Steady-State Levels of Monoamines in the Rat Lumbar Spinal Cord: Spatial Mapping and the Effect of Acute Spinal Cord Injury

Semaphorin7A: its role in the control of serotonergic circuits and functional recovery following spinal cord injury

Serotonin is a monoamine neurotransmitter synthetized in various populations of brainstem neurons. In the spinal cord, descending serotonergic projections regulate postural muscle tone, locomotion and rhythm and coordination of movements via the Central Pattern Generator. Following a spinal cord injury, serotonergic projections to the lumbar spinal cord, where the Central Pattern Generators are located, are interrupted resulting in devastating locomotor impairments and changes in the expression and activation of serotonin and its spinal receptors. The molecular cues that control the precise patterning and targeting of serotonergic inputs onto Central Pattern Generator networks in healthy animals or after injury are still unknown. In our recent research work, we have been particularly interested in Semaphorin7A, which belongs to the Semaphorins family involved in guiding growing axons and controlling plasticity of synaptic connections. In this review, we discuss the role of Semaphorin7A signaling as an important molecular actor that instructs the patterning of serotonin inputs to spinal Central Pattern Generator networks. We show that Semaphorin7A controls the wiring of descending serotonin axons in the spinal cord. Our results reveal that mistargetting of serotonin fibers in the spinal cord is compensated in healthy uninjured Semaphorin7A deficient mice so that their gross locomotion proceeds accurately. We also demonstrate that when the system is challenged with a spinal lesion, the pattern of post-injury serotonin expression is significantly altered in Semaphorin7A deficient mice with specific ectopic targeting of serotonin fibers in the lumbar spinal cord. Compensatory mechanisms in place in uninjured Semaphorin7A deficient mice are lost and injured Semaphorin7A deficient mice exhibit a worsening of their post-injury locomotor abilities. Our findings identify Semaphorin7A as a critical determinant of serotonergic circuit formation in healthy or spinal cord injured mice.

Regional Hyperexcitability and Chronic Neuropathic Pain Following Spinal Cord Injury

Spinal cord injury (SCI) causes maladaptive changes to nociceptive synaptic circuits within the injured spinal cord. Changes also occur at remote regions including the brain stem, limbic system, cortex, and dorsal root ganglia. These maladaptive nociceptive synaptic circuits frequently cause neuronal hyperexcitability in the entire nervous system and enhance nociceptive transmission, resulting in chronic central neuropathic pain following SCI. The underlying mechanism of chronic neuropathic pain depends on the neuroanatomical structures and electrochemical communication between pre- and postsynaptic neuronal membranes, and propagation of synaptic transmission in the ascending pain pathways. In the nervous system, neurons are the only cell type that transmits nociceptive signals from peripheral receptors to supraspinal systems due to their neuroanatomical and electrophysiological properties. However, the entire range of nociceptive signaling is not mediated by any single neuron. Current literature describes regional studies of electrophysiological or neurochemical mechanisms for enhanced nociceptive transmission post-SCI, but few studies report the electrophysiological, neurochemical, and neuroanatomical changes across the entire nervous system following a regional SCI. We, along with others, have continuously described the enhanced nociceptive transmission in the spinal dorsal horn, brain stem, thalamus, and cortex in SCI-induced chronic central neuropathic pain condition, respectively. Thus, this review summarizes the current understanding of SCI-induced neuronal hyperexcitability and maladaptive nociceptive transmission in the entire nervous system that contributes to chronic central neuropathic pain.

Abnormal Circadian Modification of Aδ-Fiber Pathway Excitability in Idiopathic Restless Legs Syndrome

