A voltage-clamp analysis of inward (anomalous) rectification in mouse spinal sensory ganglion neurones

Neuromedin U depolarizes rat hypothalamic paraventricular nucleus neurons in vitro by enhancing IH channel activity

The effect of neuromedin U (NMU) on rat paraventricular nucleus (PVN) neurons was examined using whole cell patch-clamp recordings. Under current-clamp, 31% of PVN parvocellular neurons (n = 243) were depolarized by 100 nM NMU, but magnocellular neurons were not affected. NMU (10 nM to 1 microM) resulted in increased basal firing rate and depolarization in a dose-dependent manner with an EC50 of 70 nM. NMU-induced depolarization was unaffected by co-perfusion with 0.5 microM TTX + 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + 10 microM bicuculline. Extracellular application of 70 microM ZD 7288 completely inhibited NMU-induced depolarization. Under voltage-clamp, 1 microM NMU produced negligible inward current but did increase the hyperpolarization-activated current (IH) at step potentials less than -80 mV. The effects of NMU on IH were voltage-dependent, and NMU shifted the IH conductance-voltage relationship (V1/2) by about 10.8 mV and enhanced IH kinetics without changing the slope constant (k). Extracellular application of 70 microM ZD 7288 or 3 mM Cs+ blocked IH and the effects of NMU in voltage-clamp. These results suggest that NMU selectively depolarizes the subpopulation of PVN parvocellular neurons via enhancement of the hyperpolarization-activated inward current.

A compartmental model of an external urethral sphincter motoneuron of Onuf's nucleus

This article discusses a model of the electrical behavior of an external urethral sphincter motoneuron, based on morphological parameters like soma size, dendritic diameters and spatial dendritic configuration, and several electrical parameters. Because experimental data about the exact ion conductance mix of external urethral sphincter neurons is scarce, the gaps in knowledge about external urethral sphincter motoneurons were filled in with known data of alpha-motoneurons. The constructed compartmental model of motoneurons of Onuf's nucleus contains six voltage-dependent ionic conductances: a fast sodium and potassium conductance and an anomalous rectifier in the soma; a fast delayed rectifier type potassium conductance and a fast sodium conductance in the initial axon segment; an L-type calcium channel in the dendritic compartments. This paper considers the simulation of external urethral sphincter motoneuron responses to current injections that evoke bistable behavior. Simulations show self-sustained discharge following a depolarizing pulse through the microelectrode; the firing was subsequently terminated by a short hyperpolarizing pulse. This behavior is highly functional for neurons that have to exhibit prolonged activation during sphincter closure. In addition to these 'on' and 'off ' responses, we also observed a particular firing behavior in response to long-lasting triangular current pulses. When the depolarizing current was slowly increased and then decreased (triangular pulse) the firing frequency was higher during the descending phase than during the initial ascending phase.

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.

Maintenance of sympathetic tone by a nickel chloride-sensitive mechanism in the rostral ventrolateral medulla of the adult rat

In urethane-anaesthetised artificially ventilated Sprague-Dawley rats, bilateral microinjection of the divalent cation nickel chloride (Ni(2+); 50 mM, 50 nl) into the rostral ventrolateral medulla elicited a dramatic inhibition of splanchnic sympathetic nerve activity (-44+/-6%) and a marked depressor response (-35+/-7 mmHg). Selective blockade of high-voltage activated Ca(2+) channels with omega-agatoxin IVA (P/Q-type), omega-conotoxin GVIA (N-type) and nifedipine (L-type) did not decrease arterial pressure or splanchnic sympathetic nerve activity when injected separately into the rostral ventrolateral medulla, or combined with kynurenate. Injection of caesium chloride or ZD 7288, a blocker of the hyperpolarization-activated cation current, into the rostral ventrolateral medulla had no effect on arterial pressure or splanchnic sympathetic nerve activity. Bilateral microinjection of nickel chloride into the caudal ventrolateral medulla/pre-Bötzinger complex elicited small increases in splanchnic sympathetic nerve activity (+17+/-13%) and arterial pressure (+12+/-4 mmHg). These were substantially smaller than those evoked by blockade of glutamatergic receptors or high-voltage activated Ca(2+) channels in this area. Injection of kynurenate or high-voltage activated Ca(2+) channel blocker, but not Ni(2+), in this area evoked respiratory termination. The results indicate the existence of a distinct mechanism maintaining the tonic activity of rostral ventrolateral medulla presympathetic neurons that is different from that maintaining the tonic activity in the caudal ventrolateral medulla/pre-Bötzinger region. We conclude that ion channels that are sensitive to Ni(2+), but are insensitive to high-voltage activated (L, P/Q, N) Ca(2+) channel blockers, and are located postsynaptically on the presympathetic rostral ventrolateral medulla neurons are responsible for the tonic activity of the presympathetic neurons in rostral ventrolateral medulla. These channels could well be the low-voltage-activated (or T-type) Ca(2+) channels although other conductances cannot be conclusively excluded.

Upregulation of the hyperpolarization-activated cation current after chronic compression of the dorsal root ganglion

A chronic compression of the DRG (CCD) produces cutaneous hyperalgesia and an enhanced excitability of neuronal somata in the compressed ganglion. The hyperpolarization-activated current (I(h)), present in the somata and axons of DRG neurons, acts to induce a depolarization after a hyperpolarizing event and, if upregulated after CCD, may contribute to enhanced neuronal excitability. Whole-cell patch-clamp recordings were obtained from acutely dissociated, retrogradely labeled, cutaneous, adult rat DRG neurons of medium size. Neurons were dissociated from L4 and L5 control DRGs or DRGs that had each been compressed for 5-7 d by L-shaped rods inserted into the intervertebral foramina. I(h), consisting of a slowly activating inward current during a step hyperpolarization, was recorded from every labeled, medium-sized neuron and was blocked by 1 mm cesium or 15 microm ZD7288. Compared with control, CCD increased the current density and rate of activation significantly without changing its reversal potential, voltage dependence of activation, or rate of deactivation. Because I(h) activation provides a depolarizing current to the neuron, thus enhancing neuronal excitability, our results are consistent with the hypothesis that I(h) contributes to hyperalgesia after CCD-induced nerve injury.

Changes of the excitability of rat trigeminal root ganglion neurons evoked by alpha(2)-adrenoreceptors

The aim of the study was to examine the effects of alpha(2)-adrenoreceptor agonists on the excitability of trigeminal root ganglion (TRG) neurons using the perforated patch-clamp technique, and to determine whether these neurons express mRNA for alpha(2)-adrenoreceptors. In current-clamp mode, the resting membrane potential was -57.4+/-1.2 mV (n=26). Most neurons (71%) were hyperpolarized by clonidine (5-50 microM) in a concentration-dependent manner. The response was associated with an increase of cell input resistance. In addition, clonidine reduced the repetitive firing evoked by depolarizing current pulses. An alpha(2)-adrenergic agonist, UK14,304, (10-20 microM) also hyperpolarized TRG neurons. The clonidine- and UK14,304-induced hyperpolarization was blocked by idazoxan (alpha(2)-adrenoreceptor antagonist). In voltage-clamp, clonidine (1-50 microM) reversibly reduced the hyperpolarization- and time-dependent cationic current. The effect was mimicked by UK14,304 (10-20 microM), and antagonized by idazoxan. Hyperpolarization-activated cationic current was blocked by extracellular Cs(+) (2 mM) or a specific blocker, ZD7288 (20 microM). Analysis of tail currents revealed that a reversal potential of the clonidine-sensitive component of hyperpolarization-activated cationic current was -46 mV. Single-cell reverse transcription-polymerase chain reaction analysis demonstrated the expression of mRNA for alpha(2A)- and alpha(2C)-adrenoreceptors. These results demonstrate that activation of alpha(2)-adrenoreceptors can hyperpolarize TRG neurons, and that the inhibitory effect is associated with inhibition of hyperpolarization-activated cationic current. Our results suggest that activation of alpha(2)-adrenoreceptors in the absence of nerve injury may have an inhibitory effect on nociceptive transmission in the trigeminal system at the level of both TRG neuronal cell bodies and primary afferent terminals.

Neuronal hyperpolarization-activated pacemaker channels drive neuropathic pain

Neuropathic pain is a common and often incapacitating clinical problem for which little useful therapy is presently available. Painful peripheral neuropathies can have many etiologies, among which are trauma, viral infections, exposure to radiation or chemotherapy, and metabolic or autoimmune diseases. Sufferers generally experience both pain at rest and exaggerated, painful sensitivity to light touch. Spontaneous firing of injured nerves is believed to play a critical role in the induction and maintenance of neuropathic pain syndromes. Using a well characterized nerve ligation model in the rat, we demonstrate that hyperpolarization-activated, cyclic nucleotide-modulated (HCN) "pacemaker" channels play a previously unrecognized role in both touch-related pain and spontaneous neuronal discharge originating in the damaged dorsal root ganglion. HCN channels, particularly HCN1, are abundantly expressed in rat primary afferent somata. Nerve injury markedly increases pacemaker currents in large-diameter dorsal root ganglion neurons and results in pacemaker-driven spontaneous action potentials in the ligated nerve. Pharmacological blockade of HCN activity using the specific inhibitor ZD7288 reverses abnormal hypersensitivity to light touch and decreases the firing frequency of ectopic discharges originating in Abeta and Adelta fibers by 90 and 40%, respectively, without conduction blockade. These findings suggest novel insights into the molecular basis of pain and the possibility of new, specific, effective pharmacological therapies.

