The ventral striatopallidothalamic projection: II. The ventral pallidothalamic link

The projection of ventral pallidal neurons to the mediodorsal nucleus of the thalamus (MD) was examined in rats by combined retrograde transport of horseradish peroxidase (HRP) after injections in the MD and glutamate decarboxylase (GAD) immunocytochemistry at light and electron microscopic levels, with and without prior exposure of the brains to colchicine. HRP was transported to the soma of medium-sized and large ventral pallidum neurons, which along with their long, large dendrites were contacted by many glutamate decarboxylase immunoreactive synaptic boutons. The retrograde tracer positive neurons bore a remarkable resemblance to the projecting cells of the globus pallidus and entopeduncular nucleus. When colchine exposure was included in the tissue preparation, some but not all tracer positive cells also exhibited cytoplasmic GAD immunoreactivity.     

Opioid and GABA modulation of accumbens-evoked ventral pallidal activity

Summary The principle output of the nucleus accumbens innervates the ventral pallidum and rostral substantia innominata. GABA and opioid peptides are among the neurotransmitter candidates for this projection. The goal of the present experiments was to delineate further the physiology and pharmacology of the accumbens projection to the ventral pallidum. The trans-synaptic responsiveness of ventral pallidal and rostral substantia innominata neurons to electrical stimulation of the nucleus accumbens was examined concurrently with the ability of microiontophoretically applied morphine (an opioid agonist), naloxone (an opioid antagonist) and bicuculline (a GABA antagonist) to modulate evoked responses. Accumbens stimulation altered the firing rate in 60% of the 132 neurons tested. Fifty-two percent of responding neurons exhibited simple excitations or inhibitions in response to accumbens stimulation, while 48% exhibited complex response sequences with two or more evoked components. Predominant responses consisted of a short latency (<10 ms) and short duration (10 ms) excitation (51 % of responding neurons) and an inhibition with a variable, onset latency and, duration (52% of responding neurons). Evoked responses often occurred within limited areas within the ventral pallidum suggesting that activation of descending afferents can influence discrete targets within the region. A large majority (>80%) of neurons evoked by accumbens stimulation also exhibited a current-dependent and naloxone-sensitive increase in spontaneous firing to microiontophoretically applied morphine. Morphine shortened the duration of the accumbens-evoked, short latency excitation and attenuated the magnitude of the long-latency inhibition. Evoked responses in the presence of morphine were opposite to those observed with naloxone, but similar to bicuculline. Thus, opioid receptor activation may be functionally antagonistic to GABAergic neurotransmission in the ventral pallidum. The prominence of accumbens-evoked and morphine-sensitive neurons within the ventral pallidum corroborates the density of accumbens and opioid input to this brain region, and demonstrates that opioids serve as an important influence on neuronal activity and information processing in the ventral-striatopallidal pathway.     

Electrophysiological and morphological studies on thalamic neurons receiving entopedunculo- and cerebello-thalamic projections in the cat

Electrophysiological studies on the entopedunculo- and cerebello-thalamic projections were performed by intracellular recordings in the thalamic VA, VL and VM nuclei of cats under sodium pentobarbital anesthesia. Identification of the thalamic neurons was performed electrophysiologically by antidromic activation on stimulation of the precruciate cortex (areas 4 and 6) and the caudate nucleus, and morphologically by intracellular staining with HRP through recording microelectrodes.One hundred and sixty-three neurons were collected in the VA, VL and VM nuclei. In 79 neurons penetrated in the medial and ventral parts of the VA and VL nuclei, stimulation of the entopeduncular nucleus induced monosynaptic IPSPs (latency of 1.1–3.5 ms, mean 2.07 ms). Sixteen neurons were identified as thalamo-cortical relay neurons and 3 were activated only orthodromically by precruciate stimulation. Seventy-eight neurons located dorsolaterally to the entopeduncular-influenced neurons received only cerebellar EPSPs. Only 6 neurons showed convergence of entopeduncular and cerebellar inputs. They were scattered around the border between the entopeduncular and cerebellar projection areas.Sixteen neurons could be stained intracellularly by HRP injection. From the pattern of dendritic arborization, two types of neurons can be distinguished: neurons whose dendrites spread radially in all directions and neurons whose dendrites extend mainly along the long axis of the soma for a long distance in the frontal plane, respectively. The former are relay cells to the cerebral cortex or the caudate nucleus (i.e. projection neurons) and the latter appear to be interneurons in the thalamus.     

