Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

Subcortical projections to lateral geniculate and thalamic reticular nuclei in the hooded rat

Restricted injections either of horseradish peroxidase conjugated with wheat germ agglutinin, or of unconjugated horseradish peroxidase were made into hooded rats in order to distinguish subcortical sources of afferents to dorsal lateral geniculate nucleus from those to the adjacent visually responsive thalamic reticular nucleus, which modulates geniculate activity. Five “nonvisual” brainstem regions project to the dorsal lateral geniculate nucleus: mesencephalic reticular formation, dorsal raphe nucleus, periaqueductal gray matter, dorsal tegmental nucleus, and locus coeruleus. Projections are generally bilateral, but ipsilateral projections dominate. Of these regions, three also project ipsilaterally to the thalamic reticular nucleus: mesencephalic reticular formation, periaqueductal gray matter, and dorsal tegmental nucleus. Similar discrete injections of horseradish peroxidase into ventral lateral geniculate nucleus allowed a comparison of afferents to dorsal and ventral lateral geniculate nuclei. In addition to the five nonvisual brainstem regions which project to the dorsal division, the ventral lateral geniculate nucleus receives afferents from the perirubral reticular formation and the central gray matter at the thalamic level. The dorsal and ventral lateral geniculate nuclei receive substantially different afferents from subcortical visual centres. The dorsal division receives projections from superior colliculus, pretectum, and parabigeminal nucleus whereas the ventral division receives afferents from superior colliculus, additional pretectal nuclei, lateral terminal nucleus of the accessory optic system, and the contralateral ventral lateral geniculate nucleus.     

Organization of the projections of a circumventricular organ: the area postrema in the rat

The projections of the rat area postrema were analysed using anterograde and retrograde axonal transport techniques. Discrete injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the area postrema produced anterograde labeling in specific medullary and pontine nuclei. In the medulla, anterograde labeling was present in the internal solitary zone and dorsal division of the medial solitary nucleus, both of which also contained a small number of retrogradely labeled perikarya. Prominent projections to the dorsal motor nucleus of the vagus were seen only if the WGA-HRP injections in the area postrema invaded dorsal solitary nuclei. In the pons, anterograde labeling was present in the parabrachial nuclei, the dorsolateral tegmental nucleus, and the pericentral division of the dorsal tegmental nucleus. By far the major pontine projection was to the dorsolateral region of the middle one-third of the rostrocaudal extent of the parabrachial nuclei. Retrograde fluorescent tracing studies indicated that most area postrema neurons take part in this parabrachial projection. The area postrema projection to the parabrachial nuclei was bilaterally distributed, whereas that from the dorsal solitary nuclei was primarily ipsilateral. The external solitary zone, immediately subadjacent to the area postrema, neither received area postrema projections nor participated in the projections to the parabrachial nuclei. Fluorescent retrograde double labeling studies confirmed the bilateral nature of the area postrema projection to the parabrachial nuclei. In addition, because no doubly labeled neurons were observed it appears that individual area postrema neurons project to either side but not both sides of the dorsal pons. Thus, numerous neuronal pathways exist for the transfer of blood-borne information (that cannot cross the blood-brain barrier) from the area postrema to other brain regions.     

Ascending projections of the brain stem reticular formation in a nonmammalian vertebrate (the lizard Varanus exanthematicus), with notes on the afferent connections of the forebrain

