Opposed locomotor asymmetries following lesions of the medial and lateral substantia nigra pars compacta or pars reticulata in the rat

In animals with lesions in the medial or lateral portions of the substantia nigra pars compacta (SNC) amphetamine produces circling in opposite directions. The present study examined the relationships between lesion site and the direction of circling using glyoxylic acid histofluorescence to visualize DA cells. Lesions were produced by 6-hydroxydopamine (2–6 μg) or 0.05% ascorbate injected into the SN. After lesions in the medial SNC, amphetamine caused rats to circle ipsiversive to the lesion while after lateral SNC lesions rats circled contraversively. When the lesion extended to the middle of the SNC, or deeper into the SN pars reticulata (SNR), the direction of circling was unpredictable. When the damage produced by the cannula track and ascorbate injection was in the lateral SNR animals circled ipsiversively while medial SNR damage led to contraversive circling. Thus the medial and lateral SN, and the pars compacta and pars reticulata, are functionally antagonistic. This four way division of the SN is consistent with the topographic mapping of SNC to striatum and striatum to SNR.     

The somatodendritic domain of substantia nigra pars reticulata projection neurons in the rat

We have examined the morphology of the somatodendritic domain of projection neurons located in different sectors of rat substantia nigra pars reticulata (SNr) or having distinct axonal arborizations. Forty-three neurons - 23 located in the dorsal half and 20 in the ventral half of SNr - were injected with biotinylated dextran amine and their somatodendritic domain was reconstructed from serial sagittal sections with a camera lucida. The axonal arborization of 14 neurons was also reconstructed. Dorsally located SNr neurons had a larger perikaryon, a higher number of primary dendrites and a more extensive dendritic arbor than the ventrally located ones. However, irrespective of their location in the SNr, the somatodendritic domain was always longer along the rostrocaudal axis than along the dorsoventral and mediolateral axes. Specific correlations between somatodendritic morphology and axonal arborization could be established for some SNr neurons, but among SNr neurons with similar efferent projections, those lying dorsally always exhibited a larger perikaryon and a more widespread dendritic arbor than those located ventrally. These results indicate that the morphology of the somatodendritic domain of SNr projection neurons is related to the location of their perikaryon within the structure rather than to the pattern of their axonal projections.     

Meloxicam reduces lipopolysaccharide-induced degeneration of dopaminergic neurons in the rat substantia nigra pars compacta

Inflammation is believed to play an important role in the etiology and pathogenesis of Parkinson's disease (PD). However, experimental and epidemiological evidences from various non-steroidal anti-inflammatory drugs, including cyclooxygenase-2 (COX-2) inhibitors, seem contradictive. Using the intranigral lipopolysaccharide (LPS) rat model, we show that meloxicam, a preferential COX-2 inhibitor, diminishes the activation of OX-42-immunoreactive (ir) microglia and reduces the loss of tyrosine hydroxylase (TH)-ir dopamine (DA) neurons in the substantia nigra pars compacta (SNpc) that is normally induced by exposure to LPS. Double-labelling immunohistochemistry identified that activated microglia rather than intact resting microglia are the main intracellular venues for COX-2 expression. These findings suggest that inhibition of COX-2 activity in activated microglial cells may be potentially neuroprotective for DA neurons in the SNpc.     

GABAergic control of rat substantia nigra dopaminergic neurons: role of globus pallidus and substantia nigra pars reticulata

