Serotonin neurons of the midbrain raphe: ascending projections.

The ascending projections of serotonin neurons of the midbrain raphe were analyzed in the rat using the autoradiographic tracing method. Axons of raphe serotonin neurons ascend in the ventral tegmental area and enter the medial forebrain bundle. A number of fibers leave the major group to ascend along the fasciculus retroflexus. Some fibers enter the habenula but the majority turn rostrally in the internal medullary lamina of the thalamus to innervate dorsal thalamus. Two additional large projections leave the medial forebrain bundle in the hypothalamus; the ansa peduncularis-ventral amygdaloid bundle system turns laterally through the internal capsule into the striatal complex, amygdala and the external capsule to reach lateral and posterior cortex, and another system of fibers turns medially to innervate medial hypothalamus and median eminence and form a contrelateral projection via the supraoptic commissures. Rostrally the major group in the medial forebrain bundle divides into several components: fibers entering the stria medullaris to terminate in thalamus; fibers entering the stria terminalis to terminate in the amygdala; fibers traversing the fornix to the hippocampus; fibers running through septum to enter the cingulum and terminate in dorsal and medial cortex and in hippocampus; fibers entering the external capsule to innervate rostral and lateral cortex; and fibers continuing forward in the medial olfactory stria to terminate in the anterior olfactory nucleus and olfactory bulb.     

Serotonergic innervation of the forebrain in the North American opossum

The forebrain distribution of axons showing serotonin-like immunoreactivity was studied in the North American opossum. Serotonergic innervation of the hypothalamus was extensive, particularly within the ventromedial nucleus, the periventricular nucleus and the rostral supraoptic nucleus. Serotonergic axons were also present within the fields of Forel and zona incerta, but they tended to avoid parts of the subthalamic nucleus. In the thalamus serotonergic innervation was dense within the midline nuclei (e.g. the central, intermediate dorsal and rhomboid nuclei) and the ventral lateral geniculate nucleus, but relatively sparse in some of the nuclei more readily associated with specific functions (e.g. the ventrobasal nucleus). Serotonergic axons innervate most areas of the rostral and dorsal forebrain. Areas containing the heaviest innervation included the interstitial nucleus of the stria terminalis and the lateral septal nucleus. Serotonergic innervation of the neocortex varied markedly from region to region and within different layers of the same regions. The retrograde transport of True Blue combined with immunofluorescence for localization of serotonin revealed that serotonergic axons within the forebrain arise mainly within the dorsal raphe and superior central nuclei, but that some originate within the midbrain and pontine reticular formation and the locus coeruleus, pars alpha. Neurons of the raphe magnus and obscurus also innervate the forebrain, but few of them are serotonergic. The use of horseradish peroxidase as a retrograde marker provided evidence that raphe projections to the forebrain are topographically organized. Our results suggest that serotonergic projections to the forebrain, like those to the spinal cord, are connectionally heterogeneous.     

Catecholamine innervation of the basal forebrain. II. Amygdala, suprarhinal cortex and entorhinal cortex

