Ascending and descending internuclear connections of the trigeminal sensory nuclei in the cat. A study with the retrograde and anterograde horseradish peroxidase technique

The distribution of cells of origin of ascending and descending internuclear connections in the trigeminal sensory nuclei was studied by the retrograde horseradish peroxidase technique in the cat. The termination of collaterals of these ascending axons was also studied by the anterograde transport of horseradish peroxidase. Following injections of horseradish peroxidase into the ventral part of the principal sensory nucleus and the adjacent reticular formation many small neurons were labeled ipsilaterally in the whole area of the caudal portion of the nucleus interpolaris and in laminae III and IV of the nucleus caudalis. Labeled neurons were also found in laminae I and V. Injections limited to either nucleus oralis, the ventral part of the principal sensory nucleus and the medial parabrachial nucleus labeled similar types of neurons in the above regions with a topographic relationship; neurons in the dorsal part of the nuclei caudalis and interpolaris project, dorsally, to rostral portions of the trigeminal sensory nuclei while those in the ventral part of the nuclei caudalis and interpolaris project ventrally. Anterograde labeling of axons arising from the nucleus caudalis demonstrates that the axons ascend in the intranuclear bundles and the adjacent reticular formation, and give off collaterals to the nuclei interpolaris and oralis, and the ventral part of the principal sensory nucleus. Injections limited to the nucleus caudalis labeled small neurons in the rostral portion of the nucleus oralis and the caudal portion of the nucleus interpolaris. The present study suggests that these ascending and descending internuclear connections of the trigeminal sensory nuclei may modulate transmission of afferent inputs to various projection sites, such as thalamus, superior colliculus, cerebellum and spinal cord.     

Trigeminal primary afferent neurons projecting directly to the solitary nucleus in the cat: A transganglionic and retrograde horseradish peroxidase study

After applying horseradish peroxidase to peripheral branches of the trigeminal nerve in the cat, the lingual and pterygopalatine nerves were found to contain fibers which ended ipsilaterally in the rostral portions of the solitary nucleus (SN); massively in the medial and ventrolateral SN, moderately in the intermediate and interstitial SN and sparsely in the ventral SN. The rostralmost part of the SN was free from labeled terminals. After injecting the enzyme into the SN portions rostral to the area postrema, small neurons were scattered in the maxillary and mandibular divisions of the trigeminal ganglion.     

The cells of origin of the trigeminothalamic,trigeminospinal and trigeminocerebellar projections in the cat

Using the retrograde horseradish peroxidase technique, we have examined the distribution of labeled thalamic-, spinal- and cerebellar-projecting neurons in the trigeminal sensory nuclei of the cat.Injections into the nucleus ventralis posterior of the thalamus resulted in labeling of neurons in lamina I (subnucleus zonalis), the deeper part of lamina IV (the subhucleus magnocellularis) of the nucleus caudalis and in lamina V (the lateral extension of the nucleus medullae oblongatae centralis) on the contralateral side. A very large number of labeled small neurons were observed mainly in the caudal part of the nucleus interpolaris and in the ventral division of the principal sensory nucleus on the contralateral side and in the dorsal division of the principal sensory nucleus on the ipsilateral side.Injections into the known projection areas of the cerebellar cortex labeled mainly ipsilaterally the trigeminocerebellar neurons in a restricted ventrolateral area of lamina IV of the nucleus caudalis at its rostral level and in lamina V. Many labeled neurons were also observed in the nucleus interpolaris. Although the distribution overlapped with that of the trigeminothalamic neurons, the greatest majority were concentrated in its rostral part where the trigeminothalamic neurons were very small in number. In addition, labeled neurons were observed in the rostral part of the nucleus oralis and the ventralmost part of the ventral division of the principal sensory nucleus. No labeled neurons were observed in the dorsal division of the principal sensory nucleus and the mesencephalic nucleus.The trigeminospinal neurons were labeled mainly ipsilaterally following injections into the upper cervical cord. They were located in laminae I and III, the deeper part of lamina IV of the nucleus caudalis and in lamina V. Only scattered labeled neurons were found in the nucleus interpolaris. The number of labeled neurons increased in the nucleus oralis at the level of the superior olive. They tended to be distributed around or dorsal to the groups of the trigeminothalamic neurons at the caudal part of the principal sensory nucleus. No neurons of the principal sensory nucleus appeared to project to the spinal cord. Based on the large size and location, the trigeminospinal neurons could be differentiated from the other projection neurons in the nucleus oralis.The present study demonstrates that the trigeminal sensory nuclei are composed of groups of neurons with different projections, since the main aggregations are localized at different levels. However, it should be examined whether the neuronal groups, which are labeled from the different structures in similar locations, are composed of individual neurons projecting to more than one of these structures.     

