Adrenergic fibers in the spinal cord of the monkey: light and electron microscopic study

Dendritic arborization and spines of the neurons of the cat and human periaqueductal gray: a light, confocal laser scanning, and electron microscope study

Neurons of the periaqueductal gray (PAG) have an extensive dendritic tree which plays an important role in the neuronal circuits supporting the functional activities of this region. The complexity of the local circuits is increased by the occurrence of dendritic spines. We have compared the dendritic and spine organization in the cat with that of man in order to verify whether an inverse relationship exists between dendritic tree extension and spine density and complexity. Sections of cat and human PAG prepared according to the Golgi-Cox method were studied with the conventional light microscope (LM) and the confocal laser scanning microscope (CLSM). The cat PAG was also studied at the electron microscopic level. The light microscopic study provided the morphoquantitative characteristics of the dendritic arborization and spines of the multipolar and fusiform neurons of the human and cat PAG. The CLSM methodology, thanks to the three-dimensional reconstruction of the neurons and the rotation of the reconstructed images, brought into view dendritic branches and spines that could not have been observed at the LM, thereby showing a wider dendritic tree and more numerous spines. The data combined from LM and CLSM demonstrate that in both species most spiny neurons are multipolar and probably projection neurons. In man, the multipolar neurons show a more extensive dendritic tree due to a wider secondary ramification, which would seem to be balanced by more numerous spines in cat. At the electron microscopic level, axo-dendritic synapses are numerous and show symmetrical and asymmetrical junctions in equal proportions; furthermore, the great majority of the spines are in contact with synaptic boutons which contain round vesicles and make predominantly asymmetrical contacts features which indicate excitatory activity. The combined use of different techniques gave a complete picture of the dendritic tree and spines of the neurons of human and cat PAG and showed a wider dendritic surface available for the receipt of the synaptic contacts than had been reported previously. Furthermore, our findings demonstrate that the PAG dendritic spines are important and specific structures in the synaptic complex of the neuropil, suggesting that they might create a local device to modulate and integrate the afferent inputs, probably in an excitatory way. The differences observed in the two species suggest that afferent information might be handled in different ways in human and cat PAG.

Molecular cloning and characterization of an L-epinephrine transporter from sympathetic ganglia of the bullfrog, Rana catesbiana

Chemical signaling by dopamine (DA) and L-norepinephrine (L-NE) at synapses is terminated by uptake via specialized presynaptic transport proteins encoded by the DA transporter (DAT) and L-NE transporter (NET) genes, respectively. In some vertebrate neurons, particularly the sympathetic neurons of amphibians, L-NE is converted to L-epinephrine (L-Epi, adrenaline) and released as the primary neurotransmitter. Although evidence exists for a molecularly distinct L-Epi transporter (ET) in the vertebrate brain and peripheral nervous system, a transporter specialized for extracellular L-Epi clearance has yet to be identified. To pursue this issue, we cloned transporter cDNAs from bullfrog (Rana catesbiana) paravertebral sympathetic ganglia and characterized functional properties via heterologous expression in non-neuronal cells. A cDNA of 2514 bp (fET) was identified for which the cognate 3.1 kb mRNA is highly enriched in frog sympathetic ganglia. Sequence analysis of the fET cDNA reveals an open reading frame coding for a protein of 630 amino acids. Inferred fET protein sequence bears 75, 66, and 48% amino acid identity with human NET, DAT, and the 5-hydroxytryptamine transporter (SERT), respectively. Transfection of fET confers Na+- and Cl--dependent catecholamine uptake in HeLa cells. Uptake of [3H]-L-NE by fET is inhibited by catecholamines in a stereospecific manner. L-Epi and DA inhibit fET-mediated [3H]-L-NE uptake more potently than they inhibit [3H]-L-NE uptake by human NET (hNET), whereas L-NE exhibits equivalent potency between the two carriers. Moreover, fET exhibits a greater maximal velocity (Vmax) for the terminal products of catecholamine biosynthesis (L-Epi > L-NE >> DA), unlike hNET, in which a Vmax rank order of L-NE > DA > L-Epi is observed. fET-mediated transport of catecholamines is sensitive to cocaine and tricyclic antidepressants, with antagonist potencies significantly correlated with hNET inhibitor sensitivity. Amino acid conservation and divergence of fET with mammalian catecholamine transporters help define residues likely to be involved in catecholamine recognition and translocation as well as blockade by selective reuptake inhibitors.

