In vitro studies on central and peripheral monoamine neurons at the ultrastructural level

Extrasynaptic Communication

Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.

Roles Played by the Na+/Ca2+ Exchanger and Hypothermia in the Prevention of Ischemia-Induced Carrier-Mediated Efflux of Catecholamines into the Extracellular Space: Implications for Stroke Therapy

The release of [³H]dopamine ([³H]DA) and [³H]noradrenaline ([³H]NA) in acutely perfused rat striatal and cortical slice preparations was measured at 37 °C and 17 °C under ischemic conditions. The ischemia was simulated by the removal of oxygen and glucose from the Krebs solution. At 37 °C, resting release rates in response to ischemia were increased; in contrast, at 17 °C, resting release rates were significantly reduced, or resting release was completely prevented. The removal of extracellular Ca²⁺ further increased the release rates of [³H]DA and [³H]NA induced by ischemic conditions. This finding indicated that the Na⁺/Ca²⁺ exchanger (NCX), working in reverse in the absence of extracellular Ca²⁺, fails to trigger the influx of Ca²⁺ in exchange for Na⁺ and fails to counteract ischemia by further increasing the intracellular Na⁺ concentration ([Na⁺]_i). KB-R7943, an inhibitor of NCX, significantly reduced the cytoplasmic resting release rate of catecholamines under ischemic conditions and under conditions where Ca²⁺ was removed. Hypothermia inhibited the excessive release of [³H]DA in response to ischemia, even in the absence of Ca²⁺. These findings further indicate that the NCX plays an important role in maintaining a high [Na⁺]_i, a condition that may lead to the reversal of monoamine transporter functions; this effect consequently leads to the excessive cytoplasmic tonic release of monoamines and the reversal of the NCX. Using HPLC combined with scintillation spectrometry, hypothermia, which enhances the stimulation-evoked release of DA, was found to inhibit the efflux of toxic DA metabolites, such as 3,4-dihydroxyphenylacetaldehyde (DOPAL). In slices prepared from human cortical brain tissue removed during elective neurosurgery, the uptake and release values for [³H]NA did not differ from those measured at 37 °C in slices that were previously maintained under hypoxic conditions at 8 °C for 20 h. This result indicates that hypothermia preserves the functions of the transport and release mechanisms, even under hypoxic conditions. Oxidative stress (H₂O₂), a mediator of ischemic brain injury enhanced the striatal resting release of [³H]DA and its toxic metabolites (DOPAL, quinone). The study supports our earlier findings that during ischemia transmitters are released from the cytoplasm. In addition, the major findings of this study that hypothermia of brain slice preparations prevents the extracellular calcium concentration ([Ca²⁺]_o)-independent non-vesicular transmitter release induced by ischemic insults, inhibiting Na⁺/Cl⁻-dependent membrane transport of monoamines and their toxic metabolites into the extracellular space, where they can exert toxic effects.

Amyloid beta peptides, locus coeruleus-norepinephrine system and dense core vesicles

The evolution of peptidergic signaling systems in the central nervous system serves a distinct and crucial role in brain processes and function. The diversity of physiological peptides and the complexity of their regulation and secretion from the dense core vesicles (DCV) throughout the brain is a topic greatly in need of investigation, though recent years have shed light on cellular and molecular mechanisms that are summarized in this review. Here, we focus on the convergence of peptidergic systems onto the Locus Coeruleus (LC), the sole provider of norepinephrine (NE) to the cortex and hippocampus, via large DCV. As the LC-NE system is one of the first regions of the brain to undergo degeneration in Alzheimer's Disease (AD), and markers of DCV have consistently been demonstrated to have biomarker potential for AD progression, here we summarize the current literature linking the LC-NE system with DCV dysregulation and Aβ peptides. We also include neuroanatomical data suggesting that the building blocks of senile plaques, Aβ monomers, may be localized to DCV of the LC and noradrenergic axon terminals of the prefrontal cortex. Finally, we explore the putative consequences of chronic stress on Aβ production and the role that DCV may play in LC degeneration. Clinical data of immunological markers of DCV in AD patients are discussed.

Early attempts to visualize cortical monoamine nerve terminals

The Falck-Hillarp, formaldehyde fluorescence method for the demonstration of monoamine neurons in a microscope was established in Lund, Sweden and published in 1962. In the same year Hillarp moved to Karolinska Institutet in Stockholm. Two years later Dahlström and Fuxe published the famous supplement in Acta Physiologica Scandinavica, describing the distribution of the dopamine, noradrenaline and serotonin cell groups in the rat brain. This landmark paper also represented an important contribution to an emerging discipline in neuroscience - chemical neuroanatomy. During the following years several modifications of the original method were developed, attempting to solve some shortcomings, one being the reproducible demonstration of noradrenaline nerve terminals in cortical regions. One result was the paper focused on in the present article, which also describes other efforts in the same direction going on in parallel, primarily, in Lund and Stockholm. As a result there was, in the mid 1970s, a fairly complete knowledge of the catecholamine systems in the rat brain. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Release of dopamine beta-hydroxylase and chromogranin A upon stimulation of the splenic nerve

Two proteins present in noradrenergic vesicles of the splenic nerve (dopamine beta-hydroxylase and chromogranin A) are released into the perfusate from the spleen when the splenic nerve is stimulated. Experiments in which drugs were added to the perfusion fluid showed that the proteins were released from terminals of the splenic nerve. There was a correlation between the amounts of the proteins released and the quantity of noradrenaline released; and the release process was dependent upon calcium. It is suggested that the proteins are released from the large dense-cored vesicles present in the terminals of the splenic nerve, and that secretion from these vesicles occurs by exocytosis.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the 'what' of brain function, the 'how' of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the 'how' that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic-extrasynaptic receptor trafficking, neuron-glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.

The fine structure of the neurons in the rat substantia nigra

Three distinct neurons were identified in the substantia nigra of the rat using Golgi, light, and electron microscopic techniques. A large neuron, found in the pars reticulata, is characterized by well-developed RER, a tubular cytoplasmic inclusion, and somatic and dendritic thorns. A medium-sized neuron, found in the pars compacta, has an eccentric nucleus, distinct Nissl bodies, and an inclusion composed of whorls of concentric cisternae. A small neuron, found in both nigral regions, contains a highly invaginated nucleus, fibrous nuclear inclusion, and paucity of cytoplasmic organelles. Its axon synapses around other nigral dendrites. The presence of these neurons was correlated with the efferent projections and function of the substantia nigra.

