Systemic Administration Of Apomorphine Research Articles

High Oestradiol Replacement Reverses Response Memory Bias in Ovariectomised Female Rats Regardless of Dopamine Levels in the DorsalStriatum.

Oestrogens influence memory system bias in female rats such that high levels of oestrogen are associated with place (or spatial) memory use, and low oestrogen levels with response (or habitual) memory use. Moreover, striatal-dependent response memory is sensitive to dopamine transmission in the dorsal striatum, and oestrogens have been shown to affect dopamine release in that brain area. In the present study, the effects of oestrogens and dopamine transmission on multiple memory system bias were explored in ovariectomised rats receiving low or high 17β-oestradiol replacement under saline, autoreceptor-activating doses of the dopamine D2 receptor agonist, apomorphine (50 and 80μg/kg), or amphetamine (0.5mg/kg) administration. Furthermore, dorsal striatal dopamine release was measured after administration of the same drug conditions using invivo microdialysis. As expected, high oestradiol rats predominantly used place memory, whereas the opposite pattern was observed in low oestradiol rats. However, the high apomorphine dose statistically significantly altered memory bias in high oestradiol rats from predominant place to predominant response memory, with a similar trend in the low apomorphine dose and the amphetamine group. There was no effect of drugs on memory bias in low oestradiol rats. Rats with high oestradiol replacement receiving amphetamine exhibited greater dorsal striatal dopamine release than low oestradiol replacement rats, and this difference was amplified in the right hemisphere. Furthermore, a logistic regression analysis revealed that oestradiol, but not dorsal striatal dopamine levels, significantly predicted response memory bias. These findings provide further evidence that oestradiol modulates memory system bias, and also that memory bias is changed by systemic apomorphine administration. However, although oestradiol affects dopamine transmission in the dorsal striatum in a lateralised manner, this does not predict memory system bias.

D1Dopamine Receptor Coupling to PLCβ Regulates Forward Locomotion in Mice

Several studies have reported the coupling of dopamine signaling to phospholipase C β (PLCβ) both in vitro and in vivo. However, the precise physiological relevance of this signaling pathway in mediating dopamine behaviors is still unclear. Here we report that stimulation of dopamine receptor signaling in vivo with systemic administration of apomorphine, amphetamine, and cocaine leads to increased production of inositol triphosphate (IP3) in the mouse striatum. Using selective antagonists and dopamine D1 and D2 receptor knock-out animals, we show that the production of IP3 is mediated by the D1 receptor, but not the D2 receptor. A selective blocker of PLCβ, U73122, was used to assess the physiological relevance of D1-mediated IP3 production. We show that U73122 inhibits the locomotor-stimulating effects of apomorphine, amphetamine, cocaine, and SKF81297. Furthermore, U73122 also suppresses the spontaneous hyperactivity exhibited by dopamine transporter knock-out mice. Importantly, the effects of U73122 are selective to dopamine-mediated hyperactivity, as this compound does not affect hyperactivity induced by the glutamate NMDA receptor antagonist MK801. Finally, we present evidence showing that an imbalance of D1- and D2-mediated signaling following U73122 treatment modifies the locomotor output of animals from horizontal locomotor activity to vertical activity, further highlighting the importance of the PLCβ pathway in the regulation of forward locomotion via dopamine receptors.

Deletion of the NMDA‐NR1 receptor subunit gene in the mouse nucleus accumbens attenuates apomorphine‐induced dopamine D1 receptor trafficking and acoustic startle behavior

The nucleus accumbens (Acb) contains subpopulations of neurons defined by their receptor content and potential involvement in sensorimotor gating and other behaviors that are dysfunctional in schizophrenia. In Acb neurons, the NMDA NR1 (NR1) subunit is coexpressed not only with the dopamine D1 receptor (D1R), but also with the µ-opioid receptor (µ-OR), which mediates certain behaviors that are adversely impacted by schizophrenia. The NMDA-NR1 subunit has been suggested to play a role in the D1R trafficking and behavioral dysfunctions resulting from systemic administration of apomorphine, a D1R and dopamine D2 receptor agonist that impacts prepulse inhibition to auditory-evoked startle (AS). Together, this evidence suggests that the NMDA receptor may regulate D1R trafficking in Acb neurons, including those expressing µ-OR, in animals exposed to auditory startle and apomorphine. We tested this hypothesis by combining spatial-temporal gene deletion technology, dual labeling immunocytochemistry, and behavioral analysis. Deleting NR1 in Acb neurons prevented the increase in the dendritic density of plasma membrane D1Rs in single D1R and dual (D1R and µ-OR) labeled dendrites in the Acb in response to apomorphine and AS. Deleting NR1 also attenuated the decrease in AS induced by apomorphine. In the absence of apomorphine and startle, deletion of Acb NR1 diminished social interaction, without affecting novel object recognition, or open field activity. These results suggest that NR1 expression in the Acb is essential for apomorphine-induced D1R surface trafficking, as well as auditory startle and social behaviors that are impaired in multiple psychiatric disorders.

