Modulation of the hyperpolarization-activated current (Ih) by cyclic nucleotides in guinea-pig primary afferent neurons

Contributions of ion conductances to the onset responses of octopus cells in the ventral cochlear nucleus: simulation results

The onset response pattern displayed by octopus cells has been attributed to intrinsic membrane properties, low membrane impedance, and/or synaptic inputs. Although the importance of a low membrane impedance generally is acknowledged as an essential component, views differ on the role that ion channels play in producing the onset response. In this study, we use a computer model to investigate the contributions of ion channels to the responses of octopus cells. Simulations using current ramps indicate that, during the "ramp-up" stage, the membrane depolarizes, activating a low-threshold K(+) channel, K(LT), which increases membrane conductance and dynamically increases the current required to evoke an action potential. As a result, the model is sensitive to the rate that membrane potential changes when initiating an action potential. Results obtained when experimentally recorded spike trains of auditory-nerve fibers served as model inputs (simulating acoustic stimulation) demonstrate that a model with K(LT) conductance as the dominant conductance produces realistic onset response patterns. Systematically replacing the K(LT) conductance by a h-type conductance (which corresponds to a hyperpolarization-activated inward rectifier current, I(h)) or by a leakage conductance reduces the model's sensitivity to rate of change in membrane potential, and the model's response to "acoustic stimulation" becomes more chopper-like. Increasing the h-type conductance while maintaining a large K(LT) conductance causes an increase in threshold to both current steps and acoustic stimulation but does not significantly affect the model's sensitivity to rate of change in membrane potential and the onset response pattern under acoustic stimulation. These findings support the idea that K(LT), which is activated during depolarization, is the primary membrane conductance determining the response properties of octopus cells, and its dynamic role cannot be provided by a static membrane conductance. On the other hand, I(h), which is activated during hyperpolarization, does not play a large role in the basic onset response pattern but may regulate response threshold through its contribution to the membrane conductance.

Modulation of the hyperpolarization-activated cation current of rat thalamic relay neurones by intracellular pH

1. Properties of the hyperpolarization-activated cation current (Ih) were investigated in thalamocortical neurones of an in vitro slice preparation of the rat ventrobasal thalamic complex (VB) before and during changes of pipette pH (pHp), intracellular pH (pHi) and bath pH (pHb) using the whole-cell patch-clamp technique and fluorescence ratio imaging of the pH indicator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. Recording of Ih with predefined pHp revealed significant shifts in the voltage dependence of Ih activation (V ) of 4-5 mV to more positive values for a pHp of 7.5 and 2-3 mV to more negative values for a pHp of 6.7 as compared to control values (pHp = 7.1). 3. Application of the weak acid lactate (20 mM), which produced a slow monophasic intracellular acidification, induced a reversible negative shift of V of up to 3 mV. Application of 20 mM TMA, which caused a distinct intracellular alkalinization, shifted V to 4-5 mV more positive values. 4. In slices bathed in Hepes-buffered saline, no significant pHo dependence of Ih was observed. Changing pHo by altering the extracellular [HCO3-] in the presence of constant pCO2 also revealed no significant pHo dependence of Ih. 5. Rhythmic stimulation of thalamocortical neurones with repetitive depolarizing pulse trains caused an intracellular acidification, which reversibly decreased the amplitude and time course of activation of Ih. 6. The results of the present study indicate that shifts in pHi result in a significant modulation of the gating properties of Ih channels in TC neurones. Through this mechanism activity-dependent shifts in pHi may contribute to the up- and downregulation of Ih.

Action of serotonin on the hyperpolarization-activated cation current (Ih) in rat CA1 hippocampal neurons

We studied the effects of serotonin (5-HT) on hippocampal CA1 pyramidal neurons. In current-clamp mode, 5-HT induced a hyperpolarization and reduction of excitability due to the opening of inward rectifier K+ channels, followed by a late depolarization and partial restoration of excitability. These two components could be dissociated, as in the presence of BaCl2 to block K+ channels, 5-HT induced a depolarization accompanied by a reduction of membrane resistance, whereas in the presence of ZD 7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride], a selective blocker of the hyperpolarization-activated cation current (Ih), 5-HT only hyperpolarized neurons. We then studied the action of 5-HT on Ih in voltage-clamp conditions. 5-HT increased Ih at -90 mV by 29.1 +/- 2.9% and decreased the time constant of activation by 20.1 +/- 1.7% (n = 16), suggesting a shift in the voltage dependence of the current towards more positive potentials; however, the fully activated current measured at -140 mV also increased (by 14.1 +/- 1.7%, n = 14); this increase was blocked by ZD 7288, implying an effect of 5-HT on the maximal conductance of Ih. Both the shift of activation curve and the increase in maximal conductance were confirmed by data obtained with ramp protocols. Perfusion with the membrane-permeable analogue of cAMP, 8-bromoadenosine 3'5'-cyclic monophosphate (8-Br-cAMP), increased Ih both at -90 and -140 mV, although the changes induced were smaller than those due to 5-HT. Our data indicate that 5-HT modulates Ih by shifting its activation curve to more positive voltages and by increasing its maximal conductance, and that this action is likely to contribute to the 5-HT modulation of excitability of CA1 cells.

