High-resolution radioautographic study of the serotonin innervation of the rat corpus striatum after intraventricular administration of [3H]5-hydroxytryptamine

Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission

Serotonin is a neurotransmitter involved in the modulation of a multitude of physiological and behavioral processes. In spite of the relatively reduced number of serotonin-producing neurons present in the mammalian CNS, a complex long-range projection system provides profuse innervation to the whole brain. Heterogeneity of serotonin receptors, grouped in seven families, and their spatiotemporal expression pattern account for its widespread impact. Although neuronal communication occurs primarily at tiny gaps called synapses, wiring transmission, another mechanism based on extrasynaptic diffusion of neuroactive molecules and referred to as volume transmission, has been described. While wiring transmission is a rapid and specific one-to-one modality of communication, volume transmission is a broader and slower mode in which a single element can simultaneously act on several different targets in a one-to-many mode. Some experimental evidence regarding ultrastructural features, extrasynaptic localization of receptors and transporters, and serotonin-glia interactions collected over the past four decades supports the existence of a serotonergic system of a dual modality of neurotransmission, in which wiring and volume transmission coexist. To date, in spite of the radical difference in the two modalities, limited information is available on the way they are coordinated to mediate the specific activities in which serotonin participates. Understanding how wiring and volume transmission modalities contribute to serotonergic neurotransmission is of utmost relevance for the comprehension of serotonin functions in both physiological and pathological conditions.

The Diversity of Spine Synapses in Animals

Here we examine the structure of the various types of spine synapses throughout the animal kingdom. Based on available evidence, we suggest that there are two major categories of spine synapses: invaginating and non-invaginating, with distributions that vary among different groups of animals. In the simplest living animals with definitive nerve cells and synapses, the cnidarians and ctenophores, most chemical synapses do not form spine synapses. But some cnidarians have invaginating spine synapses, especially in photoreceptor terminals of motile cnidarians with highly complex visual organs, and also in some mainly sessile cnidarians with rapid prey capture reflexes. This association of invaginating spine synapses with complex sensory inputs is retained in the evolution of higher animals in photoreceptor terminals and some mechanoreceptor synapses. In contrast to invaginating spine synapse, non-invaginating spine synapses have been described only in animals with bilateral symmetry, heads and brains, associated with greater complexity in neural connections. This is apparent already in the simplest bilaterians, the flatworms, which can have well-developed non-invaginating spine synapses in some cases. Non-invaginating spine synapses diversify in higher animal groups. We also discuss the functional advantages of having synapses on spines and more specifically, on invaginating spines. And finally we discuss pathologies associated with spine synapses, concentrating on those systems and diseases where invaginating spine synapses are involved.

Dopamine synapse is a neuroligin-2-mediated contact between dopaminergic presynaptic and GABAergic postsynaptic structures

Midbrain dopamine neurons project densely to the striatum and form so-called dopamine synapses on medium spiny neurons (MSNs), principal neurons in the striatum. Because dopamine receptors are widely expressed away from dopamine synapses, it remains unclear how dopamine synapses are involved in dopaminergic transmission. Here we demonstrate that dopamine synapses are contacts formed between dopaminergic presynaptic and GABAergic postsynaptic structures. The presynaptic structure expressed tyrosine hydroxylase, vesicular monoamine transporter-2, and plasmalemmal dopamine transporter, which are essential for dopamine synthesis, vesicular filling, and recycling, but was below the detection threshold for molecules involving GABA synthesis and vesicular filling or for GABA itself. In contrast, the postsynaptic structure of dopamine synapses expressed GABAergic molecules, including postsynaptic adhesion molecule neuroligin-2, postsynaptic scaffolding molecule gephyrin, and GABAA receptor α1, without any specific clustering of dopamine receptors. Of these, neuroligin-2 promoted presynaptic differentiation in axons of midbrain dopamine neurons and striatal GABAergic neurons in culture. After neuroligin-2 knockdown in the striatum, a significant decrease of dopamine synapses coupled with a reciprocal increase of GABAergic synapses was observed on MSN dendrites. This finding suggests that neuroligin-2 controls striatal synapse formation by giving competitive advantage to heterologous dopamine synapses over conventional GABAergic synapses. Considering that MSN dendrites are preferential targets of dopamine synapses and express high levels of dopamine receptors, dopamine synapse formation may serve to increase the specificity and potency of dopaminergic modulation of striatal outputs by anchoring dopamine release sites to dopamine-sensing targets.

Anatomy of Graft-induced Dyskinesias: Circuit Remodeling in the Parkinsonian Striatum

The goal of researchers and clinicians interested in re-instituting cell based therapies for PD is to develop an effective and safe surgical approach to replace dopamine (DA) in individuals suffering from Parkinson's disease (PD). Worldwide clinical trials involving transplantation of embryonic DA neurons into individuals with PD have been discontinued because of the often devastating post-surgical side-effect known as graft-induced dyskinesia (GID). There have been many review articles published in recent years on this subject. There has been a tendency to promote single factors in the cause of GID. In this review, we contrast the pros and cons of multiple factors that have been suggested from clinical and/or preclinical observations, as well as novel factors not yet studied that may be involved with GID. It is our intention to provide a platform that might be instrumental in examining how individual factors that correlate with GID and/or striatal pathology might interact to give rise to dysfunctional circuit remodeling and aberrant motor output.

