Origin of projections from the midbrain raphe nuclei to the hypothalamic paraventricular nucleus in the rat: A combined retrograde and anterograde tracing study

A number of neuronal functions governed by the hypothalamic paraventricular nucleus are influenced by serotonin, and it is generally believed that the moderate density of serotonin-immunoreactive fibres and terminals within the paraventricular nucleus originates from the midbrain dorsal and median raphe nuclei. To further evaluate the intricate anatomy of projections from brain stem raphe nuclei of the rat, a combination of retrograde and anterograde tracing experiments were conducted to determine the medullary raphe nuclei projection to the paraventricular nucleus. Rhodamine-labelled latex microspheres, Cholera toxin subunit B and FluoroGold we used as retrograde tracers. Intracerebroventricular injections into the third ventricle of all retrograde tracers labelled a distinct population of neurons in the dorsal raphe situated in the subependymal stratum adjacent to the cerebral aqueduct indicating that these cells take up the tracer from the cerebrospinal fluid. Very few retrogradely labelled neurons were seen in the median raphe after i.c.v. administration of the tracers. Retrograde tracers delivered into the medial part of the paraventricular nucleus labelled no further cells in the midbrain dorsal and median raphe nuclei, whereas a substantial number of retrogradely labelled cells emerged in the pontine raphe magnus. However, when the retrograde tracers were delivered into the lateral part of the paraventricular nucleus, avoiding leakage of the tracer into the ventricle, very few labelled neurons were seen in the dorsal and median raphe, whereas the prominent labelling of raphe magnus neurons persisted. The anatomical organization of nerve fibres terminating in the area of paraventricular nucleus originating from midbrain raphe nuclei was studied in a series of anterograde tracing experiments using the plant lectin Phaseolus vulgaris leucoagglutinin. Injections delivered into the dorsal raphe or median raphe labelled but a few fibres in the paraventricular nucleus proper. A high number of fine calibered nerve fibres overlying the ependyma adjacent to the paraventricular nucleus was, however, seen after the injections into the subependymal rostral part of the dorsal raphe. Injections delivered into the raphe magnus gave rise to a dense plexus of terminating fibres in the parvicellular parts of the paraventricular nucleus and moderately innervated the posterior magnocellular part of the paraventricular nucleus as well as the magnocellular supraoptic nucleus. Concomitant visualization of serotonin-immunoreactive neurons and retrograde FluoroGold-tracing from the paraventricular nucleus revealed that none of the serotonergic neurons of the raphe magnus projects to this nucleus, while a few of the neurons putatively projecting to the paraventricular nucleus from the median raphe are serotonergic.

The current observations suggest that the raphe magnus constitute by far the largest raphe input to the paraventricular nucleus and strongly questions the earlier held view that most raphe fibres innervating the paraventricular nucleus are derived from the midbrain dorsal and median raphe. However, the source of serotonergic innervation of the paraventricular nucleus remains elusive.     

大鼠视前内侧区的传出性神经纤维联系—WGA—HRP法研究

应用WGA-HRP顺行轴突运输研究大鼠视前内侧区传出性神经纤维投射。结果表明:视前内侧区的上行投射向嘴侧经斜角带进入外侧隔核;经髓纹进入缰核;经无名质进入杏仁前区及经终纹进入杏仁内侧核,另有标记纤维经内侧前脑束向外下行,经视束上方进入杏仁内侧核。下行投射经内侧前脑束进入下丘脑室旁核、外侧区、内侧核、后核、弓状核、乳头体前腹核和乳头体上核。继续向尾侧,标记纤维进入中脑腹侧背盖区,并投射到中缝正中核及中缝背核。     

Locomotor activity and ingestive behavior after damage to ascending serotonergic systems

The effects of electrolytic midbrain raphe lesions on ingestive behavior and locomotor activity of rats were compared to those produced by intracerebral injections of 5,7 dihydroxytryptamine (5,7 DHT) at various points along the ascending serotonergic pathways. Only electrolytic lesions of the median and/or dorsal raphe nuclei produced significant changes in food intake, water intake, body weight gain, and wheel running activity. Intracerebral injections of 5,7 DHT, a selective serotonergic neurotoxin, had no effect on any of these variables. However, 5,7 DHT induced lesions produced decreases in forebrain synaptosomal uptake of serotonin which were equivalent to, or greater than, those resulting from electrolytic lesions of the midbrain raphe nuclei. Failure of 5,7 DHT injections to replicate the behavioral changes resulting from electrolytic lesions of the midbrain raphe nuclei suggests that loss of ascending serotonergic projections was not responsible for the behavioral effects that followed the electrolytic lesions.     