The primary afferent nociceptor as pattern generator

One of the most important advances in our understanding of the pain experience was the introduction of the 'gate control' theory which stimulated analysis of activity pattern in nociceptive pathways and its modulation. Advances in cellular and molecular biology have recently begun to provide detailed information on the mechanisms of stimulus transduction within primary afferent nociceptors as well as mechanisms that modulate the transduction process. From these new insights into the sensory physiology of the nociceptive nerve ending emerges a concept of the primary afferent as the first site of pattern generation in the nociceptive pathway, in which dynamic tuning of gain in the mosaic of inputs to individual primary afferents occurs. The electrical properties of the nociceptor membrane that converts the generator potential to a pattern of action potentials is also actively adjusted. Our present understanding of the intracellular mechanisms that modulate the pattern of activity in nociceptive primary afferents is summarized, and implications for future efforts to unravel the meaning of patterning in nociceptor activity are discussed.

Serotonergic modulation of hyperpolarization-activated current in acutely isolated rat dorsal root ganglion neurons

1. The effect of serotonin (5-HT) on the hyperpolarization-activated cation current (IH) was studied in small-, medium- and large-diameter acutely isolated rat dorsal root ganglion (DRG) cells, including cells categorized as type 1, 2, 3 and 4 based on membrane properties. 5-HT increased IH in 91 % of medium-diameter DRG cells (including type 4) and in 67 % of large-diameter DRG cells, but not other DRG cell types. 2. The increase of IH by 5-HT was antagonized by spiperone but not cyanopindolol, and was mimicked by 5-carboxyamidotryptamine, but not (+)-8-hydroxydipropylaminotetralin (8-OH-DPAT) or cyanopindolol. These data suggested the involvement of 5-HT7 receptors, which were shown to be expressed by medium-diameter DRG cells using RT-PCR analysis. 3. 5-HT shifted the conductance-voltage relationship of IH by +6 mV without changing peak conductance. The effects of 5-HT on IH were mimicked and occluded by forskolin, but not by inactive 1,9-dideoxy forskolin. 4. At holding potentials negative to -50 mV, 5-HT increased steady-state inward current and instantaneous membrane conductance (fast current). The 5-HT-induced inward current and fast current were blocked by Cs+ but not Ba2+ and reversed at -23 mV, consistent with the properties of tonically activated IH. 5. In medium-diameter neurons recorded from in the current clamp mode, 5-HT depolarized the resting membrane potential, decreased input resistance and facilitated action potential generation by anode-break excitation. 6. The above data suggest that in distinct subpopulations of DRG neurons, 5-HT increases cAMP levels via activation of 5-HT7 receptors, which shifts the voltage dependence of IH to more depolarized potentials and increases neuronal excitability.

Ion channels gated by heat

All animals need to sense temperature to avoid hostile environments and to regulate their internal homeostasis. A particularly obvious example is that animals need to avoid damagingly hot stimuli. The mechanisms by which temperature is sensed have until recently been mysterious, but in the last couple of years, we have begun to understand how noxious thermal stimuli are detected by sensory neurons. Heat has been found to open a nonselective cation channel in primary sensory neurons, probably by a direct action. In a separate study, an ion channel gated by capsaicin, the active ingredient of chili peppers, was cloned from sensory neurons. This channel (vanilloid receptor subtype 1, VR1) is gated by heat in a manner similar to the native heat-activated channel, and our current best guess is that this channel is the molecular substrate for the detection of painful heat. Both the heat channel and VR1 are modulated in interesting ways. The response of the heat channel is potentiated by phosphorylation by protein kinase C, whereas VR1 is potentiated by externally applied protons. Protein kinase C is known to be activated by a variety of inflammatory mediators, including bradykinin, whereas extracellular acidification is characteristically produced by anoxia and inflammation. Both modulatory pathways are likely, therefore, to have important physiological correlates in terms of the enhanced pain (hyperalgesia) produced by tissue damage and inflammation. Future work should focus on establishing, in molecular terms, how a single ion channel can detect heat and how the detection threshold can be modulated by hyperalgesic stimuli.