Age-related changes in the synapses of the rat's neostriatum

By means of transmission electron microscopy, the age-related changes in axospinous (ASS) and axodendritic (ADS) synapses in the dorsal part of the rostral neostriatum in two groups of Wistar rats: young (3-month-old), and senescent (25-month-old) were examined. The changes in different parameters, characterizing the ASS and ADS: synaptic density (SD), number of synaptic vesicles (SV), number of synaptic contact zone (SCZ), and number of dendritic spines, bearing synapses (DS) were investigated morphometrically. The SD of the ASS decreased significantly during aging, but the SD of the ADS did not changed significantly. The mean area of the synaptic boutons increased significantly during aging in two types of synapses. The mean number of vesicles per synaptic bouton increased, but the number of vesicles per microm2 of synaptic bouton, and per microm3 of the neuropil decreased. The mean SCZ length increased in both types of synapses. The total SCZ length per 1000 microm2 of the neuropil, and the total area of the SCZ per 1000 microm3 of the neuropil decreased in ASS, but the same parameters of the ADS did not changed significantly. The mean number of synaptic DS per 1000 microm2 of the neropil decreased during aging, but the mean area of the synaptic DS increased. The present results support the hypothesis that the synaptic contacts change significantly during aging, and the ASS are more vulnerable during aging than the ADS.

Quantal release of serotonin

We have studied the origin of quantal variability for small synaptic vesicles (SSVs) and large dense-cored vesicles (LDCVs). As a model, we used serotonergic Retzius neurons of leech that allow for combined amperometrical and morphological analyses of quantal transmitter release. We find that the transmitter amount released by a SSV varies proportionally to the volume of the vesicle, suggesting that serotonin is stored at a constant intravesicular concentration and is completely discharged during exocytosis. Transmitter discharge from LDCVs shows a higher degree of variability than is expected from their size distribution, and bulk release from LDCVs is slower than release from SSVs. On average, differences in the transmitter amount released from SSVs and LDCVs are proportional to the size differences of the organelles, suggesting that transmitter is stored at similar concentrations in SSVs and LDCVs.

Prenatal cocaine exposure increases serotonergic inhibition of electrically evoked acetylcholine release from rat striatal slices at adulthood

This study tests the hypothesis that prenatal cocaine (pCOC) exposure (20 mg/kg, bidaily from embryonic days 15-21) modifies 5-HT(3) receptor regulation of electrically-evoked [(3)H]acetylcholine (ACh) overflow from adult male and female (proestrus, diestrus) rat striatal slices. Also, the influence of endogenous dopamine (DA) on serotonin (5-HT) regulation of ACh overflow was determined by assessing the effects alpha-methyl-para-tyrosine (AMPT) pretreatment or sulpiride. Phenylbiguanide (PBG, 5-HT(3) agonist) superfusion dose-dependently inhibited ACh overflow in all groups except the diestrus pCOC group in which there was an enhanced sensitivity to PBG. PBG (10, 30, and 60 microM) produced greater effects in the pCOC male than in the prenatal saline (pSAL) group. The pCOC male group also exhibited greater sensitivity to PBG (30 and 60 microM) than the pCOC proestrus group. PBG inhibition of ACh overflow was comparable in the pSAL male and female (proestrus) groups. PBG inhibition of ACh overflow was greater in the pCOC diestrus group than in the pCOC proestrus (10, 30, and 60 microM), the pSAL diestrus (10 and 30 microM), and the pCOC male (10 microM) conditions. In slices from untreated rats superfused with 30 microM PBG, AMPT pretreatment (68% DA loss) reduced inhibition of ACh overflow, and 1 microM sulpiride increased ACh overflow. ICS205-930 (5-HT(3) antagonist) reduced effectiveness of PBG indicating 5-HT(3) receptor specificity for PBG. In summary, pCOC exposure enhances modulatory effects of 5-HT (via 5-HT(3) receptors) on striatal ACh release in male and females rats and the inhibitory actions of 5-HT(3) receptors are mediated by DA.

Regulation of striatal dopamine release through 5-HT1 and 5-HT2 receptors

Dopamine (DA) release in the striatum is regulated by 5-hydroxytryptamine (5-HT, serotonin) through putative heteroreceptors. However, the effect of 5-HT is controversial. The present study investigated the effects of different 5-HT receptor ligands on DA release in the rat striatum by using in vivo microdialysis in conscious and freely moving rats. Perfusion with 5-carboxamidotryptamine, anpirtoline, pindobind-5-HT1A, and isamoltane demonstrated the involvement of 5-HT1A and 5-HT1B receptors in facilitating DA release. In contrast, 5-HT2 receptors mediated inhibition of DA efflux, as shown by experiments with DOI [R-(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] and ketanserin. A 5-HT3 agonist (1-(m-chlorophenyl)-biguanide hydrochloride) did not have any effect. None of the agonists used affected DA uptake into striatal synaptosomes. Unilateral 6-hydroxydopamine lesioning of the nigrostriatal DA pathway led to a selective decrease in 5-HT2 receptors. It is concluded that there are 5-HT2 heteroreceptors at the dopaminergic terminals that mediate inhibition of DA release. Further investigation is required to clarify the localization of the 5-HT1 receptors in the striatum.

