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

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

Distribution and density of contacts from noradrenergic and serotonergic boutons on the dendrites of neck flexor motoneurons in the adult cat

Serotonergic (5-HT) and noradrenergic (NA) input to spinal motoneurons is essential for generating plateau potentials and self-sustained discharges. Extensor motoneurons are densely innervated by 5-HT and NA synapses and have robust plateau potentials and self-sustained discharges. Conversely, plateau potentials and self-sustained discharges are very rare in flexor motoneurons. The most likely reasons for this difference are that flexor motoneurons have few 5-HT and NA synapses and/or they are distributed distant to the channels responsible for plateau potentials and self-sustained discharges. However, the distribution of 5-HT and NA synapses on flexor motoneurons is unknown. Here we describe the distribution and density of 5-HT and NA synapses on motoneurons that innervate the flexor neck muscle, rectus capitis anterior (RCA), in the adult cat. Using a combination of intracellular staining, fluorescent immunohistochemistry, and 3D reconstruction techniques, we found that 5-HT and NA synapses are widely distributed throughout the dendritic trees of RCA motoneurons, albeit with a strong bias to small-diameter dendrites and to medial dendrites in the case of NA contacts. The number of 5-HT and NA contacts per motoneuron ranged, respectively, from 381 to 1,430 and from 642 to 1,382, which is 2.3- and 1.4-fold less than neck extensor motoneurons (Montague et al., J Comp Neurol 2013;521:638-656). These results suggest that 5-HT and NA synapses on flexor motoneurons may provide a powerful means of amplifying synaptic currents without incurring plateau potentials or self-sustained discharges. This feature is well suited to meet the biomechanical demands imposed on flexor muscles during different motor tasks.

Nonuniform distribution of contacts from noradrenergic and serotonergic boutons on the dendrites of cat splenius motoneurons

The input-output properties of motoneurons are dynamically regulated. This regulation depends, in part, on the relative location of excitatory and inhibitory synapses, voltage-dependent and -independent channels, and neuromodulatory synapses on the dendritic tree. The goal of the present study was to quantify the number and distribution of synapses from two powerful neuromodulatory systems that originate from noradrenergic (NA) and serotonergic (5-HT) neurons. Here we show that the dendritic trees of motoneurons innervating a dorsal neck extensor muscle, splenius, in the adult cat are densely, but not uniformly innervated by both NA and 5-HT boutons. Identified splenius motoneurons were intracellularly stained with Neurobiotin. Using 3D reconstruction techniques we mapped the distributions of contacts formed by NA and 5-HT boutons on the reconstructed dendritic trees of these motoneurons. Splenius motoneurons received an average of 1,230 NA contacts (range = 647-1,507) and 1,582 5-HT contacts (range = 1,234-2,143). The densities of these contacts were 10 (NA) to 6 (5-HT)-fold higher on small compared to large-diameter dendrites. This relationship largely accounts for the bias of NA and 5-HT contacts on distal dendrites and is partially responsible for the higher density of NA contacts on dendrites located more than 200 μm dorsal to the soma. These results suggest that the neuromodulatory actions of NA and 5-HT are compartmentalized and regulate the input-output properties of motoneurons according to precisely arranged interactions with voltage-dependent and -independent channels that are primarily located on small-diameter dendrites.

Reduced removal of synaptic terminals from axotomized spinal motoneurons in the absence of complement C3

Complement proteins C1q and C3 play a critical role in synaptic elimination during development. Axotomy of spinal motoneurons triggers removal of synaptic terminals from the cell surface of motoneurons by largely unknown mechanisms. We therefore hypothesized that the complement system is involved also in synaptic stripping of injured motoneurons. In the sciatic motor pool of wild type (WT) mice, the immunoreactivity (IR) for both C1q and C3 was increased after sciatic nerve transection (SNT). Mice deficient in C3 (C3(-/-)) showed a reduced loss of synaptic terminals from injured motoneurons at one week after SNT, as assessed by immunoreactivity for synaptic markers and electron microscopy. In particular, the removal of putative inhibitory terminals, immunopositive for vesicular inhibitory amino acid transporter (VIAAT) and ultrastructurally identified as type F synapses, was reduced in C3(-/-) mice. In contrast, lesion-induced removal of nerve terminals in C1q(-/-) mice appeared similar to WT mice. Growth associated protein (GAP)-43 mRNA expression in lesioned motoneurons increased much more in C3(-/-) compared to WT mice after SNT. After sciatic nerve crush (SNC), the C3(-/-) mice showed a faster functional recovery, assessed as grip strength, compared to WT mice. No differences were detected regarding nerve inflammation at the site of injury or pattern of muscle reinnervation. These data indicate that a non-classical pathway of complement activation is involved in axotomy-induced adult synapse removal, and that its inhibition promotes functional recovery.

Contributions to the field of neurotransmitters by Japanese scientists, and reflections on my own research

PART I DESCRIBES IMPORTANT CONTRIBUTIONS MADE BY SOME JAPANESE PIONEERS IN THE FIELD OF NEUROTRANSMITTERS: (their achievements in parentheses) J. Takamine (isolation and crystallization of adrenaline); K. Shimidzu (early hint for acetylcholine as a neurotransmitter); F. Kanematsu (donation of the Kanematsu Memorial Institute in Sydney); T. Hayashi (discovery of the excitatory action of glutamate and the inhibitory action of GABA); and I. Sano (discovery of a high concentration of dopamine in striatum, its reduction in a patient with Parkinson's disease and the treatment with DOPA). In Part II, I present some of my reflections on my research on neurotransmitters. The work of my colleagues and myself has made some significant contributions to the establishment of neurotransmitter roles played by GABA and substance P, the first amino acid and the first peptide neurotransmitters, respectively. By the early 1960s, 3 substances, i.e., acetylcholine, noradrenaline, and adrenaline, had been established as neurotransmitters. Now the number of neurotransmitters is believed to be as many as 50 or even more mainly due to the inclusion of several amino acids and a large number of peptide transmitters.

