Electron microscopic analysis of the synaptic organization of the globus pallidus in the cat

Dopamine D4 receptor modulates inhibitory transmission in pallido-pallidal terminals and regulates motor behavior

Two major groups of terminals release GABA within the Globus pallidus; one group is constituted by projections from striatal neurons, while endings of the intranuclear collaterals form the other one. Each neurons' population expresses different subtypes of dopamine D2-like receptors: D₂ R subtype is expressed by encephalin-positive MSNs, while pallidal neurons express the D₄ R subtype. The D₂ R modulates the firing rate of striatal neurons and GABA release at their projection areas, while the D₄ R regulates Globus pallidus neurons excitability and GABA release at their projection areas. However, it is unknown if these receptors control GABA release at pallido-pallidal collaterals and regulate motor behavior. Here, we present neurochemical evidence of protein content and binding of D₄ R in pallidal synaptosomes, control of [³ H] GABA release in pallidal slices of rat, electrophysiological evidence of the presence of D₄ R on pallidal recurrent collaterals in mouse slices, and turning behavior induced by D₄ R antagonist microinjected in amphetamine challenged rats. As in projection areas of pallidal neurons, GABAergic transmission in pallido-pallidal recurrent synapses is under modulation of D₄ R, while the D₂ R subtype, as known, modulates striato-pallidal projections. Also, as in projection areas, D₄ R contributes to control the motor activity differently than D₂ R. This study could help to understand the organization of intra-pallidal circuitry.

Enhanced striatopallidal gamma-aminobutyric acid (GABA)A receptor transmission in mouse models of huntington's disease

BACKGROUND

Huntington's disease (HD) is caused by a CAG repeat expansion in the huntingtin gene. This mutation leads to progressive dysfunction that is largely attributable to dysfunction of the striatum. The earliest signs of striatal pathology in HD are found in indirect pathway gamma-Aminobutyric acid (GABA)-ergic spiny projection neurons that innervate the external segment of the globus pallidus (GPe). What is less clear is whether the synaptic coupling of spiny projection neurons with GPe neurons changes in HD.

OBJECTIVES

The principal goal of this study was to determine whether striatopallidal synaptic transmission was altered in 2 mouse models of HD.

METHODS

Striatopallidal synaptic transmission was studied using electrophysiological and optogenetic approaches in ex vivo brain slices from 2 HD models: Q175 heterozygous (het) and R6/2 mice.

RESULTS

Striatopallidal synaptic transmission increased in strength with the progression of behavioral deficits in Q175 and R6/2 mice. The alteration in synaptic transmission was evident in both prototypical and arkypallidal GPe neurons. This change did not appear attributable to an increase in the probability of GABA release but, rather, to an enhancement in the postsynaptic response to GABA released at synaptic sites. This alteration significantly increased the ability of striatopallidal axon terminals to pause ongoing GPe activity.

CONCLUSIONS

In 2 mouse models of HD, striatopallidal synaptic transmission increased in parallel with the progression of behavioral deficits. This adaptation could compensate in part for the concomitant deficit in the ability of corticostriatal signals to activate spiny projection neurons and pause GPe activity. © 2019 International Parkinson and Movement Disorder Society.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Inhibitory short-term plasticity modulates neuronal activity in the rat entopeduncular nucleus in vitro

The entopeduncular nucleus (EP) is one of the basal ganglia output nuclei integrating synaptic information from several pathways within the basal ganglia. The firing of EP neurons is modulated by two streams of inhibitory synaptic transmission, the direct pathway from the striatum and the indirect pathway from the globus pallidus. These two inhibitory pathways continuously modulate the firing of EP neurons. However, the link between these synaptic inputs to neuronal firing in the EP is unclear. To investigate this input-output transformation we performed whole-cell and perforated-patch recordings from single neurons in the entopeduncular nucleus in rat brain slices during repetitive stimulation of the striatum and the globus pallidus at frequencies within the in vivo activity range of these neurons. These recordings, supplemented by compartmental modelling, showed that GABAergic synapses from the striatum, converging on EP dendrites, display short-term facilitation and that somatic or proximal GABAergic synapses from the globus pallidus show short-term depression. Activation of striatal synapses during low presynaptic activity decreased postsynaptic firing rate by continuously increasing the inter-spike interval. Conversely, activation of pallidal synapses significantly affected postsynaptic firing during high presynaptic activity. Our data thus suggest that low-frequency striatal output may be encoded as progressive phase shifts in downstream nuclei of the basal ganglia while high-frequency pallidal output may continuously modulate EP firing.

Functional connectome of the striatal medium spiny neuron

Dopamine system disorders ranging from movement disorders to addiction and schizophrenia involve striatal medium spiny neurons (MSNs), yet their functional connectivity has been difficult to determine comprehensively. We generated a mouse with conditional channelrhodopsin-2 expression restricted to medium spiny neurons and assessed the specificity and strength of their intrinsic connections in the striatum and their projections to the globus pallidus and the substantia nigra. In the striatum, medium spiny neurons connected with other MSNs and tonically active cholinergic interneurons, but not with fast-spiking GABA interneurons. In the globus pallidus, medium spiny neurons connected strongly with one class of electrophysiologically identified neurons, but weakly with the other. In the substantia nigra, medium spiny neurons connected strongly with GABA, but not with dopamine neurons. Projections to the globus pallidus showed solely D2-mediated presynaptic inhibition, whereas projections to the substantia nigra showed solely D1-mediated presynaptic facilitation. This optogenetic approach defines the functional connectome of the striatal medium spiny neuron.

