Two pathways for the activation of small-conductance potassium channels in neurons of substantia nigra pars reticulata

Neurons in substantia nigra pars reticulata express the messenger RNA for SK2 but not for SK3 subunits that form small-conductance, Ca2+-dependent K+ channels in dopamine neurons. To determine pathways for the activation of small-conductance, Ca2+-dependent K+ channels in substantia nigra pars reticulata neurons of rats and mice, we studied effects of the selective blocker of small-conductance, Ca2+-dependent K+ channels, apamin (0.01 or 0.3 microM). Apamin diminished the afterhyperpolarization following each action potential and induced burst discharges in substantia nigra pars reticulata neurons. Apamin had a robust effect already at a low (10 nM) concentration consistent with the expression of the SK2 subunit. Afterhyperpolarizations were also reduced by the Ca2+ channel blockers Ni2+ (100 microM) and omega-conotoxin GVIA (1 microM). Depletion of intracellular Ca2+ stores did not change the afterhyperpolarization. However, we observed outward current pulses that occurred independently from action potentials and were abrogated by apamin. Apart from a faster time course, they shared all properties with spontaneous hyperpolarizations or outward currents that ryanodine receptor-mediated Ca2+ release from intracellular stores induces in juvenile dopamine neurons. Sensitization of ryanodine receptors by caffeine silenced substantia nigra pars reticulata neurons. This effect was abolished by the depletion of intracellular Ca2+ stores. We conclude that SK2 channels in substantia nigra pars reticulata neurons are activated by Ca2+ influx through at least two types of Ca2+ channels in the membrane and by ryanodine receptor-mediated Ca2+ release from intracellular stores. Ryanodine receptors do not amplify small-conductance, Ca2+-dependent K+ channel activation by the Ca2+ influx during a single spike. Yet, ryanodine receptor-mediated Ca2+ release and, thereby, an activation of small-conductance, Ca2+-dependent K+ channels by intracellular Ca2+ are available for excitability modulation in these output neurons of the basal ganglia system.     

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.     

Striatal, pallidal, and pars reticulata evoked inhibition of nigrostriatal dopaminergic neurons is mediated by GABA(A) receptors in vivo

Dopaminergic neurons express both GABA(A) and GABA(B) receptors and GABAergic inputs play a significant role in the afferent modulation of these neurons. Electrical stimulation of GABAergic pathways originating in neostriatum, globus pallidus or substantia nigra pars reticulata produces inhibition of dopaminergic neurons in vivo. Despite a number of prior studies, the identity of the GABAergic receptor subtype(s) mediating the inhibition evoked by electrical stimulation of neostriatum, globus pallidus, or the axon collaterals of the projection neurons from substantia nigra pars reticulata in vivo remain uncertain. Single-unit extracellular recordings were obtained from substantia nigra dopaminergic neurons in urethane anesthetized rats. The effects of local pressure application of the selective GABA(A) antagonists, bicuculline and picrotoxin, and the GABA(B) antagonists, saclofen and CGP-55845A, on the inhibition of dopaminergic neurons elicited by single-pulse electrical stimulation of striatum, globus pallidus, and the thalamic axon terminals of the substantia nigra pars reticulata projection neurons were recorded in vivo. Striatal, pallidal, and thalamic induced inhibition of dopaminergic neurons was always attenuated or completely abolished by local application of the GABA(A) antagonists. In contrast, the GABA(B) antagonists, saclofen or CGP-55845A, did not block or attenuate the stimulus-induced inhibition and at times even increased the magnitude and/or duration of the evoked inhibition. Train stimulation of globus pallidus and striatum also produced an inhibition of firing in dopaminergic neurons of longer duration. However this inhibition was largely insensitive to either GABA(A) or GABA(B) antagonists although the GABA(A) antagonists consistently blocked the early portion of the inhibitory period indicating the presence of a GABA(A) component. These data demonstrate that dopaminergic neurons of the substantia nigra pars compacta are inhibited by electrical stimulation of striatum, globus pallidus, and the projection neurons of substantia nigra pars reticulata in vivo. This inhibition appears to be mediated via the GABA(A) receptor subtype, and all three GABAergic afferents studied appear to possess inhibitory presynaptic GABA(B) autoreceptors that are active under physiological conditions in vivo.     

Dopamine agonist-mediated rotation in rats with unilateral nigrostriatal lesions is not dependent on net inhibitions of rate in basal ganglia output nuclei.