Looking at neurotransmitters in the microscope

This review article covers the early period of my career. I first summarize research initiated by the late Nils-Ake Hillarp, after his appointment in 1962 as professor in the Department of Histology at Karolinska Institutet. He only lived for three more years, but during this short period he started up a group of ten students who explored various aspects of the three monoamine transmitters, dopamine, noradrenaline and 5-hydroxytryptamine, using the new formaldehyde fluorescence method developed by Bengt Falck and Hillarp in Lund. This method allowed visualization of the cellular localization in the microscope of these monoamines, which introduced a new discipline in neurobiology-chemical neuroanatomy. I then deal with work aiming at localizing the monoamines at the ultrastructural level, as well as attempts to use radioactively labeled aminoacids, especially gamma-aminobutyric acid (GABA), and autoradiography, to identify, in the microscope, neurons using such transmitters. Finally, our immunohistochemical work together with Kjell Fuxe and the late Menek Goldstein, using antibodies to four monoamine-synthesizing enzymes is summarized, including some aspects on the adrenaline neurons, which had escaped detection with the Falck-Hillarp technique.

Factors influencing the occurrence of "on-off" symptoms during long-term treatment with L-dopa

"On-off" symptoms were found to have developed in 43 of 85 parkinsonian patients who had been treated with L-dopa for five years or more, the risk of such symptoms apparently being greater the younger the patient had been at the début of the disease and at the start of treatment. The dopa dose had been higher throughout the treatment in the patients developing "on-off" symptoms than in those maintaining an even effect, and, furthermore, the initial improvement had been more marked and dyskinesia had appeared earlier and in a higher frequency. The clinical observations in the present studies seemed to be related to some pharmacological findings, where different degrees of the nigrostriatal neuron degeneration and the efficacy of the remaining neurons could be of importance for a varying therapeutic response. It also seemed possible that a higher dopa dose could evoke "on-off" symptoms more easily than a lower one in thereto predisposed individuals. As the patients maintaining an even symptomatology during long-term treatment with L-dopa were older, and in particularly as they had dementia in a higher frequency than those developing "on-off" symptoms, the possibility of a more widespread neuron damage, influencing the clinical manifestation in these patients, had to be taken into consideration.

Long latency of evoked quantal transmitter release from somata of locus coeruleus neurons in rat pontine slices

The locus coeruleus (LC) harbors a compact group of noradrenergic cell bodies projecting to virtually all parts of the central nervous system. By using combined measurements of amperometry and patch-clamp, quantal vesicle release of noradrenaline (NA) was detected as amperometric spikes, after depolarization of the LC neurons. After a pulse depolarization, the average latency of amperometric spikes was 1,870 ms, whereas the latency of glutamate-mediated excitatory postsynaptic currents was 1.6 ms. A substantial fraction of the depolarization-induced amperometric spikes originated from the somata. In contrast to glutamate-mediated excitatory postsynaptic currents, NA secretion was strongly modulated by the action potential frequency (0.5-50 Hz). Somatodendritic NA release from LC upon enhanced cell activity produced autoinhibition of firing and of NA release. We conclude that, in contrast to classic synaptic transmission, quantal NA release from LC somata is characterized by a number of distinct properties, including long latency and high sensitivity to action potential frequency.

Noradrenaline storage function of species-specific protein bodies, markers of monoamine neurons in human locus coeruleus demonstrated by dopamine-beta-hydroxylase immunogold localization

Our histochemical and ultrastructural studies have identified, in human catecholamine locus coeruleus (LC) neurons, abundant and large spherical protein bodies (PB), containing histone-like, arginine-rich proteins, which originate as dense bodies in mitochondria. This species-specific phenotype in the neurons of man is highly intriguing. In the electron microscope PB are disrupted in LC neurons in depressed individuals, where noradrenaline is known to be reduced. This coincidence of ultrastructure and neurochemistry raises the question whether these bodies could qualify as noradrenaline-storing organelles in the human LC. Our rationale was to examine, in known model tissues that contain catecholamines--sympathetic ganglia and tumors of the autonomic nervous system--if vesicles show the same fine structure and histochemistry as the PB of the human LC. Hence, we selected biopsy tissues of five ganglioneuromas and postmortem tissues of LC from 25 control subjects. Since dopamine-beta-hydroxylase (DBH) is a hallmark of noradrenaline identity and present in dense core vesicles, the investigation of DBH localization with the immunogold method constituted the experiment of choice for this study. Histochemical determinations of arginine with Carmoisine L, and of lipids with Rhodamine B complemented the study of similarities between the PB of the human LC and ganglioneuromas. Our results showed, with the colloidal gold method, that DBH immunogold labeling was localized in the core and in the double membranes of the PB, and also in the adjacent mitochondria. These results indicate that protein bodies (a) are unequivocal storage vesicles of noradrenaline, and (b) derive from regular mitochondria and represent a new phenotype in man, which is probably an evolutionary adaptation of amine-storing organelles.

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.

Distribution of catecholaminergic afferent fibres in the rat globus pallidus and their relations with cholinergic neurons

The topographical distribution of catecholaminergic nerve fibres and their anatomical relationship to cholinergic elements in the rat globus pallidus were studied. Peroxidase-antiperoxidase and two-colour immunoperoxidase staining procedures were used to demonstrate tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) immunoreactivities, combined with acetylcholinesterase (AChE) pharmacohistochemistry. TH immunoreactive nerve fibres were seen to enter the globus pallidus from the medial forebrain bundle. The greatest density of such fibres was found in the ventral region of the globus pallidus, which was also characterized by the greatest density of ChAT immunoreactive neurons. TH immunoreactive nerve fibres showed varicose arborizations and sparse boutons, which were occasionally seen in close opposition to cholinergic structures. In all regions of the globus pallidus, there were also larger, smooth TH immunoreactive nerve fibres of passage to the caudate putamen. A smaller number of DBH immunoreactive nerve fibres and terminal arborizations were found in the substantia innominata, internal capsule and in the globus pallidus bordering these structures. A few PNMT immunoreactive nerve fibres in the substantia innominata and internal capsule did not enter the globus pallidus. Electron microscopy revealed TH immunoreactive synaptic profiles in the ventromedial area of the globus pallidus corresponding to the nucleus basalis magnocellularis of Meynert (nBM). These made mainly symmetrical and only a few asymmetrical synaptic contacts with dendrites containing AChE reaction product. The results indicate that cholinergic structures in the nBM are innervated by dopaminergic fibres and terminals, with only a very small input from noradrenergic fibres.