The catecholamine (CA) innervation of the posterior basal forebrain, the amygdala, suprarhinal cortex and entorhinal cortex, was studied in the rat using biochemical assay and fluorescence histochemistry. The assay studies demonstrate a moderate norepinephrine (NE) content in the amygdala and entorhinal cortex with a lower value for the suprarhinal cortex. Following destruction of the locus coeruleus, the decrease in NE content of these basal forebrain structures indicates that their principal NE innervation is from locus coeruleus. An additional small NE input arises from the medullary NE neuron groups. Ablation of dopamine (DA) cell groups (substantia nigra-ventral tegmental area, SN-VTA) indicates that the DA input to the amygdala arises from the lateral VTA and medial half of the SN. Fluorescence histochemical studies using the glyoxylic acid-Vibratome technique demonstrate the presence of four distinct types of CA neuron terminal plexus in the posterior basal forebrain. These include two different DA fiber types arising in SN-VTA, small NE fibers with small varicosities arising in locus coeruleus and NE fibers with larger varicosities arising in other brainstem NE cell groups. The large NE fibers appear to enter the amygdala via the ansa peduncularis-ventral amygdaloid bundle to innervate the central and basolateral nucleus and the anterior amygdaloid area. The locus coeruleus NE fibers appear to enter the posterior basal forebrain via both the stria terminalis and ansa peduncularis-ventral amygdaloid bundle system to form a moderately dense innervation of the central and basolateral nuclei of the amygdala and a less dense innervation of the other areas. The DA neuron axons are concentrated in the central and basal nuclei and intercalated cell groups. Other areas receive a more diffuse DA input, with the exception of the moderately dense innervation of the suprarhinal cortex and DA "islands" in the ventral-anterrior entorhinal cortex, The DA input to the posterior basal forebrain is complex and heterogeneous and the axonal morphology differs greatly among the terminal fields within the amygdala and adjacent cortical areas.     

Dopamine-β-hydroxylase-like immunoreactivity in the fetal cerebral cortex of the rat: Noradrenergic ascending pathways and terminal fields

A topographical analysis of the noradrenergic innervation in the fetal rat cerebral cortex was carried out from embryonic day 15 (E15) until birth using antibodies raised against dopamine-β-hydroxylase (DBH). During late gestation DBH-like immunoreactive axons were coursing through the basal forebrain along three pathways:

1. (1) a medial component reached the medial cortex and then ran caudally along the anlage of the cingulum bundle;

2. (2) a lateral component reached the frontal pole and curved ventro-dorsally in the primordium of the external capsule;

3. (3) a few fibers were observed along the ventral amygdaloid bundle toward the amygdaloid complex and the surrounding cortex. No DBH positive fibers were observed in the main body of the internal capsule.

The first noradrenergic axons were seen at E17 in the frontal pole, the lateral frontal cortex, and in the medial frontal cortex which also receives a dopaminergic input. The innervation then extended caudally, but the dorsal part of the cortex was reached after a 2-day delay when compared to the medial and lateral parts. The arrival of noradrenergic axons did not parallel the gradient of cortical neurogenesis; however, all cortical areas were innervated at birth. DBH positive fibers reached a given cortical region simultaneously through the marginal and intermediate zones and then invaded the cortical plate.     

Catecholaminergic systems in the zebrafish. II. Projection pathways and pattern of termination of the locus coeruleus

The locus coeruleus is a widely projecting isthmal noradrenergic nucleus. In the zebrafish, it consists of between three and ten neurons, most of which have multiple, bilaterally projecting axons. Immunohodolgical studies show that the locus coeruleus provides most, if not all, of the noradrenergic innervation of the brain rostral to the isthmus. The pathways and targets in the zebrafish are similar to ascending coeruleal projections of other vertebrates. Axons ascend through two amin pathways: the longitudinal catecholamine bundle and the periventricular catecholamine pathway. The former is a dense meshwork of varicosity-bearing axons which ascends along the lateral longitudinal fasciculus into the mesencephalon. In the posterior tuberal area, this bundle dives ventrally and assumes a lateral position. In the diencephalon, it takes up a position ventral to the medial forebrain bundle, and follows this bundle into the telencephalon, where it joins the medial olfactory tract to enter the olfactory bulb. The periventricular catecholamine pathway is a diffuse pathway consisting of thick, smooth axons. It is associated with the medial longitudianl fasciculus. Rostral to the nucleus of the medial longitudinal fasciculus, this pathway joins the longitudinal catecholamine bundle around the medial forebrain bundle. The Periventricular pathway gives rise to coarse terminal arbors with large but sparse varicosities, whereas the longitudinal catecholamine bundle gives rise to terminal plexuses with fine and dense fibers and varicosities. Among the more densely innervated regions are the raphé nucleus, the interpeduncular nucleus, the torus semicircularis, parts of the hypothalamus, and the suprachiasmatic and preoptic areas. The tours longitudinalis, optic tectum, cerebellum, habenular complex, the dorsomedial zone of area dorsalis telencephali, and the olfactory bulb are moderately innervated. The nucleus glomerulosus, the torus lateralis and lateral subnuclei of the nucleus diffusus, and the anterior tuberal nucleus are devoid of noradrenergic innervation. © 1994 Wiley-Liss, Inc.     

