Role of the substantia nigra in the expression of dopamine D1 receptor-mediated and D2 receptor-mediated behaviours

Dopamine D1-D2 receptor heteromer in dual phenotype GABA/glutamate-coexpressing striatal medium spiny neurons: regulation of BDNF, GAD67 and VGLUT1/2

In basal ganglia a significant subset of GABAergic medium spiny neurons (MSNs) coexpress D1 and D2 receptors (D1R and D2R) along with the neuropeptides dynorphin (DYN) and enkephalin (ENK). These coexpressing neurons have been recently shown to have a region-specific distribution throughout the mesolimbic and basal ganglia circuits. While the functional relevance of these MSNs remains relatively unexplored, they have been shown to exhibit the unique property of expressing the dopamine D1–D2 receptor heteromer, a novel receptor complex with distinct pharmacology and cell signaling properties. Here we showed that MSNs coexpressing the D1R and D2R also exhibited a dual GABA/glutamate phenotype. Activation of the D1R–D2R heteromer in these neurons resulted in the simultaneous, but differential regulation of proteins involved in GABA and glutamate production or vesicular uptake in the nucleus accumbens (NAc), ventral tegmental area (VTA), caudate putamen and substantia nigra (SN). Additionally, activation of the D1R–D2R heteromer in NAc shell, but not NAc core, differentially altered protein expression in VTA and SN, regions rich in dopamine cell bodies. The identification of a MSN with dual inhibitory and excitatory intrinsic functions provides new insights into the neuroanatomy of the basal ganglia and demonstrates a novel source of glutamate in this circuit. Furthermore, the demonstration of a dopamine receptor complex with the potential to differentially regulate the expression of proteins directly involved in GABAergic inhibitory or glutamatergic excitatory activation in VTA and SN may potentially provide new insights into the regulation of dopamine neuron activity. This could have broad implications in understanding how dysregulation of neurotransmission within basal ganglia contributes to dopamine neuronal dysfunction.

Dissociable effects of dopamine on neuronal firing rate and synchrony in the dorsal striatum

Previous studies showed that dopamine depletion leads to both changes in firing rate and in neuronal synchrony in the basal ganglia. Since dopamine D1 and D2 receptors are preferentially expressed in striatonigral and striatopallidal medium spiny neurons, respectively, we investigated the relative contribution of lack of D1 and/or D2-type receptor activation to the changes in striatal firing rate and synchrony observed after dopamine depletion. Similar to what was observed after dopamine depletion, co-administration of D1 and D2 antagonists to mice chronically implanted with multielectrode arrays in the striatum caused significant changes in firing rate, power of the local field potential (LFP) oscillations, and synchrony measured by the entrainment of neurons to striatal local field potentials. However, although blockade of either D1 or D2 type receptors produced similarly severe akinesia, the effects on neural activity differed. Blockade of D2 receptors affected the firing rate of medium spiny neurons and the power of the LFP oscillations substantially, but it did not affect synchrony to the same extent. In contrast, D1 blockade affected synchrony dramatically, but had less substantial effects on firing rate and LFP power. Furthermore, there was no consistent relation between neurons changing firing rate and changing LFP entrainment after dopamine blockade. Our results suggest that the changes in rate and entrainment to the LFP observed in medium spiny neurons after dopamine depletion are somewhat dissociable, and that lack of D1- or D2-type receptor activation can exert independent yet interactive pathological effects during the progression of Parkinson's disease.

Synchronous oscillations and phase reorganization in the basal ganglia during akinesia induced by high-dose haloperidol