Synaptic organization of the human striatum: a postmortem ultrastructural study

The goal of this study was to characterize the synaptic organization of the normal human adult striatum for comparison with other species and with the diseased human striatum. Samples of striatal tissue from the Maryland Brain Collection obtained at autopsy with postmortem intervals of less than 4 hours were prepared for electron microscopic analysis according to standard techniques. The caudate nucleus and the putamen were similar in terms of the proportions of synaptic subtypes, the lengths of synaptic subtypes, and the area of most types of axon terminals. The proportions of major striatal synaptic subdivisions, such as axospinous synapses (83.5%) and asymmetric synapses (77.5%), were similar to that of the monkey (82% and 77%, respectively) but slightly lower than found in the rat (90% and 89%, respectively). Interestingly, the proportion of synapses with perforated postsynaptic densities (23%), a type of synapse thought to represent synaptic plasticity, was much higher in humans than in rats (5-8%). The lengths of asymmetric synapses (0.697 micron) were significantly longer than that of symmetric synapses (0.423 microns), a relationship found in other mammals. Also, the areas of terminals forming asymmetric synapses (0.707 micron2) were larger than those forming symmetric synapses (0.401 micron2), also consistent with data from other species. The length of axospinous synapses (0.656 micron) and the area of the terminals forming them (0.611 micron2) were not significantly different from the length of axodendritic synapses (0.523 micron) or the area of terminals forming them (0.602 micron2). This study is the first quantitative study on synaptic organization in human postmortem striatum. The results indicate that the synaptic organization of the human striatum is similar, but not identical, to that of other mammalian species.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.

Ultrastructure of serotonin-immunoreactive terminals in the core and shell of the rat nucleus accumbens: cellular substrates for interactions with catecholamine afferents

The nucleus accumbens is composed of a core region involved in motor functions and a shell region implicated in emotional and motivational processes. Both of these regions receive serotonin- and dopamine-containing afferents. We examined whether the serotonin innervation or relation to catecholamine (mainly dopamine) axons in the nucleus accumbens shows common features or specializations corresponding to the noted functional differences in core and shell subregions. To address this question, we examined the ultrastructure of serotonin-containing axons and their relation to catecholamine-containing afferents in either the core or shell of the nucleus accumbens. Single coronal sections through the rat forebrain were processed for immunoperoxidase labeling of serotonin and immunogold silver labeling of tyrosine hydroxylase, the catecholamine-synthesizing enzyme. Varicose processes showing peroxidase product for serotonin by light microscopy were confirmed to be axons and terminals by electron microscopy. In a quantitative analysis of serotonin-immunoreactive terminals forming one or more contacts in single sections, some common features were observed. For the core (n = 120) and the shell (n = 82), 41% formed synaptic junctions with unlabeled dendrites, 75% were in apposition with unlabeled terminals, which often formed asymmetric junctions, and 20% were in apposition with axons or terminals containing tyrosine hydroxylase. Thus, in both the core and shell of the nucleus accumbens, serotonin terminals synapse on postsynaptic neurons and are likely to modulate or be modulated by presynaptic interactions with excitatory axons forming asymmetric junctions and by catecholaminergic afferents. Marked differences in the morphology of serotonin axons were also seen in the core versus shell of the nucleus accumbens. By light microscopy, serotonin-immunoreactive axons were thicker and more varicose than those found in the core. Ultrastructural analysis confirmed that, in contrast to the core, serotonin-immunoreactive axons and terminals in the shell were larger in cross-sectional diameter size (0.7 micron vs. 0.3 micron). Additionally, serotonin axon terminals in the shell contained more numerous immunoreactive large dense core vesicles and more frequently formed symmetric as opposed to asymmetric contacts with dendrites. The larger size and more numerous dense core vesicles in serotonin-immunoreactive terminals in the shell support the concept that serotonin or co-existing neurotransmitter may be more tonically released in the shell versus core of the nucleus accumbens.

Ultrastructural analysis of the serotonin hyperinnervation in adult rat neostriatum following neonatal dopamine denervation with 6-hydroxydopamine

Serotonin (5-HT) immunocytochemistry was used at the electron microscopic level to characterize the ultrastructural features of 5-HT axon terminals (varicosities) hyperinnervating the neostriatum of adult rats, 3 months after a neonatal destruction of the nigrostriatal dopamine system by intraventricular 6-hydroxydopamine. 5-HT-immunostained terminals from the anterior half of the hyperinnervated neostriatum were examined in single thin sections, and compared to their counterparts in vehicle-injected controls with respect to shape, size, organelle content, presence of a synaptic membrane differentiation and composition of the microenvironment. The intrinsic and relational features of the 5-HT-immunostained varicosities were essentially the same in 5-HT-hyperinnervated and control tissue. In particular, the frequency with which these varicosities made synaptic contacts was similarly low in both conditions (6-8% for whole varicosities), as already described in normal adult rat neostriatum. The distributional frequency of elements juxtaposed to the 5-HT-immunostained varicosities was also comparable in control and 5-HT-hyperinnervated tissue. However, in both conditions, there were much fewer dendritic spines in the microenvironment of 5-HT varicosities than around unlabeled terminals randomly selected from the same thin sections. This difference seemed entirely due to the numerous axo-spinous synaptic contacts made by the randomly selected, unlabeled varicosities. Together with recent observations on the 5-HT-hyperinnervation of adult rat hippocampus after grafts of fetal neurons, these data lead to the suggestion that mostly non-junctional neostriatal 5-HT terminals are not committed to a specific intratissular microenvironment. This might in part explain why they grow in excess when reinnervating adult tissue after a lesion or a graft.