6-Hydroxydopamine induces serotonergic axon sprouting in cerebral cortex of newborn rat

Newborn rats were administered the neurotoxin 6-hydroxydopamine (6-OHDA) to determine whether neonatal ablation of the noradrenergic (NE) innervation produces augmented growth (i.e., sprouting) of serotonergic (5-HT) raphe-cortical axons. Following NE denervation at birth, the density of 5-HT axons in motor cortex (AG1) was determined at 4 days postnatal. Using a computer microscope system, the positions of all 5-HT-positive axons were mapped in radial strips of cortex from treated and control rats. Cumulative axon length, expressed as a function of area inspected, was used as a parameter of innervation density. Following 6-hydroxydopamine, the cumulative length of 5-HT axons in motor cortex increases by 32% (P less than 0.05) while cortical serotonin levels measured by HPLC concomitantly increase by 29% (P less than 0.005). The combined increases in 5-HT axon density and in neurotransmitter levels indicate that NE denervation produces increased growth of the cortical 5-HT innervation by the 4th postnatal day. The amount of transmitter stored per unit length of 5-HT axons appears unchanged. In 6-OHDA-treated rats, 5-HT axons exhibit augmented growth in all layers of motor cortex. In the treated rats, the relative density of 5-HT axons in each cortical layer is roughly proportional to the normal innervation density. Accordingly, in motor cortex, the magnitude of 5-HT axon sprouting is greatest in layer VI, which normally receives a dense 5-HT innervation, and is less in layer V, which has a lower innervation density. Qualitative assessment of other cortical areas following 6-OHDA reveals that 5-HT axon density appears increased in cortical zones that normally receive a dense 5-HT innervation, while the density remains low in zones with sparse innervation. The absence of axonal sprouting is particularly striking in those zones which receive a dense NE innervation but are sparsely innervated by 5-HT axons. Thus, while 5-HT axons undergo sprouting, they do not appear to replace ablated NE terminals in areas with a sparse 5-HT innervation. Hence, normal laminar and regional specificity of 5-HT axons is preserved despite ablation of NE afferents. These data indicate that, while NE denervation may trigger serotonergic sprouting, competition between NE and 5-HT fibers for the same postsynaptic sites is not the main factor that regulates postnatal growth of these axonal projections. The present findings demonstrate that the early development of raphe-cortical projections is influenced by NE cortical innervation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Disappearance of supra-ependymal 5-HT axons in the rat forebrain after electrolytic and 5,6-DHT-induced lesions of the medial forebrain bundle.

The present study was intended to demonstrate the origin of supra-ependymal 5-hydroxytryptamine axons in the rat forebrain. Electrolytic lesions and injections of 5,6-dihydroxytryptamine (10 mug in 4 mul) were carried out unilaterally in and close to the medial forebrain bundle in the posterior hypothalamus of rats. Ten to 14 days later, terminal axons and formaldehyde-induced indolealkylamine fluorescence had virtually disappeared supra-ependymally in the lateral ventricles and interventricular foramina ipsilateral to the lesion if the indolealkylamine axons passing through the medial part of the medial forebrain bundle had been destroyed. No changes were observed, electron microscopically or fluorescence histochemically, in ventricles contralateral to the lesion. It is concluded that the supra-ependymal serotonergic nerve terminals in the lateral ventricles and interventricular foramina originate, uncrossed, from non-terminal axons passing through the medial forebrain bundle in the posterior hypothalamus.     

