Neuroendocrine Perspectives: Volume 3

Neuroendocrine Perspectives, Volume 3 provides information on amine and peptide biochemistry. This book discusses the availability of specific biochemical and histochemical techniques that have greatly advanced knowledge of central nervous system neurotransmitter and neuropeptide systems. Organized into nine chapters, this volume begins with an overview of the structure of corticotropin releasing hormone. This text then examines the possible role of the cerebrospinal fluid in the regulation of pituitary function. Other chapters consider the importance of cerebrospinal fluid as a route for the hypothalamic regulation of pituitary function. This book discusses as well the available information concerning the neuroendocrine mechanisms involved in the onset of female puberty in primate and subprimate species. The final chapter deals with pineal indole metabolism and its controlling mechanisms as well as information on the interactions of the pineal hormones with neuroendocrine-reproductive axis. This book is a valuable resource for pharmacologists, research workers, and students.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483278131

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Chapter 1

Topography of chemically identified neurons in the central nervous system: progress in 1981–1983

M. Palkovits

Publisher Summary

This chapter discusses the topography of chemically identified neurons in the central nervous system (CNS) and its progress during 1981–1983. The large number of substances that occurred in CNS neurons at first gave a chaotic impression rather than the feeling of increased understanding. Although, there was no doubt that the classical views based on the assumption of homogeneous brain centers and unidirectional interconnections between brain regions were challenged by later neurochemical findings, the conventional view of innervations of a certain brain area, nucleus, or even a single cell by one or two types of nerve terminals was a rough oversimplification. The degree of neuronal convergence was surprisingly high and median eminence of more than 40 amines, amino acids, and peptides were demonstrated. Increasing numbers of immunohistochemical observations provided evidence against the Dale principle—one transmitter/one neuron—demonstrating double or multiple coexistence of amines and peptides in the same neuronal cell body or terminal. A similar degree of neuronal divergence was revealed by histofluorescence and immunohistochemistry—a single neuron can innervate hundreds, thousands, and in certain cases ten thousands of other neurons.

INTRODUCTION

The chemical characterization of central nervous system (CNS) neurons began in the 1960′s and is continually expanding. This advancement is based on two factors: (1) important achievements in amine and peptide biochemistry and (2) the availability of sensitive and specific biochemical and histochemical techniques.

In the past few years a growing number of brain neurons has been identified chemically. This review is the third in the last 6 years. Papers published before 1978 were summarized in the first (Palkovits, 1978) and the progress between 1978–1980 in the second (Palkovits, 1980a). Here the latest data, published before August 1983, are briefly summarized.

In construction and form this review is confined to the topography of chemically identified neuronal cell bodies, nerve fibers and terminals in the CNS. It is a morphological summary with no neurophysiological or neuropharmacological comments. For mapping, only those morphological techniques were considered by which neurons could be visualized and localized light- or electronmicroscopically. Data obtained from biochemical [only radioimmunoassay (RIA) data] mapping of neuropeptides on microdissected brain nuclei are summarized in Tables 1.2–1.6. For didactical reasons chemical substances present in neurons are divided into four groups: transmitter amino acids, acetylcholine, biogenic amines and neuronal peptides.

Table 1.2

CONCENTRATIONS OF NEUROPEPTIDES IN CEREBRAL CORTEX, BASAL GANGLIA AND THALAMIC NUCLEI

Concentrations: = low.

References: 1 = Palkovits et al. (1974); 2 = Kizer et al. (1976); 3 = Selmanoff et al. (1980); 4 = Samson et al. (1980); 5 = Brownstein et al. (1974); 6 = Eskay et al. (1983); 7 = Kerdelhue et al. (1981); 8 = Brownstein et al. (1975); 9 = Kobayashi et al. (1977); 10 = Palkovits et al. (1980b); ll = Epelbaum et al. (1979); 12 = Palkovits et al. (1976); 13 = Douglas and Palkovits (1982); 14 = George and Jacobowitz (1976); 15 = Dogterom et al. (1978); 16 = Hawthorn et al. (1980); 17 = Laczi et al. (1983); 18 = Zerbe and Palkovits (1984); 19 = George et al. (1976); 20 = Palkovits et al. (1978); 21 = Dupont et al. (1980); 22 = Dorsa et al. (1981); 23 = Kerdelhue et al. (1983); 24 = Krieger et al. (1977); 25 = O’Donohue et al. (1979); 26 = Eskay et al. (1979); 27 = Zamir et al. (1983a); 28 = Zamir et al. (1983b); 29 = Zamir et al. (in prep.); 30 = Brownstein et al. (1976); 31 =Kanazawa and Jessell (1976); 32 = Douglas et al. (1982); 33 = Hong et al. (1977); 34 = Kobayashi et al. (1978); 35 = Moody et al. (1981); 36 = Beinfeld et al. (1981); 37 = Beinfeld and Palkovits (1981); 38 = Beinfeld and Palkovits (1982); 39 = Palkovits et al. (1981a); 40 = Eiden et al. (1982); 41 = Rostène et al. (1982).

Table 1.3

CONCENTRATIONS OF NEUROPEPTIDES IN THE PREOPTIC, HYPOTHALAMIC, MAMMILLARY AND SUBTHALAMIC NUCLEI

Symbols and references are as in Table 1.2.

