Electrical properties of acutely isolated, identified rat spinal dorsal horn projection neurons

Effects of M-current modulators on the excitability of immature rat spinal sensory and motor neurones

M-currents have been shown to control neuronal excitability in a variety of central and peripheral neurones. Here we studied the effects of specific M-current modulators on the excitability of spinal neurones and their response to synaptic activation. Experiments were performed in vitro using the hemisected spinal cord from 7- to 11-day-old rats. Intracellular recordings were obtained from lumbar deep dorsal horn and motor neurones. Neuronal excitability was assessed by applying outward current pulses and synaptic responses were elicited by activation of a lumbar dorsal root. The M-current antagonist 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991) and the agonist retigabine were superfused at 10 microM. Retigabine produced hyperpolarization and a large decrease in the excitability of motor (7/7) and dorsal horn neurones (11/12). The effects of retigabine were fully reversed by XE-991. XE-991 induced depolarization of most neurones tested and a large increase in the excitability of motor neurones (7/7) but only a weak increase in the excitability of a proportion of dorsal horn neurones (4/10). The effects of XE-991 were partly reversed by retigabine. Consistent with their effects on neuronal excitability, retigabine showed a general depressant effect on synaptic transmission, whereas XE-991 showed the opposite tendency to potentiate responses to dorsal root stimulation, particularly in motor neurones. The results show that retigabine can depress spinal excitability and the transmission of nociceptive information. Results also indicate a post-synaptic expression of functional M-currents in most motor neurones and a considerable proportion of deep dorsal horn neurones.

Spike frequency adaptation and signaling properties of identified neurons in rodent deep spinal dorsal horn

Using whole cell recordings, I analyzed the intrinsic discharge properties for 285 neurons in Rexed's laminae III-V of isolated hamster spinal cord preparations. Neurons were characterized by their responses to step-wise and ramp-hold depolarizing current applied through the recording pipettes. Tonic cells (133/285; 47%) fired repetitively during step-wise current application. Firing decayed linearly (-0.14 to -4.3 imp . s(-1) . s(-1)) or was bimodal, with an initial exponential phase (tau approximately 450 ms) followed by a linear decline (-0.02 to -6.3 imp . s(-1) . s(-1)); discharge frequency was unrelated to current trajectory. Phasic-firing cells (108/285; 38%) responded with a burst discharge having an initial rapid, exponential decrease (tau approximately 30 ms) and subsequent linear decline (-1 to -78 imp . s(-1) . s(-1)). Phasic cells were activated preferentially by fast current ramps (slope, 70 pA/s-2.2 nA/s) with the number and frequency of impulses increasing with current slope. Delayed-firing cells (44/285; 15%), responded to current steps with an accelerating firing following a substantial latent period (0.5-4 s) and discharged during current ramps with slopes less than approximately 100 pA/s. Intracellular staining revealed a significant association between electrophysiological profile and neuronal morphology. A majority of presumed projection cells (22/30; 73%) exhibited tonic firing to step-wise activation. The preponderance of phasic and delayed firing cells, 93% (42/45) and 71% (12/17), respectively, were interneurons with local or intersegmental terminations. Differential sensitivity to static and time-varying components of membrane current suggest differences in neuronal signaling properties that may have important implications for integration of mechanosensory information in the deep spinal dorsal horn.

Lamina-specific membrane and discharge properties of rat spinal dorsal horn neurones in vitro

Membrane and discharge properties determine the input-output relationship of neurones and are therefore of paramount importance for the functions of neural circuits. Here, we have tested the hypothesis that neurones in different laminae of the spinal dorsal horn differ in their electrophysiological properties. Whole-cell patch-clamp recordings from dorsal horn neurones in a rat transverse spinal cord slice preparation were used to record active and passive membrane properties. Neurones from superficial dorsal horn laminae had higher membrane resistances and broader action potentials than deep dorsal horn neurones. Action potential thresholds were highest in lamina II neurones, representing low membrane excitability. Five types of firing patterns were identified in response to depolarising current injections. Tonic-firing neurones discharged action potentials at regular intervals throughout the current pulse. Delayed-firing neurones showed a delayed onset of firing in response to current injections that was due to activation of a transient voltage-dependent outward current, presumably an A-current. Another group of neurones fired a short initial burst of action potentials. Single-spiking neurones discharged only one action potential at the onset of a depolarising pulse. Phasic-bursting neurones showed irregular bursts of action potentials. Firing patterns were unequally distributed among laminae. Tonic-firing neurones were numerous in lamina I and deeper laminae but were not found in lamina II. Delayed-firing neurones were encountered in laminae I and II but not in deeper laminae. Most of the neurones showing an initial burst were found in lamina II. These differences in membrane and discharge properties probably contribute to lamina-specific processing of sensory, including nociceptive, information.