Evidence for thermoregulatory dopaminergic receptors located in the preopticus medialis nucleus of the rat hypothalamus.

Systemic administration of apomorphine induces hypothermia in anaesthetized rats as well as in conscious rats exposed to an ambient temperature of 22 degrees C. The central nature of this effect is confirmed by the antagonism exerted by haloperidol and the lack of antagonism with domperidone. Dopamine receptors involved in the hypothermic effect of apomorphine seem to be located in the preopticus medialis nucleus (p.o.m.n.) of the hypothalamus since: (i) injections of apomorphine (5 ng), dopamine (100 ng) or (+)-amphetamine (50 ng) into this nucleus induce hpothermia, (ii) haloperidol injected into the p.o.m.n. antagonizes the hypothermic effect of a systemic administration of apomorphine, (iii) heat lesions of the p.o.m.n. strongly reduce the hypothermic effect of a systemic injection of apomorphine.

Preferential relocation of the N-methyl- d-aspartate receptor NR1 subunit in nucleus accumbens neurons that contain dopamine D1 receptors in rats showing an apomorphine-induced sensorimotor gating deficit

Sensorimotor gating as measured by prepulse inhibition (PPI) to startle-evoking auditory stimulation (AS) is disrupted in schizophrenia and in rodents receiving systemic administration of apomorphine, a dopamine D1/D2 receptor agonist, or MK-801, an N-methyl- d-aspartate (NMDA) receptor antagonist. The functional analogies and our prior results showing apomorphine- and AS-induced relocation of the dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell suggest that apomorphine and AS may affect the subcellular distribution of the NMDA receptor NR1 subunit, a protein that forms protein–protein interactions with the D1R. We quantitatively compared the electron microscopic immunogold labeling for NR1 in dendritic profiles distinguished with respect to presence of D1R immunoreactivity and location in the Acb shell or core of rats receiving a single s.c. injection of vehicle (VEH) or apomorphine (APO) alone, or combined with AS (VEH+AS, APO+AS). The rats in the APO+AS group were previously shown to have PPI deficits, whereas the rats in the VEH+AS group had normal PPI. A significantly higher percentage of plasmalemmal and a lower percentage of cytoplasmic NR1 immunogold particles were seen in D1R-labeled dendritic spines in the Acb shell of the APO+AS group compared with all other groups. D1R-containing small dendrites in the Acb shell of the APO+AS group also showed a significantly higher density of plasmalemmal and a lower density of cytoplasmic NR1 immunogold particles compared with VEH or APO groups. In the Acb core, the APO+AS group had significantly fewer dendritic spines co-expressing NR1 and D1R compared with VEH or VEH+AS groups. These results, together with our earlier findings, suggest that NMDA receptors are preferentially mobilized in D1R-containing Acb neurons of rats showing apomorphine-induced disruption of PPI in a paradigm using acoustic stimulation.

Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK 1) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK 1 receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK 1 immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK 1 receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK 1 immunogold distributions. The NK 1 receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK 1 receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK 1 gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK 1 receptors. These results provide ultrastructural evidence that NK 1 receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.