Table 1.4

CONCENTRATIONS OF NEUROPEPTIDES IN THE LIMBIC SYSTEM

Symbols and references are as in Table 1.2.

Table 1.5

CONCENTRATIONS OF NEUROPEPTIDES IN THE MIDBRAIN AND PONTINE NUCLEILH-RH

Symbols and references are as in Table 1.2.

Table 1.6

CONCENTRATIONS OF NEUROPEPTIDES IN THE MEDULLA OBLONGATA, THE CEREBELLUM AND THE SPINAL CORDLH

Symbols and references are as in Table 1.2.

Although this review aims to summarize progress since 1980, some basic data discussed in earlier reviews are also incorporated; 45 putative transmitter substances (some of them with subclasses or variants) listed in Table 1.1 are reviewed in the order given. The reader is referred for details and further references to recently published reviews (Elde and Hökfelt, 1978; Hökfelt et al., 1978a,b; Palkovits, 1978, 1980a,b, 1982a,b).

Table 1.1

CHEMICALLY IDENTIFIED SUBSTANCES (PUTATIVE NEUROTRANSMITTERS) IN THE CNS OF RAT

AMINO ACIDS

A considerable wealth of evidence suggests that some CNS amino acids may act as neurotransmitters (cf., Palkovits 1980a; Perry, 1982; Fagg and Foster, 1983). Neutral amino acids (GABA, glycine, taurine) are strong candidates to be inhibitory, the acidic amino acids (glutamate and aspartate) excitatory neurotransmitters. Although amino acids are widely distributed in the CNS, as has been detected biochemically, their definitive transmitter role cannot automatically be suggested since the presence of amino acids in the brain as the substrates of protein synthesis should also be taken into account. Beside biochemical mapping indirect immunohistochemical methods have been introduced in recent years to visualize biosynthetic enzymes of amino acids for use as markers for topographical localization of amino acid-containing cell bodies, nerve fibers and terminals.

Knowledge about brain amino acid pools (Perry, 1982) and amino acid transmitter pathways (Fagg and Foster, 1983) has been recently reviewed.

1 GABA

γ-Aminobutyric acid (GABA) is the most widely examined amino acid in the brain. It is unevenly distributed in all major brain areas but its level is generally lower than the concentrations of other transmitter amino acids except glycine. GABA is present in highest concentrations in the substantia nigra, globus pallidus, hypothalamic nuclei, midbrain colliculi and cerebellum, especially in the cerebellar nuclei (cf., Palkovits, 1978, 1980a). GABA itself cannot yet be visualized by immunohistochemistry. Its assumed distribution in the CNS is based on immunohistochemical demonstration of its rate-limiting enzyme, the glutamic acid decarboxylase (GAD).

In the 1970′s GAD-containing neuronal perikarya were demonstrated in various brain regions such as cerebral cortex, olfactory bulb, hippocampus, cerebellum, geniculates, superior collicle, reticular formation, spinal cord and spinal ganglia (cf., Palkovits, 1978, 1980a). Many of these observations were proved by recent immunohistochemical studies. In the rat cerebellum, GAD immunoreactivity was localized in all inhibitory (Purkinje, stellate, basket and Golgi cells) neurons (Oertel et al., 1981). Large, GAD-containing neurons were found in the spinal cord, predominantly within layers I and II and II–III border (Hunt et al., 1981b) and in neurons of all other laminae except motoneurons in the lamina IX (Barber et al., 1982). The majority of these immunostained neurons were interneurons. In the hypothalamus, GAD cells were found in the arcuate nucleus (Vincent et al., 1982c; Tappaz et al., 1983) and a major GAD-immunoreactive cell group was observed in the posterior hypothalamus along the ventral surface of the diencephalon (Vincent et al., 1982c). Anatomically, this group can be identified as the magnocellular lateral premammillary nucleus. GAD cells – in small numbers – were also reported in the lateral preoptic and hypothalamic areas, in the suprachiasmatic, periventricular, perifornical, ventromedial nuclei and in the zona incerta (Vincent et al., 1982c). GAD-immunoreactivity was demonstrated in dorsal raphe neurons in possible coexistence with serotonin (Nanopoulos et al., 1982).

Several pathways have been reported to be GABAergic: (1) extrapyramidal connections (striato-pallidal, striato-nigral, striato-entopeduncular, pallido-nigral pathways); (2) cerebellar Purkinje-cell projections (to the cerebellar nuclei and lateral vestibular nucleus); (3) hypothalamic tubero-infundibular (arcuate nucleus-median eminence projections [Vincent et al., 1982c; Tappaz et al., 1983]); (4) fibers in the stria medullaris and in the medial longitudinal fascicle (cf., Palkovits, 1980a; Fagg and Foster, 1983).

In fine varicosities, GAD is widely distributed in the CNS (mainly in the cortex, hippocampus, hypothalamus, olfactory bulb and spinal cord), likely in interneurons. High density of fairly large varicosities (nerve terminals of projecting neurons) immunostained for GAD is confined to the cerebellar nuclei, lateral vestibular nucleus, substantia nigra, globus pallidus, substantia innominata, medial forebrain bundle and olfactory tubercle (Pérez de la Mora et al., 1981). A dense network of GAD-immunopositive nerve fibers was demonstrated over the external layer of the median eminence. Positive fibers were also found in the internal layer. Fine, moderately intense varicosities were observed evenly distributed throughout the hypothalamus (Vincent et al., 1982c; Tappaz et al., 1983).