Differential localization of GABA(A) receptor subunits in relation to rat striatopallidal and pallidopallidal synapses

As a central integrator of basal ganglia function, the external segment of the globus pallidus (GP) plays a critical role in the control of voluntary movement. The GP is composed of a network of inhibitory GABA-containing projection neurons which receive GABAergic input from axons of the striatum (Str) and local collaterals of GP neurons. Here, using electrophysiological techniques and immunofluorescent labeling we have investigated the differential cellular distribution of α1, α2 and α3 GABA(A) receptor subunits in relation to striatopallidal (Str-GP) and pallidopallidal (GP-GP) synapses. Electrophysiological investigations showed that zolpidem (100 nm; selective for the α1 subunit) increased the amplitude and the decay time of both Str-GP and GP-GP IPSCs, indicating the presence of the α1 subunits at both synapses. However, the application of drugs selective for the α2, α3 and α5 subunits (zolpidem at 400 nm, L-838,417 and TP003) revealed differential effects on amplitude and decay time of IPSCs, suggesting the nonuniform distribution of non-α1 subunits. Immunofluorescence revealed widespread distribution of the α1 subunit at both soma and dendrites, while double- and triple-immunofluorescent labeling for parvalbumin, enkephalin, gephyrin and the γ2 subunit indicated strong immunoreactivity for GABA(A) α3 subunits in perisomatic synapses, a region mainly targeted by local axon collaterals. In contrast, immunoreactivity for synaptic GABA(A) α2 subunits was observed in dendritic compartments where striatal synapses are preferentially located. Due to the kinetic properties which each GABA(A) α subunit confers, this distribution is likely to contribute differentially to both physiological and pathological patterns of activity.

Functional characterization of GABAergic pallidopallidal and striatopallidal synapses in the rat globus pallidus in vitro

As a central integrator of basal ganglia function, the external segment of the globus pallidus (GP) plays a critical role in the control of voluntary movement. Driven by intrinsic mechanisms and excitatory glutamatergic inputs from the subthalamic nucleus, GP neurons receive GABAergic inhibitory input from the striatum (Str-GP) and from local collaterals of neighbouring pallidal neurons (GP-GP). Here we provide electrophysiological evidence for functional differences between these two inhibitory inputs. The basic synaptic characteristics of GP-GP and Str-GP GABAergic synapses were studied using whole-cell recordings with paired-pulse and train stimulation protocols and variance-mean (VM) analysis. We found (i) IPSC kinetics are consistent with local collaterals innervating the soma and proximal dendrites of GP neurons whereas striatal inputs innervate more distal regions. (ii) Compared to GP-GP synapses Str-GP synapses have a greater paired-pulse ratio, indicative of a lower probability of release. This was confirmed using VM analysis. (iii) In response to 20 and 50 Hz train stimulation, GP-GP synapses are weakly facilitatory in 1 mM external calcium and depressant in 2.4 mM calcium. This is in contrast to Str-GP synapses which display facilitation under both conditions. This is the first quantitative study comparing the properties of GP-GP and Str-GP synapses. The results are consistent with the differential location of these inhibitory synapses and subtle differences in their release probability which underpin stable GP-GP responses and robust short-term facilitation of Str-GP responses. These fundamental differences may provide the physiological basis for functional specialization.

Serotonin Activates Presynaptic and Postsynaptic Receptors in Rat Globus Pallidus

Although recent histological, behavioral, and clinical studies suggest that serotonin (5-HT) plays significant roles in the control of pallidal activity, only little is known about the physiological action of 5-HT in the pallidum. Our recent unit recording study in monkeys suggested that 5-HT provides both presynaptic and postsynaptic modulations of pallidal neurons. The present study using rat brain slice preparations further explored these presynaptic and postsynaptic actions of 5-HT. Bath application of 5-HT or the 5-HT1A/1B/1D/5/7 receptor (R) agonist 5-carboxamidotryptamine maleate (5-CT) depolarized some and hyperpolarized other pallidal neurons. Pretreatments of slices with blockers of the hyperpolarization–cyclic nucleotide-activated current or with the 5-HT2/7R–selective antagonist mesulergine occluded 5-CT–induced depolarization. The 5-HT1AR–selective blocker N-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N-2-pyridinylcyclohex- anecarboxamide maleate occluded the 5-CT–induced hyperpolarization. These results suggested involvement of 5-HT7R and 5-HT1AR in the postsynaptic depolarization and hyperpolarization, respectively. 5-CT presynaptically suppressed both internal capsule stimulation–induced excitatory postsynaptic currents (EPSCs) and striatal stimulation–induced inhibitory postsynaptic currents (IPSCs). The potencies of 5-CT on the presynaptic effects were 20- to 25-fold higher than on postsynaptic effects, suggesting that 5-HT mainly modulates presynaptic sites in the globus pallidus. Experiments with several antagonists suggested involvement of 5-HT1B/DR in the presynaptic suppression of EPSCs. However, the receptor type involved in the presynaptic suppression of IPSCs was inconclusive. The present results provided evidence that 5-HT exerts significant control over the synaptic inputs and the autonomous activity of pallidal neurons.