Localization of neurones projecting to the hypothalamic paraventricular nucleus area of the rat: a horseradish peroxidase study

To detect the cell bodies of neurones which project to the area of the hypothalamic para-ventricular nucleus, 10–40 nl of a solution containing horseradish peroxidase and poly-l-ornithine were pressure-injected into one paraventricular nucleus of the rat. After 24 or 48 h, the enzyme remaining at the site of injection was detected by the diaminobenzidine procedure. Retrogradely transported horse-radish peroxidase was visualized by using o-dianisidine as the chromogen substrate.The extent and the intensity of labelling correlated with the apparent volume of the injection site. Labelled cell bodies were observed, ipsilateral to the injection, in the mediobasal hypothalamus, in the limbic system (lateral septum, posteromedial amygdala, ventral subiculum) and in several cell clusters in the brain stem (dorsal raphe nucleus, locus coeruleus, parabrachial nucleus, nucleus of the solitary tract and lateral reticular nucleus). In some animals, light labelling in the organum vasculosum laminae terminalis and in the subfornical organ was observed. No labelled neurones could be detected in the spinal cord.     

The origin of the reticulo-olivary projection in the rat: a retrograde horseradish peroxidase study

Electrophoretic injections of horseradish peroxidase were made in different parts of the rat inferior olivary complex using a ventral approach. Data from these injections provide anatomical evidence for the existence of a projection to the inferior olive which takes origin from reticular nuclei in the brainstem. The majority of reticulo-olivary neurons are located in the nucleus raphe obscurus and nucleus raphe pallidus. Other reticular nuclei which contribute to this projection include the nucleus reticularis ventralis and nucleus reticularis gigantocellularis. Analysis of injections confined to specific parts of the olivary complex reveals a topographical pattern in the reticulo-olivary projection. Caudal parts of the complex receive input primarily from the nucleus reticularis ventralis. As more rostral and medial parts of the inferior olive are included in the injection, there is concomitant shifting of labeled neurons to the nucleus reticularis gigantocellularis and the raphe nuclei. The reticulo-olivary neurons may serve several non-mutually exclusive roles in olivary circuitry. They may be the source of serotonin and/or substance P to the nucleus. Physiologically, they may provide the inhibitory input observed in the nucleus. Finally, some of these neurons may be the brainstem relay of the lateral funiculus and dorsolateral funiculus spino-olivo-cerebellar pathway proposed by Larson and his co-workers (J. Physiol., Lond. 203, 611-640, 641-649).     

Spinocervical neurons and dorsal horn neurons projecting to the dorsal column nuclei through the dorsolateral fascicle: a retrograde HRP study in the cat