Immunocytochemical localization of tyrosine hydroxylase in the human striatum: a postmortem ultrastructural study

An electron microscopic evaluation of tyrosine hydroxylase (TH) immunocytochemistry was used to describe the synaptic organization of dopamine innervation of the striatum in postmortem human brain tissue. TH immunoreactivity was qualitatively and quantitatively similar in the caudate and putamen. TH immunoreactivity was present mainly in unmyelinated axons and occasionally in myelinated axons. Both TH-immunoreactive (TH-i) varicosities (0.75-1.5 microm) and intervaricose segments (0.2-0.3 microm) formed synapses with spines and dendrites. Most synapses formed by TH-i profiles were symmetric axospinous (57-62%) or symmetric axodendritic (33-35%). An occasional asymmetric axodendritic or asymmetric axospinous synapse was observed. Approximately 35-50% of all symmetric axospinous and axodendritic synapses were formed by TH-i boutons. Synapses formed by TH-i profiles were short in length (0.226 microm) and had nonperforated postsynaptic densities. TH-i profiles formed synapses with both the head (40%) and the neck (60%) of spines. Typically, the TH-i bouton was apposed to both a spine and a nonlabeled terminal which formed an asymmetric synapse with that spine. Direct, nonsynaptic appositions were often seen between TH-labeled and nonlabeled boutons forming asymmetric synapses. The general pattern ofTH immunoreactivity was similar to that of other species except for the presence of TH-i myelinated axons and the observation that the majority of TH-i synapses were formed with spines rather than with dendritic shafts.

The fine structure of the vertical lobe ofOctopusbrain

Although much is known about the structural organization and connexions of the various lobes of the octopus brain from light microscopy, this is the first attempt at a detailed analysis of one of the lobes— the vertical lobe, with the electron microscope. The vertical lobe consists of five lobules. The median superior frontal (MSF) axons enter each lobule from the MSF lobe. The MSF axons contain both microtubules and neurofilaments. The varicosities of the MSF axons contain both agranular and dense-cored vesicles and synapse with trunks of the amacrine cells. These trunks run together in bundles termed amacrine tracts into the centres of the lobules. The amacrine trunks contain microtubules but no neurofilaments. The trunks contain large and small agranular synaptic vesicles and synapse with what are in all probability branches of the trunks of the large cells. These trunks contain microtubules but no neurofilaments. They run out through the bases of the lobules probably without forming synaptic contacts within the lobule. Fibres signalling ‘pain’ (nocifensor) enter the lobules from below. They can be recognized by their content of neurofilaments. Their terminals contain numerous very small synaptic vesicles and a few larger and dense-cored ones. These ‘pain’ fibres appear to synapse mostly with processes of the large cells. J. Z. Young has shown that the vertical lobe is especially concerned with the integrative action of the visual system, linked with the chemo-tactile system. Electron microscopy supports Young’s suggestion that the superior frontal and interconnected vertical lobe systems constitute a loop which could sustain a positive feed-back mechanism (MSF —> amacrine -> large cell -> lateral superior frontal -> MSF) while the ‘pain’ (nocifensor) input could exert a suppressor (inhibitory) effect on the loop by its action on the large cells.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

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.

5-HT3 receptors in the rat central nervous system are mainly located on nerve fibres and terminals

Autoradiographic and membrane binding studies with [3H](R,S)- or [3H](S)-zacopride were performed in combination with lesions using various neurotoxins in an attempt to identify which neuronal cell types are endowed with 5-HT3 receptors in the rat central nervous system. Lesions of noradrenergic (by DSP-4), dopaminergic (by 6-hydroxydopamine) and serotonergic (by 5,7-dihydroxytryptamine) systems had little effect generally on the density of 5-HT3 receptors labelled with [3H](R,S)- or [3H](S)-zacopride in various regions of the brain and the spinal cord. The only exception was the amygdala where a significant loss (approximately -20%) of 5-HT3 receptors labelled by [3H](R,S)-zacopride was associated with the selective lesion of serotonergic fibres by 5,7-dihydroxytryptamine. Microinjection of kainic or ibotenic acid into the dorsal and ventral hippocampus reduced the density of 5-HT1A receptors labelled with [3H]8-OH-DPAT (approximately -45%) as expected from their known location on intrinsic neuronal cell bodies and/or dendrites. In contrast, the same lesion did not affect 5-HT3 receptors, suggesting their location on fibres 'en passage'. At the spinal level, 5-HT3 receptors were found to exist on primary afferent fibres terminating within the superficial layers of the dorsal horn, as shown by the marked reduction in the local autoradiographic labelling by [3H](S)-zacopride after either dorsal rhizotomy (-81%) or neonatal capsaicin treatment (-72%). These data suggest that 5-HT3 receptors in the central nervous system are generally located presynaptically on nerve terminals or fibres of non-monoaminergic neurones.

