Fine structure of noradrenaline nerve terminals in the dorsomedial portion of the nucleus tractus solitarii as demonstrated by a modified potassium permanganate method

Cardiovascular afferents cause the release of 5-HT in the nucleus tractus solitarii; this release is regulated by the low- (PMAT) not the high-affinity transporter (SERT)

The nucleus tractus solitarii (NTS) integrates inputs from cardiovascular afferents and thus is crucial for cardiovascular homeostasis. These afferents primarily release glutamate, although 5-HT has also been shown to play a role in their actions. Using fast-cyclic voltammetry, an increase in 5-HT concentrations (range 12-50 nm) could be detected in the NTS in anaesthetized rats in response to electrical stimulation of the vagus and activation of cardiopulmonary, chemo- and baroreceptor reflexes. This 5-HT signal was not potentiated by the serotonin transporter (SERT) or the noradrenaline transporter (NET) inhibitors citalopram and desipramine (1 mg kg(-1) ). However, decynium-22 (600 μg kg(-1) ), an organic cation 3 transporter (OCT3)/plasma membrane monoamine transporter (PMAT) inhibitor, increased the 5-HT signal by 111 ± 21% from 29 ± 10 nm. The effectiveness of these inhibitors was tested against the removal time of 5-HT and noradrenaline applied by microinjection to the NTS. Citalopram and decynium-22 attenuated the removal of 5-HT but not noradrenaline, whereas desipramine had the reverse action. The OCT3 inhibitor corticosterone (10 mg kg(-1) ) had no effect. Blockade of glutamate receptors with topical kynurenate (10-50 nm) reduced the vagally evoked 5-HT signal by 50%, indicating that this release was from at least two sources. It is concluded that vagally evoked 5-HT release is under the regulation of the high-capacity, low-affinity transporter PMAT, not the low-capacity, high-affinity transporter SERT. This is the first demonstration that PMAT may be playing a physiological role in the regulation of 5-HT transmission and this could indicate that 5-HT is acting, in part, as a volume transmitter within the NTS.

Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 microm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS.

Synaptic organization of the interstitial subdivision of the nucleus tractus solitarii and of its laryngeal afferents in the rat

The nucleus tractus solitarii, the first central relay for gustatory and a variety of visceral afferents, is also an integrative center for numerous functions. Its interstitial subdivision is involved in swallowing and respiratory reflexes. The ultrastructural characteristics of this subdivision and of its laryngeal afferents were investigated in adult rat by a serial-section study and by application of wheat germ agglutinin-horseradish peroxidase conjugate to the peripheral afferent fibers. The interstitial subnucleus contained scattered small neuronal cell bodies with such ultrastructural features as a large nucleus with deep indentations and an organelle-poor cytoplasm. On the basis of their size and vesicular content, the axon terminals were classified into three categories. Group I and group II terminals were small or large, respectively, and contained mainly small, round, and clear synaptic vesicles. Group III terminals were also small but contained small, pleomorphic, and clear vesicles. Axodendritic synapses were the most numerous. They were either asymmetrical, comprised of group I and II terminals, or symmetrical, comprised of group III terminals. More than 50% were part of complex synaptic arrangements in the form of rosettes or glomeruli. Axosomatic contacts involved both group I and group III terminals and were always symmetrical. A high frequency of axoaxonic synapses was found. They were symmetrical, comprised of group III terminals on group I or II terminals. Different types of symmetrical synaptic contacts made by dendrites were also found. This study indicates also that the ipsilateral interstitial subdivision constitutes the preferential site of termination for superior laryngeal afferents. The labeled axon terminals belonged exclusively to groups I and II and were involved in both axodendritic and axoaxonic synapses. Some of the axodendritic synapses were part of rosettes or glomeruli. All these synaptic arrangements may be considered a morphological substrate for important processing of afferent information in the nucleus tractus solitarii. They may account for some of the integrative functions of the interstitial subnucleus such as physiological processes triggered from the superior laryngeal nerve.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic responses of baroreceptive cells in the nucleus tractus solitarii of the rat: a microiontophoretic study