Age-associated changes in the monoaminergic innervation of rat lumbosacral spinal cord

The effects of ageing on the innervation patterns of lumbosacral spinal nuclei involved in controlling lower urinary tract functions, including micturition, were studied using immunohistochemistry for serotonin (5-HT) and tyrosine hydroxylase (TH) in male Wistar rats of 3 and 24 months. Quantitative image analysis revealed significant age-associated declines in the innervation of most regions including the intermediolateral cell nucleus, sacral parasympathetic nucleus, dorsal grey commissure and in the ventral horn including the dorsolateral nucleus which in the rat is one of the component nuclei homologous to Onuf's nucleus in man. Notable exceptions to this generalised decline were observed in the 5-HT innervation of the sacral parasympathetic nucleus, which was maintained, and in the region of the dorsolateral motor nucleus where TH-like immunoreactivity did not significantly decline. These results suggest that the changes in micturition characteristics observed in aged rats may in part be a consequence of the alterations in, and decline of, aminergic inputs to both autonomic and somatic spinal nuclei associated with bladder function.

Serotonin inputs to inspiratory laryngeal motoneurons in the rat

Serotonergic neurons are distributed widely throughout the central nervous system and exert a tonic influence on a range of activities in relation to the sleep-wake cycle. Previous morphologic and functional studies have indicated a role for serotonin in control of laryngeal motoneurons. In the present study, we used a combination of intracellular recording, dye-filling, and immunocytochemistry in rats to demonstrate close appositions between serotonin immunoreactive boutons and posterior cricoarytenoid (PCA) and cricothyroid (CT) motoneurons, both of which are located in the nucleus ambiguus and exhibit phasic inspiratory activity. PCA motoneurons received 29 +/- 5 close appositions/neuron (mean +/- SD, n = 6), with the close appositions distributed more frequently on the distal dendrites, less frequently on the proximal dendrites, and sparsely on the axons and somata. CT motoneurons received 56 +/- 15 (n = 6), with close appositions found on both the somata and dendrites, especially proximal dendrites. Close appositions on the axons were only seen on one CT motoneuron. These results demonstrate a significant serotonin input to inspiratory laryngeal motoneurons, which is more prominent on CT compared with PCA motoneurons, and may reflect the different functional role of the muscles that they innervate during the sleep-wake cycle.

On the Distribution of GAP-43 and its Relation to Serotonin in Adult Monkey and Cat Spinal Cord and Lower Brainstem

By use of a monoclonal antibody, the distribution of growth-associated protein (GAP)-43-like immunoreactivity (LI) has been studied in the spinal cord of adult grey monkeys (Macaca fascicularis) and adult cats by use of immunofluorescence and peroxidase - antiperoxidase techniques. The brainstem was also studied with in situ hybridization histochemistry. In both monkeys and cats, a dense innervation of GAP-43-immunoreactive (IR) fibres was seen in close apposition to large cell bodies and their processes in the motor nucleus of the ventral horn. Double-labelling experiments revealed a high degree of coexistence between GAP-43- and 5-hydroxytryptamine (5-HT, serotonin)-LI in the monkey motor nucleus, while in the cat no such colocalization could be verified. At the electron microscopic level, GAP-43 labelling was seen as a coating of vesicles and axolemma inside the terminals. In both monkey and cat, cell bodies expressing mRNA encoding GAP-43 were demonstrated in the medullary midline raphe nuclei. A similar location was also encountered for mRNA for aromatic l-amino acid decarboxylase, an enzyme found in both catecholamine- and serotonin-containing neurons. The present results suggest that GAP-43 is present in the 5-HT bulbospinal pathway of the monkey. In the cat, GAP-43 mRNA-expressing cell bodies were demonstrated in areas where descending 5-HT neurons are located, but no convincing colocalization of 5-HT- and GAP-43-LI was found at spinal cord levels, despite the existence of extensive fibre networks containing either of the two compounds. Possible explanations for this species discrepancy are discussed. The function of GAP-43 in nerve terminals impinging on the motoneurons is unknown. However, it may play a role in transmitter release and/or plasticity, since such roles have been proposed for this protein in other systems.

Serotoninergic and noradrenergic axons make contacts with neurons of the ventral spinocerebellar tract in the cat

Contacts between monoaminergic fibers and electrophysiologically identified neurons of the ventral spinocerebellar tract were investigated in the cat. Five neurons were labeled intracellularly with rhodamine dextran, and monoaminergic fibers were revealed with antibodies against serotonin and dopamine beta-hydroxylase. The distribution of appositions between monoaminergic varicosities and the soma and the whole length of dendrites of these neurons was examined by using a three-channel confocal microscope. The analysis showed that close appositions between monoaminergic fibers and labeled processes occurred over the whole surface of the neurons. The highest percentage of such appositions was found on proximal dendrites, for both serotonin (37%) and noradrenaline (57%). The total number of serotoninergic contacts (66-134 per neuron) by far exceeded that of noradrenergic contacts (3-36 per neuron). Contacts between serotoninergic fibers and two neurons were analyzed by using electron microscopy. These neurons were labeled intracellularly with horseradish peroxidase, and serotoninergic varicosities were identified by immunocytochemistry. Six of 10 serially analyzed boutons in apposition to proximal dendrites were found to form morphologic synapses. The identification of the remaining four was inconclusive. These results indicate that many of the appositions seen in confocal microscopy may represent direct synaptic contacts. They also indicate that monoaminergic neurons may modulate activity of neurons of the ventral spinocerebellar tract by direct postsynaptic actions in addition to any effects evoked by means of volume transmission.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

Coupling between serotoninergic and noradrenergic neurones and gamma-motoneurones in the cat

Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to gamma-motoneurones, and serotonin (5-HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of gamma-motoneurones. Eight gamma-motoneurones were labelled with rhodamine-dextran, and 50 micrometer thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine beta-hydroxylase (DBH) and 5-HT. All the cells were directly and/or indirectly excited by muscle group II afferents from the muscle they innervated and/or other muscles. Appositions between monoaminergic fibres and the labelled somata and dendrites were located with three-colour confocal laser scanning microscopy by examining series of optical sections at 1 or 0.5 micrometer intervals. DBH and 5-HT varicosities formed appositions with the somata and dendrites of all the gamma-motoneurones. The mean packing densities for 5-HT (1.12 +/- 0.11 appositions per 100 micrometer(2) for somata and 0.91 +/- 0.07 per 100 micrometer(2) for dendrites) were similar to the densities of contacts reported for alpha-motoneurones. Monoaminergic varicosities in apposition to dendrites greatly outnumbered those on the somata. The density of DBH appositions was consistently lower - corresponding means were 53% and 62% of those for 5-HT on the somata and dendrites, respectively. It is concluded from an analysis of the distribution and density of varicosities in apposition to the gamma-motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of gamma-motoneurones.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Serotoninergic and noradrenergic axonal contacts associated with premotor interneurons in spinal pathways from group II muscle afferents

We investigated the possibility that monoaminergic axons make contacts with spinal interneurons which project to motor nuclei and are monosynaptically activated by group II muscle afferents. Interneurons in midlumbar spinal segments of adult cats were characterized electrophysiologically and intracellularly labelled with tetramethylrhodamine dextran. Serotoninergic and noradrenergic axons were identified with immunofluorescence in sections containing labelled cells. Contacts between monoaminergic axons and interneurons were investigated with three-colour confocal laser scanning microscopy and analysed with a computer reconstruction program. Cell bodies and dendritic trees of five cells were reconstructed and putative contacts were plotted. The average number of contacts formed by serotoninergic axons was 140 and the average number of noradrenergic contacts was 38. The majority (95%) of contacts were formed with dendrites; these were distributed over the entire dendritic tree, even on the most distal branches. These findings provide a morphological basis for the modulatory actions of monoamines on premotor spinal interneurons in pathways from group II muscle afferents.

Ultrastructural anatomy of physiologically identified jaw-muscle spindle afferent terminations onto retrogradely labeled jaw-elevator motoneurons in the rat

Neuronal microcircuits involving jaw-muscle spindle afferents and jaw-elevator motoneurons were studied via retrograde and intracellular labeling in rats. Initially, trigeminal motoneurons were retrogradely labeled from horseradish peroxidase (HRP) injections into the temporalis and masseter muscles. The intracellular response of jaw-muscle spindle afferent neurons was then characterized during palpation, ramp and hold, and sinusoidal stretching of the jaw-closing muscles. Biotinamide was injected into these neurons, and the tissue was processed for the visualization of HRP and biotinamide. The ultrastructure of 243 intracellularly stained jaw-muscle spindle afferent boutons located within the trigeminal motor nucleus (Vmo) was examined. Eighty-five of these boutons synapsed with motoneurons retrogradely labeled with HRP, and 158 boutons synapsed with unlabeled structures within the Vmo. All spindle afferent boutons contained clear, spherical synaptic vesicles. Although the majority of boutons were S type, a few labeled jaw-muscle spindle afferent boutons possessed a long, narrow cleft, with a subsynaptic cistern comparable to previous descriptions of C-type boutons. Sixty-eight percent of spindle afferent boutons synapsed with large or medium-sized, retrogradely labeled motoneuron dendrites, and 32% synapsed with retrogradely labeled somata. In numerous instances, spindle afferent boutons synapsed with trigeminal motoneuron dendritic or somatic spines. Most of the synapses between spindle afferent boutons and trigeminal motoneuron dendrites were asymmetric, and the greatest percentage of axosomatic synapses between spindle afferents and trigeminal motoneurons were symmetric. Approximately 24% of spindle afferent boutons constituted the intermediate element of a axoaxodendritic or axoaxosomatic assemblage, implying that some jaw-muscle spindle afferent synapses with trigeminal motoneurons are presynaptically modulated.

Pharmacological characteristics and regulation of 5-HT receptor-stimulated phosphoinositide hydrolysis in the rat spinal cord

In slices from immature rat spinal cord, both 5-hydroxytryptamine (5-HT) and the 5-HT2A/C receptor agonists (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and alpha-methyl-5-HT (alpha-Me-5-HT) stimulate phosphoinositide (PI) hydrolysis. PI breakdown is also increased by the 5-HT3 receptor agonist 2-Me-5-HT but not by phenylbiguanide. The effect of either 5-HT or DOI is blocked by selective 5-HT2A receptor antagonists such as spiperone and ketanserin and more markedly by mixed 5-HT2 receptor antagonists, such as ritanserin, methysergide and mesulergine, with higher affinity at the 2C subtype. The effect of 2-Me-5-HT is blocked by 5-HT2 and not by 5-HT3 receptor antagonists, indicating that 5-HT3 receptors do not directly or indirectly take part in PI hydrolysis in the spinal cord. Moreover, lesion with neonatal capsaicin of thin primary afferents to the dorsal spinal cord enhances inositol phosphate formation stimulated by 5-HT or DOI but not by 2-Me-5-HT. This lesion also increases 5-HT2A and 5-HT2C receptor density. After neonatal injection of 5,7-dihydroxytryptamine, which results in a marked loss of 5-HT content in the cord, 5-HT and 5-HT2 receptor agonists also enhance PI breakdown without a concomitant change in receptor number. The results suggest that the 5-HT-stimulated PI response in the rat spinal cord is associated only with the 5-HT2 receptor class, in particular with the 5-HT2C subtype.

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.

Autoradiographic localization of neurotransmitter binding sites in the hypoglossal and motor trigeminal nuclei of the rat

The hypoglossal and motor trigeminal nuclei contain somatic motoneurons innervating the tongue, jaw, and palate. These two cranial motor nuclei are myotopically organized and contain neurotransmitter binding sites for thyrotropin-releasing hormone, substance P, and serotonin. Quantitative autoradiography was used to localize thyrotropin-releasing hormone, substance P, and serotonin-1A and serotonin-1B binding sites in the hypoglossal and motor trigeminal nuclei and to relate the relative distributions of these binding sites to the myotopic organizations of the two nuclei. In the hypoglossal nucleus, high-to-moderate concentrations of all four binding sites were present in the dorsal and ventromedial subnuclei, whereas low concentrations were noted in the ventrolateral subnucleus. In the motor trigeminal nucleus, high concentrations of serotonin-1B, moderate densities of thyrotropin-releasing hormone, and low levels of substance P and serotonin-1A binding sites were present in both the ventromedial and dorsolateral subnuclei. These observations demonstrate that neurotransmitter binding sites in the hypoglossal and motor trigeminal nuclei are heterogeneously localized and that their distributions correspond to the previously described myotopic organizations of each nucleus.