Current models of basal ganglia function predict that dopamine agonist-induced motor activation is mediated by decreases in basal ganglia output. This study examines the relationship between dopamine agonist effects on firing rate in basal ganglia output nuclei and rotational behavior in rats with nigrostriatal lesions. Extracellular single-unit activity ipsilateral to the lesion was recorded in awake, locally-anesthetized rats. Separate rats were used for behavioral experiments. Low i.v. doses of D1 agonists (SKF 38393, SKF 81297, SKF 82958) were effective in producing rotation, yet did not change average firing rate in the substantia nigra pars reticulata or entopeduncular nucleus. At these doses, firing rate effects differed from neuron to neuron, and included increases, decreases, and no change. Higher i.v. doses of D1 agonists were effective in causing both rotation and a net decrease in rate of substantia nigra pars reticulata neurons. A low s.c. dose of the D1/D2 agonist apomorphine (0.05 mg/kg) produced both rotation and a robust average decrease in firing rate in the substantia nigra pars reticulata, yet the onset of the net firing rate decrease (at 13-16 min) was greatly delayed compared to the onset of rotation (at 3 min). Immunostaining for the immediate-early gene Fos indicated that a low i.v. dose of SKF 38393 (that produced rotation but not a net decrease in firing rate in basal ganglia output nuclei) induced Fos-like immunoreactivity in the striatum and subthalamic nucleus, suggesting an activation of both inhibitory and excitatory afferents to the substantia nigra and entopeduncular nucleus. In addition, D1 agonist-induced Fos expression in the striatum and subthalamic nucleus was equivalent in freely-moving and awake, locally-anesthetized rats. The results show that decreases in firing rate in basal ganglia output nuclei are not necessary for dopamine agonist-induced motor activation. Motor-activating actions of dopamine agonists may be mediated by firing rate decreases in a small subpopulation of output nucleus neurons, or may be mediated by other features of firing activity besides rate in these nuclei such as oscillatory firing pattern or interneuronal firing synchrony. Also, the results suggest that dopamine receptors in both the striatum and at extrastriatal sites (especially the subthalamic nucleus) are likely to be involved in dopamine agonist influences on firing rates in the substantia nigra pars reticulata and entopeduncular nucleus.     

Hyperexcitable substantia nigra dopamine neurons in PINK1- and HtrA2/Omi-deficient mice

The electrophysiological properties of substantia nigra pars compacta (SNC) dopamine neurons can influence their susceptibility to degeneration in toxin-based models of Parkinson's disease (PD), suggesting that excitotoxic and/or hypoactive mechanisms may be engaged during the early stages of the disease. It is unclear, however, whether the electrophysiological properties of SNC dopamine neurons are affected by genetic susceptibility to PD. Here we show that deletion of PD-associated genes, PINK1 or HtrA2/Omi, leads to a functional reduction in the activity of small-conductance Ca(2+)-activated potassium channels. This reduction causes SNC dopamine neurons to fire action potentials in an irregular pattern and enhances burst firing in brain slices and in vivo. In contrast, PINK1 deletion does not affect firing regularity in ventral tegmental area dopamine neurons or substantia nigra pars reticulata GABAergic neurons. These findings suggest that changes in SNC dopamine neuron excitability may play a role in their selective vulnerability in PD.     

Neural activities in the substantia nigra modulated by stimulation of the orofacial motor cortex and rhythmical jaw movements in the rat

Neurons related to jaw movements in the substantia nigra pars reticulata were explored by examining changes in their neural activities in response to electrical stimulation of the orofacial sensorimotor cortex and during rhythmical jaw movements induced by mechanical stimulation applied to the oral cavity in the rat. Out of 80 neurons tested, 59 showed changes in their firing patterns of activities in response to the electrical stimulation of the cortex. The responding neurons were mainly located in the dorsolateral part of the substantia nigra pars reticulata. The substantia nigra pars reticulata neurons showing responses were classified into the following five types according to their response patterns: (1) an inhibition preceded by an early excitation and followed by a late excitation (n = 26), (2) an inhibition preceded by an early excitation but not followed by a late excitation (n = 7), (3) an inhibition not preceded by an early excitation but followed by a late excitation (n = 2), (4) an inhibition without early or late excitations (n = 7) and (5) an excitation without an inhibition (n = 17). Out of 18 neurons responding to the cortical stimulation, 11 (61.1%) increased or decreased their neural activities during rhythmical jaw movements. Some of these neurons had a projection to the lateral part of the superior colliculus (n = 5) and/or to the parvicellular reticular formation (n = 2). These results provide first neurophysiological evidence for neurons in the dorsolateral part of the substantia nigra pars reticulata with inputs from and outputs to the areas related to jaw movements. These neurons may participate in the control of jaw movements in the rat.     