Ultrastructure of aberrant serotonin-immunoreactive fibers in the caudate putamen complex of the aged rat

Degeneration of neurons in the central nervous system is associated with morphological changes. Previous observations made at the light microscopical level indicated degeneration of serotonin-immunoreactive (IR) fibers in the aged rat brain. In this study, a comparison at the ultrastructural level was made between serotonin-IR normal thin and aberrant swollen varicose fibers in the caudate-putamen complex of the aged rat. Ultrastructural features such as the size and content of the thin varicose fibers resembled those in the caudate-putamen complex of the young rat as reported by others. The aberrant profiles were swollen, reaching a size of 6 microns. Their vesicles varied in size and were no longer uniformly round. Moreover, distorted mitochondria and membrane-filled vacuolelike structures were a common feature of the aberrant profiles. These changes are indicative of a degenerative process and give further evidence that, whereas many serotonergic fibers are preserved at high age, other serotonergic fibers are degenerating in the caudate-putamen complex of the aged rat.

Immunohistochemical study of the serotoninergic innervation of the basal ganglia in the squirrel monkey

A specific antibody raised against 5-hydroxytryptamine (5-HT) conjugated to bovine serum albumin was used to study the serotoninergic innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus). At midbrain level, numerous fine 5-HT-immunoreactive axons were seen to arise from the immunopositive neurons of the dorsal raphe nucleus and less abundantly from those of the nucleus centralis superior. The bulk of these axons formed a rather loosely arranged bundle that arched ventrorostrally through the central portion of the midbrain tegmentum and ascended toward the ventral tegmental area. Several fascicles detached themselves from this bundle to reach the substantia nigra where they arborized into a multitude of heterogeneously distributed 5-HT terminals. The 5-HT innervation was particularly dense in the pars reticulata but much less so in the pars compacta of the substantia nigra. More rostrally other 5-HT fibers swept dorsolaterally and formed a remarkably dense network of varicose fibers within the subthalamic nucleus. A multitude of 5-HT axons continued their ascending course within the lateral hypothalamic area, and many of them swept laterally to invade the lenticular nucleus. At pallidal levels, the 5-HT axons arborized much less profusely in the external segment than in the internal segment, which contained numerous 5-HT varicose fibers and terminals arranged in a typical bandlike pattern. At striatal levels, the 5-HT terminals were particularly abundant in the ventral striatum, including the nucleus accumbens and deep layers of the olfactory tubercle. They also abounded in the ventrolateral region of the putamen and the ventromedial aspect of the caudate nucleus. Overall, the number of 5-HT fibers and terminals decreased progressively along the rostrocaudal axis of the striatum and several large and elongated zones rather devoid of 5-HT immunoreactivity were visualized, particularly in the caudate nucleus and the dorsal putamen. These zones of poor 5-HT immunoreactivity were in register with similar areas devoid of tyrosine hydroxylase immunoreactivity as seen on contiguous sections. These findings reveal that all the core structures of the basal ganglia in primates receive a significant serotoninergic input, but that the densities and patterns of innervation vary markedly from one structure to the other.

Potentiation by serotonergic inhibition of yawning induced by dopamine receptor agonists in rats

Low doses of the dopamine D2-receptor agonist, B-HT 920 (25 micrograms/kg, SC), and the dopamine D1/D2-receptor agonists, apomorphine (50 micrograms/kg, SC) and piribedil (1 mg/kg, SC), evoked yawning. However, the dopamine D1-receptor agonist, SK&F 38393 (2 mg/kg, SC), failed to induce yawning. The yawning responses induced by B-HT 920, apomorphine or piribedil were markedly increased without eliciting stereotypy by pretreatment with reserpine (5 mg/kg, IP, 24 hr). These yawning responses were also enhanced by p-chlorophenylalanine (PCPA) (300 mg/kg, IP, 72 hr), but not by alpha-methyl-p-tyrosine (300 mg/kg, IP, 6 hr). The yawning induced by B-HT 920 given alone and in combination with reserpine or PCPA was inhibited by spiperone (0.5 mg/kg, IP) or scopolamine (0.5 mg/kg, IP), but not by SCH 23390 (0.5 mg/kg, IP). The present results suggest that yawning is evoked by stimulation of dopamine D2-receptors having a high affinity and consequent muscarinic activation, and that the yawning induced by dopamine receptor agonists is potentiated by decreases in serotonergic neuron activity.

Serotonin innervation in adult rat neostriatum. II. Ultrastructural features: a radioautographic and immunocytochemical study

High-resolution radioautography after cerebroventricular administration of tritiated serotonin (5-HT) and PAP immunocytochemistry with an antiserum against 5-HT-glutaraldehyde conjugate (kindly donated by M. Geffard) were used in parallel to investigate the intrinsic and relational fine structural features of 5-HT axon varicosities (terminals) in the neostriatum of the adult rat. The uptake-labeled varicosities were examined in single thin sections from a paraventricular sector of neostriatum, whereas their immunostained counterparts were viewed in serial thin sections from the same paraventricular sector plus a dorsal neostriatal sector. The two approaches yielded complementary results in terms of varicosity dimensions, synaptic features and appositional relationships. Serotonin axon terminals were generally small and, as measured in immunostained material, even smaller in the dorsal than in the paraventricular neostriatum. Their internal features, best viewed in radioautographs, included small pleomorphic synaptic vesicles with occasional large granular vesicles and mitochondria. Junctional 5-HT terminals from both the paraventricular and the dorsal neostriatal sectors synapsed exclusively, and with equal frequency, on dendritic spines or shafts, almost always with asymmetrical membrane differentiations. The proportion of junctional varicosities, however, was very low in serial (immunocytochemical) as well as single (radioautographic) thin sections. Only 10-13% of 5-HT varicosities from either the paraventricular or the dorsal neostriatum exhibited a synaptic junction, in contrast with a junctional incidence of at least 70% for randomly selected axonal varicosities similarly sampled in the surrounding neuropil. Serotonin axon terminals, whether or not synaptic, were closely apposed to a variety of structures comprising mostly other axon terminals, dendritic spines and branches, but rarely neuronal somata. The synaptic and appositional features of immunostained 5-HT varicosities were similar for both the dorsal and the paraventricular neostriatum. In this context, it is likely that the effects of 5-HT in the neostriatum are exerted upon a multiplicity of cellular target sites in addition to the restricted number of dendritic spines and shafts synaptically contacted by this type of monoamine terminal.