Dopaminergic neurons in vivo fire spontaneously in three distinct patterns or modes. It has previously been shown that the firing pattern of substantia nigra dopaminergic neurons can be differentially modulated by local application of GABA(A) and GABA(B) receptor antagonists. The GABA(A) antagonists, bicuculline or picrotoxin, greatly increase burst firing in dopaminergic neurons whereas GABA(B) antagonists cause a modest shift away from burst firing towards pacemaker-like firing. The three principal GABAergic inputs to nigral dopaminergic neurons arise from striatum, globus pallidus and from the axon collaterals of nigral pars reticulata projection neurons, each of which appear to act in vivo primarily on GABA(A) receptors (see preceding paper). In this study we attempted to determine on which afferent pathway(s) GABA(A) antagonists were acting to cause burst firing. Substantia nigra dopaminergic neurons were studied by single unit extracellular recordings in urethane anesthetized rats during pharmacologically induced inhibition and excitation of globus pallidus. Muscimol-induced inhibition of pallidal neurons produced an increase in the regularity of firing of nigral dopaminergic neurons together with a slight decrease in firing rate. Bicuculline-induced excitation of globus pallidus neurons produced a marked increase in burst firing together with a modest increase in firing rate. These changes in firing rate were in the opposite direction to what would be expected for a monosynaptic GABAergic pallidonigral input. Examination of the response of pars reticulata GABAergic neurons to similar manipulations of globus pallidus revealed that the firing rates of these neurons were much more sensitive to changes in globus pallidus neuron firing rate than dopaminergic neurons and that they responded in the opposite direction. Pallidal inhibition produced a dramatic increase in the firing rate of pars reticulata GABAergic neurons while pallidal excitation suppressed the spontaneous activity of pars reticulata GABAergic neurons. These data suggest that globus pallidus exerts significant control over the firing rate and pattern of substantia nigra dopaminergic neurons through a disynaptic pathway involving nigral pars reticulata GABAergic neurons and that at least one important way in which local application of bicuculline induces burst firing of dopaminergic neurons is by disinhibition of this tonic inhibitory input.     

Opposite functions of histamine H1 and H2 receptors and H3 receptor in substantia nigra pars reticulata

The substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus. Inhibitory outputs from SNr are encoded in spike frequency and pattern of the inhibitory SNr projection neurons. SNr output intensity and pattern are often abnormal in movement disorders of basal ganglia origin. In Parkinson's disease, histamine innervation and histamine H3 receptor expression in SNr may be increased. However, the functional consequences of these alterations are not known. In this study, whole cell patch-clamp recordings were used to elucidate the function of different histamine receptors in SNr. Histamine increased SNr inhibitory projection neuron firing frequency and thus inhibitory output. This effect was mediated by activation of histamine H1 and H2 receptors that induced inward currents and depolarization. In contrast, histamine H3 receptor activation hyperpolarized and inhibited SNr inhibitory projection neurons, thus decreasing the intensity of basal ganglia output. By the hyperpolarization, H3 receptor activation also increased the irregularity of the interspike intervals or changed the pattern of SNr inhibitory neuron firing. H3 receptor-mediated effects were normally dominated by those mediated by H1 and H2 receptors. Furthermore, endogenously released histamine provided a tonic, H1 and H2 receptor-mediated excitation that helped keep SNr inhibitory projection neurons sufficiently depolarized and spiking regularly. These results suggest that H1 and H2 receptors and H3 receptor exert opposite effects on SNr inhibitory projection neurons. Functional balance of these different histamine receptors may contribute to the proper intensity and pattern of basal ganglia output and, as a consequence, exert important effects on motor control.     

Testosterone regulates androgen and estrogen receptor immunoreactivity in rat substantia nigra pars reticulata

At postnatal day (PN)1, there are sex differences in gonadal receptor expression in the rat substantia nigra pars reticulata (SNR). Male pups have lower levels of androgen receptor (AR) and estrogen receptor (ER)beta immunoreactivity (IR) compared to female pups, while ERalpha IR is equally expressed in the two sexes. To test whether these differences are due to sex differences in testosterone exposure, we injected female pups with testosterone propionate (TP) on the day of birth and analyzed the levels of AR and ER IR at PN1. TP-treated females have lower levels of AR and ERbeta IR than control, while there are no differences in the levels of ERalpha IR. TP treatment did not affect the number of AR and ER expressing cells. The regulation of SNR AR and ERbeta IR by testosterone may be important for the development of sex-specific functional systems involved in motor control.     

