Properties of interneurones in the intermediolateral cell column of the rat spinal cord: role of the potassium channel subunit Kv3.1

Topography of the GLP-1/GLP-1 receptor system in the spinal cord of male mice

Glucagon-like peptide-1 receptor (GLP-1R) agonists are now commonly used to treat type 2 diabetes and obesity. GLP-1R signaling in the spinal cord has been suggested to account for the mild tachycardia caused by GLP-1R agonists, and may also be involved in the therapeutic effects of these drugs. However, the neuroanatomy of the GLP-1/GLP-1R system in the spinal cord is still poorly understood. Here we applied in situ hybridization and immunohistochemistry to characterize this system, and its relation to cholinergic neurons. GLP-1R transcript and protein were expressed in neuronal cell bodies across the gray matter, in matching distribution patterns. GLP-1R-immunolabeling was also robust in dendrites and axons, especially in laminae II-III in the dorsal horn. Cerebrospinal fluid-contacting neurons expressed GLP-1R protein at exceedingly high levels. Only small subpopulations of cholinergic neurons expressed GLP-1R, including a subset of sympathetic preganglionic neurons at the rostral tip of the intermediolateral nucleus. GLP-1 axons innervated all regions where GLP-1R neurons were distributed, except laminae II-III. Scattered preproglucagon (Gcg) mRNA-expressing neurons were identified in the cervical and lumbar enlargements. The results will facilitate further studies on how GLP-1 regulates the sympathetic system and other autonomic and somatic functions via the spinal cord.

The Role and Modulation of Spinal Perineuronal Nets in the Healthy and Injured Spinal Cord

Rather than being a stable scaffold, perineuronal nets (PNNs) are a dynamic and specialized extracellular matrix involved in plasticity modulation. They have been extensively studied in the brain and associated with neuroprotection, ionic buffering, and neural maturation. However, their biological function in the spinal cord and the effects of disrupting spinal PNNs remain elusive. The goal of this review is to summarize the current knowledge of spinal PNNs and their potential in pathological conditions such as traumatic spinal cord injury (SCI). We also highlighted interventions that have been used to modulate the extracellular matrix after SCI, targeting the glial scar and spinal PNNs, in an effort to promote regeneration and stabilization of the spinal circuits, respectively. These concepts are discussed in the framework of developmental and neuroplastic changes in PNNs, drawing similarities between immature and denervated neurons after an SCI, which may provide a useful context for future SCI research.

Kv3 Channels Contribute to the Excitability of Subpopulations of Spinal Cord Neurons in Lamina VII

Autonomic parasympathetic preganglionic neurons (PGNs) drive contraction of the bladder during micturition but remain quiescent during bladder filling. This quiescence is postulated to be because of recurrent inhibition of PGN by fast-firing adjoining interneurons. Here, we defined four distinct neuronal types within Lamina VII, where PGN are situated, by combining whole cell patch clamp recordings with k-means clustering of a range of electrophysiological parameters. Additional morphologic analysis separated these neuronal classes into parasympathetic preganglionic populations (PGN) and a fast-firing interneuronal population. Kv3 channels are voltage-gated potassium channels (Kv) that allow fast and precise firing of neurons. We found that blockade of Kv3 channels by tetraethylammonium (TEA) reduced neuronal firing frequency and isolated high-voltage-activated Kv currents in the fast-firing population but had no effect in PGN populations. Furthermore, Kv3 blockade potentiated the local and descending inhibitory inputs to PGN indicating that Kv3-expressing inhibitory neurons are synaptically connected to PGN. Taken together, our data reveal that Kv3 channels are crucial for fast and regulated neuronal output of a defined population that may be involved in intrinsic spinal bladder circuits that underpin recurrent inhibition of PGN.

Compromised N-Glycosylation Processing of Kv3.1b Correlates with Perturbed Motor Neuron Structure and Locomotor Activity

Neurological difficulties commonly accompany individuals suffering from congenital disorders of glycosylation, resulting from defects in the N-glycosylation pathway. Vacant N-glycosylation sites (N220 and N229) of Kv3, voltage-gated K+ channels of high-firing neurons, deeply perturb channel activity in neuroblastoma (NB) cells. Here we examined neuron development, localization, and activity of Kv3 channels in wildtype AB zebrafish and CRISPR/Cas9 engineered NB cells, due to perturbations in N-glycosylation processing of Kv3.1b. We showed that caudal primary (CaP) motor neurons of zebrafish spinal cord transiently expressing fully glycosylated (WT) Kv3.1b have stereotypical morphology, while CaP neurons expressing partially glycosylated (N220Q) Kv3.1b showed severe maldevelopment with incomplete axonal branching and extension around the ventral musculature. Consequently, larvae expressing N220Q in CaP neurons had impaired swimming locomotor activity. We showed that replacement of complex N-glycans with oligomannose attached to Kv3.1b and at cell surface lessened Kv3.1b dispersal to outgrowths by altering the number, size, and density of Kv3.1b-containing particles in membranes of rat neuroblastoma cells. Opening and closing rates were slowed in Kv3 channels containing Kv3.1b with oligomannose, instead of complex N-glycans, which suggested a reduction in the intrinsic dynamics of the Kv3.1b α-subunit. Thus, N-glycosylation processing of Kv3.1b regulates neuronal development and excitability, thereby controlling motor activity.

Potentiation of NMDA-Mediated Responses by Amyloid-β Peptide 1-40 in Rat Sympathetic Preganglionic Neurons

The abnormal accumulation of amyloid-β peptides (Aβ) is one of the main characteristics of Alzheimer's disease (AD). Cerebro- and cardiovascular diseases may be the risk factors for developing AD. The effect of Aβ on central sympathetic control of cardiovascular function remains unclear. The present study examines the acute effects of Aβ oligomers on the function of NMDA receptors, a subtype of ionotropic glutamate receptors, in rat sympathetic preganglionic neurons (SPNs). In the in vitro electrophysiological study, Aβ1-40 but not Aβ1-42 applied by superfusion for 5 min significantly potentiated NMDA-induced depolarizations in SPNs of neonatal rat spinal cord slice preparation. Application of Aβ1-40 had little effects on AMPA-induced depolarizations or GABA-induced hyperpolarizations. Treatment with a selective protein kinase C (PKC) inhibitor applied together with Aβ1-40 blocked the augmentation by Aβ1-40 of NMDA-induced depolarizations. Western blot analysis showed an increase in the levels of phosphoserine 896, selectively regulated by PKC, without significant changes in phosphoserine 897 on GluN1 subunits in lateral horn areas of spinal cord slices following treatment with Aβ1-40. In the in vivo study, intrathecal injection of Aβ1-40 (0.2 nmol) potentiated the pressor effects induced by NMDA (2 nmol) injected intrathecally in urethane-anesthetized rats. These results suggest that different fragments of Aβ may have differential effects on the NMDA receptor function and the selective augmentation of NMDA receptor function by Aβ1-40 may involve PKC-dependent mechanisms in sympathetic preganglionic neurons.

Kv3 K+ currents contribute to spike-timing in dorsal cochlear nucleus principal cells

Exposure to loud sound increases burst-firing of dorsal cochlear nucleus (DCN) fusiform cells in the auditory brainstem, which has been suggested to be an electrophysiological correlate of tinnitus. The altered activity of DCN fusiform cells may be due to down-regulation of high voltage-activated (Kv3-like) K⁺ currents. Whole cell current-clamp recordings were obtained from DCN fusiform cells in brain slices from P15-P18 CBA mice. We first studied whether acoustic over-exposure (performed at P15) or pharmacological inhibition of K⁺ currents with tetraethylamonium (TEA) affect fusiform cell action potential characteristics, firing frequency and spike-timing relative to evoking current stimuli. We then tested whether AUT1, a modulator of Kv3 K⁺ currents reverses the effects of sound exposure or TEA. Both loud sound exposure and TEA decreased the amplitude of action potential after-hyperpolarization, reduced the maximum firing frequency, and disrupted spike-timing. These treatments also increased post-synaptic voltage fluctuations at baseline. AUT1 applied in the presence of TEA or following acoustic over-exposure, did not affect the firing frequency, but enhanced action potential after-hyperpolarization, prevented the increased voltage fluctuations and restored spike-timing. Furthermore AUT1 prevented the occurrence of bursts. Our study shows that the effect on spike-timing is significantly correlated with the amplitude of the action potential after-hyperpolarization and the voltage fluctuations at baseline. In conclusion, modulation of putative Kv3 K⁺ currents may restore regular spike-timing of DCN fusiform cell firing following noise exposure, and could provide a means to restore deficits in temporal encoding observed during noise-induced tinnitus.