Summary Spinocervical cells were identified by retrograde labelling from implants of HRP in the dorsolateral fascicle after destruction of the dorsal columns. They lay in laminae III and IV throughout the cord in estimated numbers of 700, 450 and 1100 in lumbosacral enlargement, upper lumbar and thoracic cord, and brachial enlargement respectively. In the cord enlargements dendritic trees were mainly or exclusively developed dorsally, with rostrocaudal exceeding mediolateral spread, and a gradient across the dorsal horn, lateral cells showing this contrast most strongly. Dendritic spread was limited at the II/III laminar boundary. Transition occurred at the edge of the enlargements to a shape with extreme rostrocaudal elongation of perikarya and of dendritic trees in upper lumbar and thoracic segments. Axons of Spinocervical cells ascended in the most dorsal part of the fascicle, distinguishable from the larger spinocerebellar bundle lying adjacent and ventral. The initial axonal course was tortuous, with local collateral branching, the axon sometimes travelling briefly in the dorsal column. In other experiments implants were made ipsilaterally in the dorsal column nuclei after destruction of the dorsal columns. Cells were few and relatively poorly labelled, for which the reasons are discussed. Some such cells, lying in lamina IV, were similar to spinocervical tract cells and may have projected to both lateral cervical and dorsal column nuclei. Others, at the extreme lateral edge of the mid-dorsal horn, were quite different, with dendrites greatly extended rostrocaudally and primary and higher order dendrites projecting ventrally from the perikaryon.     

Desending and intrinsic inputs to dorsal cochlear nucleus of cats: A horseradish peroxidase study

Horseradish peroxidase was injected unilaterally into the dorsal cochlear nucleus of adult cats in efforts to find neurons innervating the dorsal cochlear nucleus from (1) higher auditory nuclei or (2) other subdivisions of the cochlear nucleus. Following horseradish peroxidase injections and short survival periods, reactive neurons were most common in the dorsal and ventral nuclei of the lateral lemniscus and in the superior olivary complex of both sides of the brain stem. In the superior olivary complex, most neurons of the medial segment and border cells of the lateral segment reacted as did periolivary cells of the ventrolateral, dorsomedial, and preolivary areas, but not in the medial nucleus of the trapezoid body. Hilus neurons of the lateral superior olive reacted contralateral to the injection site. Although inferior colliculus neurons contained lightly stained granules bilaterally, more reactive neurons (including unusually large tripolar neurons) contained heavily stained granules in the contralateral colliculus. Intrinsic reactive neurons mainly included ipsilateral octopus cells, multipolar neurons of the nerve root regions, and stellate cells of the more rostra] anteroventral cochlear nucleus. All findings were confirmed by comparison to control animals.Our findings of specific neuronal types projecting to the cat dorsal cochlear nucleus suggest a relatively greater input from the nuclei of the lateral lemnisci of both sides than previously believed. Also, our results showed an unusually heavy input from the nearby superior olivary complex to the dorsal cochlear nucleus as well as inputs from specific cell types of the ipsilateral antero- and postero-ventral cochlear nucleus. By correlating these findings with those of other types of studies, we concluded that (1) too much emphasis has been placed upon inputs to the dorsal cochlear nucleus from the inferior colliculus relative to the descending pontine inputs and that (2) a new circuit involving the ventral cochlear nucleus, the dorsal cochlear nucleus and the medial superior olive may provide binaural information to large dorsal cochlear nucleus cells that terminate in their own unique areas of higher auditory nuclei.     

Nucleus tegmenti pedunculopontinus: Efferent connections with special reference to the basal ganglia,studied in the rat by anterograde and retrograde transport of horseradish peroxidase