In the present study an attempt has been made to analyze the ascending reticular projections in the lizard Varanus exanthematicus by means of the horseradish peroxidase (HRP) technique. Reticular projections ascending to the telencephalon were found to arise in the mesencephalon, but not caudal to the mesorhombencephalic border. HRP injections into the dorsal thalamus have demonstrated retrogradely labeled cells in the mesencephalic reticular formation, particularly at the level of the oculomotor nerve and in the medial magnocellular zone of the rhombencephalic reticular formation, predominantly rostrally. HRP infiltrations at the mesodiencephalic border damaged most of the fibers passing beyond this junction, resulting in the uptake of HRP by the damaged axons and subsequent labeling of the cell bodies or origin of ascending reticular projections to the diencephalon and telencephalon. From a comparison of cell-labeling patterns in cases of HRP injections of, respectively, the dorsal thalamus and the mesodiencephalic border, it seems likely that the nucleus reticularis medius and more sparsely the nucleus reticularis inferior project to ventral diencephalic structures (ventral thalamus and hypothalamus), whereas the midbrain reticular formation and the rostral parts of the rhombencephalic reticular formation (nuclei reticulares isthmi and superior) project to both the dorsal thalamus and more ventral diencephalic structures. Projections arising throughout the rhombencephalic reticular formation, but predominantly in the nucleus reticularis inferior, were found to ascend to the midbrain reticular formation. The present experimental data in the lizard Varanus exanthematicus are comparable to the findings in mammals, with the exception of the reticulo-oculomotor pathways which have not been analyzed so far in reptiles. In addition to the aforementioned ascending reticular projections, the present study has demonstrated projections ascending from monoamine cell groups, various diencephalics structures, as well as from neuronal groups involved in somatosensory, auditory, and gustatory systems. Projections were found from the locus coeruleus and the nucleus raphes superior to the telencephalon, as well as from the substantia nigra and the presumable reptilian homologue of the mammalian ventral tegmental area to the basal forebrain and the dorsal thalamus. Bilateral projections were demonstrated from the principal trigeminal nucleus to the telencephalon, reminiscent of the quintofrontal tract of birds. Ascending projections to the diencephalon were found to originate bilaterally in the descending trigeminal nucleus and the dorsal funicular nucleus. Auditory projections to the midbrain arise bilaterally in the superior olivary complex and in the cochlear nuclear complex. Finally, the ascending gustatory pathway arising in the nucleus of the solitary tract was found to project to the “parabrachial region”, which in its turn has extensive projections to the forebrain.     

Ascending projections to the mammillary nuclei in the rat: a study using retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase

Cells of origin of ascending afferents to the mammillary nuclei and the afferents' fields of termination within these nuclei were studied by using retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase in the rat. The pars compacta of the superior central nucleus projects bilaterally to the median region of the medial mammillary nucleus. The ventral tegmental nucleus projects ipsilaterally to the medial mammillary nucleus, except for its median region, in a topographic manner such that the rostrodorsolateral part of the ventral tegmental nucleus projects to the medial quadrant of the medial mammillary nucleus; the rostroventromedial part projects to the dorsal quadrant; the caudodorsolateral part projects to the ventral quadrant; and the caudoventromedial part projects to the lateral quadrant. These projection fields extend throughout the longitudinal axis of the medial mammillary nucleus, except for its most caudal region, to which only the dorsolateral part of the ventral tegmental nucleus projects. This nucleus also projects topographically to the ipsilateral dorsal premammillary nucleus; the rostral part of the ventral tegmental nucleus projects to the dorsal part of the dorsal premammillary nucleus, whereas the caudal part projects to the ventral part. The periaqueductal gray around the dorsal tegmental nucleus projects bilaterally to the supramammillary nucleus. The pars alpha of the pontine periaqueductal gray projects bilaterally to the peripheral part of the lateral mammillary nucleus, whereas the pars ventralis of the dorsal tegmental nucleus projects ipsilaterally to the lateral mammillary nucleus. The results show that the tegmentomammillary projections are organized in a gradient fashion, with the rostral to caudal position of cells of origin within the tegmental nuclei of Gudden being reflected by the medial to lateral position of fields of termination within the mammillary nuclei.     

Visual and electrosensory circuits of the diencephalon in mormyrids: an evolutionary perspective