2 Glycine

It is widely recognized that glycine functions as an inhibitory neurotransmitter in the spinal cord and brainstem. Only biochemical methods are available to demonstrate its presence in several brain regions. Compared to other amino acids the concentration of glycine is relatively low, except in the spinal cord where it is highly concentrated (cf., Palkovits, 1978, 1980a). On the basis of electrophysiological and biochemical evidence, it is generally considered that glycine may have an intrinsic, interneuronal transmitter role in the spinal cord (cf., Palkovits, 1980a). Recently, a number of authors have suggested that glycine may also mediate synaptic transmission in the cortex, cerebellum, striatum, midbrain (substantia nigra, ventral tegmental area), pons and medulla oblongata (Levi et al., 1982; Fagg and Foster, 1983). The existence of corticohypothalamic glycinergic inhibitory pathway was recently reported on the basis of electrophysiological studies (Kita and Oomura, 1982; cf., Fagg and Foster, 1983).

3 Glutamate

Glutamic acid is present in all brain areas, usually in higher concentrations than other amino acids (cf., Palkovits, 1978, 1980a). Glutamate cannot be visualized directly in neurons. It is assumed on the basis of various indirect studies that glutamate is present in short, intrinsic and long, projecting neurons. Intrinsic glutamate-containing neurons are in the cerebral cortex, amygdala, hippocampus and bed nucleus of the stria terminalis (Palkovits, 1980a). A more detailed distribution of glutamate was recently described in the rat spinal cord (Patrick et al., 1983). Its functional role there is discussed by Fagg and Foster (1983).

Glutamate-containing long projecting fibers are suggested within several pathways, such as hippocampal efferents (cf., Fagg and Foster, 1983), corticostriatal paths (Fonnum et al., 1981; Walaas, 1981; cf., Palkovits, 1980a; Fagg and Foster, 1983). The pyriform cortex appears to be the principal glutamergic input to the amygdala (Walker and Fonnum, 1983). The pyriform-thalamic pathway may utilize both glutamate and aspartate as neurotransmitters (Walker and Fonnum, 1983).

4 Aspartate

Aspartic acid is present in considerable concentrations in practically all brain regions (cf., Palkovits, 1980a; Perry, 1982). Like glutamate, aspartate is considered to be an excitatory neurotransmitter. Experimental studies suggest that aspartate exists in local neurons in the accumbens nucleus, central amygdaloid nucleus, bed nucleus of the stria terminalis, entorhinal cortex and the spinal cord (cf., Palkovits, 1980a).

Several aspartate-containing long, projecting fibers were also reported in the CNS. Recent data suggest that in the main extrinsic input to the hippocampus from the entorhinal cortex – perforant pathway – aspartate is the transmitter (Di Lavro et al., 1981). Aspartate has also been proposed as the transmitter of the commissural and associational hippocampal fibers projecting from CA4 pyramidal cells to the dentate gyrus (cf., Fagg and Foster, 1983). The cortico(pyriform)-thalamic pathway may utilize both glutamate and aspartate as neurotransmitters (Walker and Fonnum, 1983). In the rat, aspartate seems to be a transmitter of the lateral olfactory tract fibers (cf., Fagg and Foster, 1983). Aspartate has also been suggested as the transmitter of the climbing fibers in the cerebellum but this observation is controversial (cf., Fagg and Foster, 1983).

Aspartate itself cannot be visualized in neurons. Its distribution in the brain may be studied by immunohistochemical localization of aspartate aminotransferase immunoreactivity (Altschuler et al., 1981).

5 Taurine

Taurine, like the other amino acids, is unevenly distributed in the CNS. The highest taurine concentration was detected in the lateral geniculate nucleus but concentrations are fairly high in the diencephalon, the cerebral cortex and the cerebellum. Moderate levels were found in the lower brainstem, amygdala and the bed nucleus of the stria terminalis (cf., Palkovits, 1980a). Cellular distribution of taurine in the mammalian cerebellum has been visualized by autoradiography with [³H]taurine and by immunohistochemistry with antibodies against the taurine-synthesizing enzyme, cysteine-sulfinic acid decarboxylase (Chan-Palay et al., 1982).

ACETYLCHOLINE

Although acetylcholine (ACh) is one of the first neurotransmitters discovered in the CNS, its precise location and morphological distribution have remained unknown due to the lack of an appropriate labeling technique. The cerebral distribution of ACh has been determined mainly by histochemical demonstration of acetylcholinesterase or by biochemical measurements of choline acetyltransferase (ChAT) or ACh itself (cf., Palkovits, 1978, 1980a,b; Armstrong et al., 1983). Acetylcholinesterase, however, is present in some non-cholinergic neurons, thus it cannot be taken as an absolute criterion of cholinergic neurons.

