Efferent connections of the habenular nuclei in the rat.

The efferent connections of the medial (MHb) and lateral (LHb) habenular nuclei in the rat were demonstrated autoradiographically following small injections of tritiated amino acids localized within various parts of the habenular complex. Comparison of individual cases led to the following conclusions. MHb efferents form the core portion of the fasciculus retroflexus and pass to the interpeduncular nucleus (IP) in which they terminate in a topographic pattern that refects 90 degrees rotations such that dorsal MHb projects to lateral IP, medial MHb to ventral, and lateral MHb to dorsal IP. Most MHb fibers cross in the interpeduncular necleus in the "figure 8" pattern described by Cajal, and terminate throughout the width of IP with only moderate preference for the ipsilateral side. However, the most dorsal part of MHb projects almost exclusively to the most lateral IP zone in a cluster pattern that is particularly dense on the ipsilateral side. The MHb appears to have no other significant projections, but very sparse MHb fibers may pass to the supracommissural septum and to the median raphe nucleus. Except for some fibers passing ventrally into the mediodorsal nucleus, all of the LHb efferents enter the fasciculus retroflexus and compose the mantle portion of the bundle. No LHb projections follow the stria medullaris. In the ventral tegmental area LHb efferents become organized into groups that disperse in several directions: (a) Rostrally directed fibers follow the medial forebrain bundle to the lateral, posterior and dorsomedial hypothalamic nuclei, ventromedial thalamic nucleus, lateral preoptic area, substantia innominata and ventrolateral septum. (b) Fibers turning laterally distribute to the substantia nigra, pars compacta (SNC); a small number continue through SNC to adjacent tegmentum. (c) The largest contingent of LHb efferents passes dorsocaudally into paramedian midbrain regions including median and dorsal raphe nuclei, and to adjacent tegmental reticular formation. Sparse addition LHb projections pass to the pretectal area, superior colliculus, nucleus reticularis tegmenti pontis, parabrachial nuclei and locus coeruleus. No LHb projections appear to involve the interpeduncular nucleus. All of these connections are in varying degree bilateral, with decussations in the supramammillary region, ventral tegmental area and median raphe nucleus. On the basis of differential afferent and efferent connections, the LHb can be divided into a medial (M-LHb) and a lateral (L-LHb) portion. The M-LHb, receiving most of its afferents from limbic regions and only few from globus pallidus, projects mainly to the raphe nuclei, while L-LHb, afferented mainly by globus pallidus and in lesser degree by the limbic forebrain, projects predominantly to a large region of reticular formation alongside the median raphe nucleus. Both M-LHb and L-LHb, however, project to SNC. The reported data are discussed in correlation with recent histochemical findings.     

Efferent connections of the substantia innominata in the rat

The efferent connections of the substantia innominata (SI) were investigated employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The projections of the SI largely reciprocate the afferent connections described by Grove (J. Comp. Neurol. 277:315-346, '88) and thus further distinguish a dorsal and a ventral division in the SI. Efferents from both the dorsal and ventral divisions of the SI descend as far caudal as the ventral tegmental area, substantia nigra, and peripeduncular area, but projections to pontine and medullary structures appear to originate mainly from the dorsal SI. Within the amygdala and hypothalamus, which receive widespread innervation from the SI, the dorsal SI projects preferentially to the lateral part of the bed nucleus of the stria terminalis; the lateral, basolateral, and central nuclei of the amygdala; the lateral preoptic area; paraventricular nucleus of the hypothalamus; and certain parts of the lateral hypothalamus, prominently including the perifornical and caudolateral zones described previously. The ventral SI projects more heavily to the medial part of the bed nucleus of the stria terminalis; the anterior amygdaloid area; a ventromedial amygdaloid region that includes but is not limited to the medial nucleus; the lateral and medial preoptic areas; and the anterior hypothalamus. Modest projections reach the lateral hypothalamus, with at least a slight preference for the medial part of the region, and the ventromedial and arcuate hypothalamic nuclei. Both SI divisions appear to innervate the dorsomedial and posterior hypothalamus and the supramammillary region. In the thalamus, the subparafascicular, gustatory, and midline nuclei receive a light innervation from the SI, which projects more densely to the medial part of the mediodorsal nucleus and the reticular nucleus. Cortical efferents from at least the midrostrocaudal part of the SI are distributed primarily in piriform, infralimbic, prelimbic, anterior cingulate, granular and agranular insular, perirhinal, and entorhinal cortices as well as in the main and accessory olfactory bulbs. The cells of origin for many projections arising from the SI were identified as cholinergic or noncholinergic by combining the retrograde transport of WGA-HRP with histochemical and immunohistochemical procedures to demonstrate acetylcholinesterase activity or choline acetyltransferase immunoreactivity. Most of the descending efferents of the SI appear to arise primarily or exclusively from noncholinergic cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Efferent connections of the ventral medulla oblongata in the rat