Restless legs syndrome (RLS) is characterized by unpleasant sensations generally localized to legs, associated with an urge to move. A likely pathogenetic mechanism is a central dopaminergic dysfunction. The exact role of pain system is unclear. The purpose of the study was to investigate the nociceptive pathways in idiopathic RLS patients. We enrolled 11 patients (mean age 53.2 ± 19.7 years; 7 men) suffering from severe, primary RLS. We recorded scalp laser-evoked potentials (LEPs) to stimulation of different sites (hands and feet) and during two different time conditions (daytime and nighttime). Finally, we compared the results with a matched control group of healthy subjects. The Aδ responses obtained from patients did not differ from those recorded from control subjects. However, the N1 and the N2-P2 amplitudes' night/day ratios after foot stimulation were increased in patients, as compared to controls (N1: patients: 133.91 ± 50.42%; controls: 83.74 ± 34.45%; p = 0.016; Aδ-N2-P2: patients: 119.15 ± 15.56%; controls: 88.42 ± 23.41%; p = 0.003). These results suggest that RLS patients present circadian modifications in the pain system, which are not present in healthy controls. Both sensory-discriminative and affective-emotional components of pain experience show parallel changes. This study confirms the structural integrity of Aδ nociceptive system in idiopathic RLS, but it also suggests that RLS patients present circadian modifications in the pain system. These findings could potentially help clinicians and contribute to identify new therapeutic approaches.

Pharmacological insights into impulsive-compulsive spectrum disorders associated with dopaminergic therapy

Impulsive-compulsive spectrum disorders are associated with dopamine agonist therapy in some patients. These untoward outcomes occur with direct-acting, full and partial agonists at D2 dopamine family receptors. The disorders typically emerge during chronic treatment, and exhibit common features that are independent of the neurological or psychiatric pathology for which the initial therapy was indicated. It is well-documented that the brain is 'plastic', changing in response to alterations to internal factors (e.g., disease processes), as well as external factors (e.g., therapies). The complexities of these clinical scenarios have eluded a clear depiction of the neurobiology for impulsive-compulsive spectrum disorders and engendered considerable debate regarding the mechanistic underpinnings of the disorders. In this opinion, we use pharmacological concepts related to homeostatic compensation subsequent to chronic receptor activation to provide a unifying construct. This construct helps explain the occurrence of impulsive-compulsive spectrum disorders across disease states, and during therapy with full and partial agonists.

Monoamine Release in the Cat Lumbar Spinal Cord during Fictive Locomotion Evoked by the Mesencephalic Locomotor Region

Spinal cord neurons active during locomotion are innervated by descending axons that release the monoamines serotonin (5-HT) and norepinephrine (NE) and these neurons express monoaminergic receptor subtypes implicated in the control of locomotion. The timing, level and spinal locations of release of these two substances during centrally-generated locomotor activity should therefore be critical to this control. These variables were measured in real time by fast-cyclic voltammetry in the decerebrate cat's lumbar spinal cord during fictive locomotion, which was evoked by electrical stimulation of the mesencephalic locomotor region (MLR) and registered as integrated activity in bilateral peripheral nerves to hindlimb muscles. Monoamine release was observed in dorsal horn (DH), intermediate zone/ventral horn (IZ/VH) and adjacent white matter (WM) during evoked locomotion. Extracellular peak levels (all sites) increased above baseline by 138 ± 232.5 nM and 35.6 ± 94.4 nM (mean ± SD) for NE and 5-HT, respectively. For both substances, release usually began prior to the onset of locomotion typically earliest in the IZ/VH and peaks were positively correlated with net activity in peripheral nerves. Monoamine levels gradually returned to baseline levels or below at the end of stimulation in most trials. Monoamine oxidase and uptake inhibitors increased the release magnitude, time-to-peak (TTP) and decline-to-baseline. These results demonstrate that spinal monoamine release is modulated on a timescale of seconds, in tandem with centrally-generated locomotion and indicate that MLR-evoked locomotor activity involves concurrent activation of descending monoaminergic and reticulospinal pathways. These gradual changes in space and time of monoamine concentrations high enough to strongly activate various receptors subtypes on locomotor activated neurons further suggest that during MLR-evoked locomotion, monoamine action is, in part, mediated by extrasynaptic neurotransmission in the spinal cord.