Acetylcholine itself cannot be directly visualized. Recent success with immunohistochemistry for ChAT permitted us to elucidate the cholinergic system. This enzyme is currently the most reliable neurochemical marker for cholinergic neurons. Immunocytochemical methods with polyclonal antisera to ChAT of various degrees of purity and with monoclonal antibodies have been used to label putative cholinergic neurons in a number of regions in the CNS of the cat (Kimura et al., 1980, 1981) and rat (Sofroniew et al., 1982; Armstrong et al., 1983; Houser et al., 1983).

ChAT-immunostained cell bodies were demonstrated in the following brain regions:

i.   Fusiform and elongated multipolar cells are scattered in the striatum (caudate nucleus, putamen), nucleus accumbens and olfactory tubercle (Kimura et al., 1981; Sofroniew et al., 1982; Armstrong et al., 1983).

ii.    Large ChAT-immunostained neurons in the nucleus tractus diagonalis (horizontal and vertical portions) and the medial septal nucleus (Kimura et al., 1981; Sofroniew et al., 1982; Armstrong et al., 1983; Houser et al., 1983). These cells project to the hippocampus and cingulum.

iii.    Large ChAT cells in and along the ventral and medial border of the globus pallidus, the so-called ventral pallidum (substantia innominata, magnocellular cells in the lateral preoptic area and entopeduncular nucleus). These cell groups are also nominated as peripallidal cell groups (Kimura et al., 1981) or magnocellular basal nucleus (Armstrong et al., 1983). Those cells project to the neocortical areas (Kimura et al., 1981; Armstrong et al., 1983; Houser et al., 1983).

iv.    Ventral part of the medial habenular nucleus (Houser et al., 1983). ChAT fibers run along the fasciculus retroflexus into the interpeduncular nucleus which is known to contain the highest level of ChAT activity in the brain (cf., Palkovits, 1980a).

v.    ChAT-immunoreactive cells in the pontine tegmentum. These cells are scattered in the territory of cuneiform nucleus (also called pedunculopontine tegmental nucleus), parabrachial nuclei and concentrated in the dorsolateral tegmental nucleus (Kimura and Maeda, 1982; Armstrong et al., 1983). No ChAT-containing neurons were found in the locus coeruleus. Since there are densely packed numerous ChAT-immunoreactive fine granules diffusely in the neuropil, locus coeruleus cells are considered to be cholinoceptive rather than cholinergic neurons.

vi.    All cranial motor nerve nuclei (III, IV, V, VI, VII, XII motor nuclei, nucleus ambiguus and preganglionic parasympathetic nuclei (Edinger-Westphal nucleus, salivatory and intercalate nuclei, dorsal vagal nucleus)) are constituted by large ChAT-immunoreactive neurons (Armstrong et al., 1983; Houser et al., 1983).

vii.    Ventral horn motor cells and the preganglionic sympathetic cells in the intermediolateral cell column in the spinal cord are also cholinergic; they immunostain with ChAT (Houser et al., 1983).

ChAT-immunohistochemistry (Armstrong et al., 1983) did not confirm previous data on the presence of cholinergic cells in the hypothalamus (cf., Palkovits, 1978).

The major cholinergic projections described by acetylcholinesterase staining have been confirmed by ChAT immunohistochemistry:

i.   Cholinergic projections arise from neurons in the nucleus of the diagonal band and the medial septal nucleus. Small, isolated ChAT-positive varicosities are present in all layers of the hippocampus and dentate gyrus (Houser et al., 1983).

ii.   The major source of cholinergic innervation of the cerebral cortex appears to be extrinsic. The pallidal (magnocellular basal nucleus) ChAT neurons are the origin of this cholinergic projection (Lehmann et al., 1980; Johnston et al., 1981; Houser et al., 1983). The minor portion seems to be intrinsic, small, bipolar, oval-shaped neurons that have been reported throughout layers II – VI in the cortex (Eckenstein and Thoenen, 1983; Houser et al., 1983). Fine varicose fibers (both extrinsic and intrinsic) formed a network in all cortical layers: fibers are horizontally oriented in layers I and VI and vertically in layers II – V.

iii.   Habenulo-interpeduncular cholinergic path.

iv.   The existence of ascending cholinergic pathways from the cuneiform and dorsolateral tegmental nuclei to the forebrain has been proposed on the basis of acetylcholinesterase reaction (cf., Palkovits, 1978, 1980a; Armstrong et al., 1983). Recent immunohistochemical observation verified that connection (Armstrong et al., 1983).

By ChAT immunohistochemistry, cholinergic nerve terminals have been mapped throughout the entire CNS. Instead of giving a detailed description of their topography, we refer the reader to recent original publications (Kimura et al., 1981; Armstrong et al., 1983; Houser et al., 1983).

BIOGENIC AMINES

Present knowledge of the topographical distribution of biogenic amines in the CNS is based on studies carried out in the sixties and early seventies (cf., Elde and Hökfelt, 1978; Hökfelt et al., 1978a; Palkovits, 1978, 1980a,b). During the last two years several new, mainly neurophysiological observations have been published, but new morphological information has also been obtained concerning the biogenic amine-containing neuronal network in the CNS.