Mormyrids are one of two groups of teleost fishes known to have evolved electroreception, and the concomitant neuroanatomical changes have confounded the interpretation of many of their brain areas in a comparative context, e.g., the diencephalon, where different sensory systems are processed and relayed. Recently, cerebellar and retinal connections of the diencephalon in mormyrids were reported. The present study reports on the telencephalic and tectal connections, specifically in Gnathonemus petersii, as these data are critical for an accurate interpretation of diencephalic nuclei in teleosts. Injections of horseradish peroxidase into the telencephalon retrogradely labeled neurons ipsilaterally in various thalamic, preglomerular, and tuberal nuclei, the nucleus of the locus coeruleus (also contralaterally), the superior raphe, and portions of the nucleus lateralis valvulae. Telencephalic injections anterogradely labeled the dorsal preglomerular and the dorsal tegmental nuclei bilaterally. Injections into the optic tectum retrogradely labeled neurons bilaterally in the central zone of area dorsalis telencephali and ipsilaterally in the torus longitudinalis, various thalamic, pretectal, and tegmental nuclei, some nuclei in the torus semicircularis, the nucleus of the locus coeruleus, the nucleus isthmi and the superior reticular formation, basal cells in the ipsilateral valvula cerebelli, and eurydendroid cells in the contralateral lobe C4 of the corpus cerebelli. Weaker contralateral projections were also observed to arise from the ventromedial thalamus and various pretectal and tegmental nuclei, and from the locus coeruleus and superior reticular formation. Tectal injections anterogradely labeled various pretectal nuclei bilaterally, as well as ipsilaterally the dorsal preglomerular and dorsal posterior thalamic nuclei, some nuclei in the torus semicircularis, the dorsal tegmental nucleus, nucleus isthmi, and, again bilaterally, the superior reticular formation. A comparison of retinal, cerebellar, tectal, and telencephalic connections in Gnathonemus with those in nonelectrosensory teleosts reveals several points: (1) the visual area of the diencephalon is highly reduced in Gnathonemus, (2) the interconnections between the preglomerular area and telencephalon in Gnathonemus are unusually well developed compared to those in other teleosts, and (3) two of the three corpopetal diencephalic nuclei are homologues of the central and dorsal periventricular pretectum in other teleosts. The third is a subdivision of the preglomerular area, rather than an accessory optic or pretectal nucleus, and is related to electroreception. The preglomerulo-cerebellar connections in Gnathonemus are therefore interpreted as uniquely derived characters for mormyrids.     

Projections of the trapezoid body and the superior olivary complex of the Kangaroo rat (Dipodomys merriami).

Glass micropipettes filled with 2 M sodium cyanide were used to physiologically locate and iontophoretically damage the nucleus of the trapezoid body (NTB), the medial superior olive (MSO), and the lateral superior olive (LSO). Mechanical lesions were made in the trapezoid body as it leaves the cochlear nuclei. After a 3- to 10-day survival time the projections and terminal degeneration were traced with the Fink-Heimer and Nauta-Gygax stains. The ventral cochlear nucleus (VCN) projects via the trapezoid body to ipsilateral LSO, ipsilateral preolivary nuclei, ipsilateral lateral and a contralateral medial dendritic fields of MSO, and contralateral NTB; there is also a small ipsilateral projection to the ventral nucleus of the lateral lemniscus (VNLL) and the central nucleus of the inferior colliculus (CNIC). Some trapezoid body fibers ascend via the contralateral lateral lemniscus to VNLL, DNLL (dorsal nucleus of the lateral lemniscus), and CNIC. There is no projection from the ventral cochlear nucleus to the ipsilateral NTB and contralateral preolivary nuclei. All portions of NTB project ipsilaterally to LSO (ventral NTB to dorsomedial LSO, dorsal NTB to ventral LSO) and to the retro-olivary nucleus. In two animals with NTB lesions there is also degeneration in the ventromedial portion of the ipsilateral facial nucleus. NTB projects contralaterally by way of the stria of Monakow to the pyramidal and molecular cell layers of the dorsal cochlear nucleus (DCN). The NTB does not project ipsilaterally to MSO, preolivary nuclei, VNLL, DNLL and CNIC. Contralaterally there are no projections to any of the nuclei of the auditory pathway except the DCN. Most MSO projections are ipsilateral. The densest goes by way of the lateral lemniscus to the lateral aspect of the ipsilateral CNIC, terminating throughout its dorsoventral axis. MSO also projects bilaterally to the pyramidal and molecular cell layers of dorsal cochlear nucleus (DCN), and ipsilaterally to the ventral portion of the motor nucleus of V and to the facial nucleus. MSO does not project ipsilaterally to the LSO, NTB, preolivary, VCN and retro-olivary nuclei. On the contralateral side, all structures except the DCN are free of projection patterns from axons originating in the MSO. LSO projects bilaterally to the central and ventral portions of CNIC and to the nuclei of the lateral lemnisci, and ipsilaterally to the large and small spherical cell areas of anterior ventral cochlear nucleus (AVCN) and to all portions of DCN. The LSO does not project ipsilaterally to the NTB, MSO, preolivary and retro-olivary nuclei. On the side opposite, this nucleus does not project to NTB, MSO, retro-olive, VCN, preolivary and LSO. For all lesions regardless of the site, there is no degeneration found rostral to the CNIC. The medial geniculate body or other structures in the diencephalon or cortex are free of any fields of terminal degeneration.     