Movement disorders such as tremor and akinesia observed in Parkinson's disease have been attributed to dopamine (DA) depletion in the basal ganglia. The changes in subcortical neuronal discharge patterns that follow DA depletion have been a matter of much discussion. Here, we implanted rats with chronic recording electrodes bilaterally in the striatum (CPu) and external globus pallidus (GPe), and induced both acute and repeated DA blockade by administration of high-dose haloperidol. Recordings were made in baseline states, as well as before and after haloperidol injections, which rendered rats akinetic. The immediate physiological effect of pharmacological DA blockade was the development of prominent oscillatory firing in the 6-8 Hz range in both CPu and GPe. Importantly, this oscillatory pattern was not accompanied by consistent changes in the firing rate of either CPu or GPe neurons. Cross-correlation analysis further indicated that neurons within the CPu and GPe fired synchronously after DA blockade. Furthermore, although phase lags between neuronal discharges in the GPe and CPu were uniformly distributed prior to haloperidol administration, CPu significantly lagged GPe discharges after repeated DA blockade. Our results demonstrate that acute DA blockade is sufficient to produce synchronous oscillatory activity across basal ganglia neuron populations, and that prolonged DA blockade results in phase lag changes in pallidostriatal synchrony.

Binding of girisopam (a 2,3-benzodiazepine derivative) to the substantia nigra is prevented by lesioning of the striatonigral pathway

The binding sites of girisopam, a homophthalazine (2,3-benzodiazepine)-derivate have a specific distribution pattern restricted to the striato-pallido-nigral system of the rat brain. Following kainic acid lesions in the caudate-putamen or the ventral striatum (nucleus accumbens, olfactory tubercle), as well as after surgical transection of the striatonigral pathway, [3H]girisopam binding sites were reduced or completely eliminated from the substantia nigra and the entopeduncular nucleus. Kainic acid lesions of the globus pallidus failed to act on girisopam binding sites in the substantia nigra. Surgical transections or 6-hydroxydopamine lesions of the striatonigral pathway, as well as intranigral kainic acid injections did not influence binding sites in the striatum or the pallidum. These findings indicate that girisopam in the striatum to be postsynaptic on striatonigral projecting neurons. Girisopam in the striatum seems is present in striatonigral projecting neurons. The binding sites are transported from the striatum (mainly from the caudate-putamen, partly from the ventral striatum) to the substantia nigra and the entopeduncular nucleus. The exact identity of these striatonigral fibres bearing homopthalazines is uncertain.

Turning behavior in rats with unilateral lesions of the subthalamic nucleus: synergism between D1 and D2 receptors

Rats with unilateral kainic acid lesion of the subthalamic nucleus showed a dose dependent rotational response to the lesioned side (ipsilateral) after systemic administration of the non-selective dopaminergic agonist apomorphine. Both D2 and D1 selective antagonists ((-)sulpiride and SCH23390) inhibited the response to apomorphine in these rats. Selective D2 and D1 agonists (quinpirole and SKF38393) were unable to induce turning behavior. However, an ipsilateral circling response was obtained after the simultaneous application of both agonists. The interaction mechanism between dopaminergic receptor subtypes seems to be similar to that of other normosensitive models of turning previously studied (Barone et al., 1986; Robertson and Robertson, 1986; Arnt and Perregard, 1987; Asim et al., 1990; Pazo et al., 1993). It is proposed that the ipsilateral turning response to dopaminergic agonists in rats with subthalamic nucleus lesion results from an impaired behavioral expression of the action of dopaminergic agonists on one side, leading the rats to turn away from the intact hemisphere.

D1 and D2 dopamine receptors differentially regulate c-fos expression in striatonigral and striatopallidal neurons