Organization of projections from the raphe nuclei to the vestibular nuclei in rats

Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.     

Pattern of distribution of serotonergic fibers to the thalamus of the rat

It is well established that serotonergic (5-hydroxytryptamine, 5-HT) fibers, mainly originating from the dorsal and median raphe nuclei of the brainstem, distribute throughout the forebrain, most heavily to ‘limbic’ forebrain structures. Few reports have examined the distribution of 5-HT fibers to the thalamus and none to our knowledge using immunoprocedures for the detection of the serotonin transporter (SERT)—a very sensitive marker for 5-HT fibers. Using immunohistochemical methods for SERT, we examined the pattern of distribution of 5-HT fibers to the thalamus in the rat. We show that serotonergic fibers are heavily concentrated in midline, intralaminar and association nuclei of the thalamus, and with the exception of the lateral geniculate complex, weakly distributed to principal nuclei of thalamus. Specifically, we demonstrate that 5-HT fibers are densely concentrated in the anteroventral, anteromedial and interanteromedial nuclei of the anterior thalamus, the paraventricular, rhomboid and reuniens nuclei of the midline thalamus, the central medial and central lateral nuclei of the intralaminar thalamus, the intermediodorsal nucleus, the lateral dorsal nucleus, and the dorsal and ventral lateral geniculate nuclei and intergeniculate leaflet of the LGN complex. Less densely innervated sites include the mediodorsal, paracentral, parafascicular, lateral posterior and submedial nuclei of thalamus. Remaining regions of the thalamus, largely consisting of principal nuclei, contained few 5-HT fibers. This pattern of 5-HT innervation indicates that serotonin/serotonergic fibers mainly affect thalamic nuclei with connections to ‘non-principal’ or limbic regions of the cortex (or forebrain). This suggests that serotonergic fibers to the thalamus may exert a significant influence on affective and cognitive functions, possibly complementing the actions of 5-HT fibers to other parts of the brain involved in emotional and cognitive behaviors.     

Selective anterograde tracing of the individual serotonergic and nonserotonergic components of the dorsal raphe nucleus projection to the vestibular nuclei

It is well known that the dorsal raphe nucleus (DRN) sends serotonergic and nonserotonergic projections to target regions in the brain stem and forebrain, including the vestibular nuclei. Although retrograde tracing studies have reported consistently that there are differences in the relative innervation of different target regions by serotonergic and nonserotonergic DRN neurons, the relative termination patterns of these two projections have not been compared using anterograde tracing methods. The object of the present investigation was to trace anterogradely the individual serotonergic and nonserotonergic components of the projection from DRN to the vestibular nuclei in rats. To trace nonserotonergic DRN projections, animals were pretreated with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and then, after 7 days, the anterograde tracer biotinylated dextran amine (BDA) was iontophoretically injected into the DRN. In animals treated with 5,7-DHT, nonserotonergic BDA-labeled fibers were found to descend exclusively within the ventricular plexus and to terminate predominantly within the periventricular aspect of the vestibular nuclei. Serotonergic DRN projections were traced by injecting 5,7-DHT directly into DRN, and amino-cupric-silver staining was used to visualize the resulting pattern of terminal degeneration. Eighteen hours after microinjection of 5,7-DHT into the DRN, fine-caliber degenerating serotonergic terminals were found within the region of the medial vestibular nucleus (MVN) that borders the fourth ventricle, and a mixture of fine- and heavier-caliber degenerating serotonergic terminals was located further laterally within the vestibular nuclear complex. These findings indicate that fine-caliber projections from serotonergic and nonserotonergic DRN neurons primarily innervate the periventricular regions of MVN, whereas heavier-caliber projections from serotonergic DRN neurons innervate terminal fields located in more lateral regions of the vestibular nuclei. Thus, serotonergic and nonserotonergic DRN axons target distinct but partially overlapping terminal fields within the vestibular nuclear complex, raising the possibility that these two DRN projection systems are organized in a manner that permits regionally-specialized regulation of processing within the vestibular nuclei.     