Upregulation of the hyperpolarization-activated cation current after chronic compression of the dorsal root ganglion

A chronic compression of the DRG (CCD) produces cutaneous hyperalgesia and an enhanced excitability of neuronal somata in the compressed ganglion. The hyperpolarization-activated current (I(h)), present in the somata and axons of DRG neurons, acts to induce a depolarization after a hyperpolarizing event and, if upregulated after CCD, may contribute to enhanced neuronal excitability. Whole-cell patch-clamp recordings were obtained from acutely dissociated, retrogradely labeled, cutaneous, adult rat DRG neurons of medium size. Neurons were dissociated from L4 and L5 control DRGs or DRGs that had each been compressed for 5-7 d by L-shaped rods inserted into the intervertebral foramina. I(h), consisting of a slowly activating inward current during a step hyperpolarization, was recorded from every labeled, medium-sized neuron and was blocked by 1 mm cesium or 15 microm ZD7288. Compared with control, CCD increased the current density and rate of activation significantly without changing its reversal potential, voltage dependence of activation, or rate of deactivation. Because I(h) activation provides a depolarizing current to the neuron, thus enhancing neuronal excitability, our results are consistent with the hypothesis that I(h) contributes to hyperalgesia after CCD-induced nerve injury.

Neuronal hyperpolarization-activated pacemaker channels drive neuropathic pain

Neuropathic pain is a common and often incapacitating clinical problem for which little useful therapy is presently available. Painful peripheral neuropathies can have many etiologies, among which are trauma, viral infections, exposure to radiation or chemotherapy, and metabolic or autoimmune diseases. Sufferers generally experience both pain at rest and exaggerated, painful sensitivity to light touch. Spontaneous firing of injured nerves is believed to play a critical role in the induction and maintenance of neuropathic pain syndromes. Using a well characterized nerve ligation model in the rat, we demonstrate that hyperpolarization-activated, cyclic nucleotide-modulated (HCN) "pacemaker" channels play a previously unrecognized role in both touch-related pain and spontaneous neuronal discharge originating in the damaged dorsal root ganglion. HCN channels, particularly HCN1, are abundantly expressed in rat primary afferent somata. Nerve injury markedly increases pacemaker currents in large-diameter dorsal root ganglion neurons and results in pacemaker-driven spontaneous action potentials in the ligated nerve. Pharmacological blockade of HCN activity using the specific inhibitor ZD7288 reverses abnormal hypersensitivity to light touch and decreases the firing frequency of ectopic discharges originating in Abeta and Adelta fibers by 90 and 40%, respectively, without conduction blockade. These findings suggest novel insights into the molecular basis of pain and the possibility of new, specific, effective pharmacological therapies.

Postsynaptic action mechanism of somatostatin on the membrane excitability in spinal substantia gelatinosa neurons of juvenile rats

We used tight-seal, whole-cell recording in juvenile rat spinal slices to investigate the action of somatostatin on substantia gelatinosa neurons. Bath application of somatostatin caused a robust and repeatable hyperpolarization or outward current in substantia gelatinosa neurons. Somatostatin inhibited spontaneous action potentials in subpopulation of substantia gelatinosa neurons. The amplitude of dorsal root-evoked excitatory postsynaptic currents and the frequency of spontaneous excitatory postsynaptic currents were not affected by somatostatin. The current induced by somatostatin developed almost instantaneously and did not show any time-dependent inactivation. The current-voltage relationship exhibited inward rectification. The conductance of somatostatin-sensitive current increased with the concentration of external K(+). The reversal potentials in different external K(+) concentrations were close to the K(+) equilibrium potentials. The effect of somatostatin was dose-dependent, with an EC(50) of 113 nM. The somatostatin-sensitive current was blocked by low concentration of extracellular Ba(2+) but not by glibenclamide, an inhibitor of ATP-sensitive K(+) channels. Hyperpolarization-activated cation current in a subpopulation of substantia gelatinosa neurons was not affected by somatostatin. In neurons recorded with an internal solution containing GTPgammaS, somatostatin induced outward current and hyperpolarization that did not reverse on washing. When the spontaneous induction of outward current with GTPgammaS was greatest, somatostatin did not induce any outward currents. Furthermore, intracellular dialysis of GDPbetaS, a G-protein antagonist, abolished the effect of somatostatin. In addition, SST-sensitive neurons were fewer in slices incubated with pertussis toxin than in adjacent control slices incubated without pertussis toxin. These results suggest that somatostatin decreases the postsynaptic membrane excitability of substantia gelatinosa neurons by a pertussis toxin-sensitive G-protein-mediated activation of an inwardly rectifying K(+) conductance.

Ih blockers have a potential of antiepileptic effects

PURPOSE

The h current (Ih) is an inwardly mixed cationic conductance activated by membrane hyperpolarization and distributed predominantly in the apical dendrites of hippocampal pyramidal neurons. To verify a hypothesis that an anomalous hyperpolarization generates an abnormal excitation by way of Ih channels, we examined the effects of Ih blockers (CsCl and ZD7288) on electrically induced paroxysmal discharges (PADs).

METHODS

Fifty-three adult male rabbits were used. We measured the PAD threshold elicited by stimulation to the apical dendritic layer of the hippocampal CA1 region before and after injecting 50 microl of each Ih blocker or saline extracellularly into the same region.

RESULTS

In Ih blocker injection groups (n = 26), we obtained a significant increase in PAD threshold (1 mM CsCl: 163%, p < 0.01; 10 mM CsCl: 265%, p < 0.01; 100 mM CsCl: 199%, p < 0.01; 100 microM ZD7288: 192%, p < 0.05; 1 mM ZD7288: 246%, p < 0.05). Conversely, we did not obtain the increase in PAD threshold in a saline injection group (n = 10, 107%). The magnitude as well as duration of the effect had a tendency to depend on concentration of Ih blockers, although a saturated or declining tendency was observed with the 100 mM CsCl injection.

CONCLUSIONS

We concluded that Ih channels might contribute to hippocampal epileptiform discharges in vivo. Our hypothesis for epileptogenesis demonstrated in the present experiment offers an idea to develop a new type of antiepileptic drug based on Ih blockers for the treatment of epileptic disorders refractory to current medications.

NPY sensitivity and postsynaptic properties of heterotopic neurons in the MAM model of malformation-associated epilepsy

Neuronal migration disorders (NMDs) can be associated with neurological dysfunction such as mental retardation, and clusters of disorganized cells (heterotopias) often act as seizure foci in medically intractable partial epilepsies. Methylazoxymethanol (MAM) treatment of pregnant rats results in neuronal heterotopias in offspring, especially in hippocampal area CA1. Although the neurons in dysplastic areas in this model are frequently hyperexcitable, the precise mechanisms controlling excitability remain unclear. Here, we used IR-DIC videomicroscopy and whole cell voltage-clamp techniques to test whether the potent anti-excitatory actions of neuropeptide Y (NPY) affected synaptic excitation of heterotopic neurons. We also compared several synaptic and intrinsic properties of heterotopic, layer 2-3 cortical, and CA1 pyramidal neurons, to further characterize heterotopic cells. NPY powerfully inhibited synaptic excitation onto normal and normotopic CA1 cells but was nearly ineffective on responses evoked in heterotopic cells from stimulation sites within the heterotopia. Glutamatergic synaptic responses on heterotopic cells exhibited a comparatively small, D-2-amino-5-phosphopentanoic acid-sensitive, N-methyl-D-aspartate component. Heterotopic neurons also differed from normal CA1 cells in postsynaptic membrane currents, possessing a prominent inwardly rectifying K(+) current sensitive to Cs(+) and Ba(2+), similar to neocortical layer 2-3 pyramidal cells. CA1 cells instead had a prominent Cs(+)- and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride-sensitive I(h) and negligible inward rectification, unlike heterotopic cells. Thus heterotopic CA1 cells appear to share numerous physiological similarities with neocortical neurons. The lack of NPY's effects on intra-heterotopic inputs, the small contribution of I(h), and abnormal glutamate receptor function, may all contribute to the lowered threshold for epileptiform activity observed in hippocampal heterotopias and could be important factors in epilepsies associated with NMDs.

