Retrograde axonal transport following injection of [3H]-serotonin into the olfactory bulb. II. Radioautographic study

Unraveling the Role of Dopaminergic and Calretinin Interneurons in the Olfactory Bulb

The perception and discriminating of odors are sensory activities that are an integral part of our daily life. The first brain region where odors are processed is the olfactory bulb (OB). Among the different cell populations that make up this brain area, interneurons play an essential role in this sensory activity. Moreover, probably because of their activity, they represent an exception compared to other parts of the brain, since OB interneurons are continuously generated in the postnatal and adult period. In this review, we will focus on periglomerular (PG) cells which are a class of interneurons found in the glomerular layer of the OB. These interneurons can be classified into distinct subtypes based on their neurochemical nature, based on the neurotransmitter and calcium-binding proteins expressed by these cells. Dopaminergic (DA) periglomerular cells and calretinin (CR) cells are among the newly generated interneurons and play an important role in the physiology of OB. In the OB, DA cells are involved in the processing of odors and the adaptation of the bulbar network to external conditions. The main role of DA cells in OB appears to be the inhibition of glutamate release from olfactory sensory fibers. Calretinin cells are probably the best morphologically characterized interneurons among PG cells in OB, but little is known about their function except for their inhibitory effect on noisy random excitatory signals arriving at the main neurons. In this review, we will mainly describe the electrophysiological properties related to the excitability profiles of DA and CR cells, with a particular view on the differences that characterize DA mature interneurons from cells in different stages of adult neurogenesis.

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Differential serotonergic modulation across the main and accessory olfactory bulbs

KEY POINTS

There are serotonergic projections to both the main (MOB) and the accessory olfactory bulb (AOB). Current-clamp experiments demonstrate that serotonergic afferents are largely excitatory for mitral cells (MCs) in the MOB where 5-HT_2A receptors mediate a direct excitatory action. Serotonergic afferents are predominately inhibitory for MCs in the AOB. There are two types of inhibition: indirect inhibition mediated through the 5-HT₂ receptors on GABAergic interneurons and direct inhibition via the 5-HT₁ receptors on MCs. Differential 5-HT neuromodulation of MCs across the MOB and AOB could contribute to select behaviours such as olfactory learning or aggression.

ABSTRACT

Mitral cells (MCs) contained in the main (MOB) and accessory (AOB) olfactory bulb have distinct intrinsic membrane properties but the extent of neuromodulation across the two systems has not been widely explored. Herein, we investigated a widely distributed CNS modulator, serotonin (5-HT), for its ability to modulate the biophysical properties of MCs across the MOB and AOB, using an in vitro, brain slice approach in postnatal 15-30 day mice. In the MOB, 5-HT elicited three types of responses in 93% of 180 cells tested. Cells were either directly excited (70%), inhibited (10%) or showed a mixed response (13%)- first inhibition followed by excitation. In the AOB, 82% of 148 cells were inhibited with 18% of cells showing no response. Albeit located in parallel partitions of the olfactory system, 5-HT largely elicited MC excitation in the MOB while it evoked two different kinetic rates of MC inhibition in the AOB. Using a combination of pharmacological agents, we found that the MC excitatory responses in the MOB were mediated by 5-HT_2A receptors through a direct activation. In comparison, 5-HT-evoked inhibitory responses in the AOB arose due to a polysynaptic, slow-onset inhibition attributed to 5-HT₂ receptor activation exciting GABAergic interneurons. The second type of inhibition had a rapid onset as a result of direct inhibition mediated by the 5-HT₁ class of receptors. The distinct serotonergic modulation of MCs between the MOB and AOB could provide a molecular basis for differential chemosensory behaviours driven by the brainstem raphe nuclei into these parallel systems.

Dopaminergic Neurones in the Main Olfactory Bulb: An Overview from an Electrophysiological Perspective

The olfactory bulb (OB), the first center processing olfactory information, is characterized by a vigorous life-long activity-dependent plasticity responsible for a variety of odor-evoked behavioral responses. It hosts the more numerous group of dopaminergic (DA) neurones in the central nervous system, cells strategically positioned at the entry of the bulbar circuitry, directly in contact with the olfactory nerve terminals, which play a key role in odor processing and in the adaptation of the bulbar network to external conditions. Here, we focus mainly on the electrophysiological properties of DA interneurones, reviewing findings concerning their excitability profiles in adulthood and in different phases of adult neurogenesis. We also discuss dynamic changes of the DA interneurones related to environmental stimuli and their possible functional implications.