The expression of Fos, the product of the proto-oncogene c-fos, is thought to be a marker of neuronal activity. D1, but not D2, dopamine receptor agonists have previously been shown to increase Fos immunoreactivity in striatonigral neurons ipsilateral to a 6-hydroxydopamine lesion of the nigrostriatal pathway. In the present study, it was demonstrated that the D1 receptor agonist SKF 38393 rarely increased Fos in striatopallidal neurons of the 6-hydroxydopamine denervated striatum. Conversely, in the intact striatum, the D2 receptor antagonist haloperidol enhanced Fos expression predominantly in striatopallidal neurons labelled retrogradely from the globus pallidus or with an oligonucleotide probe complementary to mRNA encoding enkephalin. These results are consistent with studies suggesting that D1 receptors are located predominantly on striatonigral neurons and that D2 receptors reside principally on enkephalin-containing striatopallidal neurons. They also provide a neuroanatomical basis for neurochemical and neurophysiological observations indicating that dopamine facilitates the activity of striatonigral neurons but inhibits striatopallidal neurons. In another experiment the selective D2 receptor agonist quinpirole was found to increase Fos immunoreactivity in the globus pallidus ipsilateral to a 6-hydroxydopamine lesion. It is proposed that this may have been due to a D2 receptor-mediated inhibition of enkephalin and GABA release from striatopallidal terminals that in turn disinhibited the pallidal neurons. In a final series of experiments, brain microdialysis was used to determine the location of dopamine receptors regulating striatal Fos expression. Local application of the selective D1 receptor agonist CY 208-243 in the 6-hydroxydopamine-denervated striatum, or of haloperidol in the intact striatum via the dialysis probe increased Fos immunoreactivity in the immediate vicinity of the probe. Hence, the inductive effects of these systematically administered compounds on Fos expression in the striatum are mediated at least partly by local dopamine receptors in the striatum. Taken together, these results suggest that the differential regulation of striatonigral and striatopallidal activity by dopamine is mediated by the largely separate location of D1 and D2 receptors on these outputs.

Preoperative operant circling training facilitates recovery following unilateral substantia nigra lesion in rats

This study was undertaken to examine (1) whether pre- and/or postoperative training, using water reinforcement, to turn in circles (rotation) affects the behavioral symptoms induced by unilateral 6-hydroxydopamine (6-OHDA)-induced DA denervation of the rat substantia nigra (SN); (2) whether there was any influence of this training on the temporal pattern of recovery; and (3) whether the rotational training influences turning induced by systemic injection of dopaminergic drugs. In the first experiment, rats were trained either ipsi- or contraversive (TI or TO) to the side to be damaged 11 days before and 23 days after lesion, and tested in an open field for rotational behavior following systemic administration of apomorphine and amphetamine. In the second experiment rats were trained only before the lesion was made and tested in the open field for spontaneous circling and thigmotactic behavior. The results of the first experiment indicated maintenance of the training performance after the lesion. At the 14th day after the lesion, the ipsiversive trained group showed a higher contraversive circling frequency after apomorphine injection in relation to the contralateral trained group. In the second experiment, rats trained only before the surgery, showed asymmetrical spontaneous circling in the trained direction before and 14 days after surgery, indicating, in a drug free condition, that training direction can be restored after unilateral SN lesions, even to the contralateral body side. Moreover, thigmotactic behavior indicated a lack of habituation in an open field in unilateral lesion rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization and physiology of the substantia nigra

A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of the substantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of the zona compacta neurons. This loop can enhance the transmission of information from the striatum through the pars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.

D-2 agonists protect rodents against pilocarpine-induced convulsions by stimulating D-2 receptors in the striatum, but not in the substantia nigra

This study employed the pilocarpine model of epilepsy to determine the relative systemic anticonvulsant potencies of five different D-2 agonists in the mouse, and to investigate the site of anticonvulsant action of LY 171555 in the rat's brain following intracerebral microinjection. Control mice pretreated with saline developed motor seizures when challenged with pilocarpine (400 mg/kg, 11/13 convulsed). D-2 agonists protected mice against pilocarpine-induced seizures in the rank order of potency PHNO greater than pergolide greater than greater than lisuride = LY 171555 much greater than RU 24213, with ED50 values ranging from 0.17 mg/kg for PHNO to greater than 4.5 mg/kg for RU 24213. The response to LY 171555 was abolished by the D-2 blocker metoclopramide (1.25 mg/kg), but not by the D-1 antagonist SCH 23390 (0.25 mg/kg). All D-2 agonists induced head-down sniffing and forward locomotion, consistent with central D-2 activation. LY 171555 (ED50 0.19 mg/kg), but not RU 24213 (ED50 greater than 4.5 mg/kg), was similarly efficacious in the rat. When injected into both hemispheres of the conscious rat via indwelling cannulae, intrastriatal saline failed to afford protection against the convulsant action of pilocarpine (600 mg/kg, 13/15 convulsed), whereas LY 171555 did (1 microgram, 1/12 convulsed). Intrastriatal RU 24213 (1 microgram per side) was without effect (7/10 convulsed). Similarly, no protection resulted when saline (15/16 convulsed) or LY 171555 (1 microgram, 17/23 convulsed) were delivered into both nigras. It is concluded that in this model of limbic seizures in the mouse and rat, D-2 agonists exert a powerful anticonvulsant effect which is mediated by D-2 receptors in the striatum, but not by D-2 receptors in the substantia nigra.

