On coupling and decoupling of spinal interneuronal networks

This review addresses the question of interrelations between spinal interneuronal networks. On the basis of electrophysiological, pharmacological, morphological and immunohistochemical analysis of interneurones mediating various reflex actions from muscle receptors and of reticulospinal neurones a considerable degree of interweaving between networks of these neurones has been established. The coupling has been found to occur at the level of several sites of these networks but the review focuses on two of these sites. The first is between dorsal horn interneurones with group II input and their target ipsilaterally and contralaterally projecting intermediate zone and commissural interneurones. The second is between commissural interneurones with input from reticulospinal neurones and their target interneurones. Several ways of both strengthening and weakening of coupling between various interneuronal networks are also briefly reviewed.

Neurons in the ventral spinal cord are more depressed by isoflurane, halothane, and propofol than are neurons in the dorsal spinal cord

BACKGROUND

Volatile anesthetics act primarily in the spinal cord to produce immobility but their exact site of action is unclear. Between 0.8 and 1.2 minimum alveolar anesthetic concentration (MAC), isoflurane does not depress neurons in the dorsal horn, suggesting that it acts at a more ventral site within the spinal cord such as in premotor interneurons and motoneurons. We hypothesized that isoflurane, halothane, and propofol would exert a greater depressant effect on nociceptive responses of ventral horn neurons when compared with dorsal horn neurons.

METHODS

Rats were anesthetized with isoflurane or halothane and responses of dorsal (<1200 microm deep) and ventral (>1200 microm) lumbar neurons to noxious mechanical stimulation of the hindpaw were determined at 0.8 and 1.2 MAC. In a third group anesthetized with isoflurane at 0.8 MAC, we administered 5 mg/kg propofol while recording responses from dorsal horn or ventral horn neurons.

RESULTS

Dorsal horn neuronal responses were not significantly affected when either isoflurane or halothane was increased from 0.8 to 1.2 MAC; propofol also had no significant effect. On the other hand, with increased isoflurane or halothane concentration, responses of ventral horn neurons were depressed by 60% and 45%, respectively. Propofol profoundly depressed (>90%) ventral horn neurons.

CONCLUSIONS

These data suggest that, in the peri-MAC range, isoflurane, halothane, and propofol have little or no effect on neuronal responses to noxious mechanical stimulation in the spinal dorsal horn but depress such responses in the ventral horn. Immobility produced in the 0.8-1.2 MAC range by these anesthetics appears to result from a depressant action in the ventral horn.

Laminar organization of spinal dorsal horn neurones activated by C- vs. A-heat nociceptors and their descending control from the periaqueductal grey in the rat

The periaqueductal grey can differentially control A- vs. C-nociceptor-evoked spinal reflexes and deep spinal dorsal horn neuronal responses. However, little is known about the control of A- vs. C-fibre inputs to lamina I and the lateral spinal nucleus, and how this correlates with the control of deeper laminae. To address this, the laminar distributions of neurones expressing Fos-like immunoreactivity were determined following preferential activation of A- or C-heat nociceptors, using fast or slow rates of skin heating, respectively, in the absence or presence of descending control evoked from the periaqueductal grey. In lamina I, numbers of Fos-positive neurones following both fast and slow rates of skin heating were reduced significantly following activation in the ventrolateral and dorsolateral/lateral periaqueductal grey. In contrast, in the deep dorsal horn (laminae III–VI), activation in both the ventrolateral and dorsolateral/lateral periaqueductal grey significantly reduced the numbers of Fos-positive neurones evoked by C- but not A-nociceptor stimulation. C- but not A-heat nociceptor activation evoked Fos bilaterally in the lateral spinal nucleus. Stimulation in the ventrolateral but not the dorsolateral/lateral periaqueductal grey significantly increased the numbers of Fos-positive neurones evoked by A- and C-nociceptor stimulation bilaterally in the lateral spinal nucleus. These data have demonstrated differences in the descending control of the superficial vs. the deep dorsal horn and lateral spinal nucleus with respect to the processing of A- and C-fibre-evoked events. The data are discussed in relation to the roles of A- and C-nociceptors in acute and chronic pain.