Catecholamine-containing neurons and networks are investigated mainly with histofluorescence techniques. In the past few years, immunohistochemical methods for catecholamine biosynthetic enzymes have been developed. By the use of these techniques previous histofluorescence data have been confirmed and finer and more specific topographical distribution of catecholamine-containing neurons has been obtained (Armstrong et al., 1982). One of the major new areas is related to the immunohistochemical mapping of serotonin in the rat brain and the mapping of other biogenic amines in the CNS of primates (Felten and Sladek, 1983) and man (Pearson et al., 1983).

1 Dopamine

Dopamine (DA) is present in biochemically measurable concentrations in all brain areas. The use of histofluorescence techniques has revealed a wide, uneven distribution of DA in the CNS. Here, only the topography of DA-containing cell groups and pathways is summarized. Instead of giving a detailed description of DA nerve terminals we refer readers to original studies (cf., Hökfelt et al., 1978a; Palkovits, 1978, 1980a,b) or reviews by Moore and Bloom (1978) and Moore and Johnson (1982).

a Dopamine-containing cell groups

i Hypothalamic DA neurons

The major group of DA cells is located in the arcuate nucleus (A12 cell group) which gives rise to tuberoinfundibular DA projections. Few DA cells are present along the third ventricle in the hypothalamic and preoptic periventricular nuclei (A14 cell group). On the basis of recent histofluorescence observations, the coexistence of DA and neurotensin in A12 and A14 cell groups is proposed (Ibata et al., 1983). DA-containing cells have also been recently described in the paraventricular nucleus (PVN) by immunohistochemistry (Swanson et al., 1981). In the caudal portion of the hypothalamus and the thalamus scattered DA cells are described (A11 cell group). This cell group is the principal, and perhaps exclusive, source of DA innervation of the spinal cord (Hökfelt et al., 1979; Skagerberg et al., 1982).

ii Zona incerta DA neurons

A small number of DA-eontaining neurons exist in the zona incerta and dorsal to the dorsomedial nucleus (A13 cell group). These cells are the source of the incerto-hypothalamic DA system innervating the dorsal hypothalamic nuclei and the subthalamus.

iii Midbrain DA neurons

The largest DA cell group in the CNS; these cells occupy the territory of the ventral tegmental area (A10 cell group), the zona compacta (A9 cell group) and the pars lateralis (A8 cell group) of the substantia nigra. Single DA cells are also scattered in the adjacent areas. Immunohistochemical data were recently reported about the coexistence of cholecystokinin and DA in certain neurons mainly in the A10 cell groups (Hökfelt et al., 1980).

iv Olfactory DA neurons

DA-containing cells have been described in the olfactory bulb, among the periglomerular cells.

b Dopamine-containing pathways

i Tubero-infundibular DA fibers

The rich DA innervation of the median eminence derives from the arcuate DA cells.

ii Mesocortical DA pathways

Most of the fibers arise from the A10 cell group and ascend in the lateral part of the medial forebrain bundle to the cingulum.

iii Mesolimbic DA pathways

DA fibers from A8, A9 and A10 cells join the mesocortical fibers. After a short run at the diencephalic-midbrain junction a group of fibers turns dorsad to the habenula and periventricular thalamic nucleus. Mesohabenular DA fibers arise mainly from the ventral tegmental area (Phillipson and Griffith, 1980). DA afferents enter the hippocampus mainly through the dorsal route together with mesocortical DA fibers. The major input to the rostral hippocampus derives from the A10 DA cells while the caudal regions receive fibers from both A9 and A10 cells (Scatton et al., 1980).

iv Mesostriatal (nigrostriatal) DA pathway

This is the largest DA system. Fibers arise from the A9 and A10 DA cells and ascend dorsolaterally from the medial forebrain bundle until the ventral pallidum. Then, DA fibers pass the internal capsule and radiate in the caudate putamen. The nucleus accumbens is also innervated by this fiber system.

v Incerto-hypothalamic pathway

This is composed by fibers from the A13 cell group which terminate mainly in the dorsomedial and paraventricular nuclei.

vi Diencephalo-spinal DA pathway

This is a long descending uncrossed fiber system arising from the A11 cell group. The spinal DA innervation is mainly confined to the dorsal horn, the intermedio-lateral cell column and associated parts of the central gray of the spinal cord (Hökfelt et al., 1979; Skagerberg et al., 1982).

2 Norepinephrine

Detailed biochemical and histofluorescence maps are available on the widespread distribution of norepinephrine (NE) throughout the CNS (cf., Hökfelt et al., 1978a; Moore and Bloom, 1979; Palkovits, 1978, 1980a,b). The precise distributions of NE cells and pathways are briefly summarized here. In the past two years new data on NE innervation of the hypothalamus, hippocampus and spinal cord have been reported.

a Norepinephrine-containing cell groups

Data summarized here are based on histofluorescence observations which have recently been proved by immunohistochemistry of catecholamine synthesizing enzymes (Armstrong et al., 1982; Chamba and Renaud, 1983).

i Locus coeruleus

This is the largest NE cell group. Approximately one-half of all NE cells in the brain are located in this area, mainly ventral to the locus coeruleus (subcoeruleus area).

ii Nucleus of the solitary tract

Scattered cells (A2 cell group) are present in the commissural part of the nucleus and among the cells of the dorsal vagal nucleus.

iii Reticular formation.

