic neurons. Dopaminomimetics such as apomorphine (APO) in these animal models elicit circling behavior and locomotor activity respectively. In animals with unilateral nigrostriatal lesions (-)-2,10,11-trihydroxy-N-n-propylnoraporphine (TNPA) and (-)-10,11-methylenedioxy-N-n-propylnoraporphine (MDO-NPA) elicited weak, but prolonged, contraversive circling, whereas (-)-2,10,11-trihydroxyaporphine (2-OH.APO) was inactive. In animals with bilateral destruction of mesolimbic dopaminergic neurons TNPA and MDO-NPA elicited a strong stimulation of locomotor activity, while 2-OH.APO was inactive. The results suggest that TNPA and MDO-NPA, but not 2-OH.APO, exert central dopaminomimetic effects in vivo. The results are also consistent with previous data indicating that N-propyl substitution of aporphines causes a relative enhancement of activity in animal models which emphasise effects at mesolimbic rather than striatal dopaminergic receptors.

Time course of certain behavioral changes after hippocampal damage and their alteration by dopaminergic intervention into nucleus accumbens

Independent groups of rats with hippocampal, neocortical, or sham lesions were observed 7, 14, or 28 days after surgery in an open field-hole board apparatus and in a smaller circular apparatus. In the circular apparatus, animals were observed after unilateral injection of the dopamine agonist, 3,4-dihydroxyphenylamino-2-imidazoline (DPI) or saline into nucleus accumbens. Behavioral changes in locomotion, exploration and grooming measured in the open field were consistent with those found previously after hippocampal damage, with each behavioral anomaly demonstrating a specific pattern of change after surgery. In general, the injection of DPI into nucleus accumbens produced greater behavioral change in animals with hippocampal damage than in animals with either neocortical or sham lesions. The drug-induced changes in the hippocampally lesioned rats made their behavior more like that of control animals. These results suggest that destruction of the hippocampus may induce alterations in dopaminergic mechanisms in nucleus accumbens which can be modified by appropriate pharmacologic intervention.

The effects of prolyl-leucyl-glycine amide on drug-induced rotation in lesioned rats

1. The effects of L-prolyl-L-leucyl-glycine amide (PLG) on drug-induced rotational behavior were studied in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra. 2. Although PLG alone was found to be ineffective at inducing rotation in these animals, pretreatment with PLG at various doses was found to enhance both amphetamine and apomorphine-induced rotation. 3. This response proved to be biphasic with loss of activity in the higher dose ranges. 4. The pattern of effects observed suggests that PLG may act postsynaptically at a non-striatal site.

Effects of electrolytic and 6-hydroxydopamine lesions of the lateral hypothalamus on rotation evoked by electrical stimulation of the substantia nigra in rats

Contralateral rotation evoked by electrical stimulation of the left substantia nigra was studied in rats before and after electrolytic or 6-hydroxydopamine (6-OHDA) lesions of the lateral hypothalamus. Electrolytic lesions (2 mA DC, 15 sec) which produced mean ipsilateral striatal dopamine depletion of 58% significantly reduced the rotation at 2 h to 14 days postlesion. 6-OHDA (8 microgram in 4 microliter) which produced mean ipsilateral striatal dopamine depletion of 93% significantly increased the rotation at 3 to 14 days postlesion. Haloperidol 0.1 and 0.5 mg/kg i.p. partially reduced rotation in both control and lesioned rats in a dose-related manner. Control and lesioned rats showed no significant differences in haloperidol sensitivity. If stimulus induced rotation were mediated by activation of dopaminergic neurons, one would have expected lesion effects in the present experiments to parallel those on rotation caused by pharmacologically evoked release of dopamine. The lesions effects we obtained on stimulus induced rotation, however, parallel those on rotation evoked by the predominantly directly acting dopamine agonist, apomorphine, rather than those on rotation evoked by the indirect (presynaptic) action of amphetamine. We suggest that contralateral rotation evoked by electrical stimulation of the substantia nigra may reflect direct activation of neurons postsynaptic to the dopaminergic nigrostriatal neurons.

Morphine dependence and dopaminergic activity: tests of circling responses in rats with unilateral nigral lesions

1 Rats with unilateral electrolytic lesions involving both parts of the substantia nigra show dose-related, ipsilateral circling responses to apomorphine which are stable over time. 2 In non-tolerant rats, morphine (up to 10 mg/kg) does not elicit any circling behaviour but as tolerance develops to morphine, initially 10 mg/kg daily and then 100 ng/kg daily for about 4 months, the rats show a progressive tendency to walk more towards the side of the lesion. This behaviour is qualitatively different from apomorphine-induced circling. 3 When apomorphine (0 to 1.0 mg/kg) and morphine (10 or 100 mg/kg) are tested together, the total amounts of 'circling' are increased in an additive manner. However, after 22 h withdrawal from morphine there is a more marked increase in apomorphine-induced circling which is related to the level of dependence. 4 It is suggested that the sensitivity of striatal dopamine receptors is not altered by morphine dependence and that the increased response to apomorphine in abstinence probably reflects changes in the modulating actions of other neurotransmitter systems in the striatum.