Organization of cerebral cortical afferent systems in the rat. II. Magnocellular basal nucleus

The organization of the magnocellular basal nucleus (MBN) projection to cerebral cortex in the rat has been studied by using cytoarchitectonic, immunohistochemical, and retrograde and anterograde transport methods. The distribution of retrogradely labeled basal forebrain neurons after cortical injections of wheat germ agglutinin-horseradish peroxidase conjugate was essentially identical to that of neurons staining immunohistochemically for choline acetyltransferase. These large (20-30 micrometers perikaryon diameter) multipolar neurons were found scattered through a number of basal forebrain cell groups: medial septal nucleus, nucleus of the diagonal band of Broca, magnocellular preoptic nucleus, substantia innominata, and globus pallidus. This peculiar distribution mimics the locations of pathways by which descending cortical fibers enter the diencephalon. Each cortical area was innervated by a characteristic subset of MBN neurons, always located in close association with descending cortical fibers. In many instances anterogradely labeled descending cortical fibers appeared to ramify into diffuse terminal fields among MBN neurons which were retrogradely labeled by the same cortical injection. Double label experiments using retrograde transport of fluorescent dyes confirmed that MBN neurons innervate restricted cortical fields. Anterograde autoradiographic transport studies after injections of 3H-amino acids into MBN revealed that MBN axons reach cerebral cortex primarily via two pathways: (1) The medial pathway, arising from the medial septal nucleus, nucleus of the diagonal band, and medial substantia innominata and globus pallidus MBN neurons, curves dorsally rostral to the diagonal band nucleus, up to the genu of the corpus callosum. Most of the fibers either directly enter medial frontal cortex or turn back over the genu of the corpus callosum into the superficial medial cingulate bundle. Many of these fibers enter anterior cigulate or retrosplenial cortex, but some can be traced back to the splenium of the corpus callosum, where a few enter visual cortex but most turn ventrally and sweep into the hippocampal formation. Here they are joined by other fibers which, at the genu of the corpus callosum, remain ventrally located and run caudally through the dorsal fornix into the hippocampus. (2) The lateral pathway arises in part from medial septal, diagonal band, and magnocellular preoptic neurons whose axons sweep laterally through the substantia innominata to innervate primarily piriform, perirhinal, and endorhinal cortex. Some of these fibers may also enter the hippocampal formation from the entorhinal cortex via the ventral subiculum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Efferents of the medial preoptic area in the guinea pig: An autoradiographic study

Medial preoptic axons were traced into the diagonal band of Broca and septum, particularly lateral septum. Other labeled fibers could be followed dorsally from medial preoptic area injections adjacent to the stria medullaris, and in the periventricular fiber system and the stria terminalis and its bed nucleus. The anterior and medial amygdaloid nuclei were labeled by fibers via the stria terminalis and others arching over the optic tract and through the substantia innominata. The lateral habenula was labeled. Labeled periventricular fibers reached the periventricular nucleus of the thalamus. Descending efferents were traced principally below the fornix and in the adjacent lateral hypothalamus to label the anterior hypothalamus, the tuberal nuclei, and median eminence. Axons of the medial preoptic area joined the medial part of the medial forebrain bundle and distributed to the reticular formation and the central gray of the midbrain and pons. A small amount of contralateral connections were described.     