The efferent connections of the ventral medulla oblongata have been analyzed in the rat using the anterograde autoradiographic method and the HRP technique. Fibers originating from the nucleus interfascicularis hypoglossi (B1 serotonergic cell group) and nucleus reticularis gigantocellularis, pars a (B3 serotonergic cell group) innervate the intermediolateral cell column, ventral horn and intermediate gray matter of the spinal cord. Some fibers innervate the hypoglossal, dorsal motor vagal, and medial solitary nuclei. Ascending fibers project through the medullary and pontine reticular formation, providing inputs to the Kölliker-Fuse, lateral parabrachial, laterodorsal tegmental, subcoeruleus and locus coeruleus nuclei. In the midbrain, the fibers ascend in the central tegmental field and then divide into several fiber bundles. Some course medially to innervate the central gray matter. Others diverge laterally to innervate the external nucleus of the inferior colliculus and cuneiform nucleus as well as the deep layers of the contralateral superior colliculus. Still others course dorsally through the ventral pretectal region to reach the thalamus (laterodorsal, paraventri-cular, paracentral, and centrolateral thalamic nuclei). The remaining fibers innervate the hypothalamus (dorsal hypothalamic area, paraventricular nucleus, perifornical area, supraoptic nucleus, retrochiasmatic area, and median eminence). Some of these continue through the lateral preoptic region, shift medially as they course through the area of the nucleus of the diagonal band, septofimbrial nucleus, and medial septum, and arch around the genu of the corpus callosum to innervate the hippocampal formation.     

Efferent connections of the subthalamic region in the rat. II. The zona incerta

The efferent connections of the zona incerta (ZI) were studied experimentally in the rat by the aid of the autoradiographic tracer technique.Small microelectrophoretic injections of tritiated proline and leucine practically confined to the ZI were found to label a widespread, predominantly ipsilateral system of descending and ascending fibers distributed to reticular structures of the brain stem (mesencephalic reticular formation, nucleus tegmenti pedunculopontinus pars compacta, parabrachial area, nuclei reticularis pontis oralis, pontis caudalis, gigantocellularis and medullae oblongatae, pars ventralis), precerebellar nuclei (nucleus reticularis tegmenti pontis, pontine nuclei and inferior olivary complex), the middle and deep layers of the superior colliculus, the pretectum (anterior, posterior and medial pretectal nuclei), perioculomotor nuclei (interstitial nucleus of Cajal, nucleus of Darkschewitsch and nuclei of the posterior commissure), the parvocellular portion of the red nucleus, the central gray substance, the nucleus tegmenti dorsalis lateralis, the ventral horn of the cervical spinal cord, non-specific thalamic nuclei (parafascicular, centralis medius, paracentralis centralis lateralis and ventromedial thalamic nuclei, nucleus reuniens), basal ganglia (entopeduncular nucleus and globus pallidus), hypothalamic structures (posterior hypothalamic nucleus, dorsal and lateral hypothalamic areas), and a subpallidal district of the substantia innominata. Isotope injections centered in Forel's field H₁ resulted in the labeling of a similar set of projections. Some of the possible functional correlates of these connections are briefly discussed.     