Complementary distribution of glutamatergic cerebellar and GABAergic basal ganglia afferents to the rat motor thalamic nuclei

Motor thalamic nuclei, ventral anterior (VA), ventral lateral (VL) and ventral medial (VM) nuclei, receive massive glutamatergic and GABAergic afferents from the cerebellum and basal ganglia, respectively. In the present study, these afferents were characterized with immunoreactivities for glutamic acid decarboxylase of 67 kDa (GAD67) and vesicular glutamate transporter (VGluT)2, and examined by combining immunocytochemistry with the anterograde axonal labeling and neuronal depletion methods in the rat brain. VGluT2 immunoreactivity was intense in the caudodorsal portion of the VA-VL, whereas GAD67 immunoreactivity was abundant in the VM and rostroventral portion of the VA-VL. The rostroventral VA-VL and VM contained two types of GAD67-immunopositive varicosities (large and small), but the caudodorsal VA-VL comprised small ones alone. VGluT2-immunopositive varicosities were much larger in the caudodorsal VA-VL than those in the rostroventral VA-VL and VM. When anterograde tracers were injected into the basal ganglia output nuclei, the vast majority of labeled axon varicosities were large and distributed in the rostroventral VA-VL and VM, showing immunoreactivity for GAD67, but not for VGluT2. Only the large GAD67-immunopositive varicosities were mostly abolished by kainic acid depletion of substantia nigra neurons. In contrast, large to giant axon varicosities derived from the deep cerebellar nuclei were distributed mostly in the caudodorsal VA-VL, displaying VGluT2 immunoreactivity. The VGluT2-positive varicosities disappeared from the core portion of the caudodorsal VA-VL by depletion of cerebellar nucleus neurons. Thus, complementary distributions of large VGluT2- and GAD67-positive terminals in the motor thalamic nuclei are considered to reflect glutamatergic cerebellar and GABAergic basal ganglia afferents, respectively.     

Distribution of cerebellothalamic neurons projecting to the ventral nuclei of the thalamus: An HRP study in the cat

Distribution of cerebellothalamic neurons projecting to the ventral nuclei of the thalamus was examined in the cat, using the horseradish peroxidase (HRP) method. After injections of HRP within the lateral or ventrolateral portions of the ventroanterior and ventrolateral nuclear complex of the thalamus (VA-VL), neurons labeled retrogradely with HRP were seen contralaterally in the cerebellar nuclei; many of them were situated in the nucleus interpositus anterior and nucleus interpositus posterior, and a moderate number of them were located in the nucleus lateralis. Labeled neurons in the nucleus interpositus posterior were observed mainly in the medial and ventral portions of the nucleus. On the side ipsilateral to the injections, a few labeled neurons were seen in the nucleus interpositus anterior, nucleus interpositus posterior, and nucleus lateralis. Virtually no labeled neurons were found in the nucleus medialis of the cerebellum. After HRP injections into the medial or dorsomedial portions of the VA-VL, many labeled neurons were found contralaterally in the ventral and ventrolateral portions of the nucleus interpositus posterior, as well as in the nucleus lateralis, especially in its ventral and lateral portions. On the side ipsilateral to the injections, labeled neurons in the nucleus lateralis and nucleus interpositus posterior were small in number. In the nucleus medialis only a few labeled neurons were found bilaterally in the caudal levels of the nucleus. After HRP injections centered on the ventromedial nucleus of the thalamus, many labeled neurons occurred bilaterally in the caudal portions of the nucleus medialis, with a slight contralateral preponderance, and contralaterally in the lateral and ventral portions of the nucleus lateralis. A few labeled neurons were also seen contralaterally in the ventrolateral and lateral portions of the nucleus interpositus posterior, and ipsilaterally in the nucleus lateralis.     