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Whole cell recordings were performed in dorsal cochlear nucleus fusiform cells.

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Spike-timing is dependent on the action potential after-hyperpolarization.

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Spike-timing is dependent on synaptic baseline voltage fluctuations.

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Inhibition of K⁺ currents using TEA or acoustic over-exposure disrupt spike-timing.

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AUT1, a Kv3.1/3.2 K⁺ current modulator, counteracts the disruptive effects on spike-timing.

Kv3 Channels: Enablers of Rapid Firing, Neurotransmitter Release, and Neuronal Endurance

The intrinsic electrical characteristics of different types of neurons are shaped by the K+ channels they express. From among the more than 70 different K+ channel genes expressed in neurons, Kv3 family voltage-dependent K+ channels are uniquely associated with the ability of certain neurons to fire action potentials and to release neurotransmitter at high rates of up to 1,000 Hz. In general, the four Kv3 channels Kv3.1-Kv3.4 share the property of activating and deactivating rapidly at potentials more positive than other channels. Each Kv3 channel gene can generate multiple protein isoforms, which contribute to the high-frequency firing of neurons such as auditory brain stem neurons, fast-spiking GABAergic interneurons, and Purkinje cells of the cerebellum, and to regulation of neurotransmitter release at the terminals of many neurons. The different Kv3 channels have unique expression patterns and biophysical properties and are regulated in different ways by protein kinases. In this review, we cover the function, localization, and modulation of Kv3 channels and describe how levels and properties of the channels are altered by changes in ongoing neuronal activity. We also cover how the protein-protein interaction of these channels with other proteins affects neuronal functions, and how mutations or abnormal regulation of Kv3 channels are associated with neurological disorders such as ataxias, epilepsies, schizophrenia, and Alzheimer's disease.

Physiologic regulation of heart rate and blood pressure involves connexin 36-containing gap junctions

Chronically elevated sympathetic nervous activity underlies many cardiovascular diseases. Elucidating the mechanisms contributing to sympathetic nervous system output may reveal new avenues of treatment. The contribution of the gap junctional protein connexin 36 (Cx36) to the regulation of sympathetic activity and thus blood pressure and heart rate was determined using a mouse with specific genetic deletion of Cx36. Ablation of the Cx36 protein was confirmed in sympathetic preganglionic neurons of Cx36-knockout (KO) mice. Telemetric analysis from conscious Cx36 KO mice revealed higher variance in heart rate and blood pressure during rest and activity compared to wild-type (WT) mice, and smaller responses to chemoreceptor activation when anesthetized. In the working heart-brain stem preparation of the Cx36-KO mouse, respiratory-coupled sympathetic nerve discharge was attenuated and responses to chemoreceptor stimulation and noxious stimulation were blunted compared to WT mice. Using whole cell patch recordings, sympathetic preganglionic neurons in spinal cord slices of Cx36-KO mice displayed lower levels of spikelet activity compared to WT mice, indicating reduced gap junction coupling between neurons. Cx36 deletion therefore disrupts normal regulation of sympathetic outflow with effects on cardiovascular parameters.-Lall, V. K., Bruce, G., Voytenko, L., Drinkhill, M., Wellershaus, K., Willecke, K., Deuchars, J., Deuchars, S. A. Physiologic regulation of heart rate and blood pressure involves connexin 36-containing gap junctions.

Autonomic dysreflexia after spinal cord injury: Systemic pathophysiology and methods of management

Traumatic spinal cord injury (SCI) has widespread physiological effects beyond the disruption of sensory and motor function, notably the loss of normal autonomic and cardiovascular control. Injury at or above the sixth thoracic spinal cord segment segregates critical spinal sympathetic neurons from supraspinal modulation which can result in a syndrome known as autonomic dysreflexia (AD). AD is defined as episodic hypertension and concomitant baroreflex-mediated bradycardia initiated by unmodulated sympathetic reflexes in the decentralized cord. This condition is often triggered by noxious yet unperceived visceral or somatic stimuli below the injury level and if severe enough can require immediate medical attention. Herein, we review the pathophysiological mechanisms germane to the development of AD, including maladaptive plasticity of neural circuits mediating abnormal sympathetic reflexes and hypersensitization of peripheral vasculature that collectively contribute to abnormal hemodynamics after SCI. Further, we discuss the systemic effects of recurrent AD and pharmacological treatments used to manage such episodes. Contemporary research avenues are then presented to better understand the relative contributions of underlying mechanisms and to elucidate the effects of recurring AD on cardiovascular and immune functions for developing more targeted and effective treatments to attenuate the development of this insidious syndrome following high-level SCI.

Kv3.1/Kv3.2 channel positive modulators enable faster activating kinetics and increase firing frequency in fast-spiking GABAergic interneurons

Due to their fast kinetic properties, K_v3.1 voltage gated potassium channels are important in setting and controlling firing frequency in neurons and pivotal in generating high frequency firing of interneurons. Pharmacological activation of K_v3.1 channels may possess therapeutic potential for treatment of epilepsy, hearing disorders, schizophrenia and cognitive impairments. Here we thoroughly investigate the selectivity and positive modulation of the two small molecules, EX15 and RE01, on K_v3 channels. Selectivity studies, conducted in Xenopus laevis oocytes confirmed a positive modulatory effect of the two compounds on K_v3.1 and to a minor extent on K_v3.2 channels. RE01 had no effect on the K_v3.3 and K_v3.4 channels, whereas EX15 had an inhibitory impact on the K_v3.4 mediated current. Voltage-clamp experiments in monoclonal hK_v3.1b/HEK293 cells (34 °C) revealed that the two compounds indeed induced larger currents and faster activation kinetics. They also decrease the speed of deactivation and shifted the voltage dependence of activation, to a more negative activation threshold. Application of action potential clamping and repetitive stimulation protocols of hK_v3.1b expressing HEK293 cells revealed that EX15 and RE01 significantly increased peak amplitude, half width and decay time of K_v3.1 mediated currents, even during high-frequency action potential clamping (250 Hz). In rat hippocampal slices, EX15 and RE01 increased neuronal excitability in fast-spiking interneurons in dentate gyrus. Action potential frequency was prominently increased at minor depolarizing steps, whereas more marginal effects of EX15 and RE01 were observed after stronger depolarizations. In conclusion, our results suggest that EX15 and RE01 positive modulation of K_v3.1 and K_v3.2 currents facilitate increased firing frequency in fast-spiking GABAergic interneurons.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Disordered cardiovascular control after spinal cord injury

Damage to the spinal cord disrupts autonomic pathways, perturbing cardiovascular homeostasis. Cardiovascular dysfunction increases with higher levels of injury and greater severity. Disordered blood pressure control after spinal cord injury (SCI) has significant ramifications as cord-injured people have an increased risk of developing heart disease and stroke; cardiovascular dysfunction is currently a leading cause of death among those with SCI. Despite the clinical significance of abnormal cardiovascular control following SCI, this problem has been generally neglected by both the clinical and research community. Both autonomic dysreflexia and orthostatic hypotension are known to prevent and delay rehabilitation, and significantly impair the overall quality of life after SCI. Starting with neurogenic shock immediately after a higher SCI, ensuing cardiovascular dysfunctions include orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. Disordered temperature regulation accompanies these autonomic dysfunctions. This chapter reviews the human and animal studies that have furthered our understanding of the pathophysiology and mechanisms of orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. The cardiovascular dysfunction that occurs during sexual function and exercise is elaborated. New awareness of cardiovascular dysfunction after SCI has led to progress toward inclusion of this important autonomic problem in the overall assessment of the neurological condition of cord-injured people.