Transformation of the output of spinal lamina I neurons after nerve injury and microglia stimulation underlying neuropathic pain

BACKGROUND

Disinhibition of neurons in the superficial spinal dorsal horn, via microglia - neuron signaling leading to disruption of chloride homeostasis, is a potential cellular substrate for neuropathic pain. But, a central unresolved question is whether this disinhibition can transform the activity and responses of spinal nociceptive output neurons to account for the symptoms of neuropathic pain.

RESULTS

Here we show that peripheral nerve injury, local spinal administration of ATP-stimulated microglia or pharmacological disruption of chloride transport change the phenotype of spinal lamina I output neurons, causing them to 1) increase the gain of nociceptive responsiveness, 2) relay innocuous mechanical input and 3) generate spontaneous bursts of activity. The changes in the electrophysiological phenotype of lamina I neurons may account for three principal components of neuropathic pain: hyperalgesia, mechanical allodynia and spontaneous pain, respectively.

CONCLUSION

The transformation of discharge activity and sensory specificity provides an aberrant signal in a primarily nociceptive ascending pathway that may serve as a basis for the symptoms of neuropathic pain.

The activation of nicotinic acetylcholine receptors enhances the inhibitory synaptic transmission in the deep dorsal horn neurons of the adult rat spinal cord

Somatosensory information can be modulated by nicotinic acetylcholine receptors (nAChRs) in the superficial dorsal horn of the spinal cord. Nonetheless, the functional significance of nAChRs in the deep dorsal horn of adult animals remains unclear. Using whole-cell patch-clamp recordings from lamina V neurons in the adult rat spinal cord, we investigated whether the activation of nAChRs could modulate the inhibitory synaptic transmission in the deep dorsal horn. In the presence of CNQX and APV to block excitatory glutamatergic synaptic transmission, bath applications of nicotine (100 microM) significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in almost all neurons tested. The effect of nicotine was mimicked by N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403, 100 microM), an alpha 4 beta 2-nAChR agonist, and was also mimicked by choline (10 mM), an alpha 7-nAChR agonist. The effect of nicotine was completely blocked by the nAChR antagonist mecamylamine (5 microM). In the presence of tetrodotoxin (0.5 microM), nicotine (100 microM) significantly increased the miniature IPSC frequency. On the other hand, RJR-2403 (100 microM) or choline (10 mM) did not affect miniature IPSCs. The application of nicotine (100 microM) also evoked a large inward current in all lamina V neurons tested when cells were held at -60 mV. Similarly, RJR-2403 (100 microM) induced inward currents in the majority of lamina V neurons examined. On the other hand, choline (10 mM) did not elicit any detectable whole-cell currents. These results suggest that several nAChR subtypes are expressed on the presynaptic terminals, preterminals, and neuronal cell bodies within lamina V and that these nAChRs are involved in the modulation of inhibitory synaptic activity in the deep dorsal horn of the spinal cord.

Axon and dendrite geography predict the specificity of synaptic connections in a functioning spinal cord network

BACKGROUND

How specific are the synaptic connections formed as neuronal networks develop and can simple rules account for the formation of functioning circuits? These questions are assessed in the spinal circuits controlling swimming in hatchling frog tadpoles. This is possible because detailed information is now available on the identity and synaptic connections of the main types of neuron.