An autoradiographic study of the organization of the efferent connections of the hippocampal formation in the rat.

The efferent connections of the hippocampal formation of the rat have been re-examined autoradiographically following the injection of small quantities of 3H-amino acids (usually 3H-proline) into different parts of Ammon's horn and the adjoining structures. The findings indicate quite clearly that each component of the hippocampal formation has a distinctive pattern of efferent connections and that each component of the fornix system arises from a specific subdivision of the hippocampus or the adjoining cortical fields. Thus, the precommissural fornix has been found to originate solely in fields CA1-3 of the hippocampus proper and from the subiculum; the projection to the anterior nuclear complex of the thalamus arises more posteriorly in the pre- and/or parasubiculum and the postsubicular area; the projection to the mammillary complex which comprises a major part of the descending columns of the fornix has its origin in the dorsal subiculum and the pre- and/or parasubiculum; and finally, the medial cortico-hypothalamic tract arises from the ventral subiculum. The lateral septal nuclei (and the adjoining parts of the posterior septal complex) constitute the only subcortical projection field of the pyramidal cells in fields CA1-3 of Ammon's horn. There is a rostral extension of the pre-commissural fornix to the bed nucleus of the stria terminalis, the nucleus accumbens, the medial and posterior parts of the anterior olfactory nucleus, the taenia tecta, and the infralimbic area, which appears to arise from the temporal part of field CA1 or the adjacent part of the ventral subiculum. The projection of Ammon's horn upon the lateral septal complex shows a high degree of topographic organization (such that different parts of fields CA1 and CA3 project in an ordered manner to different zones within the lateral septal nucleus). The septal projection of "CA2" and field CA3 is bilateral, while that of field CA1 is strictly unilateral. In addition to its subcortical projections, the hippocampus has been found to give rise to a surprisingly extensive series of intracortical association connections. For example, all parts of fields CA1, CA2 and CA3 project to the subiculum, and at least some parts of these fields send fibers to the pre- and parasubiculum, and to the entorhinal perirhinal, retrosplenial and cingulate areas. From the region of the pre- and parasubiculum there is a projection to the entorhinal cortex and the parasubiculum of both sides. That part of the postsubiculum (= dorsal part of the presubiculum) which we have examined has been found to project to the cingulate and retrosplenial areas ipsilaterally, and to the entorhinal cortex and parasubiculum bilaterally.     

Intrinsic connections of the macaque monkey hippocampal formation: II. CA3 connections

We examined the topographic organization of the connections of the CA3 field of the macaque monkey hippocampus. Discrete anterograde and retrograde tracer injections were made at various positions within CA3 and CA1. The projections from CA3 to CA1 (Schaffer collaterals), which terminate in the strata radiatum, pyramidale, and oriens, are present throughout the entire transverse extent of CA1. Projections extend both rostrally and caudally from the injection site for as much as three‐fourths of the longitudinal extent of the hippocampus. The associational projections from CA3 to CA3 also travel extensively along the longitudinal axis. CA3 gives rise to more substantial projections to CA1 than to CA3. CA3 projections that originate at the level of the uncus tend to be more restricted to the rostral portions of CA1 and CA3. As in the rodent brain, projections from CA3 to CA1 are distributed along a radial gradient, depending on the transverse location of the cells of origin. CA3 cells located near the dentate gyrus generate projections that more densely terminate superficially in the terminal zone of CA1, whereas CA3 cells located closer to CA1 give rise to projections that more heavily terminate deeply in the terminal zone of CA1. The present results indicate that in the monkey, as in the rat, CA3 cells give rise to extensive projections to CA1 and CA3. Interestingly, radial, transverse, and longitudinal gradients of CA3 fiber distribution, so clear in the rat, are much more subtle in the nonhuman primate brain. J. Comp. Neurol. 515:349–377, 2009. © 2009 Wiley‐Liss, Inc.     