Nucleus raphe obscurus (nRO) regulation of anorectal motility in rats

Previous research has demonstrated that anorectal contractions in the rat are modulated by activation of spinal autonomic circuits. In the present study, anterograde tracing of descending pathways originating from the caudal nucleus raphe obscurus (nRO) revealed that this nucleus projects to cells within the intermediolateral (IML) cell column of the thoracic cord and the sacral parasympathetic nucleus (SPN). These anatomical studies suggested that the nRO may influence the regulation of spinal reflexes of the pelvic floor. In a second set of experiments, acute rat preparations were used to investigate changes in anorectal motility during electrical stimulation of the nRO. Anorectal contractions were measured by a fluid-filled manometer. Electrical stimulation of the nRO significantly reduced spontaneous anorectal activity when compared to baseline contractions recorded for 1 min prior to stimulation. Stimulation sites outside the nRO did not affect anorectal contractions when compared to either (a) the 1-min pre-stimulation baseline for that site or (b) the 1-min stimulation period for sites within the nRO. Stimulation of caudal portions of the nRO were more likely than the rostral nRO to reduce anorectal contractions. Given that the SPN contains preganglionic neurons which may be involved in control of anorectal contractions (mediated via the pelvic nerve), the studies presented here suggest a functional role for nRO regulation of preganglionic motoneurons innervating the distal gut of the rat.

Differential effects of intrathecal thyrotropin-releasing hormone (TRH) on perineal reflexes in male rats

The effects of thyrotropin-releasing hormone (TRH) on the sexual and defecatory reflexes regulated by pudendal motoneurons were investigated. Intrathecal TRH (10 microliters volume; 0.0, 0.01, 1.0 or 100 microM concentration) at lumbosacral spinal segments (L4-S1) in acute preparations produced a dose-dependent increase in external anal sphincter (EAS), but not bulbospongiosus (BS), electromyographic (EMG) activity. Intraspinal (L6) injection of 100 microM TRH (1 microliter/micropipette), significantly increased EAS EMG activity in acute preparations. Electromyographic activity of the BS muscle was unchanged. All doses of intrathecal TRH (10 microliters volume; 0, 10, 50, 100, or 500 microM concentration) in awake animals significantly reduced the proportion of responders to a penile reflex test. Subsequently, all measures of penile reflexes were significantly reduced. Glans tumescence and defecation bouts before or during penile reflex testing were unaffected by intrathecal TRH as were indices of behavioral and motor hyper-reactivity analogous to that produced by serotonin. These data indicate that pudendal motoneurons, in the dorsomedial nucleus, are differentially regulated by neuropeptides present in the lumbosacral spinal cord.

Changes in receptor levels for thyrotropin releasing hormone, serotonin, and substance P in cervical spinal cord of Wobbler mouse: a quantitative autoradiography study during early and late stages of the motoneuron disease

Receptor levels for thyrotropin releasing hormone (TRH) measured by quantitative autoradiography in the Wobbler mouse cervical spinal cord show receptor losses that may relate to the inherited loss of motoneurons, most pronounced late (at Stage 4) in the motoneuron disease. An age-related decrease of TRH and serotonin (5-HT) receptors can be seen in the ventral horn of the control specimens (normal phenotype littermate and wild-type alike). However, this pattern is missing for substance P (SP) receptors from the wild-type specimens. Therefore the age-related decrease of SP receptors detected in the Wobbler mouse strain may identify a strain-related defect in SP neuronal/receptor developmental patterns. A higher level of TRH receptors was measured in the Wobbler dorsal horn at an early stage (Stage 1) in the motoneuron disease compared with the control specimens. The data are discussed in relation to an aberrant neuronal sprouting that occurs around the degenerating motoneurons in the ventral horn during the course of the motoneuron disease.

Met-enkephalin inhibits 5-hydroxytryptamine release from the rat ventral spinal cord via delta opioid receptors

The effect of opioid receptor agonists and antagonists on the electrically evoked release of endogenous serotonin (5-hydroxytryptamine, 5-HT) was studied in superfused slices of the rat ventral lumbar spinal cord. Met-ENK (1 x 10(-8)M-1 x 10(-6)M) and DPDPE (1 x 10(-8)M-1 x 10(-6)M) reduced the evoked 5-Ht release in a concentration dependent fashion. DAMGO (1 x 10(-8)-1 x 10(-6)) and (-)-trans-(1S,2S)-U-50488 (1 x 10(-6)M) had no effect on the 5-HT release. The inhibitory effect of met-ENK was completely abolished by ICI-174,864, but neither by naloxonazine nor nor-binaltorphimine. Following i.c.v. treatment with 5,7-dihydroxytryptamine (5,7-DHT), the tissue concentration of 5-HT was reduced by 97%, whereas the concentration of noradrenaline was reduced by only 5%. The tissue concentration of met-ENK, as measured by radioimmunoassay, was not significantly altered. The results suggest that met-ENK is present in the rat ventral spinal cord mainly in non-serotonergic nerve terminals and exerts an inhibitory action on 5-HT release via delta opioid receptors.

Qualitative and quantitative analysis of glycine- and GABA-immunoreactive nerve terminals on motoneuron cell bodies in the cat spinal cord: a postembedding electron microscopic study

The distribution of glycine- and gamma-aminobutyric acid (GABA)-like immunoreactivity (LI) in nerve terminals on the cell soma of motoneurons in the aldehyde-fixed cat L7 spinal cord was examined using postembedding immunogold histochemistry in serial ultrathin sections. Quantitative examination of 405 terminals on eight neurons of alpha-motoneuron size in the L7 motor nuclei from one animal was performed. A majority of the terminals (69%) were immunoreactive to glycine and/or GABA. These terminals contained flat or oval synaptic vesicles, thus classifying them as F type or as C type in one case. In no case was a type-F terminal unlabeled for both glycine and GABA. Most of the immunolabeled terminals were immunoreactive to glycine only (62.5%), whereas 35.4% contained both glycine- and GABA-LI. A very small number of immunolabeled terminals (2%) were immunoreactive to GABA only. In those terminals, where glycine- and GABA-LI coexisted, the gold particle density for each amino acid was only half of that seen in boutons containing only one of the two amino acids. The involvement of glycine and GABA in postsynaptic inhibition of spinal alpha-motoneurons is discussed, with particular reference to the possibility that these two inhibitory amino acids may be coreleased from a significant proportion of the nerve terminals impinging on the cell bodies.