Organization of ascending hypothalamic projections to the rostral forebrain with special reference to the innervation of cholinergic projection neurons

Axonal projections from hypothalamic nuclei to the basal forebrain, and their relation to cholinergic projection neurons in particular, were studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with choline acetyltransferase (ChAT) immunocytochemistry. Discrete iontophoretic PHA-L injections were delivered to different portions of the caudal lateral hypothalamus, as well as to various medial hypothalamic areas, including the ventromedial, dorsomedial, and paraventricular nuclei, and anterior hypothalamic and medial preoptic areas. The simultaneous detection of PHA-L-labeled fibers/terminals and ChAT-positive neurons was performed by using nickel-enhanced diaminobenzidine (DAB) and nonenhanced DAB as chromogens. Selected cases were investigated at the electron microscopic level. Ascending hypothalamic projections maintained an orderly lateromedial arrangement within the different components of the medial forebrain bundle, as well as with respect to their terminal projection fields (e.g., within the bed nucleus of the stria terminalis and lateral septal nucleus). The distribution pattern of hypothalamic inputs to cholinergic projection neurons corresponded to the topography of ascending hypothalamic axons. Axons originating from neurons in the far-lateral hypothalamus reached cholinergic neurons in a zone that extended from the dorsal part of the sublenticular substantia innominata (SI) caudolaterally, to the lateral portion of the bed nucleus of the stria terminalis rostromedially, encompassing a narrow band along the ventral part of the globus pallidus and medial portion of the internal capsule. Axons originating from cells in the medial portion of the lateral hypothalamus reached cholinergic cells primarily in more medial and ventral parts of the SI, and in the magnocellular preoptic nucleus and horizontal limb of the diagonal band nucleus (HDB). Axons from medial hypothalamic cells appeared to contact cholinergic neurons primarily in the medial part of the HDB, and in the medial septum/vertical limb of the diagonal band complex. Electron microscopic double-labeling experiments confirmed contacts between labeled terminals and cholinergic cells in the HDB and SI. Individual hypothalamic axons established synapses with both cholinergic and noncholinergic neuronal elements in the same regions. These findings have important implications for our understanding of the organization of afferents to the basal forebrain cholinergic projection system.     

Efferents from medial basal forebrain and hypothalamus in the rat. I. An autoradiographic study of the medial preoptic area.

Efferent projections from the medial and periventricular preoptic area, bed nucleus of the stria terminalis and nuclei of the diagonal band were traced using tritiated amino acid autoradiography in albino rats. Medial and periventricular preoptic area efferents were not restricted to short-axon projections. Ascending projections from the medial preoptic area (mPOA) were traced through the diagonal band into the septum. Descending mPOA axons coursed in the medial parts of the medial forebrain bundle. Projections to most hypothalamic nuclei, including the arcuate nucleus and median eminence, were observed. In the midbrain, mPOA efferents were distributed in the central grey, raphe nuclei, ventral tegmental area and reticular formation. Projections from the mPOA were also observed to the amygdala through the stria terminalis, to the lateral habenula through the stria medullaris, and to the periventricular thalamus. Axons of the most medial and periventricular preoptic area (pvPOA) neurons had a distribution similar to more lateral mPOA neurons but their longest-axoned projections were weaker. The pvPOA did not send axons through the stria medullaris but did project more heavily than the more lateral mPOA to the arcuate nucleus and median eminence. Projections from the bed nucleus of the stria terminalis (nST) were in most respects similar to those from the medial preoptic area, with the major addition of a projection to the accessory olfactory bulb. The nuclei of the diagonal band of Broca (nDBB) gave a different pattern of projections than mPOA or nST, projecting, for instance, to the medial septum and hippocampus. Descending nDBB efferents ran in the ventral portion of the medial forebrain bundle. Among hypothalamic cell groups, only the medial mammillary nuclei received nDBB projections. nDBB efferents also distributed in the medial and lateral habenular nuclei and the mediodorsal thalamic nucleus.     