Restless Legs Syndrome: From Pathophysiology to Clinical Diagnosis and Management

Restless legs syndrome (RLS), a common neurological sensorimotor disorder in western countries, has gained more and more attention in Asian countries. The prevalence of RLS is higher in older people and females. RLS is most commonly related to iron deficiency, pregnancy and uremia. The RLS symptoms show a significant circadian rhythm and a close relationship to periodic limb movements (PLMs) in clinical observations, while the pathophysiological pathways are still unknown. The diagnostic criteria have been revised in 2012 to improve the validity of RLS diagnosis. Recent studies have suggested an important role of iron decrease of brain in RLS pathophysiology. Dopaminergic (DA) system dysfunction in A11 cell groups has been recognized long ago from clinical treatment and autopsy. Nowadays, it is believed that iron dysfunction can affect DA system from different pathways and opioids have a protective effect on DA system. Several susceptible single nucleotide polymorphisms such as BTBD9 and MEIS1, which are thought to be involved in embryonic neuronal development, have been reported to be associated with RLS. Several pharmacological and non-pharmacological treatment are discussed in this review. First-line treatments of RLS include DA agents and α2δ agonists. Augmentation is very common in long-term treatment of RLS which makes prevention and management of augmentation very important for RLS patients. A combination of different types of medication is effective in preventing and treating augmentation. The knowledge on RLS is still limited, the pathophysiology and better management of RLS remain to be discovered.

Effectiveness of minocycline and FK506 alone and in combination on enhanced behavioral and biochemical recovery from spinal cord injury in rats

Injury to the spinal cord results in immediate physical damage (primary injury) followed by a prolonged posttraumatic inflammatory disorder (secondary injury). The present study aimed to investigate the neuroprotective effects of minocycline and FK506 (Tacrolimus) individually and in combination on recovery from experimental spinal cord injury (SCI). Young adult male rats were subjected to experimental SCI by weight compression method. Minocycline (50mg/kg) and FK506 (1mg/kg) were administered orally in combination and individually to the SCI group daily for three weeks. During these three weeks, the recovery was measured using behavioral motor parameters (including BBB, Tarlov and other scorings) every other day for 29days after SCI. Thereafter, the animals were sacrificed and the segment of the spinal cord centered at the injury site was removed for the histopathological studies as well as for biochemical analysis of monoamines such as 5-hydroxytryptamine (5-HT) and 5-hydroxy-indolacetic acid (5-HIAA) and some oxidative stress indices, such as thiobarbituric acid-reactive substances (TBARS), total glutathione (GSH) and myeloperoxidase (MPO). All behavioral results indicated that both drugs induced significant recovery from SCI with respect to time. The biochemical and histopathological results supported the behavioral findings, revealing significant recovery in the regeneration of the injured spinal tissues, the monoamine levels, and the oxidative stress indices. Overall, the effects of the tested drugs for SCI recovery were as follows: FK506+minocycline>minocycline>FK506 in all studied parameters. Thus, minocycline and FK506 may prove to be a potential therapy cocktail to treat acute SCI. However, further studies are warranted.

The role of the serotonergic system in locomotor recovery after spinal cord injury

Serotonin (5-HT), a monoamine neurotransmitter synthesized in various populations of brainstem neurons, plays an important role in modulating the activity of spinal networks involved in vertebrate locomotion. Following spinal cord injury (SCI) there is a disruption of descending serotonergic projections to spinal motor areas, which results in a subsequent depletion in 5-HT, the dysregulation of 5-HT transporters as well as the elevated expression, super-sensitivity and/or constitutive auto-activation of specific 5-HT receptors. These changes in the serotonergic system can produce varying degrees of locomotor dysfunction through to paralysis. To date, various approaches targeting the different components of the serotonergic system have been employed to restore limb coordination and improve locomotor function in experimental models of SCI. These strategies have included pharmacological modulation of serotonergic receptors, through the administration of specific 5-HT receptor agonists, or by elevating the 5-HT precursor 5-hydroxytryptophan, which produces a global activation of all classes of 5-HT receptors. Stimulation of these receptors leads to the activation of the locomotor central pattern generator (CPG) below the site of injury to facilitate or improve the quality and frequency of movements, particularly when used in concert with the activation of other monoaminergic systems or coupled with electrical stimulation. Another approach has been to employ cell therapeutics to replace the loss of descending serotonergic input to the CPG, either through transplanted fetal brainstem 5-HT neurons at the site of injury that can supply 5-HT to below the level of the lesion or by other cell types to provide a substrate at the injury site for encouraging serotonergic axon regrowth across the lesion to the caudal spinal cord for restoring locomotion.