The efferent connections of the brain stem nucleus tegmenti pedunculopontinus were studied in the rat using the techniques of anterograde and retrograde transport of the enzyme horseradish peroxidase, laying particular emphasis on that part of pedunculopontinus which receives direct descending projections from the basal ganglia and related nuclei. In a preliminary series of experiments horseradish peroxidase was injected into either the entopeduncular nucleus or the subthalamic nucleus and, following anterograde transport of enzyme, terminal labelling was identified in nucleus tegmenti pedunculopontinus, surrounding the brachium conjunctivum in the caudal mesencephalon.In a subsequent series of experiments, horseradish peroxidase was injected into that region of nucleus tegmenti pedunculopontinus which receives entopeduncular and subthalamic efferents and its efferent projections were studied by anterograde transport of the enzyme. The results indicate that nucleus tegmenti pedunculopontinus gives rise to widely distributed efferent projections which terminate rostrally in mesencephalic, diencephalic and telencephalic structures and caudally in the pontine tegmentum. In the mesencephalon, terminal labelling was found in the pars compacta of the ipsilateral substantia nigra and sometimes in the adjoining ventral tegmental area. Labelling was also found in the ipsilateral half of the periaqueductal grey. In the diencephalon terminal labelling occurred bilaterally in the subthalamic nucleus and ipsilaterally in the intralaminar nuclei of the thalamus. Further rostrally, terminal labelling was particularly evident in the ipsilateral pallidal complex, especially in the caudal two-thirds of the entopeduncular nucleus and the ventral half of the caudal third of the globus pallidus. Caudal to pedunculopontine injection sites dense labelling was observed in the reticular formation of the pontine tegmentum.In a final series of experiments, confirmation of apparent pedunculopontine efferent projections was sought using the retrograde transport of horseradish peroxidase. Enzyme was injected into sites possibly receiving pedunculopontine efferents and the peribrachial area of the brain stem was examined for retrograde cell labelling. In this way, pedunculopontine projections were confirmed to the globus pallidus, entopeduncular nucleus, subthalamic nucleus, substantia nigra, parafascicular nucleus and pontine reticular formation. Injections into the globus pallidus or subthalamic nucleus gave rise to retrograde cell labelling bilaterally in pedunculopontinus. In addition, retrograde transport studies alone demonstrated projections from pedunculopontinus to the cerebral cortex and to the spinal cord.It is concluded that the nucleus tegmenti pedunculopontinus has reciprocal relationships with parts of the basal ganglia and some functionally related nuclei (in particular, the pallidal complex, subthalamic nucleus and substantia nigra). These connections support the view that nucleus tegmenti pedunculopontinus is likely to be involved in the subcortical regulation and mediation of basal ganglia influences upon the lower motor system. This suggests a potential role for pedunculopontine afferent and efferent pathways in the pathophysiology of basal ganglia related disorders of movement.     

Two types of rostroventral medulla neurons projecting to the trigeminal spinal nucleus as differentiated by the response to antidromic activation and the histological location

A population of neurons in the rostroventral medulla, which send their axons to the subnucleus oralis of the trigeminal spinal nucleus of rats, could be differentiated into two types on the basis of their location and the variability of antidromic latency during repetitive stimulation at 10 Hz. Type A neurons were mostly located in the raphe magnus and were activated antidromically with a relatively long latency, which gradually increased during repetitive stimulation. By contrast, type B neurons were located in the medial portion of the gigantocellular reticular nucleus, and responded with a relatively short, stable antidromic latency.     

The relationship of dorsal root afferents to motoneuron somata and dendrites in the adult bullfrog: a light and electron microscopic study using horseradish peroxidase

The relationship of lumbar dorsal root afferents to lateral motor column motoneurons was studied using anterograde injury filling of dorsal roots and retrograde injury filling of ventral roots with horseradish peroxidase. At the light microscopic level, horseradish peroxidase labelled dorsal root axons were observed to separate into a medial division of large diameter axons which enter the dorsal funiculus and a lateral division of small diameter axons which form a compact bundle in the dorsolateral funiculus which may be homologous to the mammalian tract of Lissauer. Within the spinal gray, primary afferents terminate in two distinct regions. The more ventral of these terminal fields, which receives collaterals of primary afferent axons in the dorsal funiculus, overlaps the dendritic arborizations of the lateral motor column motoneurons. Some axons leave the ventral terminal field to enter the dorsal lateral motor column. Here they terminate on the primary dendrites and somata of lateral motor column motoneurons. At the electron microscopic level, labelled primary afferent terminals were seen to synapse upon lateral motor column motoneuron dendrites as well as upon the somata of dorsally positioned lateral motor column motoneurons. These terminals contain small spherical vesicles and occasional dense-cored vesicles. The synaptic specializations are characterized by a small amount of postsynaptic material. The lateral motor column may be divided into dorsal and ventral portions on the basis of the primary afferent distribution and this is in accord with functional, physiological and developmental data.     