The firing patterns of spinal neurons: in situ patch-clamp recordings reveal a key role for potassium currents

Neuron firing patterns underpin the detection and processing of stimuli, influence synaptic interactions, and contribute to the function of networks. To understand how intrinsic membrane properties determine firing patterns, we investigated the biophysical basis of single and repetitive firing in spinal neurons of hatchling Xenopus laevis tadpoles, a well-understood vertebrate model; experiments were conducted in situ. Primary sensory Rohon-Beard (RB) neurons fire singly in response to depolarising current, and dorsolateral (DL) interneurons fire repetitively. RB neurons exhibited a large tetrodotoxin-sensitive sodium current; in DL neurons, the sodium current density was significantly lower. High-voltage-activated calcium currents were similar in both neuron types. There was no evidence of persistent sodium currents, low-voltage-activated calcium currents, or hyperpolarisation-activated currents. In RB neurons, the potassium current was dominated by a tetraethylammonium-sensitive slow component (I(Ks) ); a fast component (I(Kf) ), sensitive to 4-aminopyridine, predominated in DL neurons. Sequential current-clamp and voltage-clamp recordings in individual neurons suggest that high densities of I(Ks) prevent repetitive firing; where I(Ks) is small, I(Kf) density determines the frequency of repetitive firing. Intermediate densities of I(Ks) and I(Kf) allow neurons to fire a few additional spikes on strong depolarisation; this property typifies a novel subset of RB neurons, and may activate escape responses. We discuss how this ensemble of currents and firing patterns underpins the operation of the Xenopus locomotor network, and suggest how simple mechanisms might underlie the similar firing patterns seen in the neurons of diverse species.

Kv3.1b and Kv3.3 channel subunit expression in murine spinal dorsal horn GABAergic interneurones

GABAergic interneurones, including those within spinal dorsal horn, contain one of the two isoforms of the synthesizing enzyme glutamate decarboxylase (GAD), either GAD65 or GAD67. The physiological significance of these two GABAergic phenotypes is unknown but a more detailed anatomical and functional characterization may help resolve this issue. In this study, two transgenic Green Fluorescent Protein (GFP) knock-in murine lines, namely GAD65-GFP and GAD67-GFP (Δneo) mice, were used to profile expression of Shaw-related Kv3.1b and Kv3.3 K(+)-channel subunits in dorsal horn interneurones. Neuronal expression of these subunits confers specific biophysical characteristic referred to as 'fast-spiking'. Immuno-labelling for Kv3.1b or Kv3.3 revealed the presence of both of these subunits across the dorsal horn, most abundantly in laminae I-III. Co-localization studies in transgenic mice indicated that Kv3.1b but not Kv3.3 was associated with GAD65-GFP and GAD67-GFP immunopositive neurones. For comparison the distributions of Kv4.2 and Kv4.3 K(+)-channel subunits which are linked to an excitatory neuronal phenotype were characterized. No co-localization was found between GAD-GFP +ve neurones and Kv4.2 or Kv4.3. In functional studies to evaluate whether either GABAergic population is activated by noxious stimulation, hindpaw intradermal injection of capsaicin followed by c-fos quantification in dorsal horn revealed co-expression c-fos and GAD65-GFP (quantified as 20-30% of GFP +ve population). Co-expression was also detected for GAD67-GFP +ve neurones and capsaicin-induced c-fos but at a much reduced level of 4-5%. These data suggest that whilst both GAD65-GFP and GAD67-GFP +ve neurones express Kv3.1b and therefore may share certain biophysical traits, their responses to peripheral noxious stimulation are distinct.

Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis

The operation of neuronal networks depends on the firing patterns of the network's neurons. When sustained current is injected, some neurons in the central nervous system fire a single action potential and others fire repetitively. For example, in Xenopus laevis tadpoles, primary-sensory Rohon-Beard (RB) neurons fired a single action potential in response to 300-ms rheobase current injections, whereas dorsolateral (DL) interneurons fired repetitively at 10-20 Hz. To investigate the basis for these differences in vivo, we examined drug-induced changes in the firing patterns of Xenopus spinal neurons using whole cell current-clamp recordings. Neuron types were initially separated through cluster analysis, and we compared results produced using different clustering algorithms. We used these results to develop a predictive function to classify subsequently recorded neurons. The potassium channel blocker tetraethylammonium (TEA) converted single-firing RB neurons to low-frequency repetitive firing but reduced the firing frequency of repetitive-firing DL interneurons. Firing frequency in DL interneurons was also reduced by the potassium channel blockers 4-aminopyridine (4-AP), catechol, and margatoxin; 4-AP had the greatest effect. The calcium channel blockers amiloride and nimodipine had few effects on firing in either neuron type but reduced action potential duration in DL interneurons. Muscarine, which blocks M-currents, did not affect RB neurons but reduced firing frequency in DL interneurons. These results suggest that potassium currents may control neuron firing patterns: a TEA-sensitive current prevents repetitive firing in RB neurons, whereas a 4-AP-sensitive current underlies repetitive firing in DL interneurons. The cluster and discriminant analysis described could help to classify neurons in other systems.

GABA(B) Mediated Regulation of Sympathetic Preganglionic Neurons: Pre- and Postsynaptic Sites of Action

Modulatory influences on sympathetic nervous system activity are diverse and far reaching, acting at select points in the complex pathways controlling sympathetic outflow to enable subtle changes or more global effects. Changes in the degree of sympathetic neuromodulation can have serious consequences on homeostatic variables such as heart rate, blood pressure and gut motility. At the level of the spinal cord, the sympathetic preganglionic neurons (SPNs) can be modulated by activation of presynaptic GABA(B) heteroreceptors on glutamatergic terminals and by postsynaptic GABA(B) receptors. Here we show that a low concentration of the GABA(B) agonist baclofen (1 μM) attenuated GABAergic inhibitory postsynaptic potentials in SPNs elicited from stimulation of either the central autonomic area or descending fibers in the lateral funiculus. This low baclofen concentration also elicited three categories of postsynaptic response: a large hyperpolarization with a decrease in input resistance, a moderate hyperpolarization with no change in input resistance and no response. Using cesium-loaded, tetraethylammonium chloride containing electrodes (to block potassium conductance), baclofen elicited moderate hyperpolarizations with no change in input resistance in 50% of SPNs; the remainder were unaffected. These modest hyperpolarizations were reduced in Ca(2+) free solution or cadmium. Hyperpolarizing responses were also observed in interneurons in the vicinity of SPNs. These studies provide the first evidence for GABA(B) autoreceptors involved in inhibitory GABAergic transmission onto SPNs and for postsynaptic GABA(B) receptors on interneurons. The data also indicate that there is heterogeneity in the postsynaptic responses of SPNs.

Spontaneous rhythmogenic capabilities of sympathetic neuronal assemblies in the rat spinal cord slice

Neuronal networks generating rhythmic activity as an emergent property are common throughout the nervous system. Some are responsible for rhythmic behaviours, as is the case for the spinal cord locomotor networks; however, for others the function is more subtle and usually involves information processing and/or transfer. An example of the latter is sympathetic nerve activity, which is synchronized into rhythmic bursts in vivo. This arrangement is postulated to offer improved control of target organ responses compared to tonic nerve activity. Traditionally, oscillogenic circuits in the brainstem are credited with generating these rhythms, despite evidence for the persistence of some frequencies in spinalized preparations. Here, we show that rhythmic population activity can be recorded from the intermediolateral cell column (IML) of thoracic spinal cord slices. Recorded in slices from 10- to 12-day-old rats, this activity was manifest as 8–22 Hz oscillations in the field potential and was spatially restricted to the IML. Oscillations often occurred spontaneously, but could also be induced by application of 5-HT, α-methyl 5-HT or MK212. These agents also significantly increased the strength of spontaneous oscillations. Rhythmic activity was abolished by TTX and attenuated by application of gap junction blockers or by antagonists of GABA_A receptors. Together these data indicate that this rhythm is an emergent feature of a population of spinal neurons coupled by gap junctions. This work questions the assumption that sympathetic rhythms are dependent on supraspinal pacemaker circuits, by highlighting a surprisingly strong rhythmogenic capability of the reduced sympathetic networks of the spinal cord slice.