[Endocrine neurons and neurotransmission: 40 years of cytochemical studies]

Conceptual advances about chemical neurotransmission during the last 40 years have benefited a lot from histocytochemical approaches and from a particular experimental model, the endocrine hypothalamic neurons. New concepts like cotransmission, neuronal versatility, somatodendritic release of neurotransmitters, volume transmission, differential routing or cooperative synthesis of mediators, have often been issued from this set of methodologies and from their application to neurosecretory neurons. This review, mainly based on the results of our group, is aiming at illustrating how the peculiar metabolism of these neurons and their location at the neuro-endocrine interface have allowed discovering new aspects of neurotransmission, first considered as exceptions but then generalized to the whole nervous system. These new concepts shed lights on the normal functioning of the brain and definitely contribute to diversify pharmacological approaches of pathological neurotransmission.

[André Calas, the original path of a neuroendocrinologist]

This talk, given as an introduction to a symposium organised to honor André Calas, calls forth his personality, recalls the major events in his career and summarizes the evolution of his research.

Selective anterograde tracing of nonserotonergic projections from dorsal raphe nucleus to the basal forebrain and extended amygdala

The dorsal raphe nucleus (DRN) contains both serotonergic and nonserotonergic projection neurons. Retrograde tracing studies have demonstrated that components of the basal forebrain and extended amygdala are targeted heavily by input from nonserotonergic DRN neurons. The object of this investigation was to examine the terminal distribution of nonserotonergic DRN projections in the basal forebrain and extended amygdala, using a technique that allows selective anterograde tracing of nonserotonergic DRN projections. To trace nonserotonergic DRN projections, animals were pretreated with nomifensine, desipramine and the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), 7 days prior to placing an iontophoretic injection of biotinylated dextran amine (BDA) into the DRN. In animals treated with 5,7-DHT, numerous nonserotonergic BDA-labeled fibers ascended to the basal forebrain in the medial forebrain bundle system. Some of these labeled fibers crossed through the lateral hypothalamus, bed nucleus of the stria terminalis, and substantial innominata. These fibers entered the amygdala through the ansa peduncularis and ramified within the central and basolateral amygdaloid nuclei. Other fibers entered the diagonal band of Broca and formed a dense plexus of labeled fibers in the dorsal half of the intermediate portion of the lateral septal nucleus and the septohippocampal nucleus. These findings demonstrate that the basal forebrain and extended amygdala receive a dense projection from nonserotonergic DRN neurons. Given that the basal forebrain plays a critical role in processes such as motivation, affect, and behavioral control, these findings support the hypothesis that nonserotonergic DRN projections may exert substantial modulatory control over emotional and motivational functions.

The dorsal raphe nucleus—From silver stainings to a role in depression

Over a hundred years ago, Santiago Ramón y Cajal used a new staining method developed by Camillo Golgi to visualize, among many other structures, what we today call the dorsal raphe nucleus (DRN) of the midbrain. Over the years, the DRN has emerged as a multifunctional and multitransmitter nucleus, which modulates or influences many CNS processes. It is a phylogenetically old brain area, whose projections reach out to a large number of regions and nuclei of the CNS, particularly in the forebrain. Several DRN-related discoveries are tightly connected with important events in the history of neuroscience, for example the invention of new histological methods, the discovery of new neurotransmitter systems and the link between neurotransmitter function and mood disorders. One of the main reasons for the wide current interest in the DRN is the nucleus' involvement in depression. This involvement is particularly attributable to the main transmitter of the DRN, serotonin. Starting with a historical perspective, this essay describes the morphology, ascending projections and multitransmitter nature of the DRN, and stresses its role as a key target for depression research.

Selective anterograde tracing of the individual serotonergic and nonserotonergic components of the dorsal raphe nucleus projection to the vestibular nuclei

