A putative relay circuit providing low-threshold mechanoreceptive input to lamina I projection neurons via vertical cells in lamina II of the rat dorsal horn

Background: Lamina I projection neurons respond to painful stimuli, and some are also activated by touch or hair movement. Neuropathic pain resulting from peripheral nerve damage is often associated with tactile allodynia (touch-evoked pain), and this may result from increased responsiveness of lamina I projection neurons to non-noxious mechanical stimuli. It is thought that polysynaptic pathways involving excitatory interneurons can transmit tactile inputs to lamina I projection neurons, but that these are normally suppressed by inhibitory interneurons. Vertical cells in lamina II provide a potential route through which tactile stimuli can activate lamina I projection neurons, since their dendrites extend into the region where tactile afferents terminate, while their axons can innervate the projection cells. The aim of this study was to determine whether vertical cell dendrites were contacted by the central terminals of low-threshold mechanoreceptive primary afferents. Results: We initially demonstrated contacts between dendritic spines of vertical cells that had been recorded in spinal cord slices and axonal boutons containing the vesicular glutamate transporter 1 (VGLUT1), which is expressed by myelinated low-threshold mechanoreceptive afferents. To confirm that the VGLUT1 boutons included primary afferents, we then examined vertical cells recorded in rats that had received injections of cholera toxin B subunit (CTb) into the sciatic nerve. We found that over half of the VGLUT1 boutons contacting the vertical cells were CTb-immunoreactive, indicating that they were of primary afferent origin. Conclusions: These results show that vertical cell dendritic spines are frequently contacted by the central terminals of myelinated low-threshold mechanoreceptive afferents. Since dendritic spines are associated with excitatory synapses, it is likely that most of these contacts were synaptic. Vertical cells in lamina II are therefore a potential route through which tactile afferents can activate lamina I projection neurons, and this pathway could play a role in tactile allodynia

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region

Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation

Transduction of graded synaptic input into trains of all-or-none action potentials (spikes) is a crucial step in neural coding. Hodgkin identified three classes of neurons with qualitatively different analog-to-digital transduction properties. Despite widespread use of this classification scheme, a generalizable explanation of its biophysical basis has not been described. We recorded from spinal sensory neurons representing each class and reproduced their transduction properties in a minimal model. With phase plane and bifurcation analysis, each class of excitability was shown to derive from distinct spike initiating dynamics. Excitability could be converted between all three classes by varying single parameters; moreover, several parameters, when varied one at a time, had functionally equivalent effects on excitability. From this, we conclude that the spike-initiating dynamics associated with each of Hodgkin's classes represent different outcomes in a nonlinear competition between oppositely directed, kinetically mismatched currents. Class 1 excitability occurs through a saddle node on invariant circle bifurcation when net current at perithreshold potentials is inward (depolarizing) at steady state. Class 2 excitability occurs through a Hopf bifurcation when, despite net current being outward (hyperpolarizing) at steady state, spike initiation occurs because inward current activates faster than outward current. Class 3 excitability occurs through a quasi-separatrix crossing when fast-activating inward current overpowers slow-activating outward current during a stimulus transient, although slow-activating outward current dominates during constant stimulation. Experiments confirmed that different classes of spinal lamina I neurons express the subthreshold currents predicted by our simulations and, further, that those currents are necessary for the excitability in each cell class. Thus, our results demonstrate that all three classes of excitability arise from a continuum in the direction and magnitude of subthreshold currents. Through detailed analysis of the spike-initiating process, we have explained a fundamental link between biophysical properties and qualitative differences in how neurons encode sensory input

Correlations between neuronal morphology and electrophysiological features in the rodent superficial dorsal horn