Secondhand smoke exposure alters K+ channel function and intrinsic cell excitability in a subset of second-order airway neurons in the nucleus tractus solitarius of young guinea pigs

Extended exposure to secondhand smoke (SHS) in infants and young children increases the incidence of cough, wheeze, airway hyper-reactivity and the prevalence and earlier onset of asthma. The adverse effects may result from environmentally-induced plasticity in the neural network regulating cough and airway function. Using whole-cell patch-clamp recordings in brainstem slices containing anatomically identified second-order lung afferent neurons in the nucleus tractus solitarius (NTS), we determined the effects of extended SHS exposure in young guinea pigs for a duration equivalent to human childhood on the intrinsic excitability of NTS neurons. SHS exposure resulted in marked decreases in the intrinsic excitability of a subset of lung afferent second-order NTS neurons. The neurons exhibited a decreased spiking capacity, prolonged action potential duration, reduced afterhyperpolarization, decrease in peak and steady-state outward currents, and membrane depolarization. SHS exposure effects were mimicked by low concentrations of the K+ channel blockers 4-aminopyridine and/or tetraethyl ammonium. The data suggest that SHS exposure downregulates K+ channel function in a subset of NTS neurons, resulting in reduced cell excitability. The changes may help to explain the exaggerated neural reflex responses in children exposed to SHS.

Kv3.3 immunoreactivity in the vestibular nuclear complex of the rat with focus on the medial vestibular nucleus: targeting of Kv3.3 neurones by terminals positive for vesicular glutamate transporter 1

Kv3 voltage-gated K(+) channels are important in shaping neuronal excitability and are abundant in the CNS, with each Kv3 gene exhibiting a unique expression pattern. Mice lacking the gene encoding for the Kv3.3 subunit exhibit motor deficits. Furthermore, mutations in this gene have been linked to the human disease spinocerebellar ataxia 13, associated with cerebellar and extra-cerebellar symptoms such as imbalance and nystagmus. Kv subunit localisation is important in defining their functional roles and thus, we investigated the distribution of Kv3.3-immunoreactivity in the vestibular nuclear complex of rats with particular focus on the medial vestibular nucleus (MVN). Kv3.3-immunoreactivity was widespread in the vestibular nuclei and was detected in somata, dendrites and synaptic terminals. Kv3.3-immunoreactivity was observed in distinct neuronal populations and dual labelling with the neuronal marker NeuN revealed 28.5+/-1.9% of NeuN labelled MVN neurones were Kv3.3-positive. Kv3.3-immunoreactivity co-localised presynaptically with the synaptic vesicle marker SV2, parvalbumin, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. VGluT1 terminals were scarce within the MVN (2.5+/-1.1 per 50 microm(2)) and co-localisation was not observed. However, 85.4+/-9.4% of VGluT1 terminals targeted and enclosed Kv3.3-immunoreactive somata. Presynaptic Kv3.3 co-localisation with the GABAergic marker GAD67 was also not observed. Cytoplasmic GlyT2 labelling was observed in a subset of Kv3.3-positive neurones. Electron microscopy confirmed a pre- and post-synaptic distribution of the Kv3.3 protein. This study provides evidence supporting a role for Kv3.3 subunits in vestibular processing by regulating neuronal excitability pre- and post-synaptically.

Distribution and role of Kv3.1b in neurons in the medial septum diagonal band complex

The medial septum diagonal band complex (MS/DB) projects via cholinergic and GABAergic pathways to the hippocampus and plays a key role in the hippocampal theta rhythm. In the MS/DB we have previously described a population of fast spiking GABAergic neurons that contain parvalbumin and mediate theta frequency activity in vitro. The Kv3.1 potassium channel is a delayed rectifier channel that plays a major role in fast spiking neurons in the CNS, and has previously been localized in the MS/DB. To determine which cell types in the MS/DB express the Kv3.1b ion channel subunit, transgenic mice in which the expression of GABAergic and glutamate markers are associated with the expression of green fluorescent protein (GFP; GAD67-GFP and VGluT2-GFP mice, respectively) were used for immunofluorescence and axonal tract tracing. Electrophysiological studies were also carried out on rat MS/DB slices to examine the role of the Kv3.1 channel in theta frequency oscillations. The results for the MS/DB were as follows: (1) cholinergic cells did not express GFP in either GAD67-GFP or VGluT2-GFP mice, and there was GAD67 immunoreactivity in GFP-positive neurons in GAD67-GFP mice and in a small proportion (6%) of GFP-positive neurons in VGluT2-GFP mice. (2) Kv3.1b immunofluorescence was associated with the somata of GABAergic neurons, especially those that contained parvalbumin, and with a minority of glutamatergic neurons, but not with cholinergic neurons, and with GABAergic axonal terminal-like processes around certain GABAergic neurons. (3) Both Kv3.1b-positive and -negative GABAergic neurons were septo-hippocampal, and there was a minor projection to hippocampus from VGluT2-GFP neurons. (4) Kainate-induced theta oscillations in the MS/DB slice were potentiated rather than inhibited by the Kv3.1 blocker 4-aminopyridine, and this agent on its own produced theta frequency oscillations in MS/DB slices that were reduced by ionotropic glutamate and GABA receptor antagonists and abolished by low extracellular calcium. These studies confirm the presence of heterogeneous populations of septo-hippocampal neurons in the MS/DB, and suggest that presence of Kv3.1 in the GABAergic neurons does not contribute to theta activity through fast spiking properties, but possibly by the regulation of transmitter release from axonal terminals.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

Sympathetic-correlated c-Fos expression in the neonatal rat spinal cord in vitro

An isolated thoracic spinal cord of the neonatal rat in vitro spontaneously generates sympathetic nerve discharge (SND) at ~25 degrees C, but it fails in SND genesis at < or = 10 degrees C. Basal levels of the c-Fos expression in the spinal cords incubated at < or = 10 degrees C and ~25 degrees C were compared to determine the anatomical substrates that might participate in SND genesis. Cells that exhibited c-Fos immunoreactivity were virtually absent in the spinal cords incubated at < or = 10 degrees C. However, in the spinal cords incubated at ~25 degrees C, c-Fos-positive cells were found in the dorsal laminae, the white matter, lamina X, and the intermediolateral cell column (IML). Cell identities were verified by double labeling of c-Fos with neuron-specific nuclear protein (NeuN), glial fibrillary acidic protein (GFAP), or choline acetyltransferase (ChAT). The c-Fos-positive cells distributed in the white matter and lamina X were NeuN-negative or GFAP-positive and were glial cells. Endogenously active neurons showing c-Fos and NeuN double labeling were scattered in the dorsal laminae and concentrated in the IML. Double labeling of c-Fos and ChAT confirmed the presence of active sympathetic preganglionic neurons (SPNs) in the IML. Suppression of SND genesis by tetrodotoxin (TTX) or mecamylamine (MECA, nicotinic receptor blocker) almost abolished c-Fos expression in dorsal laminae, but only mildly affected c-Fos expression in the SPNs. Therefore, c-Fos expression in some SPNs does not require synaptic activation. Our results suggest that spinal SND genesis is initiated from some spontaneously active SPNs, which are capable of TTX- or MECA-resistant c-Fos expression.

