Daniel A. Pasquier

Dorsal raphe, substantia nigra and locus coeruleus: Interconnections with each other and the neostriatum ☆

Using a retrograde axonal transport method, direct projections to the neostriatum were demonstrated from the dorsal raphe nucleus, a large area of the ventral midbrain tegmentum (including the ventral tegmental area of Tsai, the substantia nigra pars compacta, reticulata and suboculomotoria), and the tegmentum ventral to the caudal red nucleus. A direct projection was also found from the mediodorsal part of the substantia nigra to the rostral part of the dorsal raphe nucleus. Projections from the entopeduncular nucleus (pallidum) and the lateral hypothalamic area to the lateral habenular nucleus, and from the latter to the dorsal raphe nucleus were also found. This habenular projection arises primarily from large neurons in the medial part of the lateral habenula and also from another group of small cells immediately adjacent to the medial habenular nucleus. A non-reciprocal connection of the dorsal raphe nucleus to the locus coernuleus was also found. On the basis of these results and the data available in the literature on the possible neurotransmitters used by these various structures, it is suggested that the dorsal raphe nucleus may play an important role in brain stem modulation of neostriatal function.

Horseradish peroxidase tracing of the lateral habenular-midbrain raphe nuclei connections in the rat

Connections of the habenular complex to the nuclei of the midline in the midbrain (interpeduncularis, medianus raphe, and dorsalis raphe) have been studied classically by anterograde degeneration in the monkey, the cat, and marsupials. Passing fibers from the medial septal nucleus and lateral preoptic area, however, have also been demonstrated which can complicate interpretation of these results. In this paper the habenular projections were studied in the rat by the retrograde axonal transport of horseradish peroxidase (HRP). After HRP injections in the medianus raphe nucleus labelled neurons appeared in the lateral habenular nucleus and parafascicular nucleus. Labelled neurons were also found in the lateral habenular nucleus after injections in either the dorsalis raphe nucleus or the caudal central gray substance. The habenular projections were always bilateral. There were no labelled neurons in the medial habenular nucleus after HRP injections in the medianus raphe especially on the dorsalis raphe neurons which have usually been thought of as functionally related to other brainstem structures. The present results suggest also that in the rat the lateral habenular nucleus might be the link between basal forbrain inputs and the limbic midbrain area. Thus, the raphe nuclei of the midbrain appear to be crucial regions for integrating two descending circuits: first, a limbic (through septum) circuit, and, second, a basal forebrain (through lateral habenular-preoptic area) circuit.

The topographic organization of hypothalamic and brain stem projections to the hippocampus.

Direct projections primarily ipsilateral to hippocampus from medial septal, diagonal band, supramammillary, submammillothalamic, locus coeruleus, and dorsal and medianus raphe nuclei were demonstrated. The locus coeruleus projects primarily through the cingulum and fornix superior to the dorsal posterior hippocampus, with its terminal fields in the stratum lacunosum moleculare of the subiculum and areas CA 1-CA 2 of the dorsal posterior hippocampus. LC projections to the granular layer of the dentate hilus were not found. Raphe nuclei project through the cingulum, fornix superior, and primarily the fimbria, to the dorsal and ventral posterior hippocampus, with their terminal fields in the stratum lacunosum moleculare of the dorsal posterior subicular region, stratum radiatum of CA 1-CA 3 in the dorsal hippocampus, and the stratum polymorph of the dentate gyrus, primarily in its superficial part. Raphe projections to the anterior hippocampal rudiment were found. However, no projection was found to the subiculum of the ventral posterior hippocampus, nor to stratum oriens. Hypothalamic nuclei project through the fornix superior and the fimbria, mainly to the dorsal posterior hippocampus with abundant terminal fibers in the depth of the dentate hilus. Smaller cells in these hypothalamic nuclei appear projecting to the ventral hippocampus. The number of neurons in the entorhinal area, the diagonal band, and the hypothalamic nuclei projecting to the hippocampus suggests these groups as the main sources of the extrinsic hippocampal afferents. In addition, they may also serve as relay stations for inputs from more caudal nuclei, and the topographic organization of their terminal fields as described herein may have important functional implications.