Diencephalic projections from the pontine reticular formation: autoradiographic studies in the cat

Ascending projections to the diencephalon from the pontine reticular formation were studied in the cat by autoradiographic techniques. Projections from both rostral and caudal pontine regions ascend to the caudal diencephalon and divide into two components; a dorsal leaf terminates primarily in the thalamic intralaminar complex and a ventral leaf terminates in the subthalamic region. The relative densities of the two terminal regions vary with the injection site. Fibers originating in the caudal pons (nucleus reticularis pontis caudalis) terminate relatively heavily in the intralaminar nuclei of the dorsal thalamus, particularly the centre median, central lateral, central dorsal and paracentral nuclei, and also the dorsal medial nucleus. Relatively sparse termination occurs in the subthalamic region. In contrast, fibers from the rostral pons (nucleus reticularis pontis oralis) terminate relatively heavily in the subthalamic region, including the zona incerta, the fields of Forel, the ventral part of the thalamic reticular complex, and the lateral hypothalamus. Relatively sparse termination occurs in the dorsal thalamus, but includes the centre median, parafascicular, central lateral, paracentral and dorsal medial nuclei. These data are discussed with regard to reticular control of forebrain activity and the role of the classic dorsal and ventral components of ascending reticular projections.     

Topographic organization of subcortical projections to the anterior thalamic nuclei in the rat.

Subcortical projections to the anterior thalamic nuclei were studied in the rat, with special reference to projections from the mammillary nuclei, by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The medial mammillary nucleus (MM) projects predominantly ipsilaterally to the entire anterior thalamic nuclei, whereas the lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus (AD) of the anterior thalamic nuclei. A topographic relationship was recognized between the MM and the anterior thalamic nuclei. The dorsal region of the pars mediana of the MM projects to the interanteromedial nucleus (IAM), whereas the ventral region projects to the rostral part of the anteromedial nucleus (AM). The dorsal and the ventral regions of the pars medialis project to the dorsomedial part of the AM at its caudal and rostral levels, respectively. The dorsomedial region of the pars lateralis projects to the ventral AM. The ventrolateral region of the pars lateralis projects to the ventral part of the anteroventral nucleus (AV) in such a manner that rostral cells project rostrally and caudal cells project caudally. The pars basalis projects predominantly ipsilaterally to the dorsolateral AV and bilaterally to the AD. The rostrolateral region of the pars posterior projects to the lateral AV, whereas the medial and the caudal regions of the pars posterior project to the dorsomedial AV. The rostrodorsal part of the nucleus reticularis thalami was found to project to the anterior thalamic nuclei; cells located rostrally in this part project to the IAM and AM, whereas cells located caudodorsally project to the AV and AD. The laterodorsal tegmental nucleus projects predominantly ipsilaterally to the AV, especially to its dorsolateral part. The present study demonstrates that subdivisions of the subcortical structures are connected to the subnuclei of the anterior thalamic nuclei, with a clear-cut topography arranged in the dorsoventral and the rostrocaudal dimensions.     

Efferent projections from the mammillary complex of the guinea pig: an autoradiographic study

The efferent projections from the medial and lateral mammillary nuclei of the guinea pig were traced after injecting tritiated amino acid. The major efferent started as the principal mammillary tract, but soon divided into mammillothalamic and mammillotegmental tracts. The mammillothalamic tract projected anterodorsally and terminated in the anterior dorsal, anterior ventral and anterior medial thalamic nuclei. The mammillotegmental tract projected caudally and terminated in the dorsal tegmental nucleus and central gray. The mammillary efferents in the mammillary peduncle ran via the tegmentum of the midbrain and pons. It terminated in the dorsal and ventral tegmental nuclei, basal pontine nucleus and pontine tegmental reticular nucleus. A diffuse mammillary projection had fibers directed dorsally which distributed in the midline thalamic nuclei and in central gray. Rostral projections via the medial forebrain bundle from the medial mammillary nucleus were found in the septal area and diagonal band of Broca. The lateral mammillary nucleus sent fibers which also joined the mammillothalamic and mammillotegmental tracts. These terminated bilaterally mainly in the anterior dorsal and anterior ventral nuclei of the thalamus, and caudally in the dorsal and ventral tegmental nuclei and basal pontine nucleus.     