K(+)-induced changes in the properties of the hyperpolarization-activated cation current I(h) in rat CA1 pyramidal cells

The effects of rises in external K(+) (K(ext)) on I(h) were investigated in CA1 pyramidal cells of rat hippocampal slices using the whole-cell patch clamp technique. At the basal K(ext) level (2.5 mM), hyperpolarization-activated cation current (I(h)) had a maximal amplitude of -350+/-60 pA which was enhanced by approximately 60 and approximately 95% at 5 and 7.5 mM K(ext), respectively. The midpoint activation voltage was significantly shifted from -80 mV in the negative direction to about -87 mV at both 5 and 7.5 mM K(ext), without appreciable alterations of the current kinetics. The maximal conductance was approximately 2.4 nS under control conditions and significantly increased to approximately 3.3 and approximately 5.6 nS at 5 and 7.5 mM K(ext), respectively. The reversal potential was shifted in the positive direction, from a control value of approximately -30 mV by approximately 6 and approximately 14 mV at 5 and 7.5 mM K(ext), respectively. Our data demonstrate that even moderate changes in K(ext) have a substantial effect on the properties of I(h).

Separable gating mechanisms in a Mammalian pacemaker channel

Despite permeability to both K(+) and Na(+), hyperpolarization-activated cyclic nucleotide-gated (HCN) pacemaker channels contain the K(+) channel signature sequence, GYG, within the selectivity filter of the pore. Here, we show that this region is involved in regulating gating in a mouse isoform of the pacemaker channel (mHCN2). A mutation in the GYG sequence of the selectivity filter (G404S) had different effects on the two components of the wild-type current; it eliminated the slowly activating current (I(f)) but, surprisingly, did not affect the instantaneous current (I(inst)). Confocal imaging and immunocytochemistry showed G404S protein on the periphery of the cells, consistent with the presence of channels on the plasma membrane. Experiments with the wild-type channel showed that the rate of I(f) deactivation and I(f) amplitude had a parallel dependence on the ratio of K(+)/Na(+) driving forces. In addition, the amplitude of fully activated I(f), unlike I(inst), was not well predicted by equal and independent flow of K(+) and Na(+). The data are consistent with two separable gating mechanisms associated with pacemaker channels: one (I(f)) that is sensitive to voltage, to a mutation in the selectivity filter, and to driving forces for permeating cations and another (I(inst)) that is insensitive to these influences.

Angiotensin II influences the hyperpolarization-activated current Ih in neurones of the rat paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) consists of mainly two types of neurones (type I and type II), according to their characteristic electrophysiological properties. An excitatory influence of angiotensin II (Ang II) on PVN neurones has been shown to be mediated by the AT(1) receptor. We investigated the underlying mechanism of the Ang II effect on PVN neurones with patch-clamp experiments. Using current-clamp mode, we identified a sag in 59% of only type II neurones due to a hyperpolarization-activated current I(h). Voltage-clamp measurements revealed that Ang II (1 microM) provokes a shift of the I(h) activation curve to more depolarized values by 8.1 mV, an effect reduced by the additional application of the AT(1) receptor antagonist Losartan (10 microM). These findings suggest that the excitatory effect of Ang II in type II neurones is at least partially due to an increased population of active I(h) channels.

Modulation by hyperpolarization-activated cationic currents of voltage responses in human rods

We used the whole-cell patch-clamp recording technique on surgically excised human retina to examine whether human rod photoreceptors express hyperpolarization-activated cationic currents (I(h)) and to analyze the effects of I(h) on rod's voltage responses. Hyperpolarizing voltage steps from a holding potential of -60 mV evoked a slow inward-rectifying current in both rods in retinal slices and isolated rods. The slow inward-rectifying currents induced by hyperpolarization were markedly reduced by 3 mM Cs(+) (a blocker of I(h)) in the bath, but not by 3 mM Ba(2+) (an anomalous rectifier K(+) current blocker) or 1 mM SITS (a Cl(-) current blocker). A concentration-response curve for block by Cs(+) of the inward currents could be fitted by the Hill equation with a half-blocking concentration (IC(50)) of 41 microM and a Hill coefficient of 0.91. The time course of the inward current activation was well described at all recorded voltages by the sum of two exponentials. Under current-clamp conditions, injection of steps of current, either hyperpolarizing or depolarizing, elicited an initial rapid voltage change that was followed by a gradual decay in the voltage response. The decay in the voltage responses was eliminated by bath application of 3 mM Cs(+). The voltage dependence, pharmacology, and kinetics of the slow inward-rectifying currents described above suggest that human rods express I(h). We suggest that I(h) becomes activated in the course of large hyperpolarizations generated by bright-light illumination and may modify the waveform of the photovoltage in human rods.

Postnatal changes in membrane properties of mice trigeminal ganglion neurons

Intracellular recordings from neurons in the mouse trigeminal ganglion (TG) in vitro were used to characterize changes in membrane properties that take place from early postnatal stages (P0-P7) to adulthood (>P21). All neonatal TG neurons had uniformly slow conduction velocities, whereas adult neurons could be separated according to their conduction velocity into Adelta and C neurons. Based on the presence or absence of a marked inflection or hump in the repolarization phase of the action potential (AP), neonatal neurons were divided into S- (slow) and F-type (fast) neurons. Their passive and subthreshold properties (resting membrane potential, input resistance, membrane capacitance, and inward rectification) were nearly identical, but they showed marked differences in AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and afterhyperpolarization (AHP) duration. Adult TG neurons also segregated into S- and F-type groups. Differences in their mean AP amplitude, AP overshoot, AP duration, rate of AP depolarization, rate of AP repolarization, and AHP duration were also prominent. In addition, axons of 90% of F-type neurons and 60% of S-type neurons became faster conducting in their central and peripheral branch, suggestive of axonal myelination. The proportion of S- and F-type neurons did not vary during postnatal development, suggesting that these phenotypes were established early in development. Membrane properties of both types of TG neurons evolved differently during postnatal development. The nature of many of these changes was linked to the process of myelination. Thus myelination was accompanied by a decrease in AP duration, input resistance (R(in)), and increase in membrane capacitance (C). These properties remained constant in unmyelinated neurons (both F- and S-type). In adult TG, all F-type neurons with inward rectification were also fast-conducting Adelta, suggesting that those F-type neurons showing inward rectification at birth will evolve to F-type Adelta neurons with age. The percentage of F-type neurons showing inward rectification also increased with age. Both F- and S-type neurons displayed changes in the sensitivity of the AP to reductions in extracellular Ca(2+) or substitution with Co(2+) during the process of maturation.

Characteristics of hyperpolarization-activated cation currents in portal vein smooth muscle cells

Voltage-clamp studies of freshly isolated smooth muscle cells from rabbit portal vein revealed the existence of a time-dependent cation current evoked by membrane hyperpolarization (termed I(h)). Both the rate of activation and the amplitude of I(h) were enhanced by membrane hyperpolarization. Half-maximal activation of I(h) was about -105 mV with conventional whole cell and -80 mV when the perforated patch technique was used. In current clamp, injection of hyperpolarizing current produced a marked depolarizing "sag" followed by rebound depolarization. Activation of I(h) was augmented by an increase in the extracellular K(+) concentration and was blocked rapidly by externally applied Cs(+) (1-5 mM). The bradycardic agent ZD-7288 (10 microM), a selective inhibitor of I(h), produced a characteristically slow inhibition of the portal vein I(h). The depolarizing sag recorded in current clamp was also abolished by application of 5 mM Cs(+). Cs(+) significantly decreased the frequency of spontaneous contractions in both whole rat portal vein and rabbit portal vein segments. Multiplex RT-PCR of rabbit portal vein myocytes using primers derived from existing genes for hyperpolarization-activated cation channels (HCN1-4) revealed the existence of cDNA clones corresponding to HCN2, 3, and 4. The present study shows that portal vein myocytes contain genes shown to encode for hyperpolarization-activated channels and exhibit an endogenous current with characteristics similar to I(h) in other cell types. This conductance appears to determine, in part, the rhythmicity of this vessel.

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

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

Modelling the biomechanics and control of sphincters

This paper reviews current mathematical models of sphincters and compares them with a new spatial neuromuscular control model based on known physiological properties. Almost all the sphincter models reviewed were constructed as a component of a more extensive model designed to mirror the overall behaviour of a larger system such as the lower urinary tract. This implied less detailed modelling of the sphincter component. It is concluded that current sphincter models are not suitable for mimicking detailed interactions between a neural controller and a sphincter. We therefore outline a new integrated model of the biomechanics and neural control of a sphincter. The muscle is represented as a lumped-mass model, providing the possibility of applying two- or three-dimensional modelling strategies. The neural network is a multi-compartment model that provides neural control signals at the level of action potentials.The integrated model was used to simulate a uniformly activated sphincter and a partially deficient innervation of the sphincter, resulting in a non-uniformly activated sphincter muscle. During the simulation, the pressure in the sphincter lumen was prescribed to increase sinusoidally to a value of 60 kPa. In the uniformly activated situation, the sphincter muscle remains closed, whereas the partially denervated sphincter is stretched open, although the muscle is intact.