Thalamocortical neurons projecting to superficial and to deep layers in parietal, frontal and prefrontal regions in the cat

Superficial HRP applications and deep injections were performed on symmetric foci of the same cortical region in primary somatosensory, motor and prefrontal areas. Retrogradely labeled neurons were analyzed for body size distribution and intensity of labeling. Neurons in the ventral posterior and ventralis lateralis nuclei projecting to layer I of the somatic sensory and motor cortices are smaller in size and less intensely labeled than neurons projecting to deeper layers. Neurons projecting to the prefrontal cortex from the ventromedialis and mediodorsalis nuclei had the same soma size and topography irrespectiveof the cortical layer affected by the HRP, although they varied in number and in intensity of labeling.     

The mode of cerebellar activation of neurons in the cat motor cortex: an intracellular HRP study

Intracellular recordings and morphological identification of neurons by using intracellular HRP staining were performed in the cat motor cortex. By cerebellar stimulation, stellate cells in layers II–III, pyramidal cells in layer III and fast pyramidal tract neurons (PTNs) were activated with short latency and fast rising EPSPs, while pyramidal cells in layer II and slow PTNs showed a wide range of latency and slow rising EPSPs. This difference may be related to activation through the deep and superficial thalamocortical projections.     

Mu and kappa opioid agonists modulate ventral tegmental area input to the ventral pallidum

The ventral pallidum (VP) is situated at the convergence of midbrain dopamine and accumbal opioid efferent projections. Using in vivo electrophysiological procedures in chloral hydrate-anaesthetized rats, we examined whether discrete application of mu- [D-Ala2,N-Me-Phe4,Gly-ol5 (DAMGO)] or kappa- (U50488) opioid receptor agonists could alter VP responses to electrical stimulation of ventral tegmental area. Rate suppressions occurred frequently following ventral tegmental area stimulation. Consistent with an involvement of dopamine in this effect, none of the 12 spontaneously active ventral pallidal neurons recorded in rats that had monoamines depleted by reserpine responded to electrical stimulation of ventral tegmental area. Moreover, in intact rats, the dopamine antagonist flupenthixol attenuated evoked suppression in 100% of the neurons tested; however, the GABAA antagonist bicuculline was able to slightly attenuate the response in 50% of the neurons tested. These observations concur with our previous studies in indicating that ventral tegmental area stimulation releases dopamine (and sometimes GABA) onto ventral pallidal neurons. Both DAMGO and U50488 decreased the inhibitory effects of ventral tegmental area stimulation. These effects on the endogenously released transmitter differed from those seen with exogenously applied dopamine, for DAMGO did not alter the efficacy or potency of microiontophoretically applied dopamine. Taken together, these observations suggest that the interaction between DAMGO and dopamine does not occur at a site that is immediately postsynaptic to the dopaminergic input within the VP, but rather that opioid modulation involves mechanisms governing presynaptically released dopamine. These modulatory processes would enable ventral pallidal opioids to gate the influence of ventral tegmental area dopamine transmission on limbic system outputs at the level of the VP.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.     

Prefrontal cortical efferents in the rat synapse on unlabeled neuronal targets of catecholamine terminals in the nucleus accumbens septi and on dopamine neurons in the ventral tegmental area.

Physiological and pharmacological studies indicate that descending projections from the prefrontal cortex modulate dopaminergic transmission in the nucleus accumbens septi and ventral tegmental area. We investigated the ultrastructural bases for these interactions in rat by examining the synaptic associations between prefrontal cortical terminals labeled with anterograde markers (lesion-induced degeneration or transport of Phaseolus vulgaris leucoagglutinin; PHA-L) and neuronal processes containing immunoreactivity for the catecholamine synthesizing enzyme, tyrosine hydroxylase. Prefrontal cortical terminals in the nucleus accumbens and ventral tegmental area contained clear, round vesicles and formed primarily asymmetric synapses on spines or small dendrites. In the ventral tegmental area, these terminals also formed asymmetric synapses on large dendrites and a few symmetric axodendritic synapses. In the nucleus accumbens septi, degenerating prefrontal cortical terminals synapsed on spiny dendrites which received convergent input from terminals containing peroxidase immunoreactivity for tyrosine hydroxylase, or from unlabeled terminals. In single sections, some tyrosine hydroxylase-labeled terminals formed thin and punctate symmetric synapses with dendritic shafts, or the heads and necks of spines. Close appositions, but not axo-axonic synapses, were frequently observed between degenerating prefrontal cortical afferents and tyrosine hydroxylase-labeled or unlabeled terminals. In the ventral tegmental area, prefrontal cortical terminals labeled with immunoperoxidase for PHA-L were in synaptic contact with dendrites containing immunogold reaction product for tyrosine hydroxylase, or with unlabeled dendrites. These results suggest that: (1) catecholaminergic (mainly dopaminergic) and prefrontal cortical terminals in the nucleus accumbens septi dually synapse on common spiny neurons; and (2) dopaminergic neurons in the ventral tegmental area receive monosynaptic input from prefrontal cortical afferents. This study provides the first ultrastructural basis for multiple sites of cellular interaction between prefrontal cortical efferents and mesolimbic dopaminergic neurons.     