Globus pallidus external segment

The external segment of the pallidum (GP(e)) is a relatively large nucleus located caudomedial to the neostriatum (Str). The GP(e) receives major inputs from two major basal ganglia input nuclei, the Str and the subthalamic nucleus (STN), and sends its output to many basal ganglia nuclei including the STN, the Str, the internal pallidal segment (GP(i)), and the substantia nigra (SN). Thus, the GPe can be placed at the center of the basal ganglia connection diagram (Fig. 1(A)). From the viewpoint that emphasizes the direct and indirect pathways of the basal ganglia, the GP(e) is a component of the indirect pathway that relays Str inputs to the STN. The indirect pathway can be traced in Fig. 1(A), although it comprises only a part of multiple indirect pathways. This chapter begins with a brief description of the anatomical organization of the GP(e) followed by physiological and pharmacological characterizations of GABAergic responses in the GP(e).

Striatal information signaling and integration in globus pallidus: timing matters

Advances in research on globus pallidus (GP) suggest that this 'long thought to be' relay in the 'indirect pathway' plays a unique and critical role in basal ganglia function. The traditional idea of parallel processing within the basal ganglia is also challenged by recent findings. It is now clear that axons of GP neurons form large, perisomatic baskets around target neurons in all major basal ganglia nuclei, thereby exerting a profound influence on the output of the entire basal ganglia. GP neurons are autonomously active both in vivo and in vitro. It is believed that temporal information carried along the corticostriatopallidal pathway is critical for proper motor execution. The importance of appropriately controlled discharge of GP neurons is highlighted by psychomotor disorders such as Parkinson's disease, in which alterations in the pattern and synchrony of discharge in GP neurons are thought to contribute to motor symptoms. Several lines of evidence suggest that the aberrant activity of GP neurons following dopamine depletion is caused by alteration in the synaptic input from both striatum and subthalamic nucleus. In normal subjects, the capability of striatal input in translating cortical input into precisely timed responses in GP neurons is mediated by (1) the expression of postsynaptic GABA(A) receptor composed of subunits with fast kinetic properties; (2) an effective GABA reuptake system in terminating the action of synaptically released GABA, and (3) the existence of dendritic HCN channels that actively abbreviate the time course of the inhibitory postsynaptic potentials and reset rhythmic discharge. Despite the rapid pace in uncovering the elements that shape the activity along the striatopallidosubthalamic pathway, the origin of rhythmic, synchronized bursting of GP neurons seen in parkinsonism has not been fully established experimentally. Further elucidation of the factors that control the information transfer in the striatopallidal synapses is thus critical to our understanding of basal ganglia function and establishing treatment for Parkinson's disease and other basal ganglia disorders.

Rat intralaminar thalamic nuclei projections to the globus pallidus: A biotinylated dextran amine anterograde tracing study

The topographical organization and ultrastructural features of the intralaminar thalamic nuclei (ITN) projections to the globus pallidus (GP) were studied using the biotinylated dextran amine (BDA) anterograde tracing method in the rat. To assess the functional association of BDA injection sites in the ITN, the known topographical organization of the ITN-neostriatal (Str) projections and calcium binding protein (CaBP) immunostaining patterns of the Str and GP were used. BDA injection in the lateral part of the lateral parafascicular nucleus and the caudal part of the central lateral nucleus labeled fibers and boutons mainly in the dorsolateral sensorimotor territory of the Str and the middle territories of the GP. BDA injection in the medial part of the lateral parafascicular nucleus and the central lateral nucleus labeled mainly the middle association territory of the Str and the border and the caudomedial territories of the GP. BDA injection in the medial parafascicular nucleus and the central medial nucleus labeled mainly the medial limbic territory of the Str. The medial parafascicular nucleus projected to the medial-most region of the GP, while the central medial nucleus projection to the GP was very sparse. Electron microscopic observations indicated that BDA-labeled boutons form asymmetric synapses mainly on 0.5-2.0 microm diameter dendritic shafts in the GP. The boutons were small but had a relatively long active zone. The present observations together with the known topographical organization of striatopallidal projections indicated that the ITN-GP projections were topographically organized in parallel to the ITN-Str projections. Thus, each part of the ITN projecting to the sensorimotor, the association, and the limbic territories of the Str also projects to the corresponding functional territories of the GP.