RESULTS

The probabilities of synapses between 7 types of identified spinal neuron were measured directly by making electrical recordings from 500 pairs of neurons. For the same neuron types, the dorso-ventral distributions of axons and dendrites were measured and then used to calculate the probabilities that axons would encounter particular dendrites and so potentially form synaptic connections. Surprisingly, synapses were found between all types of neuron but contact probabilities could be predicted simply by the anatomical overlap of their axons and dendrites. These results suggested that synapse formation may not require axons to recognise specific, correct dendrites. To test the plausibility of simpler hypotheses, we first made computational models that were able to generate longitudinal axon growth paths and reproduce the axon distribution patterns and synaptic contact probabilities found in the spinal cord. To test if probabilistic rules could produce functioning spinal networks, we then made realistic computational models of spinal cord neurons, giving them established cell-specific properties and connecting them into networks using the contact probabilities we had determined. A majority of these networks produced robust swimming activity.

CONCLUSION

Simple factors such as morphogen gradients controlling dorso-ventral soma, dendrite and axon positions may sufficiently constrain the synaptic connections made between different types of neuron as the spinal cord first develops and allow functional networks to form. Our analysis implies that detailed cellular recognition between spinal neuron types may not be necessary for the reliable formation of functional networks to generate early behaviour like swimming.

Morphology of inhibitory and excitatory interneurons in superficial laminae of the rat dorsal horn

If we are to stand any chance of understanding the circuitry of the superficial dorsal horn, it is imperative that we can identify which classes of interneuron are excitatory and which are inhibitory. Our aim was to test the hypothesis that there is a correlation between the morphology of an interneuron and its postsynaptic action. We used in vitro slice preparations of the rat spinal cord to characterize and label interneurons in laminae I-III with Neurobiotin. Labelled cells (n = 19) were reconstructed in 3D with Neurolucida and classified according to the scheme proposed by Grudt & Perl (2002). We determined if cells were inhibitory or excitatory by reacting their axon terminals with antibodies to reveal glutamate decrboxylase (for GABAergic cells) or the vesicular glutamate transporter 2 (for glutamatergic cells). All five islet cells retrieved were inhibitory. Of the six vertical (stalked) cells analysed, four were excitatory and, surprisingly, two were inhibitory. It was noted that these inhibitory cells had axonal projections confined to lamina II whereas excitatory vertical cells projected to lamina I and II. Of the remaining neurons, three were radial cells (2 inhibitory, 1 excitatory), two were antennae cells (1 inhibitory, 1 excitatory), one was an inhibitory central cell and the remaining two were unclassifiable excitatory cells. Our hypothesis appears to be correct only for islet cells. Other classes of cells have mixed actions, and in the case of vertical cells, the axonal projection appears to be a more important determinant of postsynaptic action.

Long-range projections of Adelta primary afferents in the Lissauer tract of the rat

Electrical microstimulation has been used to activate fine myelinated primary afferents running within the Lissauer tract. Stimulation of the tract at the L2/L3 border produced antidromic volleys which were recorded on the dorsal roots of more caudal spinal segments. Antidromic volleys were present in all cases for roots as far caudal as the S2 segment (L3, n=12; L4, n=6; L5, n=6; L6, n=9; S1, n=3; S2, n=6; observations in a total of 15 rats). These fibres were collaterals of primary afferents with conduction velocities in the dorsal root of up to 17.3+/-2.3 ms(-1) (mean+/-S.D., n=6; range 14-20 ms(-1)). Conduction velocities within the Lissauer tract were slower; the fastest contributing fibres had conduction velocities of 9.2+/-2.2 ms(-1) (range 6-12 ms(-1)). Lesions of the Lissauer tract caudal to the stimulation site abolished the volleys on roots lying caudal to the lesion. Most previous works have suggested that primary afferents project in the Lissauer tract for only one or two spinal segments. The present study shows that some fibres project rostrally for up to seven spinal segments (L2-S2).

Posterior spinal cord stimulation in a case of painful legs and moving toes

A 59-year-old woman with a 5-year history of right lower limb pain is reported. Symptoms developed initially when walking and progressively became bilateral, appeared at rest and involuntary movements of the toes became evident. A diagnosis of painful legs and moving toes was made. As several drug therapies proved unsuccessful, a therapeutic test with a tetrapolar epidural lead to stimulate the spinal cord dorsal tracts was performed. Due to the marked improvement the device and generator were implanted and she has responded satisfactorily to this therapy for the past 13 months.