Discharge Rate of Substantia Nigra Pars Reticulata Neurons Is Reduced In Non-Parkinsonian Monkeys With Apomorphine-Induced Orofacial Dyskinesia

Involuntary movements (dyskinesia) are a common symptom of dopamine-replacement therapy in parkinsonian patients, neuroleptic drug treatment of mental patients, and tic disorders. Levodopa-induced dyskinesia has been shown to be associated with substantial reduction of firing rate in the internal part of the globus pallidus. This study characterizes the changes that occur in the activity of the substantia nigra pars reticulata (SNr) of non-parkinsonian (normal) monkeys with apomorphine (APO)-induced orofacial dyskinesia. We conducted extracellular recordings of SNr neurons of two monkeys before and after induction of orofacial dyskinesia by systemic administration of APO. Involuntary orofacial movements appeared a few minutes after the injections and lasted 20-40 min. Almost all recorded neurons changed their firing rate after APO injection (96%), and most declined (70%). The mean amplitude of decreases was also larger than that of increases (40 vs. 21% of the control rate). Changes in firing pattern were not significant on average. Pairs of SNr neurons were uncorrelated before APO injection, similar to the normal pallidum. However, unlike the increased correlations in the pallidum that accompany parkinsonism, orofacilal dyskinesia in non-parkinsonian monkeys was not associated with changes in correlation between SNr neurons. We conclude that normal monkeys treated with APO can model orofacial dyskinesia and tic disorders that are a consequence of dopaminergic over-activity. These symptoms appear to be more related to reduced firing rate of SNr neurons and thus to disinhibition of their targets, than to changes in pattern and synchronization.

Infusion of apomorphine into the dorsocentral striatum produces acute drug-induced recovery from neglect produced by unilateral medial agranular cortex lesions in rats

Previous studies have shown that systemic administration of apomorphine is effective in producing acute drug-induced recovery from neglect induced by unilateral medial agranular cortex (AGm) lesions. More recent studies have demonstrated that recovery from neglect may be due to plastic changes occurring in the dorsal central striatum (DCS). Further, lesions of the DCS produce neglect that does not respond to systemic administration of apomorphine, suggesting that this area may be crucial for the therapeutic effects of apomorphine. In the present study, the behavioral effects of apomorphine infused into the DCS of animals with AGm lesion-induced neglect were examined to determine whether the DCS is a site of drug action. An infusion of 0.375 μg of apomorphine into the DCS, but not a lateral striatal control area, was effective in producing acute recovery from neglect. The results of this study support the crucial role of the DCS in recovery from neglect induced by unilateral AGm lesions and suggest that the DCS may be an important site of action for the therapeutic effects of apomorphine. Because dopamine agonist therapy has been shown to be effective in humans with neglect, the results of the current study may represent an important step in the development of future pharmacotherapies.

A Role for Electrotonic Coupling in the Striatum in the Expression of Dopamine Receptor-mediated Stereotypies

Stimulation of dopamine (DA) receptors in the striatum evokes a number of alterations in motor behavior in rats, as well as causing several alterations in cellular physiology, including changes in membrane potential, cell excitability, afferent drive, and electrotonic coupling. One cellular property that is potently modulated by DA stimulation is electrotonic coupling, a process shown to subserve motor pattern generation. To examine whether electrotonic coupling plays a role in mediating a specific set of DA receptor-mediated motor behaviors, we tested the effects of two inhibitors of gap junction conductance, carbenoxolone (CARB) and anandamide (AEA), on apomorphine (APO)-induced motor responses. We then used intra-striatal infusions of CARB to determine the role of electrotonic coupling specifically in the ventral striatum in the expression of APO-induced behaviors. APO (2.5–3.0 mg/kg, i.p.) significantly increased motor activity (a composite score) and the frequencies of oral and sniffing stereotypies. APO also disrupted grooming initiation and completion. APO-induced oral stereotypies were selectively blocked by systemic administration of CARB (7.0, 35.0 mg/kg). Moreover, although CARB alone disrupted the initiation and completion of grooming sequences, it also partially normalized APO-induced disruptions in grooming. AEA (0.5, 1.5 mg/kg) also blocked APO-induced oral stereotypies at the higher dose, but differed from CARB in that it did not restore normal grooming behaviors but, instead, appeared to “release” locomotion. Bilateral infusion of carbenoxolone (50 pmol) into the ventral striatum also blocked the oral stereotypies induced by systemic APO. We conclude from these and previous experiments that gap junctions play an important role in normal motor behavior, and furthermore that disruption of motor behavior in the form of oral and sniffing stereotypies associated with systemic APO administration may be a consequence of this heightened electrotonic coupling in the striatum. These results may be relevant to diseases and pharmacotherapies associated with disruptions of motor and possibly cognitive sequencing.