Independent neuronal oscillators of the rat globus pallidus

In vivo, neurons of the globus pallidus (GP) and subthalamic nucleus (STN) resonate independently around 70 Hz. However, on the loss of dopamine as in Parkinson's disease, there is a switch to a lower frequency of firing with increased bursting and synchronization of activity. In vitro, type A neurons of the GP, identified by the presence of I(h) and rebound depolarizations, fire at frequencies (<or=80 Hz) in response to glutamate pressure ejection, designed to mimic STN input. The profile of this frequency response was unaltered by bath application of the GABA(A) antagonist bicuculline (10 microM), indicating the lack of involvement of a local GABA neuronal network, while cross-correlations of neuronal pairs revealed uncorrelated activity or phase-locked activity with a variable phase delay, consistent with each GP neuron acting as an independent oscillator. This autonomy of firing appears to arise due to the presence of intrinsic voltage- and sodium-dependent subthreshold membrane oscillations. GABA(A) inhibitory postsynaptic potentials are able to disrupt this tonic activity while promoting a rebound depolarization and action potential firing. This rebound is able to reset the phase of the intrinsic oscillation and provides a mechanism for promoting coherent firing activity in ensembles of GP neurons that may ultimately lead to abnormal and pathological disorders of movement.

Dopamine D2 receptor mediated presynaptic inhibition of striatopallidal GABA(A) IPSCs in vitro

The modulation of GABA release within the globus pallidus (GP) by dopamine was studied using whole-cell patch clamp recordings from visually identified neurones. In sagittal slices, single shock electrical stimulation in the striatum evoked GABA(A) inhibitory postsynaptic currents (IPSCs), which were inhibited by dopamine in a dose-dependent manner (0.3-30 microM) with an IC(50) value of 0.7 microM. The inhibition was accompanied by an increase in paired pulse facilitation, indicative of a presynaptic effect. In coronal slices, stimulation within the GP adjacent to the recording site evoked GABA(A) IPSCs which were relatively unaffected by dopamine indicating the lack of modulation of GABA release from terminals of local GP axon collaterals. No consistent changes in holding current, membrane potential, firing rate or the frequency of spontaneous IPSCs was observed.Tetrodotoxin-resistant miniature (m)IPSCs were recorded in chloride-loaded cells. Dopamine (3-30 microM) reduced the frequency of mIPSCs, but was without effect on mIPSC amplitude, confirming a presynaptic effect. The addition of the "D2 like" agonist quinpirole (3 microM), but not the "D1 like" agonist SKF 38393 (10 microM), mimicked these effects. The "D2 like" antagonist sulpiride (10 microM), while having no effect alone, blocked the action of dopamine. In contrast the dopamine D4 selective antagonist L745, 870 (1 microM) or D1 antagonist SCH 23390 (10 microM) were without effect. These results indicate that dopamine acts on presynaptic D2 receptors on striatopallidal terminals to reduce the release of GABA in the GP. Attenuation of this mechanism following the depletion of dopamine may contribute to the changes in GP neuronal activity observed in animal models of Parkinson's disease.

Adenosine A(2A) receptor enhances GABA(A)-mediated IPSCs in the rat globus pallidus

1. The actions of adenosine A(2A) receptor agonists were examined on GABAergic synaptic transmission in the globus pallidus (GP) in rat brain slices using whole-cell patch-clamp recording. GP neurones were characterized into two major groups, type I and type II, according to the degree of time-dependent hyperpolarization-activated inward rectification and the size of input resistance. 2. The A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido- adenosine (CGS21680; 0.3-3 microM) enhanced IPSCs evoked by stimulation within the GP. The actions of CGS21680 were blocked by the A(2A) antagonists (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine (KF17837) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). 3. The CGS21680-induced increase in IPSCs was associated with a reduction in paired-pulse facilitation. CGS21680 (0.3 microM) increased the frequency of miniature IPSCs (mIPSCs) without affecting mIPSC amplitude. These observations demonstrated that the enhancement of IPSCs in the GP was attributable to presynaptic, but not postsynaptic, A(2A) receptors. 4. The results suggest that A(2A) receptors in the GP serve to inhibit GP neuronal activity, thereby disinhibiting subthalamic nucleus neurone activity. Thus, the A(2A) receptor-mediated presynaptic regulation in the GP, together with the A(2A) receptor-mediated intrastriatal presynaptic control of GABAergic neurotransmission described previously, may play a crucial role in controlling the neuronal functions of basal ganglia. This A(2A) receptor-mediated presynaptic dual control in the striatopallidal pathway could also afford the mode of action of A(2A) antagonists for ameliorating the symptoms of Parkinson's disease in an animal model.

Dynorphin exerts both postsynaptic and presynaptic effects in the Globus pallidus of the rat