NE cells in the ventrolateral corner of the brainstem constitute three rostro-caudal cell groups: A7 in the midbrain, A5 in the pons and A1 in the medulla oblongata. The last one is the largest; cells exist in the lateral reticular nucleus.

b Norepinephrine-containing pathways

i Ventral NE bundle (ventral tegmental tract).

Fibers arise from medullary NE cells and ascend in the dorsomedial part of the reticular formation. In the pons and midbrain, the pathway is supplemented with fibers from the A5 and A7 cell groups and from the locus coeruleus. The bundle joins the medial forebrain bundle in the ventral tegmental area and enters the lateral hypothalamus.

ii Dorsal NE bundle.

Fibers deriving from the locus coeruleus form a bundle at the lateral side of the midbrain central gray. At the diencephalic-midbrain junction, fibers diverge into several smaller pathways: two of them to the hypothalamus (one joins the ventral NE bundle, the other the dorsal periventricular NE pathways), others to the dorsal thalamus and epithalamus as well as to the tectum.

iii Dorsal periventricular NE bundle.

Ascending fibers from the locus coeruleus run rostralward in the central gray to innervate the dorsal hypothalamus and ventral thalamus.

iv Ventral periventricular NE bundle.

This forms the periventricular projection of the ventral NE bundle inside the hypothalamus. This bundle innervates the majority of the hypothalamic nuclei (Palkovits et al., 1980a).

v Cerebellar NE pathway.

This originates from the locus coeruleus and A4 cell group and reaches the cerebellum through the superior peduncle.

vi Bulbospinal NE bundle.

Fibers from the locus coeruleus descend to the medulla (also called as dorsal tegmental NE tract) and the spinal cord (Glazer and Ross, 1980; Ross et al., 1981; Westlund et al., 1982). In the cat, a major NE input to the spinal cord from the A1 cell group was revealed (Fleetwood-Walker and Coote, 1981).

Innervation pattern of the hypothalamus, hippocampus and spinal cord. The NE axons entering the hypothalamus form a dense network embracing all NE cell groups. The dorsal hypothalamic nuclei are mainly innervated by locus coeruleus and A7 NE cells throughout the dorsal periventricular NE pathway, while the ventral hypothalamus receives NE fibers mainly from the A1 cell group through the ventral NE bundle (Palkovits et al., 1980a). A certain overlap in the NE innervation patterns of the hypothalamic nuclei may be assumed, thus the lesion of a NE cell group can be compensated by other groups.

NE innervation of the hippocampus arises exclusively in the locus coeruleus. The axons course into the hippocampus by three routes: fasciculus cinguli, fornix and ventral amygdaloid bundle-ansa lenticularis (Loy et al., 1980).

A5 and A7 bulbospinal NE neurons project bilaterally to the sympathetic preganglionic cells in the intermediolateral cell column of the spinal cord. The NE innervation of the dorsal and ventral horns appears to arise from the locus coeruleus (Glazer and Ross, 1980; Ross et al., 1981; Westlund et al., 1982).

3 Epinephrine

Distribution of epinephrine in the CNS has been mapped with biochemical and immunohistochemical techniques (cf., Elde and Hökfelt, 1978; Hökfelt et al., 1978a; Moore and Bloom, 1979; Palkovits, 1978, 1980a,b).

Epinephrine-containing perikarya have been shown in only two brainstem areas. The C1 cell group consists of cells in the lateral reticular nucleus, about 500 μm rostral to the A1 cell group, in the rat. The other cells (C2 cell group) are scattered in the dorsomedial medulla oblongata mainly in and around the nucleus of the solitary tract about 1500 μm rostral to the A2 cell group (Chamba and Renaud, 1983).

The ascending epinephrine-containing fibers have been only partly visualized in the dorso-medial part of the medullary reticular formation. These fibers may join the ventral NE bundle.

Epinephrine-containing axons and terminals exist in most brain areas, especially in high concentrations in the hypothalamus. The fibers originate mainly in the C1 cell group (Palkovits et al., 1980c).

Adrenergic innervation of the spinal cord, which is exclusively directed to the intermediolateral cell column, arises from the medullary C1 cell group (Ross et al., 1981).

4 Histamine

Distribution of histamine in the CNS is much less known than that of the other biogenic amines. Information is based on biochemical measurements of histamine and enzymes responsible for its synthesis and metabolism. Histamine is present in measurable quantities in all brain areas; especially high concentrations were measured in the hypothalamus (cf., Palkovits, 1978, 1980a). There is also evidence that not all histamine is neuronal; it is highly concentrated in mast cells.

Recently, histamine-like immunoreactive cell bodies were localized in the lateral hypothalamus at the midhypothalamic level. They were distributed in an arched configuration extending from the dorsal tip of the third ventricle to the optic tract (Wilcox and Seybold, 1982). Dense networks of histamine-like fibers were demonstrated in both layers of the median eminence and in the mammillary body. Extrahypothalamic histamine-like fibers were found in the cerebral cortex, hippocampus and amygdala (Wilcox and Seybold, 1982).