Electrophysiological characteristics of cells within mesencephalon suspension grafts

Both spontaneous and evoked extracellular electrophysiological activity of neurons within fetal mesencephalon suspension grafts to the dopamine-depleted striatum of rats were examined. In some cases, extracellular recording was combined with intracellular labeling to identify recorded neurons. Grafted rats displaying a complete cessation of ipsilateral rotations following amphetamine administration were examined at post-implantation time intervals of two, four, five, eight and nine months. Four separate classes of neurons were distinguished within the transplanted striatum based on electrophysiological properties. The first of these groups, the type I cells, appeared to be non-grafted striatal neurons. When spontaneously active, these striatal-like cells fired bursts of action potentials separated by periods of decreased activity. Evoked responses in these cells were characteristic of striatal cells. Type I cells which were intracellularly labeled were found outside the grafts and displayed the characteristic morphology of the medium spiny neuron of the neostriatum. The other three cell classes displayed electrophysiological properties similar to neurons recorded in situ within the reticular formation, substantia nigra pars compacta and substantia nigra pars reticulata. Neurons from these three groups which were labeled with an intracellular marker were found to lie within the suspension grafts. The spontaneous activity of the pars compacta dopaminergic-like neurons was predominantly irregular, with some cells also firing in a regular or pacemaker-like pattern. Infrequently, irregular firing dopaminergic-like neurons displayed episodes of doublet bursting. Many of the grafted neurons responded to electrical stimulation of prefrontal cortex and striatum, indicating that the graft was receiving functional inputs from host neurons. Comparison of the firing rate and pattern of grafted neurons to in situ mesencephalic neurons as a function of time following grafting suggested that the grafted neurons and/or the neuronal circuitry is slowly developing within the host environment. A prolonged time-course for the maturation of the graft may be reflected in the time required to achieve improvements in some behavioral deficits following transplantation. However, the relatively rapid recovery of drug-induced rotational asymmetry following grafting suggests that this form of recovery may not require mature functioning of the grafted neurons.     

Anatomy and physiology of substantia nigra and retrorubral neurons studied by extra- and intracellular recording and by horseradish peroxidase labeling

The question of origin of the excitatory and inhibitory responses that occur in neostriatal neurons following electrical stimulation of the substantia nigra is complicated by the possible spread of stimulus currents to numerous unspecifiable systems of neuronal elements. The present work begins to address this problem through the study of conduction properties of specific nigral and perinigral neurons in the cat. Neurons of pars compacta of substantia nigra and of the retrorubral area were found to have similar latencies for antidromic activation, whether from caudate nucleus stimuli (6.8–8 ms) or medial forebrain bundle stimulation (2.4–6.4 ms).The soma-dendritic features of both pars compacta and retrorubral neurons (revealed by intracellular injection of horseradish peroxidase) resembled the sparsely-branched, medium-sized substantia nigra neurons known from Golgi studies to have long dendrites with scattered and mainly distally-located spine-like appendages. Two types of pars compacta neurons were found; one with an ascending axon lacking collateral branches, and another with a descending axon that issued collaterals which terminated in the compacta, in pars reticulata, and possibly in retrorubral areas. Despite failure to detect as ascending axonal trajectory for this latter neuron, both types of pars compacta cells responded antidromically to stimulation of the caudate nucleus or medial forebrain bundle.The conduction time for impulse propagation in axons of pars compacta or retrorubral neurons suggests that either may mediate at least some of the excitatory responses that are known to occur in neostriatal neurons following stimulation of the substantia nigra in the cat. However, these conduction times are not compatible with the production of other excitatory responses which are commonly observed in the cat striatum at latencies shorter than 6 to 7 ms following stimulation of the substantia nigra.     

m-Chlorophenylpiperazine excites non-dopaminergic neurons in the rat substantia nigra and ventral tegmental area by activating serotonin-2C receptors