Specific serotonergic projections to the lateral geniculate body from the lateral cell groups of the dorsal raphe nucleus

Serotonin (5-hydroxytryptamine, 5-HT) may play a role in the intrinsic processing of visual information by the lateral geniculate body (LGB). For instance, 5-HT has been related to the slow wave sleep period and to dream episodes during REM sleep, and some hallucinogenic drugs are molecules similar in structure to 5-HT 1. The biochemical demonstration of 5-HT and serotonergic fluorescent fibers in the LGB 5,9 also supports this hypothesis. However, the cell bodies giving origin to the 5-HT in the LGB have not been demonstrated, although it was suggested that they would belong to the mesencephalic raphe nucleP. To demonstrate a dorsal raphe nucleus (DR) projection to the LGB, however, would not prove the existence of a 5-HT connection between them because of the heterogeneity of the DR which receives multiple inputs from a variety of brainstem and diencephalic areas with diverse chemical substrates 11,13. GABAergic neurons, enkephalin-positive reactive neurons and norepinephrine neurons have been described in the DR which, in fact, has only some 5-HT neurons, while other indoleamines are also present (see ref. 13 for a review). Here we report HRP evidence of a projection to the LGB from the lateral parts of the dorsal raphe nucleus (DRL). The presence of serotonergic neurons in this part of the DR is also confirmed by fluorescence microscopy. Finally, we report that pretreatment of the animals with 5,6-dihydroxytryptamine (5,6-DHT), which is thought to be a specific neurotoxin for serotonergic neurons 2, blocked the retrograde axonat transport of the H R P from the LGB to

Locus coeruleus-to-dorsal raphe input examined by electrophysiological and morphological methods☆

In order to examine the hypothesis that the locus coeruleus (LC) projects directly to the nucleus raphe dorsalis (DR), electrical stimulation was applied to the LC of rats while recording from single neurons in the region of the DR. Slow firing units of the DR were not influenced by the stimulation, although faster firing units in the nearby substantia grisea centralis (SGC) were. These latter cells become oscillatory in their firing rates during LC stimulation. In parallel studies a retrograde transport technique was imployed to obtain morphological evidence regarding projections to DR. Placements of horseradish peroxidase precisely in the DR resulted in very sparse labeling in the LC, although positive transport occurred to other areas. The results indicate that the LC does not project directly to slow firing DR neurons, but does influence faster firing celles in the region of the SGC, probably by complex routes. Suggestions are made for the integration of these findings with earlier fluorescence studies.

Noradrenergic perikarya (A5-A7, subcoeruleus) projections to the rat cerebellum

Our current understanding of the identity of the cell bodies responsible for the projection of noradrenergic nerves to the cerebellum is that of the locus coeruleus (A6 group). This has been demonstrated by histochemical means 9 and by assay of the cerebellar catecholamine content 7 following locus coeruleus lesions (for review see ref. 1). A more recent method has demonstrated that the in vivo administration of a specific antibody to dopamine-fl-hydroxylase (ADflH) results in the specific uptake of the antibody to noradrenergic fibers 6 in addition to a retrograde transport from the terminal sympathetic plexus to cell bodies of origin 18. Furthermore, following intraventricular administration, the ADflH was shown to be taken up by noradrenergic nerve terminals and transported in a retrograde manner to associated cell bodies in the brainlL The phenomenon of retrograde transport of ADflH is used in this communication to reveal the cell body groups responsible for the noradrenergic innervation of the cerebellum, and reveals that in addition to the locus coeruleus, the subcoeruleus, A7 and A5 cell body groups contribute to the cerebellar noradrenergic innervation. Fourteen male albino rats (200-250 g body weight) were anesthetized with Chloropent (Fort Dodge Laboratories, Iowa), placed in a Kopf stereotaxic apparatus and their cerebellar cortices exposed for antiserum injections through glass pipettes (30-80/~m internal diameter) attached to a 2 #1 Hamilton syringe (for details see ref. 11). The antiserum was prepared in a goat and directed against bovine adrenal DflH. The pipette system was filled with light immersion oil prior to loading. The antiserum was placed in a small container and the volume to be applied was drawn through the tip of the pipette. A range of 0.0 a, 4.0 #1 was injected in this study. Most of these

