Appearance of EMG activity and motor asymmetry after unilateral lesions of the dopaminergic innervation to the subthalamic nucleus in the rat

Involvement of dopamine loss in extrastriatal basal ganglia nuclei in the pathophysiology of Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the manifestation of motor symptoms, such as akinesia, muscle rigidity and tremor at rest. These symptoms are classically attributed to the degeneration of dopamine neurons in the pars compacta of substantia nigra (SNc), which results in a marked dopamine depletion in the striatum. It is well established that dopamine neurons in the SNc innervate not only the striatum, which is the main target, but also other basal ganglia nuclei including the two segments of globus pallidus and the subthalamic nucleus (STN). The role of dopamine and its depletion in the striatum is well known, however, the role of dopamine depletion in the pallidal complex and the STN in the genesis of their abnormal neuronal activity and in parkinsonian motor deficits is still not clearly determined. Based on recent experimental data from animal models of Parkinson's disease in rodents and non-human primates and also from parkinsonian patients, this review summarizes current knowledge on the role of dopamine in the modulation of basal ganglia neuronal activity and also the role of dopamine depletion in these nuclei in the pathophysiology of Parkinson's disease.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

Alterations in dendritic morphology of the prefrontal cortical and striatum neurons in the unilateral 6-OHDA-rat model of Parkinson's disease

We have studied the morphological changes of the dendrites of the pyramidal neurons of the prefrontal cortex (PFC) and the medium spiny neurons of the caudate-putamen (CPu) and nucleus accumbens (NAcc) induced by the injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNc). The unilateral 6-OHDA-induced lesion of the SNc was made in Wistar rats to produce the Parkinson model lesion. Two weeks after the injection, the testing of rotational behavior caused by amphetamine injection was done to assess the animals with lesions. Four weeks after the 6-OHDA injection, the morphology of the pyramidal cells of Layer 5 of the PFC and the medium spiny neurons of the CPu and NAcc were quantified by modified Golgi-Cox staining. The results showed that the length of dendrites, the branching, and the density of dendritic spines on the medium spiny neurons of the same side of the caudate-putamen lesion were significantly decreased in rats with the unilateral 6-OHDA-induced lesion of the SNc. The pyramidal neurons of the PFC and medium spiny neurons of the NAcc showed a decrease in the density of dendritic spines without significant changes in dendritic length or arborization. Our data suggest that the SNc lesion with the 6-OHDA, Hemiparkinsonism animal model may lead to altered neuronal plasticity in the CPu, NAcc, and PFC that may have participated in the emergence of the behavioral changes observed in these animals.

Electrophysiological alterations in subthalamic neurons after unilateral dopamine depletion in the rat

The present study examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) on electrophysiological properties of subthalamic neurons (STN) in adult rats. Most neurons displayed regular spontaneous tonic firing patterns in both control and lesioned animals; however, the percentage of neurons with spontaneous burst firing at hyperpolarized membrane potentials was increased significantly in lesioned animals compared with controls (45% vs. 14% respectively). In the presence of bicuculline, a gamma-aminobutyric acid type A (GABAA) receptor antagonist, electrical stimulation of the internal capsule produced monosynaptic excitatory postsynaptic potentials (EPSPs) in almost all recorded neurons. DA (50 microM) increased the amplitude and/or duration of the EPSPs in neurons from both groups, whereas the DA D1 receptor agonist SKF 81297 (10 microM) produced a significant increase in amplitude and/or duration of EPSPs in neurons from the lesioned group only. This latter increase was blocked by pretreatment with the DA D1 antagonist SCH 23390 (10 microM). These data suggest that unilateral degeneration of DA neurons in the SNc changes firing properties and enhances electrophysiological responsiveness of STN neurons to activation of DA D1 receptors.

