Calcium channels in isolated rat dorsal horn neurones, including labelled spinothalamic and trigeminothalamic cells

Voltage-gated Calcium Channels as Potential Therapeutic Targets in Migraine

Migraine is a complex and highly incapacitating neurological disorder that affects around 15% of the general population with greater incidence in women, often at the most productive age of life. Migraine physiopathology is still not fully understood, but it involves multiple mediators and events in the trigeminovascular system and the central nervous system. The identification of calcitonin gene-related peptide as a key mediator in migraine physiopathology has led to the development of effective and highly selective antimigraine therapies. However, this treatment is neither accessible nor effective for all migraine sufferers. Thus, a better understanding of migraine mechanisms and the identification of potential targets are still clearly warranted. Voltage-gated calcium channels (VGCCs) are widely distributed in the trigeminovascular system, and there is accumulating evidence of their contribution to the mechanisms associated with headache pain. Several drugs used in migraine abortive or prophylactic treatment target VGCCs, which probably contributes to their analgesic effect. This review aims to summarize the current evidence of VGGC contribution to migraine physiopathology and to discuss how current pharmacological options for migraine treatment interfere with VGGC function. PERSPECTIVE: Calcitonin gene-related peptide (CGRP) represents a major migraine mediator, but few studies have investigated the relationship between CGRP and VGCCs. CGRP release is calcium channel-dependent and VGGCs are key players in familial migraine. Further studies are needed to determine whether VGCCs are suitable molecular targets for treating migraine.

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically, the action potential is initiated once voltage-activated Na⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential's all-or-none nature. Here we demonstrate that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-activated Na⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-activated Ca²⁺ channel, enabling spikes of different amplitude. T-type Ca²⁺ channels generate small amplitude spikes, whereas larger amplitude spikes require high voltage-activated Cd²⁺-sensitive Ca²⁺ channels. We demonstrate that these different amplitude spikes can signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes whereas cholinergic inputs evoke larger spikes that do not rely on T-type channels. Different synaptic inputs to CSFcNs can therefore be signaled by the spike amplitude.

Nerve injury elevates functional Cav3.2 channels in superficial spinal dorsal horn

Cav3 channels play an important role in modulating chronic pain. However, less is known about the functional changes of Cav3 channels in superficial spinal dorsal horn in neuropathic pain states. Here, we examined the effect of partial sciatic nerve ligation (PSNL) on either expression or electrophysiological properties of Cav3 channels in superficial spinal dorsal horn. Our in vivo studies showed that the blockers of Cav3 channels robustly alleviated PSNL-induced mechanical allodynia and thermal hyperalgesia, which lasted at least 14 days following PSNL. Meanwhile, PSNL triggered an increase in both mRNA and protein levels of Cav3.2 but not Cav3.1 or Cav3.3 in rats. However, in Cav3.2 knockout mice, PSNL predominantly attenuated mechanical allodynia but not thermal hyperalgesia. In addition, the results of whole-cell patch-clamp recordings showed that both the overall proportion of Cav3 current-expressing neurons and the Cav3 current density in individual neurons were elevated in spinal lamina II neurons from PSNL rats, which could not be recapitulated in Cav3.2 knockout mice. Altogether, our findings reveal that the elevated functional Cav3.2 channels in superficial spinal dorsal horn may contribute to the mechanical allodynia in PSNL-induced neuropathic pain model.

Two distinct types of depolarizing afterpotentials are differentially expressed in stellate and pyramidal-like neurons of entorhinal-cortex layer II

Two types of principal neurons, stellate cells and pyramidal-like cells, are found in medial entorhinal-cortex (mEC) layer II, and are believed to represent two distinct channels of information processing and transmission in the entorhinal cortex-hippocampus network. In this study, we found that depolarizing afterpotentials (DAPs) that follow single action potentials (APs) evoked from various levels of holding membrane voltage (Vh ) show distinct properties in the two cells types. In both, an evident DAP followed the AP at near-threshold Vh levels, and was accompanied by an enhancement of excitability and spike-timing precision. This DAP was sensitive to voltage-gated Na(+)-channel block with TTx, but not to partial removal of extracellular Ca(2+). Application of 5-μM anandamide, which inhibited the resurgent and persistent Na(+) -current components in a relatively selective way, significantly reduced the amplitude of this particular DAP while exerting poor effects on the foregoing AP. In the presence of background hyperpolarization, DAPs showed an opposite behavior in the two cell types, as in stellate cells they became even more prominent, whereas in pyramidal-like cells their amplitude was markedly reduced. The DAP observed in stellate cells under this condition was strongly inhibited by partial extracellular-Ca(2+) removal, and was sensitive to the low-voltage-activated Ca(2+)-channel blocker, NNC55-0396. This Ca(2+) dependence was not observed in the residual DAP evoked in pyramidal-like cells from likewise negative Vh levels. These results demonstrate that two distinct mechanism of DAP generation operate in mEC layer-II neurons, one Na(+)-dependent and active at near-threshold Vh levels in both stellate and-pyramidal-like cells, the other Ca(2+)-dependent and only expressed by stellate cells in the presence of background membrane hyperpolarization.

Characterisation of rebound depolarisation in mice deep dorsal horn neurons in vitro

Spinal dorsal horn neurons constitute the first relay for pain processing and participate in the processing of other sensory, motor and autonomic information. At the cellular level, intrinsic excitability is a factor contributing to network function. In turn, excitability is set by the array of ionic conductance expressed by neurons. Here, we set out to characterise rebound depolarisation following hyperpolarisation, a feature frequently described in dorsal horn neurons but never addressed in depth. To this end, an in vitro preparation of the spinal cord from mice pups was used combined with whole-cell recordings in current and voltage clamp modes. Results show the expression of H- and/or T-type currents in a significant proportion of dorsal horn neurons. The expression of these currents determines the presence of rebound behaviour at the end of hyperpolarising pulses. T-type calcium currents were associated to high-amplitude rebounds usually involving high-frequency action potential firing. H-currents were associated to low-amplitude rebounds less prone to elicit firing or firing at lower frequencies. For a large proportion of neurons expressing both currents, the H-current constitutes a mechanism to ensure a faster response after hyperpolarisations, adjusting the latency of the rebound firing. We conclude that rebound depolarisation and firing are intrinsic factors to many dorsal horn neurons that may constitute a mechanism to integrate somatosensory information in the spinal cord, allowing for a rapid switch from inhibited-to-excited states.

Ionotropic glutamate receptors and voltage-gated Ca²⁺ channels in long-term potentiation of spinal dorsal horn synapses and pain hypersensitivity

Over the last twenty years of research on cellular mechanisms of pain hypersensitivity, long-term potentiation (LTP) of synaptic transmission in the spinal cord dorsal horn (DH) has emerged as an important contributor to pain pathology. Mechanisms that underlie LTP of spinal DH neurons include changes in the numbers, activity, and properties of ionotropic glutamate receptors (AMPA and NMDA receptors) and of voltage-gated Ca²⁺ channels. Here, we review the roles and mechanisms of these channels in the induction and expression of spinal DH LTP, and we present this within the framework of the anatomical organization and synaptic circuitry of the spinal DH. Moreover, we compare synaptic plasticity in the spinal DH with classical LTP described for hippocampal synapses.

