Istvan Törk

Anatomy of the Serotonergic Systema

The serotonergic (5-hydroxytryptaminergic, 5-HT) system is one of the diffusively organized projection systems of the brain. Although the presence of serotonin itself was demonstrated in the brain more than thirty years ago’,* the full description of its neuronal localization and projections has only become possible with the availability of the histoflu~rescence’~~ and immunohistochemical’ techniques. Indeed it was only by the use of antibodies against serotonin that the detailed anatomy and topography of the serotonergic cells and their projections throughout the brain could be described. Therefore, the description of the 5-HT system below is based entirely on immunohistochemical data although some reference is made to other anatomical observations based on traditional neurohistological and histochemical techniques. The 5-HT system consists of a morphologically diverse group of neurons the cell bodies of which are located in the brainstem raphe nuclei and some regions of the reticular formation, and complex axonal systems which innervate virtually all regions of the central nervous system, but with particular density the cerebral cortex, limbic structures, basal ganglia, many regions of the brainstem and the gray matter of the spinal cord.’-’’ This review discusses, from a structural as well as hodological viewpoint, the organizational principles of the 5-HT system. Since several detailed reviews are available on the serotonergic system, the repetition of all data is largely avoided. Rather, a more salient description of the general features of the brainstem 5-HT neurons and their terminal fields is attempted. All investigators of the mammalian brain have agreed on the finding that the 5-HT system consists of two distinct subdivisions: a rostral division, with cell bodies localized in the midbrain and rostral pons and providing projections to the forebrain, and a caudal division, located primarily in the medulla oblongata, with major descending projections directed principally to the spinal cord.”.” Target areas within the brainstem itself and in the cerebellum are shared by these two divisions. The description given below is in part based on original experimental material (with particular reference to the human brain) prepared to illustrate the general morphology of the serotonergic neurons. For this reason, a brief description of the methodology used in our laboratory for the demonstration of the 5-HT neurons is given below.

The human locus coeruleus complex: an immunohistochemical and three dimensional reconstruction study.

SummaryThe CA (catecholamine/catecholaminergic) cell populations of the locus coeruleus (LC) and subcoeruleus (SubC) were studied using serial sections of the human brainstem immunostained with an antibody against tyrosine hydroxylase. The tyrosine hydroxylase-immunoreactive (TH-IR) neurons were plotted in a computer reconstruction system and their number and soma size determined. Serial section computer analysis was then used to create a three dimensional reconstruction of the LC complex. The number of cells containing neuromelanin pigment was also determined and compared with the number of TH-IR cells. In our sample there were 53,900 TH-IR cells in the LC and a further 6260 cells in the SubC. These numbers were very similar to our estimates of the number of cells containing neuromelanin pigment and we concluded that virtually all of these cells were also tyrosine hydroxylase positive. The average soma size of the TH-IR cells of the LC was 37 μm and in the SubC 34 μm. In addition to these quantitative observations the morphology of the TH-IR and the Nissl stained cells is described in some detail. We also compared the groups of immunoreactive cells in the human pons with the noradrenergic groups A5–A7 described in the rat. Although in the human these groups are contiguous, A5 is not part of the LC complex. However we did find that the A7 group is equivalent to the rostroventral part of SubC while the remainder of SubC is formed by ventral A6.

Morphology of tyrosine hydroxylase-immunoreactive neurons in the human cerebral cortex.

SummaryIn freshly fixed biopsies of human cerebral cortex obtained at surgery, immunocytochemical staining with antibodies against tyrosine hydroxylase (the rate limiting biosynthetic enzyme for catecholamines) revealed, in addition to a dense axonal plexus, a population of immunoreactive cell bodies. The neuronal nature of these cells was ascertained by: i) the presence of a rich rough endoplasmic reticulum in the cell body and of synapses on the cell body and dendrites, and ii) the demonstration of the lack of reactivity with the astroglial marker, glial fibrillary acidic protein, in the tyrosine hydroxylase-immunoreactive cells. The tyrosine hydroxylase-immunoreactive neurons were found in all areas of cortex sampled, and were located almost exclusively in the infragranular layers. Most tyrosine hydroxylase-immunoreactive cells were bipolar and were vertically oriented, but a few had a multipolar or horizontal dendritic arbor. The dendrites of these cells were varicose and aspiny, and the axons were very thin. Tyrosine hydroxylase-immunoreactive neurons were reported to be present transiently in the developing mammalian cerebral cortex and only recently in cerebral cortex of mature mammalian brains. Internuncial neurons in the human cerebral cortex containing a catecholamine synthesizing enzyme would be significant, in particular considering that catecholamines are likely to be involved in some major mental disorders.

Histochemical evidence for a catecholaminergic (presumably dopaminergic) projection from the ventral mesencephalic tegmentum to visual cortex in the cat

An histochemical technique combining the retrograde transport of horseradish peroxidase (HRP) and catecholamine histofluorescence was used to identify those neurons of the cats mesencephalic ventromedial tegmentum (VMT) which project to visual cortex. In a region corresponding to the ventral tegmental area-A10 nucleus we identified a group of fluorescent cells which were retrogradely labeled by HRP injections into visual cortex. It is suggested that these cells belong to a hitherto unknown, posterior part of the mesocortical dopaminergic system.

The internal organization of the human solitary nucleus

The present study examines the topography and cytoarchitecture of the solitary nucleus (Sol). Three human medulla oblongatae were serially sectioned and alternate sections were reacted for the enzyme acetylcholinesterase or stained with cresyl violet. The 10 component subnuclei of Sol were identified on the basis of their acetylcholinesterase reactivity and cytoarchitecture. These subnuclei are the paracommissural, commissural, gelatinosus, medial, ventral, ventrolateral, dorsal, dorsolateral, intermediate, and interstitial. Cytoarchitecturally, Sol is characterised by an abundance of small cells of varied morphology but also features some medium to large cells as well as pigmented neurons which are differentially distributed across the subnuclei. From spaced serial sections, the outlines of the subnuclei and the entire Sol were used to create three-dimensional computer reconstructions to display the position and extent of each component subnucleus. Our results show that the internal architecture of the human solitary nucleus is similar to that of rodents and carnivores, though there are some notable species differences.