Gene expression profiling of two distinct neuronal populations in the rodent spinal cord

Background

In the field of neuroscience microarray gene expression profiles on anatomically defined brain structures are being used increasingly to study both normal brain functions as well as pathological states. Fluorescent tracing techniques in brain tissue that identifies distinct neuronal populations can in combination with global gene expression profiling potentially increase the resolution and specificity of such studies to shed new light on neuronal functions at the cellular level.

Methodology/Principal Findings

We examine the microarray gene expression profiles of two distinct neuronal populations in the spinal cord of the neonatal rat, the principal motor neurons and specific interneurons involved in motor control. The gene expression profiles of the respective cell populations were obtained from amplified mRNA originating from 50–250 fluorescently identified and laser microdissected cells. In the data analysis we combine a new microarray normalization procedure with a conglomerate measure of significant differential gene expression. Using our methodology we find 32 genes to be more expressed in the interneurons compared to the motor neurons that all except one have not previously been associated with this neuronal population. As a validation of our method we find 17 genes to be more expressed in the motor neurons than in the interneurons and of these only one had not previously been described in this population.

Conclusions/Significance

We provide an optimized experimental protocol that allows isolation of gene transcripts from fluorescent retrogradely labeled cell populations in fresh tissue, which can be used to generate amplified aRNA for microarray hybridization from as few as 50 laser microdissected cells. Using this optimized experimental protocol in combination with our microarray analysis methodology we find 49 differentially expressed genes between the motor neurons and the interneurons that reflect the functional differences between these two cell populations in generating and transmitting the motor output in the rodent spinal cord.

Expression of connexin30.2 in interneurons of the central nervous system in the mouse

Electrical synapses, particularly gap junctions composed of connexin (Cx) 36, have been suggested to synchronize neuronal network oscillations. Recently, we generated Cx30.2-deficient mice which express beta-galactosidase under control of Cx30.2 gene regulatory elements. In the central nervous system beta-galactosidase activity representing Cx30.2 expression was restricted to NeuN-positive cells, thus identifying Cx30.2 as new neuronal connexin. In the hippocampus, co-immunofluorescence analyses revealed beta-galactosidase/Cx30.2 expression in GABAergic inhibitory interneurons such as parvalbumin- and somatostatin-positive basket, axo-axonic, bistratified or oriens lacunosum-moleculare cells. approximately 94% of the Cx30.2 expressing parvalbumin-positive interneurons also expressed Cx36. Performing field potential recordings from hippocampal slices we found no differences in basal excitation and excitation-inhibition balance between Cx30.2+/+ and Cx30.2LacZ/LacZ)mice. Furthermore, frequency and power of gap junction dependent gamma and ripples oscillations were similar in these animals. This suggests that the lack of Cx30.2 in interneurons can be largely compensated by other connexins, most likely Cx36.

Modulation of potassium ion channel proteins utilising antibodies

The application of antibodies to living cells has the potential to modulate the function of specific proteins by virtue of their high specificity. This specificity has proven effective in determining the involvement of many proteins in neuronal function where specific agonists and antagonists do not exist, e.g. ion channel subunits. We discuss a way to utilise subunit specific antibodies to target individual channel subunits in electrophysiological experiments to determine functional roles within native neurones. Utilising this approach, we have investigated the role of the voltage-gated potassium channel Kv3.1b subunit within a region of the brainstem important in the regulation of autonomic function. We provide some useful control experiments in order to help validate this method. We conclude that antibodies can be extremely valuable in determining the functions of specific proteins in living neurones in neuroscience research.

Histamine depolarizes neurons in the dorsal vagal complex

We sought to determine whether histamine has effects on single neurons in the dorsal vagal complex of the brainstem since previous studies have suggested a role for histamine receptors in this region. Using whole-cell patch clamp recordings from neurons within the nucleus of the tractus solitarius (NTS) and the dorsal vagal nucleus (DVN), histamine (20 microM) depolarized a small proportion of neurons in these regions accompanied by a decrease in input resistance. Although few neurons were depolarized (21% of NTS neurons and 15% of DVN neurons), those that were affected showed robust depolarizations of 13 mV. These depolarizations were antagonized by the histamine H1 receptor antagonist triprolidine (2 microM) and were subject to a level of desensitization. Neither histamine nor the H3 receptor agonist imetit caused any change in the amplitudes of excitatory or inhibitory postsynaptic potentials elicited in NTS neurons by stimulation of the solitary tract. These data indicate that histamine has a restricted but profound effect on neurons in the dorsal vagal complex.

Voltage-gated potassium currents within the dorsal vagal nucleus: inhibition by BDS toxin

Voltage-gated potassium (Kv) channels are essential components of neuronal excitability. The Kv3.4 channel protein is widely distributed throughout the central nervous system (CNS), where it can form heteromeric or homomeric Kv3 channels. Electrophysiological studies reported here highlight a functional role for this channel protein within neurons of the dorsal vagal nucleus (DVN). Current clamp experiments revealed that blood depressing substance (BDS) and intracellular dialysis of an anti-Kv3.4 antibody prolonged the action potential duration. In addition, a BDS sensitive, voltage-dependent, slowly inactivating outward current was observed in voltage clamp recordings from DVN neurons. Electrical stimulation of the solitary tract evoked EPSPs and IPSPs in DVN neurons and BDS increased the average amplitude and decreased the paired pulse ratio, consistent with a presynaptic site of action. This presynaptic modulation was action potential dependent as revealed by ongoing synaptic activity. Given the role of the Kv3 proteins in shaping neuronal excitability, these data highlight a role for homomeric Kv3.4 channels in spike timing and neurotransmitter release in low frequency firing neurons of the DVN.

Electrophysiological characteristics of vasomotor preganglionic neurons and related neurons in the thoracic spinal cord of the rat: an intracellular study in vivo

Sympathetic preganglionic neurons (SPN) represent the final central neurons in the sympathetic pathways which regulate vasomotor tone; they therefore play a pivotal role in the re-distribution of cardiac output to different vascular beds in response to environmental challenges. While the consensus view is that activity in these neurons is due mainly to supraspinal inputs, the possibility that some activity may be generated intrinsically and modified by synaptic inputs cannot be excluded. Therefore, in order to distinguish between these two possibilities, the electrophysiological properties of cardiovascular-like SPN in the upper thoracic spinal cord of the anesthetized rat were examined and their response to activation of vasodepressor inputs was investigated. Intracellular recordings were made from 22 antidromically identified SPN of which 17 displayed irregular, but maintained, spontaneous activity; no evidence of bursting behavior or pacemaker-like activity was observed. Stimulation of the aortic depressor nerve or a vasodepressor site within the nucleus tractus solitarius (NTS) resulted in a membrane hyperpolarization, decrease in cell input resistance and long-lasting cessation of neuronal firing in SPN including a sub-population which had cardiac-modulated patterns of activity patterns. Recordings were also undertaken from 80 non-antidromically-activated neurons located in the vicinity of SPN; 23% of which fired in phase with the cardiac cycle, with this peak of activity occurring before similar increases in cardiac-modulated SPN. Stimulation of vasodepressor regions of the NTS evoked a membrane hyperpolarization and decrease in cell input resistance in cardiac-modulated but not non-modulated interneurons. These studies show that activity patterns in SPN in vivo are determined principally by synaptic inputs. They also demonstrate that spinal interneurons which exhibit cardiac-modulated patterns of activity are postsynaptically inhibited following activation of baroreceptor pathways. However, the question as to whether these inhibitory pathways and/or disfacilitation of tonic excitatory drive underlies the baroreceptor-mediated inhibition of SPN remains to be determined.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

Subdivision-specific responses of neurons in the nucleus of the tractus solitarius to activation of mu-opioid receptors in the rat