Non-Hebbian plasticity at C-fiber synapses in rat spinal cord lamina I neurons

Current concepts of memory storage are largely based on Hebbian-type synaptic long-term potentiation induced by concurrent activity of pre- and postsynaptic neurons. Little is known about non-Hebbian synaptic plasticity, which, if present in nociceptive pathways, could resolve a number of unexplained findings. We performed whole-cell patch-clamp recordings in rat spinal cord slices and found that a rise in postsynaptic [Ca²⁺]_i due to postsynaptic depolarization was sufficient to induce synaptic long-term potentiation (LTP) in the absence of any presynaptic conditioning stimulation. LTP induction could be prevented by postsynaptic application of the Ca²⁺ chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid), the L-type voltage-gated calcium channel (VGCC) antagonist nifedipine, and by postsynaptic application of the NMDA receptor antagonist MK801. This indicates that synaptic potentiation was induced postsynaptically by Ca²⁺ entry likely via L-type voltage-gated Ca²⁺ channels (VGCC) and via NMDA receptor channels. The paired pulse ratio and the coefficient of variation remained unchanged in neurons expressing LTP, suggesting that this form of synaptic potentiation was not only induced, but also expressed postsynaptically. Postsynaptic depolarization had no influence on firing patterns, action potential shape, or neuronal excitability. An increase in [Ca²⁺]_i in spinal lamina I neurons induces a non-Hebbian form of synaptic plasticity in spinal nociceptive pathways without affecting neuronal active and passive membrane properties.

Omega-conotoxins as experimental tools and therapeutics in pain management

Neuropathic pain afflicts a large percentage of the global population. This form of chronic, intractable pain arises when the peripheral or central nervous systems are damaged, either directly by lesion or indirectly through disease. The comorbidity of neuropathic pain with other diseases, including diabetes, cancer, and AIDS, contributes to a complex pathogenesis and symptom profile. Because most patients present with neuropathic pain refractory to current first-line therapeutics, pharmaceuticals with greater efficacy in pain management are highly desired. In this review we discuss the growing application of ω-conotoxins, small peptides isolated from Conus species, in the management of neuropathic pain. These toxins are synthesized by predatory cone snails as a component of paralytic venoms. The potency and selectivity with which ω-conotoxins inhibit their molecular targets, voltage-gated Ca²⁺ channels, is advantageous in the treatment of neuropathic pain states, in which Ca²⁺ channel activity is characteristically aberrant. Although ω-conotoxins demonstrate analgesic efficacy in animal models of neuropathic pain and in human clinical trials, there remains a critical need to improve the convenience of peptide drug delivery methods, and reduce the number and severity of adverse effects associated with ω-conotoxin-based therapies.

Anaesthetics differentially modulate the trigeminocardiac reflex excitatory synaptic pathway in the brainstem

The trigeminocardiac reflex (TCR) occurs upon excitation of the trigeminal nerve with a resulting bradycardia and hypotension. While several anaesthetics and analgesics have been reported to alter the incidence and strength of the TCR the mechanisms for this modulation are unclear. This study examines the mechanisms of action of ketamine, isoflurane and fentanyl on the synaptic TCR responses in both neurones in the spinal trigeminal interpolaris (Sp5I) nucleus and cardiac vagal neurones (CVNs) in the Nucleus Ambiguus (NA). Stimulation of trigeminal afferent fibres evoked an excitatory postsynaptic current (EPSC) in trigeminal neurones with a latency of 1.8 ± 0.1 ms, jitter of 625 μs, and peak amplitude of 239 ± 45 pA. Synaptic responses further downstream in the reflex pathway in the CVNs occurred with a latency of 12.1 ± 1.1 ms, jitter of 0.8-2 ms and amplitude of 57.8 ± 7.5 pA. The average conduction velocity to the Sp5I neurones was 0.94 ± 0.18 mm ms(-1) indicating a mixture of A-δ and C fibres. Stimulation-evoked EPSCs in both Sp5I and CVNs were completely blocked by AMPA/kainate and NMDA glutamatergic receptor antagonists. Ketamine (10 μm) inhibited the peak amplitude and duration in Sp5I as well as more distal synapses in the CVNs. Isoflurane (300 μm) significantly inhibited, while fentanyl (1 μm) significantly enhanced, EPSC amplitude and area in CVNs but had no effect on the responses in Sp5l neurones. These findings indicate glutamatergic excitatory synaptic pathways are critical in the TCR, and ketamine, isoflurane and fentanyl differentially alter the synaptic pathways via modulation of both AMPA/kainate and NMDA receptors at different synapses in the TCR.

Multiple T-type Ca2+ current subtypes in electrophysiologically characterized hamster dorsal horn neurons: possible role in spinal sensory integration

Whole cell patch-clamp recordings were used to investigate the contribution of transient, low-threshold calcium currents (I(T)) to firing properties of hamster spinal dorsal horn neurons. I(T) was widely, though not uniformly, expressed by cells in Rexed's laminae I-IV and correlated with the pattern of action potential discharge evoked under current-clamp conditions: I(T) in neurons responding to constant membrane depolarization with one or two action potentials was nearly threefold larger than I(T) in cells responding to the same activation with continuous firing. I(T) was evoked by depolarizing voltage ramps exceeding 46 mV/s and increased with ramp slope (240-2,400 mV/s). Bath application of 200 μM Ni(2+) depressed ramp-activated I(T). Phasic firing recorded in current clamp could only be activated by membrane depolarizations exceeding ∼43-46 mV/s and was blocked by Ni(2+) and mibefradil, suggesting I(T) as an underlying mechanism. Two components of I(T), "fast" and "slow," were isolated based on a difference in time constant of inactivation (12 ms and 177 ms, respectively). The amplitude of the fast subtype depended on the slope of membrane depolarization and was twice as great in burst-firing cells than in cells having a tonic discharge. Post hoc single-cell RT-PCR analyses suggested that the fast component is associated with the Ca(V)3.1 channel subtype. I(T) may enhance responses of phasic-firing dorsal horn neurons to rapid membrane depolarizations and contribute to an ability to discriminate between afferent sensory inputs that encode high- and low-frequency stimulus information.

Formalin-induced short- and long-term modulation of Cav currents expressed in Xenopus oocytes: an in vitro cellular model for formalin-induced pain

Xenopus oocytes expressing high voltage-gated calcium channels (Ca(v)) were exposed to formalin (0.5%, v/v, 5 min.) and the oocyte death and Ca(v) currents were studied for up to 10 days. Ca(v) channels were expressed with alpha(1)beta(1)b and alpha(2)delta sub-units and the currents (I(Ba)) were studied by voltage clamp. None of the oocytes was dead during the exposure to formalin. Oocyte death was significant between day 1 and day 5 after the exposure to formalin and was uniform among the oocytes expressing various Ca(v) channels. Peak I(Ba) of all Ca(v) and A(1), the inactivating current component was decreased whereas the non-inactivated R current was not affected by 5 min. exposure to formalin. On day 1 after the exposure to formalin, Ca(v)1.2c currents were increased, 2.1 and 2.2 currents were decreased and 2.3 currents were unaltered. On day 5, both peak I(Ba) and A(1) currents were increased. Ca(v)1.2c, 2.2 and 2.3 currents were increased and Ca(v)2.1 was unaltered on day 10 after the exposure to formalin. Protein kinase C (PKC) may be involved in formalin-induced increase in Ca(v) currents due to the (i) requirement for Ca(v)beta(1)b sub-units; (ii) decreased phorbol-12-myristate,13-acetate potentiation of Ca(v)2.3 currents; (iii) absence of potentiation of Ca(v)2.3 currents following down-regulation of PKC; and (iv) absence of potentiation of Ca(v)2.2 or 2.3 currents with Ser-->Ala mutation of Ca(v)alpha(1)2.2 or 2.3 sub-units. Increased Ca(v) currents and PKC activation may coincide with changes observed in in vivo pain investigations, and oocytes incubated with formalin may serve as an in vitro model for some cellular mechanisms of pain.