Efferent connections of the caudal part of the globus pallidus in the rat

The efferent connections of the caudal pole of the globus pallidus (GP) were examined in the rat by employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L), and the retrograde transport of fluorescent tracers combined with choline acetyltransferase (ChAT) or parvalbumin (PV) immunofluorescence histochemistry. Labeled fibers from the caudal GP distribute to the caudate-putamen, nucleus of the ansa lenticularis, reuniens, reticular thalamic nucleus (mainly its posterior extent), and along a thin strip of the zona incerta adjacent to the cerebral peduncle. The entopeduncular and subthalamic nuclei do not appear to receive input from the caudal GP. Descending fibers from the caudal GP course in the cerebral peduncle and project to posterior thalamic nuclei (the subparafascicular and suprageniculate nuclei, medial division of the medial geniculate nucleus, and posterior intralaminar nucleus/peripeduncular area) and to extensive brainstem territories, including the pars lateralis of the substantia nigra, lateral terminal nucleus of the accessory optic system, nucleus of the brachium of the inferior colliculus, nucleus sagulum, external cortical nucleus of the inferior colliculus, cuneiform nucleus, and periaqueductal gray. In cases with deposits of PHA-L in the ventral part of the caudal GP, labeled fibers in addition distribute to the lateral amygdaloid nucleus, amygdalostriatal transition area, cerebral cortex (mainly perirhinal, temporal, and somatosensory areas) and rostroventral part of the lateral hypothalamus. Following injections of fluorescent tracer centered in the lateral hypothalamus, posterior intralaminar nucleus, substantia nigra, pars lateralis, or lateral terminal nucleus, a substantial number of retrogradely labeled cells is observed in the caudal GP. None of these cells express ChAT immunoreactivity, but, except for the ones projecting to the lateral hypothalamus, a significant proportion is immunoreactive to PV. Our results indicate that caudal GP efferents differ from those of the rostral GP in that they project to extensive brainstem territories and appear to be less intimately related to intrinsic basal ganglia circuits. Moreover, our data suggest a possible participation of the caudal GP in feedback loops involving posterior cortical areas, posterior striatopallidal districts, and posterior thalamic nuclei. Taken as a whole, the projections of the caudal GP suggest a potential role of this pallidal district in visuomotor and auditory processes. © 1996 Wiley-Liss, Inc.     

Efferent connections of the subthalamic region in the rat. I. The subthalamic nucleus of luys

The efferent connections of the subthalamic nucleus of Luys (STN) in the rat were investigated with the aid of the anterograde autoradiographic and the retrograde horseradish peroxidase (HRP) tracer techniques.A small microelectrophoretic injection of tritiated proline and leucine centered in the STN (case RST-4) was found to label fibers directed mainly at 3 ipsilateral structures: the substantia nigra (chiefly the ventral portions of this pars reticulata), the entopeduncular nucleus and the globus pallidus (including the ventral pallidum). In addition to this major labeling pattern, much sparser labeling was seen in striatal, thalamic, hypothalamic, pretectal, tectal and reticular territories. In another series of experiments, microelectrophoretic HRP injections confined to the substantia nigra or the globus pallidus consistently resulted in retrograde labeling of neurons in the ipsilateral STN. On the other hand, HRP injections of the vontromedial portion of the midbrain tegmentum (including the red nucleus), the superior colliculus, the pretectal area or a midbrain region at the lateral border of the central gray substance resulted in retrograde labeling of cells in the zona incerta, but no labeled cells appeared in these cases in the ventrally adjacent STN. These HRP results, together with autoradiographic data obtained in control cases, suggest that the minor projections to territories other than the substantia nigra and the pallidal complex originate in the zona incerta or the cerebral cortex rather than in STN.     

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.     