Behavioural consequences of nucleus accumbens dopaminergic stimulation and glutamatergic blocking in pigeons

Upon systemic administration of apomorphine, a potent dopamine agonist, pigeons show a bout of pecking behaviour. When the drug is repeatedly administered a sensitization takes place that is associated with pronounced discrimination learning. Here we show that intra-cerebral injections of apomorphine in the periphery of the nucleus accumbens of pigeons also elicit pecking. We additionally show that injections of 5-amino-phosphonohepatnoic acid, a NMDA-glutamate receptor blocker, into the Acc impairs the performance of a learned visual discrimination incorporating pecking as a choice response. We conclude that, as it is the case in mammals, the control mechanisms of learned sensory-motor behaviour in birds involves dopaminergic and glutamatergic synaptic transmission within the nucleus accumbens area.

Nucleus accumbens dopamine and discriminated approach learning: interactive effects of 6-hydroxydopamine lesions and systemic apomorphine administration.

Although dopaminergic mechanisms have been implicated in incentive motivational processes, their role in appetitive conditioning remains poorly understood. To investigate the effects of dopamine (DA) depleting lesions of the nucleus accumbens and the direct acting dopamine receptor agonist apomorphine on the learning of discriminated appetitive approach behavior in a Pavlovian autoshaping paradigm. Rats received bilateral infusions of either phosphate-buffered saline (shams) or 6-hydroxydopamine (lesions) directly into the nucleus accumbens. Ten days later, rats were trained on an autoshaping task whereby a previously neutral light stimulus was paired with food reward. Presentation of another stimulus (CS-) was never followed by reward. Over 100 pairings subjects developed a conditioned response of approaching the reward-predictive stimulus (CS+). Prior to each autoshaping session subjects were administered either saline (1 ml/kg SC) or apomorphine (30 microg/kg; 100 microg/kg), in a between-subjects design. Lesioned subjects showed a delay in the acquisition of discriminated approach and were insensitive to a subsequent change in the contingency of the task whereby approaches to the CS+ now prevented food delivery. Low dose apomorphine profoundly impaired learning in both sham and lesioned subjects. Despite increasing the overall number of CS+ and CS- approaches, the higher dose of apomorphine allowed discriminated approach in sham-operated animals only. These data are compatible with the hypothesis that nucleus accumbens dopamine serves to energize rather than guide conditioned approach to appetitive cues. They also support the notion that DA inputs in this region confer flexibility of approach to cues predictive of reward.

Pharmacological involvement of the calcium channel blocker flunarizine in dopamine transmission at the striatum

Single intrastriatal microinjections of 25, 50 and 100 nmol/μl of flunarizine in normal rats produced a dose-dependent turning behavior toward the injected side when they were challenged with apomorphine (1 mg/kg, s.c). This effect was seen at 1, 3 and 7 days following administration of the high dose of flunarizine, but had subsided by 24 h after administration of the intermediate dose; the low dose was ineffective. However, intrastriatal injection of the high dose of flunarizine resulted in a local lesion and thereafter this dose was not used. A similar dose-response relationship was determined for nifedipine, an L-type calcium channel antagonist. Injection of this antagonist did not result in apomorphine-elicited rotational behavior, reflecting its lack of antidopaminergic action. Intrastriatal injections of haloperidol (5 μg/μl), an antagonist of dopamine D 2 receptors, or the sodium channel blocker lidocaine (40 μg/μl), were given in order to compare their effects to those observed with flunarizine. Intracerebral injection of haloperidol produced ipsilateral turning in response to systemic administration of apomorphine given 60 min after. The same response was obtained with the injection of apomorphine 10 min after the injection of intracerebral lidocaine. This effect was no longer apparent 24 h after the microinjection of haloperidol and 60 min after the injection of lidocaine. In rats rendered hemiparkinsionian by lesioning the nigrostriatal pathway with 6OHDA, intrastriatal microinjection of flunarizine (50 nmol/μl) significantly reduced apomorphine (0.2 mg/kg, s.c.)-elicited turning behavior towards the non-lesioned side. These results suggest an antidopaminergic effect of flunarizine mediated by antagonistic action of post-synaptic striatal dopamine receptors. However, an action of the drug on sodium channels may not be ruled out. These studies offer additional supporting evidence for the induction or aggravation of extrapyramidal side-effects in patients receiving flunarizine.