Effect of muscle denervation on the expression of substance P in the ventral raphe-spinal pathway of the rat

The medullary raphe nuclei, wherein serotonin (5-HT) coexists with substance P (SP) and thyrotropin-releasing hormone (TRH), innervate lower motor neurons in the spinal cord ventral horn by means of the ventral raphe-spinal pathway. Destruction of the ventral raphe-spinal pathway is associated with deficient recovery of denervated muscle, indicating that it may exert a trophic effect upon lower motor neurons. To determine whether SP could be a trophic factor for lower motor neurons within the ventral raphe-spinal pathway, the effect of muscle denervation with botulinum toxin type A on SP-encoding beta-preprotachykinin mRNA in the rat medullary raphe was examined by in situ hybridization histochemistry. Silver grain density over hybridized medullary raphe neurons was increased by up to 11%, although the number of hybridized neurons did not change in denervated as compared to control rats. Increased SP gene expression in the medullary raphe in response to motor unit lesioning suggests that raphe-spinal SP may be trophic to lower motor neurons.

Thyrotropin-releasing hormone (TRH) and CNS regulation of anorectal motility in the rat

The effect of thyrotropin-releasing hormone (TRH) upon anorectal motility was investigated in acute male rat preparations. Micromolar doses of TRH were intrathecally (i.t.) infused at the L6 spinal level at a rate of 1 microliter/min over 8 min. TRH infusions in 1.0-1000 microM concentrations elicited biphasic, dose-dependent anorectal contractions as measured by a rectal manometer. The 100 microM dose yielded the most significant increase in contractions over the greatest period of time. Atropine, administered as a pretreatment (100 micrograms s.c.), blocked contractions normally produced by i.t. infusion of TRH (1000 microM). Intravenous infusions of atropine (10 micrograms) through a jugular catheter immediately blocked anorectal contractions produced by i.t. infusion of 100 microM TRH. Sectioning of the hypogastric nerve, which supplies sympathetic innervation to the colon and internal anal sphincter, did not significantly affect contractions induced by 100 microM TRH applied intrathecally. Disruption of the major pelvic ganglion fibers, however, completely abolished the contractions induced by 100 microM TRH, either through the interruption of preganglionic parasympathetic fibers in the pelvic nerve, or by disrupting postganglionic fibers. These findings extend the role of TRH in the regulation of defecatory behaviors.

Thyrotropin-releasing hormone-immunoreactive varicosities synapse on rat phrenic motoneurons

The relationship between retrogradely labelled or intracellularly filled phrenic motoneurons and varicosities containing thyrotropin-releasing hormone immunoreactivity was investigated in rats by light and electron microscopy. Phrenic motoneurons were identified via retrograde tracing from the diaphragm with cholera toxin B subunit, which was followed by immunocytochemistry to visualise retrogradely labelled motoneurons and thyrotropin-releasing hormone-immunoreactive nerve fibres in their vicinity. At the light microscopic level, varicose thyrotropin-releasing hormone-immunoreactive nerve fibres were distributed sparsely in the phrenic motor nucleus, with some axons surrounding retrogradely labelled motoneurons. In separate intracellular experiments, four phrenic motoneurons identified by antidromic activation from the C5 phrenic nerve root were subsequently filled with Neurobiotin, and nerve fibres that contained thyrotropin-releasing hormone immunoreactivity were identified by immunocytochemistry. The numbers and locations of thyrotropin-releasing hormone-immunoreactive varicosities that were closely appeared to the intracellularly labelled motoneurons were mapped using a camera lucida technique. Close appositions by thyrotropin-releasing hormone-immunoreactive varicosities were seen on somata as well as on proximal and distal dendrites. The closely apposed varicosities were usually present in tight clusters, which were formed by single varicose axons. However, the distribution was nonuniform, in that some dendrites did not receive any close appositions. Ultrastructural analysis of random ultrathin sections through retrogradely labelled neurons showed that varicosities with thyrotropin-releasing hormone immunoreactivity made 1.8% of all synapses and direct contacts on somata and 2.3% of synapses and contacts with dendrites of the retrogradely labelled phrenic motoneurons. The results of these experiments suggest that thyrotropin-releasing hormone-immunoreactive varicosities provide similar numbers of inputs to both the somata and dendrites of phrenic motoneurons. These thyrotropin-releasing hormone-containing inputs seen via light and electron microscopy could modulate the excitability of phrenic motoneurons.

Effects of serotonin on dorsal horn dorsal spinocerebellar tract neurons

Effects of ionophoretic application of serotonin and of one of its agonists were tested on responses of dorsal horn dorsal spinocerebellar tract neurons evoked by electrical stimulation of peripheral nerves. Both drugs depressed monosynaptically evoked actions of group II muscle afferents; they decreased the number and/or increased the latency of spike potentials evoked by these afferents. In contrast, synaptic actions of low-threshold cutaneous afferents (mono- or oligosynaptic) were facilitated in the majority of the neurons, as judged by decrease in the latency of spike potentials evoked by stimulation of a cutaneous nerve and/or an increase in the number of these potentials. It is proposed that facilitatory actions assist in maintaining tonic discharges of dorsal spinocerebellar tract neurons in some movements and that the selective control of group II input is used to correlate activity of spinal and supraspinal neurons. Both actions may be subserved by tight contacts between serotoninergic nerve fibres and dorsal spinocerebellar tract neurons, which have been revealed in a parallel study.