Response properties and morphological identification of neurons in the cat motor cortex

Intracellular recordings and morphological identification of neurons using intracellular HRP staining were performed in the cat motor cortex. By thalamic ventrolateral (VL) or cerebellar nucleus stimulation, pyramidal cells in layer III, fast pyramidal tract neurons (PTNs) and stellate cells in layers II and III were activated with short latency and fast rising EPSPs, while pyramidal cells in layer II and slow PTNs showed longer latency and slow rising EPSPs. This difference may be related to activation through the deep and superficial thalamocortical projections. Although pyramidal cells in layer VI did not respond orthodromically to VL or cerebellar stimulation, some of them proved to receive the recurrent action of PTNs because of the response to stimulation of the cerebral peduncle (CP). One aspinous stellate cell in layer III was activated by CP as well as VL stimulation. This cell was supposed to be an inhibitory interneuron responsible for both recurrent and VL-evoked inhibition.     

The organization of afferents to the cerebellar cortex in the cat: Projections from the deep cerebellar nuclei

The topography of the cerebellar nucleo-cortical projection was investigated in the cat by experiments employing the horseradish peroxidase (HRP) technique or by combined HRP-autoradiographic methods. The results of the HRP studies extend previous findings showing that neurons in the deep nuclei project to the cerebellar cortex in an orderly way. Thus, it appears that the cortex of the vermis-proper receives projections fron neurons located predominately in the fastigial nucleus. Intermediate and lateral zones of mid-vermal cerebellar cortex are projected on by neurons located in the interposed and dentate nuclei. Crus II receives input from neurons located predominately in the dentate nucleus, while the paramedian lobule is projected on by neurons located in a large postero-dorsal sector of the interposed nucleus and in a smaller medial strip of the dentate nucleus. Neurons in the ventral part of the dentate nucleus and the lateral part of the interposed nucleus send fibers to the paraflocculus. The nucleo-cortical pathway to the flocculus and nodulus arises largely from a population of neurons located in a ventral region stretching from the medial border of the dentate nucleus to the lateral border of the fastigial nucleus. The results of experiments using the combined HRP-autoradiographic method show that clusters of neurons in the deep cerebellar nuclei project back to the cerebellar cortical areas from which they receive input, establishing a fairly precise feedback loop between the cerebellar cortex and deep nuclei.     

Maturation of rat visual cortex. III. Postnatal morphogenesis and synaptogenesis of local circuit neurons

The postnatal development of 3 types of local circuit neurons in rat visual cortex was examined in Golgi and electron microscopic preparations. During the first postnatal week, smooth and sparsely spinous stellate, bitufted and bipolar neurons were identified in Golgi material by their characteristic dendritic arborizations. Morphological differentiation begins during this week, as each neuron sprouts dendrites which extend, branch and produce spines, and ends by day 21. This differentiation was traced by quantifying the somatic area and number of primary dendrites on stellate, bitufted and bipolar neurons in layer II/III or layer V. Neurons in deep cortex differentiate earlier than those in superficial laminae. On day 3, axons are evident as short, straight processes, however, by day 6, many axons have branches and varicosities. The increase in the complexity of the axonal trees continues during the second and third postnatal weeks. Since the axons of stellate and bitufted neurons form synapses with the somata of pyramidal neurons, an index of the synaptogenesis of these neurons was traced by counting the numbers of synapses on the somata of pyramidal neurons. The mean number of axosomatic synapses increases steadily from day 3 to day 30. Layer V pyramidal neurons form axosomatic synapses before pyramidal neurons in layer II/III. In conclusion, the morphology of local circuit neurons develops during the period after they migrate into cortex. The principle that cortical local circuit neurons develop after projection neurons only applies for the synaptogenesis of the axon, but not for the maturation of the cell body and dendrites.     