Anatomical and pharmacological characterization of catecholamine transients in the medial prefrontal cortex evoked by ventral tegmental area stimulation

Voltammetric measurements of catecholamines in the medial prefrontal cortex (mPFC) are infrequent because of lack of chemical selectivity between dopamine and norepinephrine and their overlapping anatomical inputs. Here, we examined the contribution of norepinephrine to the catecholamine release in the mPFC evoked by electrical stimulation of the ventral tegmental area (VTA). Initially, electrical stimulation was delivered in the midbrain at incremental depths of -5 to -9.4 mm from bregma while catecholamine release was monitored in the mPFC. Although catecholamine release was observed at dorsal stimulation sites that may correspond to the dorsal noradrenergic bundle (DNB, containing noradrenergic axonal projections to the mPFC), maximal release was evoked by stimulation of the VTA (the source of dopaminergic input to the mPFC). Next, VTA-evoked catecholamine release was monitored in the mPFC before and after knife incision of the DNB, and no significant changes in the evoked catecholamine signals were found. These data indicated that DNB fibers did not contribute to the VTA-evoked catecholamine release observed in the mPFC. Finally, while the D2-receptor antagonist raclopride significantly altered VTA-evoked catecholamine release, the α₂-adrenergic receptor antagonist idazoxan did not. Specifically, raclopride reduced catecholamine release in the mPFC, opposite to that observed in the striatum, indicating differential autoreceptor regulation of mesocortical and mesostriatal neurons. Together, these findings suggest that the catecholamine release in the mPFC arising from VTA stimulation was predominately dopaminergic rather than noradrenergic.

Locomotor-activated neurons of the cat. II. Noradrenergic innervation and colocalization with NEα 1a or NEα 2b receptors in the thoraco-lumbar spinal cord

Norepinephrine (NE) is a strong modulator and/or activator of spinal locomotor networks. Thus noradrenergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the noradrenergic innervation of functionally related, locomotor-activated neurons within the thoraco-lumbar spinal cord. This was accomplished by immunohistochemical colocalization of noradrenergic fibers using dopamine-β-hydroxylase or NEα(1A) and NEα(2B) receptors with cells expressing the c-fos gene activity-dependent marker Fos. Experiments were performed on paralyzed, precollicular-postmamillary decerebrate cats, in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. The majority of Fos labeled neurons, especially abundant in laminae VII and VIII throughout the thoraco-lumbar (T13-L7) region of locomotor animals, showed close contacts with multiple noradrenergic boutons. A small percentage (10-40%) of Fos neurons in the T7-L7 segments showed colocalization with NEα(1A) receptors. In contrast, NEα(2B) receptor immunoreactivity was observed in 70-90% of Fos cells, with no obvious rostrocaudal gradient. In comparison with results obtained from our previous study on the same animals, a significantly smaller proportion of Fos labeled neurons were innervated by noradrenergic than serotonergic fibers, with significant differences observed for laminae VII and VIII in some segments. In lamina VII of the lumbar segments, the degree of monoaminergic receptor subtype/Fos colocalization examined statistically generally fell into the following order: NEα(2B) = 5-HT(2A) ≥ 5-HT(7) = 5-HT(1A) > NEα(1A). These results suggest that noradrenergic modulation of locomotion involves NEα(1A)/NEα(2B) receptors on noradrenergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments. Further study of the functional role of these receptors in locomotion is warranted.

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.