Lipopolysaccharide induces a spinal learning deficit that is blocked by IL-1 receptor antagonism

Previous studies have shown that spinal neurons are capable of supporting a form of instrumental conditioning. Subjects receiving a spinal transection will learn to maintain a flexion response after exposure to shock contingent on leg position. In contrast, subjects receiving shock irrespective of leg position will not show increased flexion duration. Activation of the immune system has deleterious effects on learning in intact animals, but the impact of immune system activation on learning spinal animals is not known. We found that a large dose of i.p. LPS (1.0mg/kg) significantly disrupted the acquisition of the instrumental flexion response. The LPS-induced learning deficit was not prevented by preexposure to contingent shock (i.e. immunization) (Experiment 2). Co-administration of the iNOS inhibitor L-NIL (0.1, 1.0 and 10.0 microg/microL) failed to block the deficit (Experiment 3). Co-administration of an IL-1 receptor antagonist (r-metHuIL-1ra [10.0, 30.0 and 100.0 microg/microL) prevented the LPS-induced learning deficit when given in a dose of 100.0 microg/microL(i.t.) only (Experiment 4). Findings indicate a role for spinal IL-1 in the decreased plasticity following LPS administration.

GABAA and glycine receptor-mediated transmission in rat lamina II neurones: relevance to the analgesic actions of neuroactive steroids

Analgesic neurosteroids such as 5alpha-pregnan-3alpha-ol-20-one (5alpha3alpha) are potent selective endogenous modulators of the GABA(A) receptor (GABA(A)R) while certain synthetic derivatives (i.e. minaxolone) additionally enhance the function of recombinant glycine receptors (GlyR). Inhibitory transmission within the superficial dorsal horn has been implicated in mediating the analgesic actions of neurosteroids. However, the relative contribution played by synaptic and extrasynaptic receptors is unknown. In this study, we have compared the actions of 5alpha3alpha and minaxolone upon inhibitory transmission mediated by both GABA(A) and strychnine-sensitive GlyRs in lamina II neurones of juvenile (P15-21) rats. At the near physiological temperature of 35 degrees C and at a holding potential of -60 mV we recorded three kinetically distinct populations of miniature IPSCs (mIPSCs): GlyR-mediated, GABA(A)R-mediated and mixed GABA(A)R-GlyR mIPSCs, arising from the corelease of both inhibitory neurotransmitters. In addition, sequential application of strychnine and bicuculline revealed a small (5.2 +/- 1.0 pA) GlyR- but not a GABA(A)R-mediated tonic conductance. 5alpha3alpha (1-10 microm) prolonged GABA(A)R and mixed mIPSCs in a concentration-dependent manner but was without effect upon GlyR mIPSCs. In contrast, minaxolone (1-10 microm) prolonged the decay of GlyR mIPSCs and, additionally, was approximately 10-fold more potent than 5alpha3alpha upon GABA(A)R mIPSCs. However, 5alpha3alpha and minaxolone (1 microm) evoked a similar bicuculline-sensitive inhibitory conductance, indicating that the extrasynaptic GABA(A)Rs do not discriminate between these two steroids. Furthermore, approximately 92% of the effect of 1 microm 5alpha3alpha upon GABAergic inhibition could be accounted for by its action upon the extrasynaptic conductance. These findings are relevant to modulation of inhibitory circuits within spinally mediated pain pathways and suggest that extrasynaptic GABA(A)Rs may represent a relevant molecular target for the analgesic actions of neurosteroids.

Convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn

The convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn was investigated in urethane/chloralose-anesthetized rats. Electrical stimulation was applied to facial, neck, shoulder and forepaw skin, cornea (COR), dura, second cervical (C2) nerve, hypoglossal nerve, temporomandibular joint, masseter (MAS) muscle and superior laryngeal nerve. In addition, acetic acid was injected intraperitoneally and microinjection of glutamate was applied to the tongue, MAS muscle, splenius cervicis muscle, dura and intrapericardial area. A total of 52 nociceptive neurons classified as wide dynamic range (n = 28) or nociceptive-specific (n = 24) was studied. All nociceptive neurons received afferent input from the skin and at least one COR, musculoskeletal, dural or visceral afferent source in the trigeminal (V) or cervical area but input from afferent sources caudal to the C2 innervation territory was sparse. The proportion of neurons responding to COR, dural, C2 nerve, hypoglossal nerve, temporomandibular joint, MAS muscle and superior laryngeal nerve stimulations was 87, 54, 85, 52, 73, 64 and 31%, respectively. Electrical stimulation of all tested sites showed a double logarithmic stimulus-response relation, and cluster analysis of the excitability to COR, musculoskeletal, dural and visceral stimulations revealed two groups of neurons, one mainly containing wide dynamic range neurons and one mainly containing nociceptive-specific neurons. These findings indicate that afferent convergence in first cervical dorsal horn nociceptive neurons may be limited to the craniofacial area and that they may play an important role in the integration of craniofacial and upper cervical nociceptive inputs.

Long-term potentiation of neuronal excitation by neuron-glia interactions in the rat spinal dorsal horn

By imaging neuronal excitation in rat spinal cord slices with a voltage-sensitive dye, we examined the role of glial cells in the P2X receptor agonist alphabeta-methylene ATP (alphabetameATP)-triggered long-term potentiation (LTP) in the dorsal horn. Bath application of alphabetameATP potentiated neuronal excitation in the superficial dorsal horn. The potentiation was inhibited in the presence of the P2X receptor antagonists TNP-ATP, PPADS and A-317491, and was not induced in slices taken from rats neonatally treated with capsaicin. These results suggest that alphabetameATP acts on P2X receptors, possibly P2X(3) and/or P2X(2/3), in capsaicin-sensitive primary afferent terminals. Furthermore, the potentiation was inhibited by treatment with the glial metabolism inhibitor monofluoroacetic acid. Results obtained with the p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB203580, tumour necrosis factor-alpha (TNF-alpha) and interleukin (IL)-6, and antibodies to TNF-alpha and IL-6, as well as by double immunolabelling of activated p38 MAPK with markers of astrocytes and microglia, demonstrated that alphabetameATP activated p38 MAPK in astrocytes, and that the presence of proinflammatory cytokines and p38 MAPK activation were necessary for the induction of alphabetameATP-triggered LTP. These findings indicate that glial cells contribute to the alphabetameATP-induced LTP, which might be part of a cellular mechanism for the induction of persistent pain.

Rostral ventromedial medulla control of spinal sensory processing in normal and pathophysiological states

Complex networks of pathways project from various structures in the brain to modulate spinal processing of sensory input in a top-down fashion. The rostral ventromedial medulla (RVM) in the brainstem is one major final common output of this endogenous modulatory system and is involved in the relay of sensory information between the spinal cord and brain. The net output of descending neurons that exert inhibitory and facilitatory effects will determine whether neuronal activity in the spinal cord is increased or decreased. By pharmacologically blocking RVM activity with the local anesthetic lignocaine, and then measuring evoked responses of dorsal horn neurons to a range of applied peripheral stimuli, our aim was to determine the prevailing descending influence operating in normal anesthetized animals and animals with experimental neuropathic pain. The injection of 0.8 microl 2% lignocaine into the RVM caused a reduction in deep dorsal horn neuronal responses to electrical and natural stimuli in 64% of normal animals and in 81% of spinal-nerve-ligated (SNL) animals. In normal animals, responses to noxious input were predominantly reduced, while in SNL animals, reductions in spinal cord activity induced by intra-RVM lignocaine further included responses to non-noxious stimuli. This suggests that in terms of activity at least, if not number, descending facilitations are the predominant RVM influence that impacts the spinal cord in normal animals. Moreover, the increase in the proportion of neurons showing a post-lignocaine reduction in dorsal horn activity in SNL rats suggests that the strength of these facilitatory influences increases after neuropathy. This predominant inhibitory spinal effect following the injection of lignocaine into the RVM may be due to blockade of facilitatory On cells.