Electrophysiological and morphological properties of neurons in the substantia gelatinosa of the mouse trigeminal subnucleus caudalis

The excitability of the second order neurons within the trigeminal subnucleus caudalis underlies pain perception and processing in migraine and trigeminal neuralgia. These neurons were studied with whole-cell patch-clamp technique in slices from mouse brain stem. Electrical and morphological characteristics of 56 neurons were determined. Four categories were distinguished from electrophysiological properties: tonic (39%), phasic (34%), delayed (16%) and single spiking (11%). These categories did not show distinct morphological properties. Neurons had tetrodotoxin-sensitive sodium currents that activated and inactivated within milliseconds. They also showed a high voltage-activated, slowly inactivating calcium current: up to half of this current was blocked by omega-conotoxin GVIA (1microM) and omega-agatoxin IVA (100-300 nM), but it was not affected by nifedipine (10microM). Exogenously applied capsaicin (1microM) and alphabetamethylene-5'-adenosine triphosphate (100microM) elicited large amplitude, spontaneous excitatory postsynaptic currents that were blocked by capsazepine (10microM) and 5-[(3-phenoxybenzyl)-(1,2,3,4-tetrahydro-naphthalen-1-yl)-carbamoyl]-benzene-1,2,4-tricarboxylic acid (A-317491: 10microM), respectively. Thus, neurons of the mouse trigeminal subnucleus caudalis substantia gelatinosa exhibit N-type and P/Q-type voltage-gated calcium channels, and receive presynaptic afferents that express TRPV1 and P2X(2/3) receptors. These results suggest possible therapeutic interventions, and serve as a basis for the characterization of cellular changes that may underlie trigeminal neuropathic pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Spinal cord injury causes plasticity in a subpopulation of lamina I GABAergic interneurons

Dysfunction of the spinal GABAergic system has been implicated in pain syndromes following spinal cord injury (SCI). Since lamina I is involved in nociceptive and thermal signaling, we characterized the effects of chronic SCI on the cellular properties of its GABAergic neurons fluorescently identified in spinal slices from GAD67-GFP transgenic mice. Whole cell recordings were obtained from the lumbar cord of 13- to 17-day-old mice, including those having had a thoracic segment (T8-11) removed 6-9 days prior to experiments. Following chronic SCI, the distribution, incidence, and firing classes of GFP+ cells remained similar to controls, and there were minimal changes in membrane properties in cells that responded to current injection with a single spike. In contrast, cells displaying tonic/initial burst firing had more depolarized membrane potentials, increased steady-state outward currents, and increased spike heights. Moreover, higher firing frequencies and spontaneous plateau potentials were much more prevalent after chronic SCI, and these changes occurred predominantly in cells displaying a tonic firing pattern. Persistent inward currents (PICs) were observed in a similar fraction of cells from spinal transects and may have contributed to these plateaus. Persistent Na+ and L-type Ca2+ channels likely contributed to the currents as both were identified pharmacologically. In conclusion, chronic SCI induces a plastic response in a subpopulation of lamina I GABAergic interneurons. Alterations are directed toward amplifying neuronal responsiveness. How these changes alter spinal sensory integration and whether they contribute to sensory dysfunction remains to be elucidated.

Is there a role for T-type calcium channels in peripheral and central pain sensitization?

Following tissue injury, both peripheral and central sensory neurons can become hyperexcitable, or "sensitized." Sensitization can lead to long-term pathological changes in pain sensation. Because many chronic pain conditions are refractory to most currently available treatments, there is great interest in identifying molecular targets that contribute to the sensitization of sensory neurons. Among these, several classes of ion channels have emerged as potential targets. Recent in vitro and in vivo studies have demonstrated a role for T-type Ca2+ channels in sensory pathways and have suggested that these channels may contribute to pain processing and sensitization. Therefore, T-type channels may represent an opportunity for the development of novel pain therapeutics and may help to address an unmet medical need.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors modulate release of inhibitory amino acids in rat spinal cord dorsal horn

Local inhibition within the spinal cord dorsal horn is mediated by the neurotransmitters GABA and glycine and strongly influences nociceptive and temperature signaling. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are expressed by inhibitory interneurons and have been shown to modulate GABA release in other regions of the CNS. In the spinal cord, there is morphological evidence for presynaptic AMPA receptor subunits in GABAergic dorsal horn neurons, but functional data are lacking. To determine if AMPA receptors are indeed functional at presynaptic terminals of inhibitory neurons, we recorded evoked and miniature inhibitory postsynaptic currents (mIPSPs) in the superficial dorsal horn of the rat spinal cord. We show that AMPA receptor activation enhances spontaneous release of inhibitory amino acids in the presence of tetrodotoxin onto both lamina II neurons and NK1 receptor-expressing (NK1R+) lamina I neurons. This effect is sensitive to the concentration of extracellular Ca2+, yet is not fully blocked in most neurons in the presence of Cd2+, suggesting possible Ca2+ entry through AMPA receptors. Postsynaptic Ca2+ elevation is not required for these changes. AMPA-induced increases in mIPSP frequency are also seen in more mature dorsal horn neurons, indicating that these receptors may play a role in nociceptive processing in the adult. In addition, we have observed AMPA-induced depression of evoked release of GABA and glycine onto lamina I NK1R+ neurons. Taken together these data support a role for presynaptic AMPA receptors in modulating release of GABA and glycine in the superficial dorsal horn. Because inhibition in the dorsal horn is important for controlling pain signaling, presynaptic AMPA receptors acting to modulate the inhibitory inputs onto dorsal horn neurons would be expected to impact upon pain signaling in the spinal cord dorsal horn.

Pre- and postsynaptic contributions of voltage-dependent Ca2+ channels to nociceptive transmission in rat spinal lamina I neurons

Activation of voltage-dependent Ca2+ channels (VDCCs) is critical for neurotransmitter release, neuronal excitability and postsynaptic Ca2+ signalling. Antagonists of VDCCs can be antinociceptive in different animal pain models. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the processing of pain-related information, but the role of VDCCs to the activity-dependent Ca2+ increase in lamina I neurons and to the synaptic transmission between nociceptive afferents and second order neurons in lamina I is not known. This has now been investigated in a lumbar spinal cord slice preparation from young Sprague-Dawley rats. Microfluorometric Ca2+ measurements with fura-2 have been used to analyse the Ca2+ increase in lamina I neurons after depolarization of the cells, resulting in a distinct and transient increase of the cytosolic Ca2+ concentration. This Ca2+ peak was reduced by the T-type channel blocker, Ni2+, by the L-type channel blockers, nifedipine and verapamil, and by the N-type channel blocker, omega-conotoxin GVIA. The P/Q-type channel antagonist, omega-agatoxin TK, had no effect on postsynaptic [Ca2+]i. The NMDA receptor channel blocker D-AP5 reduced the Ca2+ peak, whereas the AMPA receptor channel blocker CNQX had no effect. Postsynaptic currents, monosynaptically evoked by electrical stimulation of the attached dorsal roots with C-fibre and Adelta-fibre intensity, respectively, were reduced by N-type channel blocker omega-conotoxin GVIA and to a much lesser extent, by P/Q-type channel antagonist omega-agatoxin TK, and the L-type channel blockers verapamil, respectively. No difference was found between unidentified neurons and neurons projecting to the periaqueductal grey matter. This is the first quantitative description of the relative contribution of voltage-dependent Ca2+ channels to the synaptic transmission in lamina I of the spinal dorsal horn, which is essential in the processing of pain-related information in the central nervous system.

Mechanical and thermal anti-nociception in rats after systemic administration of verapamil

Voltage-gated Ca(2+) channels expressed in neurons may contribute to nociceptive information processing. However, the role of L-type Ca(2+) channels in pain transmission is not well understood. In this study, we examined the effects of systemically administered verapamil, an antihypertensive agent and L-type Ca(2+) channel blocker, on mechanical and thermal withdrawal thresholds in rats. Intraperitoneal injections of verapamil induced dose-dependent (3-18 mg/kg) mechanical and thermal anti-nociception in adult rats without altering their sensorimotor abilities. Our data suggest that L-type Ca(2+) channels contribute to acute nociceptive signaling and that anti-nociceptive effects may result from the blockade of these channels.