It is well known that the dorsal raphe nucleus (DRN) sends serotonergic and nonserotonergic projections to target regions in the brain stem and forebrain, including the vestibular nuclei. Although retrograde tracing studies have reported consistently that there are differences in the relative innervation of different target regions by serotonergic and nonserotonergic DRN neurons, the relative termination patterns of these two projections have not been compared using anterograde tracing methods. The object of the present investigation was to trace anterogradely the individual serotonergic and nonserotonergic components of the projection from DRN to the vestibular nuclei in rats. To trace nonserotonergic DRN projections, animals were pretreated with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and then, after 7 days, the anterograde tracer biotinylated dextran amine (BDA) was iontophoretically injected into the DRN. In animals treated with 5,7-DHT, nonserotonergic BDA-labeled fibers were found to descend exclusively within the ventricular plexus and to terminate predominantly within the periventricular aspect of the vestibular nuclei. Serotonergic DRN projections were traced by injecting 5,7-DHT directly into DRN, and amino-cupric-silver staining was used to visualize the resulting pattern of terminal degeneration. Eighteen hours after microinjection of 5,7-DHT into the DRN, fine-caliber degenerating serotonergic terminals were found within the region of the medial vestibular nucleus (MVN) that borders the fourth ventricle, and a mixture of fine- and heavier-caliber degenerating serotonergic terminals was located further laterally within the vestibular nuclear complex. These findings indicate that fine-caliber projections from serotonergic and nonserotonergic DRN neurons primarily innervate the periventricular regions of MVN, whereas heavier-caliber projections from serotonergic DRN neurons innervate terminal fields located in more lateral regions of the vestibular nuclei. Thus, serotonergic and nonserotonergic DRN axons target distinct but partially overlapping terminal fields within the vestibular nuclear complex, raising the possibility that these two DRN projection systems are organized in a manner that permits regionally-specialized regulation of processing within the vestibular nuclei.

5-hydroxytryptamine action in the rat olfactory bulb: In vitro electrophysiological patch-clamp recordings of juxtaglomerular and mitral cells

The olfactory bulb, first relay of olfactory pathways, is densely innervated by serotoninergic centrifugal fibers originating from the raphe nuclei. Although serotonin innervation was reported to be involved in olfactory learning in mammals, the action of this neurotransmitter on its putative cellular targets has been never described through unitary recordings. This lack of data initiated the present study where the effects of 5HT on juxtaglomerular and mitral cells are analyzed using whole-cell recordings on olfactory bulb slices. Serotonin depolarizes 34% of 525 JG cells. A multivariate statistical analysis of juxtaglomerular cells characteristics shows that the serotonin responsive cell group can be individualized regarding their tonic discharge-mode in response to a direct current injection, their lower expression of hyperpolarization-activated cation current and their low membrane capacities. The use of ion channel blockers and ramp voltage protocol indicate that serotoninergic depolarization of juxtaglomerular cells may be due to a nonselective cation current with a reversal potential of -44 mV. Pharmacological tests with serotonin receptor antagonists and agonists reveal that 5HT action on juxtaglomerular cells would be mainly mediated by 5HT2C receptors. In mitral cells, serotonin acts on 49.1% of the 242 tested cells, inducing two types of responses. A first subset of mitral cells (26.8%, n=65) were hyperpolarized by serotonin. This response would be indirect and mediated by action of GABA on GABAA receptors since it was antagonized by bicuculline. The involved GABAergic neurons are hypothesized to be juxtaglomerular and granular cells, on which serotonin would act mainly via 5HT2C and via 5HT2A receptors respectively. The second subset of mitral cells (22.3%, n=54) were directly depolarized by serotonin acting through 5HT2A receptors. Our data on serotonin action on juxtaglomerular cells and mitral cells reveal a part of functional mechanisms whereby serotonin can act on olfactory bulb network. This is expected to enrich the understanding of its determining role in olfactory learning.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

Changes in steady-state levels of tryptophan hydroxylase protein in adult rat brain after neonatal 6-hydroxydopamine lesion

A recently developed technique of immunoautoradiography on nitrocellulose transfers of serial frozen sections was used to determine tryptophan hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopamine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nucleus raphe dorsalis, containing the cell bodies of origin for these nerve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive transverse levels across the entire caudorostral extent of all these regions. Similar measurements of tyrosine hydroxylase protein across the substantia nigra and the neostriatum verified the disappearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin-hyperinnervated neostriatum was up by 36% above control, i.e. significantly less than the number of its serotonin axon terminals or varicosities. This was therefore indicative of a lowering of the tryptophan hydroxylase protein content per serotonin ending. Interestingly, a tight correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the control neostriatum allowed the prediction the tryptophan hydroxylase concentration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably at those raphe dorsalis levels known to give rise to the serotonin hyperinnervation of neostriatum. It is hypothesized that the lower steady-state level of tryptophan hydroxylase inside the terminals and cell bodies of hyperinnervating serotonin neurons was the result of a feedback inhibition of the synthesis of the enzyme by its end-product, presumably because of the increased amount of serotonin in these terminals.