Specific involvement of PKC-epsilon in sensitization of the neuronal response to painful heat

Pain is unique among sensations in that the perceived intensity increases, or sensitizes, during exposure to a strong stimulus. One important mediator of sensitization is bradykinin (BK), a peptide released as a consequence of tissue damage. BK enhances the membrane ionic current activated by heat in nociceptive neurons, using a pathway that involves activation of protein kinase C (PKC). We find that five PKC isoforms are present in sensory neurons but that only PKC-epsilon is translocated to the cell membrane by BK. The heat response is sensitized when constitutively active PKC-epsilon is incorporated into nociceptive neurons. Conversely, BK-induced sensitization is suppressed by a specific peptide inhibitor of PKC-epsilon. We conclude that PKC-epsilon is principally responsible for sensitization of the heat response in nociceptors by bradykinin.

The HCN gene family: molecular basis of the hyperpolarization-activated pacemaker channels

The molecular basis of the hyperpolarization-activated cation channels that underlie the anomalous rectifying current variously termed Ih, Iq, or I(f) is discussed. On the basis of the expression patterns and biophysical properties of the newly cloned HCN ion channels, an initial attempt at defining the identity and subunit composition of channels underlying native Ih is undertaken. By comparing the sequences of HCN channels to other members of the K channel superfamily, we discuss how channel opening may be coupled to membrane hyperpolarization and to direct binding of cyclic nucleotide. Finally, we consider some of the questions in cardiovascular physiology and neurobiology that can be addressed as a result of the demonstration that Ih is encoded by the HCN gene family.

Contribution of the hyperpolarization-activated current to the resting membrane potential of rat nodose sensory neurons

1. The voltage- and time-dependent characteristics of the hyperpolarization-activated current (IH) and its contribution to the resting membrane potential of neonatal rat nodose sensory neurons were investigated using the whole-cell tight seal method of voltage and current clamp recording. 2. IH was found in all neonatal nodose neurons in vitro, contrary to previous reports where its presence was particular for A-type neurons. We used the presence of both tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents to distinguish C- from A-type neurons (TTX-S only). We obtained further support for the presence of IH in C-type neurons with experiments in which IH was demonstrated in a subset of neurons sensitive to capsaicin. 3. In both groups IH activated at potentials negative to -50 mV, developed slowly with time and was inhibited by 1-5 mM extracellular caesium. At -120 mV, IH activated with a fast time constant of 73 +/- 3 ms in A-type neurons and 163 +/- 37 ms in C-type neurons (P < 0.05). A second, slower time constant of 682 +/- 83 ms was observed in A-type neurons and 957 +/- 122 ms in C-type neurons. 4. A- and C-type neurons differed in the amplitude of IH. The mean magnitude of IH at -110 mV was -2338 +/- 258 pA in A-type neurons but only -241 +/- 40 pA (P < 0.001) in C-type neurons. This disparity persisted when currents were normalized for capacitance. The reversal potentials for IH were -39 +/- 4 mV for A-type neurons and -37 +/- 5 mV for C-type neurons (P > 0.05). 5. During current clamp recording IH caused time-dependent rectification in response to hyperpolarizing current injections from the resting membrane potential. CsCl abolished the rectification and hyperpolarized the resting potential of A-type neurons from -55 +/- 3 mV to -61 +/- 4 mV and C-type neurons from -62 +/- 2 mV to -71 +/- 3 mV. Taken together, the results in these studies indicate that IH contributes to the resting membrane potential in all nodose neurons.

Dopamine modulates inwardly rectifying hyperpolarization-activated current (Ih) in cultured rat olfactory receptor neurons