In vivo electrophysiological techniques were used to study the effect of m-chlorophenylpiperazine, a non-selective serotonin-2C receptor agonist, on the activity of non-dopaminergic neurons in the substantia nigra pars reticulata and the ventral tegmental area of anesthetized rats. Intravenous administration of m-chlorophenylpiperazine (5-320 microg/kg) caused a dose-dependent increase in the basal firing rate of a subpopulation of nigral neurons which do not respond to a footpinch stimulus [P(0) neurons], whereas it did not affect the activity of neurons which are responsive to the footpinch [P(+) neurons]. However, m-chlorophenylpiperazine (5-320 microg/kg) excited all non-dopaminergic neurons sampled in the ventral tegmental area. Moreover, microiontophoretic application of m-chlorophenylpiperazine (10-40 nA) caused an excitation of P(0) nigral and ventral tegmental area neurons. Pretreatment with the selective serotonin-2C receptor antagonist SB 242084 (200 microg/kg, i.v.) completely blocked the excitatory effect of i.v. m-chlorophenylpiperazine (5-320 microg/kg), both in the substantia nigra pars reticulata and in the ventral tegmental area. It is concluded that stimulation of serotonin-2C receptors by m-chlorophenylpiperazine activates non-dopaminergic (presumably GABA-containing) neurons in the substantia nigra pars reticulata and ventral tegmental area.     

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.     

Activation of metabotropic glutamate receptor 1 inhibits glutamatergic transmission in the substantia nigra pars reticulata

The substantia nigra pars reticulata is a primary output nucleus of the basal ganglia motor circuit and is controlled by a fine balance between excitatory and inhibitory inputs. The major excitatory input to GABAergic neurons in the substantia nigra arises from glutamatergic neurons in the subthalamic nucleus, whereas inhibitory inputs arise mainly from the striatum and the globus pallidus. Anatomical studies revealed that metabotropic glutamate receptors (mGluRs) are highly expressed throughout the basal ganglia. Interestingly, mRNA for group I mGluRs are abundant in neurons of the subthalamic nucleus and the substantia nigra pars reticulata. Thus, it is possible that group I mGluRs play a role in the modulation of glutamatergic synaptic transmission at excitatory subthalamonigral synapses. To test this hypothesis, we investigated the effects of group I mGluR activation on excitatory synaptic transmission in putative GABAergic neurons in the substantia nigra pars reticulata using the whole cell patch clamp recording approach in slices of rat midbrain. We report that activation of group I mGluRs by the selective agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM) decreases synaptic transmission at excitatory synapses in the substantia nigra pars reticulata. This effect is selectively mediated by presynaptic activation of the group I mGluR subtype, mGluR1. Consistent with these data, electron microscopic immunocytochemical studies demonstrate the localization of mGluR1a at presynaptic sites in the rat substantia nigra pars reticulata.From this finding that group I mGluRs modulate the major excitatory inputs to GABAergic neurons in the substantia nigra pars reticulata we suggest that these receptors may play an important role in basal ganglia functions. Studying this effect, therefore, provides new insights into the modulatory role of glutamate in basal ganglia output nuclei in physiological and pathophysiological conditions.     

Effect of high-frequency stimulation of the subthalamic nucleus on the neuronal activities of the substantia nigra pars reticulata and ventrolateral nucleus of the thalamus in the rat

Electrophysiological recordings were made in anaesthetized rats to investigate the mode of function of high-frequency stimulation of the subthalamic nucleus used as a therapeutic approach for Parkinson's disease. High-frequency electrical stimulation of the subthalamic nucleus (130 Hz) induced a net decrease in activity of all cells recorded around the site of stimulation in the subthalamic nucleus. It also caused an inhibition of the majority of neurons recorded in the substantia nigra pars reticulata in normal rats (94%) and in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (90%) or with ibotenic acid lesions of the globus pallidus (79.5%). The majority of cells recorded in the ventrolateral nucleus of the thalamus responded with an increase in their activity (84%).These results show that high-frequency stimulation of the subthalamic nucleus induces a reduction of the excitatory glutamatergic output from the subthalamic nucleus which results in deactivation of substantia nigra pars reticulata neurons. The reduction in tonic inhibitory drive of nigral neurons induces a disinhibition of activity in the ventrolateral motor thalamic nucleus, which should result in activation of the motor cortical system.     