The opioids contained in striato-pallidal axons are thought to play a significant role in motor control. We examined post- and presynaptic effects of the kappa (kappa)-receptor agonist dynorphin A (1-13) (DYN13) on the globus pallidus (GP) neurons in rat brain slice preparations using the whole cell recording method. DYN13 hyperpolarized and decreased the input resistance of approximately one-quarter of neurons examined. All of these DYN13-sensitive neurons had medium-sized somata, large aspiny dendrites and generated repetitive firing without strong accommodation. The hyperpolarization was blocked by barium and was independent of TTX and intracellular chloride levels. The hyperpolarization was also selectively blocked by the kappa-antagonist nor-binaltorphimine dihydrochloride but not by the mu- or delta-antagonists. These data suggested that DYN13 activates barium-sensitive potassium currents in some GP neurons. Low- and high-intensity stimulation of the neostriatum (Str) evoked long- and short-latency GABAergic responses, respectively. Previous data suggested that the long- and the short-latency responses were due to activation of the striato-pallidal axons and the local collaterals of pallido-striatal axons, respectively. DYN13 diminished the amplitude of both the short- and long-latency GABAergic responses in all the neurons tested. The effects of DYN13 on GABAergic postsynaptic responses were also selectively blocked by a kappa-antagonist. To investigate whether the effects were pre- or postsynaptic, the effects of DYN13 on spontaneous inhibitory postsynaptic potentials (IPSPs) and TTX-independent miniature-inhibitory postsynaptic currents (IPSCs) were examined. DYN13 decreased the frequency, but not the amplitude, of spontaneous IPSCs and calcium-dependent miniature-IPSCs. However, DYN13 did not alter the cadmium-insensitive miniature-IPSCs. These results suggested that DYN13 suppressed GABA release from presynaptic terminals. This possibility was tested using a paired-stimulation test. DYN13 reduced the probability of evoking IPSCs to the first stimulation and greatly increased the success probability to the second stimulus. The amplitude of successfully evoked IPSCs was not changed with DYN13. DYN13 did not affect the excitatory postsynaptic potentials (EPSPs) or the response to iontophoretically applied GABA and glutamate. Together, these results suggest that DYN released from striato-pallidal axons controls the activity of GP neurons 1) by directly hyperpolarizing a population of neurons and 2) by presynaptically inhibiting GABA release from striato-pallidal and intrapallidal terminals.

GABA(A) receptors in the primate basal ganglia: an autoradiographic and a light and electron microscopic immunohistochemical study of the alpha1 and beta2,3 subunits in the baboon brain

The distribution of gamma-aminobutyric acid(A) (GABA(A)) receptors was investigated in the basal ganglia in the baboon brain by using receptor autoradiography and the immunohistochemical localisation of the alpha1 and beta2,3 subunits of the GABA(A) receptor by light and electron microscopy. In the caudate-putamen, the alpha1 subunit was distributed in high densities in the matrix compartment, and the beta2,3 subunits were more homogeneously distributed; the globus pallidus showed lower levels of the alpha1 and beta2,3 subunits. Four types of alpha1 subunit immunoreactive neurons were identified in the baboon striatum: the most numerous (75%) were type 1 medium-sized aspiny neurons; type 2 (2%) were large aspiny neurons with an indented nuclear membrane located in the ventral striatum; type 3 neurons were the least numerous (1%) and were comprised of large neurons in the ventromedial regions of the striatum; and type 4 (22%) neurons were medium to large aspiny neurons located in striosomes. At the ultrastructural level, alpha1 and beta2,3 subunit immunoreactivity was localised in the neuropil of the striatum in both symmetrical and asymmetrical synaptic contacts. In the globus pallidus, alpha1 and beta2,3 subunits were localised on large neurons and were found in three types of synaptic terminals: type 1 terminals were small and established symmetrical synapses; type 2 terminals were large; and type 3 terminals formed small synaptic terminals with subjunctional dense bodies. These results show that the subunit composition of GABA(A) receptors varies between the striosome and the matrix compartments in the striatum and that there is receptor subunit homogeneity in the globus pallidus.

Neuronal activity in the striatum and pallidum of primates related to the execution of externally cued reaching movements

We studied changes in basal ganglia neuronal activity associated with reaching movements of the arm in two monkeys. Data were obtained from 427 single neuronal units in putamen, 199 in caudate nucleus, and 216 in globus pallidus with multiwire electrodes allowing simultaneous recordings from multiple neurons. In all structures, changes in activity related to movement occurred most often after the onset of EMG: 43% of tested neurons in the putamen, 32% in the caudate nucleus, and 38% in the globus pallidus. Less frequently, changes began before EMG activation: 20% of neurons in the putamen, 19% in caudate nucleus, and 17% in globus pallidus. In general, these changes in neuronal activity lasted longer than EMG activity associated with reaching. The proportions of neurons activated were significantly larger in the putamen than the caudate nucleus. In the pallidum, the proportions were not statistically different from either the putamen or caudate nucleus, and no significant difference was found between the internal and external pallidal segments. Significant selectivity for movements to different targets was observed in 36% of neurons in the putamen, 28% in the caudate nucleus and 9% in the globus pallidus. The lower proportion in the globus pallidus compared to the striatum was significant (P < 0.002). Clusters of activated neurons were found in the striatum, however, the timing of changes was often different for individual neurons in these clusters. A cross-correlation analysis of the activity of neurons in the clusters revealed no evidence of common inputs, suggesting that striatal neurons in close proximity with neurons showing similar changes in activity are driven by different populations of neurons. In the putamen, the anatomical locations of neurons with changes in activity related to movement execution were on average significantly more posterior and lateral than neurons with changes related to the preparation of movement described earlier. These findings support the view that the putamen and the caudate nucleus contain distinct functional areas. The present studies show that most anatomical regions in both the striatum and pallidum participate in the control of executing reaching movements.

