Effects of intranigral injections of dopamine agonists and antagonists, glycine, muscimol and N-methyl-D,L-aspartate on locomotor activity

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Motor activity is modulated via different neuronal circuits in rats with chronic liver failure than in normal rats

The mechanisms by which liver failure alters motor function remain unclear. It has been suggested that liver disease alters the neuronal circuit between basal ganglia and cortex that modulates motor function. Activation of group I metabotropic glutamate receptors in the nucleus accumbens (NAcc) by injecting (S)-3,5-dihydroxyphenylglycine (DHPG) activates this circuit and induces locomotion We analysed by in vivo brain microdialysis the function of the circuits that modulate motor function in rats with liver failure due to portacaval shunt (PCS). We inserted cannulae in the NAcc and microdialysis probes in the NAcc, ventral pallidum (VP), substantia nigra pars reticulata (SNr), medio-dorsal thalamus (MDT), ventro-medial thalamus (VMT) or prefrontal cortex (PFCx). We injected DHPG in the NAcc and analysed extracellular neurotransmitters concentration in these areas. The results indicate that in control rats DHPG induces locomotion by activating the 'normal' neuronal circuit: NAcc --> VP --> MDT --> PFCx. In PCS rats this circuit is not activated. In PCS rats, DHPG injection activates an 'alternative' circuit: NAcc --> SNr --> VMT --> PFCx. This circuit is not activated in control rats. DHPG injection increases dopamine in the NAcc of control but not of PCS rats, and glutamate in PCS but not in control rats. DHPG-induced increase in dopamine would activate the 'normal' neuronal circuit, while an increase in glutamate would activate the 'alternative' circuit. The identification of the mechanisms responsible for altered motor function and coordination in liver disease would allow designing treatments to improve motor function in patients with hepatic encephalopathy.

Somatodendritic dopamine release in rat substantia nigra influences motor performance on the accelerating rod

The physiological role of somatodendritic dopamine release in the rat substantia nigra was evaluated with a combination of dual probe microdialysis and simultaneous motor performance tests on an accelerating rod. Three main findings support a modulating influence of somatodendritic dopamine release on motor coordination. (1) The rod performance tests were associated with an increase in extracellular dopamine but not 5-hydroxytryptamine concentrations in substantia nigra and with increases in both dopamine and 5-hydroxytryptamine concentrations in the striatum. (2) Nigral application of dopamine antagonists without intrinsic activity resulted in changed performances on the accelerating rod. The response to nigral perfusion with low concentrations (0.1, 1.0 microM) of the D(2)/D(3)-antagonist raclopride consisted of an impairment in rod performance to 63% of the pre-perfusion performance. Higher concentrations (10, 100 microM), however, were not associated with impaired rod performance, but with increased striatal dopamine concentrations. Perfusion of the substantia nigra with 1, 10 and 100 microM of the D(1)/D(5)-antagonist SCH 23390 dose-dependently impaired rod performance. SCH 23390 consistently increased dopamine and 5-hydroxytryptamine concentrations in substantia nigra but did not change the dialysate in the striatum. (3) In unilaterally 6-hydroxydopamine-lesioned rats, a dose-dependent improvement in rod performance was observed during perfusion of the substantia nigra with the non-selective dopamine agonist apomorphine.

Role of the substantia nigra pars reticulata in sensorimotor gating, measured by prepulse inhibition of startle in rats

The substantia nigra pars reticulata (SNR) is one of the major output nuclei of the basal ganglia. It connects the dorsal and ventral striatum with the thalamus, superior colliculus and pontomedullary brainstem. The SNR is therefore in a strategic position to regulate sensorimotor behavior. We here assessed the effects of SNR lesions on prepulse inhibition (PPI) of the acoustic startle response (ASR), stereotypy and locomotion in drug-free rats, as well as after systemic administration of the dopamine agonist DL-amphetamine (2 mg/kg), and the NMDA receptor antagonists dizocilpine (0.16 mg/kg) and CGP 40116 (2 mg/kg). SNR lesions reduced PPI, enhanced spontaneous sniffing and potentiated the locomotor stimulation by dizocilpine and CGP 40116. PPI was impaired by dizocilpine and CGP 40116 in controls. The ASR was enhanced in controls by dizocilpine and amphetamine. SNR lesions prevented the enhancement of the ASR by amphetamine. A second experiment tested the hypothesis that the SNR mediates PPI via a GABAergic inhibition of the startle pathway. Infusion of the GABA(B) antagonist phaclofen but not the GABA(A) antagonist picrotoxin into the caudal pontine reticular nucleus reduced PPI. Hence, lesion of the SNR reduces sensorimotor gating possibly by elimination of a nigroreticular GABAergic projection interacting with GABA(B) receptors. Moreover, destruction of the SNR enhances the motor stimulatory effects of amphetamine and of the NMDA antagonists dizocilpine and CGP 40116. We conclude that the SNR exerts a tonic GABAergic inhibition on sensorimotor behavior that is regulated by the dorsal and the ventral striatum.