Moving from an averaged to specific view of spinal cord pain processing circuits

Neurons in the superficial dorsal horn (SDH) of the spinal cord play a critical role in processing potentially painful or noxious signals from skin, muscle, and viscera. Many acute pain therapies are based on the notion that altering the excitability of SDH neurons can block or gate these signals and reduce pain. This same notion also underlies treatments for certain chronic pain states. Basic scientists are now beginning to identify a number of potential molecular targets for spinal cord-based pain therapies with a focus on ion channels and receptors that can alter neuronal excitability. The current challenge in pain research is to identify which are the most promising targets and how their manipulation alters pain processing. In this review, we propose that our understanding of spinal pain processing mechanisms and translation of these discoveries into pain therapies could be improved by 1) better appreciating and understanding neuronal heterogeneity in the SDH; 2) establishing connectivity patterns among SDH neuron types; and 3) testing and extending findings made in vitro to intact (in vivo) animal models. As this information becomes available, it will be possible to determine the precise distribution of potential therapeutic targets on various SDH neuron types within specific circuits known to be functionally important in spinal pain processing.

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 microg/10 microl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

Nitrous oxide inhibits glutamatergic transmission in spinal dorsal horn neurons

The effects of nitrous oxide (N2O) are thought to be mediated by several pharmacological pathways at different levels of the central nervous system. Here, we focus on excitatory glutamatergic transmission in the superficial dorsal horn of the spinal cord with respect to its importance for the nociceptive processing. The effects of 50% N2O on electrically evoked and spontaneous excitatory glutamatergic transmission and on the response to exogenous administration of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA) receptor agonists were examined in lamina II neurons of adult rat spinal cord slices using the whole-cell patch-clamp technique. Peak amplitudes of Adelta- and C-fiber evoked monosynaptic NMDA- and AMPA-receptor-mediated excitatory postsynaptic currents (EPSCs) were decreased in the presence of N2O. N2O reduced the peak amplitude and integrated area of exogenous NMDA- and AMPA-induced currents. Moreover N2O changed the distribution of miniature EPSC amplitude, but not frequency distribution in most neurons. N2O inhibits glutamatergic transmission in the superficial dorsal horn by modulating the NMDA- and AMPA-receptors. Our findings raise the possibility that the antinociceptive effect of N2O may be directly mediated at the level of the spinal cord.

Clonidine reduces the excitability of spinal dorsal horn neurones

BACKGROUND

Clonidine has often been applied in combination with local anaesthetics for spinal or epidural anaesthesia. This study was designed to investigate the local anaesthetic-like action of clonidine in superficial dorsal horn neurones. The superficial laminae of the dorsal horn contain three groups of neurones: tonic-, adapting-, and single-spike-firing neurones which are important neuronal structures for pain transmission, receiving most of their primary sensory input from Adelta and C fibres.

METHODS

Whole cell patch clamp recordings from spinal cord slices of Wistar rats were used to study the action of clonidine on the generation of single and series of action potentials. Voltage clamp recordings in isolated somata were performed to study the effect of clonidine on voltage-gated Na(+) and different types of K(+) currents.

RESULTS

Firing frequencies of trains of action potentials in tonic-firing neurones are reduced at low concentrations (10 microM) of clonidine, but not in adapting- or single-spike-firing neurones. High concentrations of clonidine (700 microM) are necessary to modify the shape of single action potentials. Low concentrations of clonidine shift the steady-state inactivation curve of Na(+) currents to more negative potentials. At clinical concentrations (6-100 microM) clonidine partially inhibits voltage-gated Na(+) and K(+) channels.