Morphological features of layer V pyramidal neurons in the cat parietal cortex: an intracellular HRP study

Layer V pyramidal neurons in the cat parietal cortex (areas 5 and 7) were investigated with intracellular HRP staining. Antidromic responses were recorded intracellularly as well as extracellularly with pontine stimulation under Nembutal anesthesia. The relationship between the latency of antidromic responses and the morphology of HRP-stained neurons was analyzed. A total of 65 neurons were stained with HRP, and sixteen of these neurons were activated antidromically with pontine stimulation. Two distinct groups of layer V pyramidal neurons were detected morphologically by intracellular HRP staining; i.e., one (F type) consisted of neurons with relatively large somata (58.4 +/- 8.1 micron X 24.5 +/- 5.1 micron, N = 11) and aspiny or sparsely spinous apical dendrites, and the other (S type) consisted of neurons with smaller somata (44.6 +/- 7.6 micron X 19.3 +/- 3.9 micron, N = 22) and richly spinous apical dendrites. These two groups showed different electrophysiological properties; i.e., the former responded antidromically to pontine stimulation at a latency shorter than 1.5 ms (namely, with a conduction velocity faster than 18 m/second) and the latter responded at a latency longer than 1.5 ms. The two neuronal types in the parietal cortex corresponded respectively to fast and slow pyramidal tract neurons (PTNs) investigated in the sensorimotor cortex. Although their morphological features were almost similar to those of PTNs, the branching pattern of apical dendrites of the F-type pyramidal neuron seemed to be different from that of fast PTNs. In the parietal cortex, apical dendrites of F-type neurons showed rather frequent branching in layer I. This was similar to the pattern of branching in slow PTNs. Such a characteristic branching pattern suggested that, in the cat parietal cortex, layer V pyramidal neurons of both types are adapted to receive cerebellar inputs through the ventroanterior (VA) thalamic nucleus to the superficial cortical layers.     

Glutamic acid decarboxylase-immunoreactive neurons and horseradish peroxidase-labeled projection neurons in the ventral posterior nucleus of the cat and Galago senegalensis

Immunocytochemical methods were used to identify neurons in the ventral posterior nucleus of the cat and Galago senegalensis that contain glutamic acid decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter, GABA. In both species GAD-immunoreactive neurons make up about 30% of the total neurons in the ventral posterior nucleus and form a distinct class of small cells. After cortical injections of horseradish peroxidase (HRP), GAD-immunoreactive cells are not labeled with HRP and may, therefore, be GABAergic local circuit neurons. Comparison of the dendritic morphology of GAD-immunoreactive neurons with that of HRP-filled projection neurons reveals that the morphology of the GAD-containing neurons is distinct and, in particular, that the GAD-immunoreactive neurons display fewer primary dendrites. The relay neurons, in turn, can be divided into classes based on dendritic morphology and cell body size.     

Morphology and physiology of single neurons in the medial interlaminar nucleus of the cat's lateral geniculate nucleus