Anatomical and pharmacological evidence suggests a role for catecholamines (CAs) in the modulation of the baroreceptor reflex within the nucleus tractus solitarii (NTS). Single neurons in the NTS of the rat were studied for their responses to activation of the baroreceptor reflex and to iontophoretic administration of dopamine, norepinephrine (NE), and epinephrine (EPI) to determine the relationship between the effects of baroreflex activation and CA application on baroreceptive neurons in the vagal sensory nucleus. Of 269 cells studied, 104 (38.7%) exhibited decreases and 41 cells (15.2%) showed increases in firing rate in response to baroreflex activation, while the remaining 124 neurons showed no response. All 3 CAs inhibited spike activity in the majority (68.5%) of NTS cells. These inhibitory effects on spontaneous firing were observed regardless of the response profile of a particular neuron to baroreflex activation. The inhibitory effects of NE and EPI on NTS neuronal activity were specifically blocked by the alpha-adrenergic receptor antagonist tolazoline, but not by the beta-adrenergic antagonist sotalol. These results indicate that CAs may interact at several sites within the NTS to influence baroreflex integration, and that the effects of NE and EPI on neuronal activity are mediated by an alpha-adrenergic receptor.

Inhibitory influence of monoamines and brainstem monoaminergic regions on the medullary swallowing reflex

Swallowing is a polysynaptic reflex organized by a neuronal network localized mainly within the lateral region of the nucleus tractus solitarius (NTS). Monoamine microinjections (serotonin, noradrenaline; 1 nmol, 50 nl) within the swallowing region of the NTS produced a significant decrease in the number of swallows elicited by repetitive stimulation of the superior laryngeal nerve. Stimulation, with concentric bipolar electrodes, of several brainstem structures overlapping monoaminergic regions such as the nucleus raphe magnus, the nucleus raphe pallidus, the lemniscus medialis, the ventrolateral reticular formation, the locus coeruleus and the nucleus commissuralis, induced an inhibition of the laryngeal-initiated swallowing. These results indicate the existence, within the NTS, of a monoaminergic inhibition of the swallowing reflex which might originate from monoaminergic brainstem regions.

X-ray microanalysis of monoaminergic terminals in the nucleus tegmentalis dorsalis of the chicken

X-ray microanalysis after aldehyde-chromate-dichromate treatment served to confirm the presence of monoaminergic terminals in the nucleus tegmentalis dorsalis (NTD) of the chicken. The monoaminergic terminals were represented as neuronal elements with electron-dense vesicles (EDVs) of several different shapes as seen in Eponembedded semi-thin sections. Conventional electron microscopic observations of the adjacent ultra-thin sections showed the EDVs to be comprised of spherical medium-sized (about 80 nm in diameter), large dense-cored (about 120 nm) and elongated granular vesicles (100-220 nm) in the same nerve varicosities. It is probable that the NTD, being a center of catecholaminergic efferent projections, may also receive direct monoaminergic inputs from an unknown area of the brain and/or from recurrent collaterals of the same catecholamine-containing neurons in the NTD.

Adrenaline involvement in the presynaptic beta-adrenoceptor-mediated mechanism of dopamine release from slices of the rat hypothalamus