Tracing specific transmitter pathways in the rat CNS: combination of [3H]serotonin retrograde labelling with immunocytochemical detection of endogenous transmitters

Selective retrograde labelling with [3H]serotonin ([3H]5-HT) can be used to identify serotonergic cell bodies after specific [3H]5-HT uptake by the corresponding nerve terminals. In the present study, we demonstrate that autoradiography of this [3H]5-HT radiolabelling can be combined with immunocytochemical detection of endogenous serotonin, GABA or substance P on the same tissue section. The midbrain raphe serotonergic projections to the olfactory bulb and the spinal projections of medullary serotonergic nuclei were investigated. The specificity of retrograde labelling with [3H]5-HT was confirmed by immunoreactivity of the radiolabelled cells for serotonin, using an antiserum specific for formaldehyde-fixed serotonin. After spinal injections of [3H]5-HT, many retrogradely labelled cells in the medullary raphe were immunopositive for substance P, and a few for GABA. These results are in agreement with the available information on the co-existence of putative transmitters in the spinal projections of caudal raphe neurons. Therefore, autoradiography of [3H]5-HT retrograde labelling combined with immunocytochemistry offers a possibility to test the specificity of transmitter-selective retrograde labelling, to identify transmitter-defined neuronal interactions and to investigate the projection fields of multitransmitter containing neurons.

[3H]-serotonin retrograde labelling in serotonergic fibers

Autoradiography, following retrograde axonal transport with [3H]-serotonin (10(-4) M) was used to identify, selectively, the serotonergic neurons of medullary raphe that innervate the spinal cord. In combination with this technique, immunocytochemical detection of endogenous serotonin showed some raphe magnus cell bodies immunostained by 5-HT antibodies and [3H]5-HT radiolabelled. Serotonergic fibers of lateral and dorsolateral funiculus of ligated spinal cord were also characterized following retrograde axonal transport with 10(-4) M [3H]-serotonin. At the ultrastructural level, mainly myelinated, although unmyelinated labelled fibers were also identified as swollen axons. Clusters of large membranous organelles, predominantly mitochondria, some dense lamellar bodies, multivesicular bodies and lysosomes accumulated on the distal segment of the ligation. Autoradiographic reaction was always observed to be very intense and silver grains overlapped mainly mitochondria (41%) and clusters of membranous organelles (37%). However, smooth endoplasmic reticulum seemed to be devoid of silver grains. From our results, serotonergic myelinated spinal fibers were labelled after uptake-retrograde axonal transport with [3H]5-HT. Mitochondria and membrane organelles could convey tritiated compounds derived from [3H]-serotonin uptake towards the serotonergic cell bodies.

Reversibility of para-chlorophenylalanine-induced insomnia by intrahypothalamic microinjection of L-5-hydroxytryptophan

Para-chlorophenylalanine, a blocker of serotonin biosynthesis by inhibiting tryptophan hydroxylase, induced total insomnia which was accompanied in cat by a permanent discharge of ponto-geniculo-occipital activity. L-5-Hydroxytryptophan microinjection (1-4 micrograms/0.5 microliters) in the anterior hypothalamus 72 h after para-chlorophenylalanine administration, restored both slow wave sleep and paradoxical sleep with variable latencies for each state of sleep. On the contrary, ponto-geniculo-occipital activity was never suppressed. The hypnogenic effects of L-5-hydroxytryptophan were always followed by a return of the para-chlorophenylalanine-induced insomnia. On the other hand, the temperature recording did not show any alteration of the cerebral temperature after para-chlorophenylalanine treatment but the subsequent L-5-hydroxytryptophan microinjection was followed by hyperthermia. Using immunohistochemistry for serotonin after intrahypothalamic L-5-hydroxytryptophan microinjection in parachlorophenylalanine-pretreated cat, we defined a restricted region of the anterior hypothalamus possibly responsible for the hypnogenic effect. This region included the lateral hypothalamus and the anterior hypothalamic area. It is suggested that the reversible hypersomnia after L-5-hydroxytryptophan microinjection in the anterior hypothalamus in para-chlorophenylalanine-pretreated cat is due to a neurohormonal action of serotonin: serotonin could act upon the anterior hypothalamus which secondarily inhibits a waking system located in the ventrolateral hypothalamus leading to the appearance of paradoxical sleep.