Electrophysiological and morphological properties of pre-autonomic neurones in the rat hypothalamic paraventricular nucleus

1. The cellular properties of pre-autonomic neurones in the hypothalamic paraventricular nucleus (PVN) were characterized by combining in vivo retrograde tracing techniques, in vitro patch-clamp recordings and three-dimensional reconstruction of recorded neurones in adult hypothalamic slices. 2. The results showed that PVN pre-autonomic neurones constitute a heterogeneous neuronal population. Based on morphological criteria, neurones were classified into three subgroups. Type A neurones (52 %) were located in the ventral parvocellular (PaV) subnucleus, and showed an oblique orientation with respect to the third ventricle (3V). Type B neurones (25 %) were located in the posterior parvocellular (PaPo) subnucleus, and were oriented perpendicularly with respect to the 3V. Type C neurones (23 %) were located in both the PaPo (82 %) and the PaV (18 %) subnuclei, and displayed a concentric dendritic configuration. 3. A morphometric analysis revealed significant differences in the dendritic configuration among neuronal types. Type B neurones had the most complex dendritic arborization, with longer and more branching dendritic trees. 4. Several electrophysiological properties, including cell input resistance and action potential waveforms, differed between cell types, suggesting that the expression and/or properties of a variety of ion channels differ between neuronal types. 5. Common features of PVN pre-autonomic neurones included the expression of a low-threshold spike and strong inward rectification. These properties distinguished them from neighbouring magnocellular vasopressin neurones. 6. In summary, these results indicate that PVN pre-autonomic neurones constitute a heterogeneous neuronal population, and provide a cellular basis for the study of their involvement in the pathophysiology of hypertension and congestive heart failure disorders.

Ionic basis of the caesium-induced depolarisation in rat supraoptic nucleus neurones

1. The effects of external Cs(+) on magnocellular neurosecretory cells were studied during intracellular recordings from 93 supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Bath application of 3-5 mM Cs(+) provoked reversible membrane depolarisation and increased firing rate in all of the neurones tested. Voltage-current analysis revealed an increase in membrane resistance between -120 and -55 mV. The increase in resistance was greater below -85 mV than at more positive potentials. 3. Voltage-clamp analysis showed that external Cs(+) blocked the hyperpolarisation-activated inward current, I(H). Under current clamp, application of ZD 7288, a selective blocker of I(H), caused an increase in membrane resistance at voltages < or = -65 mV. Voltage-current analysis further revealed that blockade of I(H) caused hyperpolarisation when the initial voltage was < -60 mV but had no effect at more positive values. 4. Current- and voltage-clamp analysis of the effects of Cs(+) in the presence of ZD 7288, or ZD 7288 and tetraethyl ammonium (TEA), revealed an increase in membrane resistance throughout the range of voltages tested (-120 to -45 mV). The current blocked by Cs(+) in the absence of I(H) was essentially voltage independent and reversed at -100 mV. The reversal potential shifted by +22.7 mV when external [K(+)] was increased from 3 to 9 mM. We conclude that, in addition to blocking I(H), external Cs(+) blocks a leakage K(+) current that contributes significantly to the resting potential of rat magnocellular neurosecretory cells.

Hyperpolarisation-activated current in glomerular cells of the rat olfactory bulb

Whole-cell patch-clamp recordings were carried out in visually identified periglomerular and external tufted cells of rat olfactory bulb. Most of the neurones showed a slowly developing hyperpolarisation-activated current with a threshold generally positive to resting potential and with a strongly voltage-dependent activation time constant. The current, identified as Ih, was sodium- and potassium-sensitive, suppressed by external caesium, and insensitive to barium. Under current-clamp conditions, perfusion with caesium induced a 10 mV hyperpolarisation and a marked reduction of the rate of low-frequency oscillations induced experimentally. It is concluded that most of the cells in the rat glomerular layer present a distinct h-current, which is tonically active at rest and which may contribute to the oscillatory behaviour of the bulbar network.

Correlation between electrophysiology and morphology of three groups of neuron in the dorsal commissural nucleus of lumbosacral spinal cord of mature rats studied in vitro

The dorsal commissural nucleus (DCN) in the lumbosacral spinal cord receives afferent inputs from the pelvic organs via pudendal and pelvic nerves. Electrophysiological and morphological properties of neurons in the DCN of L6-S1 were examined using whole-cell recordings with biocytin-filled electrodes in transverse slices of mature rat spinal cord. Neurons were categorized into three groups according to their discharge in response to suprathreshold depolarizing pulses; neurons with tonic (19/42) and phasic (13/42) firing patterns, and neurons (10/42) that fired in bursts arising from a Ca(2+)-dependent hump. The predominantly fusiform somata of neurons labeled during recording (n = 31) had on average 3.1 primary dendrites, 7.5 terminating dendritic branches, 3.1 axon collaterals, and 14.2 axon terminations per neuron. The groups were morphologically distinct on the basis of their dendritic branching patterns. Phasic neurons (n = 10) had the most elaborate dendritic branching and the largest numbers of axon collaterals. All tonic neurons (n = 11) had axons/collaterals projecting to the intermediolateral area but none to the funiculi, suggesting that they function as interneurons in local autonomic reflexes. Many axons/collaterals of all phasic neurons lay within the DCN, suggesting that they integrate segmental and descending inputs. Seven of 10 neurons with Ca(2+)-dependent humps had axons/collaterals extending into one of the funiculi, suggesting that they project intersegmentally or to the brain. Ca(2+) hump neurons also had more axons/collaterals within the DCN and fewer in the intermediolateral area than tonic neurons. This correlation between firing pattern and morphology is an important step toward defining the cellular pathways regulating pelvic function.

Contribution of the hyperpolarization-activated current (I(h)) to membrane potential and GABA release in hippocampal interneurons

Intrinsic GABAergic interneurons provide inhibitory input to the principal neurons of the hippocampus. The majority of interneurons located in stratum oriens (s.o.) of the CA1 region express the hyperpolarization-activated cation current known as I(h). In an effort to elucidate the role of this current in regulating the baseline excitability of these neurons and its participation in the regulation of the release of GABA onto CA1 pyramidal neurons, we utilized whole cell electrophysiological recordings from both populations of cells. In voltage-clamp experiments, hyperpolarization of the interneuron membrane initiated a large inward current with an estimated activation threshold of 51.6 +/- 7.6 mV and a half-maximal voltage of -73.0 +/- 7.0 mV. This current was blocked by bath application of the I(h) inhibitors ZD 7288 (50 microM) or cesium (2 mM). Current-clamp experiments at the interneuron resting membrane potential (-61.3 +/- 1.2 mV) revealed a significant hyperpolarization, a decrease in the rate of spontaneous action potential discharge, an increase in the cellular input resistance, and the elimination of rebound afterdepolarizations during blockade of I(h) with ZD 7288 (50 microM). The hyperpolarizing effect of ZD 7288 was also substantially larger in interneurons clamped near -80 mV using current injection through the pipette. In addition to neurons exhibiting I(h), recordings were obtained from a small population of s.o. interneurons that did not exhibit this current. These cells demonstrated resting membrane potentials that were significantly more negative (-73.6 +/- 5.5 mV) than those observed in neurons expressing I(h), suggesting that this current contributes to more depolarized membrane potentials in these cells. Recordings from postsynaptic pyramidal neurons demonstrated that blockade of I(h) with ZD 7288 caused a substantial reduction (approximately 43%) in the frequency of spontaneous action potential-dependent inhibitory postsynaptic currents (IPSCs), without altering their average amplitude. However, miniature action-potential-independent IPSC frequency, amplitude, and decay kinetics were unaltered by ZD 7288. These data suggest that I(h) is active at the resting membrane potential in s.o. interneurons and as a result contributes to the spontaneous activity of these cells and to the tonic inhibition of CA1 pyramidal neurons in the hippocampus.