Microinjection of opioid receptor agonists into the nucleus tractus solitarius (NTS) has differential effects on cardiovascular, respiratory, and gastrointestinal responses. This can be achieved either by presynaptic modulation of inputs onto neurons or by postsynaptic activation of receptors on neurons in specific regions. Therefore we sought to determine whether responses of neurons to activation of opioid receptors were dependent on their location within the NTS. Using whole cell patch-clamp recordings from neurons within the NTS, the mu opioid receptor (MOR) agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO, 100 nM) hyperpolarized a proportion of neurons in the medial, dorsomedial and dorsolateral NTS, whereas no postsynaptic responses were observed in remaining subdivisions. DAMGO reduced the amplitude of solitary tract-evoked excitatory postsynaptic potentials (EPSPs) in all neurons tested, regardless of subdivision. The kappa opioid receptor (KOR) agonist U69593 (10-20 microM) also hyperpolarized a small fraction of neurons (6/79) and decreased the amplitude of EPSPs in 50% of neurons. In contrast, the delta-opioid receptor agonist DPDPE (1-4 microM) had no presynaptic or postsynaptic effects on NTS neurons even after preincubation with bradykinin. Anatomical data at the light and electron microscopic level complemented electrophysiological observations with respect to MOR location and further showed that MORs were present at both presynaptic and postsynaptic sites in the dorsolateral NTS, often at the same synapse. These data demonstrate site specific responses of neurons to activation of MORs and KORs, which may underlie their ability to modulate different autonomic reflexes.

Multi-tasking in the spinal cord--do 'sympathetic' interneurones work harder than we give them credit for?

The role of interneurones in the control of sympathetic activity has been somewhat of a mystery since, for many years, it was difficult to target these cells for study. Recently scientists have started to unravel the action potential properties of these neurones, where they receive their inputs from and where they project to. This review looks at the information known to date about sympathetic interneurones. The locations of these neurones and their local axonal ramifications suggest that they play a more widespread function than previously thought. Therefore the data to support such a theory are also examined.

Ischaemia-induced sympathoexcitation in spinalyzed rats

Systemic ischaemia increases sympathetic activity via both reflex and direct effects on the nervous system, which include the hypothalamus and brainstem structures that provide excitatory drive to sympathetic pre-ganglionic motoneurones. Using an arterially perfused working heart-brainstem preparation (WHBP), we evaluated the sympathoexcitatory response recorded from the thoracic sympathetic chain (tSC) in response to systemic ischaemia (produced by arresting perfusion for 30 s) before and after transecting consecutively at both the ponto-medullary and medullary-spinal cord junctions. Ischaemia produced a striking increase in tSC activity that persisted after transecting at both the ponto-medullary and medullary-spinal cord levels (intact: 70+/-3%; ponto-medullary: 77+/-7%; medullary-spinal cord: 61+/-6%; n=9). In sino-aortic denervated (SAD) rats (n=4), sympathoexcitatory responses were smaller in both intact and ponto-medullary, but not in medullary-spinal cord transected versus intact rats. Following administration of a ganglionic blocker [hexamethonium (hex), 25 mg/kg] after medullary-spinal cord transection the ischaemia-induced sympathoexcitatory response was reduced (12+/-6% increase relative to control, n=4). In medullary-spinal cord transected preparations, intrathecal injection of N2-saturated saline increased tSC discharge (22+/-3%, n=4), which was attenuated by hex (5+/-1%). We propose that neural mechanisms within the cervical-thoracic segments can make a substantial contribution to the sympathoexcitatory response during systemic ischaemia.

Behavioral motor dysfunction in Kv3-type potassium channel-deficient mice

The voltage-gated potassium channels Kv3.1 and Kv3.3 are expressed in several distinct neuronal subpopulations in brain areas known to be involved in motor control such as cortex, basal ganglia and cerebellum. Depending on the lack of Kv3.1 or Kv3.3 channel subunits, mutant mice show different Kv3-null allele-dependent behavioral alterations that include constitutive hyperactivity, sleep loss, impaired motor performance and, in the case of the Kv3.1/Kv3.3 double mutant, also severe ataxia, tremor and myoclonus (Espinosa et al. 2001, J Neurosci 21, 6657-6665, Genes, Brain Behav 3, 90-100). The lack of Kv3.1 channel subunits is mainly responsible for the constitutively increased locomotor activity and for sleep loss, whereas the absence of Kv3.3 subunits affects cerebellar function, in particular Purkinje cell discharges and olivocerebellar system properties (McMahon et al. 2004, Eur J Neurosci 19, 3317-3327). Here, we describe two sensitive and non-invasive tests to reliably quantify normal and abnormal motor functions, and we apply these tests to characterize motor dysfunction in Kv3-mutant mice. In contrast to wildtype and Kv3.1-single mutants, Kv3.3-single mutants and Kv3 mutants lacking three and four Kv3 alleles display Kv3-null allele-dependent gait alterations. Although the Kv3-null allele-dependent gait changes correlate with reduced motor performance, they appear to not affect the training-induced improvement of motor performance. These findings suggest that altered cerebellar physiology in the absence of Kv3.3 channels is responsible for impaired motor task execution but not motor task learning.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

Developmental changes in the expression of calbindin and potassium-channel subunits Kv3.1b and Kv3.2 in mouse Renshaw cells

One class of spinal interneurons, the Renshaw cells, is able to discharge at very high frequencies in adult mammals. Neuronal firing at such high frequencies requires voltage-gated potassium channels to rapidly repolarize the membrane potential after each action potential. We sought to establish the pattern of expression of calbindin and potassium channels with Kv3.1b and Kv3.2 subunits in Renshaw cells at different developmental stages of postnatal mice. The pattern of expression of calbindin changed dramatically during early postnatal development. An adult pattern of calbindin reactive neurons started to emerge from postnatal day 10 to postnatal day 14, with cells in laminae I and II of superficial dorsal horn and the ventral lamina VII. Renshaw cells were identified immunohistochemically by their expression of calbindin and their location in the ventral horn of the spinal cord. Western blot results of the lumbar spinal cord showed that Kv3.1b expression became faintly evident from postnatal day 10, reached a maximum at postnatal day 21 and was maintained through postnatal day 49. Double labeling results showed that all Renshaw cells expressed Kv3.1b weakly from postnatal day 14, and strongly at postnatal day 21. Western blot results showed that Kv3.2 expression became detectable in the lumbar cord from postnatal day 12, and increased steadily until reaching an adult level at postnatal day 28. In contrast to the Kv3.1b results, Kv3.2 was not expressed in Renshaw cells, although some neurons located at laminae VIII and VI expressed Kv3.2. We conclude that Renshaw cells express Kv3.1b but not Kv3.2 from postnatal day 14.

Immunohistochemical localisation of the voltage gated potassium ion channel subunit Kv3.3 in the rat medulla oblongata and thoracic spinal cord

Voltage gated K+ channels (Kv) are a diverse group of channels important in determining neuronal excitability. The Kv superfamily is divided into 12 subfamilies (Kv1-12) and members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique expression pattern. Since the localisation of Kv subunits is important in defining the roles they play in neuronal function, we have used immunohistochemistry to determine the distribution of the Kv3.3 subunit in the medulla oblongata and spinal cord of rats. Kv3.3 subunit immunoreactivity (Kv3.3-IR) was widespread but present only in specific cell populations where it could be detected in somata, dendrites and synaptic terminals. Labelled neurones were observed in the spinal cord in laminae IV and V, in the region of the central canal and in the ventral horn. In the medulla oblongata, labelled cell bodies were numerous in the spinal trigeminal, cuneate and gracilis nuclei whilst rarer in the lateral reticular nucleus, hypoglossal nucleus and raphe nucleus. Regions containing autonomic efferent neurones were predominantly devoid of labelling with only occasional labelled neurones being observed. Dual immunohistochemistry revealed that some Kv3.3-IR neurones in the ventral medullary reticular nucleus, spinal trigeminal nucleus, dorsal horn, ventral horn and central canal region were also immunoreactive for the Kv3.1b subunit. The presence of Kv3.3 subunits in terminals was confirmed by co-localisation of Kv3.3-IR with the synaptic vesicle protein SV2, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. Co-localisation of Kv3.3-IR was not observed with VGluT1, tyrosine hydroxylase, serotonin or choline acetyl transferase. Electron microscopy confirmed the presence of Kv3.3-IR in terminals and somatic membranes in ventral horn neurones, but not motoneurones. This study provides evidence supporting a role for Kv3.3 subunits in regulating neuronal excitability and in the modulation of excitatory and inhibitory synaptic transmission in the medulla oblongata and spinal cord.