Ca2+ release-dependent hyperpolarizations modulate the firing pattern of juvenile GABA neurons in mouse substantia nigra pars reticulata in vitro

A phasic activation of small-conductance Ca²⁺-dependent K⁺ channels (SK channels) underlies spike-afterhyperpolarizations and spike-independent, transient hyperpolarizations in juvenile substantia nigra neurons. Outward current pulses that cause the spike-independent hyperpolarizations result from ryanodine receptor-mediated Ca²⁺ release from intracellular stores. To study the modulation of excitability by the outward current pulses, we recorded from GABAergic pars reticulata neurons of mice at postnatal days 12–16. We induced a prolongation of SK channel open states by 1-ethyl-2-benzimidazolinone (1-EBIO). In addition to a prolongation of spike-afterhyperpolarizations, 1-EBIO (200 μ m ) potentiated outward current pulses by increasing their duration. Neurons were manipulated by current injection to display continuous or discontinuous discharge. Despite the prolongation of the outward current pulses by 1-EBIO, continuous action potential discharge became more regular, although its frequency declined. Durations of silent periods (periods of >2× average interspike interval) increased. Caffeine (1 m m ) further increased the duration of such silent periods. Caffeine, however, had no effect at short interspike intervals (<600 ms). Cyclopiazonic acid (10 μ m ) silenced discharge in 1-EBIO, but discharge reappeared with the depletion of Ca²⁺ stores. We conclude that the modulation of excitability by an activation of SK channels through ryanodine receptor-mediated release of Ca²⁺ critically depends on the frequency of discharge. Outward current pulses occur only if interspike intervals exceed the duration of spike-afterhyperpolarizations. In this instance, the phasic, spike-independent activation of SK channels supports pauses to interrupt autonomous discharge in juvenile GABAergic pars reticulata neurons.     

The role of dopamine released from distal and proximal dendrites of nigrostriatal dopaminergic neurons in the control of GABA transmission in the thalamic nucleus ventralis medialis in the cat

Halothane-anaesthetized cats implanted with push-pull cannulae were used in this study. Amphetamine was applied in the pars reticulata or pars compacta of the substantia nigra in order to determine the role of dopamine released from distal or proximal dendrites of dopaminergic cells in the control of GABAergic transmission in the nucleus ventralis medialis of the thalamus. When applied for 30 min in either the pars reticulata or the pars compacta, amphetamine (10(-6) M) enhanced to a similar extent the local release of [3H]dopamine synthesized from [3H]tyrosine, these effects being seen mainly during the drug application. The amphetamine-evoked release of dopamine in the pars reticulata produced a long lasting reduction in the release of [3H]GABA synthesized from [3H]glutamine in the nucleus ventralis medialis as well as in the paralamellar zone of the nucleus ventralis lateralis. Opposite effects were observed when amphetamine (10(-6) M) was applied in the pars compacta. In complementary experiments, single unit recordings were made in the intermediate part of the pars reticulata, some of the cells being identified by antidromic activation from the nucleus ventralis medialis. Whether applied in the pars reticulata or pars compacta, amphetamine (10(-6) M, 10 min) evoked a reversible decrease in the firing rate of most recorded cells whether or not they were identified as projecting to the nucleus ventralis medialis. Therefore, the decreased release of [3H]GABA in the nucleus ventralis medialis seen following application of amphetamine in the pars reticulata of the substantia nigra could result from an inhibition of nigrothalamic GABAergic neurons. Since the nucleus ventralis medialis is also innervated by GABAergic neurons originating in the entopeduncular nucleus, single unit recordings were made from cells in this nucleus during the application of amphetamine (10(-6) M, 10 min) into the pars compacta of the substantia nigra, some of which were identified antidromically as projecting to the nucleus ventralis medialis. Most cells identified or not were found to be activated during this treatment. These results suggested that the increased release of [3H]GABA seen in the nucleus ventralis medialis following application of amphetamine in the pars compacta of the substantia nigra might be linked to the enhanced firing rate of entopeduncular-thalamic GABAergic neurons.     

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.     

