Synaptic relationships between serotonin-immunoreactive axons and dorsal horn spinocerebellar tract cells in the cat spinal cord

Locomotor-activated neurons of the cat. I. Serotonergic innervation and co-localization of 5-HT7, 5-HT2A, and 5-HT1A receptors in the thoraco-lumbar spinal cord

Monoamines are strong modulators and/or activators of spinal locomotor networks. Thus monoaminergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the serotonergic innervation of locomotor-activated neurons within the thoraco-lumbar spinal cord following induction of hindlimb locomotion. This was determined by immunohistochemical co-localization of serotonin (5-HT) fibers or 5-HT(7)/5-HT2A/5-HT1A receptors with cells expressing the activity-dependent marker c-fos. Experiments were performed on paralyzed, decerebrate cats in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. Abundant c-fos immunoreactive cells were observed in laminae VII and VIII throughout the thoraco-lumbar segments of locomotor animals. Control sections from the same segments showed significantly fewer labeled neurons, mostly within the dorsal horn. Multiple serotonergic boutons were found in close apposition to the majority (80-100%) of locomotor cells, which were most abundant in lumbar segments L3-7. 5-HT7 receptor immunoreactivity was observed on cells across the thoraco-lumbar segments (T7-L7), in a dorsoventral gradient. Most locomotor-activated cells co-localized with 5-HT7, 5-HT2A, and 5-HT1A receptors, with largest numbers in laminae VII and VIII. Co-localization of c-fos and 5-HT7 receptor was highest in the L5-L7 segments (>90%) and decreased rostrally (to approximately 50%) due to the absence of receptors on cells within the intermediolateral nucleus. In contrast, 60-80 and 35-80% of c-fos immunoreactive cells stained positive for 5-HT2A and 5-HT1A receptors, respectively, with no rostrocaudal gradient. These results indicate that serotonergic modulation of locomotion likely involves 5-HT(7)/5-HT2A/5-HT1A receptors located on the soma and proximal dendrites of serotonergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments.

Genetically expressed transneuronal tracer reveals direct and indirect serotonergic descending control circuits

Despite the evidence for a significant contribution of brainstem serotonergic (5HT) systems to the control of spinal cord "pain" transmission neurons, attention has turned recently to the influence of nonserotonergic neurons, including the facilitatory and inhibitory controls that originate from so-called "on" and "off" cells of the rostroventral medulla (RVM). Unclear, however, is the extent to which these latter circuits interact with or are influenced by the serotonergic cell groups. To address this question we selectively targeted expression of a transneuronal tracer, wheat germ agglutinin (WGA), in the 5HT neurons so as to study the interplay between the 5HT and non-5HT systems. In addition to confirming the direct medullary 5HT projection to the spinal cord we also observed large numbers of non-5HT neurons, in the medullary nucleus reticularis gigantocellularis and magnocellularis, that were WGA-immunoreactive, i.e., were transneuronally labeled from 5HT neurons. FluoroGold injections into the spinal cord established that these reticular neurons are not only postsynaptic to the 5HT neurons of the medulla, but that most are also at the origin of descending, bulbospinal pathways. By contrast, we found no evidence that neurons of the midbrain periaqueductal gray that project to the RVM are postsynaptic to midbrain or medullary 5HT neurons. Finally, we found very few examples of WGA-immunoreactive noradrenergic neurons, which suggests that there is considerable independence of the monoaminergic bulbospinal pathways. Our results indicate that 5HT neurons influence "pain" processing at the spinal cord level both directly and indirectly via feedforward connections with multiple non-5HT descending control pathways.

Serotonin inputs to laryngeal constrictor motoneurons in the rat

OBJECTIVES/HYPOTHESIS

The objective was to demonstrate close appositions between serotonin-immunoreactive boutons and laryngeal constrictor (LCon) motoneurons in Sprague-Dawley rats.

STUDY DESIGN

Animal experimental.

METHODS

LCon motoneurons were identified functionally by their antidromic responses to stimulation of the recurrent laryngeal nerve and postinspiratory modulation and were filled by intracellular injection of biotin amide (n = 6). The medulla was sectioned and, using immunohistochemical analysis, examined by light microscopy.

RESULTS

Serotonin appositions were found on all 6 LCon motoneurons, with an average number of 17 +/- 6 close appositions per neuron.

CONCLUSION

In comparison with the authors' previous study of inspiratory laryngeal motoneurons, the number of serotonin close appositions with LCon motoneurons was similar to that found with posterior cricoarytenoid motoneurons, but significantly less than that found with cricothyroid motoneurons. This finding may represent a basis for differences in tonic activity of laryngeal muscles observed in relation to the sleep-wake cycle.