Biochemical mapping of cholecystokinin-, substance P-, [Met]enkephalin-, [Leu]enkephalin- and dynorphin A (1-8)-like immunoreactivities in the human cerebral cortex

The distribution of immunoreactive cholecystokinin, substance P, [Met]enkephalin, [Leu]-enkephalin and dynorphin was determined in the cerebral cortex of the human brain post mortem. Peptide radioimmunoassays in three selected zones of the cortical gray mantle (frontal, temporal, occipital) revealed significant regional differences, prompting to the development of a new dissection procedure for the complete mapping of peptide-like materials throughout the entire cerebral cortex. For this purpose, frozen cerebral hemispheres were cut rostrocaudally in 21 verticofrontal serial sections, from which the cortical gray matter was divided into 4-5 distinct zones. The peptides could be measured in each of the 93 dissected pieces of tissue, but their distribution was uneven. The most abundant was cholecystokinin, particularly in the anterior part of the frontal lobe and in the temporal cortex, where its levels reached 0.5 ng/mg of tissue. The regional distribution of cholecystokinin resembled that of substance P with a decreasing gradient from the frontal to the occipital pole, but absolute levels of substance P were hardly one tenth of cholecystokinin levels. The mean concentrations of the three opioid peptides were even less than those of substance P, and their regional distributions were markedly different. [Met]Enkephalin was concentrated in the occipital cortex, and [Leu]enkephalin in the temporal cortex. Dynorphin was the least abundant, even in the temporal cortex where the highest levels were found. The widespread and heterogeneous distribution of these peptides strongly suggests that each of them exerts specific functions in the human cerebral cortex.

Serotonin innervation in adult rat neostriatum. I. Quantified regional distribution

The distributional features of the serotonin (5-HT) innervation in adult rat neostriatum were examined and quantified using two complementary chemoanatomical methods: 5-HT-immunohistochemistry on serial histological sections and radioautography after [3H]5-HT uptake in whole cerebral hemisphere slices. As visualized and measured after peroxidase-antiperoxidase immunostaining, the neostriatal 5-HT fiber network pervading the entire neostriatum was 2-3 times denser in its ventral than dorsal parts, and showed a slight rostrocaudal increase in density. Its axonal length ranged from 1.06 to 4.18 m per mm3 of striatal tissue. Radioautographic counts of the [3H]5-HT-labeled axon varicosities within comparable sectors of the neostriatum showed good correlation with this distribution pattern. As extrapolated after appropriate corrections for incomplete detection at the chosen exposure time and from the thickness of sections examined, the number of neostriatal 5-HT varicosities (innervation density) ranged from 1.5 to 4.8 millions and averaged 2.6 millions per mm3 of tissue. These quantitative results provided new insights into the topographical organization of the dorsal raphe-neostriatal 5-HT projection system. They already allow for meaningful correlations with currently available microchemical data on intrastriatal 5-HT levels and should also be of considerable significance when as precise information becomes available on the number and localization of different 5-HT receptors and uptake carriers within rat neostriatum.

Relation between yawning behavior and central serotonergic neuronal system in rats

Subcutaneously (s.c.) administered apomorphine (0.0125-0.4 mg/kg) or physostigmine (0.025-0.4 mg/kg) to rats elicited yawning. The dose-response curves were bell-shaped. The peak effects of apomorphine and physostigmine were observed with a dose of 0.1 mg/kg of each drug. Yawning elicited by apomorphine (0.1 mg/kg) or physostigmine (0.1 mg/kg) was reduced by intraperitoneally (i.p.) administered 5-hydroxytryptophan (5-HTP, 50-200 mg/kg, given 30 min before). Yawning elicited by apomorphine but not by physostigmine was enhanced by p-chlorophenylalanine (p-CPA, 25-400 mg/kg i.p., given 24 h before). Apomorphine elicited but not physostigmine-elicited yawning was enhanced by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, 8 micrograms/rat, given 14 days before into the dorsal raphe). This treatment led to a 35% depletion of serotonin (5-HT) in the striatum. 5-HTP, p-CPA or 5,7-DHT given alone did not elicit yawning. Bilateral, intrastriatal microinjection of apomorphine (1.5-50 micrograms/site) but not physostigmine (5-50 micrograms/site) elicited yawning. The dose-response curve was also bell-shaped. These results indicate that central serotonergic pathways play an important role in modulating drug-elicited yawning in rats.