Spinal sympathetic interneurons: Their identification and roles after spinal cord injury

Primary afferent neurons rarely, if ever, synapse on the sympathetic preganglionic neurons that regulate the cardiovascular system, nor do sympathetic preganglionic neurons normally exhibit spontaneous activity in the absence of excitatory inputs. Therefore, after serious spinal cord injury "spinal sympathetic interneurons" provide the sole excitatory and inhibitory inputs to sympathetic preganglionic neurons. Few studies have addressed the anatomy and physiology of spinal sympathetic interneurons, to a great extent because they are difficult to identify. Therefore, this chapter begins with descriptions of both neurophysiological and neuroanatomical criteria for identifying spinal sympathetic interneurons, and it discusses the advantages and disadvantages of each. Spinal sympathetic interneurons also have been little studied because their importance in intact animals has been unknown, whereas the roles of direct projections from the brain to sympathetic preganglionic neurons are better known. This chapter presents evidence that spinal sympathetic interneurons play only a minor role in sympathetic regulation when the spinal cord is intact. However, they play an important role after spinal cord injury, both in generating ongoing activity in sympathetic nerves and in mediating segmental and intersegmental sympathetic reflexes. The spinal sympathetic interneurons that most directly influence the activity of sympathetic preganglionic neurons after spinal cord injury are located close to their associated sympathetic preganglionic neurons, and the inputs from distant segments that mediate multisegmental reflexes are relayed to sympathetic preganglionic neurons multisynaptically via spinal sympathetic interneurons. Finally, spinal sympathetic interneurons are more likely to be excited and less likely to be inhibited by both noxious and innocuous somatic stimuli after chronic spinal transection. The onset of this hyperexcitability corresponds to morphological changes in both sympathetic preganglionic neurons and primary afferents, and it may reflect the pathophysiological processes that lead to autonomic dysreflexia and the hypertensive crises that may occur with it in people after chronic spinal injury.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.

GABAergic neurons in the central region of the spinal cord: a novel substrate for sympathetic inhibition

Homeostatic maintenance of widespread functions is critically dependent on the activity of the sympathetic nervous system. This activity is generated by the CNS acting on the sole output cells in the spinal cord, sympathetic preganglionic neurons (SPNs). SPNs are subject to control from both supraspinal and spinal inputs that exert effects through activation of direct or indirect pathways. A high proportion of indirect control is attributable to activation of spinal interneurons in a number of locations. However, little is known about the different groups of interneurons with respect to their neurochemistry or function. In this study, we report on a novel group of GABAergic interneurons located in the spinal central autonomic area (CAA) that directly inhibit SPN activity. In situ hybridization studies demonstrated a group of neurons that contained mRNA for glutamic acid decarboxylase (GAD)65 and GAD67 within the CAA. Combining in situ hybridization with trans-synaptic labeling from the adrenal gland using pseudorabies virus identified presympathetic GABAergic neurons in the CAA. Electrical stimulation of the CAA elicited monosynaptic IPSPs in SPNs located laterally in the intermediolateral cell column. IPSPs were GABAergic, because they reversed at the chloride reversal potential and were blocked by bicuculline. Chemical activation of neurons in the CAA hyperpolarized SPNs, an effect that was also bicuculline sensitive. We conclude that the CAA contains GABAergic interneurons that impinge directly onto SPNs to inhibit their activity and suggest that these newly identified interneurons may play an essential role in the regulation of sympathetic activity and thus homeostasis.

Kv3 voltage-gated potassium channels regulate neurotransmitter release from mouse motor nerve terminals

Voltage-gated potassium (Kv) channels are critical to regulation of neurotransmitter release throughout the nervous system but the roles and identity of the subtypes involved remain unclear. Here we show that Kv3 channels regulate transmitter release at the mouse neuromuscular junction (NMJ). Light- and electron-microscopic immunohistochemistry revealed Kv3.3 and Kv3.4 subunits within all motor nerve terminals of muscles examined [transversus abdominus, lumbrical and flexor digitorum brevis (FDB)]. To determine the roles of these Kv3 subunits, intracellular recordings were made of end-plate potentials (EPPs) in FDB muscle fibres evoked by electrical stimulation of tibial nerve. Tetraethylammonium (TEA) applied at low concentrations (0.05-0.5 mM), which blocks only a few known potassium channels including Kv3 channels, did not affect muscle fibre resting potential but significantly increased the amplitude of all EPPs tested. Significantly, this effect of TEA was still observed in the presence of the large-conductance calcium-activated potassium channel blockers iberiotoxin (25-150 nM) and Penitrem A (100 nM), suggesting a selective action on Kv3 subunits. Consistent with this, 15-microM 4-aminopyridine, which blocks Kv3 but not large-conductance calcium-activated potassium channels, enhanced evoked EPP amplitude. Unexpectedly, blood-depressing substance-I, a toxin selective for Kv3.4 subunits, had no effect at 0.05-1 microM. The combined presynaptic localization of Kv3 subunits and pharmacological enhancement of EPP amplitude indicate that Kv3 channels regulate neurotransmitter release from presynaptic terminals at the NMJ.

Localization and function of the Kv3.1b subunit in the rat medulla oblongata: focus on the nucleus tractus solitarii

The voltage-gated potassium channel subunit Kv3.1 confers fast firing characteristics to neurones. Kv3.1b subunit immunoreactivity (Kv3.1b-IR) was widespread throughout the medulla oblongata, with labelled neurones in the gracile, cuneate and spinal trigeminal nuclei. In the nucleus of the solitary tract (NTS), Kv3.1b-IR neurones were predominantly located close to the tractus solitarius (TS) and could be GABAergic or glutamatergic. Ultrastructurally, Kv3.1b-IR was detected in NTS terminals, some of which were vagal afferents. Whole-cell current-clamp recordings from neurones near the TS revealed electrophysiological characteristics consistent with the presence of Kv3.1b subunits: short duration action potentials (4.2 +/- 1.4 ms) and high firing frequencies (68.9 +/- 5.3 Hz), both sensitive to application of TEA (0.5 mm) and 4-aminopyridine (4-AP; 30 mum). Intracellular dialysis of an anti-Kv3.1b antibody mimicked and occluded the effects of TEA and 4-AP in NTS and dorsal column nuclei neurones, but not in dorsal vagal nucleus or cerebellar Purkinje cells (which express other Kv3 subunits, but not Kv3.1b). Voltage-clamp recordings from outside-out patches from NTS neurones revealed an outward K(+) current with the basic characteristics of that carried by Kv3 channels. In NTS neurones, electrical stimulation of the TS evoked EPSPs and IPSPs, and TEA and 4-AP increased the average amplitude and decreased the paired pulse ratio, consistent with a presynaptic site of action. Synaptic inputs evoked by stimulation of a region lacking Kv3.1b-IR neurones were not affected, correlating the presence of Kv3.1b in the TS with the pharmacological effects.