Modulation of dorsal spinocerebellar responses to limb movement. I. Effect of serotonin

Spinocerebellar neurons (DSCT) receive converging sensory information from various sensory receptors in the hindlimbs and lower trunk. Previous studies have shown that sensory processing by DSCT neurons results in a representation of global hindlimb kinematic parameters such as the length and the orientation of the limb axis. In addition to the sensory input, the DSCT circuitry also receives a descending input from the raphe nuclei in the brain stem. Recent studies have demonstrated that the raphe serotonergic terminals synapse directly on DSCT neurons and exert a differential modulatory influence on their sensory inputs. We examined the role of serotonergic modulation on the DSCT representation of hindlimb kinematic parameters by recording DSCT activity during passive hindlimb movements before and after perturbing serotonergic transmission. We used two types of perturbation: electrical stimulation of the raphe areas in the brain stem to release serotonin in the spinal cord (42 neurons) and intravenous administration of serotonergic agonists or antagonists, mostly the 5HTP2 antagonist ketanserin (30 neurons). We found that movement responses were altered in approximately 70% of the DSCT units studied with each protocol. Changes could include shifts in mean firing rate, increases or decreases in response amplitude, and changes in response waveform. We used a principal component analysis (PCA) to examine waveform components and to determine how they contributed to the response waveform changes caused by serotonin perturbation. Such changes could be explained by new or different response components that might indicate a modification in the data processing or by a different weighting of existing components that might indicate a modification of synaptic weighting. The results were consistent with the second alternative. We found that the same underlying response components could account for both control responses and those altered by serotonin perturbations. The observed changes in waveform could be entirely accounted for by a re-weighting of response components. In particular, the changes observed after raphe stimulation could be accounted for by selective changes in the weighting of the first principal component (PC) with only minor changes of the weighting of the second PC. Because these response components were shown previously to correlate with the limb axis orientation and length trajectories respectively, the finding is consistent with the idea that limb axis length and orientation information are processed separately within the spinal circuitry.

Substance P inputs to laryngeal motoneurons in the rat

Substance P terminals have previously been demonstrated around retrogradely labelled posterior cricoarytenoid (PCA) motoneurons, but little is known regarding substance P inputs to other functionally identified laryngeal motoneurons. In the present study, we determined the number and distribution of close appositions between substance P immunoreactive boutons and three types of laryngeal motoneuron by using a combination of intracellular recording, dye-filling and immunocytochemistry in the rat. Cricothyroid (CT) motoneurons received 15+/-5 substance P appositions/neuron (mean+/-S.D., n = 6), PCA motoneurons received 13+/-5 (n = 6), and laryngeal constrictor (LCS) motoneurons received 11+/-4 (n = 5). In contrast to our previous finding of a preferential serotonin innervation of CT motoneurons, we found no significant difference between the substance P inputs to CT, PCA and LCS motoneurons. Our results indicate a modest role for substance P in control of laryngeal motoneuronal 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.

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.

Spinal interneuronal systems: identification, multifunctional character and reconfigurations in mammals

This review focuses on the flexibility of operation of spinal interneuronal networks and their multifunctional character in mammals. It concerns, in particular, two ways in which spinal interneuronal networks may be functionally reorganised, namely by modulating the synaptic actions of primary afferents by monoamines and by GABAergic presynaptic inhibition. The evidence will be reviewed for topographical and target-related differences in modulatory effects in various interneuronal networks and these will be related to differences in the intrinsic properties of different functional types of interneurones in these networks and to the role played by them.

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.

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.

Serotonergic transmission in the periaqueductal gray matter in relation to aversive behaviour: morphological evidence for direct modulatory effects on identified output neurons

Intracellular recordings were made from 21 cells in the dorsolateral periaqueductal gray matter in coronal midbrain slices. In the majority (n = 20) bath application of 5-hydroxytryptamine (30 or 150 mM) evoked either hyperpolarizing (n = 11) or depolarizing (n = 9) responses. Reconstructions of 11 neurons in the dorsolateral periaqueductal gray matter after filling with biocytin revealed a population of output neurons whose axons followed a dorsolateral trajectory towards the perimeter of the ipsilateral periaqueductal gray matter. In seven cells, the axon could be followed into the adjacent mesencephalic reticular formation. At the light microscopic level, immunostaining for 5-hydroxytryptamine revealed immunoreactive processes throughout the dorsolateral periaqueductal gray matter but no labelled somata or dendrites. Close associations (i.e. no discernible gap) were observed between serotonergic profiles and the somata and dendrites of biocytin-filled cells. At the ultrastructural level, serial sections through 21 appositions on to biocytin-filled dendrites in three slices revealed 19 true appositions (i.e. having closely parallel plasma membranes with no intervening glial cell profiles) with the biocytin-filled dendrite. Only four of the appositions (21%) showed evidence of synaptic specializations which included aggregations of synaptic vesicles, and some thickening of the apposing membrane. The dense reaction product in the biocytin-filled cells precluded identification of the ultrastructure of postsynaptic elements. However, examination of contacts between 5-hydroxytryptamine-immunoreactive profiles and unlabelled elements in material taken from the contralateral side of the periaqueductal gray matter (i.e. no biocytin present) or in material taken from perfusion-fixed whole brain, in which ultrastructural preservation was superior compared with slices, revealed a similar incidence (21% and 23%, respectively) of synaptic specializations. The data indicate that serotonergic transmission on to output neurons in the dorsolateral periaqueductal gray matter is largely mediated by non-junctional contacts, suggesting that the actions of 5-hydroxytryptamine on these cells are mediated predominantly by volume rather than wiring transmission.