Organization of ascending serotonin systems in the adult rat brain. A radioautographic study after intraventricular administration of [3h]5-hydroxytryptamine

Light microscope radioautography was used to visualize ascending serotonin (5-HT) systems in adult rat brain, following prolonged latero-ventricular instillation of [³H]5-HT. In paraventricular and paracisternal regions, the cell bodies, axonal projections and terminal fields of 5-HT neurons were distinctly labelled, allowing comprehensive analysis of their organizational features. Two major systems of ascending 5-HT fibers could be defined: a transtegmental system originating mainly from nucleus raphe dorsalis (B-7 and B-6) and less prominently from nucleus raphe medianus (B-8), and a periventricular system predominantly issuing from the rostral pole of nucleus raphe dorsalis (B-7). The transtegmental system converges ventral-ward across the decussation of the superior cerebellar peduncles and sweeps rostral-ward, through the ventral tegmental decussation, to enter the ventral tegmental area. The periventricular system closely follows the dorsal longitudinal fasciculus of Schu¨tz. It branches off dorsally toward the superior and inferior colliculi and the subcommissural organ, but the bulk of its fibres arch abruptly ventralward, beneath the posterior commissure, to reach the dorsal hypothalamus. Along their way, these axons most likely contribute to the dense 5-HT innervation of the periventricular gray matter of the midbrain and caudal diencephalon. Both ascending 5-HT systems merge in the medial forebrain bundle area of caudal hypothalamus, wherefrom their fibres borrow several limbic pathways to reach distant territories of innervation: fasciculus retroflexus to medial habenula; striae medullaris and terminalis toward the habenula and the amygdala, respectively; fornix to hippocampus; diagonal band of Broca to septum; induseum griseum and cingulate bundle to hippocampus and neocortex. Other fibers spread more laterally, across the internal capsule, to innervate the globus pallidus and neostriatum. Finally, at the base of anterior forebrain, some 5-HT axons traverse the anterior olfactory nucleus and end in the glomerular layer of olfactory bulb.These and additional results concerning the heterogeneous distribution of 5-HT axonal varicosities in various parts of forebrain (e.g. thalamus, hypothalamus, septal area and amygdaloid complex) complement earlier data gathered by means of other neuroanatomical techniques and make it possible to present a useful model of the general organization of ascending 5-HT systems in the mammalian brain.     

Substance P afferents from the habenula innervate the dorsal raphe nucleus

Summary The lateral habenula nuclei of the diencephalon innervate the median and dorsal raphe nuclei of the brainstem. Habenula lesions lead to decreased substance P levels in the dorsal but not median raphe within 24 hours. From this data, we propose a peptidergic innervation of the dorsal raphe nucleus by the habenula nuclei.     

The human raphe nuclei and the serotonergic system

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.     

Non-serotonergic dorsal and median raphe projection onto parvalbumin- and calbindin-containing neurons in hippocampus and septum

The median raphe nucleus is involved in controlling and maintaining hippocampal activity through its projection to inhibitory neurons in medial septum and hippocampus. It has been shown that anterogradely axonal-traced fibers originating in the median raphe nucleus project onto calbindin-containing neurons in hippocampus and parvalbumin-containing neurons in medial septum. Parallel immunohistochemistry studies showing serotonin fibers contacting calbindin- and parvalbumin-positive neurons have led to the assumption that raphe fibers projecting on these types of neurons are mainly serotonergic. However, in both dorsal and median raphe nucleus there is a large amount of non-serotonergic neurons which also are projecting neurons, indicating that a part of the raphe fibers projecting to hippocampus and septum may be non-serotonergic. Our aim was to determine whether there is a non-serotonergic projection from the raphe nucleus onto calbindin- and parvalbumin-containing neurons in hippocampus and septum. Biotin dextran amine was used as the anterograde neuronal tracer and injected into either dorsal or median raphe nucleus. By use of triple immunofluorescence-labeling we analyzed the serotonergic content of the biotin dextran amine-labeled fibers contacting parvalbumin- and calbindin-positive neurons. Surprisingly, we found a significant non-serotonergic projection from both dorsal and median raphe nuclei onto calbindin- and parvalbumin-containing interneurons in septum and hippocampus, with a preference in hippocampus for projecting onto calbindin-positive neurons. These results indicate that the raphe nuclei may exert their control on hippocampal and septal activity not only through a serotonergic projection, but also through a significant non-serotonergic pathway.     