Retrograde signaling changes the venue of postsynaptic inhibition in rat substantia nigra

Both endocannabinoids through cannabinoid receptor type I (CB1) receptors and dopamine through dopamine receptor type D1 receptors modulate postsynaptic inhibition in substantia nigra by changing GABA release from striatonigral terminals. By recording from visually identified pars compacta and pars reticulata neurons we searched for a possible co-release and interaction of endocannabinoids and dopamine. Depolarization of a neuron in pars reticulata or in pars compacta transiently suppressed evoked synaptic currents which were blocked by GABA(A) receptor antagonists (inhibitory postsynaptic currents [IPSCs]). This depolarization-induced suppression of inhibition (DSI) was abrogated by the cannabinoid CB1 receptor antagonist AM251 (1 microM). A correlation existed between the degree of DSI and the degree of reduction of evoked IPSCs by the CB1 receptor agonist WIN55,212-2 (1 microM). The cholinergic receptor agonist carbachol (0.5-5 microM) enhanced DSI, but suppression of spontaneous IPSCs was barely detectable pointing to the existence of GABA release sites without CB1 receptors. In dopamine, but not in GABAergic neurons DSI was enhanced by the dopamine D1 receptor antagonist SCH23390 (3-10 microM). Both the antagonist for CB1 receptors and the antagonist for dopamine D1 receptors enhanced or reduced, respectively, the amplitudes of evoked IPSCs. This tonic influence persisted if the receptor for the other ligand was blocked. We conclude that endocannabinoids and dopamine can be co-released. Retrograde signaling through endocannabinoids and dopamine changes inhibition independently from each other. Activation of dopamine D1 receptors emphasizes extrinsic inhibition and activation of CB1 receptors promotes intrinsic inhibition.     

Evidence for a GABAergic inhibitory nigrotectal pathway in the rat

The aim of the present study was to test the GABAergic nature of the inhibitory projection from substantia nigra, pars reticulata (SNr) to superior colliculus (SC) in the rat, through the use of extracellular recordings and microiontophoresis. The effect of SN stimulation on the spontaneous or glutamate-evoked firing of SC units was analyzed. Among 28 SC cells inhibited by SNr stimulation, 27 decreased their firing rate following iontophoretic application of either GABA or glycine. The effect of the iontophoretic administration of bicuculline on SNr-evoked inhibition was studied on 14 of these GABA- and glycine-sensitive neurons. Bicuculline reversibly blocked nigral inhibition on 12 neurons, with iontophoretic current which did not affect glycine depression.These results are consistent with the hypothesis that GABA is the inhibitory transmitter of the nigrotectal projection.     

Novel vistas of calcium-mediated signalling in the thalamus

Traditionally, the role of calcium ions (Ca²⁺) in thalamic neurons has been viewed as that of electrical charge carriers. Recent experimental findings in thalamic cells have only begun to unravel a highly complex Ca²⁺ signalling network that exploits extra- and intracellular Ca²⁺ sources. In thalamocortical relay neurons, interactions between T-type Ca²⁺ channel activation, Ca²⁺-dependent regulation of adenylyl cyclase activity and the hyperpolarization-activated cation current (I_h) regulate oscillatory burst firing during periods of sleep and generalized epilepsy, while a functional triad between Ca²⁺ influx through high-voltage-activated (most likely L-type) Ca²⁺ channels, Ca²⁺-induced Ca²⁺ release via ryanodine receptors (RyRs) and a repolarizing mechanism (possibly via K⁺ channels of the BK_Ca type) supports tonic spike firing as required during wakefulness. The mechanisms seem to be located mostly at dendritic and somatic sites, respectively. One functional compartment involving local GABAergic interneurons in certain thalamic relay nuclei is the glomerulus, in which the dendritic release of GABA is regulated by Ca²⁺ influx via canonical transient receptor potential channels (TRPC), thereby presumably enabling transmitters of extrathalamic input systems that are coupled to phospholipase C (PLC)-activating receptors to control feed-forward inhibition in the thalamus. Functional interplay between T-type Ca²⁺ channels in dendrites and the A-type K⁺ current controls burst firing, contributing to the range of oscillatory activity observed in these interneurons. GABAergic neurons in the reticular thalamic (RT) nucleus recruit a specific set of Ca²⁺-dependent mechanisms for the generation of rhythmic burst firing, of which a particular T-type Ca²⁺ channel in the dendritic membrane, the Ca²⁺-dependent activation of non-specific cation channels (I_CAN) and of K⁺ channels (SK_Ca type) are key players. Glial Ca²⁺ signalling in the thalamus appears to be a basic mechanism of the dynamic and integrated exchange of information between glial cells and neurons. The conclusion from these observations is that a localized calcium signalling network exists in all neuronal and probably also glial cell types in the thalamus and that this network is dedicated to the precise regulation of the functional mode of the thalamus during various behavioural states.     

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.