Cortical relay neurons and interneurons in the N. ventralis posterolateralis of cats: a horseradish peroxidase, electron-microscopic, Golgi and immunocytochemical study

After injections of horseradish peroxidase involving the whole primary (SI) and secondary somatosensory (SII) areas of adult cats, 16-21% out of 2220 counted neurons in the nucleus ventralis posterolateralis were unlabelled. The mean areas of perikarya of these neurons varied between 111.8 +/- 32.3 microns2 and 180.8 +/- 48.6 microns2. The size of perikarya of retrogradely-labelled neuron ranged from 256.9 +/- 100.4 microns2 to 409 +/- 163 microns2. Retrogradely-labelled and unlabelled neurons were examined under light- and high-voltage electron-microscopy. Besides 'large', mainly multipolar or oval fusiform perikarya, retrogradely-labelled neurons may display perikarya of 'small' size. Both types of neurons correlate well with Golgi-impregnated cells with a tufted dendritic pattern usually identified as thalamocortical neurons. On the other hand, the size and morphology of perikarya and initial dendrites of neurons unlabelled by retrograde transport of horseradish peroxidase correlate well with that of Golgi-impregnated neurons which are markedly different from the thalamocortical neurons, have very characteristic and profuse dendritic appendages and have been identified by previous investigators as Golgi Type II neurons. In order to probe further whether these may correspond to the GABAergic interneurons proposed by previous evidence, an immunocytochemical approach was also applied at the light- and electron-microscope level, using an antiserum prepared in sheep against rat brain glutamate decarboxylase. By this method it is shown that 19-21% of neurons in the nucleus ventralis posterolateralis of adult cats are glutamate decarboxylase-positive and that the perikaryal size of these labelled neurons ranges between 134.6 +/- 44.5 microns2 and 164.4 +/- 47.3 microns2. Histogram distribution of the number and areas of the counted immunoreactive neurons closely matches that of unlabelled neurons in experiments with retrograde transport of horseradish peroxidase. The results give support to previous evidence suggesting that part of population of neurons in the nucleus ventralis posterolateralis is represented by a distinct class of neurons which are apparently GABAergic.     

The efferent vestibular system in the cat: a horseradish peroxidase and fluorescent retrograde tracers study

Neurons of the efferent vestibular system were investigated in the cat using retrograde axonal transport of horseradish peroxidase and fluorescent retrograde double labelling techniques. The number of efferent neurons was clearly higher than previously reported. A three dimensional reconstruction of the location of these neurons showed that they constitute a single group and did not give evidence of an eventual specialization based on neuron subpopulations. However, a study of cross-sectional areas of the horseradish peroxidase-labelled efferent neurons detected that the ipsilateral population contained a larger number of small neurons than the contralateral one. Double labelling by means of either 4',6-diamidino-2- phenylindol 2HCl in combination with horseradish peroxidase or Fast Blue in combination with Nuclear Yellow showed that 20% of efferent neurons project to both labyrinths. Such a high percentage raises the question of the role of these double-projecting cells and the specificity of their branching on vestibular receptors. This study expands previous work in the cat demonstrating that a much greater number of efferent neurons exists than had hitherto been assumed, among them 20% have both crossed and uncrossed projections.     

Projections from the nucleus parafascicularis prerubralis to medullary raphe nuclei and inferior olive in the rat: A horseradish peroxidase and autoradiography study

Horseradish peroxidase gel implants into either the nucleus raphe magnus (NRM) or inferior olive (IO) led to large numbers of retrogradely labeled cells in the prerubral subthalamic region in a cell group that surrounds the fasciculus retroflexus, which we suggest should be referred to as the nucleus parafascicularis prerubralis (nPfPr) to avoid the confusion of previous terminology. Autoradiographic studies, following injections of tritiated leucine into the nPfPr, confirmed projections to both medullary raphe nuclei (MRN) and IO. This common prerubral projection to IO, a well-established precerebellar nucleus, and MRN, again raises the question of whether the latter are involved in brainstem-mediated reflexes in addition to their well documented role in analgesic mechanisms.     