Tectal connections in Python reticulatus

The origins of the axons terminating in the mesencephalic tectum in Python reticulatus were examined by unilateral tectal injections of horseradish peroxidase. Kutrogradely labeled cells were observed bilaterally throughout the spinal cord in all subdivisions of the trigeminal system, with the exception of nucleus principalis, which showed labeled cells only on the ipsilateral side. Labeling of the reticular formation occurred bilaterally in nucleus reticular is interiormagnocellularis, nucleus reticularis lateralis, nucleus reticularis medius and the mesencephalic reticular formation. The tectum also receives bilateral projections from the dorsal tegmentaJ field, the nucleus of the lateral lemniscus and nucleus isthmi, and ipsilateral projections from nucleus profundus mesencephali. A few labeled cells were found ipsilaterally in the locus coeruleus and in nuclei vestibulares ventrolateralis and centromedialis. In the diencephalon labeled cells were observed ipsilaterally in nucleus ventrolateralis thalami, nucleus ventromedialis thalami, nucleus suprapeduncularis, and in the dorsal and ventral lateral geniculate nuclei. Bilateral labeling was observed in nucleus periventricularis hypo-thalami. Furthermore, labeling was ipsilaterally present in the ventral telen-cephalic areas. The tectum in Python reticulatus receives a wide variety of afferent connections which confirm the role of the tectum as an integration center of visual and exteroceptive information.     

Projections from the superior olivary complex to the cochlear nucleus in the tree shrew

The origins and targets of projections from the superior olivary complex to the cochlear nuclei were studied in the tree shrew by placing small injections of horseradish peroxidase (HRP) in the cochlear nucleus and small injections of ³H-leucine in the superior olivary complex. The results show that the descending pathways originate in periolivary cell groups surrounding the medial and lateral superior olives and that the periolivary nuclei differ from one another in their patterns of projections to the cochlear nucleus. For example, cell groups may project either ipsilaterally or bilaterally. Cells in the lateral nucleus of the trapezoid body project only to the ipsilateral cochlear nucleus. Other periolivary cell groups project bilaterally, although some of these may project more heavily to one side than the other. Some pathways have widespread targets in the cochlear nucleus whereas others have relatively specific targets. Diffuse projections to all divisions of the cochlear nucleus arise from the lateral nucleus of the trapezoid body ipsilaterally and from the medial perioliviary nucleus bilaterally. The targets of other descending pathways are more restricted. The anterolateral, dorsal, and dorsolateral periolivary nuclei project mainly to the anteroventral cochlear nucleus; the ventral nucleus of the trapezoid body and the posterior periolivary nucleus project mainly to the dorsal and posteroventral cochlear nuclei. All of these specific projections are bilateral. These results suggest that projections from the periolivary cell groups to the cochlear nucleus consist of multiple components with different degrees of specificity.     

Organization of the superior olivary complex in the guinea pig: II. Patterns of projection from the periolivary nuclei to the inferior colliculus.

The superior olivary complex is a major source of auditory projections to the inferior colliculus. Although the projections from the medial and lateral superior olivary nuclei have been well characterized, projections from the surrounding periolivary nuclei have received relatively little attention. In the guinea pig, cytoarchitectonic criteria can be used to distinguish 11 periolivary nuclei that can be divided into four groups. These are: 1) a lateral group that comprises the anterolateral and posteroventral periolivary nuclei and the lateral nucleus of the trapezoid body; 2) a dorsal group that comprises the dorsal and dorsolateral periolivary nuclei; 3) a ventral group that comprises the rostral, ventromedial, and anteroventral periolivary nuclei and the ventral nucleus of the trapezoid body; and 4) a medial group that comprises the superior paraolivary nucleus and the medial nucleus of the trapezoid body. In the present study we used horseradish peroxidase and fluorescent tracers to identify olivocollicular cells in each of the periolivary nuclei. The lateral, dorsal, and medial periolivary groups project bilaterally, and the ventral periolivary group projects ipsilaterally. Within groups, individual nuclei contain different numbers of olivocollicular cells. The posteroventral periolivary nucleus is the only periolivary nucleus that does not project to the inferior colliculus. The superior paraolivary nucleus is the only periolivary nucleus that contains significant numbers of individual cells that project to both inferior colliculi. The remaining periolivary nuclei project only ipsilaterally or contain separate populations of cells that project to the two inferior colliculi.     