Serotonergic projections to the spinal cord but not those to the olfactory bulb also contain substance P. A combined immunocytochemical and autoradiographic study following retrograde axonal transport of [3H]serotonin labeled products

Co-localization of substance P with serotonin in raphe projection neurons was studied by combining substance P immunocytochemistry and autoradiography following uptake and retrograde axonal transport of [3H]serotonin and/or its products from target areas. In this study, two central pathways in the rat were investigated: the serotonergic projections of the midbrain raphe to the olfactory bulb and those of the medullary raphe that innervate the thoracic spinal cord. Two hours after pargyline pretreatment, injections of 10(-4) M [3H]serotonin were made either into the olfactory bulb or into the spinal cord and respectively 24 or 60 h thereafter, rats were administered with colchicine. After a 24 h survival time, the paraformaldehyde fixed brains were investigated for substance P immunocytochemistry and then treated for light and electron microscopy autoradiography. Combining both methods, we can define on the same tissue sections at least three labeled neuronal populations: substance P immunolabeled neurons, radiolabeled neurons and doubly immuno-radiolabeled neurons. In the midbrain raphe cells as well as in the olfactory bulb nerve terminals, two kinds of labeled profiles were detected: substance P immunoreactive profiles and radiolabeled ones. The radiolabeled cell bodies of the midbrain raphe (403 counted cells) were never reactive to substance P antibodies. Moreover, they were distributed caudally to substance P stained perikarya. In contrast, in the medullary raphe, of the 336 radiolabeled cell bodies 162 were stained after substance P antibody treatment. They represent about 48% of the serotonin radiolabeled neurons projecting to the thoracic spinal cord, where a great number of varicosities were observed immunolabeled, radiolabeled and doubly immuno-radiolabeled in the dorsal horn. At the ultrastructural level, cell bodies and dendritic processes were also doubly labeled. Both labelings were observed over the cytoplasm and some organelles or perikarya. These observations provide a morphological basis to support the hypothesis that substance P can occur within some but not all serotonergic neurons and raise questions about the expression of this peptide in these systems as well as the modes of interaction of these transmitter molecules.

Immunocytochemical demonstration of the presence of catecholamine and serotonin neurons in the sheep olfactory bulb

The catecholamine and serotonin innervation of the sheep olfactory bulb was studied using immunocytochemistry. Specific antisera raised against tyrosine hydroxylase, dopamine beta-hydroxylase, phenylethanolamine N-methyl transferase and serotonin were used. Tyrosine hydroxylase-positive cell bodies were present in all cell layers except in the anterior olfactory nucleus, the greatest number being found in the glomerular layer. Neither dopamine beta-hydroxylase-positive nor serotonin-positive cell bodies were observed. Dopamine beta-hydroxylase-positive fibers were widely distributed in the granule cell layer but less widely in other layers. The glomerular layer contained the greatest distribution of serotonergic positive fibers, but such fibers were also visualized in other cell layers. No phenylethanolamine N-methyl transferase-positive structures were found in this investigation.

Selective retrograde labeling in some afferents to the rabbit lateral geniculate nucleus following injections of tritiated neurotransmitter-related compounds

Retrogradely labeled neurons were found in the visual cortex and superior colliculus following D-[3H]aspartate injections in the lateral geniculate nucleus (LGN). In these experiments labeling was also observed over the optic tract. [3H]dopamine and [3H]serotonin injections in the LGN caused weak labeling in a small number of superior colliculus neurons. Furthermore, in [3H]serotonin cases, labeled neurons were also found in the dorsal raphé nucleus. In contrast, when other amino acids or monoamines were injected, no retrograde labeling occurred in any of the afferents to the LGN. These results are largely consistent with the idea of D-[3H]aspartate being a useful marker for pathways using excitatory amino acids as neurotransmitters. The findings in [3H]dopamine and [3H]serotonin experiments indicate that these substances may induce retrograde labeling patterns, which are not related to the transmitter specificity of the pathways concerned.