Physiological studies have shown that the cat's retinogeniculocortical system is comprised of at least three parallel and independent pathways, the W-, X-, and Y-cell pathways. The morphological correlates of the constituent W-, X-, and Y-cells have been determined both in the retina and in the A and C laminae of the lateral geniculate nucleus. The aim of this study was to extend these structure/function relationships to neurons in laminae 1 and 2 of the medial interlaminar nucleus (MIN), which is a division of the cat's dorsal lateral geniculate nucleus. We used intracellular injection of horseradish peroxidase (HRP) into individual, physiologically identified MIN neurons. Since this procedure may yield an unrepresentative sample of MIN neurons, two controls were performed. First Nissl staining showed that the soma sizes of intracellularly labeled cells were representative of those of all MIN cells. Second, retrograde labeling following HRP injections into the optic radiations or specific visual cortical areas showed that the intracellularly labeled MIN cells were representative of MIN relay neurons. Many of the retrogradely labeled cells were so well filled that their entire dendritic arbors were revealed. Of 70 MIN neurons recorded physiologically, 22 were injected with HRP and successfully recovered. We also completely labeled the somata and dendrites of 114 MIN neurons from HRP injections into the optic radiations and retrogradely labeled 165 MIN neurons by injection of HRP into visual cortical areas. Our sample of intracellularly injected neurons, which were all Y-cells, were morphologically representative of all MIN relay cells. We thus conclude that laminae 1 and 2 of the MIN contain a nearly homogeneous population of Y-cells with properties essentially identical to those of Y-cells in the A and C laminae of the lateral geniculate nucleus. When viewed in the coronal plane, MIN projection neurons typically exhibited oval or elongated somata. In the medial and ventral parts of the MIN, these somata were smaller and more flattened. MIN soma sizes extended over the full range of those seen in the A laminae. Dendritic arbors of most MIN relay neurons radiated in a fairly spherical fashion. In the medial and ventral parts of the MIN, however, dendrites were oriented in a more bipolar fashion, but intermediate forms between spherical and bipolar arbors were also common. Dendrites of MIN projection neurons were typically smooth; most primary dendrites were straight, but secondary dendrites were more variable in structure.(ABSTRACT TRUNCATED AT 400 WORDS)     

The cytoarchitecture and some efferent projections of the centromedian-parafascicular complex in the lesser bushbaby (Galago senegalensis)

The morphology of neurons in the centromedian (CM) and parafascicular (PF) nuclei in the lesser bushbaby (Galago senegalensis) is described in coronal and horizontal brain sections using Golgi-, horseradish peroxidase (HRP)-, and Nissl-staining procedures. The CM contains two types of cells referred to as principal neurons and Golgi type II (like) neurons. Cell bodies of principal neurons are relatively large in cross-sectional area (mean = 130.42 micron2), round to spindle in shape, support short somatic spines, and give rise to three to five primary dendrites. The dendrites branch in a "radiate" pattern and possess numerous appendages consisting of narrow, stalk-supported swellings. The presumed axons of these cells are impregnated only in their initial segments. On the basis of the similarity of principal neuron soma shapes and cross-sectional areas with those of HRP-reactive somata following cortical HRP implantation, it is concluded that at least some of the principal neurons in Galago CM project to somatic sensory-motor cortex. Golgi type II (like) neurons have small (mean = 79.43 micron2), round somata which support several spines and give rise to three to four small-diameter dendrites. The dendrites are infrequently branched, sinuous in their courses, and give rise to complex appendages and beaded processes. However, the axons of these cells could not be seen to ramify in the immediate vicinity of the dendritic field or soma, and there is considerable overlap in the cross-sectional areas of Golgi type II (like) neurons seen in Golgi preparations and HRP-stained cells following cortical implant of HRP pellets. Consequently, although Golgi type II (like) cells have traits characteristic of classically described intrinsic neurons, a cortical projection of these cells cannot be ruled out by the present study. The parafascicular nucleus contains two groups of large, radiate cells characterized by the presence or absence of somatic spines. Cells with somatic spines also contain numerous appendages on the dendrites. Cells without somatic spines support only a few, isolated, short dendritic appendages. Numerous small cell-bodied neurons are present in Nissl-stained sections of PF; however, cells which resemble Golgi type II neurons were not observed in the PF in the present Golgi-impregnated material. In contrast to the CM, the large cell-bodied neurons in PF were not found to project to somatic sensory-motor cortex in Galago.     

Modulation of responses of feline ventral spinocerebellar tract neurons by monoamines.