The existence of histamine-containing pathways is argued on the basis of biochemical measurements made after various surgical lesions. On the basis of such experiments, the presence of projecting histaminergic fibers in the lateral hypothalamus were suggested. Fibers run mainly in the medial forebrain bundle and probably in the fornix interconnecting the hypothalamus, cerebral cortex, hippocampus and amygdala (cf., Palkovits, 1980a; Roberts and Calcutt, 1983).

5 Serotonin

The most significant advancement in studies on central biogenic amines during the last 2 years has been unquestionably the immunohistochemical mapping of serotonin (5HT) in the CNS (Steinbusch, 1981). Previous observations based on histofluorescence and biochemistry in microdissected brain nuclei are in relatively good agreement with recent findings (cf., Hökfelt et al., 1978a; Palkovits, 1978, 1980a; Steinbusch, 1981).

a Serotonin-containing cell bodies

5HT-immunoreactive cells are first detected early on the 13th embryonic day (Wallace and Lauder, 1983). In adult rats, several 5HT cell groups and scattered cells can be visualized (Steinbusch, 1981).

i Raphe nuclei.

5HT cells exist in highest numbers in the dorsal raphe nucleus. In the midbrain, 5HT cells were demonstrated in the midbrain raphe nucleus (central superior nucleus) and the nuclei lineares. Few 5HT cells are present in the pontine raphe nucleus but more exist in the nucleus raphe magnus. In the obscurus and pallidus raphe nuclei and in their vicinity, several 5HT cells were also visualized. Immunohistochemical support has been presented for three putative transmitters in the same pontine-medullary raphe neuron. 5HT, substance P and thyrotropin releasing hormone-like immunoreactivity was found in cells that project to the spinal cord (Johansson et al., 1981).

ii Medullary 5HT cells.

A compact group of relatively small monopolar 5HT-immunostained cells were found in the area postrema and a few cells throughout the reticular formation of the medulla oblongata.

iii Pontine 5HT cells.

Other than pontine raphe cells, 5HT-containing cells are present in the subcoeruleus area and around the dorsal tegmental nucleus.

iv Midbrain 5HT cells.

Besides the rostral raphe cells, a few 5HT cells were found in the ventral tegmental area, interpeduncular nucleus and among the fibers of the medial lemniscus.

v Hypothalamic 5HT cells.

Immunoreactive 5HT cells were described in the hypothalamic dorsomedial nucleus by Frankfurt et al. (1981). Recently, the presence of 5HT cells in the ventrolateral hypothalamus (close to the basal surface) was reported (Sakumoto et al., 1982).

b Serotonin-containing pathways

The topography of 5HT-containing pathways has been described on the basis of histofluorescence and biochemical measurements following surgical brain lesions and of autoradiography (cf., Palkovits, 1978, 1980a).

i Medial 5HT pathway.

This pathway originates in the midbrain and pontine raphe cells and passes the ventral tegmental area close to the midline. Fibers ascend to the forebrain in a position ventromedial to the main portion of the medial forebrain bundle.

ii Lateral 5HT pathway.

This pathway derives together with the medial 5HT pathway but these fibers run more laterally throughout the lateral hypothalamus.

iii Descending 5HT pathway.

Fibers from the magnus, obscurus and pallidus raphe nuclei descend through the reticular formation and the lateral funiculus to the spinal cord where they terminate in the intermediolateral cell column and in both ventral and dorsal horns (Bowker et al., 1982).

A widespread occurrence of 5HT nerve terminals throughout the entire CNS was demonstrated by an immunofluorescent technique (Steinbusch, 1981). Dense 5HT networks were demonstrated in the hypothalamus (suprachiasmatic nucleus, medial subdivision of the ventromedial nucleus, the lateral hypothalamic area), several thalamic nuclei (partly or entirely), subthalamic nucleus, the medial mammillary nucleus, basal amygdaloid nucleus, lateral geniculate, substantia nigra, pontine reticular formation, motor nuclei of Vth and VIIth nerves and the nucleus of the solitary tract (Steinbusch, 1981).

In the cerebral cortex, 5HT innervation is relatively dense and uniform across all layers (Lidov et al., 1980). Fine 5HT fibers in low density are distributed throughout the entire cerebellum, including the nuclei (Takeuchi et al., 1982).

6 Minor biogenic amines

Octopamine, phenylethanolamine, phenylethylamine and tryptamine are synthesized, stored and released in neurons and are regarded as putative neurotransmitters. All of them are present in the mammalian CNS but their concentrations are low, amounting to only 0.5–2% of the norepinephrine levels.

Minor biogenic amines distribute in the brain unevenly (cf., Palkovits, 1978, 1980a):

i.   Octopamine is highly concentrated in the hypothalamus, 50% less in the thalamus, the brainstem and the spinal cord, and low concentrations are found in the cerebral cortex, hippocampus, striatum and cerebellum.

ii.   Phenylethylamine concentrations are generally low in the CNS. It is mostly found in the hypothalamus and less in the caudal midbrain and the spinal cord (Karoum et al., 1981); the lowest detectable amounts were measured in the cerebral cortex and cerebellum.

iii.   Phenylethanolamine is present in minute concentrations in the brain.

iv.   Tryptamine exists mainly in the brainstem as detected in rat and human brains.