Slices of rat hypothalamus were superfused and endogenous release of dopamine (DA) was measured by high performance liquid chromatography combined with electrochemical detection. The K+ (20 mM)-evoked release in the presence of tetrodotoxin was Ca2+-dependent. The evoked release was facilitated by a beta-agonist, isoproterenol and this effect was completely abolished by a beta-antagonist, 1-propranolol. Isoproterenol also concentration-dependently facilitated the electrically (at 5Hz) evoked release of DA. The pretreatment with 1-propranolol, beta 1-antagonist, atenolol and beta 2-antagonist, butoxamine shifted the concentration-effect curve of isoproterenol to the right. On the other hand, beta 1-agonist, tazolol, beta 2-agonist, salbutamol and low concentration (10(-9) M) of adrenaline also facilitated the release. 1-Propranolol alone reduced the electrically (at 2 Hz) evoked release, and this effect was completely abolished when the adrenaline content in the brain was drastically reduced by use of a potent PNMT inhibitor, DCMB. These findings suggest that in the rat hypothalamus adrenaline released from adrenaline-containing nerve terminals probably modulates DA release via presynaptic beta 1- and beta 2-adrenoceptors on DA nerve terminals.

Immunohistochemical localization of putative neurotransmitters within the feline nucleus tractus solitarii

With the aid of immunohistochemical techniques the distribution of substance P, met-enkephalin, serotonin, somatostatin, alpha-melanocyte stimulating hormone, neurotensin and neurophysin immunoreactivities were mapped throughout the rostro-caudal extent of the cat's nucleus tractus solitarii. Three of the putative neurotransmitters (substance P, enkephalin and serotonin) were found to be widely distributed as varicose fibers and punctate structures. The densities of their immunoreactivities were plotted in a range from very dense, dense, moderate, occasional, to none, at different levels of the nucleus of the solitary tract. Substance P immunoreactivity was the most varied and dense of all the neurotransmitters studied. Its accumulations ranged from very dense in the lateral, dense in portions of the parvocellular and lateral, moderate in medial and commissural and occasional in ventrolateral and portions of the parvocellular subdivisions. Both the enkephalin and serotonin immunoreactivities had patterns similar to that of substance P immunoreactivity, although their amounts were not as great. Following colchicine treatment neurons containing substance P and enkephalin immunoreactivity were found in many subdivisions of the nucleus of the solitary tract. Somatostatin, alpha-melanocyte stimulating hormone, neurotensin and neurophysin immunoreactivities were present in the nucleus of the solitary tract as isolated varicose fibers scattered throughout the nucleus. Immunoreactive neurons were not found for these putative neurotransmitters after colchicine treatment. The presence of substance P immunoreactivity within subdivisions which receive visceral afferent input is discussed in relation to the role of substance P as a possible transmitter of the afferent limb of the vagus nerve. The distribution of enkephalin and serotonin immunoreactivities in the nucleus of the solitary tract reflect their involvement in the regulation or modulation of cardiovascular and respiratory functions. While the significance of somatostatin, alpha-melanocyte stimulating hormone, neurotensin and neurophysin immunoreactivities within the nucleus of the solitary tract is not understood at present, these substances might possibly play a role in visceral functions.

Electron microscopic cytochemical correlates of histofluorescence

The biogenic amines, norepinephrine and dopamine, have been localized electron microscopically in the substantia nigra, pineal body, adrenal medulla and stellate ganglion of squirrel monkeys. The method of localization has been by the use of a specific cytochemical technique, utilizing a chromium (Cr) complex with an isoquiniline derivative formed by the reaction of the unsubstituted biogenic amine with glutaraldehyde. The final reaction product, i.e., amine-glutaraldehyde-Cr, has been identified using energy dispersive X-ray analysis and the positive Cr reactive sites have been correlated with light microscopy conducted by using glyoxylic acid histofluorescence. All tissues were taken from squirrel monkeys which had either been perfused with the fixative or had been perfused with an F-12 buffer prior to fixation and/or histochemical treatment. The best results were obtained from the F-12 perfusion followed by tissue being taken through the various histochemical-cytochemical procedures. The electron microscopic localization of deposits shows significantly large structures containing the Cr positive amine material. These large deposits have not heretofore been described and correlate well in size and location with what is seen with histofluorescence. This electron microscopic technique should lead to further advances in the study of biogenic amines in nervous tissue when fine structural localization, plus well preserved morphology, are essential.

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.