Pharmacological and biophysical characterization of voltage-gated calcium currents in the endopiriform nucleus of the guinea pig

The endopiriform nucleus (EPN) is a well-defined structure that is located deeply in the piriform region at the border with the striatum and is characterized by dense intrinsic connections and prominent projections to piriform and limbic cortices. The EPN has been proposed to promote synchronization of large populations of neurons in the olfactory cortices via the activation of transient depolarizations possibly mediated by Ca(2+) spikes. It is known that principal cells in the EPN express both a low- and high-voltage-activated (HVA) Ca(2+) currents. We further characterized HVA conductances possibly related to Ca(2+)-spike generation in the EPN with a whole cell, patch-clamp study on neurons acutely dissociated from the EPN of the guinea pig. To study HVA currents in isolation, experiments were performed from a holding potential of -60 mV, using Ba(2+) as the permeant ion. Total Ba(2+) currents (I(Ba)) evoked by depolarizing square pulses peaked at 0/+10 mV and were completely abolished by 200 microM Cd(2+). The pharmacology of HVA I(Ba)s was analyzed by applying saturating concentrations of specific Ca(2+)-channel blockers. The L-type blocker nifedipine (10 microM; n = 11), the N-type-channel blocker omega-conotoxin GVIA (0.5 microM; n = 24), and the P/Q-type blocker omega-conotoxin MVIIC (1 microM; n = 16) abolished fractions of total I(Ba)s equal on average to 24.7 +/- 5.4%, 27.1 +/- 3.4%, and 22.2 +/- 2.4%, respectively (mean +/- SE). The simultaneous application of the three blockers reduced I(Ba) by 68.5 +/- 6.6% (n = 10). Nifedipine-sensitive currents and most N- and P/Q-type currents were slowly decaying, the average fractional persistence after 300 ms of steady depolarization being 0.77 +/- 0.02, 0.60 +/- 0.06, and 0.68 +/- 0.04, respectively. The residual, blocker-resistant (R-type) currents were consistently faster inactivating, with an average fractional persistence after 300 ms of 0.30 +/- 0.08. Fast-decaying R-type currents also displayed a more negative threshold of activation (by about 10 mV) than non-R-type HVA currents. These results demonstrate that EPN neurons express multiple pharmacological components of the HVA Ca(2+) currents and point to the existence of an R-type current with specific functional properties including fast inactivation kinetics and intermediate threshold of activation.

Sustained L-type calcium currents in dissociated deep dorsal horn neurons of the rat: characteristics and modulation

Deep dorsal horn neurons present plateau properties involved in non-linear integration of nociceptive inputs, in the windup of the discharge, and in the expression of long-lasting afterdischarges. In vitro experiments using intracellular recordings in a slice preparation of the rat spinal cord have established that they are supported in part by voltage-dependent calcium currents, and positively modulated by metabotropic glutamate receptor activation. In the present study, whole-cell patch-clamp recordings in acutely isolated soma of dorsal horn neurons (n=48) were used to analyse the voltage-dependent calcium currents involved.Deep dorsal horn neurons expressed both inactivating and non-inactivating calcium currents with Ca(2+) or Ba(2+) used as a charge carrier. The non-inactivating component activated at intermediate threshold (-55mV), and was blocked mostly by nifedipine (61+/-6%). Although voltage-dependent facilitation of whole-cell calcium currents could be obtained by prepulses to +100mV, repetitive depolarization at potentials compatible with the plateau (-45mV and -10mV) failed to induce facilitation of calcium currents. No direct modulation of somatic calcium currents by application of (S)-3,5-dihydroxyphenylglycine, a selective group I metabotropic glutamate receptor agonist and 1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid, a group I and II metabotropic glutamate receptor agonist, was found, while application of the metabotropic GABA(B) receptor agonist baclofen induced a significant decrease in calcium currents.Thus, the present voltage-clamp study shows that rat deep dorsal horn neurons express a non-inactivating, nifedipine sensitive, intermediate threshold (-55mV) calcium current which could provide the depolarizing drive to generate plateau potentials near threshold. Our results also indicate that calcium currents are not sensitized following repetitive stimulation, and not modulated by metabotropic glutamate receptor activation. They provide, however, the first evidence for a direct modulation of voltage-gated calcium channels in dorsal horn neurons by GABA(B) receptor activation, which may contribute to the mechanism of baclofen's antinociceptive activity.

Methylmercury affects multiple subtypes of calcium channels in rat cerebellar granule cells

We tested the ability of methylmercury (MeHg) to block calcium channel current in cultures of neonatal cerebellar granule cells using whole-cell patch clamp techniques and Ba(2+) as charge carrier. Low micromolar concentrations of MeHg (0.25-1 microM) reduced the amplitude of whole cell Ba(2+) current in a concentration- and time-dependent fashion; however, this effect was not voltage-dependent and the current-voltage relationship was not altered. Increasing the stimulation frequency hastened the onset and increased the magnitude of block at both 0.25 and 0.5 microM MeHg but not at 1 microM. In the absence of stimulation, all concentrations of MeHg were able to decrease current amplitude. The ability of several Ca(2+) channel antagonists (omega-conotoxin GVIA, omega-conotoxin MVIIC, omega-agatoxin IVA, calcicludine, and nimodipine) to alter the MeHg-induced effect was tested in an effort to determine if MeHg targets a specific subtype of Ca(2+) channel. Each of the antagonists tested was able to decrease a portion of whole cell Ba(2+) current under control conditions. However, none were able to attenuate the MeHg-induced block of whole cell Ba(2+) current, suggesting either that the mechanism of MeHg-induced block involves sites other than those influenced specifically by Ca(2+) channel antagonists or that MeHg was able to "outcompete" these toxins for their binding sites. These results show that acute exposure to submicromolar concentrations of MeHg can block Ba(2+) currents carried through multiple Ca(2+) channel subtypes in primary cultures of cerebellar granule cells. However, it is unlikely that the presence of a specific Ca(2+) channel subtype is able to render granule cells more susceptible to the neurotoxicologic actions of MeHg.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

Visualization of lamina I of the dorsal horn in live adult rat spinal cord slices

The superficial dorsal horn of the spinal cord, particularly lamina I, plays a key role in the integration and relay of pain related sensory input. To study the physiology of lamina I neurons in slices, a clear delineation of this layer can be greatly advantageous. Yet, it has remained difficult to distinguish this layer in live tissue in conventional transverse spinal slices because of its very narrow thickness at the edge of the dorsal horn. We describe here the criteria we used to delineate lamina I in live tissue using gradient contrast videomicroscopy in 400 microm-thick parasagittal spinal cord slices from adult rats (30-60-day-old). Because of the longitudinal orientation of the neurons in this layer, the resulting distinctive reticulated appearance of lamina I made it possible to readily distinguish it from lamina II. The usefulness of this distinguishing parameter is demonstrated by our ability to contrast synaptic properties of neurons in lamina I from those in lamina II. Complete morphological identification of lamina I neurons however also requires visualization of the cell in the horizontal plane. To maintain compatibility with the parasagittal slice, we used 3D reconstructions from confocal images of the recorded neurons. Rotation of the neuron in space allowed for its morphological characterization in all three planes (horizontal, parasagittal, and transverse). This approach therefore presents optimal conditions for systematic electrophysiological recording from visually identified lamina I neurons.

Spatial distribution of NA+ and K+ channels in spinal dorsal horn neurones: role of the soma, axon and dendrites in spike generation

Spinal dorsal horn neurones play an important role in processing sensory information received from primary afferent fibers. The application of the patch-clamp technique to thin slices of rat spinal cord has enabled the study of ionic channels in visually identified dorsal horn neurones. The small soma of these neurones isolated from the slice by means of a novel method of 'entire soma isolation' has become a convenient model for investigating the properties and distributions of ionic channels. The present review summarizes results of recent experiments studying different types of voltage-gated Na+ and K+ channels expressed in dorsal horn neurones. Uneven distribution of the channels between the soma. axon and dendrites appears to play a major role in determining the neuronal excitability. The contribution of the soma, axon and dendrites to generation and propagation of the action potentials in central neurones is discussed.