Distribution of two morphologically distinct subsets of serotoninergic axons in the cerebral cortex of the marmoset

The serotoninergic innervation of the marmoset (New World monkey, Callithrix jacchus) cerebral cortex has been analyzed by using immunocytochemistry. The use of a sensitive monoclonal antibody against serotonin allowed the visualization of the fine morphology of individual axons. Two types of terminal axons were demonstrated: one has sparse, small, ovoid varicosities (dia. less than 1 micron), and the other has large, spheroidal varicosities (up to 5 microns in dia.), which are more densely clustered. The first type of axon is distributed through all cortical layers, with a characteristic laminar distribution that varies from area to area. The second type of axons was distributed sparsely in all regions but was markedly denser in the frontal and anterior parietal lobes, and in the hippocampal formation. Axons with large varicosities typically surrounded certain cell bodies and proximal dendrites, forming pericellular arrays, or baskets. These morphological specializations were most frequent in the frontal and anterior parietal cortex, where they were found around stellate and horizontal cells in layer I and around stellate and bipolar cells in layer II and III. Similar baskets were also found in the hippocampal formation, mainly along the border between the hilus and the granule cell layer of the dentate gyrus, across the CA4 field, and at each side of the pyramidal cell layer of the CA3 regions. The distribution and cellular morphology of the cell surrounded by the 5-HT basket fibres were suggestive of a subpopulation of interneurons, possibly GABAergic and/or peptidergic. In agreement with previous reports on the innervation of the cerebral cortex of other mammalian species, the marmoset cerebral cortex is innervated by two separate subsystems of serotoninergic axons. One of these may have a strong and specific influence on the cortical inhibitory circuitry, via relay through cortical interneurons.     

Spatially and temporally restricted chemoattractive and chemorepulsive cues direct the formation of the nigro-striatal circuit

Identifying cellular and molecular mechanisms that direct the formation of circuits during development is thought to be the key to reconstructing circuitry lost in adulthood to neurodegenerative disorders or common traumatic injuries. Here we have tested whether brain regions situated in and around the developing nigro-striatal pathway have particular chemoattractive or chemorepulsive effects on mesencephalic dopamine axons, and whether these effects are temporally restricted. Mesencephalic explants from embryonic day (E)12 rats were either cultured alone or with coexplants from the embryonic, postnatal or adult medial forebrain bundle region (MFB), striatum, cortex, brain stem or thalamus. Statistical analysis of axon growth responses revealed a potent chemoattraction to the early embryonic MFB (i.e. E12-15) that diminished (temporally) in concert with the emergence of chemoattraction to the striatum in the late embryonic period (i.e. E19+). Repulsive responses by dopaminergic axons were obvious in cocultures with embryonic brain stem and cortex, however, there was no effect by the thalamus. Such results suggest that the nigro-striatal circuit is formed via spatially and temporally distributed chemoattractive and chemorepulsive elements that: (i) orientate the circuit in a rostral direction (via brain stem repulsion); (ii) initiate outgrowth (via MFB attraction); (iii) prevent growth beyond the target region (via cortical repulsion); and (iv) facilitate target innervation (via striatal chemoattraction). Subsequent studies will focus on identifying genes responsible for these events so that their products may be exploited to increase the integration of neuronal transplants to the mature brain, or provide a means to (re)establish the nigro-striatal circuit in vivo.     

Correspondence between 5-HT2 receptors and serotonergic axons in rat neocortex

The anatomic relationship between serotonergic (5-HT) axons and 5-HT2 receptors in the rat forebrain was determined by a combined analysis of transmitter immunocytochemistry and receptor autoradiography. High densities of 5-HT2 receptors, localized by the ligand N1-methyl-2-125I-LSD (125I-MIL), are found in neocortex and striatum; these regions also receive a dense serotonergic innervation. Regional variations in the density of 5-HT2 receptors and 5-HT axons correspond closely in most, but not all, areas of the forebrain. In somatosensory cortex (SI), the laminar distribution of 5-HT2 receptors closely matches that of 5-HT axons: in particular, a dense band of 5-HT2 receptors in layer Va of SI is in precise register with a dense plexus of fine 5-HT axons. We have also observed a close spatial relationship between 5-HT2 receptors and fine axons in other areas of the forebrain, suggesting that 5-HT2 receptors may be selectively linked to a particular type of 5-HT axon terminal. Since fine axons of this type have been reported to arise from the dorsal raphe nucleus, it appears likely that 5-HT2 receptors may mediate the effects of dorsal but not median raphe projections.     