Association of potassium channel Kv3.4 subunits with pre- and post-synaptic structures in brainstem and spinal cord

Voltage-gated K+ channels (Kv) are divided into eight subfamilies (Kv1-8) and play a major role in determining the excitability of neurones. Members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique pattern of expression, although single neurones can express more than one subtype. Of the Kv3 subunits relatively little is known of the Kv3.4 subunit distribution in the nervous system, particularly in the brainstem and spinal cord of the rat. We performed immunohistochemistry to determine both the cellular and sub-cellular distribution of the Kv3.4 subunit in these areas. Kv3.4 subunit immunoreactivity (Kv3.4-IR) was widespread, with dense, punctate staining in many regions including the intermediolateral cell column (IML) and the dorsal vagal nucleus (DVN), nucleus ambiguus (NA) and nucleus tractus solitarius (NTS). In the ventral horn a presynaptic location was confirmed by co-localization of Kv3.4-IR with the synaptic vesicle protein, SV2 and also with the glutamate vesicle markers vesicular glutamate transporter (VGluT) 1, VGluT2 or the glycine transporter GlyT2, suggesting a role for the channel in both excitatory and inhibitory neurotransmission. Electron microscopy confirmed a presynaptic terminal location of Kv3.4-IR in the VH, IML, DVN, NA and NTS. Interestingly however, patches of Kv3.4-IR were also revealed postsynaptically in dendritic and somatic structures throughout these areas. This staining was striking due to its localization at synaptic junctions at terminals with morphological features consistent with excitatory functions, suggesting an association with the postsynaptic density. Therefore the pre and postsynaptic localization of Kv3.4-IR suggests a role both in the control of transmitter release and in regulating neuronal excitability.

Evidence for inhibition mediated by coassembly of GABAA and GABAC receptor subunits in native central neurons

Fast inhibition in the nervous system is commonly mediated by GABA(A) receptors comprised of 2alpha/2beta/1gamma subunits. In contrast, GABA(C) receptors containing only rho subunits (rho1-rho3) have been predominantly detected in the retina. However, here using reverse transcription-PCR and in situ hybridization we show that mRNA encoding the rho1 subunit is highly expressed in brainstem neurons. Immunohistochemistry localized the rho1 subunit to neurons at light and electron microscopic levels, where it was detected at synaptic junctions. Application of the GABA(C) receptor agonist cis-4-aminocrotonic acid (100-800 microM) requires the rho1 subunit to elicit responses, which surprisingly are blocked independently by antagonists to GABA(A) (bicuculline, 10 microM) and GABA(C) [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA); 40-160 microM] receptors. Responses to GABA(C) agonists were also enhanced by the GABA(A) receptor modulator pentobarbitone (300 microM). Spontaneous and evoked IPSPs were reduced in amplitude but never abolished by TPMPA, but were completely blocked by bicuculline. We therefore tested the hypothesis that GABA(A) and GABA(C) subunits formed a heteromeric receptor. Immunohistochemistry indicated that rho1 and alpha1 subunits were colocalized at light and electron microscopic levels. Electrophysiology revealed that responses to GABA(C) receptor agonists were enhanced by the GABA(A) receptor modulator zolpidem (500 nm), which acts on the alpha1 subunit when the gamma2 subunit is also present. Finally, coimmunoprecipitation indicated that the rho1 subunit formed complexes that also containedalpha1 and gamma2 subunits. Taken together these separate lines of evidence suggest that the effects of GABA in central neurons can be mediated by heteromeric complexes of GABA(A) and GABA(C) receptor subunits.

Spinal interneurons infected by renal injection of pseudorabies virus in the rat

The potency of spinal sympathetic reflexes is increased after spinal injury, and these reflexes may result in life-threatening hypertensive crises in humans. Few, if any, primary afferents project directly to sympathetic preganglionic neurons (SPN). Therefore, spinal sympathetic interneurons (IN) must play a major role in generating dysfunctional sympathetic activity after spinal cord injury. Furthermore, these IN are potentially aberrant targets, either for ascending and descending axons that may sprout after spinal cord injury or for axons that regenerate after spinal cord injury. We identified IN via the transsynaptic retrograde transport of pseudorabies virus (PRV) injected into the kidneys of rats. The proportion of infected IN ranged from approximately 1/3 to approximately 2/3 of the number of infected SPN. IN were heavily concentrated among the SPN in spinal lamina VII. However, IN were located in all lamina of the dorsal horn. The longitudinal distribution of infected IN was closely correlated with the longitudinal distribution of infected SPN. Few infected IN were found rostral or caudal to the longitudinal range of infected SPN. Infected IN were heterogeneous in both their sizes and the extent of their dendritic trees. The strong correlation between longitudinal distributions of infected IN and SPN supports physiological data demonstrating a segmental organization of spinal sympathetic reflexes. The paucity of infected IN in segments distant from SPN suggests that multisegmental sympathetic reflexes are mediated by projections onto IN rather than onto SPN themselves. The morphological heterogeneity of IN probably manifests the variety of systems that affect spinal sympathetic regulation.

Input-specific modulation of neurotransmitter release in the lateral horn of the spinal cord via adenosine receptors

Activation of adenosine A2A receptors (A2ARs) in the CNS produces a variety of neuromodulatory actions dependent on the region and preparation examined. In autonomic regions of the spinal cord, A1R activation decreases excitatory synaptic transmission, but the effects of A2AR stimulation are unknown. We sought to determine the location and function of the A2ARs in the thoracic spinal cord, focusing on the intermediolateral cell column (IML). A2AR immunoreactivity was observed throughout the gray matter, with particularly dense immunostaining in regions containing sympathetic preganglionic neurons (SPNs), namely, the IML and intercalated nucleus. Electron microscopy revealed A2AR immunoreactivity within presynaptic terminals and in postsynaptic structures in the IML. To study the functional relevance of these A2ARs, visualized whole-cell patch-clamp recordings were made from electrophysiologically identified SPNs and interneurons within the IML. The A2AR agonist c2-[p-(carboxyethyl)phenethylamino]-5'-N-ethylcarboxyamidoadenosine (CGS 21680) had no significant effect on EPSPs but increased the amplitude of IPSPs elicited by stimulation of the lateral funiculus. These effects were attributable to activation of presynaptic A2ARs because CGS 21680 application altered the paired pulse ratio. Furthermore, neurons in the IML that have IPSPs increased via A2AR activation also receive excitatory inputs that are inhibited by A1R activation. These data show that activating A2ARs increase inhibitory but not excitatory transmission onto neurons in the IML. Simultaneous activation of A1Rs and A2ARs therefore could facilitate inhibition of the postsynaptic neuron, leading to an overall reduction of sympathetic nervous activity.

Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K+ channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1-50 microM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 microM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217 +/- 65%) by 5 microM 4-AP without changes in axon conduction. 4-AP (5 microM) accelerated fictive locomotion induced by N-methyl-d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.

Inhibition by ethanol of NMDA-induced responses and acute tolerance to the inhibition in rat sympathetic preganglionic neurons in vitro and in vivo

N-methyl-d-aspartate (NMDA) receptors have been demonstrated to be a pivotal target for ethanol action. The present study examined the actions of acute ethanol exposure on NMDA-induced responses and the acute tolerance to ethanol actions in rat sympathetic preganglionic neurons (SPNs) in vitro and in vivo. NMDA (50 microM) applied every 5 min induced reproducible membrane depolarizations of SPNs in neonatal spinal cord slice preparations. Ethanol (50 - 100 mM) applied by superfusion for 15 min caused a sustained decrease in NMDA-induced depolarizations in a dose-dependent and reversible manner. When the superfusion time of ethanol (100 mm) was increased to 50 min, NMDA-induced depolarizations were attenuated initially but a gradual recovery was seen in approximately 40% of SPNs tested. Repeated injections of NMDA (2 nM) intrathecally at 30 min interval caused reproducible increases in mean arterial pressure (MAP) in urethane-anesthetized rats. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood concentration-dependent manner. The inhibition by ethanol of NMDA-induced pressor effects was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol. Ethanol, at concentrations significantly inhibited NMDA-induced responses, had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced responses. The study demonstrated the selective inhibition by ethanol of NMDA-induced responses and the development of acute tolerance to the inhibitory effects in SPNs both in vitro and in vivo. These effects may play important roles in the ethanol regulation of cardiovascular function.