Use of NK1 receptor antagonists in the exploration of physiological functions of substance P and neurokinin A

Tachykinin NK1 receptor antagonists were used to explore the physiological functions of substance P (SP) and neurokinin A (NKA). Pharmacological profiles of three NK1 receptor antagonists, GR71251, GR82334, and RP 67580, were examined in the isolated spinal cord preparation of the neonatal rat. These tachykinin receptor antagonists exhibited considerable specificities and antagonized the actions of both SP and NKA to induce the depolarization of ventral roots. Electrical stimulation of the saphenous nerve with C-fiber strength evoked a depolarization lasting about 30 s of the ipsilateral L3 ventral root. This response, which is referred to as saphenous-nerve-evoked slow ventral root potential (VRP), was depressed by these NK1 receptor antagonists. In contrast, the saphenous-nerve-evoked slow VRP was potentiated by application of a mixture of peptidase inhibitors, including thiorphan, actinonin, and captopril in the presence of naloxone, but not after further addition of GR71251. Likewise, in the isolated coeliac ganglion of the guinea pig, electrical stimulation of the mesenteric nerves evoked in some ganglionic cells slow excitatory postsynaptic potentials (EPSPs), which were depressed by GR71251 and potentiated by peptidase inhibitors. These results further support the notion that SP and NKA serve as neurotransmitters producing slow EPSPs in the neonatal rat spinal cord and guinea pig prevertebral ganglia.

Contacts between serotoninergic fibres and dorsal horn spinocerebellar tract neurons in the cat and rat: a confocal microscopic study

Contacts between serotoninergic nerve fibres and dorsal horn dorsal spinocerebellar tract neurons were analysed in order to investigate the morphological basis of actions of serotonin upon dorsal spinocerebellar tract neurons. In a series of experiments dorsal spinocerebellar tract neurons were labelled with intracellularly injected rhodamine-dextran in the cat. The neurons were monosynaptically excited by group II muscle afferents and cutaneous afferents and were identified by antidromic activation following stimuli applied in the cerebellum. In the second series of experiments dorsal spinocerebellar tract neurons were labelled by retrograde transport of Fluorogold injected into the cerebellum in the rat. In both series, serotoninergic fibres were labelled by using a specific anti-serotonin antiserum and were revealed by immunofluorescence. Appositions between the serotoninergic fibres and the cells were inspected with a dual channel confocal microscope. The merged images obtained with the two channels of the microscope were viewed in single optical planes 2 microns apart and in rotated three-dimensional reconstructions. Serotoninergic nerve fibres were found in apposition to cell bodies of all feline dorsal spinocerebellar tract neurons (n = 7) and of 75% of rat dorsal spinocerebellar tract neurons (n = 90). The numbers of putative contacts on cell bodies varied between less than 100 and nearly 300 (mean 160) in the cat and between about five and 30 in the rat. Contacts with dendrites of feline neurons were seen on 96% of 72 dendrites within 300 microns from soma and on 91% of 23 dendrites at distances of 300-500 microns. The number of such contacts varied from less than five to 150 on a single dendrite within these ranges of distances. Their total number within 100 microns from the soma was comparable or exceeded the number of contacts on the soma.

Tachykininergic slow depolarization of motoneurones evoked by descending fibres in the neonatal rat spinal cord

1. In the isolated spinal cord of the neonatal rat, repetitive electrical stimulation of the upper cervical region elicited a prolonged depolarization of lumbar motoneurones (L3-5) lasting 1-2 min, which was recorded extracellularly from ventral roots, or intracellularly. 2. This depolarizing response was markedly depressed by the excitatory amino acid receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (D-APV, 30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). The remaining response was further depressed by a 5-hydroxytryptamine (5-HT) receptor antagonist, ketanserin (3 microM). 3. In the presence of these antagonists, a small part of the depolarizing response of slow time course remained, and this response was partially blocked by the tachykinin NK1 receptor antagonists GR71251 (0.3-5 microM) and RP67580 (0.3-1 microM). In contrast, RP68651 (0.3-1 microM), the inactive enantiomer of RP67580, had no effect on the depolarizing response. 4. The slow depolarizing response in the presence of D-APV, CNQX and ketanserin was markedly potentiated by a peptidase inhibitor, thiorphan (1 microM). 5. This descending fibre-evoked slow depolarization became smaller after prolonged treatment (5-7 h) with 5,7-dihydroxytryptamine (10 microM), a neurotoxin for 5-HT neurones. Under such conditions, the effects of thiorphan and GR71251 on the slow depolarization were virtually absent. 6. Under the action of D-APV, CNQX and ketanserin, applications of tachykinins, substance P and neurokinin A produced depolarizing responses of lumbar motoneurones, and the responses were depressed by GR71251 and potentiated by thiorphan. 7. These results suggest that tachykinins contained in serotonergic fibres serve as neurotransmitters mediating the descending fibre-evoked slow excitatory postsynaptic potentials in motoneurones.

5-HT1A receptor localization on the axon hillock of cervical spinal motoneurons in primates

Serotonin (5-HT) has direct and specific effects on the activity of spinal cord motoneurons. The 5-HT1A receptor has been shown to mediate motoneuron responses in spinal reflex pathways using the highly selective 5-HT1A receptor agonist 8-OH-DPAT. We have developed an antipeptide antibody that recognizes a specific region (the second external loop) of the 5-HT1A receptor. This 5-HT1A receptor antibody labels populations of neurons and glia in the primate cervical spinal cord. The highest receptor density is present in the superficial lamina of the dorsal horn, around the central canal, and on the axon hillock of large ventral horn motoneurons. The cellular labeling pattern on motoneurons shows a single, densely stained, tapering process emanating from the perikaryon. A more diffuse label is also present throughout the soma. Dendritic labeling was not apparent. These results suggest that post-synaptic 5-HT1A receptors may be involved in modulating spinal motoneuron activity at the key site of action potential initiation, the axon hillock.