CONCLUSIONS

Clonidine suppresses the generation of action potentials in tonic-firing spinal dorsal horn neurones. This may be explained, in part, by an interaction with voltage-gated Na(+) and K(+) currents. Clonidine could therefore contribute to analgesia during local anaesthesia.

Superficial dorsal horn neurons with double spike activity in the rat

Superficial dorsal horn neurons promote the transfer of nociceptive information from the periphery to supraspinal structures. The membrane and discharge properties of spinal cord neurons can alter the reliability of peripheral signals. In this paper, we analyze the location and response properties of a particular class of dorsal horn neurons that exhibits double spike discharge with a very short interspike interval (2.01+/-0.11 ms). These neurons receive nociceptive C-fiber input and are located in laminae I-II. Double spikes are generated spontaneously or by depolarizing current injection (interval of 2.37+/-0.22). Cells presenting double spike (interval 2.28+/-0.11) increased the firing rate by electrical noxious stimulation, as well as, in the first minutes after carrageenan injection into their receptive field. Carrageenan is a polysaccharide soluble in water and it is used for producing an experimental model of semi-chronic pain. In the present study carrageenan also produces an increase in the interval between double spikes and then, reduced their occurrence after 5-10 min. The results suggest that double spikes are due to intrinsic membrane properties and that their frequency is related to C-fiber nociceptive activity. The present work shows evidence that double spikes in superficial spinal cord neurones are related to the nociceptive stimulation, and they are possibly part of an acute pain-control mechanism.

Selective vulnerability to ischemia in the rat spinal cord: a comparison between ventral and dorsal horn neurons

STUDY DESIGN

Whole-cell patch-clamp recordings were performed from ventral horn (VH) and dorsal horn (DH) neurons obtained from the rat spinal cord slices.

OBJECTIVE

This study investigated which is more vulnerable to ischemia, spinal VH neurons or DH neurons.

SUMMARY OF BACKGROUND DATA

Spinal cord ischemia or injury sometimes causes a greater loss of motor function than of sensory function in patients. However, it is difficult to evaluate whether spinal motor neurons are more vulnerable than sensory neurons because of the anatomic complexity and a variety of physiologic factors in the spinal cord.

METHODS

Whole-cell patch-clamp recordings were performed from VH and DH neurons obtained from the spinal cord slices. Ischemia was simulated by superfusing an oxygen- and glucose-deprived medium (ischemia simulating medium [ISM]).

RESULTS

Perfusion with ISM generated an agonal depolarization in all VH and DH neurons recorded in current-clamp mode. Following ISM superfusion, an agonal inward current was produced at a holding potential of -70 mV in all VH and DH neurons tested in voltage-clamp mode. The agonal inward current consisted of a slow and subsequent rapid inward current. The average latency of the rapid inward currents after ISM exposures in VH neurons was significantly shorter than that in DH neurons. The average amplitude of the agonal inward currents in VH neurons was significantly bigger than that of DH neurons. Moreover, the recovery ratio by the reintroduction of oxygen and glucose in VH neurons was smaller than that in DH neurons.

CONCLUSIONS

These results suggest that VH neurons are more vulnerable to ischemia than DH neurons. This finding may help in achieving a better understanding of the difference between motor and sensory disturbance in spinal cord ischemia or injury patients.