Developmental changes in hyperpolarization-activated currents I(h) and I(K(IR)) in isolated rat intracardiac neurons

The hyperpolarization-activated nonselective cation current, I(h), was investigated in neonatal and adult rat intracardiac neurons. I(h) was observed in all neurons studied and displayed slow time-dependent rectification. I(h) was isolated by blockade with external Cs(+) (2 mM) and was inhibited irreversibly by the bradycardic agent, ZD 7288. Current density of I(h) was approximately twofold greater in neurons from neonatal (-4.1 pA/pF at -130 mV) as compared with adult (-2.3 pA/pF) rats; however, the reversal potential and activation parameters were unchanged. The reversal potential and amplitude of I(h) was sensitive to changes in external Na(+) and K(+) concentrations. An inwardly rectifying K(+) current, I(K(IR)), was also present in intracardiac neurons from adult but not neonatal rats and was blocked by extracellular Ba(2+). I(K(IR)) was present in approximately one-third of the adult intracardiac neurons studied, with a current density of -0.6 pA/pF at -130 mV. I(K(IR)) displayed rapid activation kinetics and no time-dependent rectification consistent with the rapidly activating, inward K(+) rectifier described in other mammalian autonomic neurons. I(K(IR)) was sensitive to changes in external K(+), whereby raising the external K(+) concentration from 3 to 15 mM shifted the reversal potential by approximately +36 mV. Substitution of external Na(+) had no effect on the reversal potential or amplitude of I(K(IR)). I(K(IR)) density increases as a function of postnatal development in a population of rat intracardiac neurons, which together with a concomitant decrease in I(h) may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.

Molecular diversity of pacemaker ion channels

Ionic currents activated by hyperpolarization and regulated by cyclic nucleotides were first discovered more than 20 years ago. Recently the molecular identity of the underlying channels has been unveiled. The structural features of the protein sequences are discussed and related to the mechanisms of activation, selectivity for cyclic nucleotides, and ion permeation. Coverage includes a comparison of the biophysical properties of recombinant and native channels and their significance for the physiological functions of these channels.

New trends in the etiopathogenesis of diabetic peripheral neuropathy

Neuropathy is well recognized as a major complication of insulin-dependent diabetes mellitus in adults, resulting in significant morbidity and possibly an increased mortality. Both the peripheral and autonomic nervous systems can be involved, and adolescents with diabetes can show early evidence of neuropathy. The pathogenesis of diabetic neuropathy remains unclear but is thought to involve various mechanisms. This complication can be traced to the metabolic effects of hyperglycemia and/or other effects of insulin deficiency on the various constituents of the peripheral nerve. The polyol pathway and/or nonenzymatic glycation affecting one or more cell types in the multicellular constituents of the peripheral nerve appear likely to have an inciting role. The role of other factors, such as possible direct neurotrophic effects of insulin and insulin-related growth factors, seems to be relevant.

Persistently modified h-channels after complex febrile seizures convert the seizure-induced enhancement of inhibition to hyperexcitability

Febrile seizures are the most common type of developmental seizures, affecting up to 5% of children. Experimental complex febrile seizures involving the immature rat hippocampus led to a persistent lowering of seizure threshold despite an upregulation of inhibition. Here we provide a mechanistic resolution to this paradox by showing that, in the hippocampus of rats that had febrile seizures, the long-lasting enhancement of the widely expressed intrinsic membrane conductance Ih converts the potentiated synaptic inhibition to hyperexcitability in a frequency-dependent manner. The altered gain of this molecular inhibition-excitation converter reveals a new mechanism for controlling the balance of excitation-inhibition in the limbic system. In addition, here we show for the first time that h-channels are modified in a human neurological disease paradigm.

Cellular expression and functional characterization of four hyperpolarization-activated pacemaker channels in cardiac and neuronal tissues

Hyperpolarization-activated cation currents (I(h)) have been identified in cardiac pacemaker cells and a variety of central and peripheral neurons. Four members of a gene family encoding hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN1--4) have been cloned recently. Native I(h) currents recorded from different cell types exhibit distinct activation kinetics. To determine if this diversity of I(h) currents may be caused by differential expression of HCN channel isoforms, we investigated the cellular distribution of the transcripts of HCN1--4 in the murine sinoatrial node, retina and dorsal root ganglion (DRG) by in situ hybridization. In the sinoatrial node, the most prominently expressed HCN channel is HCN4, whereas HCN2 and HCN1 are detected there at moderate and low levels, respectively. Retinal photoreceptors express high levels of HCN1, whereas HCN2, 3 and 4 were not found in these cells. In DRG neurons, the dominant HCN transcript is HCN1, followed by HCN2. We next determined the functional properties of recombinant HCN1--4 channels expressed in HEK293 cells. All four channel types gave rise to I(h) currents but displayed marked differences in their activation kinetics. Our results suggest that the heterogeneity of native I(h) currents is generated, at least in part, by the tissue-specific expression of HCN channel genes.

Ionic currents and spontaneous firing in neurons isolated from the cerebellar nuclei

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly ( approximately 20 Hz), with shallow interspike hyperpolarizations (approximately -60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections. During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltage-independent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage- and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mm TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Axotomy- and autotomy-induced changes in the excitability of rat dorsal root ganglion neurons

The spontaneous, ectopic activity in sensory nerves that is induced by peripheral nerve injury is thought to contribute to the generation of "neuropathic" pain in humans. To examine the cellular mechanisms that underlie this activity, neurons in rat L(4)-L(5) dorsal root ganglion (DRG) were first grouped as "large," "medium," or "small" on the basis of their size (input capacitance) and action potential (AP) shape. A fourth group of cells that exhibited a pronounced afterdepolarization (ADP) were defined as AD-cells. Whole cell recording was used to compare the properties of control neurons with those dissociated from rats in which the sciatic nerve had been sectioned ("axotomy" group) and with neurons from rats that exhibited self-mutilatory behavior in response to sciatic nerve section ("autotomy" group). Increases in excitability in all types of DRG neuron were seen within 2-7 wk of axotomy. Resting membrane potential (RMP) and the amplitude and duration of the afterhyperpolarization (AHP) that followed the AP were unaffected. Effects of axotomy were greatest in the small, putative nociceptive cells and least in the large cells. Moderate changes were seen in the medium and AD-cells. Compared to control neurons, axotomized neurons exhibited a higher frequency of evoked AP discharge in response to 500-ms depolarizing current injections; i.e., "gain" was increased and accommodation was decreased. The minimum current required to discharge an AP (rheobase) was reduced. There were significant increases in spike width in small cells and significant increases in spike height in small, medium, and AD-cells. The electrophysiological changes promoted by axotomy were intensified in animals that exhibited autotomy; spike height, and spike width were significantly greater than control for all cell types. Under our experimental conditions, spontaneous activity was never encountered in neurons dissociated from animals that exhibited autotomy. Thus changes in the electrical properties of cell bodies alone may not entirely account for injury-induced spontaneous activity in sensory nerves. The onset of autotomy coincided with alterations in the excitability of large, putative nonnociceptive, neurons. Thus large cells from the autotomy group were much more excitable than those from the axotomy group, whereas small cells from the autotomy group were only slightly more excitable. This is consistent with the hypothesis that the onset of autotomy is associated with changes in the properties of myelinated fibers. Changes in Ca2+ and K+ channel conductances that contribute to axotomy- and autotomy-induced changes in excitability are addressed in the accompanying paper.

Effects of substance P on excitability and ionic currents of normal and axotomized rat dorsal root ganglion neurons

Substance P (SP) may act within dorsal root ganglia (DRG) to modulate the transmission of nociceptive information. Because peripheral nerve injury (axotomy) alters the peptide content of sensory neurons, we used whole-cell recording to examine the effects of sciatic nerve section on the sensitivity of rat lumbar DRG neurons to SP (0.3--1 microM). At 1 microM, SP increased the excitability of 'small', putative nociceptive neurons but had little effect on the excitability of 'large' neurons. Two-four weeks after sciatic nerve section, however, the effect of SP on 'large' axotomized neurons was increased and its effect on 'small' neurons was decreased. SP did not affect Ca(2+) channel currents in control or axotomized neurons. The effects of SP on the current-voltage (I--V) relationship of 77% of neurons involved increased inward current at potentials below -30 mV and suppressed outward current at potentials above -20 mV. The effects of SP on the I--V relationship were similar in control and in axotomized neurons and the altered sensitivity of 'small' and 'large' cells could not be attributed to axotomy-induced changes in input resistance or membrane potential. The possible relevance of alterations in sensitivity, of 'large' DRG neurons to SP, to the generation of neuropathic pain is discussed.