Lasting increase in D1 dopamine receptors in the lateral part of the substantia nigra pars reticulata after subchronic methamphetamine administration

To examine the possible involvement of D1 dopamine receptors in behavioral sensitization induced by subchronic methamphetamine (MAP) administration, regional D1 receptors labeled with [3H]SCH 23390 were examined using binding assay and quantitative autoradiography. Rats received 4 mg/kg/day MAP (i.p.) for 14 days, and were decapitated after an abstinence period of 24 h, 7 days or 21 days. In MAP-treated rats, a significant decrease in Kd in the mesolimbic area was observed 24 h but not 7 days after the last injection. Neither Kd nor Bmax changed in the striatum or medial prefrontal cortex of MAP-treated rats after any period of abstinence. Autoradiography revealed a significant increase in specific [3H]SCH 22390 binding in the lateral part of the substantia nigra pars reticulata (SNr) of MAP-treated rats. Since this increase lasted up to 21 days after cessation of subchronic MAP administration, it is suggested that lasting increase in the nigral D1 receptors may be associated with the biological changes underlying MAP-induced behavioral sensitization.

Behavioral demonstration of a reciprocal interaction between dopamine receptor subtypes in the mouse striatum: possible involvement of the striato-nigral pathway

It is well known that stimulation of the D-2 dopamine receptor in vitro inhibits the increased efflux of cyclic adenosine monophosphate caused by D-1 receptor agonists. Furthermore, behavioral data suggest that the striato-nigral pathway is more involved with the dopamine agonist-induced expression of oral behaviors, which are, in turn, mediated by stimulation of the D-1 receptor. We examined an in vivo model to determine whether this D-1/D-2 reciprocal interaction is detectable at a behavioral level. First, mice were pretreated with wide range of doses of the D-2 antagonist, spiperone, and then injected with a behaviorally active dose of apomorphine (a nonspecific direct dopamine agonist) and were observed for incidence of oral behavior and rated for stereotypic behavior. A biphasic effect of spiperone pretreatment was observed, at some low doses both stereotypy and oral behavior were enhanced, while at high doses, both agonist-induced behaviors were progressively inhibited. To test the specificity of this effect for the striato-nigral pathway, mice were administered discrete electrolytic lesions in the ventral portion of the internal capsule in one hemisphere. The animals that responded to apomorphine by rotating ipsilaterally to the lesion were used in two, five-point apomorphine dose-response curves, one with, and one without, pretreatment with the dose of spiperone which most enhanced stereotypic behavior and incidence of oral behavior. The spiperone pretreatment caused a clear increase in the maximum rotational response to apomorphine without affecting the ED50. These data suggest that behavior associated with the striato-nigral efferent from striatum is marked by the opposition of D-1 and D-2 receptors.

Responses of substantia nigra pars reticulata neurons to GABA and SKF 38393 in 6-hydroxydopamine-lesioned rats are differentially affected by continuous and intermittent levodopa administration

Systemic administration of the selective D1 agonist, SKF 38393, to rats with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopamine pathway induces contralateral turning and reduces firing rates of substantia nigra pars reticulata neurons. Previous studies have shown that chronically administered levodopa diminishes the contralateral turning induced by SKF 38393 in these animals. The present study demonstrates that twice daily injections (45-50 mg/kg, i.p.) of levodopa for 19 days also diminishes the effects of SKF 38393 on substantia nigra pars reticulata activity. Concomitant with this change, chronic levodopa injections reversed the lesion-induced supersensitivity of substantia nigra pars reticulata neurons to iontophoresed GABA. Neither of these effects were produced by the continuous infusion of levodopa (90-100 mg/kg/day, i.p. by osmotic pump) for 19 days, a treatment that produces average daily blood levodopa levels similar to those produced by chronic levodopa injection. These results suggest that large variations in circulating levodopa levels in 6-hydroxydopamine lesioned rats may desensitize the behavioral responses to D1 dopamine agonist administration by down-regulating D1 and GABA receptor-mediated mechanisms of basal ganglia output through the substantia nigra pars reticulata.