Neonatal ventral hippocampus lesion alters the dopamine content in the limbic regions in postpubertal rats

The neonatal ventral Hippocampus (nVH) lesion in rats has been used as a model to test the hypothesis that early neurodevelopmental abnormalities lead to behavioral changes putatively linked to schizophrenia. The schizophrenic patients tend to social isolation. In addition, considerable evidence from behavioral and neurochemistry studies strongly implicate the dopamine (DA) system and the medial part of the prefrontal cortex (mPFC) in the pathophysiology of the social isolation syndrome. In order to assess effects of the postweaning social isolation (pwSI) on the DA system of the nVH lesions, we investigated the DA content and its metabolite, DOPAC in different limbic subregions in rats postpubertally at postnatal day (P) 78 following nVH lesions at P7 with and without pwSI for 8 weeks. The DA and DOPAC were measured by HPLC with electrochemical detection. The nVH lesion induces increase in the DA content in the hippocampus with no effect in the mPFC, nucleus accumbens and caudate-putamen, while the pwSI induces major increase in the DA content in limbic subregions such as the mPFC, nucleus accumbens and hipocampus with opposite effect in the caudate-putamen. These results suggest that while pwSI has an effect in the postpubertal content of DA in both sham and nVH lesions in rats, the nVH-lesioned rats appear to be affected to a greater extent than the sham animals underscoring the influence of pwSI differences in the development of behaviors in the nVH-lesioned animals.

Contribution of the serotonin 5-HT1A receptor agonism of 8-OH-DPAT and EMD 128130 to the regulation of haloperidol-induced muscle rigidity in rats

The aim of the present study was to find out whether (+/-)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a prototypical 5-HT1A agonist, and (R)-(-)-2-[5-(4-fluorophenyl)-3-pyridylmethylaminomethyl]-chromane HCl (EMD 128130), a compound with serotonin 5-HT1A-agonist and dopamine D2-like antagonist properties, are able to attenuate the haloperidol-induced (1 mg/kg) muscle rigidity in rats. Muscle tone was examined using a combined mechano- and electromyographic (EMG) method that simultaneously measured the mechanical muscle resistance (MMG) of the rat's hind foot to passive movements in the ankle joint, and the EMG activity of two antagonist muscles. Both 8-OH-DPAT (0.125-0.5 mg/kg i.p.) and EMD 128130 (1-10 mg/kg i.p.) dose-dependently decreased the haloperidol-enhanced MMG to passive movements, as well as the tonic and the long-latency reflex EMG activities. Provided these results can be extrapolated to humans, the efficacy of EMD 128130 in relieving the haloperidol-induced muscle rigidity supports the concept that novel antipsychotics with 5-HT1A agonist and dopamine D2 antagonist activities should have a favourable extrapyramidal side-effect profile.

Excitation by dopamine of rat subthalamic nucleus neurones in vitro-a direct action with unconventional pharmacology

Recent anatomical and physiological studies have pointed to a functional innervation of the subthalamic nucleus by dopamine. This nucleus has a pivotal role in basal ganglia function and voluntary movement control and the possibility that dopamine, and dopaminergic medication used in Parkinson's disease, might directly influence its activity is of considerable interest. We have evaluated electrophysiologically the action and pharmacology of dopamine on single subthalamic neurones in rat brain slices. Dopamine increased firing rate to up to a mean of 60% in 98% of the 261 neurones tested when examined using extracellular single-unit recording. This excitation was unaffected by the GABA antagonist picrotoxin, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted in a low Ca(2+)/raised Mg(2+) solution, indicative of a direct action, independent of synaptic transmission. Of the 33 cells examined using whole patch-clamp recording, only 13 showed measurable increases in firing rate and/or depolarisations in response to dopamine. Dopamine-responsive cells displayed significantly greater access resistance, suggesting that an unidentified cytoplamic constituent, removed by whole-cell dialysis, was required for the response. Using extracellular recording, the D2-like dopamine receptor agonists quinpirole and bromocryptine, but not the D1-like receptor agonist 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, also consistently caused an excitation. This was mimicked by the catecholamine releaser amphetamine in 60% of cells tested. However, the dopamine excitation was not significantly reduced either by the D1-like receptor antagonist 7-chloro8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine or the D2-like receptor antagonists (-)-sulpiride, eticlopride and (+)-butaclamol, and the quinpirole excitation was also unaffected by (-)-sulpiride. In contrast, (-)-sulpiride, eticlopride and (+)-butaclamol all abolished the D2-like receptor-mediated inhibition by dopamine of substantia nigra pars compacta neurones. The alpha-adrenoceptor antagonist phentolamine was a weak antagonist of dopamine excitations, but not of those caused by quinpirole. Dopamine excitations also showed weak sensitivity to the 5-HT(2) antagonist ritanserin, but were unaffected by the alpha(1)-adrenoceptor antagonist prazocin and the beta-adrenoceptor antagonist propranolol. The pharmacology of this dopamine excitation is inconsistent with an action on any known catecholamine receptor. However, the effect of amphetamine indicates that an unidentified monamine--possibly dopamine--can be released within the subthalamic nucleus to cause an excitation. The anomalies of its pharmacological characterisation do not strongly support a physiologically relevant direct action of dopamine in the rat subthalamic nucleus.