Axotomy- and autotomy-induced changes in Ca2+ and K+ channel currents of rat dorsal root ganglion neurons

Sciatic nerve section (axotomy) increases the excitability of rat dorsal root ganglion (DRG) neurons. The changes in Ca2+ currents, K+ currents, Ca2+ sensitive K+ current, and hyperpolarization-activated cation current (I(H)) that may be associated with this effect were examined by whole cell recording. Axotomy affected the same conductances in all types of DRG neuron. In general, the largest changes were seen in "small" cells and the smallest changes were seen in "large" cells. High-voltage-activated Ca2+ channel current (HVA-I(Ba)) was reduced by axotomy. Although currents recorded in axotomized neurons exhibited increased inactivation, this did not account for all of the reduction in HVA-I(Ba). Activation kinetics were unchanged, and experiments with nifedipine and/or omega-conotoxin GVIA showed that there was no change in the percentage contribution of L-type, N-type, or "other" HVA-I(Ba) to the total current after axotomy. T-type (low-voltage-activated) I(Ba) was not affected by axotomy. Ca2+ sensitive K+ conductance (g(K,Ca)) appeared to be reduced, but when voltage protocols were adjusted to elicit similar amounts of Ca2+ influx into control and axotomized cells, I(K,Ca)(s) were unchanged. After axotomy, Cd2+ insensitive, steady-state K+ channel current, which primarily comprised delayed rectifier K+ current (I(K)), was reduced by about 60% in small, medium, and large cells. These data suggest that axotomy-induced increases in excitability are associated with decreases in I(K) and/or decreases in g(K,Ca) that are secondary to decreased Ca2+ influx. Because I(H) was reduced by axotomy, changes in this current do not contribute to increased excitability. The amplitude and inactivation of I(Ba) in all cell types was changed more profoundly in animals that exhibited self-mutilatory behavior (autotomy). The onset of this behavior corresponded with significant reduction in I(Ba) of large neurons. This finding supports the hypothesis that autotomy, that may be related to human neuropathic pain, is associated with changes in the properties of large myelinated sensory neurons.

5-HT modulation of multiple inward rectifiers in motoneurons in intact preparations of the neonatal rat spinal cord

This study introduces novel aspects of inward rectification in neonatal rat spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cell tight-seal recordings were made from MNs in an isolated lumbar spinal cord preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step commands were generated from holding potentials of -50 to -40 mV. Discordant with previous reports involving slice preparations, fast inward rectification was commonly expressed and in 44% of the MNs co-existed with a slow inward rectification related to activation of I(h). The fast inward rectification is likely caused by an I(Kir). Thus it appeared around E(K) and was sensitive to low concentrations (100-300 microM) of Ba2+ but not to ZD 7288, which blocked I(h). Both I(Kir) and I(h) were inhibited by Cs2+ (0.3-1.5 mM). Extracellular addition of 5-HT (10 microM) reduced the instantaneous conductance, most strongly at membrane potentials above E(K). Low [Ba2+] prevented the 5-HT-induced instantaneous conductance reduction below, but not that above, E(K). This suggests that 5-HT inhibits I(Kir), but also other instantaneous conductances. The biophysical parameters of I(h) were evaluated before and under 5-HT. The maximal I(h) conductance, G(max), was 12 nS, much higher than observed in slice preparations. G(max) was unaffected by 5-HT. In contrast, 5-HT caused a 7-mV depolarizing shift in the activation curve of I(h). Double-exponential fits were generally needed to describe I(h) activation. The fast and slow time constants obtained by these fits differed by an order of magnitude. Both time constants were accelerated by 5-HT, the slow time constant to the largest extent. We conclude that spinal neonatal MNs possess multiple forms of inward rectification. I(h) may be carried by two spatially segregated channel populations, which differ in kinetics and sensitivity to 5-HT. 5-HT increases MN excitability in several ways, including inhibition of a barium-insensitive leak conductance, inhibition of I(Kir), and enhancement of I(h). The quantitative characterization of these effects should be useful for further studies seeking to understand how neuromodulation prepares vertebrate MNs for concerted behaviors such as locomotor activity.

Inhibition of the I(h) current in isolated peripheral nerve: a novel mode of peripheral antinociception?

Although the alpha2-adrenergic agonist clonidine has been shown to promote peripheral antinociception, its mechanism of action has not yet been clearly elucidated. By the use of the sucrose-gap method, we have shown that in C fibers of the rabbit vagus nerve, clonidine at micromolar concentrations enhances activity-dependent hyperpolarizations generated by the Na+-K+ pump during and after repetitive stimulation. Similar results were obtained with 10 microM of ZD 7288, a specific blocker of the hyperpolarization-activated cation current (I(h)) and with 2 mM of Ba2+ that blocks the inwardly rectifying potassium current (I(KIR)). Furthermore, clonidine had no added effect on the ZD 7288-induced response, whereas it produced a marked enhancement of Ba2+induced response. From these results, it can be concluded that clonidine enhances activity-dependent hyperpolarization by inhibiting the current I(h). We propose that clonidine, by increasing the threshold for initiating the action potential, induces a slowing or block of conduction and that this mechanism is the origin of the clonidine-induced antinociception. Finally, this study suggests a novel role for inwardly rectifying hyperpolarization-activated conductances in peripherally mediated antinociception.

Blocker state dependence and trapping in hyperpolarization-activated cation channels: evidence for an intracellular activation gate

Hyperpolarization-activated cation currents (I(h)) are key determinants of repetitive electrical activity in heart and nerve cells. The bradycardic agent ZD7288 is a selective blocker of these currents. We studied the mechanism for ZD7288 blockade of cloned I(h) channels in excised inside-out patches. ZD7288 blockade of the mammalian mHCN1 channel appeared to require opening of the channel, but strong hyperpolarization disfavored blockade. The steepness of this voltage-dependent effect (an apparent valence of approximately 4) makes it unlikely to arise solely from a direct effect of voltage on blocker binding. Instead, it probably indicates a differential affinity of the blocker for different channel conformations. Similar properties were seen for ZD7288 blockade of the sea urchin homologue of I(h) channels (SPIH), but some of the blockade was irreversible. To explore the molecular basis for the difference in reversibility, we constructed chimeric channels from mHCN1 and SPIH and localized the structural determinant for the reversibility to three residues in the S6 region likely to line the pore. Using a triple point mutant in S6, we also revealed the trapping of ZD7288 by the closing of the channel. Overall, the observations led us to hypothesize that the residues responsible for ZD7288 block of I(h) channels are located in the pore lining, and are guarded by an intracellular activation gate of the channel.

Swelling-activated calcium signalling in cultured mouse primary sensory neurons

The effects of hypo-osmotic membrane stretch on intracellular calcium concentration ([Ca(2+)](i)), cell volume and cellular excitability were investigated in cultured mouse primary sensory trigeminal neurons. Hypotonic solutions (15--45%) led to rapid cell swelling in all neurons. Swelling was accompanied by dose-dependent elevations in [Ca(2+)](i) in a large fraction of neurons. Responses could be classified into three categories. (i) In 57% of the neurons [Ca(2+)](i) responses had a slow rise time and were generally of small amplitude. (ii) In 21% of the neurons, responses had a faster rise and were larger in amplitude. (iii) The remaining cells (22%) did not show [Ca(2+)](i) responses to hypo-osmotic stretch. Slow and fast [Ca(2+)](i) changes were observed in trigeminal neurons of different sizes with variable responses to capsaicin (0.5 microM). The swelling-induced [Ca(2+)](i) responses were not abolished after depletion of intracellular Ca2+ stores with cyclopiazonic acid or preincubation in thapsigargin, but were suppressed in the absence of external Ca(2+). They were strongly attenuated by extracellular nickel and gadolinium. Hypotonic stimulation led to a decrease in input resistance and to membrane potential depolarization. Under voltage-clamp, the [Ca(2+)](i) elevation produced by hypotonic stimulation was accompanied by the development of an inward current and a conductance increase. The time course and amplitude of the [Ca(2+)](i) response to hypo-osmotic stimulation showed a close correlation with electrophysiological properties of the neurons. Fast [Ca(2+)](i) responses were characteristic of trigeminal neurons with short duration action potentials and marked inward rectification. These findings suggest that hypo-osmotic stimulation activates several Ca(2+)-influx pathways, including Gd(3+)-sensitive stretch-activated ion channels, in a large fraction of trigeminal ganglion neurons. Opening of voltage-gated Ca(2+) channels also contributes to the response. The pattern and rate of Ca(2+) influx may be correlated with functional subtypes of sensory neurons.