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.

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.

Functional distribution of three types of Na+ channel on soma and processes of dorsal horn neurones of rat spinal cord

1. Voltage-gated Na+ channels and their distribution were studied by the patch-clamp technique in intact dorsal horn neurones in slices of newborn rat spinal cord and in neurones isolated from the slice by slow withdrawal of the recording pipette. This new method of neurone isolation was further used to study the roles of soma and axon in generation of action potentials. 2. Tetrodotoxin (TTX)-sensitive Na+ currents in intact neurones consisted of three components. A fast component with an inactivation time constant (tau f) of 0.6-2.0 ms formed the major part (80-90%) of the total Na+ current. The remaining parts consisted of a slowly inactivating component (tau s of 5-20 ms) and a steady-state component. 3. Single fast and slow inactivating Na+ channels with conductances of 11.6 and 15.5 pS, respectively, were identified in the soma of intact neurones in the slice. Steady-state Na+ channels were not found in the soma, suggesting an axonal or dendritic localization of these channels. 4. In the whole-cell recording mode, the entire soma of a dorsal horn neurone could be isolated from the slice by slow withdrawal of the recording pipette, leaving all or nearly all of its processes in the slice. The isolated structure was classified as: (1) 'soma' if it lost all of its processes, (2) 'soma+axon' complex if it preserved one process and at least 85% of its original peak Na+ current or (3) 'soma+dendrite' complex if it preserved one process but the remaining Na+ current did not exceed those observed in the isolated 'somata'. 5. The spatial distribution of Na+ channels in the neurone was studied by comparing Na+ currents recorded before and after isolation. The isolated 'soma' contained 13.8 +/- 1.3% of inactivating Na+ current but no steady-state Na+ current. 'Soma+axon' complexes contained 93.6 +/- 1.4% of inactivating and 46% of steady-state Na+ current. 6. In current-clamp experiments, the intact neurones and isolated 'soma+axon' complexes responded with 'all-or-nothing' action potentials to current injections. In contrast, isolated 'somata' showed only passive or local responses and were unable to generate action potentials. 7. It is concluded that dorsal horn neurones of the spinal cord possess three types of TTX-sensitive voltage-gated Na+ channels. The method of entire soma isolation described here shows that the majority of inactivating Na+ channels are localized in the axon hillock and only a small proportion (ca 1/7) are distributed in the soma. Steady-state Na+ channels are most probably expressed in the axonal and dendritic membranes. The soma itself is not able to generate action potentials. The axon or its initial segment plays a crucial role in the generation of action potentials.

Patch-clamp recording from identified rat ciliary ganglion neurons in primary culture

Adult rat parasympathetic ciliary ganglion (CG) neurons were retrogradely labelled by intraocular injection of the carbocyanine fluorescent dye 1,1-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI). Whole-cell and nystatin perforated patch recording techniques were then used to examine the electrophysiological properties of labelled CG neurons growing in primary culture. The resting membrane potential of CG neurons in dissociated cell culture was -50 +/- 8 mV, and isolated neurons fired overshooting action potentials in response to depolarizing current injection. Voltage-clamp recordings of membrane currents revealed a transient tetrodotoxin-sensitive Na+ inward current and both sustained and transient outward K+ currents. Sustained outward K+ current was reduced (55-77%) by 5 mM tetraethylammonium and to a lesser extent (42-46%) by superfusion with nominally Ca2+ free external solution. Transient outward current was blocked by 100 microns 4-aminopyridine and exhibited steady-state inactivation at potentials depolarized to -50 mV. These data demonstrate that identified adult mammalian CG neurons can be successfully maintained in culture. Cultured CG neurons retain electrical excitability, with voltage-sensitive Na+ and K+ currents giving rise to action potentials.