Pharmacological identification of inward current evoked by dopamine in rat subthalamic neurons in vitro

Dopaminergic mechanisms in the subthalamic nucleus (STN) are implicated in the pathophysiology of Parkinson's disease. Here, electrophysiological responses of STN neurons to dopamine (DA) were investigated by using whole-cell patch-clamp recordings in the rat brain slice preparation. Under current-clamp, DA depolarized membrane potential and increased the frequency of spontaneous action potentials of STN neurons. Under voltage-clamp, DA (3-300 microM) produced a reversible concentration-dependent inward current (I(DA); 6-40 pA) with an EC(50) of 13 microM. This DA-induced current had a negative slope conductance which reversed at -102 mV. It was partially reduced by barium and by superfusion with an elevated concentration of extracellular K(+). Moreover, TTX and glutamate receptor antagonists (CNQX and AP5) did not significantly affect the DA responses, indicating that I(DA) is not dependent upon afferent synaptic activity in the STN. Quinpirole, a D(2) receptor agonist, mimicked the DA action more effectively than did the D(1) agonist SKF-38393. The D(2) antagonist sulpiride, but not the D(1) antagonist SCH-23390, blocked responses induced by DA. Intracellular application of G-protein inhibitor GDP-beta-S also suppressed I(DA). GTP-gamma-S, added to the pipette solution, evoked a sustained inward shift in the absence of DA. These results suggest that DA increases the activity of STN neurons via activation of G-protein-coupled D(2)-like receptors which reduce a K(+) conductance.

Dopamine receptors in the subthalamic nucleus are involved in the regulation of muscle tone in the rat

The effects of injecting an irreversible dopamine receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) bilaterally into the subthalamic nucleus (STN) on changes in muscle tone and behaviour were investigated. Increased muscle tone (i.e. rigidity) was assessed quantitatively as increases in tonic electromyographic (EMG) activity in the hindlimb muscles of conscious, unrestrained rats. Significant, sustained increases in EMG activity were observed for 3-5 h post-injection in the tibialis and gastrocnemius muscles following injection of EEDQ into the STN. This effect was associated with 73% dopamine D1 and 66% dopamine D2 receptor occupancy in the STN, while dopamine receptors in surrounding regions were minimally occupied. These results provide new evidence that dopamine receptors located in the STN play an important role in the regulation of normal muscle tone.

Compactotomy in Huntington's chorea

Advances in neuroradiological and neurosurgical techniques have lead to a growing interest in functional neurosurgical interventions for medically intractable movement disorders. The majority of these procedures are performed in patients with hypokinetic movement disorders, especially Parkinson's disease. However, relatively few interventions were done in hyperkinetic disorders such as Huntington's disease (HD), mainly owing to the lack of an adequate target nucleus. We have recently described the case of a reversible chorea in a genetically confirmed HD patient. We subsequently identified a marked bilateral degeneration of the substantia nigra as the probable reason for choreatic cessation. We therefore suggest that primary striatal atrophy causing hyperkinesia and secondary substantia nigra atrophy favouring hypokinesia were balanced in this patient, thus resulting in a close-to-physiologic GABAergic basal ganglia output. We postulate that deep brain stimulation of the substantia nigra pars compacta may ameliorate hyperkinesia in choreatic movement disorders, thus representing the first effective therapy in Huntington's chorea. Several lines of evidence in recent neurophysiological research support our hypothesis and are discussed below.

Intranigral transplantation of solid tissue ventral mesencephalon or striatal grafts induces behavioral recovery in 6-OHDA-lesioned rats