Evidence for corticocortical connections between areas 7 and 17 in cerebral cortex of the cat

The serotonergic system regulates processing in components of the vestibular nuclear complex, including the medial vestibular nucleus (MVe) and nucleus prepositus hypoglossi (PH). Recent studies using anterograde and retrograde tracers have shown that vestibular nuclei are targeted by regionally selective projections from the serotonergic dorsal raphe nucleus. The objective of the present investigation was to determine whether the DRN is targeted by projections from the vestibular nuclear complex in rats, using the anterograde tracer biotinylated dextran amine (BDA). After injection of BDA into PH or the caudal parvicellular division of MVe, labeled fibers and terminals were observed in the ventromedial and lateral subdivisions of DRN. These findings indicate that projections from the vestibular nuclei and PH are organized to modulate processing within specific functional domains of the DRN.     

Forebrain and brainstem afferents to the arcuate nucleus in the rat: potential pathways for the modulation of hypophyseal secretions

Doxorubicin, an anti-oncogenic agent, was used as a retrograde marker to identify arcuate nucleus afferent projections. Injections of this tracer into the arcuate nucleus indicated that the subfornical organ, the organum vasculosum of the lamina terminalis, the nucleus raphe dorsalis and median raphe send projections to the arcuate nucleus. Immunocytochemical procedures were used to demonstrate that the raphe projections to the arcuate nucleus are serotoninergic. This anatomical investigation provides evidence that neural pathways exist between forebrain body fluid and mineral nuclei, mesencephalic serotonin nuclei and the arcuate nuclei.     

Topographic principles in the spinal projections of serotonergic and non-serotonergic brainstem neurons in the rat

The spinal projections from the raphe-associated brainstem areas containing serotonergic neurons were studied with aldehyde-induced fluorescence in combination with the retrograde fluorescent tracer True Blue in the rat. This technique makes it possible to determine simultaneously the projections of individual neurons and to detect whether serotonin is present in the same neurons. After tracer injections into the spinal cord retrogradely labeled serotonergic and non-serotonergic neurons were found in the medullary raphe nuclei and adjacent regions and to a lesser extent in association with the dorsal and median raphe nuclei in the mesencephalon. Large True Blue injections that covered one side of the spinal cord at mid-cervical level labeled about 60% of the ipsilaterally situated serotonergic neurons in the medullary raphe regions while the corresponding figure contralaterally was about 25%. On both sides a larger number of labeled non-serotonergic neurons were found; these were sometimes located dorsal to, but often intermingled with, the serotonergic cells. While the serotonergic projection from the mesencephalon could not be labeled from injections below cervical levels, the labeling in more caudal brainstem regions exhibited only minor variations depending on the rostrocaudal level of the spinal segment injected. Furthermore, quantitative data from injections at different levels indicate that the majority of the spinal-projecting neurons traverse most of the length of the cord. Summarizing the results obtained from small injections restricted to subregions of the cord we feel that it is possible to distinguish three fairly distinct pathways for spinal projections from the medullary raphe and adjacent regions: The dorsal pathway originates mainly from cells in the caudal pons and rostral medulla oblongata (rostral part of nucleus raphe magnus, nucleus raphe magnus proper, nucleus reticularis gigantocellularis pars alpha and nucleus paragigantocellularis). This pathway, which contains a large non-serotonergic component, descends through the dorsal part of the lateral funiculus and terminates mainly in the dorsal horn at all spinal cord levels. The intermediate pathway is largely serotonergic with its cell bodies located within the arcuate cell group (situated just ventral and lateral to the pyramids very close to the ventral surface of the brainstem) and in the nucleus raphe obscurus and pallidus and terminates in the intermediate grey at thoracolumbar and upper sacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serotonergic and nonserotonergic neurons in the dorsal raphe nucleus send collateralized projections to both the vestibular nuclei and the central amygdaloid nucleus