Selective retrograde axonal transport of free glycine in identified neurons of Aplysia

The specific retrograde axonal transport of free glycine within the identified neurons R3-14 of Aplysia californica was studied. The soma of the R3-14 neurons are located in the parietovisceral ganglion and their axons project down the branchial nerve to end in a large peripheral field. Using a double-chambered apparatus, the peripheral tissue was incubated in medium containing a 3H-amino acid for 4-48 hr, while the nerve and ganglion were isolated and perfused with plain or chemically altered medium. The nerve and ganglion were then either rapidly frozen for scintillation counting or fixed for autoradiography. When 3H-glycine was used, radioactivity entered the nerve rapidly, reached the ganglion in 3 hr, and was transported largely (greater than 80%) in the free amino acid form [trichloroacetic acid (TCA) soluble]. The right parietovisceral hemiganglion accumulated up to nine times more radioactivity than the left hemiganglion, reflecting the presence of the R3-14 axons and soma. Two phases of radioactivity were observed, a fast component moving at about 3 mm/hr and a slower (but larger) component moving at about 0.4 mm/hr. Light microscope autoradiography on nerves containing 3H-glycine revealed that the R3-14 axons accounted for more than 30% of the total label in the nerve but occupied less than 7% of the total cross-sectional area of the axonal core. Electron microscope autoradiography showed a close association of silver grains and dense core vesicles in the R3-14 axons. Retrograde axonal transport of free glycine was inhibited by (in decreasing order of effectiveness) mercuric chloride, vinblastine, colchicine, Nocodazole, and 2,4-dinitrophenol (2,4-DNP). Comparative studies of other amino acids [3H-leucine, 3H-serine, 3H-glutamic acid, 3H-gamma-aminobutyric acid (3H-GABA), and 3H-alanine] showed that 3H-glycine is the only amino acid that is rapidly axonally transported in large quantities within the R3-14 axons. This work demonstrates, for the first time, that a free amino acid, glycine, is transported in the retrograde direction within a select group of axons. The significance of this transport of glycine is discussed in relation to its use as a neural messenger by neurons R3-14.

Selective retrograde labeling of neurons of the cat vestibular ganglion with [3H]D-aspartate

D-[2,3-3H]Aspartate [( 3H]D-Asp) was injected in the cat vestibular nuclei. Labeling patterns resulting from retrograde axonal transport by the vestibular nerve fibers were observed in the vestibular ganglion neurons and also in the nerve fibers. The selectivity of such labeling, related to the neurotransmitter's specificity, is strongly indicated.

Substance P and catecholaminergic expression in neurons of the hamster main olfactory bulb

A coordinated series of immunohistochemical and biochemical analyses have been conducted in the hamster to examine the dependence of substance P and tyrosine hydroxylase (TH) expression by second-order olfactory neurons, and the level of dopamine in the main olfactory bulb (MOB), on the integrity of carnosine- and olfactory marker protein (OMP)-containing primary afferent neurons. Substance P-like immunoreactivity (SPLI) is localized in external tufted cells and centrifugal afferents of the MOB; TH immunoreactivity has a wider distribution, in external tufted, middle tufted, periglomerular, and deep short-axon cells as well as in centrifugal afferents. To characterize the SPLI, this material was isolated by guanidine-HCl extraction and passage over a C18 SEP-PAK. The SPLI coelutes on HPLC with authentic substance P and, following oxidation, coelutes with substance P sulfoxide. It is sensitive to alpha-chymotrypsin and is resistant to trypsin. Thus, the SPLI in the MOB behaves as authentic substance P. Intranasal irrigation with 0.17 M ZnSO4 results in peripheral deafferentiation of the MOB for up to 8 months as evidenced by a persistent loss of OMP immunoreactivity and shrinkage of the olfactory nerve layer and glomeruli. By these criteria, the vomeronasal inputs to the accessory olfactory bulb are not destroyed and the spared vomeronasal receptor neurons do not innervate the vacated peripheral projection field in the MOB. The loss of peripheral inputs to the MOB is accompanied by marked and parallel reductions in the incidences of SPLI- and TH-positive second-order neurons despite an increase in the density of neuronal somata in the glomerular layer. Biochemical quantifications following peripheral deafferentation also demonstrate significant decreases of both substance P and dopamine, together with the expected decrease of carnosine. In contrast, the SPLI and the TH and serotoninlike immunoreactivities in centrifugal afferents as well as the TH immunoreactivity in deep interneurons do not appear to be reduced, and the MOB content of norepinephrine in centrifugal afferents is unaffected. These results collectively indicate that the loss of inputs from the primary olfactory receptor neurons can reduce the levels of at least two different, putatively neuroactive compounds (substance P and dopamine) in at least three classes of second-order neurons (external tufted, middle tufted, and periglomerular cells). The control of central neuron phenotype by the peripheral olfactory neurons thus appears to be a phenomenon of broad influence. It may play a role in processing chemosensory information as well as offering a system in which to study neuronal plasticit