Parkinson's disease (PD) is characterized by a degeneration of the dopamine (DA) pathway from the substantia nigra (SN) to the basal forebrain. Prior studies in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats have primarily concentrated on the implantation of fetal ventral mesencephalon (VM) into the striatum in attempts to restore DA function in the target. We implanted solid blocks of fetal VM or fetal striatal tissue into the SN to investigate whether intra-nigral grafts would restore motor function in unilaterally 6-OHDA-lesioned rats. Intra-nigral fetal striatal and VM grafts elicited a significant and long-lasting reduction in apomorphine-induced rotational behavior. Lesioned animals with ectopic grafts or sham surgery as well as animals that received intra-nigral grafts of fetal cerebellar cortex showed no recovery of motor symmetry. Subsequent immunohistochemical studies demonstrated that VM grafts, but not cerebellar grafted tissue expressed tyrosine hydroxylase (TH)-positive cell bodies and were associated with the innervation by TH-positive fibers into the lesioned SN as well as adjacent brain areas. Striatal grafts were also associated with the expression of TH-positive cell bodies and fibers extending into the lesioned SN and an induction of TH-immunolabeling in endogenous SN cell bodies. This finding suggests that trophic influences of transplanted fetal striatal tissue can stimulate the re-expression of dopaminergic phenotype in SN neurons following a 6-OHDA lesion. Our data support the hypothesis that a dopaminergic re-innervation of the SN and surrounding tissue by a single solid tissue graft is sufficient to improve motor asymmetry in unilateral 6-OHDA-lesioned rats.

Increased behavioral response to dopaminergic stimulation of the subthalamic nucleus after nigrostriatal lesions

Local infusions of the nonselective dopaminergic agonist apomorphine into the subthalamic nucleus of rats has been shown to elicit orofacial dyskinesia which can be blocked by D1 but not D2 receptor antagonists. In the present study, we show that the selective D1 agonist A77636 also induces orofacial dyskinesia when injected into the subthalamic nucleus of awake rats, thus confirming a role for D1 receptors in this effect. We also examined the dyskinesia induced by intrasubthalamic injections of apomorphine in rats with an ipsilateral lesion of the nigrostriatal pathway. The orofacial response to local administration of apomorphine (1.0 microg) into the subthalamic nucleus was markedly increased in the lesioned rats. As in control rats, the enhanced behavioral response seen in lesioned rats was blocked by peripheral administration of D1 antagonists. Although D1 receptor binding autoradiography revealed no difference in D1 receptor binding in the subthalamic nucleus on the side of the lesion compared to controls, D1 binding was higher in the subthalamic nucleus on the side of the lesion compared to the contralateral side. The increased behavioral response observed after unilateral dopamine denervation suggests that the subthalamic nucleus is tonically regulated by dopaminergic projections from the substantia nigra. Furthermore, the data suggest that subthalamic D1 receptors may be involved in the development of dyskinesia induced by dopaminergic drugs.

Simultaneous intrastriatal and intranigral grafting (double grafts) in the rat model of Parkinson's disease

Experimental and clinical studies of neural transplantation in Parkinson's disease have focused on the placement of fetal dopaminergic grafts not in their ontogenic site (substantia nigra) but in the main nigral target area (striatum). The reason for this is the apparent inability of intranigral nigral grafts to extend axons for long distances reinnervating the ipsilateral striatum. This review presents previous work by our laboratory [I. Mendez, M. Hong, Reconstruction of the striato-nigro-striatal circuitry by simultaneous double dopaminergic grafts: a tracer study using fluorogold and horseradish peroxidase, Brain Res. 778 (1997) 194-205; I. Mendez, D. Sadi, M. Hong., Reconstruction of the nigrostriatal pathway by simultaneous intrastriatal and intranigral dopaminergic transplants, J. Neurosci. 16 (1996) 7216-7227] using a new transplantation strategy aimed at restoring dopaminergic innervation of the nigra and striatum by simultaneous dopaminergic transplants placed in the substantia nigra and ipsilateral striatum (double grafts) in the 6-hydroxydopamine lesioned adult rat brain. These double grafts achieve not only greater striatal reinnervation than the standard intrastriatal grafts but also produce a faster and more complete behavioural recovery six weeks after transplantation. Injection of the retrograde tracer fluorogold into the striatum and nigra resulted in fluorescent labeled cells within the intranigral graft and the intrastriatal graft and surrounding striatum, respectively suggesting that these double grafts promote at least partial reconstruction of the nigrostriatal dopaminergic pathway. This double graft strategy may have potential implications in clinical neural transplantation for Parkinson's disease.