D1-receptor antagonists: comparison of [3H]SCH39166 to [3H]SCH23390.

A radiolabeled form of the benzonaphthazephine, SCH39166 was used to characterize the binding of this D1 antagonist in cortex, and an autoradiographic comparison of the localization of [3H]SCH39166 to [3H]SCH23390 (D1 antagonist and forerunner of SCH39166) binding was performed. The Kd for [3H]SCH39166, calculated from dissociation and association rate constants (1.09 nM), was comparable to the Kd value derived from Scatchard analyses of saturation data (1.74 nM). [3H]SCH39166 binds to brain tissue in a saturable manner with high affinity and low non-specific binding. Inhibition of [3H]SCH39166 binding by dopaminergic and serotonergic agents supports the hypothesis that this is indeed a D1-specific compound with little overlap onto serotonin (5-HT) receptors. The affinity of [3H]SCH39166 for 5-HT2 and 5-HT1c receptors is at least an order of magnitude lower than the affinity of [3H]SCH23390 for these same receptor sites. Quantitative autoradiographic analysis of [3H]SCH39166 and [3H]SCH23390 binding indicates high D1-receptor density in the caudate-putamen, nucleus accumbens, olfactory tubercle, substantia nigra and entopeduncular nucleus. Low levels of binding (not significantly above background) were detected with [3H]SCH39166 in lamina IV of the cortex and in choroid plexus; areas which had significant [3H]SCH23390 binding and are known to have a high density of 5-HT (5-HT2 and 5-HT1c respectively) receptors.     

Cortically induced inhibition of neurons of rat substantia nigra (pars compacta)

The effects of electrical stimulation of prefrontal cortex upon neurons of substantia nigra (pars compacta) in anesthetized rats were mostly inhibition without antidromic excitation. By studying nigral neurons in which the inhibition from caudate-putamen was antagonized by iontophoretic bicuculline, it was found in only half of them that the same drug also antagonized the inhibition from prefrontal cortex.     

Cannabinoids inhibit excitatory neurotransmission in the substantia nigra pars reticulata

The substantia nigra pars reticulata belongs to the brain regions with the highest density of CB(1) cannabinoid receptors. Since the level of CB(1) receptor messenger RNA is very low in the pars reticulata, most of the receptors are probably localized on terminals of afferent axons. The hypothesis was tested that terminals of glutamatergic afferents of substantia nigra pars reticulata neurons possess CB(1) cannnabinoid receptors, the activation of which presynaptically modulates neurotransmission.Rat midbrain slices were superfused and the electrophysiological properties of substantia nigra pars reticulata neurons were studied with the patch-clamp technique. Focal electrical stimulation in the presence of bicuculline evoked excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate glutamate receptors. The excitatory postsynaptic currents were reduced by the metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 10(-4)M). The mixed CB(1)/CB(2) cannabinoid receptor agonists R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2, 3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone (WIN55212-2; 10(-8)-10(-5)M) and (-)-cis-3-[2-hydroxy-4-(1, 1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55940; 10(-6)M) also produced inhibition. The maximal inhibition by WIN55212-2 was 54+/-6%. The CB(1) cannabinoid antagonist N-piperidino-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR141716A; 10(-6)M) prevented the effect of WIN55212-2, but had no effect when superfused alone. WIN55212-2 (10(-6)M) increased the amplitude ratio of two excitatory postsynaptic currents evoked with an interstimulus interval of 100ms. Currents evoked by short ejection of glutamate on to the surface of the slices were not changed by WIN55212-2.The results show that activation of CB(1) cannabinoid receptors inhibits glutamatergic synaptic transmission between afferent axons and neurons in the substantia nigra pars reticulata. The lack of effect of the cannabinoids on glutamate-evoked currents and the increase of the paired-pulse ratio indicate that the mechanism of action is presynaptic inhibition of transmitter release.     