Efferent connections of the substantia nigra and ventral tegmental area in the rat

Small injections of tritiated leucine and proline confined to the ventral tegmental area (AVT) were found to label fibers ascending: (a) to the entire ventromedial half of the striatum, but most massively to the ventral striatal zone that includes the nucleus accumbens; (b) to the thalamus: lateral habenular nucleus, nuclei reuniens and centralis medius, and the most medial zone of the mediodorsal nucleus; (c) to the posterior hypothalamic nucleus and possibly the lateral hypothalamic and preoptic region; (d) to the nuclei amygdalae centralis, lateralis and medialis; (e) to the bed nucleus of the stria terminalis, the nucleus of the diagonal band, and the medial half of the lateral septal nucleus; (f) to the anteromedial (frontocingulate) cortex; and (g) to the entorhinal area. Further AVT efferents descend to the medial half of the midbrain tegmentum including an anterior region of the median raphe nucleus, to the ventral half of the central grey substance including the dorsal raphe nucleus, to the parabrachial nuclei, and to the locus coeruleus. Similar injections centered in the pars compacta of the substantia nigra (SNC) label fibers that are distributed in the striatum in an orderly medial-to-lateral arrangement, and almost entirely avoid the nucleus accumbens and olfactory tubercle. With the exception of the lateral quarter of the substantia nigra, which apparently does not project to the extreme rostral pole of the striatum, each small SNC locus, regardless of its anteroposterior localization, distributes nigrostriatal fibers throughout the length of the striatum. Descending SNC efferents are distributed to the same general regions that receive descending AVT projections, except that no SNC fibers appear to enter the locus coeruleus. Isotope injections confined to the pars reticulata (SNR) label sparse nigrostriatal fibers, and numerous nigrothalamic fibers ascending mainly to the nucleus ventromedialis and in lesser number to the parafascicular nucleus and the paralamellar zone of the nucleus mediodorsalis. Descending SNR fibers leave the nigra as a voluminous fiber bundle that bifurcates into a large nigrotectal and a smaller nigrotegmental component, the latter terminating largely in the pedunculopontine nucleus of the pontomesencephalic tegmentum.     

Efferent connections of the caudate nucleus in the Virginia opossum.

The efferents of the opossum's caudate nucleus were investigated by charting the the fiber degenerations produced by electrolytic lesions in various parts of this nucleus. By the aid of a modified Fink-Heimer procedure, degenerating fibers were traced from each of the lesions to the small globus pallidus in which they appeared to be distributed in an orderly dorsoventral pattern. Fibers from all lesion sites in the caudate nucleus were found to terminate in the entopedunucular nucleus. In the substantia nigra, caudatofugal-fiber degeneration was confined in all cases to the rostal part of the pars reticulata, and was densest in the medial one-half of this nucleus. Only from lesions in the ventromedial part of the caudate nucleus could degenerating fibers be traced to the nucleus ansae lenticularis. No fiber degeneration could be traced rostrally from the lesions, or to the putamen, red nucleus, subthalamic nucleus, or pons.     

Efferent connections of the vestibular nuclei in the rat: a neuromorphological study using PHA-L

The efferent connections of the superior, medial, lateral, and descending vestibular nuclei were studied with anterograde tracing methods in rats. The following areas of termination could be discerned: (1) In the diencephalon, labeled terminals were detected in the thalamus. (2) In the mesencephalon, the red nucleus and motor nuclei involved in eye movements were richly supplied by the vestibular nuclei. (3) In the rhombencephalon, extensive intrinsic connections of all vestibular nuclei were demonstrated. Strong commissural connections were found among the medial, superior, and descending vestibular nuclei. The inferior olive received labeled fibers exclusively from the lateral vestibular nuclei. Individual differences were demonstrated in the termination areas in the reticular formation. (4) In the spinal cord, most of the descending vestibular fibers were found in the ipsilateral anterior funiculus.     