The presence of dopamine receptors in olfactory receptor neurons (ORNs) suggests that odor sensitivity may be modulated by neurotransmitters at the level of primary sensory neurons. Using standard patch-clamp techniques on rat ORNs, we found that 1 microM dopamine, 500 microM SQ 22536 (SQ, an adenylyl cyclase inhibitor), 20 and 50 microM quinpirole (a selective dopamine D2 receptor agonist), and 1 mM adenosine 3', 5'-cyclic monophosphate (cAMP) modulate the hyperpolarization-activated current Ih. On hyperpolarizing from a holding potential of -58 mV, a small Cs+-sensitive inwardly rectifying current (Ih) was observed. Increases in extracellular K+ increased Ih amplitude without shifting its voltage dependence of activation, whereas increases in temperature produced an increase in Ih amplitude and a hyperpolarizing shift in the activation curve. Application of 1 microM dopamine reversibly shifted Ih activation to more negative potentials and decreased Ih current amplitudes. These effects were blocked by concomitant application of dopamine with sulpiride, a selective dopamine D2 receptor antagonist. The effects of dopamine were mimicked by quinpirole. Quinpirole (20 microM) decreased Ih current amplitude, but was without effect on Ih voltage dependence of activation. However, 50 microM quinpirole produced both a reduction of Ih peak currents and a hyperpolarizing shift in the activation curve for Ih. External application of the adenylyl cyclase inhibitor SQ 22536 produced a reversible decrease in peak currents but had no effect on Ih voltage dependence of activation, whereas internal application of cAMP shifted Ih activation to more depolarized potentials. Because Ih modulates cell excitability and spike frequency adaptation, our findings support a role for dopamine in modulating the sensitivity and output of rat ORNs to odorants.

Dendritic hyperpolarization-activated currents modify the integrative properties of hippocampal CA1 pyramidal neurons

Step hyperpolarizations evoked slowly activating, noninactivating, and slowly deactivating inward currents from membrane patches recorded in the cell-attached patch configuration from the soma and apical dendrites of hippocampal CA1 pyramidal neurons. The density of these hyperpolarization-activated currents (Ih) increased over sixfold from soma to distal dendrites. Activation curves demonstrate that a significant fraction of Ih channels is active near rest and that the range is hyperpolarized relatively more in the distal dendrites. Ih activation and deactivation kinetics are voltage-and temperature-dependent, with time constants of activation and deactivation decreasing with hyperpolarization and depolarization, respectively. Ih demonstrated a mixed Na+-K+ conductance and was sensitive to low concentrations of external CsCl. Dual whole-cell recordings revealed regional differences in input resistance (Rin) and membrane polarization rates (taumem) across the somatodendritic axis that are attributable to the spatial gradient of Ih channels. As a result of these membrane effects the propagation of subthreshold voltage transients is directionally specific. The elevated dendritic Ih density decreases EPSP amplitude and duration and reduces the time window over which temporal summation takes place. The backpropagation of action potentials into the dendritic arborization was impacted only slightly by dendritic Ih, with the most consistent effect being a decrease in dendritic action potential duration and an increase in afterhyperpolarization. Overall, Ih acts to dampen dendritic excitability, but its largest impact is on the subthreshold range of membrane potentials where the integration of inhibitory and excitatory synaptic inputs takes place.

Dendritic hyperpolarization-activated currents modify the integrative properties of hippocampal CA1 pyramidal neurons

Step hyperpolarizations evoked slowly activating, noninactivating, and slowly deactivating inward currents from membrane patches recorded in the cell-attached patch configuration from the soma and apical dendrites of hippocampal CA1 pyramidal neurons. The density of these hyperpolarization-activated currents (Ih) increased over sixfold from soma to distal dendrites. Activation curves demonstrate that a significant fraction of Ih channels is active near rest and that the range is hyperpolarized relatively more in the distal dendrites. Ih activation and deactivation kinetics are voltage-and temperature-dependent, with time constants of activation and deactivation decreasing with hyperpolarization and depolarization, respectively. Ih demonstrated a mixed Na+-K+ conductance and was sensitive to low concentrations of external CsCl. Dual whole-cell recordings revealed regional differences in input resistance (Rin) and membrane polarization rates (taumem) across the somatodendritic axis that are attributable to the spatial gradient of Ih channels. As a result of these membrane effects the propagation of subthreshold voltage transients is directionally specific. The elevated dendritic Ih density decreases EPSP amplitude and duration and reduces the time window over which temporal summation takes place. The backpropagation of action potentials into the dendritic arborization was impacted only slightly by dendritic Ih, with the most consistent effect being a decrease in dendritic action potential duration and an increase in afterhyperpolarization. Overall, Ih acts to dampen dendritic excitability, but its largest impact is on the subthreshold range of membrane potentials where the integration of inhibitory and excitatory synaptic inputs takes place.