Transmitter localization and vesicle turnover at a serotoninergic synapse between identified leech neurons in culture

An electron microscopic study has been made of chemical synapses that develop between identified nerve cells isolated from the CNS of the leech and maintained in culture. Structures resembling synapses were observed in pairs of Retzius cells and P sensory cells at which chemical transmission had been demonstrated by recording with microelectrodes. Vesicle recycling was shown by following the uptake of extracellular markers after stimulation. The membrane separation between the presynaptic Retzius cell (which is known to liberate serotonin) and the postsynaptic P cell was wider in synaptic than in extrasynaptic regions. The Retzius cell contained clusters of clear vesicles apposed to thickenings of the presynaptic membrane. These clear vesicle clusters were capped by a layer of dense core vesicles that did not contact the presynaptic membrane thickenings. Subsynaptic cisternae were found in the postsynaptic cell opposite the presynaptic membrane thickenings. Occasional slight postsynaptic membrane thickenings were seen. Extracellular material was observed within the synaptic cleft. Similar synaptic structures developed between pairs of Retzius cells in culture; even a single Retzius cell was able to form autapses upon itself. Structures resembling transmitter release sites were found in Retzius cells at a distance from any postsynaptic membranes. These are presumed to be locations for the diffuse release of transmitter. Presynaptic structures resembling release sites were never observed in P cells apposed to Retzius cells. Antibody to serotonin (5-HT) labelled with colloidal gold showed serotonin to be localized in the dense core vesicles in Retzius cells. Stimulation of pairs of Retzius and P cells by raised concentrations of K+ resulted in uptake of extracellular markers. Only Retzius cells became labelled. Ferritin was found in cisternae, in dense core vesicles, and in clear vesicles. HRP was found in cisternae and in clear vesicles. Colloidal gold was taken up by coated vesicles and was occasionally found in both clear and dense core vesicles. The uptake of extracellular markers following stimulation was blocked by high Mg++. These results show that structures develop between pairs of cells at which chemical transmission develops and that transmitter release leads to turnover of dense core and clear vesicles.

Abundance in the embryonic brainstem of adrenaline during the absence of detectable tyrosine hydroxylase activity

The activities of the catecholamine synthetic enzymes tyrosine hydroxylase and phenylethanolamine N-methyltransferase, and the concentrations of the catecholamines and their respective metabolites, have been measured in the dorsal and ventral halves of the brainstem at various ages in the embryonic and adult rat. The activity of phenylethanolamine N-methyltransferase in both parts of the brainstem at day 14 of gestation is at or greater than adult levels and thereafter displays relatively small variations during ontogeny. Tyrosine hydroxylase activity, in contrast, is undetectable at day 14 and increases slowly, achieving only 20-25% of adult values by day 18 of gestation. Adrenaline concentrations correlate well with the activity of phenylethanolamine N-methyltransferase, showing a precocious development, whereas noradrenaline and 3,4-dihydroxyphenylethylamine (dopamine) concentrations are more closely related to the enhancement of tyrosine hydroxylase activity; at day 18 of gestation, for example, they are only 5 and 10%, respectively, of the adult values. The concentrations of the metabolites of noradrenaline and dopamine are suggestive of a high rate of turnover. These results confirm previous immunocytochemical evidence of a tardy appearance of tyrosine hydroxylase-like immunoreactivity in the phenylethanolamine N-methyltransferase-positive perikarya of the embryonic medulla oblongata. In addition, the abundance of adrenaline in this area at early gestational stages strongly suggests that, despite the paucity of tyrosine hydroxylase, phenylethanolamine N-methyltransferase is active in vivo and is utilizing a substrate other than noradrenaline. It is likely, however, that at later stages of gestation, when tyrosine hydroxylase is present at sufficient activity to supply noradrenaline, the conventional synthetic pathway for adrenaline formation comes into being.

Monoamine innervation of the oculomotor nucleus in the rat. A radioautographic study

The serotonin and noradrenaline innervations of the rat oculomotor nucleus were examined by high resolution radioautography after in vivo labeling with tritiated 5-hydroxytryptamine and dopamine, respectively. Noradrenaline as well as serotonin endings (axonal varicosities) pervaded the entire nucleus, but the latter were at least six times more numerous (1.3 X 10(6) per mm3 of tissue) and were often found in the immediate vicinity of neuronal somata and proximal dendrites. The axon terminals of both types were of similar size and exhibited some large dense-cored vesicles in association with aggregated small and clear vesicles. The dense-cored vesicles were, however, more frequent and the content in clear vesicles more pleomorphic in serotonin than noradrenaline endings. In single thin sections, the proportion of noradrenaline and serotonin profiles exhibiting a synaptic junction was relatively small (15%). These were either symmetrical or asymmetrical when made on dendritic branches but invariably symmetrical on spines. In addition, a significant number of serotonin terminals were seen in close apposition or synaptic contact with neuronal perikarya and large dendrites, allowing for a direct, "proximal" action of serotonin. Moreover, many such terminals appeared to be coupled with unlabeled endings of another category, characterized by dispersed, uniformly round and clear synaptic vesicles, providing an alternate route for a proximal effect of serotonin in the oculomotor nucleus. In line with previous investigations on other motor nuclei, these data support the likelihood of a close involvement of both noradrenaline and serotonin in the control of motoneuronal activity.