Evidence for functional differences between entopeduncular nucleus and substantia nigra: effects of APV (DL-2-amino-5-phosphonovaleric acid) microinfusion on reaction time performance in the rat

Overactivity of the excitatory amino acid outputs of the subthalamic nucleus (STN) has recently been found to be one of the cascade of subsequent disruptions caused by nigrostriatal dopaminergic degeneration in Parkinson's disease. The respective contribution of the excitatory glutamatergic output structures of the STN [i.e. the globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr)] to the control of movement is not known, however. To investigate further the function of glutamatergic transmission through NMDA receptor subtypes in these three structures, the effects of discrete local infusion of a competitive receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), into the EP, GP and SNr were tested in rats performing a reaction time task. Bilateral infusion of APV into the different output structures of the STN differentially impaired the performance of rats trained to release a lever after the onset of a visual stimulus within a time limit to obtain a food reward. Infusion of APV (0.25 and 0.5 microgram/0.5 microliter) into the SNr was found to induce behavioural deficits characterized by a dramatic increase in the number of premature lever releases and decreased mean reaction time. In contrast, the infusion of APV at a dose of 0.25 microgram into the GP or EP was found to induce a motor initiation deficit characterized by an increased number of delayed responses (lever release after the time limit) and increased mean reaction time. At a dose of 0.5 microgram, a premature responding deficit was added to the previous motor impairment. Interestingly, when APV was infused simultaneously into the GP and SNr in the same animals, the behavioural effects tended to be similar to those observed after a single infusion into the SNr. Altogether, these results reveal that the different functional weight of the three main output pathways originating at the STN level is t.o. The behavioural deficits induced by NMDA receptor blockade in the SNr were similar to those observed previously after a neurotoxic lesion of the STN, suggesting that NMDA receptors in this structure play a major role as a functional output of the STN. Furthermore, regarding the differential effects produced by the same dose of APV in the SNr and the EP, these two structures, which are classically believed to be functionally linked should not be considered as the same functional entity in the organization of basal ganglia outflow.

Differential involvement of voltage-dependent calcium channels in apomorphine-induced hypermotility and stereotypy

The involvement of the voltage-dependent calcium channel in behavioral effects of apomorphine was tested in naive rats and in animals which were morphine-abstinent or were subjected to chronic electroconvulsive treatment (ECS). In naive rats a calcium channel blocker, nifedipine, which by itself does not affect locomotor activity, inhibited the locomotor stimulation induced by apomorphine, while it facilitated stereotyped behavior. Morphine-abstinent and ECS-treated rats displayed elevated responsiveness to apomorphine, reflected by hypermotility and stereotyped behavior after a dose of 1 mg/kg IP that does not produce overt behavioral effects in naive animals. Nifedipine, 5 mg/kg IP, significantly reduced hypermotility produced by apomorphine in morphine abstinent or ECS-treated rats. The calcium channel blocker did not, however, antagonize enhanced stereotyped behavior. The results indicate that apomorphine hypermotility is controlled by dihydropyridine calcium channels and that enhancement of calcium channel density produced by morphine abstinence and by chronic ECS potentiates the hypermotility response. Calcium channels seem to be differently involved in control of apomorphine-induced hypermotility and stereotypy.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Microinjections of Sch-23390 into the ventral tegmental area and substantia nigra pars reticulata attenuate the development of sensitization to the locomotor activating effects of systemic amphetamine

Pre-exposure of rats to systemic injections of D-amphetamine sulfate in the presence of bilateral injections of Sch-23390 (0.5 or 1.0 micrograms/side) into the ventral tegmental area (VTA) attenuated the acute locomotor effects of amphetamine and blocked the development of sensitization to amphetamine in a test when only amphetamine was administered, in a dose-dependent manner. Similar, but less potent, effects were observed following injections into substantia nigra pars reticulata. These findings suggest that dopamine released from somatodendritic regions brings about changes in local circuitry in the VTA that underlie the development of sensitization to amphetamine, and that Sch-23390 acts at D1 receptors in these regions to block these changes.

Neuropeptide modulation of apomorphine-induced stereotyped behavior

The nucleus accumbens contains many neuropeptides whose functions are presently unknown. The purpose of this study was to determine the extent to which these neuropeptides act in conjunction with the mesolimbic dopamine system. Microinjections of cholecystokinin, neurotensin, met-enkephalin, somatostatin, bombesin, as well as glutamate and muscimol, were made into the medial nucleus accumbens after systemic injection of apomorphine. Cholecystokinin and neurotensin, in nanogram doses, potentiated apomorphine-induced stereotypy. Met-enkephalin reduced, while somatostatin and bombesin were without effect on, apomorphine-induced stereotypy. In addition, both glutamate and muscimol potentiated this effect. These results suggest that several neuropeptides and amino acids act in the nucleus accumbens to modulate apomorphine-induced stereotyped behaviors.

Role of mesencephalic glycine in locomotor activity

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