Membrane properties of physiologically classified rat dorsal horn neurons in vitro: correlation with cutaneous sensory afferent input

Dorsal horn neurons in the young rat spinal cord-hindlimb preparation were physiologically classified as wide dynamic range (WDR), nociceptive specific (NS) or low threshold (LT) according to their excitatory responses to low and high intensity mechanical stimuli applied to the hindlimb skin. Two additional types were classified: neurons displaying only sub-threshold excitations (SUB) and neurons displaying inhibitory events (INH), such as inhibitory post-synaptic potentials or interruption of spontaneous spiking following cutaneous stimulation. Direct intracellular current injection revealed four different patterns of spiking behaviour: group A neurons were characterized by tonic firing in response to depolarizing current pulses; group B neurons were strongly phasic, producing only one spike at the beginning of the pulse; group A-B neurons generated an early unsustained (< 300 ms) burst of spikes; and group C neurons exhibited anomalous rectification in response to hyperpolarizing current which was followed by a voltage-dependent rebound excitation. A statistically significant (P < or = 0.01) association existed between a neuron's physiological classification and its electrophysiological profile. The majority of WDR neurons responded with tonic firing and were assigned to group A, while NS neurons were strongly represented in group A-B. All INH neurons were assigned to group C. LT neurons were distributed between groups A and A-B, and SUB neurons were distributed between groups A and B. These data indicate, firstly, that dorsal horn neurons possess heterogeneous membrane properties and, secondly, that a relationship exists between a neuron's biophysical profile and its excitatory or inhibitory response to peripheral cutaneous afferent stimulation. The implications for dorsal horn somatosensory processing are discussed.

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.

A retrograde labeling technique for the functional study of airway-specific visceral afferent neurons

The development of a method is described whereby primary afferent neurons that specifically innervate the airways in the guinea pig can be retrogradely labeled, acutely dissociated and studied functionally with electrophysiological techniques. Following administration of either dextran-tetramethylrhodamine, Fast Blue, or Fluorogold dye into the tracheal lumen, dye-labeled neurons can be visualized in 100 microns serial nodose ganglion sections. Control experiments show that labeling does not result from the undesirable spread of the dyes to target innervation fields in the gastrointestinal (GI) or cardiovascular (CV) systems. Neuronal somata retain dye label when acutely dissociated. Microelectrode studies provide evidence that the presence of the Rhodamine dye label and its fluorescent excitation neither alter basic electrophysiological membrane parameters nor the chemoreceptive properties of isolated neurons. Thus this new method will allow the isolation of individual airway-specific primary visceral afferent neurons for functional studies with multidisciplinary techniques.

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.

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.

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.

Double-labelling with rhodamine beads and biocytin: a technique for studying corticospinal and other projection neurons in vitro

Corticospinal neurons retrogradely labelled with rhodamine-labelled latex microspheres (RLMs) in vivo were studied intracellularly in a slice preparation up to 13 months later with electrodes containing biocytin. The physiological properties of these double-labelled corticospinal neurons were indistinguishable from those of comparable neurons which were impaled with biocytin-containing electrodes without prior RLM-labelling, and neurons studied with potassium acetate-filled electrodes in similar areas. Thus, neither labelling with RLMs nor injection of biocytin affected neuronal properties. This important advantage of RLMs makes them suitable for prelabelling projection neurons in vivo for subsequent studies that take advantage of the versatility of a brain slice preparation. In addition to its lack of effects on neuronal properties, intracellular labelling with biocytin also provides high-quality morphological details ideal for anatomical analysis. The compatibility of retrograde labelling with RLMs and intracellular staining with biocytin make this a useful combined technique for tracking electrophysiological and anatomical changes in identified projection neurons over time.

Ionic currents in retrogradely labeled trigeminothalamic neurons in slices of rat medulla

We have recorded the ionic currents of identified trigeminothalamic neurons in medulla slices in vitro. Trigeminothalamic cells were first retrogradely labeled by injecting fluorescent latex microspheres in the thalamus of a 7- to 10-day-old rat. Two days later, thin slices (80-100 microns) were prepared from the lower medulla of the injected rat. Whole cell recordings were performed on the labeled cells located in the spinal trigeminal nucleus caudalis using the patch clamp technique. The voltage dependent inward sodium, inward calcium and outward potassium currents are qualitatively similar to those obtained from the enzymatically dissociated trigeminothalamic neurons. Successful application of this thin slice method opens the opportunity of studying synaptic circuitry in the trigeminothalamic system.