Effect of microiontophoretic application of dopamine on subthalamic nucleus neuronal activity in normal rats and in rats with unilateral lesion of the nigrostriatal pathway.

The subthalamic nucleus (STN) receives dopamine inputs from the substantia nigra but their implication in the pathophysiology of parkinsonism is still debated. Extracellular microrecordings were used to study the effect of microiontophoretic injection of dopamine and the D1 receptor agonist SKF 38393 on the activity of STN neurons in normal and 6-hydroxydopamine-lesioned rats under urethane anaesthesia. Dopamine and SKF induced an increase in the firing rate of the majority of STN neurons in both normal and 6-OHDA rats. In rats with 6-OHDA lesions, the percentage of firing rate increase did not differ from that of controls. When GABA, glutamate and dopamine were all applied to the same individual STN neurons, GABA induced an inhibitory effect and glutamate and dopamine caused an excitatory effect in both groups. This excitatory response was suppressed by the application of GABA. Systemic administration of apomorphine provoked a decrease in the firing rate of STN neurons in rats with 6-OHDA lesions. These results show that dopamine exerts an excitatory influence on STN neurons, suggesting that the inhibitory effect induced by the systemic injection of apomorphine is due to the GABAergic inputs from the globus pallidus as predicted by the current model of basal ganglia organization. In addition, we show that dopamine, GABA and glutamate can act on the same STN neuron and that GABA can reverse the excitatory effect of dopamine and glutamate, suggesting the predominant influence of GABAergic inputs to the subthalamic nucleus.

Fetal striatal transplants reinstate the electrophysiological response of pallidal neurons to systemic apomorphine challenge in rats with excitotoxic striatal lesions.

Previous studies with single-unit recording and 2-[14C]deoxyglucose quantitative autoradiography have shown that systemic administration of apomorphine increases the functional activity of pallidal neurons, and that the enhancement in the globus pallidus (GP) activity is abolished by striatal lesions. The present study employing electrophysiological techniques tested whether embryonic striatal tissue implanted in the excitotoxically damaged striatum of rats may affect the lesion-induced alteration in the neuronal response of GP to apomorphine. Systemically administered apomorphine significantly increased spontaneously firing rates of GP cells. The blockade of dopamine receptors with haloperidol reversed the increased rate to baseline levels. Quinolinate-induced striatal lesions attenuated the rate-increasing effect of apomorphine. Embryonic striatal grafts placed in the lesioned striatum restored the response of GP cells to systemic apomorphine. The graft-mediated restoration of the GP neuron response to apomorphine were accompanied by an improvement in the motor asymmetry induced by this drug. Considering previous anatomical data to demonstrate extensive innervation of the GP by embryonic striatal grafts, the present results suggest that the grafts reconstruct the functional striatopallidal pathway which is capable of transmitting apomorphine-induced changes in the neuronal activity.

Systemic administration of apomorphine improves the hemodynamic mechanism of clitoral and vaginal engorgement in the rabbit.

Clitoral and vaginal engorgement during sexual stimulation depend in part on the increase of arterial inflow. It has been shown that apomorphine (APO), a non-selective dopamine receptor agonist, produces penile erection by activating dopaminergic receptors in the central nervous system. Our aim was to study whether systemic administration of APO improves the hemodynamic mechanism of clitoral and vaginal engorgement in the rabbit. Female New Zealand white rabbits (3.5-4 kg, n=6) were anesthetized. To examine sexual arousal function, the vaginal/clitoral branch of the pelvic nerve was stimulated electrically and maximal increases in clitoral intracavernosal and vaginal wall blood flows and pressures were recorded. After this APO was injected intravenously in a dose-response manner (0.05, 0.1, 0.2, 0.3 and 0.4 mg/kg) and nerve stimulation was performed after each dose. Changes in nerve-stimulated increase of clitoral intracavernosal and vaginal blood flows and pressures after APO was compared to those recorded before APO. Electrical stimulation of the vaginal/clitoral branch of the pelvic nerve significantly increased clitoral intracavernosal and vaginal wall blood flows in the rabbit. Intravenous administration of APO caused concentration dependent increase in nerve stimulation-induced peak clitoral intracavernosal and vaginal wall blood flows reaching to statistically significant at the concentration of 0.1 and 0.2 mg/kg. Inravenous administration of APO greater than 0.2 mg/kg (0.3 and 0.4 mg/kg) were less effective or produced adverse effects on clitoral intracavernosal and vaginal wall blood flows. Intravenous APO also tended to increase nerve-stimulated increase of clitoral intracavernosal and vaginal wall pressures, but the effect was not statistically significant. In conclusion, our studies suggest that systemic administration of APO may improve clitoral and vaginal engorgement by increasing clitoral intracavernosal and vaginal wall arterial inflow.