Dopaminergic and non-dopaminergic neurons in substantia nigra: differential response to bromocriptine

Bromocriptine reduces the spontaneous firing rate of neurons in the pars compacta of the substantia nigra but does not change the electrical activity of the neurons located in the pars reticulata. On the other hand, bromocriptine induces contralateral circling behaviour in rats with unilateral 6-hydroxydopamine nigral lesion. This increased motor activity follows an initial period of hypomotility. The decrease of the neuronal firing rate in the pars compacta of the substantia nigra coincides with the hypomotility observed in the lesioned rats.

On the ability of N-chloroethyl aporphine derivatives to cause irreversible inhibition of dopamine receptor mechanisms

The potential dopamine inhibitory properties of (-)N-(2-chloroethyl)-norapomorphine [(-)NCA], (-)N-(hydroxyethyl)norapomorphine [(-)NHA], (-)N-(2-chloroethyl)-norapocodeine[(-)NCC] and 6-[2-bis-(2-chIoroethyl)-amino]acetyl-11-acetoxy-2-hydroxy-10-methoxynoraporphine (I) were assessed in behavioural (ability to antagonize apomorphine climbing, stereotypy and circling after unilateral electrolesions of the striatum in the mouse, ability to initiate circling/asymmetry alone or after challenge with apomorphine when injected unilaterally into the striatum of rat) and biochemical (ability to inhibit the binding of [3H] (-)N-n-propylnorapomorphine, 3H-NPA, to rat striatal homogenates) tests. (-) NCA, 10–20 mg kg−1 s.c., antagonized apomorphine climbing for a period of 5 days, the response recovering to control values by the 7th day. 10 mg kg−1 s.c. (-)NHA, (-)NCC or I failed to modify apomorphine climbing. Similarly, 2–4 mg kg−1 s.c. (-)NCA caused a long-lasting inhibition of apomorphine circling in the mouse (up to 5 days) whilst (-)NHA, (-)NCC and I were inactive. (-)NCA (10–40 μg) (but not (-)NHA, (-)NCC or I) also caused ipsilateral circling/asymmetry when injected unilaterally into the striatum of rat: this effect was enhanced by apomorphine. However, all agents, including (-)NCA, failed to consistently modify apomorphine stereotypy in the mouse. Non-labelled (-)NPA, (-)NCA and (-)NHA were shown to inhibit the ‘specific’ binding of 3H-NPA to rat striatal homogenates; (-)NCC and I were ineffective. A single washing removed the (-)NHA inhibition whilst repeated washing caused only a modest reversal of the inhibition afforded by (-)NCA. It is concluded that N-chloroethylation in the aporphine series can abolish dopamine agonist action and confer a long-lasting dopamine antagonist potential.

"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

The ability of clozapine and haloperidol to antagonize the depression of firing rate produced by d-amphetamine and apomorphine in the neostriatum and nucleus accumbens was tested in immobilized, locally anesthetized rats. In the neostriatum, an intraperitoneal injection of 2.5 mg/kg d-amphetamine or 1.0 mg/kg apomorphine produced a prolonged inhibition of neuronal activity that was reversed by a subjsequent injection of either 20 mg/kg clozapine or 2.0 mg/kg haloperidol. An analysis of the onset and magnitude of the blockade revealed that clozapine was more effective than haloperidol in reversing the amphetamine response but that both antipsychotic drugs produced a comparable blockade of the apomorphine-induced depression. Similar results were obtained in the nucleus accumbens. The data indicate that although clozapine acts equieffectively in the neostriatum and nucleus accumbens, this atypical antipsychotic drug, aside from blocking postsynaptic dopamine receptors, may exert at least some of its effects by preventing dopamine release.

GABAergic and glycinergic mechanisms within the substantia nigra: pharmacological specificity of dopamine-independent contralateral turning behavior and interactions with other neurotransmitters

The pharmacological specificity of the GABA agonist muscimol-induced contralateral turning behavior after unilateral injection into substantia nigra pars reticulata (SNR) has been studied. Muscimol-induced turning was antagonized by intranigral bicuculline methochloride (BMC) and picrotoxin, whereas antagonists of glycine, morphine, dopamine, noradrenaline, and serotonin were ineffective. Glycine induced a qualitatively similar turning behavior which was strychnine-sensitive but relatively BMC and picrotoxin-insensitive. Other drugs, including substance P, kainic acid, clonidine, oxymetazoline, serotonin, and carbachol, induced turning that could be dissociated from the effect of muscimol. Muscimol-induced turning was dopamine-independent, indicated by resistance to haloperidol (1 mg/kg), to pretreatment with reserpine (7.5 mg/kg) plus alpha-methyl-p-tyrosine (200 mg/kg), to haloperidol injections into the SNR, striatum and nucleus accumbens, and finally to kainic acid lesions of the striatum. 6-Hydroxydopamine lesions increased the efficacy of intranigral muscimol, while kainic acid lesions of the SNR antagonized muscimol. Muscimol-induced turning was inhibited by oxotremorine (0.25 mg/kg), by intranigral carbachol, and by apomorphine (0.1--0.5 mg/kg), but only moderately by intranigrally injected apomorphine. These data suggest specificity of GABA-agonist-induced contralateral turning and indicate an interaction between nigral GABA and other neurotransmitters, particularly dopamine and acetylcholine.