Responses of rat substantia nigra pars reticulata units to cortical stimulation.

In order to study the function of multiple pathways between the sensorimotor cortex (Cx) and the substantia nigra pars reticulata (SNr), responses of SNr units to stimulation of the Cx were studied in anesthetized rats. Most of the units (229 of 236) exhibited repetitive firing with fairly short, regular intervals. The other 7 units displayed long duration spikes, irregular firing intervals and slow spontaneous firing. Stimulation of the Cx usually resulted in a short latency excitation and occasionally a long latency inhibition in both types of units. When strong stimulation was applied, multiple excitatory and inhibitory responses alternating each other with about a 25 ms interval were observed. SNr units responded with different patterns and latencies to stimulation of different sites of the sensorimotor Cx. The results indicate that signals derived from the sensorimotor Cx reach the SNr via multiple pathways and converge on many SNr neurons.     

Laminar organization of the substantia nigra pars reticulata in the cat

Using a semihorizontal section plane tangential to the ventral surface of the cerebral peduncle, the authors re-examined cyto-, myelo- and dendroarchitecture, acetylcholinesterase activity, afferent fibers, and efferent projection neurons of the substantia nigra pars reticulata. In the semihorizontal section plane, the substantia nigra pars reticulata was a disc-shaped nucleus and contained two to three myelinated fiber bundles running from anteromedial to posterolateral. Bands of high acetylcholinesterase activity existed parallel to the anteromedial-posterolateral direction. The Golgi silver impregnation study revealed that many nigral neurons extended their varicose dendrites anteromedially and posterolaterally. In cases with injections of wheat germ agglutinated horseradish peroxidase into the neostriatum or injections of tritiated leucine into the subthalamic nucleus, anterogradely labeled afferent fibers and axon terminals in the substantia nigra pars reticulata were organized into bands in the same anteromedial-posterolateral direction. In cases with injections of wheat germ agglutinated horseradish peroxidase into either the superior colliculus, the pedunculopontine tegmental nucleus or the ventromedial nucleus of the thalamus, retrogradely labeled neurons were also clustered along the anteromedial-posterolateral direction with their dendrites extending anteromedially and posterolaterally. The present findings strongly suggest that the substantia nigra pars reticulata has a laminar organization.     

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200–500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABA_A receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABA_A receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.     

Electrophysiological properties of identified output neurons of the rat substantia nigra (pars compacta and pars reticulata): Evidences for the existence of branched neurons

Summary In Ketamine-anaesthetized rats the nigral efferents to the ipsilateral striatum, to both VL/VM thalamic nuclei and to both superior colliculi were studied. Nigral output neurons were antidromically activated from these target nuclei and characterized by their spontaneous activity and cellular localization within the substantia nigra. Neurons in the pars compacta, which give rise to the dopaminergic nigro-striatal pathway, exhibited a low spontaneous discharge rate (3 to 6/sec) and their axon had a slow conduction velocity (0.33 to 1 m/sec). They were antidromically activated from the striatum only. Neurons in the pars reticulata were characterized by a higher spontaneous activity (20 to 40/sec) and a faster conduction velocity (1.9 m/sec to 10 m/sec). They were antidromically activated from the thalamus, superior colliculus and striatum. Furthermore, some of these neurons were found to have branching axons projecting at least to two of the different target nuclei studied.This work was supported by INSERM Grant 76. I. 205. 6IBRO — UNESCO travel fellow     

Habenular lesion attenuates methamphetamine-induced inhibition of dopamine neuronal activity in the substantia nigra pars compacta of rats

The relation between the habenula and central dopaminergic system was investigated by examining the inhibitory effects of methamphetamine on the firing rate of dopamine neurons in the substantia nigra of habenular lesioned and unlesioned rats. Electrolytic lesions of the habenula attenuated the methamphetamine-induced inhibition of dopamine neurons in the substantia nigra. This, along with other findings, indicates possibly that the habenula is involved in a feedback pathway from the striatum to the substantia nigra and regulates the activity of dopamine neurons in the substantia nigra.     