GABAergic axons in the ventral forebrain of the rat: an electron microscopic study

The ventral forebrain, including the ventral striatum, the ventral pallidum and the substantia innominata, is an important region involved in the functions of the basal ganglia and the limbic system, as well as the magnocellular corticopetal neurons of the nucleus basalis of Meynert. Although previous studies have shown that this region is richly innervated by GABAergic fibers, little is known with respect to the relative densities of GABAergic to non-GABAergic axon terminals in this region. To address this issue, we have developed a specific rabbit antiserum to GABA and used a postembedding immunocytochemical reaction to analyse the distribution of GABA-like immunoreactive axon terminals in the rat ventral striatum, ventral pallidum and substantia innominata. Of all axon terminals that form identifiable synapses within single ultrathin sections taken from these regions, 11.6% in the ventral striatum, 85.5% in the ventral pallidum and 64.8% in the substantia innominata were GABAergic. Differences were also found in the distribution patterns of these terminals with respect to the size of their synaptic target dendrites. These findings are consistent with previous findings that a majority of inputs to the ventral striatum are excitatory, and that a majority of inputs to the ventral pallidum are inhibitory. Our results provide a first approximation of the anatomical substrate for the physiology and pharmacology of GABA actions in the ventral forebrain region. These results also show that GABA may play an important role in the substantia innominata, where both the cholinergic and the non-cholinergic magnocellular corticopetal neurons reside within a neuropil innervated by many different non-cholinergic fibers.

Differential synaptic innervation of neurons in the internal and external segments of the globus pallidus by the GABA- and glutamate-containing terminals in the squirrel monkey

The present study aimed at comparing the pattern of synaptic innervation of neurons in the external (GPe) and internal (GPi) pallidum by gamma-aminobutyric acid (GABA)- and glutamate-immunoreactive terminals in the squirrel monkey. Four major populations of terminals were encountered in GPe and GPi. Our findings combined with those obtained in previous tract-tracing studies reveal that the synaptic innervation of perikarya in GPe is strikingly different from that in GPi. Although the GABA-positive type I boutons (from the striatum) represent 85% of the terminals in contact with somata in GPe, only 32% of the axosomatic synapses involve this type of terminal in GPi. However, the type II terminals (from GPe), which display a moderate level of GABA and glutamate immunoreactivities, account for 48% of the boutons in contact with perikarya in GPi but only 10% in GPe. In both pallidal segments, less than 10% of the axosomatic synapses involve the glutamate-immunoreactive type III terminals (from the subthalamic nucleus). Finally, the type IIa boutons (unknown source), which show levels of immunoreactivities similar to the type II terminals, account for 12% of the boutons in contact with perikarya in GPi but only 4% in GPe. In contrast to perikarya, the innervation of dendritic shafts is similar in both GPe and GPi; more than 80% of the axodendritic synapses involve the type I terminals, 10-15% involve the type III terminals, less than 5% are formed by the type II boutons, and less than 1% involve the type IIa terminals. Three other categories of boutons (types IV, V, VI) account for less than 1% of the total population of terminals in GPe and GPi. In conclusion, our findings demonstrate a differential synaptic innervation of neuronal perikarya in GPe and GPi in primates. These data suggest that the two pallidal segments are separate functional entities of which the neuronal activity is largely controlled by extrinsic inputs that are differentially distributed at the level of single cells.

Parvalbumin-immunopositive neurons in rat globus pallidus: a light and electron microscopic study

To add to our understanding of the anatomical organization of the globus pallidus (GP) of the rat, a light and electron microscopic analysis of parvalbumin (PV, a Ca-binding protein) immunoreactive neurons in the GP was performed. Light microscopic analysis revealed that the GP contains PV-positive and PV-negative neurons. Approximately two-thirds of the GP neurons were PV-positive. The somata of PV-positive neurons were, on average, larger than PV-negative ones. The proximal dendrites of PV-positive neurons were smooth and often lay parallel to the border between the GP and the neostriatum. Distal dendrites of PV-positive neurons were varicose. Thin PV-positive fibers with large boutons (with average diameter of 1.7 microns) were observed in the neuropil of the GP. Some PV-positive boutons formed basket-like aggregates surrounding the somata of PV-positive or negative neurons. Electron microscopic observations revealed that PV-positive neurons were often large and contained deeply indented nuclei and a large volume of cytoplasm. PV-negative neurons had smaller somata that were occupied by deeply indented nuclei and a small volume of cytoplasm. Both PV-positive and negative neurons were contacted by synaptic boutons identical to the known striato-pallidal, subthalamo-pallidal, and local collateral boutons. The PV-positive boutons contained small round or elongated vesicles and often more than one mitochondrion. Most of the boutons (i.e. 86%) formed symmetric synapses with somata and large dendrites and, the other (14%) formed asymmetric synapses with small dendrites. The study indicated that GP projection neurons can be divided into two subgroups according to their PV-immunoreactivity. PV-positive and negative neurons received similar extrinsic synaptic inputs and both types of neurons were connected through their local collateral axons. It is conceivable that the physiology of PV-positive and negative neurons might be different because of a difference in the Ca-buffering mechanisms in these neurons.