Serotonergic modulation of spinal ascending activity and sacral reflex activity evoked by pelvic nerve stimulation in cats

Serotonin (5-HT) may be inhibitory to micturition at a spinal level. A potential mechanism of action for serotonergic inhibition of bladder function is a depression of the ascending limb of the supraspinal reflex mediating micturition. Ascending activity evoked by pelvic nerve stimulation was recorded in the thoracic spinal cord of anesthetized cats. For comparison, spinal reflex activity evoked by pelvic nerve stimulation was recorded on the pudendal nerve. The effects of intrathecal administration of serotonergic agents were examined to determine whether spinal and supraspinal responses to bladder afferent activation were modulated by 5-HT. Methysergide (60 nmol), a non-selective serotonergic antagonist, increased ascending activity by 61+/-7% and depressed spinal reflex activity by 38+/-6%. Zatosetron (10 nmol), a 5-HT3 antagonist had a similar effect on both activities (increased by 93+/-24% and decreased by 77+/-7%, respectively). The effect on ascending activity of blocking 5-HT3 receptors was also confirmed with ICS 205930 and MDL 72222. 2-Methyl-5-HT (800 nmol), a 5-HT3 agonist, depressed ascending activity to 46+/-9% of control, but enhanced spinal reflex activity by 73+/-92%. These results demonstrate that stimulation of 5-HT3 and methysergide-sensitive 5-HT receptors can inhibit ascending activity and facilitate spinal reflex activity elicited by activation of bladder afferents. It is suggested that descending serotonergic pathways may participate in the spinal coordination of urinary continence.

Modulation of responses of four types of feline ascending tract neurons by serotonin and noradrenaline

Modulation of responses of four types of ascending tract cells by noradrenaline and serotonin was compared in order to investigate how information forwarded by these cells may be gated by monoaminergic tract neurons. Spinocervical tract, postsynaptic dorsal column and dorsal spinocerebellar tract neurons located in Clarke's column and in the dorsal horn were identified by their axonal projections. Noradrenaline and serotonin were applied ionophoretically close to a selected neuron, and their effects were tested on extracellularly recorded responses of this neuron to electrical stimulation of low-threshold skin afferents and group II muscle spindle afferents. The modulatory actions of noradrenaline and serotonin were estimated from changes in the number of responses evoked by 30 successive stimuli, the minimal latencies of these responses, and their firing frequency. All four populations of ascending tract neurons investigated were modulated by serotonin and noradrenaline, but not in the same way. The responses were most often depressed by noradrenaline and facilitated by serotonin, but in some types of neuron they were affected in the same direction. Transmission from low-threshold skin and group II muscle afferents changed in the same direction in some types of neuron but in the opposite direction in other types. The results indicate that transfer of information from skin and group II muscle afferents to supraspinal centres may be gated by descending monoaminergic pathways in a highly differentiated manner, and is adjusted to the requirements of various behavioural situations.

Afferent inputs modulate the activity of a rhythmic burst generator in the rat disinhibited spinal cord in vitro

Application of strychnine and bicuculline to the isolated spinal cord of the neonatal rat induces spontaneous bursting of regular rhythmicity (cycle period approximately 30 s). This phenomenon is important because it shows that a spinal network, made up by excitatory connections only, generates a very reliable rhythmic pattern. To find out how signals from the periphery or higher centres might influence the operation of the rhythmogenic network, the present experiments examined whether synaptic inputs from dorsal root (DR) or ventrolateral (VL) afferent fibers could modulate this spontaneous rhythmicity. This issue was addressed with intracellular recording from motoneurons or extracellular recording from ventral roots after eliciting bursting with strychnine plus bicuculline. Single electrical shocks (0.1 ms; intensity 1-4 times threshold) applied to one DR reset spontaneous bursting without altering its period or duration. Repetitive stimulations at periods ranging from 20 to 2 s entrained bursts on a 1:1 basis. Burst duration was shorter at lower stimulation periods whereas burst amplitude was unchanged. The lowest stimulation period compatible with burst entrainment depended on stimulus strength. At stimulation periods <2-s entrainment was always lost and spontaneous bursts unexpectedly returned even if electrical pulses still elicited ventral root reflexes. Such spontaneous bursts had similar properties as those recorded in the absence of electrical pulses. Analogous results were obtained with VL stimulations. It is concluded that the spinal rhythmogenic network was highly susceptible to external synaptic inputs, which paced burst generation whereas burst duration was adapted to interstimulus interval. A scheme is provided to explain the modulatory role of synaptic inputs as well as the escape of bursting from fast stimulus entrainment in terms of a rhythmogenic network functionally separated from reflex pathways activated by DR or VL tracts.