Ionic basis for plateau potentials in deep dorsal horn neurons of the rat spinal cord

Approximately 28% of dorsal horn neurons (DHNs) in lamina V of the rat spinal cord generate voltage-dependent plateau potentials underlying accelerating discharges and prolonged afterdischarges in response to steady current pulses or stimulation of nociceptive primary afferent fibers. Using intracellular recordings in a transverse slice preparation of the cervical spinal cord, we have analyzed the ionic mechanisms involved in the generation and maintenance of plateau potentials in lamina V DHNs. Both the accelerating discharges and afterdischarges were reversibly blocked by Mn(2+) and enhanced when Ca(2+) was substituted with Ba(2+). The underlying tetrodotoxin-resistant regenerative depolarization was sensitive to dihydropyridines, being blocked by nifedipine and enhanced by Bay K 8644. Substitution of extracellular Na(+) with N-methyl-D-glucamine or choline strongly decreased the duration of the plateau potential. Loading the neurons with the calcium chelator BAPTA did not change the initial response but clearly decreased the maximum firing frequency and the duration of the afterdischarge. A similar effect was obtained with flufenamate, a specific blocker of the calcium-activated nonspecific cation current (I(CAN)). We conclude that the plateau potential of deep DHNs is supported by both Ca(2+) influx through intermediate-threshold voltage-gated calcium channels of the L-type and by subsequent activation of a CAN current. Ca(2+) influx during the plateau is potentially of importance for pain integration and the associated sensitization in spinal cord.

Ionic basis for plateau potentials in deep dorsal horn neurons of the rat spinal cord

Approximately 28% of dorsal horn neurons (DHNs) in lamina V of the rat spinal cord generate voltage-dependent plateau potentials underlying accelerating discharges and prolonged afterdischarges in response to steady current pulses or stimulation of nociceptive primary afferent fibers. Using intracellular recordings in a transverse slice preparation of the cervical spinal cord, we have analyzed the ionic mechanisms involved in the generation and maintenance of plateau potentials in lamina V DHNs. Both the accelerating discharges and afterdischarges were reversibly blocked by Mn(2+) and enhanced when Ca(2+) was substituted with Ba(2+). The underlying tetrodotoxin-resistant regenerative depolarization was sensitive to dihydropyridines, being blocked by nifedipine and enhanced by Bay K 8644. Substitution of extracellular Na(+) with N-methyl-D-glucamine or choline strongly decreased the duration of the plateau potential. Loading the neurons with the calcium chelator BAPTA did not change the initial response but clearly decreased the maximum firing frequency and the duration of the afterdischarge. A similar effect was obtained with flufenamate, a specific blocker of the calcium-activated nonspecific cation current (I(CAN)). We conclude that the plateau potential of deep DHNs is supported by both Ca(2+) influx through intermediate-threshold voltage-gated calcium channels of the L-type and by subsequent activation of a CAN current. Ca(2+) influx during the plateau is potentially of importance for pain integration and the associated sensitization in spinal cord.

Mechanism underlying increased neuronal activity in the rat ventrolateral periaqueductal grey by a mu-opioid

1. The overall effect of the mu-opioid receptor agonist DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) on ventrolateral periaqueductal grey (PAG) neurons in brain slices was studied using the whole-cell patch-clamp recording technique. 2. Under current-clamp conditions, DAMGO (1 microM) increased cell firing in many PAG neurons even though the opioid induced hyperpolarization and inhibited excitatory postsynaptic potentials (EPSPs) in these cells. 3. The increase in cell activity by DAMGO was observed in both transverse and horizontal slices. The increase persisted when the membrane potential was re-depolarized to the control level. Thus, different planes of sections or the removal of Na+ channel inactivation could not account for the observation. 4. The GABA antagonist bicuculline caused cell firing, mimicking the excitatory effect of DAMGO. Unlike DAMGO, however, bicuculline depolarized PAG cells. 5. Under voltage-clamp conditions, with the same driving force, the evoked inhibitory postsynaptic currents (IPSCs) in these neurons were 2.3 times larger than the evoked excitatory postsynaptic currents (EPSCs). Furthermore, DAMGO inhibited IPSCs by 60.7% while it inhibited EPSCs by 35.3%. 6. We propose that the overall effect of an opioid depends on the dynamic balance of its excitatory and inhibitory actions. In the PAG, the blockade of the inhibitory drive of GABAergic inputs by DAMGO is large. It overcomes the DAMGO-induced hyperpolarization and inhibition of EPSCs and thus results in the excitation of these neurons.

Nociceptive integration in the rat spinal cord: role of non-linear membrane properties of deep dorsal horn neurons

Deep dorsal horn neurons (DHNs) involved in nociception can relay long-lasting inputs and generate prolonged afterdischarges believed to enhance the transfer of nociceptive responses to the brain. We addressed the role of neuronal membrane properties in shaping these responses, by recording lamina V DHNs in a slice preparation of the rat cervical spinal cord. Of 256 neurons, 102 produced accelerating discharges in response to depolarizing current pulses, whereas the other neurons showed spike frequency adaptation. Two mechanisms mediated the firing acceleration: a slow inactivation of a K+ current expressed upon activation of the neuron from hyperpolarized holding potentials, and the expression of a regenerative plateau potential activating around resting membrane potential. The increase in firing frequency was much stronger when sustained by the plateau potential (71 DHNs, 28%). A few neurons produced adaptation and both types of acceleration, in different membrane potential domains, showing that the firing pattern of a deep DHN is not a rigid characteristic. Plateau potentials could be elicited by stimulation of nociceptive primary afferent fibres. The bistability associated with plateau potentials permitted afterdischarges. Because plateau potentials had slow activation kinetics and were voltage-dependent, the neurons had non-linear input-output relationships in both the amplitude and time domains. Nociceptive primary afferent stimulation elicited intense and prolonged responses in plateau-generating DHNs, while brief bursts of spikes were evoked otherwise. These results indicate that in a population of deep DHNs, intense firing and prolonged afterdischarges in response to nociceptive stimulation depend on non-linear intrinsic membrane properties.

Cross-modulation of glycine-activated Cl- channels by protein kinase C and cAMP-dependent protein kinase in the rat

1. The cross-modulation of glycine responses by cyclic-AMP-dependent protein kinase (PKA) and protein kinase C (PKC) was determined in acutely dissociated trigeminal neurons. 2. Whole-cell glycine-evoked Cl- current (IGly) was recorded using the patch clamp technique. Protein kinases and their inhibitors were intracellularly perfused into the cells. 3. Both PKA and PKC when applied separately potentiated IGly. 4. When PKA and PKC were sequentially applied, PKC could not increase the IGly any further after the glycine responses were enhanced by PKA. 5. In 42% of our cells, IGly increased spontaneously. Endogenous PKA was found to mediate the increase. PKC had no effects on IGly in these cells. 6. The effect of PKA on IGly was studied in PKC-pretreated cells. PKA failed to potentiate IGly in these cells, suggesting that the PKA action also depends on the activity of PKC inside the cells. 7. These results suggest that the PKC action on IGly is conditional upon the modulation of the currents by PKA and vice versa. This cross-regulation of ligand-gated channel activity by protein kinases may play a role in neuronal integration and synaptic plasticity.

Membrane properties of deep dorsal horn neurons from neonatal rat spinal cord in vitro

Whole-cell patch-clamp recordings were undertaken to characterize and compare the membrane properties of deep dorsal horn neurons in transverse slices of rat lumbar spinal cord in two age groups, postnatal days (P) 3-6 and 9-16. In both age groups, significant correlations were observed between membrane time constant and cell resistance and between action potential height and its duration at half-maximal amplitude. Cell resistance and action potential half-width values were lower in the P9-16 age group. Neurons were divided into four categories based on their firing properties in response to intracellular current injection: single spike, phasic firing, repetitive firing, and delayed firing. The distribution of neurons within these categories was similar in both age groups which suggests that the firing properties of deep dorsal horn neurons are functionally differentiated at an early postnatal age.