Conceptual history of the nigrostriatal dopamine system

The history of the nigrostriatal dopamine system may provide a prime example of the two faces of scientific development. First, a given concept is replaced by another simply as a result of methodologies being improved, and second, successive technical improvements make seemingly settled controversies even more complicated and disputable. The nigrostriatal pathway, which had been unrecognizable with Nauta's silver impregnation method, became apparent by use of the more sensitive silver impregnation method of Fink-Heimer. The sensitivity of the latter method, however, was still insufficient to reveal the whole extent of another ascending dopamine system, the mesocortical dopamine system, until its existence was established through the application of glyoxylic acid fluorescent histochemistry. Electron microscopic analysis of nigrostriatal dopamine synapses in properly fixed tissue was initiated by the demonstration of dark type terminal degeneration, which was induced by either electrolytic lesions or chemical destruction with a specific toxin (6-hydroxydopamine) of the substantia nigra and medial forebrain bundle. The degenerating terminal boutons, thus produced, invariably formed postsynaptic membrane specializations of asymmetric type. However, the asymmetric nature of the synaptic morphology, although later confirmed by the combined study of chemical lesions and autoradiographic anterograde tracing, was seriously challenged with the introduction of electron microscopic immunohistochemistry. The latter method has consistently revealed that symmetric en passant synapses or axonal varicosities with no synaptic membrane specializations are the only tissue compartments immunoreactive to antibodies against dopamine and its synthetic enzyme tyrosine hydroxylase. In view of the fact that more than 95% of the nigrostriatal projection neurons are dopaminergic, it is difficult to satisfactorily interpret all the available and seemingly paradoxical fine structural data. In this context, a novel concept has emerged in the process of eliminating all the possible alternative interpretations. The concept is that single nigrostriatal neurons form two chemically distinct types of synapses, one dopaminergic symmetric en passant bouton and another non-dopaminergic (still chemically unclassified) asymmetric terminal bouton. If the concept is a valid one, it contradicts Dale's long standing principle, as defined by Eccles: at all the axonal branches of a neuron there is liberation of the same transmitter substance or substances. Furthermore, a certain population of substantia nigra pars reticulata neurons has recently been recognized to be immunoreactive to both dopamine synthetic tyrosine hydroxylase and GABA synthetic glutamate decarboxylase. These single neurons send projections to both the striatum and superior colliculus by way of axon collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartmentalization of monoaminergic synaptic vesicles in the storage and release of neurotransmitter

Monoaminergic nerves are characterized by the presence of a population of small synaptic vesicles (40-60 nm in diameter) containing a few large vesicles (80-90 nm in diameter). Thus, although both types of vesicles contain monoamines, the small vesicles must be considered as the organoid responsible for the storage and release of the neurotransmitter, whereas the large ones possibly are involved in the modulation of the process. The small vesicles are electron-lucent or have an osmiophilic electron-dense core that is always linked to the vesicle membrane. Considering morphological and histochemical evidence under different experimental conditions, we proposed the existence of two compartments in the small vesicles: the core and the matrix, corresponding respectively to the electron-dense core and the electron-lucent space between the core and the vesicle membrane in osmium tetroxide fixations. The sizes of both compartments are inversely related, i.e., the smaller the core, the larger the matrix and vice versa. The core even disappears, giving way to a small electron-lucent vesicle made exclusively by the matrix. Thus, the matrix is a constant component of the vesicle, whereas the core is a transient one. Each compartment has a different pool of amine: a loosely bound, easily releasable pool in the matrix and a tightly bound, more resistant pool in the core. These two pools subserve, respectively, a tonic or phasic release of the neurotransmitter, correlated with a tonic or phasic stimulation of the receptor. The core may be considered as a storage or reserve pool. Experimental evidence from our laboratory supports the concept that different mechanisms are operative in both compartments in the release of the neurotransmitter. For instance, a Ca2(+)-independent release would be primarily concerned with the neurotransmitter contained in the matrix, and a Ca2(+)-dependent efflux would be primarily related with the neurotransmitter stored in the core. However, it still must be established that a simple relationship exists between each kind of stimulus and each vesicle compartment, rather than both compartments being integrated in a dynamic functional unit.

Subcellular distribution of serotonin in the lamprey spinal cord

The subcellular distribution of serotonin (5-hydroxytryptamine; 5-HT) in the lamprey (Ichtyomyzon unicuspis, Lampetra fluviatilis) spinal cord was investigated by using ultracentrifugation on continuous density gradients combined with an electron microscopic analysis of the gradients and of immunostained tissue. Endogenous 5-HT was analyzed by high-performance liquid chromatography with electrochemical detection. After differential centrifugation, the highest levels of 5-HT were found in the particulate fractions. After ultracentrifugation of lysed synaptosomal fractions on continuous sucrose gradients and the subsequent sedimentation of the individual fractions, 5-HT showed a biphasic distribution in the gradient. The two peaks corresponded to 0.30-0.40 M and 0.85-1.05 M sucrose. Electron microscopy of intact tissue showed that some of the boutons were strongly immunoreactive to 5-HT with dense precipitates over large granular vesicles. The area around these large vesicles, however, also showed reaction product. Large granular vesicles could be clearly distinguished in the immunostained axonal varicosities. In tissue not processed for 5-HT immunoreactivity it was seen that the varicosities contained not only large dense-cored vesicles, but also small agranular vesicles. An electron microscopical analysis of the subcellular fractions revealed that the fraction corresponding to the "light" 5-HT peak contained numerous vesicular structures, which in most cases were electron lucent. In the "heavy" fractions, nerve ending particles containing vesicles of various sizes were observed. The results suggest that 5-HT in the lamprey spinal cord may be distributed in more than one subcellular compartment which, apart from the cytosol, possibly corresponds to small and large synaptic vesicles.

Fine structure of the pinealopetal innervation of the mammalian pineal gland

The mammalian pineal gland is innervated by peripheral sympathetic and parasympathetic nerve fibers as well as by nerve fibers originating in the central nervous system (central innervation). The perikarya of the sympathetic fibers are located in the superior cervical ganglia, while the fibers terminate in boutons containing small granular vesicles and a few large granular vesicles. Both noradrenaline and neuropeptide Y are contained in these neurons. The parasympathetic fibers originate from perikarya in the pterygopalatine ganglia. The neuropeptides, vasoactive intestinal peptide and peptide histidine isoleucine, are present in these fibers, the boutons of which contain small clear transmitter vesicles and larger granular vesicles. The fibers of the central innervation originate predominantly from perikarya located in hypothalamic and limbic forebrain structures as well as from perikarya in the optic system. These fibers terminate in boutons containing small clear and, in certain fibers, an abundant number of large granular vesicles. In rodents, the majority of the central fibers terminate in the deep pineal gland and the pineal stalk. From these areas impulses might be transmitted further caudally to the superficial pineal gland via neuronal structures or processes from pinealocytes. Several hypothalamic neuropeptides and monoamines might be contained in the central fibers. The intrapineal nerve fibers are located both in the perivascular spaces and intraparenchymally. The majority of the intraparenchymally located fibers terminate freely between the pinealocytes. However, some nerve terminals make synaptic contacts with the pinealocytes and in some species with intrapineal neurons. In fetal mammals, sympathetic, parasympathetic, and central fibers are also present. In addition, an unpaired nerve, connecting the caudal part of the pineal gland with the extreme rostral part of the mesencephalon, is present. This nerve is a homologue to the pineal nerve (nervus pinealis) observed in lower vertebrates.