There are no specific methods available to visualize minor biogenic amines in the CNS, therefore their topographical and cellular distributions are unknown.

NEUROPEPTIDES

More than 30 neuropeptides have been identified in the CNS, with the number constantly increasing. The classification of neuropeptides in five groups (Table 1.1) is rather arbitrary. ‘Hypothalamic peptides’ are synthesized mainly in the hypothalamus and act as neurohormones at the pituitary level (releasing and release inhibiting hormones) or in the periphery (posterior pituitary hormones). The second group is constituted by those peptides that are characteristically of pituitary origin but are synthesized by nerve cells in the CNS, even if in orders of magnitude less than in the pituitary. (β-Endorphin is also synthesized by pituitary cells in high concentrations but on the basis of its physiological and chemical characteristics it is classified under the ‘opioid peptide’ group together with dynorphins, α– and β–neo-endorphins and enkephalins.) A separate group is constituted by the brain-borne ‘gastrointestinal’ peptides.

Concentrations of a number of neuropeptides have been measured by RIA in microdissected individual brain nuclei; these data are summarized in Tables 1.2–1.6. Their topographical distributions have been visualized by immunohistochemical techniques. The specificity and validity of this information is out of the scope of this review. It must be emphasized, however, that negative results with the immunohistochemical technique do not necessarily indicate the absence of a peptide.

1 Hypothalamic neuropeptides

Neuropeptides classified in this group are present in highest concentrations in the hypothalamus, especially in the median eminence. These substances are intimately involved in regulating pituitary function (releasing and release inhibiting hormones) or acting as neurohormones on the periphery (posterior pituitary hormones).

Evidence based on morphological, physiological and pharmacological experiments indicates that the role of these hypothalamic neuropeptides is not restricted to that of a neurohormone. The presence of these peptides in extrahypothalamic brain regions (Tables 1.2–1.6), the modification of neuronal activity of individual neurons induced when they are applied microiontophoretically and their numerous physiological and behavioral effects have led to the suggestion that these peptides may act as neurotransmitters in addition to their neurohormonal actions.

In the past two years, two hypothalamic neuropeptides – corticotropin releasing hormone (CRF) and growth hormone releasing hormone (GRF) – were identified chemically and subsequently localized in CNS neurons immunohistochemically.

a Leuteinizing hormone releasing hormone (LH-RH)

The decapeptide (LH-RH) is concentrated mainly in the hypothalamus, especially in the median eminence. Recent studies have revealed the presence of LH-RH at very low levels in several extrahypothalamic areas (Table 1.3). Distribution of LH-RH in the CNS was summarized by several reviews (cf., Barry, 1978; Elde and Hökfelt, 1978; Hökfelt et al., 1978a,b; Palkovits, 1980a,b, 1982a,b).

i LH-RH containing cell bodies.

The major group [almost 70% of the total (Shivers et al., 1983)] of rat LH-RH neurons is found in the preoptic area (medial preoptic nucleus) and its vicinity (nucleus of the diagonal band, septum, anterior hypothalamic nucleus). Few LH-RH cells can be demonstrated in rat arcuate nucleus (Kelly et al., 1982; Shivers et al., 1983) while in bird, dog, monkey and human brains, LH-RH cells predominate. Immunoreactive LH-RH cells were found in small numbers in the olfactory tubercle and the main and accessory bulbs (Barry, 1978; Dluzen and Ramirez, 1981; Witkin et al., 1982).

ii LH-RH containing pathways.

The majority of LH-RH fibers in the rat proceed from the preoptic area to the median eminence. Only a limited number of fibers reach the median eminence from a midsagittal location, by a periventricular route. Most LH-RH fibers originating in preoptic cells take a lateral course upon leaving that area and travel along the medial forebrain bundle; then they turn medial and enter the medial basal hypothalamus through the lateral retrochiasmatic area. This area, at the caudal edge of the optic chiasm, serves as a small gate for fibers entering the median eminence. Besides LH-RH fibers, several neuropeptide-containing axons [thyrotropin releasing hormone (TRH), somatostatin, CRF, vasopressin, oxytocin, cholecystokinin, dynorphin, α-neo-endorphin] pass through that particular small area (Palkovits, 1982c). From the lateral retrochiasmatic area LH-RH fibers enter the median eminence anterolaterally. That loop-like course of the preopticoinfundibular LR-RH path has been demonstrated with various techniques by several laboratories (Ibata et al., 1979; Merchenthaler et al., 1980; Kawano and Daikoku, 1981; Réthelyi et al., 1981; King et al., 1982; Palkovits et al., 1984b). Fibers from the medial preoptic nucleus traverse the neighboring Organum vasculosum laminae terminalis (Barry, 1978; Ibata et al., 1979; Kawano and Daikoku, 1981; King et al., 1982; Palkovits et al., 1984b). LH-RH fibers also descend through the medial forebrain bundle to the mammillary body and the ventral tegmental area (Liposits and Sétáló, 1980; Witkin et al., 1982). LH-RH fibers are also found in the medial and lateral olfactory tracts (Barry, 1978; Witkin et al., 1982). Scattered LH-RH axons of various origins and destinations can be traced extrahypothalamically (cf., Palkovits, 1980a; Witkin et al.,