Immunocytochemical localization of serotonin in the rat dentate gyrus following raphe transplants

The ultrastructural features of the serotoninergic innervation of the rat dentate gyrus in normal adults and in animals receiving raphe nuclear area transplants was investigated using an antibody to serotonin (5-HT). Neonatal rats received a lesion of the fimbria-fornix and entorhinal cortex. Three days later, a portion of embryonic (E-16-18) raphe nuclear area was transplanted to the entorhinal cavity and the animals were allowed to survive for 60 days. Animals were processed for the immunocytochemical localization of 5-HT using the peroxidase-antiperoxidase method. Light microscopic observation showed that 5-HT-containing fibers from transplanted raphe neurons densely innervated the hilar and molecular zones of the dentate gyrus. Electron microscopic analysis showed that 5-HT immunoreactivity was contained only in axons and axon varicosities. There were no differences in the ultrastructural characteristics of axons and axon terminals between normal animals and those which had received raphe transplants. A mixture of both conventional synaptic junctions and non-synaptic axonal swellings were found in both groups.

Localization of [3H]serotonin in neuroblastoma x glioma hybrid cells

Synthesis, localization and release of serotonin (5-HT) were studied in cholinergic neuroblastoma X glioma NG108-15 cells. The content of 5-HT and tryptophan hydroxylase activity rose substantially when hybrid cells were differentiated by prostaglandin E1 plus theophylline, or dibutyryl cAMP. Localization of [3H]5-HT taken up into differentiated NG108-15 cells was examined by electron microscopic radioautography. Silver grains were observed mostly in neurites, indicating that neurites of differentiated NG108-15 cells are the preferential uptake site of [3H]5-HT. Statistical analysis of the results of electron microscopic radioautographs revealed that silver grains had a high affinity for dense core vesicles of 60-170 nm diameter, though grains were also located over endoplasmic reticulum, mitochondria and cytosol. Dense core vesicles were abundant in neurites, and less numerous in cell bodies of the hybrid cells. [3H]5-HT taken up into NG108-15 cells was released by potassium stimulation in the presence of Ca2+. The results indicate that NG108-15 hybrid cells manifest many properties comparable to those of serotonergic neurons.

Chemical anatomy of the brain

The development of sensitive histochemical-neuroanatomical techniques has made it possible to analyze the content of specific compounds in single nerve cells and their processes. In consequence, it has been possible to construct detailed maps of the distribution of various types of neurons on the basis of their transmitter substance. There are now many examples of neurons containing both a classical transmitter and a peptide. In some instances the peptides seem to support the action of the classical transmitters. This interaction may have applications in the prevention and treatment of nervous disease states.

Uptake and retention of [3H]adrenaline by central monoaminergic neurons: a light- and electron-microscope radioautographic study after intraventricular administration in the rat

Paraventricular and paracisternal regions of adult rat central nervous system were investigated by light- and electron-microscope radioautography after intraventricular administration of tritiated adrenaline. In tissue primarily fixed by glutaraldehyde perfusion and post-fixed by immersion in osmium tetroxide, there were no aggregates of silver grains indicative of intraneuronal accumulation of the tracer, except over perivascular nerve terminals at the base of the brain. In contrast, when both fixation and postfixation were carried out by rapid vascular perfusion, preferentially labeled nerve cell bodies and axonal varicosities (i.e. terminals) were detected in various anatomical areas known to contain dopaminergic and/or noradrenergic neurons. Serotoninergic axonal varicosities in the supraependymal plexus and subcommissural organ, as well as a small group of nerve cell bodies of undetermined chemical identity in the n. paraventricularis thalami were also found to be labeled. Addition of a ten-fold higher concentration of non-radioactive serotonin to the solution of [3H]adrenaline suppressed the reactivity in the subcommissural organ and the supraependymal plexus but had no such effect elsewhere in brain. Lesioning of the nigrostriatal dopaminergic system with 6-hydroxydopamine prior to [3H]adrenaline injection eradicated axon terminal labeling in the ipsilateral neostriatum. Electron-microscopic examination of [3H]adrenaline-labeled varicosities in the neostriatum, lateral septum, arcuate nucleus and median eminence extended earlier observations on the ultrastructure of the catecholaminergic innervation of these regions. It was concluded that both dopaminergic and noradrenergic neurons as well as certain serotonin-containing axon terminals can take up and retain [3H]adrenaline, although they probably have lesser affinity for this amine than for their own transmitter. Due to the fact that presumptive adrenergic neurons are intermingled with dopaminergic and noradrenergic elements, further work will be needed to determine to which extent they also contributed to [3H]adrenaline uptake in the present experimental conditions.

Serotonergic excitation from dorsal raphe stimulation recorded intracellularly from rat caudate-putamen

Similar to other afferents to rat caudate-putamen, stimulation of the dorsal raphe nucleus evokes a series of 3 responses which can be recorded intracellularly. An initial depolarization is followed by a long-lasting inhibition which is, in turn, terminated by another period of depolarization. Pharmacological manipulations demonstrate that the initial depolarizing potential is serotonergic. Depletion of serotonin by means of prior treatment with para-chlorophenylalanine leads to a reduction in the amplitude of the depolarization which can be evoked by maximal stimulation of dorsal raphe. Neither the long-lasting hyperpolarization nor the late excitation which follow the initial depolarization is affected. Replacement of serotonin in levels by injection of 5-hydroxytryptophan results in a restoration of the amplitude of the depolarizing response. The latency of the initial depolarization is, however, unchanged in serotonin-depleted animals. This together with the observation in some cells of a component of the initial depolarization resistant to para-chlorophenylalanine treatment, suggests that there is a non-serotonergic excitation which precedes that mediated by serotonin.