Intrinsic connections of the macaque monkey hippocampal formation: I. Dentate gyrus

We have carried out a detailed analysis of the intrinsic connectivity of the Macaca fascicularis monkey hippocampal formation. Here we report findings on the topographical organization of the major connections of the dentate gyrus. Localized anterograde tracer injections were made at various rostrocaudal levels of the dentate gyrus, and we investigated the three-dimensional organization of the mossy fibers, the associational projection, and the local projections. The mossy fibers travel throughout the transverse extent of CA3 at the level of the cells of origin. Once the mossy fibers reach the distal portion of CA3, they change course and travel for 3-5 mm rostrally. The associational projection, originating from cells in the polymorphic layer, terminates in the inner one-third of the molecular layer. The associational projection, though modest at the level of origin, travels both rostrally and caudally from the injection site for as much as 80% of the rostrocaudal extent of the dentate gyrus. The caudally directed projection is typically more extensive and denser than the rostrally directed projection. Cells in the polymorphic layer originate local projections that terminate in the outer two-thirds of the molecular layer. These projections are densest at the level of the cells of origin but also extend several millimeters rostrocaudally. Overall, the topographic organization of the intrinsic connections of the monkey dentate gyrus is largely similar to that of the rat. Such extensive longitudinal connections have the potential for integrating information across much of the rostrocaudal extent of the dentate gyrus.     

Efferent connections of the lateral hypothalamic area of the rat: An autoradiographic investigation

The efferent projections of the lateral hypothalamic area (LHA) at mid-tuberal levels were examined with the autoradiographic tracing method. Connections were observed to widespread regions of the brain, from the telencephalon to the medulla. Ascending fibers course through LHA and the lateral preoptic area and lie lateral to the diagonal band of Broca. Fibers sweep dorsally into the lateral septal nucleus, cingulum bundle and medial cortex. Although sparse projections are found to the ventromedial hypothalamic nucleus, a prominent pathway courses to the dorsal and medial parvocellular subnuclei of the paraventricular nucleus. Labeled fibers in the stria medullaris project to the lateral habenular nucleus. The central nucleus of the amygdala is encapsulated by fibers from the stria terminalis and the ventral amygdalofugal pathway. The substantia innominate, nucleus paraventricularis of the thalamus, and bed nucleus of the stria terminalis also receive LHA fibers. Three descending pathways course to the brainstem: (1) periventricular system, (2) central tegmental tract (CTT), and (3) medial forebrain bundle (MFB). Periventricular fibers travel to the ventral and lateral parts of the midbrain central gray, dorsal raphe nucleus, and laterodorsal tegmental nucleus of the pens. Dorsally coursing fibers of CTT enter the central tegmental field and the lateral and medial parabrachial nuclei. The intermediate and deep layers of the superior colliculus receive some fibers. Fibers from CTT leave the parabranchial region by descending in the ventrolateral pontine and medullary reticular formation; some of these fibers sweep dorsomedially into the nucleus tractus solitarius, dorsal motor nucleus of the vagus, and nucleus commissuralis. From MFB, fibers descend into the ventral tegmental area and to the border of the median raphe and raphe magnus nuclei.     

Efferent connections from the anterior pretectal nucleus to the diencephalon and mesencephalon in the rat

The anterior pretectal nucleus has been described as part of the visual pretectal complex. However, several electrophysiological and behavioural studies showed that this area is involved in somatosensory modulation, more specifically, antinociception. The efferents of the anterior pretectal nucleus have not been identified taking into account the different function of this nucleus in relation to the rest of the pretectal complex. In the study herein described, a sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin was used to trace the mesencephalic and diencephalic efferents of the anterior pretectal nucleus in the rat. The majority of the connections were ipsilateral. Fibres with varicosities were observed in discrete areas of the thalamus (central lateral, posterior complex), hypothalamus (lateral, posterior and ventromedial), zona incerta, parvocellular red nucleus, intermediate and deep layers of the superior colliculus, central grey, deep mesencephalon, pontine parabrachial region, and pontine nuclei. Fibres en passant were detected in the medial lemniscus, from the level of the injection site to rostral medullary levels. Some labelled axons were seen coursing to the contralateral side through the posterior commissure and the decussation of the superior cerebellar peduncle. These results show that the anterior pretectal nucleus projects principally to areas involved in somatosensory and motor control in a manner that permits sensory modulation at higher and lower levels of the brain. These connections may explain the antinociceptive and antiaversive effects of stimulating the anterior pretectal nucleus in freely moving animals.     