The tridimensional structure of Nissl bodies: a stereoscopic study in ventral horn cells of rat spinal cord

The tridimensional structure of rough endoplasmic reticulum was examined with both high and low voltage electron microscopes in large ventral horn cells of rat spinal cord, by combining stereoscopic techniques with the use of thick sections selectively impregnated with heavy metal salts. In all neurons examined Nissl bodies appeared as well defined clusters of densely stained and profusely anastomosed plate-, ribbon-, and thread-like cisternae. Plate-like cisternae were variable in size, often showed a shallow curvature, and usually ran in short parallel arrays, separated from one another by fairly constant intervals. All gave rise at their edges to several ribbon-like extensions which occasionally decreased in width distally, turning into thin, thread-like cisternae. Characteristically, these ribbon-like structures would emerge at an angle from their plate of origin and smoothly curve away from the plane of the plate to merge with ribbons or threads arising from adjacent or more distant plates. Most plate-like cisternae were found at the periphery of Nissl bodies and tended to be oriented parallel to their surface. In contrast, the center of Nissl bodies was almost exclusively occupied by a complex network of ribbon- and thread-like cisternae. It is suggested that the basic plate/ribbon association here described in spinal motoneurons might be a constant feature of Nissl body architecture in various neuronal types, while the size, orientation, and relative proportion of plate-like cisternae may vary according to the metabolic state and/or functional specialization of the cells.

Quantitative autoradiographic studies of 5-HT-accumulating neurones in the nodose ganglia of the cat after perikaryal or terminal uptake

The distributions of ganglionar cell bodies, specifically labeled with 3H-5-HT, were studied in light microscope autoradiographs of the cat nodose ganglion. The size, number and localization of labeled cells were examined under different experimental conditions: after in vitro incubation of the ganglion with 3H-5-HT and after retrograde transport of 3H-5-HT injected into the nucleus of the solitary tract (NST) which receives primary visceral sensory projections from nodose ganglia cells of the vagal nerve. Following incubation of the nodose ganglia with a low concentration of 3H-5-HT (10(-6) M), some ganglionar cell bodies took up and retained the tracer. In both the right and left ganglia, they were significantly smaller in size than the unreactive neurones. The mean diameter of their perikaryon was 36.97 +/- 0.52 microns, compared with 45.76 +/- 0.87 microns in unreactive neurones. About 600 labeled cell bodies were counted in each ganglion, corresponding to 2-3% of the total nodose ganglion cell population. These reactive neurones were not localized in one particular area of the ganglia, but scattered throughout both of them. Following bilateral or unilateral microinjections of 3H-5-HT in the NST, retrogradely labeled cell bodies were observed, 24 h later, in the nodose ganglia. Their mean diameter was estimated to be 36.14 +/- 0.69 microns and they represented approximately 2% of the total ganglion cell population. As in in vitro experiments, the labeled cells were not grouped in any particular region of the ganglion. These experiments show that the distribution of both populations of labeled cells, observed under these conditions, are comparable. On an anatomical and quantitative basis one may reasonably suppose that the perikaryal or terminal uptakes concern the same neuronal population.

Selective retrograde labelling of vestibular efferent neurons with [3H]choline

Following administration of [3H]choline in the lateral semicircular canal of the cat labyrinth, bidirectional axoplasmic transport [3H]choline and its derivatives was shown by radioautography in the vestibular system. Light-microscopic radioautographs exhibited various patterns of radioautographic labelling. First, a diffuse reaction was observed in vestibular nuclei representing anterograde-labelled, vestibular nerve endings. Second, a heavy labelling limited to perikarya was detected in efferent vestibular neurons and corresponded to retrograde transport. The anterograde migration of [3H]choline is known to be non-selective and is related to synthesis of phospholipids, non-diffusable molecules. In contrast, the retrograde perikaryal labelling seems highly selective and related to the cholinergic specificity of the transmitter. The selectivity of such labelling offers a further possibility of identifying cholinergic neurons and is additional evidence that cholinergic mechanisms are involved in the efferent vestibular control.