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.     

Eye movements induced by microinjection of GABA agonist in the rat substantia nigra pars reticulata

Injection of muscimol (GABA agonist) in the substantia nigra pars reticulata (SNr) of the alert rat induced a continual repetition of fast eye movements to the contralateral side, each of which was followed by a slow returning movement. The fast eye movements were similar to spontaneous saccades. Larger saccades were accompanied by contralateral neck muscle activity. We suggest that the SNr plays an important role in control of eye movements in the rat.     

Separate neuronal populations of the rat substantia nigra pars lateralis with distinct projection sites and transmitter phenotypes.

The topographic organization of the nigral cells sending axons to the striatum, amygdala and inferior colliculus was studied in the rat substantia nigra pars lateralis by using retrograde fluorescent tracers. Nigral perikarya projecting to the inferior colliculus were located dorsolaterally within the substantia nigra pars lateralis, whereas nigral perikarya projecting to the striatum or to the amygdala were mostly situated ventromedially within the substantia nigra pars lateralis. The transmitter substances of the nigrotectal cells were examined by combining a retrograde tracing method with immunohistochemistry for tyrosine hydroxylase or glutamate decarboxylase. Nigral neurons projecting to the inferior colliculus lacked tyrosine hydroxylase immunoreactivity, but exhibited immunoreactivity for glutamate decarboxylase. The substantia nigra pars lateralis is made up of different neuronal populations: one projecting to the inferior colliculus and another directed to the striatum and amygdala. The pars lateralis pathway to the inferior colliculus utilized GABA as a neurotransmitter, whereas the previously characterized nigral cells projecting to the striatum and superior colliculus use GABA and dopamine as neurotransmitters.     

Activity of neurons in the cat substantia nigra pars reticulata during drinking

Summary Extracellular activity of single neurons in the pars reticulata of the Substantia Nigra (SNpr) was recorded in cats during drinking. Two groups of cells were distinguished: I. Somatosensory cells which responded by a short decrease in firing rate to the arrival of water against the upper lip. We suggest that these stimulus-related responses reflect a complex process linked to preparation of buccolingual movements. II. Action-related cells which were de-activated at the beginning or during the entire drinking period but without modulation in relation to the individual movements of jaws and tongue. We suggest that de-activation of these cells during drinking operates as a gating mechanism which allows implementation of complex motor sequences by cortical and/or subcortical structures.     

Connexin mRNA expression in single dopaminergic neurons of substantia nigra pars compacta

Dopaminergic neurons of the substantia nigra pars compacta play a major role in goal-directed behavior and reinforcement learning. The study of their local interactions has revealed that they are connected by electrical synapses. Connexins, the molecular substrate of electrical synapses, constitute a multigenic family of 20 proteins in rodents. The permeability and regulation properties of electrical synapses depend on their connexin composition. Therefore, the knowledge of the molecular composition of electrical synapses is fundamental to the understanding of their specific functions. We have investigated the connexin mRNA expression pattern of dopaminergic neurons by single-cell RT-PCR analysis, during two periods in which dopaminergic neurons are electrically coupled in vitro (P7-P10 and P17-P21). Our results show that dopaminergic neurons express mRNAs of various connexins (Cx26, Cx30, Cx31.1, Cx32, Cx36 and Cx43) in a developmentally regulated manner. Furthermore, we have observed that dopaminergic neurons display different connexin expression patterns, and that multiple connexins can be expressed in a single dopaminergic neuron. These observations underline the importance of electrical coupling in the development of dopaminergic neurons and raise the question of the existence of functionally distinct electrically coupled networks in the substantia nigra pars compacta.