Functional MRI of apomorphine activation of the basal ganglia in awake rhesus monkeys

Functional magnetic resonance imaging (fMRI) was used to analyze blood oxygen level-dependent (BOLD) responses in the nigrostriatal system (caudate nucleus, putamen and substantia nigra) of awake rhesus monkeys to systemic apomorphine administration. The study (1) measured BOLD responses as an index of neuronal activity in the three structures following injections of the mixed D1/D2 agonist, and (2) assessed the effects of isoflurane anesthesia on the fMRI responses. Compared to control saline injections, 0.1 mg/kg apomorphine significantly activated the caudate nucleus ( P≤0.005), putamen ( P≤0.001) and substantia nigra ( P≤0.005). The responses were consistent with activation of GABAergic neurons in these three structures seen in other animal models. Isoflurane gas measurably blunted the response to apomorphine, so that a significant apomorphine activation was only seen in the substantia nigra of anesthetized animals. Even there, the mean MR signal change was reduced from 9.8% in awake monkeys to 2.3% in anesthetized animals. The data support the hypothesis that fMRI can be used to study the effects of drugs that alter basal ganglia activity in awake rhesus monkeys.

Modifications of local cerebral metabolic rates for glucose and motor behavior in rats with unilateral lesion of the subthalamic nucleus.

Inactivation of the subthalamic nucleus (STN) has attracted interest as a therapeutic tool in Parkinson's disease. The functional consequences of the inactivation, however, are uncertain. In this study definition of the pattern of changes of cerebral functional activity associated with lesion of the STN and dopaminergic stimulation, by using the [14C]deoxyglucose method, was sought. Six or 7 days following unilateral lesion of the STN, the animals were divided into two groups: One group (n = 10) was administered apomorphine (1 mg/kg) subcutaneously; the second group (n = 10) received saline. The [14C]deoxyglucose procedure was initiated 10 minutes following the drug or saline injection. The results show that systemic administration of apomorphine to rats with unilateral lesion of the STN causes ipsiversive rotational behavior and asymmetries of glucose utilization of defined brain areas, including the substantia nigra reticulata, globus pallidus, and entopeduncular nucleus. These nuclei are the main targets of the subthalamic excitatory projections. Lesion of the nucleus per se (without challenge with apomorphine) has no significant consequences on glucose utilization. The findings indicate that the STN is involved in the activation of the basal ganglia output nuclei induced by systemic dopaminergic stimulation.

Do silent dopaminergic neurons exist in rat substantia nigra in vivo?

A subpopulation of inactive or “silent” dopaminergic neurons has been reported to exist in vivo in rat substantia nigra, comprising up to 50% of nigral dopaminergic neurons. The existence of this large proportion of silent neurons has been inferred from various experimental manipulations, but never demonstrated directly. In the present study, striatal or medial forebrain bundle stimulation was used to activate antidromically substantia nigra dopaminergic neurons in vivo. Antidromic spikes of dopaminergic neurons observed by extracellular single-unit recordings in the absence of spontaneous activity were employed as indicators of the presence of a silent cell. A total of 312 dopamine neurons were recorded, including 190 neurons that could be antidromically activated from the striatum and/or the medial forebrain bundle. All neurons exhibited spontaneous activity. The firing rates were unimodally distributed about the mean of 4 spikes/s, and very few cells were observed to fire at less than 0.5 spikes/s. The numbers of spontaneously active and antidromically activated dopaminergic neurons per track were recorded and compared with the number of antidromically responding silent dopaminergic neurons per track after systemic apomorphine administration. Under control conditions, 0.80 ± 0.10 or 1.36 ± 0.13 spontaneously active neurons per track could be antidromically activated at 1.0 mA by striatal or medial forebrain bundle stimulation, respectively. After apomorphine completely suppressed spontaneous activity, 0.69 ± 0.08 and 1.39 ± 0.14 antidromic neurons per track were detected by stimulating the striatum or medial forebrain bundle respectively at 1.0 mA, demonstrating that silent dopaminergic neurons can be reliably identified through antidromic activation. In sharp contrast to previous reports, these data suggest that silent neurons do not comprise a substantial proportion of the total number of dopaminergic neurons in the substantia nigra. Reverse χ 2 analysis revealed that, if they exist at all, silent dopaminergic neurons make up less than 2% of the dopaminergic cells in the substantia nigra. These findings are related to current theories of the mechanisms of action of antipsychotic drugs and the maintenance of near-normal levels of dopamine in the striatum following large-scale loss of nigral dopaminergic neurons.