Brain stem afferents of hypoglossal neurons in the rat

The origin of afferent connections of the hypoglossal nucleus in rats was investigated using horseradish peroxidase (HRP) as a retrograde tracer. Pressure injections (0.15–0.17 μl) of 15% HRP were introduced into the rostral, middle and caudal portions of the nucleus. Projections to the hypoglossal nucleus originated from 3 regions of the brainstem: the reticular formation, the spinal V complex and the nucleus of the solitary tract. Bilateral projections with ipsilateral predominance came from the lateral reticular formation: the dorsal aspect of the nucleus reticularis parvocellularis and its caudal continuation, the nucleus reticularis dorsalis. Fewer projections emerged from two nuclei of the medial reticular formation. The dorsal part of the nucleus reticularis ventralis at the spinomedullary junction contributed bilateral with mainly contralateral input to hypoglossal neurons. A few labeled neurons were situated bilaterally in the nucleus reticularis gigantocellularis of the rostral medulla. The input from the spinal V complex originated from the dorsal aspect along most of its length but particularly from the pars interpolaris and oralis subdivisions. Labeled neurons were located primarily in the posterior portion of the nucleus of the solitary tract. Projections from the spinal V complex and the solitary nucleus exhibited ipsilateral predominance. These results suggest that somatic and visceral centers of the rat brainstem play an important role in the control of the activity of hypoglossal motoneurons.     

The central connections of the olfactory bulbs in cod, Gadus morhua L.

The central projections of the "classical" olfactory system of the cod, Gadus morhua were examined with horseradish peroxidase and cobalt tracing techniques. Label was applied to the olfactory bulb or selectively to central stumps of sectioned individual olfactory tract bundlets. The olfactory bulb projects bilaterally to restricted areas of the dorsolateral, ventromedial and basolateral telencephalon, anterior commissural and preoptic areas, habenular nuclei, dorsal thalamus and to the nucleus posterior tuberis in the diencephalon. An interbulbar connection courses in the medial olfactory tract (MOT). Contralateral projections were less pronounced than on the ipsilateral side. More specifically, the lateral olfactory tract (LOT) projects ipsilaterally to the telencephalon into the Dlv, Dc, Vs and Dp areas. The lateral bundlet of the medial olfactory tract (IMOT) terminates in the Dlv and Dc areas. The medial bundlet of the medial olfactory tract (mMOT) terminates in Vv and Vd. The fused lMOT and mMOT project to the caudal telencephalon in the Vs and Dp. Neurons projecting to the olfactory bulb were located bilaterally in the telencephalon. The majority of the bulbopetal fibers course via the lateral part of the MOT; a few neurons also project to the bulb through the other bundlets of the olfactory tract. The results are compared with previous studies on the olfactory projections of other teleost species and discussed with respect to the reported functional differentiation of the olfactory system in teleosts.     

Afferent connections to the amygdaloid complex of the rat with some observations in the cat. III. Afferents from the lower brain stem

The projections from the medulla oblongata, pons, and mesencephalon to each nucleus of the amygdaloid complex of the rat were investigated by the use of retrograde transport of horseradish peroxidase (HRP). The enzyme was injected stereotactically by microiontophoresis using four different approaches. The findings indicate that the majority of the ascending fibers terminate in the central and medial amygdalar nuclei. Injections in the central nucleus label neurons at the dorsal aspect of substantia nigra, pars compacta, and in the adjacent ventral tegmental area and peripeduncular nucleus. At more caudal levels, reactive neurons are found in the periaqueductal gray substance, various raphe nuclei, the locus coeruleus, the parabrachial nucleus, the nucleus of the solitary tract, and the mesencephalic and bulbar reticular formation. Injections in the medial nucleus lead to labeling of neurons in the peripeduncular nucleus, the dorsal raphe and superior central nuclei, the parabrachial nucleus, and in the dorsomedial extreme of the dorsal nucleus of the lateral lemniscus. The parabrachial nucleus is the most important lower brain stem source of amygdalopetal fibers. This nucleus projects to the ipsilateral as well as the contralateral amygdala in a topographical manner. Most of the lower brain stem structures found to project to the amygdala in the rat are identified as sources of amygdalopetal fibers in the cat as well.     