Dihydropyridine- and neurotoxin-sensitive and -insensitive calcium currents in acutely dissociated neurons of the rat central amygdala

The central amygdala (CeA) is an area involved in emotional learning and stress, and identification of Ca2+ currents is essential to understanding interneuronal communication through this nucleus. The purpose of this study was to separate and characterize dihydropyridine (DHP)- and neurotoxin-sensitive and -resistant components of the whole cell Ca2+ current (ICa) in acutely dissociated rat CeA neurons with the use of whole cell patch-clamp recording. Saturating concentrations of nimodipine (NIM, 5 microM), a DHP antagonist, blocked 22% of ICa: this NIM-sensitive (L-type) current was recorded in 68% of CeA neurons. The DHP agonist Bay K 8644 (5 microM) produced a 36% increase in ICa in a similar proportion of CeA neurons (70%). omega-Conotoxin GVIA (CgTx GVIA, 1 microM) in saturating concentrations inhibited 30% of ICa, whereas omega-agatoxin IVA (Aga IVA, 100 nM), in concentrations known to block P-type currents, did not affect ICa. Higher concentrations of Aga IVA (1 microM) alone reduced ICa by 34%, but in the presence of NIM (5 microM) and CgTx GVIA (1 microM) blocked only 18% of ICa. omega-Conotoxin MVIIC (CgTx MVIIC, 250 nM) reduced ICa by 13% in the presence of CgTx GVIA (1 microM). Application of NIM (5 mM), CgTx GVIA (1 microM); and Aga IVA (1 microM) blocked approximately 67% of ICa. A similar portion (63%) of Ca2+ current was blocked with CgTx MVIIC (250 nM) in the presence of NIM (5 microM) and CgTx GVIA (1 microM). The current resistant to NIM and the neurotoxins represented 37% of ICa, whereas in neurons not having L-type currents the resistant current made up approximately 53% of ICa (49 +/- 2%, mean +/- SE). The resistant current activated at around -40 mV and peaked at approximately 0 mV with half-activation and -inactivation potentials of -17 and -58 mV and slopes for activation and inactivation of -5 and 13 mV, respectively. The resistant current was sensitive to Cd2+ (IC50 = 2.5 microM) and Ni2+ (IC50 = 86 microM), was larger in Ca2+ than in Ba2+ (ratio = 1.31:1), and showed a moderate rate of decay. In summary, our results show that the high-voltage-activated calcium current in rat CeA neurons is composed of at least four pharmacologically distinct components: L-type current (NIM sensitive, 22%), N-type current (CgTx GVIA sensitive, 30%), Q-type current [Aga IVA (1 microM) and CgTx MVIIC sensitive, approximately 13-18%], and a resistant current (Non-L, -N, and -Q current, 33 approximately 37%), amounting to 37-53% of the total current. The resistant current has some electrophysiological and pharmacological characteristics in common with doe-1, alpha 1E, and R-type calcium currents, but remains unclassified.

Burst-generating neurones in the dorsal horn in an in vitro preparation of the turtle spinal cord

1. In transverse slices of the spinal cord of the turtle, intracellular recordings were used to characterize and analyse the responses to injected current and activation of primary afferents in dorsal horn neurones. 2. A subpopulation of neurones, with cell bodies located centrally in the dorsal horn, was distinguished by the ability to generate a burst response following a hyperpolarization from rest or during a depolarization from a hyperpolarized holding potential. The burst response was inactivated at the resting membrane potential. 3. The burst response was mediated by a low threshold Ca2+ spike assumed to be mediated by T-type Ca2+ channels since it resisted tetrodotoxin and was blocked by 3 mM Co2+ or 100-300 microM Ni2+ and resembled the low threshold spike (LTS) described elsewhere. 4. Some burst-generating cells also displayed plateau potentials mediated by L-type Ca2+ channels. In these cells the burst following a hyperpolarizing current pulse, applied from the resting membrane potential, facilitated the activation of the plateau potential. Wind-up of the plateau potential was produced when the hyperpolarizing pulse generating the burst was repeated at 0.1-0.3 Hz or faster. 5. The burst response and the underlying low threshold Ca2+ spike were activated synaptically by primary afferent stimuli in a voltage range hyperpolarized from the resting membrane potential. 6. Cells with bursts were morphologically distinguishable from cells with bursts and plateau properties. 7. Our findings in this and the preceding paper show that the intrinsic response properties of particular subtypes of neurones in the dorsal horn have a profound influence on the amplitude and time course of the responses mediated by primary afferent fibres. We predict that these postsynaptic properties are probable targets for synaptic modulation.

Plateau-generating neurones in the dorsal horn in an in vitro preparation of the turtle spinal cord

1. In transverse slices of the spinal cord of the turtle, intracellular recordings were used to characterize and analyse the responses to injected current and activation of primary afferents in dorsal horn neurones. 2. A subpopulation of neurones, with cell bodies located laterally in the deep dorsal horn and dendrites radiating towards the pial surface, was distinguished by the ability to generate plateau potentials. Activation of the plateau potential by a suprathreshold depolarizing current pulse produced an increasing firing frequency during the first few seconds and a sustained after-discharge. 3. The plateau potential was assumed to be mediated by L-type Ca2+ channels since it was blocked by Co2+ (3 mM) and nifedipine (10 microM) and enhanced by Bay K 8644 (0.5-2 microM). 4. The threshold for activating the plateau potential declined during the first few seconds of depolarization. The decline in threshold gradually subsided over 3-10 s after repolarization. 5. Frequency potentiation of the plateau potential contributed to wind-up of the response to depolarizing current pulses and primary afferent stimuli repeated at frequencies higher than 0.1-0.3 Hz. 6. The sustained after-discharge mediated by the plateau potential was curtailed by a slow after-hyperpolarization (sAHP) evoked by strong depolarizations. The relative strength of the plateau potential and sAHP varied among cells. In some cells the plateau potential and sAHP interacted to produce damped oscillations upon depolarization. The sAHP was mediated by both apamin and tetraethylammonium (TEA)-sensitive K+ channels. 7. Our findings suggest that basic properties of sensory integration may reside with the specialized intrinsic response properties of particular subtypes of neurones in the dorsal horn.

Modulation of the activity of midbrain central gray substance neurons by calcium channel agonists and antagonists in vitro

Changes in the background impulse activity of midbrain central gray substance neurons have been studied on slice preparations from the rat midbrain upon application of calcium-free solution, an activator of calcium channels, BAY-K 8644 (10 nM), organic (verapamil, 40 microM; D600, 10 microM; nifedipine, 1-10 microM; amiloride, 1 microM) and inorganic (Co2+, 1.5 mM) calcium channel blockers. Besides BAY-K 8644, all the substances inhibited most of the neurons studied. Verapamil, BAY-K 8644 and Co2+ also revealed facilitatory effects. Facilitatory action of BAY-K was most effective in silent neurons and in those previously inhibited by amiloride. Latent period values of inhibition in calcium-free solution and upon application of organic and inorganic blockers have the following sequence: D600 > amiloride > verapamil > Co2+ > nifedipine > calcium-free solution. Maximum rise time had the following order: amiloride > D600 > nifedipine > verapamil > Co2+ > calcium-free solution. Complete suppression of the neuronal activity induced by amiloride lasted twice as long as that induced by calcium-free solution, Co2+ and nifedipine, and six times as long as verapamil-induced suppression. Preliminary application of calcium channel blockers reduced facilitatory and increased inhibitory effects of serotonin and substance P. Data obtained are discussed with the supposition in mind that inhibition of the function of calcium channels in central gray substance neurons could be one of the mechanisms underlying the analgesic effect of a series of neurotropic agents after their introduction into this structure.