Electrophysiological actions of the rubrospinal tract in the anaesthetised rat

The rubrospinal tract (RST) of the rat is widely used in studies of regeneration and plasticity, but the electrophysiology of its spinal actions has not been described. In anaesthetised rats with neuromuscular blockade, a tungsten microelectrode was located in the region of the red nucleus (RN) by combining stereotaxis with recording of antidromic potentials evoked from the contralateral spinal cord. Single stimuli through this electrode typically elicited two descending volleys in the contralateral dorsolateral funiculus (DLF) separated by about 1 ms, and one volley recorded from the ipsilateral DLF. Latencies of the ipsilateral and the early contralateral volley were similar. The activation of these volleys depended on the location of the stimulation site in or near the RN. Evidence is adduced to show that: (a) the late contralateral volley is carried by fibres of RST neurones, synaptically activated; (b) the early contralateral volley is mostly carried by RST fibres stimulated directly; (c) the ipsilateral volley is sometimes carried by RST fibres from the RN on the side contralateral to the stimulus; (d) otherwise, either early volley may derive from fibres in other tracts. Synaptic potentials related to the volleys were recorded within the cervical enlargement and their distribution plotted on cross-sections of the spinal cord. These measurements suggest that the great majority of RST terminations are on interneurones in the intermediate region contralateral to the RN. Direct synaptic actions on motoneurones are likely to be weak. Stimulation parameters appropriate for specific activation of the RST in future studies are suggested.

Serotonin concentrations in the lumbosacral spinal cord of the adult rat following microinjection or dorsal surface application

Application of neuroactive substances, including monoamines, is common in studies examining the spinal mechanisms of sensation and behavior. However, affected regions and time courses of transmitter activity are uncertain. We measured the spatial and temporal distribution of serotonin [5-hydroxytryptamine (5-HT)] in the lumbosacral spinal cord of halothane-anesthetized adult rats, following its intraspinal microinjection or surface application. Carbon fiber microelectrodes (CFMEs) were positioned at various locations in the spinal cord and oxidation currents corresponding to extracellular 5-HT were measured by fast cyclic voltammetry. Intraspinal microinjection of 5-HT (100 microM, 1-3 microl) produced responses that were most pronounced at CFMEs positioned Collapse

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MESH Headings

• Animals
• Clomipramine/pharmacology
• Electrophysiology/methods
• Female
• Halothane
• Injections, Spinal
• Microinjections
• Rats
• Rats, Inbred Strains
• Serotonin/administration & dosage
• Serotonin/metabolism
• Serotonin/pharmacokinetics
• Spinal Cord/anatomy & histology
• Spinal Cord/drug effects
• Spinal Cord/physiology

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Grants

• R01 NS046404 NINDS NIH HHS
• R56 NS046404 NINDS NIH HHS
• NS-46404 NINDS NIH HHS

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Affiliation(s)

Michele R Brumley The Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, 1095 NW 14th Terrace, Miami, FL 33136, USA
Ian D Hentall
Alberto Pinzon
Brijesh H Kadam
Anthony Blythe
Francisco J Sanchez
Annette M Taberner
Brian R Noga

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Collaborators Collapse

Differential modulation by monoamine membrane receptor agonists of reticulospinal input to lamina VIII feline spinal commissural interneurons

Noradrenaline and serotonin have previously been demonstrated to facilitate the transmission between descending reticulospinal tracts fibres and commissural interneurons coordinating left-right hindlimb muscle activity. The aim of the present study was to investigate the contribution of subclasses of monoaminergic membrane receptors to this facilitation. The neurons were located in Rexed lamina VIII in midlumbar segments and identified by their projections to the contralateral gastrocnemius-soleus motor nuclei and by lack of projections rostral to the lumbosacral enlargement. The effects of ionophoretically applied membrane receptor agonists [phenylephrine (noradrenergic alpha(1)), clonidine (noradrenergic alpha(2)), 8-OH-DPAT (5-HT(1A), 5-HT(7)), 2-me-5-HT (5-HT(3)), 5-me-5-HT (5-HT(2)) and alpha-me-5-HT (5-HT(2))] were examined on extracellularly recorded spikes evoked monosynaptically by electric stimulation of descending reticulospinal fibres in the medial longitudinal fascicle. Application of alpha(1) and 5-HT(2) agonists resulted in a facilitation of responses in all investigated neurons while application of alpha(2), 5-HT(1A/7) and 5-HT(3) agonists resulted in a depression. These opposite modulatory effects of different agonists suggest that the facilitatory actions of noradrenaline and serotonin on responses of commissural interneurons reported previously following ionophoretic application are the net outcome of the activation of different subclasses of monoaminergic membrane receptors. As these receptors may be distributed predominantly, or even selectively, at either pre- or postsynaptic sites their differential modulatory actions could be compatible with a presynaptically induced depression and a postsynaptically evoked enhancement of synaptic transmission between reticulospinal neurons and commissural interneurons.