An autoradiographic analysis of the differential ascending projections of the dorsal and median raphe nuclei in the rat

The differential projections from the dorsal raphe and median raphe nuclei of the midbrain were autoradiographically traced in the rat brain after ³H-proline micro-injections. Six ascending fiber tracts were identified, the dorsal raphe nucleus being the sole source of four tracts and sharing one with the median raphe nucleus. The tracts can be classified as those lying within the medial forebrain bundle (dorsal raphe forebrain tract and the median raphe forebrain tract) and those lying entirely outside (dorsal raphe arcuate tract, dorsal raphe periventricular tract, dorsal raphe cortical tract, and raphe medial tract). The dorsal raphe forebrain tract lies in the ventrolateral aspect of the medial forebrain bundle (MFB) and projects mainly to lateral forebrain areas (e.g., basal ganglion, amygdala, and the pyriform cortex). The median raphe forebrain tract lies in the ventromedial aspect of the MFB and projects to medial forebrain areas (e.g., cingulate cortex, medial septum, and hippocampus). The dorsal raphe cortical tract lies ventrolaterally to the medial longitudinal fasciculus and projects to the caudate-putamen and the parieto-temporal cortex. The dorsal raphe periventricular tract lies immediately below the midbrain aqueduct and projects rostrally to the periventricular region of the thalamus and hypothalamus. The dorsal raphe arcuate tract curves laterally from the dorsal raphe nucleus to reach the ventrolateral edge of the midbrain and projects to ventrolateral geniculate body nuclei and the hypothalamic suprachiasmatic nuclei. Finally, the raphe medial tract receives fibers from both the median and dorsal raphe nuclei and runs ventrally between the fasciculus retroflexus and projects to the interpeduncular nucleus and the midline mammillary body. Further studies were done to test whether the fiber tracts travelling in the MFB contained 5-HT. Unilateral (left) injections of 5,7-dihydroxytryptamine (5 μgm/400 nl) 18 days before midbrain raphe microinjections of ³H-proline produced a reduction in the grain concentrations in all the ascending fibers within the MFB. Furthermore, pharmacological and behavioural evidence was obtained to show that the 5-HT system had been unilaterally damaged; these animals displayed preferential ipsilateral turning in a rotameter which was strongly reversed to contralateral turning after 5-hydroxytryptophan administration. The results show that DR and MR nuclei have numerous ascending projections whose axons contain the transmitter 5-HT. The results agree with the neuroanatomical distribution of the 5-HT system previously determined biochemically, histochemically, and neurophysiologically. The midbrain serotonin system seems to be organized by a series of fiber pathways. The fast transport rate in these fibers was found to be about 108 mm/day.     

Receptor tyrosine phosphatases regulate birth order-dependent axonal fasciculation and midline repulsion during development of the Drosophila mushroom body

Receptor tyrosine phosphatases (RPTPs) are required for axon guidance during embryonic development in Drosophila. Here we examine the roles of four RPTPs during development of the larval mushroom body (MB). MB neurons extend axons into parallel tracts known as the peduncle and lobes. The temporal order of neuronal birth is reflected in the organization of axons within these tracts. Axons of the youngest neurons, known as core fibers, extend within a single bundle at the center, while those of older neurons fill the outer layers. RPTPs are selectively expressed on the core fibers of the MB. Ptp10D and Ptp69D regulate segregation of the young axons into a single core bundle. Ptp69D signaling is required for axonal extension beyond the peduncle. Lar and Ptp69D are necessary for the axonal branching decisions that create the lobes. Avoidance of the brain midline by extending medial lobe axons involves signaling through Lar.     