Ionic currents and spontaneous firing in neurons isolated from the cerebellar nuclei

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly ( approximately 20 Hz), with shallow interspike hyperpolarizations (approximately -60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections. During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltage-independent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage- and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mm TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Subclassified acutely dissociated cells of rat DRG: histochemistry and patterns of capsaicin-, proton-, and ATP-activated currents

We used a "current signature" method to subclassify acutely dissociated dorsal root ganglion (DRG) cells into nine subgroups. Cells subclassified by current signature had uniform properties. The type 1 cell had moderate capsaicin sensitivity (25.9 pA/pF), powerful, slowly desensitizing (tau = 2,300 ms), ATP-activated current (13.3 pA/pF), and small nondesensitizing responses to acidic solutions (5.6 pA/pF). Type 1 cells expressed calcitonin gene-related peptide immunoreactivity (CGRP-IR), manifested a wide action potential (7.3 ms), long duration afterhyperpolarization (57.0 ms), and were IB4 positive. The type 2 cell exhibited large capsaicin activated currents (134.9 pA/pF) but weak nondesensitizing responses to protons (15.3 pA/pF). Currents activated by ATP and alphabeta-m-ATP (51.7 and 44.6 pA/pF, respectively) had fast desensitization kinetics (tau = 214 ms) that were distinct from all other cell types. Type 2 cells were IB4 positive but did not contain either substance P (SP) or CGRP-IR. Similar to capsaicin-sensitive nociceptors in vivo, the afterhyperpolarization of the type 2 cell was prolonged (54.7 ms). The type 3 cell expressed, amiloride-sensitive, rapidly desensitizing (tau = 683 ms) proton-activated currents (127.0 pA/pF), and was insensitive to ATP or capsaicin. The type 3 cell was IB4 negative and contained neither CGRP nor SP-IR. The afterhyperpolarization (17.5 ms) suggested nonnociceptive function. The type 4 cell had powerful ATP-activated currents (17.4 pA/pF) with slow desensitization kinetics (tau = 2, 813 ms). The afterhyperpolarization was prolonged (46.5 ms), suggesting that this cell type might belong to a capsaicin-insensitive nociceptor population. The type 4 cell did not contain peptides. The type 7 cell manifested amiloride-sensitive, proton-activated currents (45.8 pA/pF) with very fast desensitization kinetics (tau = 255 ms) and was further distinct from the type 3 cell by virtue of a nondesensitizing amiloride-insensitive component (6.0 pA/pF). Capsaicin and ATP sensitivity were relatively weak (4.3 and 2.9 pA/pF, respectively). Type 7 cells were IB4 positive and contained both SP and CGRP-IR. They exhibited an exceptionally long afterhyperpolarization (110 ms) that was suggestive of a silent (mechanically insensitive) nociceptor. We concluded that presorting of DRG cells by current signatures separated them into internally homogenous subpopulations that were distinct from other subclassified cell types.

The calcium receptor modulates the hyperpolarization-activated current in subfornical organ neurons

Here we report that neurons of the subfornical organ (SFO), a circumventricular structure devoid of a blood-brain barrier, show time-dependent, inward rectification indicative of the presence of a subthreshold, hyperpolarization-activated inward current (Ih). In whole-cell patch clamp experiments of isolated SFO neurons, we observed a Cs+-sensitive Ih in 47% of cells tested. Furthermore, we show that Ih is involved in the generation of evoked bursts in SFO neurons. An allosteric agonist of the extracellular calcium-sensing receptor (CaR) was found to potentiate Ih consistent with our previous observations of CaR-mediated bursting in SFO neurons. These studies indicate that a proportion of SFO neurons express Ih, and this may be one ionic mechanism through which bursting is regulated by various extracellular messengers.

Characterization of a hyperpolarization-activated inward current in Cajal-Retzius cells in rat neonatal neocortex

Cajal-Retzius cells are among the first neurons appearing during corticogenesis and play an important role in the establishment of cortical lamination. To characterize the hyperpolarization-activated inward current (I(h)) and to investigate whether I(h) contributes to the relatively positive resting membrane potential (RMP) of these cells, we analyzed the properties of I(h) in visually identified Cajal-Retzius cells in cortical slices from neonatal rats using the whole cell patch-clamp technique. Membrane hyperpolarization to -90 mV activated a prominent inward current that was inhibited by 1 mM Cs(+) and was insensitive to 1 mM Ba(2+). The activation time constant for I(h) was strongly voltage dependent. In Na(+)-free solution, I(h) was reduced, indicating a contribution of Na(+). An analysis of the tail currents revealed a reversal potential of -45.2 mV, corresponding to a permeability coefficient (pNa(+)/pK(+)) of 0. 13. While an increase in the extracellular K(+) concentration ([K(+)](e)) enhances I(h), it was reduced by a [K(+)](e) decrease. This [K(+)](e) dependence could not be explained by an effect on the electromotive force on K(+) but suggested an additional extracellular binding site for K(+) with an apparent dissociation constant of 7.2 mM. Complete Cl(-) substitution by Br(-), I(-), or NO(3)(-) had no significant effect on I(h), whereas a complete Cl(-) substitution by the organic compounds methylsulfate, isethionate, or gluconate reduced I(h) by approximately 40%. The I(h) reduction observed in gluconate could be abolished by the addition of Cl(-). The analysis of the [Cl(-)](e) dependence of I(h) revealed a dissociation constant of 9.8 mM and a Hill-coefficient of 2.5, while the assumption of a gluconate-dependent I(h) reduction required an unreasonably high Hill-coefficient >20. An internal perfusion with the lidocaine derivative lidocaine N-ethyl bromide blocks I(h) within 1 min after establishment of the whole cell configuration. An inhibition of I(h) by 1 mM Cs(+) was without an effect on RMP, action potential amplitude, threshold, width, or afterhyperpolarization. We conclude from these results that Cajal-Retzius cells express a prominent I(h) with characteristic properties that does not contribute to the RMP.

A role for chloride in the hyperpolarizing effect of acetylcholine in isolated frog vestibular hair cells

Acetylcholine (ACh) is the dominant transmitter released from inner ear efferent neurons. In frog vestibular organs, these efferent neurons synapse exclusively with type II hair cells. Hair cells isolated from the frog saccule hyperpolarize following the application of 50 microM ACh, thereby demonstrating the presence of an ACh receptor. A role for Cl(-) in the response of hair cell-bearing organs to efferent nerve activation or ACh application was suggested some years ago. Perfusion with solutions in which most of the Cl(-) was replaced by large impermeant anions decreased the cholinergic inhibition of afferent firing in the cat and turtle cochleas, and frog semicircular canal. Our previous work in the intact organ demonstrated that substitution of large impermeant anions for Cl(-) or use of Cl(-) channel blockers reduced the effect of ACh on saccular afferent firing. Using the perforated-patch clamping technique, replacement of Cl(-) by methanesulfonate, iodide, nitrate, or thiocyanate attenuated the hyperpolarizing response to ACh in hair cells isolated from the frog saccule. The chloride channel blockers picrotoxin and 4,4'-dinitrostilbene-2,2'-disulfonic acid were also tested and found to inhibit the ACh response. Thus, the present work demonstrates that the effects of Cl(-) substitutions or Cl(-) channel blockers on the ACh response in the intact saccule can be explained completely by effects on the hair cell. Evidence is also presented for the presence of the messenger RNA for a calcium-dependent chloride channel in all hair cells but especially saccular hair cells. This channel may be involved in the response to ACh. The precise role for chloride in this response, whether as a distinct ion current, as a transported ion, or as a permissive ion for other components, is discussed.

Pacemaker oscillations in heart and brain: a key role for hyperpolarization-activated cation channels

Rhythmic activity of single cells or multicellular networks is a common feature of all organisms. The oscillatory activity is characterized by time intervals of several seconds up to many hours. Cellular rhythms govern the beating of the heart, the swimming behavior of sperm, cycles of sleep and wakefulness, breathing, and the release of hormones. Many neurons in the brain and cardiac cells are characterized by endogenous rhythmic activity, which relies on a complex interplay between several distinct ion channels. In particular, one type of ion channel plays a prominent role in the control of rhythmic electrical activity since it determines the frequency of the oscillations. The activity of the channels is thus setting the "pace" of the oscillations; therefore, these channels are often referred to as "pacemaker" channels. Despite their obvious important physiological function, it was not until recently that genes encoding pacemaker channels have been identified. Because both hyperpolarization and cyclic nucleotides are key elements that control their activity, pacemaker channels have now been designated hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. The molecular identification of the channels and the upcoming studies on their properties in heterologous systems will certainly enhance our understanding of "pacemaking" in physiological systems. This review gives a brief insight into the physiological importance of these channels and sums up what we have learned since the first cloning of genes succeeded (for recent reviews, see also Clapham 1998; Luthi and McCormick 1998a; Biel et al. 1999; Ludwig, Zong, Hofmann, et al. 1999; Santoro and Tibbs 1999).

The role of the hyperpolarization-activated current in modulating rhythmic activity in the isolated respiratory network of mice

We examined the role of the hyperpolarization-activated current (I(h)) in the generation of the respiratory rhythm using a spontaneously active brainstem slice of mice. This preparation contains the hypoglossus (XII) nucleus, which is activated in-phase with inspiration and the pre-Bötzinger complex (PBC), the presumed site for respiratory rhythm generation. Voltage-clamp recordings (n = 90) indicate that cesium (Cs) (5 mM) blocked 77.2% of the I(h) current, and ZD 7288 (100 microM) blocked 85.8% of the I(h) current. This blockade increased the respiratory frequency by 161% in Cs and by 150% in ZD 7288 and increased the amplitude of integrated population activity in the XII by 97% in Cs and by 162% in ZD 7288, but not in the PBC (Cs, by 19%; ZD 7288, by -4.56%). All inspiratory PBC neurons (n = 44) recorded in current clamp within the active network revealed a significantly decreased frequency of action potentials during the interburst interval and an earlier onset of inspiratory bursts after I(h) current blockade. However, hyperpolarizing current pulses evoked only in a small proportion of inspiratory neurons (0% of type I; 29% of type II neurons) a depolarizing sag. Most of the neurons expressing an I(h) current (86%) were pacemaker neurons, which continued to generate rhythmic bursts after inactivating the respiratory network pharmacologically with CNQX alone or with CNQX, AP-5, strychnine, bicuculline, and carbenoxolone. Cs and ZD 7288 increased the frequency of pacemaker bursts and decreased the frequency of action potentials between pacemaker bursts. Our findings suggest that the I(h) current plays an important role in modulating respiratory frequency, which is presumably mediated by pacemaker neurons.