Characterization of the distribution of G alpha o in rat striatal synaptosomes and its colocalization with tyrosine hydroxylase

Dopaminergic striatal synaptosomes can be detected and isolated with a fluorescence-activated cell sorter (FACS). In the present study, two antigens were detected simultaneously with primary antisera raised in different species and species-specific fluorescent secondary antibodies with different emission spectra. Double-label FACS analysis was used to determine whether tyrosine hydroxylase (TH) and the alpha subunit of Go (G alpha o) are colocalized in striatal synaptosomes. Rabbit antibodies generated against a synthetic fragment of G alpha o (corresponding to amino acids 22-35) combined with fluorescein-conjugated secondary antibodies were used to detect G alpha o-containing striatal synaptosomes. Preadsorption of G alpha o antiserum with the synthetic peptide antigen reduced labeling to the level obtained with preimmune serum. Approximately 65-75% of striatal synaptosomes were specifically labeled by G alpha o antiserum. Tyrosine hydroxylase-containing synaptosomes were detected with a mouse monoclonal antibody to TH and R-phycoerythrin-conjugated secondary antibody. They comprised 15-17% of total striatal synaptosomes. Double-label studies indicated that at least 50% of TH-containing synaptosomes also contained G alpha o. These findings suggest that G alpha o may not be a protein component of all striatal nerve terminals, and provide a basis for a role for G alpha o in signal transduction within subpopulations of intrinsic and afferent nerve terminals, including those of nigrostriatal dopamine neurons.

Monoamine synaptic structure and localization in the central nervous system

The monoamines dopamine, noradrenaline, adrenaline, and serotonin as well as the diamine histamine have a widespread distribution in the central nervous system within synaptic terminals and nonsynaptic varicosities. In certain regions of the central nervous system the monoamines are contained in varicosities that have no synaptic specialization associated with them, suggesting a possible neuromodulatory role for some of the monoamines. The majority of monoamine labelled structures are synaptic terminals which are characterized by the presence of small, clear vesicles (40-60 nm) and large, granular vesicles (70-120 nm) within the terminal. A third population of vesicles--small, granular vesicles--which are visible only after histochemical staining, are probably the equivalent of the small, clear vesicles present after either autoradiographic or immunohistochemical labelling. Most monoamine containing terminals contact dendrites and dendritic spines and, less frequently, neuronal somata and other axons. Both asymmetrical and symmetrical membrane specializations are associated with monoaminergic terminals; however, asymmetrical contacts are the most frequent type found. These ultrastructural results indicate that monoamine containing terminals and varicosities in general share many common morphological features, but still have diverse functions.

5-hydroxytryptamine immunoreactive varicosities in the lamprey spinal cord have no synaptic specializations--an ultrastructural study

The distribution and fine structure of 5-hydroxytryptamine (5-HT) immunoreactive cell bodies and axonal varicosities have been studied in the lamprey spinal cord, using the peroxidase-antiperoxidase (PAP) immunohistochemical technique and subsequent analysis of ultrathin serial sections. Immunostained cell bodies were found in the ventral spinal cord close to the central canal. Immunostained varicosities were found throughout the spinal cord with the highest density in the ventromedial plexus and the dorsal horn. Only large granular vesicles could be clearly distinguished in immunostained cell bodies and varicosities, but it was concluded based on a comparison with unstained normal tissue that these boutons also contained small, pleomorphic agranular vesicles. Immunoreactive varicosities were studied in the ventromedial plexus, the dorsal horn, the dorsal column, the dorsolateral and ventrolateral funiculi and the grey matter. No morphological differences could be observed between varicosities in the different loci. The varicosities were in no case seen to make synaptic contact with surrounding neuronal elements, even when followed through serial sections. Consequently, 5-HT released from boutons in all parts of the spinal cord could be expected to act on 5-HT receptors located on nearby as well as distant receptors.

Tyrosine hydroxylase content of residual striatal dopamine nerve terminals following 6-hydroxydopamine administration: a flow cytometric study

Fluorescence-activated cell sorting based on immunolabeling with a monoclonal antibody to tyrosine hydroxylase and a fluorescein-conjugated secondary antibody was used to identify striatal synaptosomes derived from nigrostriatal dopamine nerve terminals. The amount of tyrosine hydroxylase immunoreactivity in dopaminergic striatal synaptosomes prepared from control rats was compared to the amount in dopaminergic synaptosomes prepared from rats that had received intraventricular injections of 6-hydroxydopamine. Although the absolute number of dopaminergic synaptosomes was decreased in lesioned animals, those residual dopamine terminals present contained more tyrosine hydroxylase than did dopamine terminals from control rats. Both the decrease in the absolute number of dopamine terminals and the increase in tyrosine hydroxylase immunoreactivity in residual terminals were proportional to the extent of the lesion, as determined by measurement of striatal dopamine levels. These results suggest that an increase in the amount of tyrosine hydroxylase protein in residual terminals may represent one compensatory mechanism by which residual dopamine neurons maintain normal striatal function after partial destruction of the nigrostriatal dopamine projection.

Tyrosine hydroxylase phosphorylation in rat brain striatal synaptosomes

The present studies were carried out to determine if tyrosine hydroxylase phosphorylation in rat brain striatal synaptosomes is activated by dibutyryl cyclic AMP treatment. Incubation of synaptosomes with [32P]orthophosphate, followed by immunoprecipitation and sodium dodecyl sulfate polyacrylamide gel electrophoresis, produced a band of radioactivity associated with a 62 kDa polypeptide. Treatment with the catecholamine neurotoxin, 6-hydroxydopamine, produced parallel losses of: (1) tyrosine hydroxylase enzyme activity, (2) dopamine content, and (3) the 62 kDa band of radioactivity. These data support the identification of this band as a tyrosine hydroxylase-derived polypeptide. Incubation with dibutyryl cyclic AMP produced an increase in soluble tyrosine hydroxylase activity and phosphorylation. These results suggest that the increase in synaptosomal catecholamine synthesis produced by dibutyryl cyclic AMP is mediated by an increase in tyrosine hydroxylase phosphorylation.