Nigral D1 and striatal D2 receptors mediate the behavioral effects of dopamine agonists

The mediation of behavior by nigral and striatal dopamine (DA) D1 and D2 receptors was investigated in rats that had sustained extensive unilateral 6-hydroxydopamine-induced injury to ascending DA neurons. Selective D1 and D2 agonists and antagonists were injected directly into the DA-denervated substantia nigra pars reticula or the caudate-putamen via a chronically indwelling cannula. Contralateral rotation resulting from unilateral stimulation of supersensitive DA receptors was quantified over 46 min. Intrastriatal apomorphine (5 micrograms) or the selective D2 agonist quinpirole (5 micrograms), but not the selective D1 agonist (+/-)-SKF 38393 (15 micrograms), induced vigorous rotation. The rotation induced by intrastriatal quinpirole was greatly diminished by systemic administration of the selective D2 antagonist eticlopride (0.5 mg/kg, i.p.) and could not be enhanced by additional injection of intrastriatal (+/-)-SKF 38393. Intranigral administration of apomorphine or (+/-)-SKF 38393, but not quinpirole (same doses as above), elicited vigorous rotation. However, the rotation induced by intranigral (+/-)-SKF 38393 could not be blocked by systemic administration of the selective D1 antagonist SCH 23390 (0.5 mg/kg, s.c.), and was mimicked by intranigral (-)-SKF 38393 (15 micrograms), which exhibits 100-fold less activity than the dextrorotatory enantiomer at the D1 receptor. In order to circumvent the problem of this drug's apparent non-D1-mediated action when injected intranigrally, rotation was induced by systemic (+/-)-SKF 38393 (2.0 mg/kg, i.p.) 10 min after intranigral administration of selective antagonists. Intranigral SCH 23390 (10 micrograms), but not eticlopride (10 micrograms), powerfully antagonized the rotation induced by systemic (+/-)-SKF 38393. Conversely, rotation induced by systemic quinpirole (0.5 mg/kg, i.p.) was potently blocked by intrastriatal eticlopride but not SCH 23390. Rotation induced by systemic apomorphine (0.25 mg/kg, i.p.) was not attenuated by either antagonist alone, regardless of intracerebral injection site. The results indicate that both nigral D1 and striatal D2 receptors mediate the behavioral effects of DA agonists. These data may be useful in elucidating the mechanism(s) underlying the D1/D2 synergism observed in neurologically intact animals, as well as in understanding the action of drugs used in the treatment of Parkinson's disease.

Rotation in response to selective dopamine receptor agonists in rats with electrolytic substantia nigra lesions

Rats with unilateral, electrolytic substantia nigra (ESN) lesions were tested for rotation following systemic injections of the D1 dopamine receptor agonist SKF38393 or the D2 agonist quinpirole. Only quinpirole produced significant levels of rotation, which was ipsilateral in direction. This rotation was potentiated by coadministration of the D1 agonist, and was significantly reduced by injections of either a D1 or D2 receptor antagonist. Other groups of lesioned animals were treated with reserpine for 5 days and were then tested for rotation in response to the agonists. In this case, SKF38393 produced significant levels of contralateral rotation, while quinpirole-induced rotation remained ipsilateral; coadministration of the D1 and D2 agonists resulted in pronounced ipsilateral rotation. These results stress a role for D1 receptor mechanisms in producing rotation, and suggest that different striatal efferent pathways mediate rotation in response to selective agonists following ESN lesions.

Motor depression: a new role for D1 receptors?