The opioid peptide dynorphin directly blocks NMDA receptor channels in the rat

1. The actions of dynorphin on N-methyl-D-aspartate (NMDA) responses were examined in acutely dissociated trigeminal neurons in rat. Whole-cell and single-channel currents were recorded using the patch clamp technique. 2. Dynorphins reduced NMDA-activated currents (INMDA). The IC50 was 0.25 microM for dynorphin (1-32), 1.65 microM for dynorphin (1-17) and 1.8 microM for dynorphin (1-13). 3. The blocking action of dynorphin is voltage independent. 4. The inhibitory action of dynorphin cannot be blocked by high concentration of the non-selective opioid receptor antagonist naloxone, nor by the specific kappa-opioid receptor antagonist nor-Binaltorphimine (nor-BNI). 5. Single-channel analyses indicate that dynorphin reduces the fraction of time the channel is open without altering the channel conductance. 6. We propose that dynorphin acts directly on NMDA receptors.

T-type Ca2+ channels are abnormal in genetically determined cardiomyopathic hamster hearts

Although there is substantial evidence of abnormal Ca2+ homeostasis in heart cells of the cardiomyopathic Syrian hamster (Bio 14.6 strain), the mechanism by which these myocytes become Ca(2+)-overloaded is not known. To elucidate the role of voltage-sensitive Ca2+ channels in the pathogenesis of myopathy, whole-cell Ca2+ currents were measured in myopathic and normal control cardiac myocytes. These studies demonstrate the presence of two voltage-sensitive Ca2+ channel types in ventricular myocytes isolated from 200- to 300-day-old cardiomyopathic and age-matched normal hamsters. The two Ca2+ channel types were identified by their unitary conductance properties and pharmacologic sensitivities. Both L-type and T-type Ca2+ channels were present in cardiomyopathic and normal cells. Current density through L-type Ca2+ channels was the same in cardiomyopathic and normal control myocytes. However, the mean current density of T-type Ca2+ channels in cardiomyopathic cells was significantly higher than in normal cells (myopathic, 12.3 +/- 1.8 pA/pF; normal, 5.8 +/- 1.1 pA/pF; n = 8; P < .01). The T-type Ca2+ current in cardiomyopathic myocytes was activated and inactivated at more negative potentials than in cells from normal hamster hearts. These findings demonstrate no abnormality of the dihydropyridine-sensitive voltage-dependent L-type Ca2+ channel. In contrast, the observed abnormalities in T-type Ca2+ channel function in cardiomyopathic hamster myocytes suggest that this alteration may be related to the pathogenesis of Ca2+ overload and the arrhythmias in this genetically determined form of cardiomyopathy.

Mechanisms of GABA and glycine depolarization-induced calcium transients in rat dorsal horn neurons

1. The mechanisms and effects of GABA- and glycine-evoked depolarization were studied in cultured rat dorsal horn neurons using indo-1 recordings of [Ca2+]i and patch clamp recordings in conventional whole-cell or perforated-patch mode. 2. Application of GABA to unclamped neurons caused [Ca2+]i increases that were dose dependent and exhibited GABAA receptor pharmacology. Calcium entered the neurons via high-threshold voltage-gated calcium channels (conotoxin and nimodipine sensitive). 3. In perforated-patch recordings employing cation-selective ionophores, GABAA receptor activation depolarized 123 of 132 cells to membrane potentials as depolarized as -33 mV (mean -50 mV in all 132 cells, +12 mV above resting potential). The ionic basis of the depolarization was determined by extracellular ion substitution; increased anionic conductance could account fully for the results. 4. Glycine, acting at a strychnine-sensitive receptor, also caused Ca2+ entry into these neurons through voltage-gated Ca2+ channels. Glycine and GABA both evoked [Ca2+]i responses in the same cells and the responses were highly correlated in amplitude. Glycine also depolarized all five cells tested with perforated recording. Each of the five cells was also depolarized by muscimol to a value similar to that obtained for glycine. 5. Both the depolarization and the increases in [Ca2+]i caused by GABA and glycine could potentially play a role in processes of development and differentiation and sensory transmission in the spinal cord dorsal horn.

Whole-cell patch-clamp recordings from visualized bulbospinal neurons in the brainstem slices

The purpose of this study was to develop a method for electrophysiological characterization of retrogradely labeled bulbospinal neurons in the specific cytoarchitectonic regions in the brainstem slices. Several days after the spinal cord was injected with the carbocyanine dye, DiI, retrogradely labeled bulbospinal neurons were visualized by epifluorescence optics in the brainstem slices with the aid of a silicon intensifier tube (SIT) camera. Labeled somata were routinely seen in the caudal raphe nuclei, rostroventral medial and lateral portions of the medulla, the A5 group and in other medullary sites known to project to the spinal cord. Electrophysiological properties of the DiI-labeled neurons were assessed by whole-cell recordings using micropipettes filled with biocytin. The slices were subsequently processed for dual visualization of biocytin and serotonin or a marker for noradrenergic neurons, tyrosine hydroxylase (TH). The electrophysiological properties of bulbospinal neurons were correlated with their morphology and neurochemical content. This technique may be useful in other areas of CNS for studying morphology, neurochemical content and physiology of retrogradely labeled neurons.

High-voltage-activated calcium current in developing neurons is insensitive to nifedipine

We have analyzed the effect of nifedipine on the macroscopic high-threshold, voltage-activated (HVA) calcium current in four cell types: postnatal rat Purkinje and dorsal root ganglion (DRG) neurons, embryonic chick DRG neurons, and adult cat ventricular myocytes. As is consistent with previous reports, nifedipine reduced HVA current in myocytes in a voltage-sensitive manner. Analysis of nifedipine actions on neurons, however, was compromised by slow inactivation of the current at holding potentials between -80 mV and -40 mV. The slow inactivation was voltage-dependent, irreversible after 5 min, and contributed to "rundown" of the current. At -40 mV, slow inactivation displayed two time constants: 12 +/- 8 s and 7 +/- 4 min. When slow inactivation was taken into account, we found no evidence for a nifedipine-sensitive component of the HVA current in these neurons. Consistent with previous studies, DRG neurons were reduced irreversibly by omega-conotoxin, whereas cardiac and Purkinje cells were unaffected. Our biophysical and pharmacological results are consistent with two types of neuronal HVA currents (N type and P type) in developing neurons that are distinct from cardiac HVA currents (L type).

Brief calcium transients evoked by glutamate receptor agonists in rat dorsal horn neurons: fast kinetics and mechanisms

1. The calcium indicator dye, indo-1, was used to analyse the receptor-specific mechanisms of intracellular calcium ion ([Ca2+]i) responses evoked by excitatory amino acid (EAA) stimulation of dorsal horn neurons. Measurements of somal changes in [Ca2+]i were made on a subsecond time scale under conditions designed to allow membrane potential to mediate interactions between agonist-gated channels and voltage-gated calcium channels (VGCCs). 2. Voltage-gated calcium channels were activated in a receptor-independent manner using elevated extracellular [K+]. The concentration-dependence of K(+)-evoked [Ca2+]i transients was steep and variable among cells, with a mean maximal [Ca2+]i response of 1400 nM and a rapid maximal rate of rise. These data indicate that VGCCs provide a high-capacity route for Ca2+ entry that is very sensitive to small changes in membrane potential. 3. Stimulation of non-NMDA receptors using the non-desensitizing agonist kainate also evoked large [Ca2+]i responses (mean, 840 nM) that were predominantly due to indirect activation of VGCCs. However, in 60% of neurons tested, a component of the [Ca2+]i transient evoked by kainate at concentrations above 10 microM was not blocked by the potent VGCC blocker, lanthanum (La3+). The La(3+)-resistant [Ca2+]i responses to kainate rose exponentially, required extracellular Ca2+, and were caused neither by evoked release of EAA transmitters nor by reversal of Na(+)-Ca2+ exchange. These responses may be mediated by a Ca(2+)-permeable conformation of non-NMDA receptors and can also be evoked by quisqualate, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and glutamate. 4. Non-NMDA receptors were activated in a desensitizing manner using quisqualate or AMPA. Quisqualate evoked small [Ca2+]i transients (210 nM) with a slow rate of rise. Typically, above 3 microM quisqualate, the size of the responses decreased, reflecting desensitization of the receptor. Responses to quisqualate were blocked by removal of extracellular Ca2+ indicating that mobilization of intracellular Ca2+ stores does not occur in the majority of dorsal horn neurons. However, trans-(+-)-1-amino-1,3-cyclopentane dicarboxylic acid (trans-ACPD) was occasionally able to evoke modest Ca2+ release. 5. Activation of the Ca(2+)-permeable NMDA receptors evoked [Ca2+]i transients that were large (780 nM), with a moderate rate of rise, and that generally achieved a maximum amplitude at NMDA concentrations around 300 microM. 6. Glutamate was used to examine [Ca2+]i responses to the activation of mixed EAA receptor subtypes by an endogenous ligand.(ABSTRACT TRUNCATED AT 400 WORDS)