Using a combination of double retrograde tracing and serotonin immunofluorescence staining, we examined whether individual serotonergic and nonserotonergic neurons in the dorsal raphe nucleus are sources of collateralized axonal projections to vestibular nuclei and the central amygdaloid nucleus in the rat. Following unilateral injections of Diamidino Yellow into the vestibular nuclei and Fast Blue into the central amygdaloid nucleus, it was observed that approximately one-fourth of the dorsal raphe nucleus neurons projecting to the vestibular nuclei send axon collaterals to the central amygdaloid nucleus. Immunofluorescence staining for serotonin revealed that more than half of the dorsal raphe nucleus neurons from which these collateralized projections arise contain serotonin-like immunoreactivity. These findings indicate that a subpopulation of serotonergic and nonserotonergic dorsal raphe nucleus cells may act to co-modulate processing in the vestibular nuclei and the central amygdaloid nucleus, regions implicated in the generation of emotional and affective responses to real and perceived motion.     

Central serotonergic projections to the nucleus tractus solitarii: evidence from a double labeling study in the rat

Projections from several brainstem serotonergic nuclei to the nucleus tractus solitarii were investigated in the rat. Experiments were performed using a double labeling method combining retrograde radioautographic tracing and serotonin immunohistochemistry. After injection of the radioactive tracer ([3H] wheat germ agglutinin) into the lateral nucleus tractus solitarii, nerve cell bodies exhibiting both radioautographic labeling and immunostaining were detected in all the serotonergic nuclei investigated, namely the nucleus raphe magnus, the ventromedial paragigantocellular nucleus, the nuclei raphe pontis, medianus and dorsalis, the medial lemniscus and the reticulotegmental nucleus of the pons. Most of the double labeled perikarya observed were in the nucleus raphe magnus, the adjacent part of the paragigantocellular nucleus and the nucleus raphe dorsalis. Nerve cell bodies retrogradely labeled but devoid of immunostaining were also observed, together with the double labeled perikarya, within serotonergic nuclei. These results provide direct evidence that brainstem serotonergic neurons contribute to the innervation of the nucleus tractus solitarii. They indicate that the nucleus raphe magnus and the nucleus raphe dorsalis constitute two major sources of central serotonergic projections to the nucleus tractus solitarii.     

Afferents to the median raphe nucleus of the rat: retrograde cholera toxin and wheat germ conjugated horseradish peroxidase tracing, and selective D-[3H]aspartate labelling of possible excitatory amino acid inputs

Afferents to the median-paramedian raphe nuclear complex, which contains the B8 serotonergic cell group, were investigated in the rat with neuroanatomical and transmitter-selective retrograde labelling techniques. Injection of sensitive retrograde tracers, cholera toxin genoid or wheat germ agglutinin conjugated horseradish peroxidase into the median raphe resulted in labelling of neurons in a large number of brain regions. Projections from 26 of these regions are supported by available orthograde tracing data; the cingulate cortex, bed nucleus of stria terminalis, medial septum and diagonal band of Broca, ventral pallidum, medial and lateral preoptic areas, lateral hypothalamus, dorsomedial nucleus of hypothalamus, lateral habenula, interpeduncular nucleus, substantia nigra, central (periaqueductal) gray, and laterodorsal tegmental nucleus seem to represent major sources of afferents to the median-paramedian raphe complex. Retrogradely labelled cells were also observed in a number of regions for which anterograde tracing data are not available, including the perifornical hypothalamic nucleus, ventral premammillary nucleus, supramammillary and submammillothalamic nuclei and the B9 area. Possible excitatory amino acid afferents were identified with retrograde D-[3H]aspartate labelling. Microinjection of D-[3H]aspartate at a low concentration, 10(-4) M in 50 nl, resulted in retrograde labelling of a limited number of median raphe afferents. The most prominent labelling was observed in the lateral habenula and the interpeduncular nucleus, but retrogradely labelled cells were also noted in the medial and lateral preoptic areas, lateral and dorsal hypothalamus, ventral tegmental area, laterodorsal tegmental nucleus, medial parabrachial nucleus, and the pontine tegmentum. After injections of 10(-3) M D-[3H]aspartate selective labelling also appeared in more distant afferent regions, including cells in cingulate cortex, and in some regions located at shorter distances, such as the supramammillary nucleus. Injections of D-[3H]aspartate at high concentration, 10(-2) M, resulted in the appearance of weakly to moderately labelled cells in most afferent areas which were devoid of labelled cells after injections of lower concentrations, suggesting that this labelling may be non-specific. It was concluded that the median-paramedian raphe receives afferents from a large number of forebrain and hypothalamic regions, while relatively few brain stem regions project to this nuclear complex. The selectivity of retrograde labelling with D-[3H]aspartate was found to be concentration dependent, and it is suggested that the connections showing high affinity for D-[3H]aspartate may use excitatory amino acids as transmitters. Excitatory amino acid inputs from lateral habenula and interpeduncular nucleus may play predominant roles in the control of ascending serotonergic and non-serotonergic projections originating in the median and paramedian raphe nuclei.     