Synaptic terminal coverage of primate triceps surae motoneurons

This study examined the synaptic terminal coverage of primate triceps surae (TS) motoneurons at the electron microscopic level. In three male pigtail macaques, motoneurons were labeled by retrograde transport of cholera toxin-horseradish peroxidase that was injected into TS muscles bilaterally and visualized with tetramethylbenzidine stabilized with diaminobenzidine. Somatic, proximal dendritic, and distal dendritic synaptic terminals were classified by standard criteria and measured. Overall and type-specific synaptic terminal coverages and frequencies were determined. Labeled cells were located in caudal L5 to rostral S1 ventral horn and ranged from 40 to 74 microns in diameter (average, 54 microns). The range and unimodal distribution of diameters, the label used, and the presence of C terminals on almost all cells indicated that the 15 cell bodies and associated proximal dendrites analyzed here probably belonged to alpha-motoneurons. Synaptic terminals covered 39% of the cell body membrane, 60% of the proximal dendritic membrane, and 40% of the distal dendritic membrane. At each of these three sites, F terminals (flattened or pleomorphic vesicles, usually symmetric active zones, average contact length 1.6 microns) were most common, averaging 52%, 56%, and 58% of total coverage and 56%, 57%, and 58% of total number of cell bodies, proximal dendrites, and distal dendrites respectively. S terminals (round vesicles, usually asymmetric active zones, average contact length 1.3 microns) averaged 24%, 29%, and 33% of coverage and 33%, 35%, and 36% of number at these three sites, respectively. Thus, S terminals were slightly more prominent relative to F terminals on distal dendrites than on cell bodies. C terminals (spherical vesicles, subsynaptic cisterns associated with rough endoplasmic reticulum, average contact length 3.5 microns) constituted 24% and 11% of total terminal coverage on cell bodies and proximal dendrites, respectively, and averaged 11% and 6% of terminal number at these two locations. M terminals (spherical vesicles, postsynaptic Taxi bodies, some with presynaptic terminals, average contact length 2.7 microns) were absent on cell bodies and averaged 3% and 7% of total coverage and 2% and 5% of terminals on proximal and distal dendrites, respectively. Except for M terminals, which tended to be smaller distally, terminal contact length was not correlated with location. Total and type-specific coverages and frequencies were not correlated with cell body diameter. Primate TS motoneurons are similar to cat TS motoneurons in synaptic terminal morphology, frequency, and distribution. However, primate terminals appear to be smaller, so that the fraction of membrane covered by them is lower.

Distribution of glutamate- and GABA-immunoreactive neurons projecting to the cardioacceleratory center of the intermediolateral nucleus of the thoracic cord of SHR and WKY rats: a double-labeling study

We aimed at (1) determining the distribution of glutamate (Glu)- and gamma-aminobutyric acid (GABA)-containing neurons in the brainstem with projections to the cardioacceleratory sympathetic preganglionic neurons in the intermediolateral nucleus (IML) of the upper thoracic cord and (2) determining whether such afferent projections in spontaneously hypertensive rats (SHR) differ from those of control Wistar-Kyoto (WKY) rats. We used a combination of electrophysiological methods to determine the site of HRP injection in the spinal cord and double-labeling methods for plotting the distribution of Glu- and GABA-immunoreactive neurons with projections to this site. HRP/Glu-labeled neurons (possibly glutamatergic) and HRP/GABA-labeled neurons (possibly GABAergic) were detected in 27% and 7% of the total HRP-labeled neurons of the central autonomic nuclei of 3 SHR rats and 3 WKY rats. HRP/Glu-labeled neurons were distributed predominantly ipsilaterally in 20 nuclei of the medulla oblongata, pons and hypothalamus, while HRP/GABA-labeled neurons were distributed in 7 nuclei of the medulla oblongata. No significant differences were found between the average percentages of HRP/Glu-labeled and HRP/GABA-labeled neurons in SHR and WKY rats. These findings indicate that: (1) the Glu-containing neurons represent a greater proportion than the GABA-containing neurons, (2) the proportions of these neurons appear to be similar in WKY and SHR rats and (3) generation of inbred tachycardia and hypertension in SHR rats can not be attributed to the topological and quantitative differences in the distribution of the glutamatergic and GABAergic neurons in the central autonomic nuclei.