Miniature postsynaptic currents recorded from identified rat spinal dorsal horn projection neurons in thin-slice preparations

Whole-cell voltage-clamp recordings were made from spinothalamic and spinomesencephalic tract neurons in thin-slice preparations of rat spinal cord. In the presence of tetrodotoxin, spontaneous inward and outward postsynaptic currents were observed near the resting membrane potential. These currents were divided into miniature excitatory postsynaptic currents (mEPSCs) mediated by glutamate, and miniature inhibitory postsynaptic currents (mIPSCs) mediated by glycine or gamma-aminobutyric acid (GABA). Glutamatergic mEPSCs had two components mediated by NMDA and non-NMDA receptors. Analyzing these miniature synaptic currents, valuable information concerning the pre- and postsynaptic mechanisms underlying modulation of synaptic transmission in the spinal dorsal horn could be obtained.

Localization and retention in vitro of fluorescently labeled aortic baroreceptor terminals on neurons from the nucleus tractus solitarius

The anterograde fluorescent tracer DiA was used to visualize baroreceptor fibers and synaptic terminals both in living and fixed tissue. Baroreceptor fibers labeled with DiA terminated as a dense synaptic field in the medial nucleus tractus solitarius (NTS), making synaptic contact on the soma, as well as processes of neurons that they innervated. A similar distribution and morphology was observed in baroreceptor fibers and terminals labeled with horseradish peroxidase. DiA also identified baroreceptor terminals and the neurons receiving these synaptic contacts in vitro. NTS neurons were dissociated from their surrounding tissue and identified by attached baroreceptor terminals that retained the fluorescent dye. These results will enable us to study the electrophysiological properties of dispersed neurons that receive identified baroreceptor synaptic terminals.

Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 microM tetrodotoxin (TTX) to the bath or by replacing Na+ in the bath with N-methyl-D-glucamine and were classified as Na+ currents. Sustained inward currents, observed in most neurons examined when Na+ inward currents were blocked with TTX and outward currents were blocked by replacing K+ in the pipette solution with Cs+ and by addition of 10 mM Ba2+ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 microM nimodipine. These currents were classified as L-type Ca2+ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K+ in the pipette with Cs+ and addition of Ba2+ to the bath were observed in all olfactory neurons examined. The outward K+ currents activated over approximately the same range as the Na+ currents (-60 to -50 mV), but the Na+ currents were larger at the normal resting potential of the neurons (-45 +/- 11 mV, mean +/- SD, n = 52). Four different types of K+ currents could be differentiated: a Ca(2+)-activated K+ current, a transient K+ current, a delayed rectifier K+ current, and an inward rectifier K+ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K+ = 100 mM) in the pipette, but frequently appeared when K+ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 +/- 5.2 G omega, n = 20) and low membrane capacitance (2.1 +/- 1.1 pF, n = 46) of the cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Labelling and recording from dissociated target-specific rat superior cervical ganglion neurons

A population of neurons was retrogradely labelled in the superior cervical ganglia (SCG) of the adult rat following the injection of the fluorescent dye Fast blue into the submandibular salivary glands (SMG). The neurons retained the fluorescent label following dissociation and culture. Electrical and chemosensitive properties of the labelled neurons were studied with the whole-cell patch-clamp technique.

Voltage-gated calcium currents in cultured embryonic Xenopus spinal neurones

1. Voltage-gated Ca2+ currents were studied in cultured embryonic Xenopus spinal neurones using whole-cell gigaohm seal techniques. Cultures of neural plate cells were established from stage 15-17 embryos (see Methods), and were studied for up to 80 h in vitro. During this period neural precursor cells morphologically differentiate and commence expression of multiple types of voltage- and ligand-gated ion channels. 2. Embryonic Xenopus neurones studied during the first 20-40 h in culture display Ca2+ currents that correspond to the low-voltage-activated (T-type) and high-voltage-activated forms described in other neurones and excitable cells. These Ca2+ current types could be separated based on voltage dependencies and pharmacological sensitivities. 3. T-type Ca2+ current was activated at voltages positive to -50 mV, and was selectively blocked by 200 microM-Ni2+. Curves describing the voltage dependencies of activation and steady-state inactivation overlapped in a region centred on -40 mV. A small sustained Ca2+ current could be recorded within this voltage region. 4. High-voltage-activated (HVA) Ca2+ currents were observed at voltages positive to -10 mV, and could be separated into relaxing and sustained components (denoted as HVA-relaxing and HVA-sustained). HVA-relaxing current was selectively reduced by Met-enkephalin (17.5 microM). Both components of HVA current were sensitive to verapamil (100 microM), were almost completely blocked by omega-conotoxin (3 microM) and were insensitive to nifedipine (20 microM). 5. The data indicate that T-type Ca2+ current is present in the membrane during the initial period of channel and receptor expression, process outgrowth, and synaptogenesis, and is the dominant influence on voltage-gated Ca2+ influx during subthreshold voltage excursions. Further, at more positive voltages, T-type Ca2+ current contributes to inward Ca2+ current during the first 5-10 ms after depolarizing voltage steps, and thus to inward Ca2+ current during the rising phase of the long-lasting Ca(2+)-dependent embryonic action potential. HVA Ca2+ currents (particularly the relaxing component) influence the plateau phase.

Identification and dissociation of cardiovascular neurons from the medulla for patch clamp analysis

This study describes a preparation that will enable us to study, using voltage clamp techniques, ionic currents from dissociated cardiovascular neurons that have retained their anatomical and functional identity of the intact animal. To identify dispersed preganglionic cardiac motoneurons various fluorescent dyes (rhodamine, fluorogold, microspheres, bizbenzimide and dextrans) were examined to determine which can be absorbed by preganglionic cardiac motorneuron nerve terminals (without surgical penetration of cardiac tissue), transported retrogradely to their soma in the medulla and retained during dissociation of the neurons. Rhodamine fulfilled these criteria. Dissociated preganglionic cardiac motorneurons had resting membrane potentials of -52.4 +/- 3 mV and input resistances of 236 +/- 71 M omega (mean +/- S.E.M., n = 10). Depolarizing voltage steps to -50 mV or above evoked a tetrodotoxin (TTX) sensitive inward sodium current followed by a biphasic outward current.

Sustained potentiation of NMDA receptor-mediated glutamate responses through activation of protein kinase C by a mu opioid

mu opioids, such as morphine and certain enkephalin analogs, are known to modulate glutamate-evoked activity in dorsal horn neurons in the spinal cord and caudal brain stem. Yet the molecular mechanism by which this modulation occurs is not understood. We examined the interactions between glutamate and a selective mu opioid receptor agonist, D-Ala2-MePhe4-Gly-ol5-enkephalin (DAGO), in spinal trigeminal neurons in thin medullary slices of rats. DAGO caused a sustained increase in glutamate-activated currents that are mediated by N-methyl-D-aspartate receptors. Intracellularly applied protein kinase C (PKC) mimics the effect of DAGO, and a specific PKC inhibitor interrupts the sustained potentiation produced by DAGO. Thus, PKC plays a key role in mediating the action of mu opioid peptides.