Organization of projections from the principal sensory trigeminal nucleus to the hypoglossal nucleus in the rat: an experimental light and electron microscopic study with axonal tracer techniques

Summary The organization of projections from the principal sensory trigeminal nucleus (PSN) to the hypoglossal nucleus (XII) in the rat was investigated at the light and electron microscopic level with retrograde and anterograde axonal tracer techniques. Microiontophoretic injection of horseradish peroxidase (HRP) into XII resulted in retrograde labeling of neurons confined to the dorsal one-third of the PSN. Labeled neurons were found bilaterally, although a clear preponderance for ipsilateral distribution was evident. Most labeled neurons were found in the medial one-third and caudal two-thirds of the PSN. Labeled neurons were large (30–50 m), round-to-pear shaped multipolar cells with dendrites oriented primarily in the mediolateral direction. At the electron microscopic level, HRP reaction product was found throughout the cytoplasm of soma and processes of PSN projection neurons. The ultrastructural characteristics of these cells included a round, centrally placed nucleus and invaginated nuclear envelope, sparse Nissl bodies, numerous free ribosomes, mitochondria, lysosomes and Golgi complexes. Three to four main stem dendrites gradually tapered from the cell body and numerous synaptic terminals impinged upon soma and dendrites of labeled PSN neurons. Microiontophoretic injection of tritiated amino acids or HRP into the dorsal one-third of the PSN resulted in moderately dense terminal labeling in XII bilaterally, although mainly ipsilaterally. Terminal labeling was found diffusely throughout all regions of XII. Fibers descended the brainstem in the dorsolateral reticular formation and entered XII ventrolaterally. At the electron microscopic level, boutons containing HRP reaction product were found to synapse on dendritic processes in XII. Labeled boutons were characterized by clear, spherical vesicles and an asymmetrical postsynaptic density. The significance of these results are discussed in relation to oro-lingual motor behavior.Abbreviations used in Figures Am Nucleus ambiguus - Ax Axon - Den Dendrite - dlRF dorsolateral reticular formation - G Golgi apparatus - IO Inferior olive - ITR Intertrigeminal region - IV Fourth ventricle - Lf Lipofuscin - LRN Lateral reticular nucleus - mRF Medial reticular formation - mPB Medial parabrachial nucleus - MV Motor trigeminal nucleus - MVN Medial vestibular nucleus - Nu Nucleus - PSN Principal sensory trigeminal nucleus - Py Pyramid - R Ribosomes - RF Reticular formation - SC Subnucleus caudalis - SI Subnucleus interpolaris - SO Subnucleus oralis - SOL Solitary tract nucleus - STR Supratrigeminal region - T Terminal - TB Trapezoid body - VII Facial nucleus - VIIn Facial nerve - X Dorsal vagal nucleus - XII Hypoglossal nucleus     

Amygdaloid projections to the motor,premotor and prefrontal areas of the cat's cerebral cortex: A topographical study using retrograde transport of horseradish peroxidase

The topographic organization of the projections from the amygdaloid complex to the frontal (motor, premotor and prefrontal) cortex has been investigated in the cat by means of the horseradish peroxidase retrograde transport technique. While most of these projections arise from the magnocellular component of the basal nucleus, some arise also from other nuclei, such as the parvocellular basal nucleus, the corticoamygdaloid transition area and the cortical nucleus. The projections from the latter nuclei are directed to the central portions of the prefrontal cortex, both laterally and medially. No clear-cut topographic segregation appeared to exist in the distribution within the magnocellular basal nucleus of the cells of origin of projections to the motor, premotor and prefrontal cortex.The gross topographic arrangement of the amygdalocortical projections seems to reciprocate, to some extent at least, the organization of corticoamygdaloid projections from high-order sensory and polymodal association areas.     