Dorsal mesencephalic projections to pons, medulla, and spinal cord in the cat: limbic and non-limbic components.

The vertebrate dorsal mesencephalon consists of the superior colliculus, the dorsal portion of the periaqueductal gray, and the mesencephalic trigeminal neurons in between. These structures, via their descending pathways, take part in various behavioral responses to environmental stimuli. This study was undertaken to compare the origins and trajectories of these pathways in the cat. Injections of horseradish peroxidase into the cervical spinal cord and upper medullary medial tegmentum retrogradely labeled cells mainly in the contralateral intermediate and deep superior colliculus, and in the ipsilateral dorsal and lateral periaqueductal gray and adjacent tegmentum. Only injections in the medullary lateral tegmental field labeled mesencephalic trigeminal neurons ipsilaterally. Autoradiographic tracing results, based on injections across the dorsal mesencephalon, revealed three efferent fiberstreams. A massive first fiberstream (limbic pathway), consisting of thin fibers, descended ipsilaterally from the dorsal and lateral periaqueductal gray and adjacent superior colliculus through the mesencephalic and pontine lateral tegmentum, terminating in these areas as well as in the ventral third of the caudal pontine and medullary medial tegmentum. A few fibers from the dorsal periaqueductal gray matter (PAG) were distributed bilaterally to the dorsal vagal, solitary, and retroambiguus nuclei. The second fiberstream (the predorsal bundle) descended contralaterally from the superior colliculus (SC) and consisted of both thick and thin labeled fibers. The thin fibers terminated bilaterally in the dorsomedial nucleus reticularis tegmenti pontis and the medial half of the caudal medial accessory inferior olive. The thick fibers targeted the contralateral dorsal two thirds of the caudal pontine and medullary medial tegmental fields, and the facial, abducens, lateral reticular, subtrigeminal, and prepositus hypoglossi nuclei. A few fibers recrossed the midline to terminate in the ipsilateral medial tegmentum. Caudal to the obex, fibers terminated laterally in the tegmentum and upper cervical intermediate zone. From the lateral SC, fibers terminated bilaterally in the lateral tegmental fields of the pons and medulla and lateral facial subnuclei. The third fiberstream (mesencephalic trigeminal or Probst tract) terminated in the supratrigeminal and motor trigeminal nuclei, and laterally in the tegmentum and upper cervical intermediate zone. In summary, neurons in the PAG and in the deep layers of the SC give rise to a massive ipsilateral descending pathway, in which a medial-to-lateral organization exists. A similar topographical pattern occurs in the crossed SC projections. The possibility that these completely different descending systems cooperate in producing specific defensive behaviors is discussed.     

Immunohistochemical localization of calretinin in the rat hindbrain

The localization of calretinin in the rat hindbrain was examined immunohistochemically with antiserum against calretinin purified from the guinea pig brain. Calretinin immunoreactivity was found within neuronal elements. The distribution of calretinin-immunoreactive cell bodies and fibers is presented in schematic drawings and summarized in a table. Major calretinin-immunoreactive neurons were found in the lateral and medial geniculate nuclei, substantia nigra, ventral tegmental area, interpeduncular nucleus, periaqueductal gray, mesencephalic trigeminal nucleus, superior and inferior colliculi, pontine nuclei, parabrachial nucleus, dorsal and laterodorsal tegmental nuclei, cochlear nuclei, vestibular nuclei, medullary reticular nuclei, nucleus of the solitary tract, area postrema, substantia gelatinosa of the spinal trigeminal nucleus, and cerebellum. These results show that distinct calretinin-immunoreactive neurons are widely distributed in the rat hindbrain.     

Morphology and projection pattern of medial and dorsal pallial neurons in the frog Discoglossus pictus and the salamander Plethodon jordani