Radioautographic investigation of monoaminergic neurons: an evaluation

This review outlines the most relevant contributions currently available on the detection and ultrastructural characterization of monoaminergic neurons by radioautography after administration of radiolabeled monoamines. It includes methodological considerations and then a critical analysis of the diagnostic value of the radioautographic method for catecholaminergic and serotoninergic neurons, emphasizing in particular its recent applications to the visualization of dopaminergic axon terminals. An attempt is then made to evaluate the method in terms of specificity, sensitivity and resolution and its possibilities with regard to quantitative analysis. Lastly, its value for approaching the dynamic and metabolic properties of monoaminergic neurons is stressed.

Supra-ependymal serotoninergic nerves in mammalian brain: morphological, pharmacological and functional studies

Supra-ependymal nerves in mammals (mainly rats) have been shown to contain serotonin (5-hydroxytryptamine, 5-HT) by combined Falck-Hillarp fluorescence histochemistry, ultrastructural monoamine cytochemistry and pharmacology as well as by immunohistochemistry and autoradiography. Supra-ependymal 5-HT cells do not occur. At least in rats, virtually all supra-ependymal nerves contain 5-HT and in our opinion the occasionally described non-5-HT supra-ependymal nerve cells and their processes contribute little to the supra-ependymal nerve plexus (with the possible exception of those cells above the median eminence). The cells of origin of the supra-ependymal 5-HT nerves are situated in raphe nuclei. The axons and terminals (varicosities) contain small and large dense core vesicles in both of which 5-HT is stored. A co-transmitter has not been found among the candidates investigated so far (leu- and met-enkephalin, substance P and gamma-aminobutyric acid (GABA)). The nerves possess uptake mechanisms specific for 5-HT and possibly GABA. Occasionally desmosome-like junctions are observed between 5-HT nerve terminals and ependymal cells but no true synapses. The function of these nerves is not known. They do not appear to regulate ciliary movement, but might influence the shape of ependymal cells.

In vivo release of serotonin in two raphe nuclei (raphe dorsalis and magnus) of the cat

The release of 3H-serotonin (3H-5-HT) endogenously synthesized from 3H-tryptophan was estimated in both dorsalis and magnus (MRN) raphe nuclei of anaesthetized "encéphale isolé" cats, by using push-pull cannulae. Resting steady state in the release of 3H-5-HT was observed 30 min after the beginning of superfusion with L-3H-tryptophan. The amounts of 3H-5-HT released in the DRN and the MRN are much greater than those measured simultaneously in the caudate nucleus. A marked increase either in the 5-HT release was seen in the presence of fluoxetine, a potent reuptake blocker of 5-HT, or during local depolarization with potassium chloride. The spontaneous release was diminished by removing Ca++ and by adding cobalt to the medium. Tetrodotoxin (TTX) decreased the 5-HT release in the DRN and, based on previously established, blocked the stimulating effect of batrachotoxin. According to the pharmacological characteristics of the monoamine dendritic release determined for dopamine in the substantia nigra [17], our results suggest that 5-HT release processes in the DRN correspond to a release from nerve endings, not from dendrites. The purpose of this study was to determine if the 5-HT released in the DRN is released from either axon terminals or dendrites. Morphological studies performed on the DRN do not consistently demonstrate the high density of serotoninergic varicosities in the DRN. In addition, two types of 5-HT axonal varicosities, characterized by their synaptic or non-synaptic junctions, are present in the brain. The concept that the quantities of 5-HT released could vary from one type compared to the other is discussed.

Evidence of the modulatory role of serotonin in acetylcholine release from striatal interneurons

The release of acetylcholine was studied in isolated striatal slices of the rat. The spontaneous and ouabain-stimulated release of acetylcholine was higher in those slices where serotonergic input was somehow impaired: raphe nuclei lesion or p-chlorophenylalanine pretreatment or 5, 7-dihydroxytryptamine pretreatment resulted in a higher release. L-(m-chlorophenyl)-piperazine, a pure serotonin receptor stimulant and D-fenfluramine, a serotonin releaser significantly reduced the release of acetylcholine evoked by ouabain. Serotonin antagonists (cyproheptadine, mianserine and methysergide) prevented the effect of serotonin agonists. When the serotonergic neurons were destroyed either by p-chlorophenylalanine or by 5, 7-dihydroxytryptamine pretreatment D-fenfluramine had no inhibitory action; however, the effect of L-(m-chlorophenyl)-piperazine was not affected. It is suggested that there is a link between serotonergic and cholinergic neurons in the striatum: serotonin released from raphe-striatal neurons is able to inhibit the release of acetylcholine from striatal interneurons.

Characterisation of inhibitory 5-hydroxytryptamine receptors that modulate dopamine release in the striatum

The effect of a series of indoleamines on the potassium-evoked tritium release of previously accumulated [3H]dopamine from rat striatal slices has been investigated. The indoleamines 5-hydroxytryptamine, 5-methoxytryptamine, 5-methoxy-N,N'-dimethyltryptamine and tryptamine (10(-7) to 10(-5) M) all reduced potassium-evoked release of tritium, to a maximum of 50%. The uptake of [3H]dopamine was unaffected by these compounds. A series of 5-hydroxytryptamine antagonists were examined for their ability to reduce the inhibition of potassium-evoked tritium release induced by 5-methoxytryptamine. The relative order of antagonist potency obtained was methysergide greater than metergoline greater than methiothepin greater than cinanserin greater than cyproheptadine greater than mianserin, and was consistent with an action on 5-hydroxytryptamine receptors. It is concluded that there are inhibitory 5-hydroxytryptamine receptors located on the terminals of dopaminergic neurones in the striatum.