The cortico-pallidal projection in the rat: an anterograde tracing study with biotinylated dextran amine

The projections from the frontal cortex to the pallidum (e.g. the globus pallidus and the entopeduncular nucleus) were studied in the rat using the biotinylated dextran amine (BDA) anterograde tracing method. Injections of BDA into the precentral medial and precentral lateral cortices consistently yielded labeling of thin fibers with small boutons in the ipsilateral pallidum although not in the contralateral pallidum. Electron microscopic observation revealed that BDA-labeled boutons form asymmetric synapses mainly with small dendrites and dendritic spines. When the injection sites included the insular, orbital, and prelimbic cortices, the labeled fibers and boutons were also seen in the ventral pallidum, the basal nucleus of Meynert, and the substantia innominata. Injections of BDA in the parietal and occipital cortices resulted in either very few labeling or no labeled boutons in the pallidum. The cortico-pallidal projections were topographically organized. The density of labeled boutons in the globus pallidus was found to be approximately 10% of the density of cortical terminals in the neostriatum.

Synaptic innervation of neurones in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys

In order to better understand the way by which the subthalamic nucleus interacts with the globus pallidus to control the output of the basal ganglia, we carried out a series of experiments to investigate the pattern of synaptic innervation of the pallidal neurones by the subthalamic terminals in the squirrel monkey. To address this problem we used the anterograde transport of biocytin. Following injections of biocytin in the subthalamic nucleus, rich plexuses of labelled fibres and varicosities formed bands that lay along the medullary lamina in both segments of the ipsilateral pallidum. At the electron microscopic level, two populations of biocytin-containing terminals were identified in the internal pallidum (GPi). A first group of small to medium-sized terminals (type 1; mean cross-sectional area +/- S.D. = 0.41 +/- 0.04 microns 2) contained round vesicles and formed asymmetric synapses with dendritic shafts (95%) of mixed sizes (maximum diameter ranging from 0.3 to 4.0 microns) and spine-like structures (5%). The second group of terminals (type 2) contained pleiomorphic vesicles, had a larger cross-sectional area (mean +/- S.D. = 0.9 +/- 0.4 micron 2) and formed symmetric synapses predominantly with perikarya (41%) and large dendrites (57%). In some cases, the two types of terminals converged at the level of single GPi neurones. Postembedding immunogold method revealed that the type 2 terminals displayed gamma-aminobutyric acid (GABA) immunoreactivity, whereas the type 1 terminals did not. In the external pallidum (GPe), injections in the subthalamic nucleus labelled both type 1 or type 2 terminals. However, the labelled type 2 boutons were much less abundant in GPe than in GPi. The presence of biocytin-labelled perikarya in GPe and the fact that the type 2 terminals displayed GABA immunoreactivity led us to suspect that these terminals were derived from axons of GPe neurones. In agreement with this hypothesis, injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in GPe labelled terminals in GPi that displayed the morphological features and a pattern of synaptic organization similar to the type 2 terminals. In conclusion, the results of our study demonstrate that the subthalamopallidal terminals form asymmetric synapses that are distributed along the dendritic tree of GPe and GPi neurones. In contrast, the GPe projection to GPi gives rise to large GABA-containing terminals that form symmetric synapses predominantly with the proximal region of pallidal neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Convergence of synaptic inputs from the striatum and the globus pallidus onto identified nigrocollicular cells in the rat: a double anterograde labelling study

Two major sources of afferent synaptic inputs to projection neurons in the rat substantia nigra reticulata are the striatum and the globus pallidus. In order to understand better the functional relationships between these two afferents in the control of the activity of nigrofugal neurons, experiments have been performed to test the possibility that single nigrofugal cells receive convergent synaptic inputs from the striatum and the globus pallidus. To address this question we have used two different approaches. First, we have developed a double anterograde labelling technique suitable for both light and electron microscopy and combined this procedure with the retrograde transport of lectin-conjugated horseradish peroxidase in order to retrogradely label the nigrocollicular cells. Second, we have combined the anterograde transport of Phaseolus vulgaris-leucoagglutinin from the globus pallidus and immunocytochemistry for DARPP-32 as a marker for the striatal terminals, with the retrograde transport of lectin-conjugated horseradish peroxidase from the superior colliculus. In the double anterograde labelling experiment, biocytin was injected in the striatum, Phaseolus vulgaris-leucoagglutinin in the globus pallidus and lectin-conjugated horseradish peroxidase in the superior colliculus. Following these injections, rich plexuses of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals were found in the ventral two-thirds of the substantia nigra. The biocytin-positive terminals (striatonigral) were generally small and formed rich plexuses without any apparent neuronal association whereas the Phaseolus vulgaris-leucoagglutinin-labelled terminals (pallidonigral) were much larger and formed baskets around the perikarya of retrogradely and non retrogradely labelled cells in the substantia nigra reticulata. In areas of the substantia nigra reticulata where the fields of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals overlapped, the perikarya and the proximal dendrites of retrogradely and non retrogradely labelled cells were found to be apposed by numerous Phaseolus vulgaris-leucoagglutinin-immunoreactive pallidonigral terminals and a few biocytin-labelled striatonigral terminals. In the sections prepared for electron microscopy, the biocytin was localized using 3,3'-diaminobenzidine tetrahydrochloride whereas Phaseolus vulgaris-leucoagglutinin was localized using benzidine dihydrochloride. It was thus possible to distinguish the biocytin- from the Phaseolus vulgaris-leucoagglutinin-labelled terminals in the electron microscope by the texture of the reaction product associated with them.4+ Examination of 231 biocytin-labelled (striatonigral) terminals and 105 Phaseolus vulgaris-leucoagglutinin-immunoreactive (pallidronigral) terminals revealed that the striatonigral terminals were generally small, contained few mitochondria and formed symmetric synapses predominantly with the distal dendrites (77%) and far less frequently with the perikarya (3%) of substantia nigra reticulata cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous neuronal unit activity in the primate basal ganglia and the effects of precentral cerebral cortical ablations