In the frog Discoglossus pictus and the salamander Plethodon jordani, the morphology and axonal projection pattern of neurons in the medial and dorsal pallium were determined by intracellular biocytin labeling. A total of 77 pallial neurons were labeled in the frog and 58 pallial neurons in the salamander. Within the medial pallium (MP) of the frog, four types of neurons were identified on the basis of differences in their axonal projection pattern. Type I neurons have bilateral projections into telencephalic and diencephalic areas; type II neurons have bilateral projections to telencephalic areas and ipsilaterally descending projections to diencephalic regions; type III neurons have only intratelencephalic connections, and a single type IV neuron has ipsilaterally descending projections. The somata of the four types occupy four nonoverlapping zones. Neurons of the dorsal pallium (DP) project exclusively to the ipsilateral MP and to the dorsal edge of the lateral pallium. In the ventral MP of the salamander, neurons have mostly intratelencephalic projections. Neurons in the dorsal MP project bilaterally to diencephalic and telencephalic regions. Neurons in the medial DP project ipsilaterally to the MP, lateral septum, nucleus accumbens, medial amygdala, and the internal granule layer of the olfactory bulb. In five cases, fibers were found in the commissura hippocampi, but in only two cases could these fibers be followed toward the contralateral MP and septum. Neurons in the lateral DP had no contralateral projections; they projected to the ipsilateral MP and in eight cases to the ipsilateral septum as well. Based on similarities of cytoarchitecture and projection pattern in neurons of the MP and DP, it is proposed that both frogs and salamanders have an MP subdivided into a ventral and dorsal portion, and a DP subdivided into a medial and a lateral portion.     

Efferents and afferents of the ventral tegmental-A10 region studied after local injection of [H]leucine and horseradish peroxidase

The efferents and the afferents of the VMT-A10 region were studied by using anterograde ([³H]leucine) and retrograde (HRP) tracing techniques. In order to produce very small injections in various parts of the VMT-A10 region, a slow diffusion technique for [³H]leucine labelling and a microiontophoretic injection for horseradish peroxidase labelling were developed. According to the histochemical and biochemical data, the [³H]leucine anterograde results were separated into three main types of projections.

(1) Projections to regions rich in DA terminals. These projections certainly correspond to the efferents of the dopaminergic A10 neurones. According to various injection sites, we have been able to identify mesolimbic projections originating from the VMT-A10, pars medialis and mesostriatal-mesolimbic projections originating from the VMT-A10, pars lateralis.

The mesolimbic projections include the prefrontal cortex, the medial part of the lateral septum, the interstitial nucleus of the stria terminalis, the accumbens nucleus and the olfactory tubercle. The mesostriatal-mesolimbic projections include the anteromedial part of the caudate nucleus, the cingular cortex, the entorhinal cortex, the amygdaloid complex, the accumbens nucleus, the olfactory tubercle and the piriform cortex to a lesser extent.

(2) Projections to regions suspected of containing DA terminals. These ascending and descending projections which could represent the dopaminergic efferents of the VMT-A10 neurones have been demonstrated. Ascending projections originating either from the VMT-A10 pars medialis or pars lateralis region were found in the claustrum, the nucleus of the tractus diagonalis, the olfactory nuclei, the lateral habenula, the medial hypothalamus and the median eminence. The projections observed in the medial hypothalamus included the periventricular region, the arcuate nucleus, the ventral part of the ventromedial nucleus and the dorsomedial nucleus. The labelling of the anteromedial part of the dorsal hippocampus appeared to originate from the VMT-A10, pars posterior. The projections to the medial hypothalamus, median eminence and hippocampus may have a great functional significance, but further proof of their dopaminergic nature is needed. Descending projections were found ipsilateally to the dorsal raphe and to the cerebellum, and bilaterally to the locus coeruleus. The projections to the cerebellum are distributed to the nuclei interpositus and dentatus and to the Purkinje cell layer and granular layer of the cortex. These results raise the problem of descending dopaminergic projections from the A10 neurones.

(3) Projections to regions not known to contain DA terminals. Anterior projections were found ipsilaterally to the supraoptic nucleus and bilaterally to the anterodorsal thalamic nucleus. Posterior projections were traced ipsilaterally to the limbic midbrain area, including the median raphe, the ventral and dorsal tegmental nucleus and the central gray.

The horseradish peroxidase experiment supplied some clues as to the posterior afferents of the VMT-A10 region. Some labelled cells were found ipsilaterally in the substantia nigra, the medain raphe and the ventral tegmental nucleus. Numerous cells were labelled ipsilaterally in the dorsal raphe nucleus, and nuclei interpositus and dentatus of the cerebellum, and contralaterally in the locus coeruleus. These structures are likely to play an important role in the modulation of the activity of VMT-A10 neurones.

The results of [³H]leucine and HRP experiments permitted us to demonstrate reciprocal connections between VMT-A10 region and anterior raphe nuclei, locus coeruleus and cerebellum.