Developmental interactions between sweat glands and the sympathetic neurons which innervate them: effects of delayed innervation on neurotransmitter plasticity and gland maturation

The neurotransmitter properties of the sympathetic innervation of sweat glands in rat footpads have previously been shown to undergo a striking change during development. When axons first reach the developing glands, they contain catecholamine histofluorescence and immunoreactivity for catecholamine synthetic enzymes. As the glands and their innervation mature, catecholamines disappear and cholinergic and peptidergic properties appear. Final maturation of the sweat glands, assayed by secretory competence, is correlated temporally with the development of cholinergic function in the innervation. To determine if the neurotransmitter phenotype of sympathetic neurons developing in vivo is plastic, if sympathetic targets can play a role in determining neurotransmitter properties of the neurons which innervate them, and if gland maturation is dependent upon its innervation, the normal developmental interaction between sweat glands and their innervation was disrupted. This was accomplished by a single injection of 6-hydroxy-dopamine (6-OHDA) on Postnatal Day 2. Following this treatment, the arrival of noradrenergic sympathetic axons at the developing glands was delayed 7 to 10 days. Like the gland innervation of normal rats, the axons which innervated the sweat glands of 6-OHDA-treated animals acquired cholinergic function and their expression of endogenous catecholamines declined. The change in neurotransmitter properties, however, occurred later in development than in untreated animals and was not always complete. Even in adult animals, some fibers continued to express endogenous catecholamines and many nerve terminals contained a small proportion of small granular vesicles after permanganate fixation. The gland innervation in the 6-OHDA-treated animals also differed from that of normal rats in that immunoreactivity for VIP was not expressed in the majority of glands. It seems likely that following treatment with 6-OHDA sweat glands were innervated both by neurons that would normally have done so and by neurons that would normally have innervated other, noradrenergic targets in the footpads, such as blood vessels. Contact with sweat glands, therefore, appears to suppress noradrenergic function and induce cholinergic function not only in the neurons which normally innervate the glands but also in neurons which ordinarily innervate other targets. Effects of delayed innervation were also observed on target development. The appearance of sensitivity to cholinergic agonists by the sweat glands was coupled with the onset of cholinergic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

Early effects of experimental diabetes on central catecholamine concentrations

Six days after induction of diabetes norepinephrine (NE), 3,4-dihydroxyphenylalanine and dopamine concentrations were determined in spinal cord, cerebellum, pons/medulla oblongata and the remaining brain of diabetic rats. Only cerebellar NE levels were significantly increased by 21% compared to age-matched controls. This effect was not seen when diabetic rats received insulin replacement therapy. Fifty-two days after induction of diabetes cerebellar NE levels in diabetic rats were further increased (+37%), implying an enduring and progressive effect. The data show that even with a short period of uncontrolled diabetes central neurochemical alterations occur.

Radioautographic method for quantifying regional monoamine innervations in the rat brain. Application to the cerebral cortex

Conditions leading to selective and complete labeling of the noradrenaline (NA) and serotonin (5-HT) innervations in rat cerebral cortex were sought by incubating 200-micron-thick whole hemisphere slices with various combinations of tritiated monoamines and uptake blockers at different concentrations in the presence of a monoamine oxidase inhibitor. After fixation with glutaraldehyde, post-fixation with osmium tetroxide and flat-embedding in Epon, 4-micron-thick sections of the entire slices were radioautographed by dipping in nuclear emulsion. As previously reported, dopamine (DA) terminals could be specifically visualized and counted following incubation with 1 micron [3H]DA and 5 microM desipramine (DMI) with or without 5 microM citalopram (CITAL). The number of NA terminals could thus be obtained by subtracting DA varicosities from the total number of sites labeled in adjacent slices incubated without DMI but in presence of CITAL to eliminate some interspecific labeling of 5-HT terminals. NA terminals could also be identified exclusively and counted after labeling with 1 microM [3H]NA in the presence of 10 microM benztropine. 5-HT terminals were specifically detected after incubation with 1 microM [3H]5-HT in the presence of 10 microM non-radioactive NA. The labeled varicosities were counted in areas FR1 and PAR1 of the frontal and the parietal neocortex, respectively, with the aid of a microcomputer-based image analysis system. DA varicosities were concentrated mainly in layer VI of these regions and were more numerous in the frontal than the parietal area. NA terminals were equally distributed in the two regions but approximately twice as numerous in layer I than subjacent layers. The 5-HT innervation also showed a comparable overall density in the two cortical regions but with a differing intracortical distribution. In the frontal area, 5-HT terminals were slightly more concentrated in layer I (1.3-fold) than underlying layers where they were rather uniformly distributed. In the parietal area, layer I was again the most densely innervated (1.8 times the average), but a second zone of higher density (1.5 times average) was present in the outer part of layer V. The remaining layers showed lower numbers of 5-HT terminals than in the frontal region. To obtain absolute estimates of these innervation densities, the number of detected varicosities was assessed experimentally as a function of radioautographic exposure time and of histological section thickness, and their 'equivalent circle diameter' was measured in electron microscope radioautographs.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of collidine (2,4,6-trimethylpyridine) on rat pineal gland and vas deferens nerves. Further evidence for a monoamine-releasing effect

In previous work of our laboratory it was demonstrated that collidine (2,4,6-trimethylpyridine) abolishes the core osmiophilia and chromaffin reaction from rat pinal gland and vas deferens nerves. This abolition was apparent when tissues were briefly incubated in collidine or when they wer fixed in glutaraldehyde or osmium tetroxide using collidine as a buffer substance. These and other results strongly suggested that the histochemical effect of collidine was due to depletion of monoamines stored in the vesicles core. To examine this hypothesis we studied in this work the effect of collidine on tissues that have taken up tritiated noradrenaline. It was found that tritium was released very rapidly to the incubation medium when collidine was applied to fresh tissues. This effect was not observed with other commonly used buffers such as cacodylate or phosphate. It was also found that tritium release also occurred, although to a lesser extent, when tissues were fixed in glutaraldehyde or osmium tetroxide using collidine as a buffer, and this release was not significant when collidine was applied to previously fixed tissues. Paper chromatographic analysis showed that the radioactive compound(s) extracted from tissues by collidine corresponded to noradrenaline and/or closely related compounds. An abstract of this work was sent to the 17th Annual Meeting of the Society for Neuroscience, New Orleans, Nov 16-21, 1987. Tomsig J.L. and Pellegrino de Iraldi A. Abstract 369-11.