Ventral spinocerebellar tract neurons located in laminae V-VII of cat lumbar spinal cord were tested for the effects of ionophoretically applied monoamines and receptor selective agonists. Extracellularly recorded responses, monosynaptically evoked by group I afferents in a muscle nerve, were compared before, during, and after ionophoresis. They were analyzed with respect to changes in the number of evoked spikes and in the latency. Both serotonin (5-HT) and noradrenaline (NA) were found to facilitate responses of all neurons tested. Ionophoresis of three serotonin subtype receptor agonists (5-carboxamidotryptamine maleate, 5 methoxytryptamine HCl, and alpha-methyl 5-hydroxytryptamine) and of two NA receptor agonists (phenylephrine and isoproterenol) likewise had a facilitatory effect. However, three other 5-HT receptor agonists (8-hydroxy-dipropylaminotetraline hydrobromide), 2-methyl 5-hydroxytryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl and two NA receptor agonists (tizanidine and clonidine) had the opposite effect because they depressed responses of the tested neurons. These results show that information forwarded by means of the ventral spinocerebellar tract may be modulated by monoamines and that several receptor subtypes, located pre- or postsynaptically, may be involved. The results also demonstrate that transmission by means of group I muscle afferents may not only be facilitated by monoamines but also depressed by selective receptor subtype activation.     

The pretectal nucleus lentiformis mesencephali of Rana pipiens

The pretectal nucleus lentiformis mesencephali (nLM) of Rana pipiens was investigated with autoradiographic, horseradish peroxidase (HRP), and Golgi techniques. Retinal afferents to nLM originate primarily from the central retina. The primary projection is contralateral with a small ipsilateral component. Following optic nerve transection and HRP impregnation, contralateral retinal afferents show a restricted, dense core of HRP label in the superficial portion of the nucleus with sparser HRP label in the surround. Ipsilateral retinal afferents arborize throughout nLM, except in the dense-core region. Additional afferents to nLM originate from the ipsilateral tectum, the nucleus rotundus, the mesencephalic pretectal gray, the contralateral nLM, and the nucleus of the basal optic root. Afferents from the accessory optic system arborize only in the dense-core region, following HRP injections into the nucleus of the basal optic root, while afferents from the mesencephalic pretectal gray arborize in all parts of nLM except the dense core. Afferents from the tectum and anterior thalamus appear to arborize throughout the nucleus without discernible pattern. The lamination of afferent terminals in nLM was correlated with Nissl-stained cytoarchitectural material in which the majority of large neurons cluster around the dense core of nLM. Three types of neurons occur in nLM: large neurons (25-micron dia.), fusiform neurons (12.5-micron dia.), and stellate neurons (10-micron dia.). Additionally, two cell groups outside nLM which send dendrites into the nucleus were observed: cells of the posterior lateral nucleus and cells of the posterior thalamic pretectal gray. Both large and fusiform neurons project to the deep layers of the optic tectum as well as to the ventral rhombencephalon superficial to the abducens nucleus. While a small number of fusiform neurons project to the nucleus of the basal optic root, the stellate neurons appear to be intrinsic to nLM. The anuran nLM strongly resembles the nucleus of the optic tract in mammals in terms of the site of origin of its retinal afferents, lamination of afferent terminations, its central connections, and its demonstrated involvement in horizontal optokinetic nystagmus.     

Striatal efferents preferentially innervate neurons in the ventral pallidum containing GABAA receptor α 1 subunit-like immunoreactivity

The γ-aminobutyric acid-A receptor consists of several subunits. In this immunohistochemical study we investigated the regional distribution of the α1 and α2 subunits with subunit-specific antibodies in the ventral pallidum, and compared the staining patterns to those of substance P (SP). α1 subunit antigenic sites were found to be localized to pallidal neurons, varicosities, and varicose fibers. α1 immunopositive fibers mainly appeared “tubulus-like” due to the intense staining of the membranes of the long pallidal dendrites. Double labelling of α1 subunit and substance P revealed that α1 subunit-like immunoreactive (IR) dendrites and somata of the pallidal neurons were often invested by SP-IR striatal efferents. Subcellularly, the dendritic and somatic membranes of pallidal neurons were strongly immunopositive for the α1 subunit, whereas only a few axon terminals exhibited α1-IR. α2-IR was restricted to a low number of ventral pallidal neurons. The distributional patterns obtained for the α1 and α2 subunits suggest that striatal efferent neurons directly influence pallidal neurons displaying a distinct GABA_A subunit composition, which may be of pharmacological importance since the α1βxγ2-subunits containing receptors have mainly a benzodiazepine type I pharmacology. © 1996 Wiley-Liss, Inc.