Immunohistochemical study of the catecholaminergic innervation of the spinal cord of the rat using specific antibodies against dopamine and noradrenaline

We have assessed the relative contributions of dopaminergic and noradrenergic descending systems to the catecholaminergic innervation of the rat spinal cord. Fibres and terminals were labelled with their own neurotransmitter by using specific antibodies raised against dopamine (DA) and noradrenaline (NA) respectively. For this purpose, immunohistochemistry according to the peroxidase anti-peroxidase technique was performed in different experimental conditions. Two group of rats received intracisternal 6-hydroxy-dopamine (6-OHDA) injections either with or without benzatropine pretreatment. Animals of a third group were not pretreated at all. While 6-OHDA induced a complete disappearance of spinal NA-like immunoreactivity (NA-LI), except for scarce residual fibres in the thoracic intermedio-lateral cell column, DA-like immunoreactivity (DA-LI) was unaffected by the lesion. This strongly suggests that the antisera used specifically labelled NA-containing and DA-containing fibres respectively. Spinal DA-LI and NA-LI innervations differed markedly in their topographical distributions and in the morphology of the corresponding fibres. DA-LI innervation was restricted to laminae I, III and IV and to the intermediate zone, especially the autonomic areas. In the ventral horn, it was sparse and more visible after acidification of the fixation solution. NA-LI innervation was much more widely spread. In addition, the organization of NA-LI fibres suggests that the innervation of the whole dorsal horn comes from a group of fibres travelling, at least partially, in the superficial dorsal horn.

Pre- and postnatal development of noradrenergic projections to the rat spinal cord: an immunocytochemical study

Using immunocytochemistry with a specific antiserum against noradrenaline, the pre- and postnatal development of noradrenergic (NA) projections to the rat spinal cord was studied from embryonic day 16 (E16) to adulthood (the day following nocturnal mating being considered as E0). In this study, pregnant animals were pre-treated with the MAO inhibitor pargyline (200 mg/kg i.p.), whereas postnatal animals received 100 mg/kg. In vibratome sections, noradrenaline-immunoreactive (NA-IR) axons were seen to invade the spinal cord at E16, at cervical and upper thoracic levels, from the ventral funiculus. At E18, small caliber NA-IR fibers were present in the ventral horn at all cord levels, and some fibers were seen in the intermediolateral cell column (IML) at thoracic level. The growth of axons towards the dorsal horn became noticeable by postnatal day 0 (P0). At P3, fine beaded and radially orientated NA-IR fibers were observed at all levels. The pattern of NA innervation of the dorsal horn was similar to that of the adult by P7. The segregation of noradrenaline immunoreactivity in the ventral and dorsal horns, the IML and the periependymal area was more obvious at all levels by P14 and P20. From P30 the NA innervation was similar to that found in the adult spinal cord. Thus, noradrenaline, like serotonin, was present early in the spinal cord before the onset of specific functions. In addition to and prior to its transmitter function, it might play a trophic role in the neurogenesis of the spinal cord.

Descending adrenergic input to the primate spinal cord and its possible role in modulation of spinothalamic cells

The present study focuses on 3 different aspects of the descending adrenergic system in the primate: (1) the distribution of adrenergic fibers and terminals in the spinal cord, (2) the source of this input and (3) the possible physiological effects of this system on spinal nociceptive processing. Antibodies to the enzyme phenylethanolamine-N-methyltransferase (PNMT) were employed to map the distribution of epinephrine-containing axonal profiles in the primate spinal cord. Smooth longitudinally oriented fibers were localized to the outer edge of the lateral funiculus. PNMT-containing axonal enlargements were distributed to the superficial dorsal horn, intermediate gray matter and the region surrounding the central canal at all spinal cord levels. PNMT-immunostained profiles were also observed in the intermediolateral cell column. A double labeling study employing retrograde transport of HRP from the spinal cord and PNMT immunohistochemistry identified a small population of HRP-PNMT-labeled neurons in the 'C1' region at the levels of the medulla and ponto-medullary junction. Thus, these cells are a probable source of adrenergic input to the spinal cord. Electrophysiological studies demonstrated that iontophoresis of epinephrine onto identified primate spinothalamic tract neurons in the lumbar dorsal horn resulted in inhibition of the glutamate-induced firing of these cells. The data from these studies support the hypothesis that adrenergic (PNMT-containing) cells in the caudal brainstem project to all levels of the cord and may contribute to descending modulation of nociceptive processing at these levels.