Entopeduncular nucleus of the cat: A Golgi study

Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: response to subthalamic stimulation

Responses of rat entopeduncular nucleus (EP) neurons after stimulation of the subthalamic nucleus (STh) and the morphology of the EP neurons were studied using brain slice preparations. EP neurons were classified into two types based on their electrophysiological properties as reported previously. Of 87 EP neurons, 72 were Type I and the rest were Type II. Synaptic responses to STh stimulation were different in these two cell types. STh stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by strong inhibitory postsynaptic potentials (IPSPs) in Type I neurons and EPSPs without strong IPSPs in Type II neurons. The EPSPs were considered to be monosynaptic because no large change in the latency (1.7 +/- 0.5 ms) resulted by alteration of stimulus intensity. The EPSPs were reversibly suppressed by kynurenic acid in a dose-dependent manner. Bath application of (+)-tubocurarine (10-50 microM) had no effect on EPSPs or IPSPs. Bath application of bicuculline methiodide (50-100 microM) markedly suppressed IPSPs evoked by STh stimulation and at the same time increased the amplitude and duration of EPSPs without affecting the latency. In the presence of bicuculline methiodide, EPSPs could induce plateau potentials and slow action potentials. Some type I and Type II neurons were intracellularly labeled by biocytin. Type I neurons were located throughout the EP but Type II neurons were located mainly in the dorsal portion of the EP. Medium sized somata of both Type I and Type II neurons were spine-free and fusiform or round in shape. They had 3-4 thick primary dendrites with diameters of 2-5 micron that branched into thin secondary dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic organization of the globus pallidus

The synaptic organization of the globus pallidus is reviewed with respect to present knowledge about neurons, fibers, axon terminals, and their intrinsic synaptic relationships. Information derived from studies employing Nissl stains, Golgi impregnations, lesion degeneration techniques, immunohistochemistry, and anterograde axonal labeling in various species are presented along with ultrastructural data. Studies indicate that the globus pallidus contains a principal efferent neuron with smooth or spiny dendrites and simple or complex terminal dendritic arborizations. This cell type receives convergent inputs from intrinsic and extrinsic sources and uses gamma-aminobutyric acid as a transmitter. A smaller and separate population of pallidal projection neurons contains acetylcholine. Two other less frequent neuronal types, of small and medium size, have also been recognized. Three to six types of axonal boutons forming synaptic contacts with pallidal neurons have been recognized in various studies. Among these, three types (types I, II, and III) are the most prevalent. Studies indicate that the most frequent category (type I) originates from neostriatal neurons via radial fiber projections and contains immunoreactive GABA and enkephalins. The synaptic architecture of the globus pallidus is dominated by a mosaic-like arrangement of long dendrites that are ensheathed by longitudinally oriented axons making synapses en passant. Triadic synapses involving dendrites that are pre- and postsynaptic are encountered infrequently. Because both striatopallidal and pallidothalamic connections are inhibitory, pallidal target neurons in the thalamus may be "disinhibited" when the neostriatum is activated.

Synaptic organization of the cat entopeduncular nucleus with special reference to the relationship between the afferents to entopedunculothalamic projection neurons: an electron microscope study by a combined degeneration and horseradish peroxidase tracing technique

The synaptic organization of the feline entopeduncular nucleus was studied electron microscopically. After horseradish peroxidase injections into the ventral anterior and ventral lateral nuclear complex of the thalamus, normal axon terminals synapsing with entopedunculothalamic projection neurons were classified into four types on the basis of the size and shape of synaptic vesicles in them, and types of the postsynaptic membrane differentiation. Type I and type II axon terminals were characterized by symmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. Type II axon terminals were further classified into two subtypes as to their sizes: one was small (IIa), the other large (IIb). Type III and type IV axon terminals were characterized by asymmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. To determine the origin of each type of terminal, electrolytic lesions of the caudate nucleus or the subthalamic nuclear region were combined with horseradish peroxidase injections into the thalamus or the subthalamic nuclear region. After electrolytic lesions of the caudate nucleus, degeneration was seen in type I axon terminals contacting entopedunculothalamic projection neurons. Following electrolysis or horseradish peroxidase injection into the subthalamic nuclear region, type IIa and type IV axon terminals showed degenerations or horseradish peroxidase labelling. Such terminals also synapsed with entopedunculothalamic projection neurons. It was demonstrated that these projection neurons relay the striatal or subthalamic inputs directly to the thalamus. After horseradish peroxidase injection into the thalamus, many labelled type II axon terminals were observed to synapse with entopedunculothalamic projection neurons. Type III axon terminals were left unchanged throughout these experiments. In addition, the entopeduncular neuron was observed to receive convergent inputs from both the caudate nucleus and probably the subthalamic nucleus. Axoaxonal synapses were also found to be involved in the synaptic triad. These results indicate that type I axon terminals originate from the caudate nucleus, part of type IIa and type IV axon terminals originate from the subthalamic nucleus or caudal to the subthalamic nuclear region, and part of type IIa and type IIb terminals come from intrinsic axon collaterals.