Efferent connections of the caudate nucleus in cats]

The structural and ultrastructural changes in the frontal cortex and globus pallidus were investigated after local or extensive destructions of the caudate nucleus. Using the Fink-Heimer method few degenerating fibres of medium and small sizes were observed in the frontal cortex following the local destruction of the caudate nucleus with preliminary implanted electrodes (2-16 months prior to electrolytic destruction). The extensive destruction of the caudate nucleus without preliminary implanted electrodes is accompanied by mass degeneration of fibres with different calibre in the frontal cortex. Light and electron microscopy of the globus pallidus confirmed the existence of thin degenerating axons 0.5-0.6 mum in diameter after the local lesion of the caudate nucleus. Degenerating changes in axo-dendritic and axo-somatic terminals of the pallidal region occur by "dark" type. The degenerating fibres of the medium size in the frontal cortex following the caudate nucleus destruction are suggested to be the axons of thalamic neurons but not those of the destructed nucleus cells.     

Efferent connections of the periaqueductal gray matter in the cat

Electrolytic lesions were stereotaxically placed in the dorsal and ventral areas of the mesencephalic periaqueductal gray in the cat. A dorsal angular approach was made so that mechanical electrode damage was contralateral to the lesion, or a posterior approach through the fourth ventricle was employed to avoid electrode damage. The brains were sectioned coronally and sagittally and stained with a modifield Nauta-Gygax or Fink-Heimer stain. Degeneration from the dorsal lesions was chiefly in a radial pattern to the superior colliculus, inferior colliculus and mesencephalic reticular area. Also, fibers angled out of the dorsal gray, crossed the midline and joined the commissure of the superior colliculus, some taking a ventrolateral course through the colliculi and others running ventrally along the borders of the gray. Degeneration was traced caudally to the cuneiform nucleus and adjacent reticular area. Rostrally, fibers traveled in the dorsal longitudinal fasciculus to the pretectal area, lateral habenular nucleus and finally, the posterior hypothalamic area. Ventral lesions showed the same radial pattern of degeneration and fibers in the superior colliculus commissure. Caudally, fibers could be traced to the cuneiform nucleus, reticular area and inferior olive. The rostral course of fibers in the dorsal longitudinal fasciculus was similar, though there were additional connections with the ventral tegmental area of Tsai, the fields of Forel, and the parafascicular nucleus of the thalamus.     

Hodological characterization of the septum in anuran amphibians: II. Efferent connections

The efferent connections of the septum of the gray treefrog Hyla versicolor were studied by combining anterograde and retrograde tracing with biotin ethylendiamine (Neurobiotin). The lateral septal complex projects mainly to the medial pallium, limbic regions (e.g., amygdala and nucleus accumbens), and hypothalamic areas but also to sensory nuclei in the diencephalon and midbrain. The central septal complex strongly innervates the medial pallium, limbic, and hypothalamic areas but also specific sensory (including olfactory) regions. The medial septal complex sends major projections to all olfactory nuclei and a weaker projection to the hypothalamus. Our results indicate that all septal nuclei may modify the animal's internal state via efferents to limbic and hypothalamic areas. Via projections to the medial pallium, lateral and central septal complexes may be involved in learning processes as well. Because of their connections to specific sensory areas, all septal areas are in a position to influence sensory processing. Furthermore, our data suggest that both the postolfactory eminence and the bed nucleus of the pallial commissure are not part of the septal complex, rather, the postolfactory eminence seems to be comparable to the mammalian primary olfactory cortex, whereas the bed nucleus may be analogous to the mammalian subfornical organ.