Enkephalin-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal innervation of the ventrolateral dendritic bundle in the cat sacral spinal cord: an ultrastructural study

The distribution and synaptic arrangement of thyrotropin-releasing hormone-, substance P- and enkephalin-immunoreactive axonal boutons have been studied in the ventrolateral nucleus (Onuf's nucleus) of the upper sacral spinal cord segments in the cat. For this purpose, the peroxidase-antiperoxidase immunohistochemical technique was used. Immunoreactive axonal boutons were traced in complete series of sections in order to reveal synaptic contacts with the bundled dendrites of the ventrolateral nucleus. As judged from the cross-sectional diameter of the postsynaptic dendrites, the distribution of immunoreactive boutons was non-random. Enkephalin-immunoreactive axonal boutons, presumed to be mostly of segmental origin, displayed a rather restricted distribution to mainly (> 80%) medium-to-large dendrites. Thyrotropin-releasing hormone-immunoreactive boutons, that derive from supraspinal levels, were also found to impinge on medium-to-large dendrites (> 80%), indicating a proximal location within the dendritic trees. The skewness toward large postsynaptic dendrites was even more marked for thyrotropin-releasing hormone- than for enkephalin-immunoreactive boutons. Substance P-immunoreactive boutons, that are of either supraspinal or spinal origin, showed a more even distribution throughout the dendritic trees, including both thin distal branches and thick proximal dendrites. In view of the well-known fact that virtually all thyrotropin-releasing hormone-immunoreactive boutons in the ventral horn co-contain substance P (and serotonin) it was assumed that substance P-immunoreactive boutons in synaptic contact with the finest-calibre dendrites as well as most of those with a very proximal juxtasomatic location on the dendritic trees were of segmental origin, while those impinging on medium-to-large dendrites could be of either spinal or supraspinal origin. Fine-calibre dendrites (< 1 micron) represent about 25% of the dendritic branches in the ventrolateral nucleus, but receive, with the exception of substance P (8%), very little (< 3%) peptidergic or GABAergic (Ramírez-León and Ulfhake, 1993) input, although the degree of dendritic membrane covering by bouton profiles in the ventrolateral nucleus does not seem to vary much with the calibre of the postsynaptic dendrite (Ramírez-León and Ulfhake, 1993). Both substance P- and enkephalin-immunoreactive axonal boutons established synaptic contact with more than one dendrite. Furthermore, one and the same bouton could be found in contact with two dendrites that were coupled to each other by a dendro-dendritic contact of desmosomal or puncta adherentia type.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Modulation of calcitonin gene-related peptide release from dorsal thoraco-lumbar spinal cord slices by 5-HT3 receptors

The release of endogenous calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) from slices of dorsal and ventral thoraco-lumbar spinal cord was examined using a fixed volume incubation technique and radioimmunoassay. Incubation with potassium (25-100 mM) produced a dose-related increase in basal CGRP-LI in the supernatant of dorsal slices which was calcium dependent and release was also evoked by incubation with capsaicin (10 microM). Pre-incubation with the 5-HT3 agonist 2-methyl-5-hydroxytryptamine (10(-5) M) attenuated potassium-induced release from dorsal spinal cord slices, which was prevented by additional pre-incubation with the 5-HT3 antagonist ondansetron (10(-8) M). In contrast, the low level of CGRP-LI released from ventral spinal cord slices was not significantly enhanced by incubation with either potassium or capsaicin.

The serotoninergic bulbospinal system and brainstem-spinal cord content of serotonin-, TRH-, and substance P-like immunoreactivity in the aged rat with special reference to the spinal cord motor nucleus

The 5-hydroxytryptamine (5HT) containing bulbospinal pathway was studied with immunohistochemical (IF) and chemical techniques in 2-3 and 30 months old male Sprague-Dawley rats. The coexisting neuropeptides substance P (SP), thyrotropin-releasing hormone (TRH) and galanin were also analysed. Furthermore, the expression of mRNA encoding aromatic L-amino acid decarboxylase (AADC), prepro-TRH, and preprotachykinin (prepro-SP) was analysed with in situ hybridization (ISH) in the midline raphé nuclei inthe lower brainstem. The results showed a decreased number of axonal 5HT fibers with a normal morphology in the ventral horn of the aged rat lumbosacral spinal cord, and several 5HT immunoreactive (IR) fibers with an aberrant morphology, suggestive of axonal degeneration, were intermingled. This was evident in both the dorsal and ventral horn of the spinal cord. The 5HT-IR fibers with an aberrant morphology usually also contained TRH-and/or SP- and/or galanin-like immunoreactivity (LI) in the ventral horn. These signs of degeneration were clearly less evident in the thoracic and cervical spinal cord segments. Moreover, these changes varied between aged litter-mates. This was in agreement with behavioural signs of motor disturbances, present in about 40% of the aged rats and which in all cases were confined to the hindlimbs. Chemical analyses disclosed significantly lower levels of TRH-LI and, in particular, SP-LI in both the ventral and dorsal quandrants of the spinal cord in the aged rat compared to young adults. The differences were largest in the lumbar regions of the spinal cord. Corresponding analysis of 5HT and 5-hydroxyindoleacetic acid (5HIAA) in the same tissue specimens revealed largely unaltered levels of 5HT and a slight increase in 5HIAA, indicating the possibility of an increased 5HT turnover in the aged rat spinal cord. Neurons in nucleus raphé obscurus and nucleus raphé pallidus were immunoreactive to 5HT, and after pretreatment with colchicine to TRH-, SP-, and galanin-LI as well. There was no obvious difference in number of labeled cells, or labeling intensity, between colchicine-treated young adult and aged rats, although, in the corresponding region of medulla oblongata, chemical analysis disclosed significantly lower levels of 5HT, TRH, and, in particular, SP in untreated aged rats. In contrast, in situ hybridization analysis revealed increased mRNA levels encoding prepro-TRH and prepro-SP in old rats, while mRNA content encoding AADC mRNA was similar in young adult and aged rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of the serotonergic innervation of spinal neurons in rats--III. Differential serotonergic innervation of somatic and parasympathetic preganglionic motoneurons as determined by patterns of co-existing peptides