Morphology and electrophysiological properties of hamster spinal dorsal horn neurons that express VGLUT2 and enkephalin

The excitatory amino acid glutamate mediates transmission at spinal synapses, including those formed by sensory afferent fibers and by intrinsic interneurons. The identity and physiological properties of glutamatergic dorsal horn neurons are poorly characterized despite their importance in spinal sensory circuits. Moreover, many intrinsic spinal glutamatergic synapses colocalize the opioid peptide enkephalin (ENK), but the neurons to which they belong are yet to be identified. Therefore, we used immunohistochemistry and confocal microscopy to investigate expression of the VGLUT2 vesicular glutamate transporter, an isoform reported in nonprimary afferent spinal synapses, and ENK in electrophysiologically identified neurons of hamster spinal dorsal horn. VGLUT2 immunoreactivity was localized in restricted fashion to axon varicosities of neurons recorded from laminae II-V, although the occurrence of immunolabeling in individual varicosities varied widely between cells (39 +/- 36%, n = 31 neurons). ENK colocalized with VGLUT2 in up to 77% of varicosities (17 +/- 21%, n = 21 neurons). The majority of neurons expressing VGLUT2 and/or ENK had axons with dense local terminations or projections consistent with propriospinal functions. VGLUT2 and ENK labeling were not correlated with cellular morphology, intrinsic membrane properties, firing patterns, or synaptic responses to sensory afferent stimulation. However, VGLUT2 expression was significantly higher in neurons with depolarized resting membrane potential. The results are new evidence for a population of dual-function dorsal horn interneurons that might provide another mechanism for limiting excitation within dorsal horn circuits during periods of strong sensory activation.

Inward currents induced by ischemia in rat spinal cord dorsal horn neurons

Hypoxia and ischemia occur in the spinal cord when blood vessels of the spinal cord are compressed under pathological conditions such as spinal stenosis, tumors, and traumatic spinal injury. Here by using spinal cord slice preparations and patch-clamp recordings we investigated the influence of an ischemia-simulating medium on dorsal horn neurons in deep lamina, a region that plays a significant role in sensory hypersensitivity and pathological pain. We found that the ischemia-simulating medium induced large inward currents in dorsal horn neurons recorded. The onset of the ischemia-induced inward currents was age-dependent, being onset earlier in older animals. Increases of sensory input by the stimulation of afferent fibers with electrical impulses or by capsaicin significantly speeded up the onset of the ischemia-induced inward currents. The ischemia-induced inward currents were abolished by the glutamate receptor antagonists CNQX (20 μM) and APV (50 μM). The ischemia-induced inward currents were also substantially inhibited by the glutamate transporter inhibitor TBOA (100 μM). Our results suggest that ischemia caused reversal operation of glutamate transporters, leading to the release of glutamate via glutamate transporters and the subsequent activation of glutamate receptors in the spinal dorsal horn neurons.

Spinal interneurons that are selectively activated during fictive flexion reflex

Behavioral choices in invertebrates are mediated by a combination of shared and specialized circuitry, including neurons that are inhibited during competing behaviors. Less is known, however, about the neural mechanisms of behavioral choice in vertebrates. The spinal cord can appropriately select among several types of limb movements, including limb withdrawal (flexion reflex), scratching, and locomotion, and thus is conducive to examination of vertebrate mechanisms of behavioral choice. Flexion reflex can interrupt and reset the rhythm of scratching and locomotion, suggesting that a combination of shared and specialized circuitry contributes to these behaviors, but little is known about the interneurons involved. Here, I used in vivo intracellular recording and dye injection to identify a group of spinal interneurons that are strongly activated during fictive flexion reflex but inhibited during fictive scratching and fictive swimming. These flexion-selective interneurons are typically rhythmically hyperpolarized during fictive scratching and fictive swimming. This hyperpolarization can be maximal during the ipsilateral hip flexor bursts of rhythmic limb motor patterns, although these cells are strongly activated during the ipsilateral hip flexor bursts of fictive flexion reflex. Thus, these interneurons are relatively specialized for fictive limb withdrawal, rather than contributing to the hip flexor phase of multiple types of limb movements. These flexion-selective cells are physiologically and morphologically distinguishable from a recently described group of spinal interneurons (transverse interneurons) that are strongly activated during both fictive flexion reflex and fictive scratching. Thus, spinal interneurons with distinct behavioral roles may to some extent be morphologically distinguishable.