Central connections of the toad neural lobe as shown by retrograde neuronal labeling: classical and new connections.

Iontophoretic deposit of horseradish peroxidase (HRP) in the hypophysial neural lobe (NL) of the toad labeled neurons in the classical neurosecretory preoptic magnocellular nucleus. Further, a number of labeled neurons appeared in the whole extension of the periventricular and medial preoptic nuclei. The latter were continuous dorsally with two other labeled areas known as the bed nucleus of the hippocampal commissure (which appears to be homologous to the subfornical organ in mammals) and the ventromedial thalamic area of Frontera. The possible functional role of many of these afferents to the neural lobe is unknown because all types of neurons were labeled in the projecting zones and, in addition, many are outside of the generally recognized neurosecretory groups.

Afferent connections of the hypothalamic retrochiasmatic area in the rat

The afferent projections to the retrochiasmatic area (RCA) of the rat hypothalamus have been studied by means of retrograde axonal transport of horseradish peroxidase. Iontophoretic deposit of the marker was used in most animals, and consistent projections from the hypothalamic ventromedial nucleus, the lateral part of the substantia nigra and the parabigeminal nucleus (NPB) were observed. A comparison between NPB projections and projections from the neighboring tegmentum suggests that some neurons of the NPB project to both the RCA and the superior colliculus. Consequently, these NPB neurons might link visual information on its way toward the retrochiasmatic area which, in view of its strategic position, could play a role in neuroendocrine processes.

Effect of a low protein diet on the anatomical development of subcortical formations

The effects of a low protein diet during gestation, lactation, and after weaning on the anatomical development of a subcortical nucleate formation, the neostriatum, and a reticulate formation, the diagonal band of Broca, were studied. At 10, 30, and 90 days the volume of the neostriatum was decreased in the experimental rats. However, the percent of the brain volume that was neostriatum was unaffected at each of these ages. In a rapid Golgi study of individual neurons at 90 days of age there was no significant effect of the low protein diet on the dendritic length of three different types of neurons within the neostriatum. However, its heavily spined dominant neuron showed a significant decrease in synaptic spine density. In the reticular formation, there was also no significant effect on dendritic length. A minute, apparently axonless cell corresponding to the neurogliaform cell of Ramón y Cajal, showed a decrease in extent of its cell processes only in the neostriatum. When compared to cortical formations, these phylogenetically more conservative neuronal substrates appear to be more resistant to the effects of undernutrition.

Effect of raphe lesions on brain serotonin in the cat

A quantitative study of the regional distribution of serotonin (5-HT) in the forebrain, was performed in adult cats, following brainstem lesions. Seven to 10 days survival times were used to avoid nonspecific variations of the amine levels, as were observed in chronic preparations. Significant decreases of 5-HT levels were found after lesions of the nucleus centralis superior (CS), in hypothalamus, preoptic area, amygdala and hippocampus. After lesions of the nucleus dorsalis raphe (Dr), significant 5-HT decreases were also found in the hypothalamus and hippocampus. There was no change of 5-HT levels after lesions placed in the lateral midbrain, pontine tegmentum, or caudal pontine raphe, contrary to that was reported for chronic lesioned cats. These results suggest that CS and Dr may integrate different serotonergic subsystems and, probably, each one of these subsystems is related to specific functional phenomena.