Ascending projections of the locus coeruleus in the rat. II. Autoradiographic study.

The ascending projections of the locus coeruleus were studied using an autoradiographic method. The major projection of locus coeruleus neurons ascends in a dorsal pathway traversing the midbrain tegmentum in a position ventrolateral to the periaqueductal gray. At the caudal diencephalon the locus coeruleus axons descend to enter the medial forebrain bundle at a caudal tuberal hypothalamic level. They are jointed in the medial forebrain bundle by a much smaller locus coeruleus projection which takes a ventral course through the midbrain tegmentum and enters the medial forebrain bundle via the mammillary peduncle and ventral tegmental area. Terminal projections are evident in the midbrain to the periaqueductal gray, tegmentum and raphe nuclei. There are widespread projections to the dorsal thalamus. The heaviest of these are to the intralaminar nuclei, the anteroventral and anteromedial nuclei, the dorsal lateral geniculate and the paraventricular nucleus. In the hypothalamus the largest projections are to the lateral hypothalamic area, periventricular nucleus, supraoptic nucleus and paraventricular nucleus. As the locus coeruleus projection ascends in the medial forebrain bundle, fibers leave it to traverse the lateral hypothalamus and zona incerta and enter the internal capsule, the ventral amygdaloid bundle and ansa peduncularis. These appear to terminate in the amygdaloid complex and, via the external capsule, in the lateral and dorsal neocortex. At the level of the septum 4 projections are evident. One group of fibers enters the stria medullaris to terminate in the paraventricular nucleus and habenular nuclei. A second group joins the stria terminalis to terminate in the anygdaloid complex. The third group turns into the diagonal band and medial septum; some fibers terminate in the septal nuclei and others continue into the fornix to termimate in hippocampus. A large component continues around the corpus callosum into the cingulum to terminate in the cingulate and adjacent neocortex, the subiculum and hippocampus. The remaining fibers continue rostrally in the medial forebrain bundle to terminate in olfactory forebrain and frontal neocortex. Commissural projections arise at 4 locations. The first decussation occurs in the dorsal tegmentum just below the central gray rostral to the locus coeruleus. The crossing fibers enter the contralateral dorsal bundle. A second group of fibers leaves the ipsilateral dorsal pathway, crosses in the posterior commissure and enters the contralateral dorsal pathway at the level. The third commissural projection arises more rostrally and crosses in the dorsal supraoptic commissure to enter the contralateral medial forebrain bundle. The fourth commissural projection is through the anterior commissure. The termination of the contralateral projection appears similar to that of the ipsilateral projection.     

Basal forebrain inputs to the interpeduncular nucleus in the rat studied with the Phaseolus vulgaris-leucoagglutinin tracing method

The connections between the basal forebrain and the interpeduncular nucleus (IP) were studied in the rat using the anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L). PHA-L was injected in the septum-diagonal band complex, the preoptic area, the substantia innominata, the globus pallidusand the ventral pallidum. In a number of cases sections of the IP were double-immunostained for PHA-L and serotonin. Only following PHA-L injections in the medial septal nucleus and the nucleus of the vertical limb of the diagonal band, were substantial terminations found throughout all subnuclei of IP. Following injections in the medial and lateral preoptic area labeling was confined to the caudal part of IP. This finding suggests that the area in the basal forebrain that contributes to these projections is smaller than has been indicated by previous retrograde tracing studies. Labeled fibers reach the IP predominantly via the medial forebrain bundle. Only a very small number of fibers reaches the ventral mesencephalonand possibly the IP, via the stria medullaris and the fasciculus retroflexus. The highest density of terminations was seen in the apical subnucleus. The apical subnucleus contains serotonin-immunoreactive neurons, in the direct vicinity of which varicosities on the labeled fibers were seen. This finding suggests innervation of the serotonergic neurons by fibers from the medial septum and preoptic area.