The spinal cord is innervated by brainstem serotonergic neurons, some of which contain substance P and/or thyrotropin-releasing hormone in addition to serotonin. These neurons project at least three types of axons to the spinal cord: those containing both substance P and thyrotropin-releasing hormone, those containing thyrotropin-releasing hormone but not substance P, and those containing neither substance P nor thyrotropin-releasing hormone. However, the organization of the different types of serotonergic processes is unclear. In the present studies, the types of serotonergic axons projecting to two kinds of spinal neurons were examined. Somatic and parasympathetic preganglionic motoneurons were labeled retrogradely from the pelvic or sciatic nerve, respectively. Sections containing these neurons were stained either for serotonin and substance P, or for serotonin and thyrotropin-releasing hormone. Of a total of 428 profiles examined that were retrogradely labeled from the sciatic nerve, 425 (99%) were apposed by serotonin-immunoreactive varicosities; similarly, of a total of 382 profiles examined that were retrogradely labeled from pelvic nerve, 353 (92%) were apposed by serotonin-immunoreactive varicosities. However, differences appeared to exist between the types of serotonergic varicosities innervating these two groups of neurons. Among the profiles labeled from the sciatic nerve, it was estimated that over 97% were apposed by serotonin-immunoreactive varicosities in which serotonin co-existed with substance P and thyrotropin-releasing hormone. In contrast, among the profiles labeled from pelvic nerve that were apposed by serotonin-immunoreactive varicosities, it was estimated that less than 1% were apposed by serotonin-immunoreactive varicosities containing both thyrotropin-releasing hormone and substance P. We estimate that most of the remainder (about 80%) were apposed by serotonin-immunoreactive varicosities containing thyrotropin-releasing hormone but not substance P. We conclude that both the cell bodies of neurons retrogradely labeled from the pelvic nerve and those labeled from the sciatic nerve were apposed by serotonin varicosities. However, these two systems of neurons appear to be innervated largely by two different populations of serotonergic cells. This suggests that the raphe-spinal serotonergic system may independently modulate the activities of somatic motoneurons and parasympathetic preganglionic motoneurons.

Quantitative synaptology of functionally different types of cat medial gastrocnemius alpha-motoneurons

The aim of this ultrastructural investigation was to study quantitatively the synaptology of the cell bodies and dendrites of cat medial gastrocnemius (MG) alpha-motoneurons of functionally different types. In electrophysiologically classified and intracellularly HRP-labelled MG alpha-motoneurons of the FF (fast twitch, fatigable), FR (fast twitch, fatigue resistant) and S (slow twitch, very fatigue resistant) types, the synaptic covering of the soma as well as that of dendritic segments located within 100 microns and at 300, 700, and 1,000 microns distance, respectively from the soma, was analyzed. The synaptic boutons were classified into the L-(apposition length > 4 microns) and S-types (< 4 microns) with spherical synaptic vesicles, and the F-type with flat or pleomorphic synaptic vesicles. The length of apposition towards the motoneuron membrane was measured for each bouton profile. Approximately 1,000 boutons contacted the soma and a similar number of boutons contacted the proximal dendrites within 50 microns from the soma. The number of dendritic boutons was larger at the 300 microns distance than at the 100 and 700 microns distances. The three types of motoneurons showed similar values for percentage synaptic covering and synaptic packing density in the proximal dendrites, while in the most distal dendritic regions the S motoneurons had more than 50% higher values for percentage covering, packing density and total number of boutons. The S motoneurons also exhibited a larger preponderance of F-type boutons on the soma. The ratio between the F- and S-types of boutons decreased somatofugally along the dendrites in the type FF and FR motoneurons, while in the S motoneurons it remained fairly constant.

Pre- and post-natal ontogeny of thyrotropin-releasing-hormone in the rat spinal cord: an immunocytochemical study

This work aimed at providing by means of immunocytochemical techniques a detailed study of the ontogeny of thyrotropin-releasing hormone (TRH) in the spinal cord of the rat. We report the first appearance of TRH-immunoreactive fibers in the ventral funiculus of thoracic and lumbar levels at embryonic day 17. At embryonic day 18, fibers penetrated the ventral gray matter towards the central canal. At embryonic day 19, the first immunoreactive fibers were seen in the intermediolateral cell column at upper thoracic levels. This region was invaded at lower thoracic levels on the day of birth. At this time, TRH-immunoreactive axodendritic synapses were observed in the ventral horn and in the intermediolateral cell column. Immunoreactivity increased in these regions until post-natal day 21 when the adult pattern of TRH immunoreactivity was established in the sympathetic nuclei and in the ventral horn. However, a transient TRH-like immunoreactivity was detected in lamina IIi of the dorsal horn between post-natal days 14 and 30: at ultrastructural level, immunoreactive varicosities were seen to establish axodendritic synapses. In conclusion, TRH is one of the earliest peptidergic systems established in the spinal cord and it presents extensive temporal and topographical similarities with the serotonergic system with which it could be colocalized.

Distribution of thyrotropin-releasing hormone receptor messenger RNA in the rat brain: an in situ hybridization study

Based on the recent cloning of the mouse thyrotropin-releasing hormone receptor, oligonucleotide probes complementary to the DNA sequence were constructed and used for in situ hybridization studies on the rat brain. Thyrotropin-releasing hormone receptor messenger RNA was found in many areas of the brain, mostly showing high degree of overlap with the distribution thyrotropin-releasing hormone binding sites as previously revealed in autoradiographic studies. Thus, a strong signal was observed in the accessory olfactory bulb, the perirhinal sulcus, the ventral aspects of the hippocampal formation, some amygdaloid nuclei, the diagonal band nucleus, parts of nucleus accumbens, the bed nucleus of the stria terminalis, dorsomedial, lateral and perifornical hypothalamic regions, the septohippocampal nucleus, parts of the vestibular complex, as well as many bulbar motoneurons including the facial, dorsal vagal, ambiguus and hypoglossal nuclei, the superficial layer of the spinal trigeminal nucleus, and motoneurons and dorsal horn neurons in the spinal cord. Cells within one and the same nucleus expressed varying levels of thyrotropin releasing hormone receptor messenger RNA suggesting marked differences in rate of receptor synthesis. Most of these areas receive an input by thyrotropin-releasing hormone-positive nerve endings. Taken together these results suggest that thyrotropin-releasing hormone receptors are mostly localized in the vicinity of the cell bodies which express thyrotropin-releasing hormone receptor messenger RNA and mediate the wide range of actions that have been recorded after administration of exogenous thyrotropin-releasing hormone.