Origin of thalamically projecting somatosensory relay neurons in the immature rat

The locations of spinothalamic (STT) and trigeminothalamic (TTT) neurons in 14-18-day rats using retrograde transport of fluorescent latex microspheres were examined. The aim of this study was to determine whether the connections between the somatosensory relay neurons and the thalamus were established in these immature rats. The majority of the labeled STT and TTT neurons was found in the brainstem and upper cervical cord (C1-C4). These cells were distributed to a number of distinct groups. Among them, the nucleus of trigeminal spinal tract interpolaris (SP51) had the highest number of TTT cells, and the internal basilar nucleus (IB) comprised the largest population of STT cells. Except for the cells located in the dorsal portion of the ventral horn (VHd), most of the labeled STT and TTT cells were contralateral to the thalamic injection sites. This pattern of distribution of the projecting neurons in immature rats is very similar to the pattern observed in adult rats. The development of the STT and TTT projections in 14-18-day rats is therefore largely complete. This result will allow us to extrapolate our understanding of the membrane properties of projection cells obtained in the immature rats to the adult rats.

Excitatory and inhibitory amino acids and peptide-induced responses in acutely isolated rat spinal dorsal horn neurons

The responses to excitatory and inhibitory amino acids and peptides were investigated in isolated rat spinal dorsal horn neurons (laminae I-V) of young rats using the whole-cell voltage-clamp technique. The treatment of spinal slices with low concentrations of enzymes and mechanical dissociation yielded isolated neurons that were sensitive to excitatory amino acids (glutamate, kainate, quisqualate and N-methyl-D-aspartate (NMDA), inhibitory amino acids (gamma-aminobutyric acid (GABA), glycine) and peptides (substance P, calcitonin gene-related peptide (CGRP). The responses of dorsal horn neurons to NMDA were potentiated by glycine and CGRP, whereas GABAA responses were enhanced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Our observations indicate that there is reasonable agreement between many of the responses of isolated neurons and those studied in in vivo and in vitro slice and culture preparations.

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

1. Single isolated neurones were prepared from the spinal trigeminal nucleus and the dorsal horn of cervical spinal cord of the rat. Spinothalamic and trigeminothalamic neurones were identified using rhodamine-labelled fluorescent latex microspheres. 2. Calcium currents in these cells were examined by the whole-cell patch-clamp technique. Three types of calcium currents, transient (T) and slow inactivating (N and L) types, were identified by their sensitivities to inorganic blockers and rates of inactivation at two different holding potentials (temperature = 21-25 degrees C). 3. From a holding potential of -100 mV, the ICa,T began to activate at -60 mV. The current reached its maximum amplitude around -30 mV and was inactivated completely when the cell was held more positive than -60 mV. The time constant of the inactivation was between 10 and 50 ms. 4. The slow inactivating component of ICa was dissociated into two components by eliciting ICa from two holding potentials of -100 and of -40 mV. The current (ICa,L) activated from -40 mV was characterized by positive activation potentials and a very slow inactivation (time constant, 700-4000 ms). The current (ICa,N) elicited from a holding potential of -100 mV started to activate at -30 mV and inactivated slowly with time constants ranging between 400 and 1000 ms. 5. Compared with the ICa,T, the inactivation curve for ICa,N was shifted about 30 mV in the depolarizing direction. ICa,N inactivated over a broader range of potentials, and its inactivation and activation curves overlapped. 6. Cadmium blocked ICa,T at a concentration 24 times higher than that which was needed to block slow inactivating currents. The apparent dissociation constant of nickel for ICa,T is twofold lower than that for the slow inactivating currents. 7. Nimodipine (2 microM) decreased the slow inactivating currents, but had no effect on ICa,T. (-)-Bay K 8644 (200 nM) increased both ICa,N and ICa,L and shifted the current activation in the hyperpolarizing direction. This result is different from that obtained in sensory and sympathetic neurones in which ICa,N is insensitive to Bay K 8644.