Pathophysiological concepts of restless legs syndrome

Pathophysiological concepts of restless legs syndrome (RLS) are based mainly on neuroimaging and on neurophysiological data. Furthermore treatment effects contribute essentially to the present understanding of the disease, unless the genetic progress expected in the near future will clarify substantially open issues. The concept agreed on assumes a dysfunction of the dopaminergic system, possibly on the level of striatal and/or spinal dopamine receptors, and the A11 neuron group localized in the hypothalamus as an integrated part of the system. These neurons modulate spinal excitability, alterations of which in turn affect sensory processing predominantly of leg afferents in brain stem structures. Neurophysiologically excitability alterations can be measured by a variety of methods such as determination of pain thresholds, H-reflex testing, and quantitative sensory testing.

Spatial and temporal patterns of serotonin release in the rat's lumbar spinal cord following electrical stimulation of the nucleus raphe magnus

The monoamine neurotransmitter serotonin is released from spinal terminals of nucleus raphe magnus (NRM) neurons and important in sensory and motor control, but its pattern of release has remained unclear. Serotonin was measured by the high-resolution method of fast cyclic voltammetry (2 Hz) with carbon-fiber microelectrodes in lumbar segments (L3-L6) of halothane-anesthetized rats during electrical stimulation of the NRM. Because sites of serotonin release are often histologically remote from membrane transporters and receptors, rapid emergence into aggregate extracellular space was expected. Increased monoamine oxidation currents were found in 94% of trials of 50-Hz, 20-s NRM stimulation across all laminae. The estimated peak serotonin concentration averaged 37.8 nM (maximum 287 nM), and was greater in dorsal and ventral laminae (I-III and VIII-IX) than in intermediate laminae (IV-VI). When measured near NRM-evoked changes, basal monoamine levels (relative to dorsal white matter) were highest in intermediate laminae, while changes in norepinephrine level produced by locus ceruleus (LC) stimulation were lowest in laminae II/III and VII. The NRM-evoked monoamine peak was linearly proportional to stimulus frequency (10-100 Hz). The peak often occurred before the stimulus ended (mean 15.6 s at 50 Hz, range 4-35 s) regardless of frequency, suggesting that release per impulse was constant during the rise but fell later. The latency from stimulus onset to electrochemical signal detection (mean 4.2 s, range 1-23 s) was inversely correlated with peak amplitude and directly correlated with time-to-peak. Quantitative modeling suggested that shorter latencies mostly reflected the time below detection threshold (5-10 nM), so that extrasynaptic serotonin was significantly elevated well within 1 s. Longer latencies (>5 s), which were confined to intermediate laminae, appeared mainly to be due to diffusion from distant sources. In conclusion, except possibly in intermediate laminae, serotonergic volume transmission is a significant mode of spinal control by the NRM.

Less is more: pathophysiology of dopaminergic-therapy-related augmentation in restless legs syndrome

Therapy-related augmentation of the symptoms of restless legs syndrome (RLS) is an important clinical problem reported in up to 60% of patients treated with levodopa and, to a lesser extent, with dopamine agonists. The efficacy of low-dose dopaminergic drugs for RLS has been established, but the mode of action is unknown. Here, we review the existing data and conclude that augmentation is a syndrome characterised by a severely increased dopamine concentration in the CNS; overstimulation of the dopamine D1 receptors compared with D2 receptors in the spinal cord may lead to D1-related pain and generate periodic limb movements; iron deficiency may be a main predisposing factor of augmentation, probably caused by a reduced function of the dopamine transporter; therapy with levodopa or dopamine agonists should remain at low doses and; iron supplementation and opiates are the therapy of choice to counter augmentation.