The connections between the suprachiasmatic, ventrolateral geniculate and raphe nuclei studied by uptake of [14C]2-deoxyglucose

The [14C]2-deoxyglucose method was used to investigate the role of the ventrolateral geniculate and raphe nuclei in the control of the metabolism of the suprachiasmatic nuclei in adult female Wistar rats anaesthetized with alphaxalone. Three to seven days before the [14C]2-deoxyglucose studies a stimulating electrode was implanted or a lesion was made in the ventrolateral geniculate nucleus, or the ascending projection from the raphe nuclei was severed. Stimulation of the ventrolateral geniculate nucleus (biphasic rectangular pulses, 30 s on and 30 s off, 50 Hz, 500 microA pulse amplitude and 1 ms pulse duration) led to a significant increase in the relative metabolic activity of the ipsilateral suprachiasmatic nucleus and a smaller increase in the relative metabolic activity of the contralateral suprachiasmatic nucleus. The stimulus also increased significantly the relative metabolic activities of mainly the ipsilateral hypothalamus, midbrain central gray and reticular formation, all of which are too remote from the ventrolateral geniculate nucleus to be affected by current spread. In animals in which the ventrolateral geniculate nucleus had been lesioned, the relative metabolic activity of the suprachiasmatic nuclei was not significantly different from normal. In animals in which the ascending projection from the raphe nuclei had been severed, there was a slight, though significant increase in the relative metabolic activity of the suprachiasmatic nucleus of one side. These results, together with the effects of stimulating the suprachiasmatic nuclei [R. C. Maxwell and G. Fink, Neuroscience 23, 241-263 (1987)], show that the connections between the ventrolateral geniculate, raphe nuclei and suprachiasmatic nuclei are "metabolically functional", but that the integrity of the ventrolateral geniculate nucleus is not essential for maintaining the relative metabolic activity of the suprachiasmatic nuclei. The raphe nuclei may reduce the relative metabolic activity of the suprachiasmatic nucleus.

An ultrastructural study of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal boutons in the motor nucleus of spinal cord segments L7-S1 in the adult cat

The distribution and fine structure of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive synaptic boutons and varicosities were studied in the motor nucleus of the spinal cord segments L7-S1 in the cat, using the peroxidase-antiperoxidase immunohistochemical technique and analysis of ultrathin serial sections. The 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive boutons had a similar ultrastructural appearance as judged from serial section analysis. The boutons could be classified into two types on the basis of their vesicular content, with one type containing a large number of small agranular vesicles together with only a few, if any large granular vesicles, while the other type contained a large number of large granular vesicles in addition to small agranular vesicles. The vesicles were spherical or spherical-to-pleomorphic. Postsynaptic dense bodies (Taxi bodies) were occasionally observed in relation to all three types of immunoreactive boutons, which almost invariably formed synaptic junctions with dendrites. Judged by the calibre of the postsynaptic dendrites, the boutons were preferentially distributed to the proximal dendritic domains of motoneurons. In one case, a substance P-immunoreactive bouton formed an axosomatic synaptic contact. In addition to synaptic boutons, 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal varicosities containing a large number of large granular and small agranular vesicles but lacking any form of conventional synaptic contact were observed. Such varicosities were either directly apposing surrounding neuronal elements or separated from the neurons by thin glial processes. The origin of the immunoreactive boutons was not traced, but it was thought likely that the main source of the boutons was neurons with their cell bodies located in the medullary raphe nuclei.

Light and electron microscopic immunocytochemical analysis of the serotonin innervation of the rat visual cortex

The serotonin afferents of the rat visual cortex were examined immunocytochemically at the light and electron microscopic levels. Immunoreactive fibres were typically thin, tortuous and varicose. Occasionally, some thicker fibres were found. The orientation of labelled axons varied according to laminar position, with fibres running parallel to the pial surface present mainly in layers I and VI, and radially oriented fibres prominent in layers II and III. Branches arising from horizontal or radially oriented fibres were seen to form irregularly shaped loops particularly in layers IV and V. The density of innervation and the prevailing axonal orientation in each cortical layer were similar in both coronal and parasagittal planes. The ultrastructural features of serotonin-labelled axon terminals were examined in single and serial ultrathin sections. While in single sections the majority did not exhibit synaptic specializations, extensive serial section analysis showed that virtually all of these terminals were engaged in junctional complexes. Postsynaptic elements were spines and dendritic shafts, including pyramidal cell apical dendrites, with both symmetrical and asymmetrical membrane specializations. In axospinous synapses, the labelled terminals were usually adjacent to unstained axon terminals contacting the same postsynaptic element.