Evolution of changes in neuronal activity in the subthalamic nucleus of rats with unilateral lesion of the substantia nigra assessed by metabolic and electrophysiological measurements

Cellular expression of cytochrome oxidase subunit I (COI) mRNA has recently been used as a metabolic marker for neuronal activity to study the functional changes in the subthalamic nucleus (STN) in parkinsonism. The previous experimental studies have been performed when the pathological state was stabilized at a maximal level. In order to determine the evolution of changes in neuronal activity in the STN after nigrostriatal denervation, we analysed by in situ hybridization the cellular expression of COI mRNA in the subthalamic neurons at different times, from 6 h to 14 days, after unilateral intranigral microinjection of 6-hydroxydopamine (6-OHDA) in rats. In parallel, the time-dependent changes of the unit neuronal activity of subthalamic neurons have been recorded. Levels of COI mRNA increased by 41% in subthalamic neurons from 24 h after 6-OHDA intoxication, to 14 days (+26%). Similarly, electrical activity started to increase slightly 24 h after lesion (+20%) and remained significantly higher at 14 days after the lesion (+189%). Changes in neuronal mean discharge rate were associated with changes in the pattern of spiking activity, from a regular firing pattern to an irregular one with a high bursting activity. These results show that: (i) the hyperactivity of the STN represents a very early phenomenon in the physiopathology of parkinsonian syndromes; and (ii) that changes in COI mRNA expression slightly precede changes in electrical neuronal activity.

Expression of dopamine receptors in the subthalamic nucleus of the rat: characterization using reverse transcriptase-polymerase chain reaction and autoradiography

We analysed the expression of dopamine receptor subtypes in the subthalamic nucleus by means of reverse transcriptase-polymerase chain reaction. We also studied, using autoradiography, all pharmacologically characterized dopamine receptors in four subregions of the subthalamic nucleus. For comparison, dopamine receptor subtypes were also evaluated in brain regions where they are more abundant and well characterized. The radioligands used were: [3H]SCH-23390, [3H]emonapride and [3H]2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene for dopamine D1, D2 and D3 receptors, respectively; and [3H]YM-09151-2 in the presence of raclopride for dopamine D4 receptors. Finally, we also evaluated the effect of unilateral 6-hydroxydopamine injection into the medial forebrain bundle on dopamine receptor levels expressed in the ipsilateral subthalamic nucleus. The lesion was estimated by decrease in the binding of [3H]WIN-35428, a specific dopamine transporter label. D1, D2 and D3 receptor messenger RNAs and binding sites were present in the subthalamic nucleus, but no messenger RNA for D4 receptors was found, although specific binding sites for these receptors were observed. As compared to the intact side, the 6-hydroxydopamine lesion did not change D1 receptors, increased D2 receptors, and decreased D3 receptors and the dopamine transporter. The results suggest that postsynaptic D1, D2 or D3 receptors can mediate the effect of dopamine on subthalamic nucleus neuronal activity. D4 receptors would mediate exclusively presynaptic effects. These results reinforce the idea that dopamine receptors in the subthalamic nucleus may play an important role in the physiology of the basal ganglia and in the pathophysiology of Parkinson's disease.

Intranigral transplantation of fetal substantia nigra allograft in the hemiparkinsonian rhesus monkey

Current clinical protocols for fetal cell transplantation for Parkinson's disease (PD) have focused on restoring dopamine in the striatum. However, there are now a number of human transplant recipients who have had robust innervation of the striatum by dopaminergic grafts (documented by positron emission tomography or by autopsy), but only a partial improvement in parkinsonian motor signs. Thus, there is a need for improved transplant strategies. In animal models of PD, there is recent evidence that restoring dopamine in the substantia nigra, instead of or in addition to the striatum, may be important to correct abnormal motor behavior. This pilot study examined the morphological features and behavioral effects of fetal dopaminergic neuronal allografts placed into the substantia nigra of three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated hemiparkinsonian rhesus monkeys. We show that grafts can survive in host substantia nigra. Characteristics of the graft-host interface were variable. In one animal, reinnervation of host substantia nigra was observed, and this animal showed behavioral improvement in a reach-and-retrieval task.