The aims of this study were two-fold. Firstly, to characterize the behavioral properties of a potential new dopamine D1 receptor agonist, (-)-4,6,6a,7,8,12b-hexahydro-7-methyl-indolo[4,3-ab]phenanth ridine (CY 208-243), to determine its suitability as a tool for investigating D1 receptor function in vivo. Secondly, to investigate how the behavioural properties of D1 agonists are modified in the presence of D2 receptor blocking drugs. For this purpose, using mice, we employed CY 208-243 and 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393) as reference D1 agonists, and the substituted benzamides metoclopramide and sulpiride as selective D2 antagonists. CY 208-243 (0.25-10 mg/kg) caused only a modest increase in grooming in non-habituated mice, but stimulated locomotion, rearing, grooming and orofacial activities in habituated animals. These responses were inhibited by a D1 antagonist, but not by D2 antagonists, suggesting CY 208-243 behaves as a selective agonist of D1 receptors in vivo. In non-habituated mice, doses of metoclopramide and sulpiride which had little or no effect on motor behaviour by themselves, interacted synergistically with CY 208-243 (4 mg/kg) and SKF 38393 (30 mg/kg) to cause extended periods of immobility. Other species-typical behaviours were not affected in this way. For example, grooming was decreased by metoclopramide and increased by sulpiride, indicating that an increase in behavioural competition from this parameter was not the cause of the hypokinesia. To explain the apparent ability of D1 receptor stimulation to increase exploratory activity in earlier experiments and to decrease it here, it is proposed that this behaviour is regulated by D1 receptors coupled to two functionally opposite postsynaptic D2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of striatal dopamine receptor function by central injection of an irreversible receptor antagonist

The stereotypic head-down sniffing response to systemically administered apomorphine (0.65 mumol/kg) was assessed in rats 48 h after the bilateral injection of 0.2-0.5 microliters of the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (60 micrograms/microliters) into the caudate-putamen and nucleus accumbens. This response was significantly attenuated in animals that had received injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into the anterior/ventral part of the caudate-putamen but not in those that received injections into regions more dorsal/posterior. Animals were killed after apomorphine challenge and the region of dopamine D1 or D2 receptor reduction due to N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline mapped and quantified. This analysis revealed that the dopamine receptors involved in the apomorphine-induced stereotyped head-down sniffing response were located in a discrete region of the ventrolateral caudate-putamen and the dorsolateral nucleus accumbens. Animals that were pretreated with the selective dopamine D2 receptor antagonist raclopride (0 20 mumol/kg, i.p.) 20 min prior to central injection of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into this area showed a dose-dependent protection of the stereotyped sniffing response to systemic apomorphine 48 h later. This combination of techniques constitutes a novel way to investigate striatal function and the results obtained support the concept of a functional subdivision of both the caudate-putamen and the nucleus accumbens.

Differential effects of pallidal lesions on the behavioural responses to SKF 38393, LY 171555 and apomorphine in the rat

The purpose of this study was to determine the role of the globus pallidus in the expression of dopamine D1- and D2-receptor mediated motor events. Rats were first injected stereotaxically with 6-hydroxydopamine in one medial forebrain bundle to denervate the ascending dopamine pathways in that hemisphere. Apomorphine and selective D1 and D2 agonists were then administered, at two dose levels, to establish characteristic response patterns. Subsequently the animals were given a secondary lesion by injecting kainic acid (0.2-1 microgram) into the ipsilateral globus pallidus and retested with the dopamine agonists over a period of two months. The kainate treatment itself caused spontaneous motor asymmetries, followed by aphagia, adipsia and hypersensitivity to touch. Contraversive circling, contralateral posture and grooming induced by systemic apomorphine were all abolished by the kainate treatment, whilst sniffing and head movements were facilitated. All activities induced by D1 stimulation were abolished or severely reduced under these conditions. By contrast, the contralateral posture and grooming elicited by D2 stimulation were spared, and only D2-dependent contraversive rotation, sniffing and head movements were reduced. All behavioural deficits were temporary and recovered partially or completely during the course of the experiment, but could not be overcome by increasing the dose of dopamine agonist. Post mortem histology revealed a consistent loss of pallidal neurons, together with more variable damage to extrapyramidal structures and the thalamus. The results show that all the D1-mediated, and certain of the D2-mediated motor responses depend on the integrity of the pallidum for their expression in the unilaterally 6-hydroxydopamine-treated rat.