Relationships between the morphology of individual neurones of the spinal superficial dorsal horn (SDH), laminae I and II, and their electrophysiological properties were studied in spinal cord slices prepared from anaesthetized, free-ranging hamsters. Tight-seal, whole-cell recordings were made with pipette microelectrodes filled with biocytin to establish electrophysiological characteristics and to label the studied neurones. Neurones were categorized according to location and size of the somata, the dendritic and axonal pattern of arborization, spontaneous synaptic potentials, evoked postsynaptic currents, pattern of discharge to depolarizing pulses and current-voltage relationships. Data were obtained for 170 neurones; 13 of these had somata in lamina I and 157 in lamina II. Stimulation of the segmental dorsal root evoked a prompt excitatory response in almost every neurone sampled (161/166) with nearly 3/4 displaying putative monosynaptic EPSCs. The majority of neurones (133/170) fitted one of several distinctive morphological categories. To a considerable extent, neurones with a common morphological configuration and neurite disposition shared electrophysiological characteristics. Five of the 13 lamina I neurones were relatively large with extensive dendritic arborization in the horizontal dimension and a prominent axon directed ventrally and contralaterally. These presumptive ventrolateral projection neurones differed structurally and electrophysiologically from the other lamina I neurones, which had ipsilateral, locally arborizing axons and/or branches entering the dorsal lateral funiculus. One hundred and twenty lamina II neurones fitted one of five morphological categories: islet, central, medial-lateral, radial or vertical. Central cells were further divided into three groups on functional features. We conclude that the spinal SDH comprises many types of neurones whose morphological characteristics are associated with specific functional features implying diversity in functional organization of the SDH and in its role as a major synaptic termination for thin primary afferent fibres

Hypocretin-2 (orexin-B) modulation of superficial dorsal horn activity in rat

The hypothalamic peptides hypocretin-1 (orexin A) and hypocretin-2 (Hcrt-2; orexin B) are important in modulating behaviours demanding arousal, including sleep and appetite. Fibres containing hypocretin project from the hypothalamus to the superficial dorsal horn (SDH) of the spinal cord (laminae I and II); however, the effects produced by hypocretins on SDH neurones are unknown. To study the action of Hcrt-2 on individual SDH neurones, tight-seal, whole-cell recordings were made with biocytin-filled electrodes from rat lumbar spinal cord slices. In 19 of 63 neurones, Hcrt-2 (30 nm to 1 μm) evoked an inward (excitatory) current accompanied by an increase in baseline noise. The inward current and noise were unaffected by TTX but were blocked by the P2X purinergic receptor antagonist suramin (300–500 μm). Hcrt-2 (30 nm to 1 μm) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in the majority of neurones. The sIPSC increase was blocked by strychnine (1 μm) and by TTX (1 μm), suggesting that the increased sIPSC frequency was glycine and action potential dependent. Hcrt-2 increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in a few neurones but had no effect on dorsal root-evoked EPSCs in these or in other neurones. Neurones located in outer lamina II, particularly radial and vertical cells, were most likely to respond to Hcrt-2. We conclude that Hcrt-2 has excitatory effects on certain SDH neurones, some of which exert inhibitory influences on other cells of the region, consistent with the perspective that hypocretin has a role in orchestrating reactions related to arousal, including nociception, pain and temperature sense

Cannabinoid actions on rat superficial medullary dorsal horn neurons in vitro

This study examined the cellular actions of cannabinoids on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis, using whole-cell and perforated patch recording in brain slices.The cannabinoid agonist WIN55,212-2 (3 μm) decreased the amplitude of both GABAergic and glycinergic electrically evoked inhibitory postsynaptic currents (IPSCs) by 35 and 41%, respectively. This inhibition was completely reversed by the CB1 receptor-selective antagonist N-piperidino-5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide) (SR141716A, 3 μm). WIN55,212-2 also produced relative facilitation of the second evoked IPSC to paired stimuli.WIN55,212-2 decreased the rate of both GABAergic and glycinergic miniature IPSCs by 44 and 34%, respectively, without changing their amplitude distributions or kinetics.WIN55,212-2 did not affect the amplitude of electrically evoked non-NMDA glutamatergic excitatory postsynaptic currents (EPSCs).WIN55,212-2 produced no postsynaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (–60 to –130 mV).These results suggest that, within the superficial medullary dorsal horn, cannabinoids presynaptically inhibit GABAergic and glycinergic neurotransmission. At the cellular level, the analgesic action of cannabinoids on these medullary dorsal horn neurons therefore differs from that of μ-opioids, which have both pre- and postsynaptic actions