Noradrenergic innervation of serotoninergic neurons in the myenteric plexus

The monoaminergic innervation of the guinea pig small intestine was investigated to determine if there is an anatomical basis for the hypothesis that serotoninergic and noradrenergic neurons physiologically interact in the enteric nervous system. Initial rates of uptake of tritiated 5-hydroxytryptamine (3H-5-HT) or norepinephrine (3H-NE) by segments of guinea pig small intestine were measured in order to estimate the regional density of the serotoninergic and noradrenergic innervation. No change was found in the uptake of 3H-5-HT as a function of distance between duodenum and ileum, whereas the relative uptake of 3H-NE declined. The pattern of serotoninergic elements demonstrated radioautographically was compared with that obtained by visualizing 5-HT immunoreactivity. Both methods revealed that a small number of serotoninergic neurons, located in 35.3% +/- 1.5% of myenteric ganglia, give rise to many fibers that form thick bundles in interganglionic connectives. Moreover, there was a pronounced heterogeneity in the serotoninergic innervation of individual myenteric neurons and ganglia. In material fixed with aldehydes and postfixed with NaMnO4, noradrenergic axon terminals were identified by their characteristic small dense-cored vesicles. Following incubation with 3H-NE only terminals with small dense-cored vesicles were radioautographically labeled, confirming that these terminals are noradrenergic. When 3H-5-HT was substituted for 3H-NE, noradrenergic terminals were not labeled, showing that nonspecific uptake of 3H-5-HT into noradrenergic axons did not occur in the presence of 5-hydroxydopamine. The combination of aldehyde-NaMnO4 fixation with the radioautographic localization of 3H-5-HT thus permitted the simultaneous identification of serotoninergic and noradrenergic neural elements. Serotoninergic varicosities were found to differ from noradrenergic varicosities in the size, appearance, and packing density of their synaptic vesicles. In addition, recognizable but rudimentary pre- and postsynaptic membrane specializations were associated with serotoninergic but not noradrenergic varicosities. Most serotoninergic neuronal cell bodies were contacted both by serotoninergic synapses and noradrenergic varicosities. Similar appositions of noradrenergic varicosities with nonserotoninergic neurons appeared to be rare. In view of earlier observations that sympathetic nerves affect the release of 5-HT from stimulated enteric serotoninergic neurons, it seems likely that the noradrenergic appositions with serotoninergic neurons are the anatomical substrate for this effect.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effect of collidine (2,4,6-trimethylpyridine) on the osmiophilia and chromaffin reaction in the synaptic vesicles of rat pineal nerves

Rat pineal nerve endings contain a population of small and of large synaptic vesicles that are either electron lucent or have electron-dense cores. It has been reported that their osmiophilia is eliminated when collidine buffer is used in the fixation procedure. We investigated this effect and found that osmium tetroxide and potassium dichromate reactivity were abolished when excised pineal glands were briefly incubated with collidine buffer before glutaraldehyde-cacodylate fixation. Such an effect was not observed when collidine was applied after fixation. Glands that had been fixed in glutaraldehyde or osmium tetroxide buffered with collidine exhibited a peripheral zone containing reactive synaptic vesicles and a deeper, central zone where such reactivity was absent. These results indicate that the effect of collidine is due to depletion of monoamines rather than to chemical blockage of their reactivity, and further suggest that collidine has a higher rate of penetration into tissues than the tested fixatives.

Quantification of the dopamine innervation in adult rat neostriatum

Conditions leading to specific and integral visualization of dopamine axon terminals (varicosities) were tested in adult rat cerebral hemisphere slices incubated with [3H]dopamine and processed for high resolution radioautography. Specific visualization of the dopamine endings was achieved after incubation with 10(-6) M [3H]dopamine in the presence of a monoamine oxidase inhibitor (pargyline 10(-4) M), and of desipramine (5 X 10(-6) M), an inhibitor of catecholamine uptake by noradrenaline and serotonin neurons. [3H]Dopamine varicosity labeling was eliminated by the addition of 5 X 10(-5) M benztropine (an inhibitor of catecholamine uptake by catecholamine neurons), and was almost absent when dopamine nerve cell bodies of the midbrain had been previously destroyed with 6-hydroxydopamine. In dopamine-denervated neostriatum incubated without desipramine, a second set of labeled terminals was also visible. These were identified as serotoninergic, since their labeling was suppressed by citalopram, an inhibitor of monoamine uptake highly specific for serotonin neurons. There was no desipramine-sensitive but citalopram-resistant varicosity labeling suggestive of neostriatal noradrenaline innervation. In normal striatum, incubation at 35 degrees C always resulted in a labeling of dopamine varicosities restricted to a narrow band which followed the contours and cut surface of this anatomical region. This unusual distribution was the result of an uptake barrier generated by the tightly packed dopamine varicosities. Indeed, the striatal dopamine varicosity labeling was more widespread after partial 6-hydroxydopamine denervation or in normal tissue incubated either with a higher [3H]dopamine concentration (5 X 10(-6) M), in the presence of relatively low benztropine concentrations (10(-5) M), or at lower temperature (15 degrees C). Material incubated at 15 degrees C for 90 min was suitable for purposes of quantification: labeled varicosities were then visualized throughout the striatum and across the full thickness of the slices; moreover, the number of labeled varicosities plotted against radioautographic exposure time increased in parallel and reached a plateau at the same time in neostriatal sectors with widely different innervation densities. At a rostral transverse level across neostriatum, the dorsolateral quadrant showed hyperdense "patches" of labeled terminals distinguishable from an already dense surrounding "matrix", whereas, ventromedially, the dopamine innervation appeared more uniform and somewhat less dense.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAergic and catecholaminergic synaptic interactions in the macaque hypothalamus: double label immunostaining with peroxidase-antiperoxidase and colloidal gold

Immunogold staining (IGS) for glutamic acid decarboxylase (GAD) was combined with the peroxidase-antiperoxidase (PAP) technique for tyrosine hydroxylase (TH) to analyze gamma-aminobutyric acid-catecholaminergic neuronal interactions in the rhesus hypothalamus. At the light-microscopic level, TH-immunoreactive (-IR) perikarya and their fibers (brown) were observed in the anterior ventral periventricular area (AVPV), the arcuate nucleus (ARC) and the adjacent periventricular zone (ARC-PVZ). GAD-IR processes (light red) were also present throughout the hypothalamus and appeared to contact some TH-IR neurons. At the electron-microscopic level, PAP was present in perikarya, dendrites, axons and axon terminals of TH-IR neurons. Colloidal gold particles (15 nm) were found only in dendrites and axon terminals of GAD-IR neurons. Labeled GAD terminals typically contained small, clear synaptic vesicles, while TH terminals contained these and sometimes one or two dense-core vesicles. In the ARC and ARC-PVZ, asymmetrical (Gray I) axodendritic synapses occurred between GAD and TH-IR profiles, with TH/GAD directionality more prevalent. Symmetrical (Gray II) synapses were less common, with either TH or GAD presynaptic in axodendritic and dendrodendritic contacts. GAD/GAD interactions were not observed, but TH/TH contacts appeared to be mostly dendrodendritic. In the AVPV, only symmetrical synapses were encountered, and their directionality was difficult to determine. GAD- and TH-IR dendrites frequently established dendrodendritic synapses, but GAD/TH dendrosomatic synapses were seldom seen. These results illustrate the complex interactions of GAD- and TH-containing elements in the neuroendocrine hypothalamus.