Serotonin-like immunoreactive cells and fibres in the rat ventromedial mesencephalic tegmentum.

The distribution and morphology of serotonin-like immunoreactive (5HT-IR) nerve cells, fibres and terminals in the rat ventromedial mesencephalic tegmentum (VMT) was studied using qualitative and quantitative immunohistochemical methods at light and electron microscopic levels. All five component nuclei were examined and the size, number and density of immunoreactive neurons and terminals determined. Thirty percent of all neurons in the caudal linear nucleus and 1% in the interfascicular nucleus were immunoreactive for serotonin. Different regions of the VMT had morphologically distinct 5HT-IR fibres and quantitative differences between the VMT nuclei were seen in the density of 5HT-IR terminals. In the lateral VMT, many thick, nonvaricose 5HT-IR fibres were found in the parabrachial pigmented nucleus while many fine, varicose 5HT-IR fibres were found in the paranigral nucleus. Fine, varicose 5HT-IR fibres were also seen in the rostral and caudal linear nuclei. Many 5HT-IR axon profiles formed synapses with small calibre dendrites. 5HT-IR fibres in the interfascicular nucleus were thick with variable sized varicosities at irregular intervals. Few 5HT-IR axon profiles formed synapses in this nucleus. A comparison between the number of 5HT-IR terminals and the total number of axon terminals in the VMT (25) reveals that the majority of all terminals in the paranigral and rostral linear nuclei can be labelled with serotonin. The effect of serotonin on VMT cells is therefore likely to be mediated by different types of 5HT-IR fibres which preferentially innervate particular VMT nuclei.

Morphology and distribution of neuropeptide-containing neurons in human cerebral cortex

Biopsies of human cerebral cortex were fixed by immersion and immunostained for the detection of neuropeptides in neuronal cell bodies and axons. Four neuropeptides (neuropeptide Y, somatostatin, , substance P and cholecystokinin) were visualized in a series of adjacent sections. All populations of immunoreactive neurons had a morphology characteristic of interneurons, with variations in dendritic arborizations and laminar distribution. The cholecystokinin-immunoreactive neurons were most numerous in the supragranular layers, whereas neurons containing the other three peptides occurred mainly in infragranular layers, or even in neurons populating the subcortical white matter. Quantitatively, each population of neuropeptide-containing neurons accounted for 1.4-2.5% of the total neuronal population. The distribution of these neurons varied slightly between cytoarchitectonic divisions, with substance P- and somatostatin-immunoreactive neurons dominating in the temporal lobe and cholecystokinin-immunoreactive neurons in the frontal lobe. Neuropeptide Y-immunoreactive neurons dominated in the gray matter of the frontal half of the hemisphere and in the subcortical white matter of the caudal half of the hemisphere. Furthermore, co-existence of neuropeptide Y or substance P immunoreactivity within somatostatin-immunoreactive neurons could be demonstrated using double labeling immunofluorescence techniques. The axonal plexuses immunoreactive for neuropeptide Y, somatostatin, or substance P were distributed in all layers, with a strong predominance of horizontally oriented fibers in layer I, a moderate plexus of randomly oriented fibers in the supra- and infragranular layers, and a slightly weaker innervation of layer IV. Immunoreactive axons formed, in addition, complex terminal arbors, mostly in older subjects, suggesting that they resulted from an as yet undefined aging process. The present study underlines several aspects of the organization of the neuropeptide-containing neurons of the human cerebral cortex, which are of particular interest in the light of the involvement of these neurons in several neurodegenerative diseases.

Neurotransmitters in the human brain

The proceedings of a February 1994 conference in New South Wales comprise 13 papers reporting some recent basic research results on neurotransmitters in the brain and how they may contribute to the understanding of such diseases as Alzheimers, Parkinsons, Huntingtons, depression, and schizophreni

Brain stem afferents to visual cortical areas 17, 18 and 19 in the cat, demonstrated by horseradish peroxidase.

The origins of brain stem projections to the cytoarchitectonically different areas 17, 18 and 19 of the cats visual cortex were studied following small horseradish peroxidase (HRP) injections. Labelled cells were counted in a dopaminergic nucleus (nucleus linearis rostralis (NLR)), other catecholaminergic nuclei (locus coeruleus, parabrachialis nuclei and nucleus subcoeruleus) and serotonergic nuclei (nucleus raphe dorsalis (NRD) and nucleus centralis superior (NCS)). Area 18 receives afferents from more locus coeruleus cells than either of areas 17 or 19. The number of labelled cells in the catecholaminergic nuclei far exceeds that in the serotonergic nuclei.

Serotonergic axons form basket-like terminals in cerebral cortex

Serotonergic axons in the posterior cerebral cortex of the cat were demonstrated immunohistochemically using a monoclonal antibody to serotonin (5-HT). This technique reveals the presence of a dense serotonergic innervation of single cortical neurons at the light microscopic level. 5-HT axons with large varicosities (1-6 microns in diameter) form distinct, basket-like arrays around counterstained somata principally in layer I. In each basket one or more axons encircle and make repeated contact with the soma. Some axons extend from the soma and apparently climb along the dendrites of the target neuron. The climbing 5-HT axons form a stellate or horizontal pattern suggesting that the target cells are non-pyramidal neurons of the supragranular layers.