Intrastriatal Mesencephalic Grafts Affect Neuronal Activity in Basal Ganglia Nuclei and Their Target Structures in a Rat Model of Parkinson’s Disease

Nigrostriatal dopamine (DA) lesions lead to changes of neuronal activity in basal ganglia nuclei such as the globus pallidus (GP, the rodent homolog of lateral globus pallidus), entopeduncular nucleus (EP, the rodent homolog of medial globus pallidus), substantia nigra pars reticulata (SNR), and subthalamic nucleus (STN). We investigated in rats whether embryonic mesencephalic DA neurons grafted in the striatum may affect the lesion-induced alterations of neuronal activity in these structures. Regional neuronal activity was determined by use of quantitative cytochrome oxidase histochemistry. It was also examined in lesioned rats whether the grafts may regulate the expression of c-Fos after systemic administration of apomorphine in the basal ganglia nuclei as well as their target structures, including the ventromedial thalamic nucleus (VM), superior colliculus (SC), and pedunculopontine nucleus (PPN). Lesioned rats exhibited an increased activity of CO in the GP, EP, SNR, and STN ipsilateral to the lesion. Intrastriatal nigral grafts reversed the increases in the CO activity in the EP and SNR, whereas the grafts failed to affect the enzyme activity in the GP or STN. Apomorphine induced an increased expression of c-Fos in the GP, STN, VM, SC, and PPN on the lesioned side. The enhanced expression of this protein in all the structures except for the STN was attenuated by nigral grafts. The present results indicate that intrastriatal DA neuron grafts can normalize the lesion-induced changes of neuronal activity in the output nuclei of the basal ganglia as well as their target structures.

Reduced striatal tyrosine hydroxylase activity is not accompanied by change in responsiveness of dopaminergic receptors following chronic treatment with deprenyl

Deprenyl is the only selective monoamine oxidase B (MAO-B) inhibitor that is in clinical use for the treatment of Parkinson's disease. Our previous studies showed that chronic treatment of rats with low (MAO-B selective) doses of deprenyl inhibited dopamine (DA) re-uptake and enhanced DA release in the striatum. These changes could affect DA synthesis rate by activation of negative feedback loops. Chronic deprenyl treatment has also been suggested to cause down-regulation of release-modulating DA receptors. The effects of chronic and acute treatment with deprenyl on ex vivo striatal tyrosine hydroxylase activity were therefore studied, by determination of steady-state tissue level of DOPA following administration of NSD-1015 (100 mg/kg i.p.). In addition, we assessed changes in the in vivo sensitivity of dopaminergic receptors from the reduction in DOPA extracellular level after systemic apomorphine administration (2.5 mg/kg s.c.), following elevation of microdialysate DOPA by systemic or local aromatic amino acid decarboxylase inhibition with NSD-1015. Chronic treatment with deprenyl (0.25 mg/kg s.c. daily for 21 days) caused a significant reduction in tyrosine hydroxylase activity to 60% of control, with no change in the apomorphine-induced reduction of microdialysate DOPA and DOP AC. The reduction in tyrosine hydroxylase activity is compatible with our previous results showing an increase in striatal DA extracellular level following chronic treatment with deprenyl. The increased extracellular striatal DA level could reduce tyrosine hydroxylase activity through activation of a negative feedback loop, by activation of either presynaptic or postsynaptic DA receptors.