The entopeduncular nucleus: Golgi morphometrics of serially reconstructed neurons in adult cats

Computer-assisted morphometrics were used to characterize mature somatodendritic architecture in Golgi-stained neurons of the entopeduncular nucleus (EN) of the adult cat. Only one form of adult EN neuron was apparent and characterized by common features including: relatively large conical somata, long aspiny and moderately branched dendrites and discoid to spherical dendritic fields oriented randomly within the EN. These results indicate that feline EN neurons have some properties in common with large neurons of the primate medial pallidal segment.

Postnatal differentiation and growth of cat entopeduncular neurons. A transient spiny period associated with branch elongation

Qualitative and computer-assisted analyses were performed on Golgi-impregnated neurons which were serially reconstructed in 3 dimensions. Analysis of the temporal pattern of growth indicated that the initial outgrowth, formation of the adult number of dendrites and virtually all dendritic branching occurred in the prenatal period. About 40% of the total growth of the dendrites occurred in the postnatal period. Maturation was completed by 90-120 days. Analyses of the mode of dendritic growth and of the morphological changes associated with growth revealed two significant findings. First, the outward expansion of the dendritic tree was not due to the addition of new branches but resulted from the elongation of terminal and non-terminal branches. Thus, growth occurred between branch points as well as on terminal portions of dendrites. Second, a transient population of spines was found during the period of postnatal growth. These spines may play an integral role in synaptogenesis and dendritic branch elongation. We suggest that developing afferent fibers initially contact spines. As spines retract, axon terminals are brought to the shaft of the dendrites. Further, the dendrites elongated because membrane associated with spines is incorporated into the shafts of dendrites. Striopallidal projections and other afferents may provide an important trophic influence for the normal dendritic differentiation of pallidal neurons by inducing the elaboration or retraction of spines.

Microiontophoretic studies on the nature of the neurotransmitter in the subthalamo-entopeduncular pathway of the rat

The aim of the present study was to determine the identity of the neurotransmitter released by the pathway from the subthalamic nucleus to the entopeduncular nucleus in the rat, using extracellular stimulating and recording techniques and microiontophoresis. In order to avoid stimulation of passing fibers at the level of the subthalamic nucleus, (collaterals to the entopeduncular nucleus of the caudato-nigral pathway, or direct projections to the entopeduncular nucleus from the substantia nigra or nucleus tegmenti pedunculopontinus), the experiments were performed in rats bearing chronic ipsilateral lesions in order to make these pathways degenerate. Under such conditions, subthalamic nucleus stimulation suppressed the spontaneous firing of all the entopeduncular nucleus cells studied (n = 40) for 15-25 ms (mean duration +/- S.E.M.:21.88 +/- 1.57 ms). Entopeduncular nucleus cells were identified by antidromic activation from the ventral anterior thalamic nucleus (40%) or lateral habenula nucleus (68%). Low doses of iontophoretically applied GABA (60 cells) or glycine (15 cells) were inhibitory upon entopeduncular cells, while acetylcholine or carbamylcholine were poorly excitatory (18 cells), or had no effect (28 cells). The subthalamic nucleus-evoked inhibition of entopeduncular neurons was reversed by microiontophoretically applied bicuculline or picrotoxin, at doses which blocked the GABA-induced response, but not that produced by glycine or acetylcholine. With similar experiments, strychnine and atropine were ineffective. This excludes a possible role of glycine or acetylcholine in the subthalamic-evoked inhibitory response of entopeduncular cells. The present study strongly suggest that GABA is a neurotransmitter in the inhibitory subthalamo-entopeduncular pathway.

Branched output neurons of the rat subthalamic nucleus: electrophysiological study of the synaptic effects on identified cells in the two main target nuclei, the entopeduncular nucleus and the substantia nigra

The synaptic responses of entopeduncular and nigral cells to subthalamic stimulation were studied with extracellular recording techniques in rats with and without chronic lesions. Entopeduncular output cells were identified by antidromic activation from the lateral habenula, ventral anterior thalamic nucleus and tegmenti pedunculopontine nucleus. Nigral cells projecting to superior colliculus were identified by antidromic discharge. Stimulation of the subthalamic nucleus produced a short latency suppression of spontaneous activity (10-60 ms duration) of 89% of the entopeduncular cells tested in chronically lesioned rats. Of these cells, 50% were identified as projecting to lateral habenula. On the other hand, subthalamic nucleus stimulation produced a short latency excitation of 73% of the nigral cells tested (4.16 +/- 0.07 ms). Forty-eight percent of these cells projected to superior colliculus. The subthalamic fibres which terminate in entopeduncular nucleus and substantia nigra, come from the same neuronal population since the majority, if not all, rat subthalamic neurones send branched projections to both these nuclei. Therefore, the two different types of responses recorded in these nuclei are elicited by the activation of a single neuronal population. This dual effect could be easily explained if one of the responses is mediated by local interneurones. If not, the same transmitter induces the two responses. The entopeduncular nucleus and substantia nigra which are the main target nuclei of the subthalamic nucleus, are also the only known outputs of the striatum. The subthalamic efferent cells could thus modulate the activity of the entire striatal descending output. It is noteworthy that this subthalamic control is different in entopeduncular nucleus than in substantia nigra.