Postural effects of unilateral blockade of glutamatergic neurotransmission in the subthalamic nucleus on haloperidol-induced akinesia in rats

The present study examined the postural effects of the local application of glutamatergic antagonists unilaterally into the subthalamic nucleus (STN), on haloperidol-induced akinesia in rats. After intracerebral injections of MK-801, a selective antagonist of N-methyl-D-aspartate (NMDA) receptor, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) disodium, a selective alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist, or vehicle, unilaterally into the STN, haloperidol was administered systemically and the elicited behaviors were assessed quantitatively. In rats which received injections of MK-801 or CNQX, but not vehicle, unilaterally into the STN, the administration of haloperidol induced contraversive dystonic posturing. The severity of the deviated posturing was dose-dependent. The present findings revealed that the overactivity of the STN under conditions of dopamine blockade is suppressed by interruptions of glutamatergic inputs, mediated via both NMDA or AMPA receptors, to the STN. Therefore, the present study may provide functional evidence in support of a recently proposed hypothesis, that not only disinhibition from the inhibitory globus pallidus efferents but also excitatory glutamatergic inputs to the STN actually contribute to the overactivity of the STN under dopamine-depleted conditions.

Dystonic posturing and circling behaviors induced by dopaminergic agents in rats with unilateral globus pallidus lesions

The present study examined the behavioral effects of dopamine receptor agonists, antagonists, or N-methyl-D-aspartate (NMDA) glutamate receptor antagonist in rats with a unilateral excitotoxic lesion of the globus pallidus (GP). After the unilateral GP lesions were made by injections of the ibotenic acid, drugs were systemically given and the elicited behaviors were quantitatively assessed. Systemic administration of haloperidol, but not SCH23390, dose-dependently induced contraversive dystonic posturing in unilateral GP-lesioned rats. On the other hand, systemic administration of quinpirole, but not SKF38393, induced ipsiversive circling. MK-801, only when given at a high dose, unilateral GP-lesioned rat, the D2 receptor agonist and antagonist caused ipsiversive and contraversive posturing or circling, respectively. Since the rotational behavior is induced on the basis of asymmetry of the basal ganglia output activity, there must be a marked difference between the GP ablation and the administration of D2 receptor blockade on the basal ganglia output activity, supporting a speculation that overactivity of the basal ganglia under dopamine depletion is not solely a result of the disinhibition from the inhibitory GP efferents. The present unilateral GP-lesion model appears to be a useful one for the pharmacobehavioral investigation of D2-mediated mechanisms.

The regulation of motor control: an evaluation of the role of dopamine receptors in the substantia nigra

The importance of the nigrostriatal dopaminergic pathway in motor control is widely accepted and it is generally believed that the motor symptoms of Parkinson's disease result solely from reduced release of dopamine from terminals in the striatum. Over recent years there has been a growing body of evidence which suggests that dendritic dopamine release in the substantia nigra is of importance in the regulation of neuronal activity and behaviour. This evidence is reviewed together with a description of our recent findings that show nigral dopamine receptors are essential for the maintenance of normal muscle tone. It is concluded that current views of the basal ganglia circuitry involved in motor control need to be re-evaluated to take into account these recent reports. A scheme is suggested to explain how dopamine mechanisms in the substantia nigra regulate motor activity.

Increased subthalamic neuronal activity after nigral dopaminergic lesion independent of disinhibition via the globus pallidus

Electrophysiological records of unit activity were used to compare the effects of excitotoxic pallidal lesions and 6-hydroxydopamine-induced damage to the midbrain dopaminergic neurons on the discharge rates and patterns of the subthalamic neurons. Removal of the pallidal input induced a slight, but statistically significant, increase (19.5%) in the discharge rate and no change in the firing pattern when compared to control animals. The rats with a dopaminergic lesion showed greater increase (105.7%) while the firing pattern activity of the subthalamic neurons became more irregular, with burst. These results indicate that the increased activity of the subthalamic neurons following a midbrain dopaminergic lesion cannot be due solely to inhibition-disinhibition involving the striato-pallido-subthalamic pathway and induced by the striatal dopaminergic depletion.

5-Hydroxytryptamine increases spontaneous activity of subthalamic neurons in the rat

The effect of 5-hydroxytryptamine on the spontaneous activity of neurons of the subthalamic nucleus was examined by recording the extracellular unitary activity in an in vitro slice preparation. The most frequent response to 5-hydroxytryptamine (84% of 57 neurons tested) was an increase (twofold of basal at 10 microM) of the discharge frequency. The EC50 for the 5-hydroxytryptamine-induced effect was 1.8 +/- 0.5 microM (mean +/- SEM). The response was dose-dependently blocked by the serotoninergic antagonist mianserin and was not prevented by removal of calcium ions from the perfusing buffer. These results indicate that the serotoninergic input to the rat subthalamic nucleus exerts a postsynaptic excitatory action on most neurons of the nucleus.