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: horseradish peroxidase labelling of identified cell types

Golgi impregnations of the posterior part of the cat's anteroventral cochlear nucleus have revealed two types of neurons, bushy cells with short bush-like dendrites and stellate cells with long, tapered processes; Nissl stains have revealed globular and multipolar cell bodies with dispersed and clumped ribosomal patterns, respectively. In the present study, we injected horseradish peroxidase into the trapezoid body. Ipsilaterally, retrograde, diffuse labelling of neurons, presumably through damaged fibers, yielded Golgi-like profiles of numerous bushy cells with typical dendrites and with thick axons projecting toward the trapezoid body. Stellate cells were almost never labelled in this way. Anterograde diffuse labelling of thick axons demonstrated calyx endings in the contralateral medial nucleus of the trapezoid body. In the electron-microscope, the perikarya of diffusely-filled bushy neurons were found to have the dispersed ribosomal pattern and the kinds of synaptic endings typical of globular cells, including large profiles of end-bulbs from cochlear nerve axons. After injections restricted to the medial trapezoid nucleus, granularly-labelled cells in the cochlear nucleus were almost completely confined to the contralateral side; Nissl counterstaining showed them to be globular cells in the posterior part of the anteroventral cochlear nucleus. After larger injections, involving surrounding regions of the superior olivary complex, granular labelling occurred throughout the ventral cochlear nucleus on both sides. There is also evidence that stellate cells in Golgi impregnations correspond to multipolar cell bodies in Nissl stains. We conclude that bushy cells typically correspond to globular cells, which receive end-bulbs from the cochlea and send thick axons to the contralateral medial trapezoid nucleus, where they form calyces on principal cells. Principal cells, in turn, are known to project to the lateral superior olive and to one of the nuclei of origin of the crossed olivo-cochlear bundle, which feeds back to the cochlea. In this circuit, correlations between synaptic patterns and particular physiological signal transfer characteristics can be suggested. These could be related to binaural intensity interactions in the lateral superior olive and to a regulatory loop involving the olivo-cochlear bundles.     

The distribution of the projection from the hippocampal formation to the nucleus accumbens in the rat: an anterograde- and retrograde-horseradish peroxidase study

We have investigated the distribution and organization of the projection from the hippocampal formation to the nucleus accumbens in the rat. In the first experiments, horseradish peroxidase was injected into the fimbria fornicis. This procedure resulted in massive anterograde labelling of fornix fibers, and enabled the hippocampo--accumbens projection to be charted in detail. Labelled fornix fibers are distributed to the entire length of the nucleus accumbens and do not spread lateral to the anterior limb of the anterior commissure, that is, the projection is limited to the medial nucleus accumbens. Terminal labelling is particularly dense in a caudal dorsomedial sector of accumbens immediately adjacent to the septum. In the second part of the study, horseradish peroxidase was microelectrophoretically injected into the nucleus accumbens, in order to confirm the findings from the anterograde experiments. When the deposit was localized within the fornix distribution field, as indicated by the first experiment, retrogradely-labelled cells were observed in the subiculum, prosubiculum, and to a lesser extent, in hippocampal field CAI. When the deposit was located outside of the fornix distribution field, no hippocampal labelling was noted. The topography of the projection, suggested by the retrograde labelling experiments, is discussed. The findings confirm earlier reports of such a projection from the hippocampal formation to the nucleus accumbens, and with the use of a relatively novel anterograde-horseradish peroxidase technique, provide a picture of fiber labelling representing the entire field of origin of the fornix system. The striatal projection field of this pathway is discussed in relation to other striatal afferents, with particular emphasis on limbic afferentation of the striatum. Finally, the so-called 'hippocampal district' of striatum is discussed with respect to its neurotransmitter composition, as well as its functional importance regarding limbic system influence on central motor mechanisms.     