The organization of serotonergic projections to cerebral cortex in primates: retrograde transport studies.

Retrograde axonal transport and immunocytochemical methods were utilized to determine the origin of serotonergic afferents to selected primary projection and association areas of cerebral cortex in macaque monkeys. After injections of Fast Blue or Diamidino Yellow in primary motor, somatosensory, or visual cortex, retrogradely labeled neurons are found in both the dorsal and median raphe nuclei. The sets of dorsal raphe neurons which innervate these cortical areas differ in their spatial distributions along the rostrocaudal axis of the brainstem; a coarse rostrocaudal topographic relationship is found between these groups of dorsal raphe neurons and their cortical targets. In contrast, neurons in the median raphe which innervate these primary projection areas are not differentially distributed along the rostrocaudal axis. However, in both the median and dorsal raphe nuclei, most neurons projecting to primary visual cortex are situated lateral to the cells which project to motor and somatosensory areas; many of these visually projecting neurons lie among the fascicles of the medial longitudinal fasciculus. For comparison with the serotonergic innervation of primary projection areas, the locations of raphe cells projecting to three areas of association cortex were examined: dorsolateral prefrontal cortex, area 5 and area 7b. Neurons projecting to each of these association areas are found throughout the dorsal and median raphe nuclei. Their distributions are similar to one another; however, more cells projecting to dorsolateral prefrontal cortex are in the rostral part of the dorsal raphe. The dorsal and median raphe neurons projecting to these association areas are intermingled with neurons projecting to motor and somatosensory cortex, but are medial to most of those projecting to visual cortex. Thus, separate cortical areas are innervated by different sets of raphe neurons; these sets partially overlap, yet differ in their rostrocaudal and mediolateral distributions. Ascending serotonergic projections to cerebral cortex form a widely distributed system which exhibits a highly intricate anatomic organization. The present observations support the hypothesis that the dorsal raphe nucleus is comprised of distinct sets of neurons whose output is distributed to multiple, interconnected cortical areas; these serotonergic projections may play a role in the coordination of excitability in functionally related areas of cortex. In contrast, the serotonergic projections arising from the median raphe appear to be more divergent and are likely to have a global influence on cortical activity. Since these individual raphe nuclei have different projection patterns, they are likely to have distinct functional roles.     

A lectin horseradish peroxidase study of the origin of ascending fibers in the medial forebrain bundle of the rat. The upper brainstem