The discharge properties of single neuronal units in the putamen, caudate nucleus, and globus pallidus were studied in awake primates. The effects of restricted deafferentation of the striatum were determined by recording single unit activity in animals with unilateral ablation of areas 4 and 6 of Brodmann. The most striking change was on the regularity of unit firing in the putamen. Units in the normal putamen exhibited a wide range of firing rates and variability. In many units discharge rate was very slow. After the lesion, putaminal units discharged in steady spike trains with highly regular patterns of interspike intervals having on average a 63% reduction in the coefficient of variation. Contrary to expectations, average firing rates actually increased slightly (22%) from a median value of 4.88 Hz in controls to 5.95 Hz in lesioned animals. Although the rates and variability observed in lesioned animals completely overlapped the range of the sample observed in controls, the distributions were shifted such that there were more units with regular discharge patterns and slightly faster firing rates. The caudate nucleus showed no significant change in firing rate or variability. In the globus pallidus, firing rate decreased significantly in the internal segment, and both segments showed an increase in discharge variability. The findings demonstrate that the cerebral cortex strongly influences the spontaneous discharge properties in the basal ganglia. The effects on the variability of spontaneous activity are greater than on the maintenance of tonic firing.

Immunocytochemical localization of cholinergic terminals in the region of the nucleus basalis magnocellularis of the rat: a correlated light and electron microscopic study

The cholinergic circuitry in the nucleus basalis magnocellularis of the rat was investigated in a correlated light and electron microscopic study by using monoclonal antibodies against the acetylcholine-synthesizing enzyme, choline acetyltransferase, following the unlabelled antibody peroxidase-antiperoxidase immunocytochemical procedure. After the immunocytochemical approach, large cholinergic cells and a few immunoreactive fibres exhibiting a varicose appearance, were detected by light microscopy in portions of the nucleus basalis magnocellularis located within the anatomical limits of the globus pallidus, mostly in its ventromedial part. Cholinergic neurons and fibre-like structures were also found within the substantia innominata on the edge of globus pallidus. The same material studied by light microscopy was analysed with the electron microscope. At the ultrastructural level, the immunopositive neurons showed the same cytological characteristics and pattern of synaptic input as cholinergic basal forebrain cells. Additionally, scarce immunoreactive preterminal axons and terminal boutons were detected in the region. The immunoreactive terminals were scattered or formed occasional clusters and appeared as heavily immunostained vesicle-filled boutons making exclusively axodendritic synaptic contacts principally with immunonegative distal dendrites. Both symmetric and asymmetric synaptic contacts established between these structures were detected, although the symmetric contacts were the more numerous. The surface of postsynaptic immunonegative dendrites in asymmetric synaptic contact with immunoreactive terminals was generally covered by terminals that lacked detectable immunoreactivity. In contrast, those in symmetric synaptic contact with labelled terminals showed much sparser input from immunonegative terminals, suggesting that they may belong to interneurons. Very rarely, cholinergic terminals were detected in asymmetric synaptic contact with dendrites which also contained positive immunoreaction product. Asymmetric contacts were frequently characterized by the presence of subjunctional dense bodies. The detection of cholinergic terminals in the region of the nucleus basalis magnocellularis of the rat indicates that this region not only contains cholinergic projecting neurons, but receives a cholinergic input itself. Results of this study provide evidence of the existence of a cholinergic transmission in the basal forebrain of the rat, and also that acetylcholine might play a role in the regulation of the extrinsic cortical cholinergic innervation. The possible sources of this innervation are discussed.

Dyskinesia in the primate following injection of an excitatory amino acid antagonist into the medial segment of the globus pallidus

Injection of an excitatory amino acid antagonist, kynurenic acid, into the medial segment of the globus pallidus of the conscious monkey elicited dyskinesia of the contralateral limbs. In most respects the dyskinesia was indistinguishable from the disorder that is produced by ablation of the subthalamic nucleus, or injection of a GABA antagonist into the subthalamic nucleus. Injections of kynurenic acid into the lateral segment of the globus pallidus, by contrast, did not provoke dyskinesia. The effect of kynurenic acid is attributed to the blockade of neurotransmission from the subthalamic nucleus to the medial pallidal segment, and the results suggest that the neurotransmitter utilised by this pathway is an excitatory amino acid.