Noradrenergic axon terminals in the substantia gelatinosa of the rat spinal cord: an electron-microscopic study using glyoxylic acid-potassium permanganate fixation

The noradrenergic terminals in the substantia gelatinosa of the dorsal horn of the cervical spinal cord of the rat were investigated by means of the histofluorescence technique and electron-microscopic cytochemistry using the glyoxylic acid-KMnO4 fixation technique. In accordance with the topographical distribution of fluorescent catecholaminergic fibers, noradrenergic terminals containing small granular vesicles were frequently observed electron microscopically in the outer layer of the substantial gelatinosa. These terminals were most frequently found to appose (without showing typical synaptic features, small-caliber dendrites, spine apparatus, and rarely, large caliber dendrites. Only in a few cases, the noradrenergic terminals exhibited typical synaptic contacts with dendritic elements of small size. In addition, noradrenergic terminals apposed non-noradrenergic terminals containing small agranular vesicles. In rats bearing surgical lesions of the dorsal roots, no noradrenergic terminal were found in contact with the degenerated axon terminals in the substantia gelatinosa. These findings suggest that the noradrenergic afferents to the substantia gelatinosa may exert their influence on sensory transmission via dorsal horn cells.

Morphological study of noradrenaline innervation in the caudal raphe nuclei with special reference to fine structure

Previous histofluorescence studies have demonstrated that the caudal raphe nuclei (CRa) of the rat, particularly the ventral portion (VCRa), contains a very high density of noradrenaline (NA) terminals and in the present study we attempted to elucidate the origins and free structure of the NA terminals in this region. The majority of NA terminals found in the VCRa originated from A1 and A3 NA neurons and disclosed that a small number of very strongly fluorescent fibers located along the blood vessels arise from the superior cervical ganglion (SGC). Electron microscopic analysis after potassium permanganate fixation demonstrated that the NA terminals originated from A1 and A3 NA neurons are connected with neuronal elements, while NA fibers from the SGC were identified among the vascular elements. Axodendritic contacts were found to be predominant among the synapselike contacts of NA terminals in this area.

Experimental and morphological studies of te noradrenaline innervations in the nucleus tractus spinalis nervi trigemini of the rat with special reference to their fine structures

An attempt was made to elucidate the origins and fine structure of the noradrenaline (NA) terminals in the nucleus tractus spinalis nervi trigemini (Vsp) of the rat. The present study revealed that the NA in the Vsp is supplied from two NA neuron systems; bilateral locus coeruleus (LC) and medullary NA neuron group, on the grounds that: (1) two types of fluorescent fibers (fine and coarse) were identified in the Vsp; and (2) HRP injected in to the Vsp resulted in labeling of neurons of the bilateral LC and bulbal NA areas, particularly the ipsilateral A1 area. The fine structure of the NA terminals in the Vsp was investigated by a modified potassium permanganate fixation method. The NA terminals in this area contained a number of small cored vesicles together with a small number of large cored vesicles. The most frequent feature of the NA terminals found in this area wa axo-dendritic contact. A lesser, but still substantial, number with axo-axonic contact were also noticed, in connection with which, it should be stressed that the non-NA terminals examined in this study were always presynaptic in structure, since the clear vesicles found in the non-NA terminals sometimes aggregated close to the contact membrane, where some of them fused or touched the membrane. As the dendrites showing a close relationship with the NA terminals were relatively large in size and contained a number of cytoplasmic organelles, these dendrites were assumed to be the proximal segments. At the contact zone, although no typical synaptic specialization was identified, the following profiles suggestive of synaptic-like contact were demonstrated: (1) somewhat dense material between contact membranes (intersynaptic filaments); (2) a slight accumulation of the dense materials adjacent to the postcontact membrane; (3) disarrangement of the contact membranes and enlargement of the space between these membranes; and (4) aggregation of the synaptic vesicles at the contact membrane with some of them fusing or touching the contact membrane.