The origins of projections within the medial forebrain bundle from the upper brainstem were examined with the horseradish peroxidase technique. Labeled cells were found in approximately 15 upper brainstem nuclei following injections of a conjugate of horseradish peroxidase and wheat germ agglutinin at various levels of the medial forebrain bundle. Labeled nuclei included (from caudal to rostral): dorsal and ventral parabrachial nuclei; Kolliker-Fuse nucleus; dorsolateral tegmental nucleus; A7 (lateral pontine tegmentum medial to lateral lemniscus); median and dorsal raphe nuclei; distinct group of cells oriented mediolaterally in the dorsal pontine tegmentum below the central gray; B9 (ventral midbrain tegmentum dorsal to medial lemniscus); retrorubral nucleus; nucleus of Darkschewitsch, interfascicular nucleus; rostral and caudal linear nuclei; ventral tegmental area; medial part of substantia nigra, pars compacta; and the supramammillary nucleus. With the exception of the ventral parabrachial nucleus, Kolliker-Fuse, A7, B9 and substantia nigra, pars compacta, each of the nuclei mentioned above sent strong projections along the medial forebrain bundle to the rostral forebrain. Sparse labeling was observed throughout the pontine and midbrain reticular formation. With the exception of the dorsal raphe nucleus, projections to the most anterior regions of the medial forebrain bundle (level of the anterior commissure) essentially only arose from presumed dopamine-containing nuclei-retrorubral nucleus (A8 area), interfascicular nucleus, rostral and caudal linear nuclei, substantia nigra pars compacta, and ventral tegmental area. Evidence was reviewed indicating that major forebrain sites of termination for these dopaminergic nuclei are structures that have been collectively referred to as the 'ventral striatum'. It is concluded from the present findings that several pontine and mesencephalic cell groups are in a position to exert a strong, direct effect on structures in the anterior forebrain and that the medial forebrain bundle is the main communication route between the upper brainstem and the forebrain.     

Behavioral and biochemical studies of the substrates of median raphe lesion induced hyperactivity

Many authors have demonstrated that electrolytic lesions of the median raphe nucleus lead to dramatic hyperactivity, but little is known as to the neural substrates of this effect. In the current series of experiments we investigated this question by examining locomotor activity and forebrain serotonin levels after the placement of wire knife cuts in various locations around the median raphe. Activity was measured in a five minute open field test and a one hour tilt cage test. Knife cuts designed to transect the major ascending serotonergic projections of the median raphe led to a pronounced depletion of forebrain serotonin, but had no effect on locomotor activity in either testing situation. Knife cuts located antero-ventral to the median raphe, designed to interrupt raphe connections with the ventral tegmental area and interpeduncular nucleus, increased activity in the tilt cage but not in the open field test. These cuts produced only small effects on forebrain serotonin levels. Knife cuts caudal to the median raphe failed to influence forebrain serotonin levels, but produced a significant increase in both open field and tilt cage activity. The effects of the posterior and the anteroventral cuts on tilt cage locomotion were additive, suggesting that different fiber systems were damaged by the two cuts. These results demonstrate that it is possible to double dissociate changes in forebrain serotonin levels and locomotor activity with lesions in the vicinity of the median raphe and further show that ascending projections are unlikely to be the only pathways involved in the effects of median raphe lesions on locomotor behavior.     

The distribution of serotonin in the brain of Apteronotus leptorhynchus: an immunohistochemical study

The immunohistochemical distribution of serotonin (5-HT) cells, fiber tracts and terminal fields was mapped in the brain of the gymnotiform electric fish. Two major types of 5-HT cells were found: the small paraventricular organ (PVO) cells of the diencephalon, and the large raphe cells of the brain stem. Six diencephalic nuclei were identified: the nucleus preopticus periventricularis, anterior division, nucleus posterioris periventricularis, nucleus recessus lateralis medial subdivisions 1, 2 and 3, and nucleus recessus posterioris. In the brainstem, raphe centralis, between the arms of the medial longitudinal fasciculus (MLF), the raphe medialis, lateral to MLF, and the diffuse raphe posterioris, were described. Five 5-HT fiber tracts were identified. The tract rising from PVOs projected rostrally through the medial forebrain bundle (MFB). The central tegmental bundle arising from the raphe centralis-medialis complex projected rostrally and also joined the MFB. The lateral tegmental tract, the ventrolateral and the subtrigeminal tracts arose from brain stem raphe groups and innervated the brainstem nuclei. The densest 5-HT innervation occurred in the hypothalamus in the neuropil of the PVOs and in the brainstem in the interpeduncular nucleus, cranial nerve nuclei V motor, V tractus spinalis, VII, X sensory and lateral inferior olive. Electrosensory nuclei including the electrosensory lateral line lobe, the nucleus praeminentialis, dorsal torus semicircularis, optic tectum, nucleus electrosensorius and prepacemaker nucleus received light to medium 5-HT innervation. Serotonergic terminal fields appeared to be conserved across phyla with additional innervation evident in specialized sensory regions such as the electrosensory nuclei of gymnotiform and mormyriform fish.