The striatonigral projection and nigrotectal neurons in the rat. A correlated light and electron microscopic study demonstrating a monosynaptic striatal input to identified nigrotectal neurons using a combined degeneration and horseradish peroxidase procedure

In a light and electron microscopic study of the substantia nigra of the rat, the distribution and morphology of nigrotectal neurons and the pattern of termination of striatonigral fibres have been examined following the placement of horseradish peroxidase injections in the superior colliculus and kainic acid lesions in the dorsal striatum. In confirmation of previous findings, nigrotectal neurons which had been identified by the retrograde transport of horseradish peroxidase from the superior colliculus had mainly medium sized somata, varied from fusiform to stellate in shape and were found in mainly ventral regions of the rostral two-thirds of the substantia nigra pars reticulata. On electron microscopic examination, single and multiple (from two to six) degenerating striatonigral boutons were found in synaptic contact with the soma, proximal mainstem dendrites and small dendrites (but mainly on small dendrites) of labelled nigrotectal and unlabelled nigral neurons in the ventral region of the pars reticulata. In addition, a small number of degenerating striatonigral boutons formed axoaxonic synapses with degenerating or normal boutons which were presynaptic to nigral dendrites. Almost all of the identified striatonigral synapses were of the symmetrical type, although a few degenerating boutons established asymmetrical synaptic contacts on unlabelled dendrites. These findings provide evidence of a monosynaptic input from the dorsal striatum to nigrotectal projection neurons in the substantia nigra and thus demonstrate the existence of a bineuronal pathway from the striatum through the substantia nigra to the superior colliculus. The possible significance of the pattern of termination of striatonigral fibres in the substantia nigra is discussed with reference to the known dendritic arborization of nigral neurons.     

Study of the efferent connections of the enkephalinergic magnocellular dorsal nucleus in the guinea-pig hypothalamus using lesions, retrograde tracing and immunohistochemistry: evidence for a projection to the lateral septum

The efferents of enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus of the guinea-pig were studied using different neuroanatomical methods and indirect immunocytochemical technique. Following unilateral implantation of the fluorescent dye 4',6-diamidino-2-phenylindole in the lateral septal nucleus, retrogradely-labeled perikarya were found in the magnocellular dorsal nucleus. These labeled perikarya reacted with antiserum against enkephalin, demonstrating that enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus project to the lateral septal nucleus. In other experiments, complete bilateral lesions were produced in the magnocellular dorsal nucleus by electrocoagulation. Enkephalin-immunoreactive nerve fibers and terminals were totally depleted in the lateral septal nucleus. This confirms that septal enkephalin-immunoreactive terminals originate in the magnocellular dorsal nucleus and further suggests that this nucleus is the source of all the enkephalin-immunoreactive material found in the septum. Experiments utilizing two different fluorescent dyes, 4',6-diamidino-2-phenylindole and propidium iodide, injected in each side of the lateral septal nucleus, respectively, demonstrated that the magnocellular dorsal nucleus gives off axon collaterals to both sides of the septum, since double-labeling of individual cell bodies was detected in the nucleus. By relating this finding to the results obtained after unilateral destruction of the nucleus, which caused an incomplete loss of enkephalin- immunoreactive material in the lateral septal nucleus ipsilaterally, it is suggested that the enkephalinergic hypothalamo-septal pathway contains unbranching neurons projecting ipsilaterally and branching neurons distributing fibers ipsilaterally and contralaterally. Lesion experiments, and experiments based on the retrograde axonal transport of horseradish peroxidase after intravenous injections, demonstrated that the magnocellular dorsal nucleus contributes neither to the tubero-infundibular nor to the hypothalamo-neurohypophyseal tracts. The lateral septal nucleus receives numerous aminergic and peptidergic projections, indicating the potential importance of this region in physiological and behavioral events. In the guinea-pig, the well-demarcated enkephalinergic pathway demonstrated in this study provides a convenient model for the experimental study of the enkephalinergic innervation of the lateral septal nucleus.