Transsynaptic regulation of olfactory bulb catecholamines in mice and rats

Norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylalanine (DOPA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured simultaneously by high performance liquid chromatography with electrochemical detection in extracts of olfactory bulbs at various intervals after chemical or surgical deafferentation. Chemical deafferentation of mice by intranasal irrigation with Triton X-100 or of rats by olfactory axotomy resulted in a rapid progressive decline of DA and DOPAC and an associated rise in NE in the olfactory bulb. However, after several weeks, these values returned to prelesion levels concomitant with reinnervation of the bulb by the afferent neurons. In contrast, deafferentation by procedures known to prevent reinnervation of the bulb by the afferent chemoreceptor neurons (i.e., a ZnSo4 solution in mice or a surgical procedure in rats) completely blocked the return to pre-lesion values of DA, DOPAC, and NE. The specificity of these effects was demonstrated by the inability of intranasal administration of the neurotoxin 6-hydroxydopamine to alter DA levels, resulting instead in a significant decline in olfactory bulb NE content. These data demonstrate that the DA content of the olfactory bulb can be influenced by either chemical or surgical modulation of the afferent pathway in two different species. This offers additional support for our hypothesis of transsynaptic regulation of intrinsic DA neurons of the bulb by the afferent olfactory chemoreceptor neurons.

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.

Selective retrograde transport of 3H-serotonin in vagal afferents

A new serotonergic afferent vagal component has been demonstrated in the cat by radioautography. Twenty-four hours after a bilateral injection of tritiated serotonin (3H-5-HT) into the area of the nucleus of the solitary tract (NST), heavily and lightly labelled cell bodies were observed in the nodose ganglia. After unilateral injections of 3H-5-HT into the same area, labelled ganglionar cell bodies were found in the ipsilateral nodose ganglion. Some were also found in the contralateral one, suggesting a serotonergic crossed fibers component. Dense clusters of silver grains, depicting typical labelling of neuronal varicosities, were observed in the NST. After destruction of the serotonergic terminals with 5,7-dihydroxytryptamine, followed by injection of 3H-5-HT, the number of labelled cell bodies decreased dramatically in the ipsilateral nodose ganglia and the clusters of silver grains disappeared in the NST. After ligature or section of the supranodose vagal nerve, following injection of 3H-5-HT into the NST, no radioautographic reaction was observed in the homolateral nodose ganglia. The present study demonstrates the existence of a peripheral serotonergic system in vagal afferents. The physiological implications of this new serotonergic visceral pathway remain to be studied.

Bilateral serotonergic projections to the dorsal hippocampus of the rat: simultaneous localization of 3H-5HT and HRP after retrograde transport

Horseradish peroxidase (HRP, Sigma VI, 30-70 nl of a 10-15% solution in saline) or 3H-5HT (30 Ci/mmole, 2.5 X 10 -3 M containing 3.3 X 10(-3) M norepinephrine in saline, 50-100 nl) was injected unilaterally into the dorsal hippocampus in separate groups of rats. HRP-labeled cells were seen in the hippocampus, medial septal nucleus, nucleus of the diagonal band, supramammilary nucleus, median raphe nucleus, interfascicular portion of the dorsal raphe nucleus, and the locus coeruleus. In contrast, 3H-5HT-labeled cells were largely restricted to the raphe nuclei. In this nucleus an equal number of ipsilateral and bilateral cells were found. Occasionally, these labeled cells stretched across the midline (bridge pattern). In another series, the 3H-5HT and HRP were injected into the same hippocampus either as a mixture or sequentially. This resulted in double labeling of the median and dorsal raphe neurons. A final group of rats received injections of 3H-5HT and HRP into opposing hippocampi. Double-labeled cells accounted for 10% of the neurons labeled. In addition, closely paired neurons composed of an HRP- and 3H-5HT-containing cell were found. In summary, the serotonergic fibers may play a key role in harmonizing the electrical activity of the hippocampi by use of bilateral projections, paired neurons with differential projections, and bridging neurons stretching across the midline but with unilateral projections.

Radioautographic study of uptake and storage of indoleamines in the rabbit enterochromaffin cells

After in vitro intra-arterial injection of tritium-labeled 5-hydroxytryptamine or -hydroxytryptophan ([3H]5-HT or [3H]5-HTP) (5 x 10(-7) M) in the presence of cold noradrenaline (5 x 10(-6) M) into rabbit colon, a clear-cut labeling pattern was observed by radioautography. Labelled cells were observed within the mucosa. The labeling was less important after [3H]5-HTP than after [3H]5-HT injection. Ultrastructural study indicates that labeling is confined to the cytoplasm and coincides with polymorphic secretory granules. Hence these labeled cells display not only APUD cells characteristics but also true neuronal properties.