Properties and role of I(h) in the pacing of subthreshold oscillations in entorhinal cortex layer II neurons

Various subsets of brain neurons express a hyperpolarization-activated inward current (I(h)) that has been shown to be instrumental in pacing oscillatory activity at both a single-cell and a network level. A characteristic feature of the stellate cells (SCs) of entorhinal cortex (EC) layer II, those neurons giving rise to the main component of the perforant path input to the hippocampal formation, is their ability to generate persistent, Na(+)-dependent rhythmic subthreshold membrane potential oscillations, which are thought to be instrumental in implementing theta rhythmicity in the entorhinal-hippocampal network. The SCs also display a robust time-dependent inward rectification in the hyperpolarizing direction that may contribute to the generation of these oscillations. We performed whole cell recordings of SCs in in vitro slices to investigate the specific biophysical and pharmacological properties of the current underlying this inward rectification and to clarify its potential role in the genesis of the subthreshold oscillations. In voltage-clamp conditions, hyperpolarizing voltage steps evoked a slow, noninactivating inward current, which also deactivated slowly on depolarization. This current was identified as I(h) because it was resistant to extracellular Ba(2+), sensitive to Cs(+), completely and selectively abolished by ZD7288, and carried by both Na(+) and K(+) ions. I(h) in the SCs had an activation threshold and reversal potential at approximately -45 and -20 mV, respectively. Its half-activation voltage was -77 mV. Importantly, bath perfusion with ZD7288, but not Ba(2+), gradually and completely abolished the subthreshold oscillations, thus directly implicating I(h) in their generation. Using experimentally derived biophysical parameters for I(h) and the low-threshold persistent Na(+) current (I(NaP)) present in the SCs, a simplified model of these neurons was constructed and their subthreshold electroresponsiveness simulated. This indicated that the interplay between I(NaP) and I(h) can sustain persistent subthreshold oscillations in SCs. I(NaP) and I(h) operate in a "push-pull" fashion where the delay in the activation/deactivation of I(h) gives rise to the oscillatory process.

Electrophysiological properties of cholinergic and noncholinergic neurons in the ventral pallidal region of the nucleus basalis in rat brain slices

The ventral pallidum is a major source of output for ventral corticobasal ganglia circuits that function in translating motivationally relevant stimuli into adaptive behavioral responses. In this study, whole cell patch-clamp recordings were made from ventral pallidal neurons in brain slices from 6- to 18-day-old rats. Intracellular filling with biocytin was used to correlate the electrophysiological and morphological properties of cholinergic and noncholinergic neurons identified by choline acetyltransferase immunohistochemistry. Most cholinergic neurons had a large whole cell conductance and exhibited marked fast (i.e., anomalous) inward rectification. These cells typically did not fire spontaneously, had a hyperpolarized resting membrane potential, and also exhibited a prominent spike afterhyperpolarization (AHP) and strong spike accommodation. Noncholinergic neurons had a smaller whole cell conductance, and the majority of these cells exhibited marked time-dependent inward rectification that was due to an h-current. This current activated slowly over several hundred milliseconds at potentials more negative than -80 mV. Noncholinergic neurons fired tonically in regular or intermittent patterns, and two-thirds of the cells fired spontaneously. Depolarizing current injection in current clamp did not cause spike accommodation but markedly increased the firing frequency and in some cells also altered the pattern of firing. Spontaneous tetrodotoxin-sensitive GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) were frequently recorded in noncholinergic neurons. These results show that cholinergic pallidal neurons have similar properties to magnocellular cholinergic neurons in other parts of the forebrain, except that they exhibit strong spike accommodation. Noncholinergic ventral pallidal neurons have large h-currents that could have a physiological role in determining the rate or pattern of firing of these cells.

The role of the hyperpolarization-activated current in modulating rhythmic activity in the isolated respiratory network of mice

We examined the role of the hyperpolarization-activated current (I(h)) in the generation of the respiratory rhythm using a spontaneously active brainstem slice of mice. This preparation contains the hypoglossus (XII) nucleus, which is activated in-phase with inspiration and the pre-Bötzinger complex (PBC), the presumed site for respiratory rhythm generation. Voltage-clamp recordings (n = 90) indicate that cesium (Cs) (5 mM) blocked 77.2% of the I(h) current, and ZD 7288 (100 microM) blocked 85.8% of the I(h) current. This blockade increased the respiratory frequency by 161% in Cs and by 150% in ZD 7288 and increased the amplitude of integrated population activity in the XII by 97% in Cs and by 162% in ZD 7288, but not in the PBC (Cs, by 19%; ZD 7288, by -4.56%). All inspiratory PBC neurons (n = 44) recorded in current clamp within the active network revealed a significantly decreased frequency of action potentials during the interburst interval and an earlier onset of inspiratory bursts after I(h) current blockade. However, hyperpolarizing current pulses evoked only in a small proportion of inspiratory neurons (0% of type I; 29% of type II neurons) a depolarizing sag. Most of the neurons expressing an I(h) current (86%) were pacemaker neurons, which continued to generate rhythmic bursts after inactivating the respiratory network pharmacologically with CNQX alone or with CNQX, AP-5, strychnine, bicuculline, and carbenoxolone. Cs and ZD 7288 increased the frequency of pacemaker bursts and decreased the frequency of action potentials between pacemaker bursts. Our findings suggest that the I(h) current plays an important role in modulating respiratory frequency, which is presumably mediated by pacemaker neurons.

Hyperpolarization-activated current, Ih, in inspiratory brainstem neurons and its inhibition by hypoxia

A hyperpolarization-activated current, Ih, is often implied in pacemaker-like depolarizations during rhythmic oscillatory activity. We describe Ih in the isolated respiratory centre of immature mice (P6-P11). Ih was recorded in 15% (22/146) of all inspiratory neurons examined. The mean half-maximal Ih activation occurred at -78 mV and the reversal potential was -40 mV. Ih was inhibited by Cs+ (1-5 mM) and by organic blockers N-ethyl-1,6-dihydro-1, 2-dimethyl-6-(methylimino)-N-phenyl-4-pyrimidinamine (ZD 7288; 0.3-3 microM) and N,N'-bis-(3,4-dimethylphenylethyl)-N-methylamine (YS 035, 3-30 microM), but not by Ba2+ (0.5 mM). The organic Ih blockers did not change the inspiratory bursts recorded from the XIIth nerve and synaptic drives in inspiratory neurons. Hypoxia reversibly inhibited Ih but, in the presence of organic blockers, the hypoxic reaction remained unchanged. We conclude that although Ih channels are functional in a minority of inspiratory neurons, Ih does not contribute to respiratory rhythm generation or its modulation by hypoxia.

Hyperpolarisation-activated inward current in isolated sheep mesenteric lymphatic smooth muscle

1. Freshly isolated sheep lymphatic smooth muscle cells were studied using the perforated patch-clamp technique. Hyperpolarisation with constant-current pulses caused a time-dependent rectification evident as a depolarising 'sag' followed by an anode-break overshoot at the end of the pulse. Both sag and overshoot were blocked with 1 mM Cs+. 2. Cells were voltage clamped at -30 mV and stepped to -120 mV in 10 mV steps of 2 s duration. Steps negative to -60 mV evoked a slowly activating, non-inactivating inward current which increased in size and rate of activation with increasing hyperpolarisation. 3. The slowly activating current was reduced in Na+-free bathing solution but enhanced when the extracellular K+ concentration was increased to 60 mM. The current was significantly reduced by 1 mM Cs+ and 1 microM ZD7288 but not by 1.8 mM Ba2+. 4. The steady-state activation curve of the underlying conductance showed a threshold at -50 mV and half-maximal activation at -81 mV. Neither threshold nor half-maximal activation was significantly affected by increasing the external K+ concentration to 60 mM. 5. The frequency of spontaneous contractions and fluid propulsion in isolated cannulated segments of sheep mesenteric lymphatics were decreased by 1 mM Cs+ and by 1 microM ZD7288. 6. We conclude that sheep lymphatics have a hyperpolarisation-activated inward current similar to the If seen in sinoatrial node cells of the heart. Blockade of this current slows spontaneous pumping in intact lymphatic vessels suggesting that it is important in normal pacemaking.