Hyperpolarization-activated cationic currents (Ih) in neurones of the trigeminal mesencephalic nucleus of the rat

1. We studied the voltage-dependent current activated by membrane hyperpolarization in sensory proprioceptive trigeminal mesencephalic nucleus (MNV) neurones. 2. Membrane hyperpolarization (from -62 to -132 mV in 10 mV steps) activated slowly activating and non-inactivating inward currents. The hyperpolarization-activated currents could be described by activation curves with a half-maximal activation potential (V ) of -93 mV, slope (k) of 8.4 mV, and maximally activated currents (Imax) of around 1 nA. The reversal potential of the hyperpolarization-activated currents was -57 mV. 3. Extracellular Cs+ blocked hyperpolarization-activated currents rapidly and reversibly in a concentration-dependent manner with an IC50 of 100 microM and Hill slope of 0.8. ZD7288 (1 microM; 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride), the compound developed as an inhibitor of the cardiac hyperpolarization-activated current (If), also blocked the hyperpolarization-activated currents in MNV neurones. Extracellular Ba2+ (1 mM) did not affect hyperpolarization-activated currents. We tested whether the hyperpolarization-activated currents contribute to the somatic membrane properties of MNV neurones by performing some experiments using current-clamp recording. In such experiments application of Cs+ (1 mM) produced no effect on neuronal resting membrane potentials. 4. During the course of our experiments we noticed that activating ATP-gated non-selective cation channels (P2X receptors) caused an inhibition of Ih associated with a V shift of 10 mV in the hyperpolarizing direction. This P2X receptor-mediated inhibition of Ih was blocked in recordings made with the rapid calcium chelator BAPTA (11 mM) in the pipette solution. 5. We conclude that the current activated by membrane hyperpolarization in MNV neurones is Ih on the basis of its similarity to Ih observed in other neuronal preparations. Activation of Ih can account for the anomalous time-dependent inward rectification that has previously been described in MNV neurones.

Response of thalamocortical neurons to hypoxia: a whole-cell patch-clamp study

The effect of hypoxia (3-4 min of 95% N2, 5% CO2) on thalamocortical (TC) neurons was investigated using the whole-cell patch-clamp technique in rat dorsal lateral geniculate nucleus slices kept submerged at 32 degreesC. The predominant feature of the response of TC neurons to hypoxia was an increase in input conductance (DeltaGN = 117 +/- 15%, n = 33) that was accompanied by an inward shift in baseline holding current (IBH) at -65 and -57 mV (DeltaIBH = -45 +/- 6 pA, n = 18, and -25 +/- 8 pA, n = 33, respectively) but not at -40 mV. The hypoxia-induced increase in GN (as well as the shift in IBH) was abolished by procedures that are known to block Ih, i.e., bath application of 4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino)-pyrimidinium chloride (100-300 microM) (DeltaGN = 5 +/- 13%, n = 11) and CsCl (2-3 mM) (DeltaGN = 16 +/- 16%, n = 5), or low [Na+]o (DeltaGN = 10 +/- 10%, n = 5), whereas bath application of BaCl2 (0.1-2.0 mM) had no significant effect (DeltaGN = 128 +/- 14%, n = 8). The hypoxic response was also abolished in low [Ca+2]o (DeltaGN = 25 +/- 16%, DeltaIBH = -6 +/- 8 pA, n = 13), but was unaffected by recording with electrodes containing EGTA (10 mM), BAPTA (10-30 mM), Cs+, or Cl-, as well as in the presence of external tetraethylammonium and 4-aminopyridine. Furthermore, preincubation of the slices with botulinum toxin A (100 nM), which is known to reduce Ca2+-dependent transmitter release, blocked the hypoxic response (DeltaGN = -3 +/- 15%, DeltaIBH = 10 +/- 5 pA, n = 4). We suggest that a positive shift in the voltage-dependence of Ih and a change in its activation kinetics, which transforms it into a fast activating current, may be responsible for the hypoxia-induced changes in GN and IBH, probably via an increase in Ca+2-dependent transmitter release.

Response of thalamocortical neurons to hypoxia: a whole-cell patch-clamp study

The effect of hypoxia (3-4 min of 95% N2, 5% CO2) on thalamocortical (TC) neurons was investigated using the whole-cell patch-clamp technique in rat dorsal lateral geniculate nucleus slices kept submerged at 32 degreesC. The predominant feature of the response of TC neurons to hypoxia was an increase in input conductance (DeltaGN = 117 +/- 15%, n = 33) that was accompanied by an inward shift in baseline holding current (IBH) at -65 and -57 mV (DeltaIBH = -45 +/- 6 pA, n = 18, and -25 +/- 8 pA, n = 33, respectively) but not at -40 mV. The hypoxia-induced increase in GN (as well as the shift in IBH) was abolished by procedures that are known to block Ih, i.e., bath application of 4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino)-pyrimidinium chloride (100-300 microM) (DeltaGN = 5 +/- 13%, n = 11) and CsCl (2-3 mM) (DeltaGN = 16 +/- 16%, n = 5), or low [Na+]o (DeltaGN = 10 +/- 10%, n = 5), whereas bath application of BaCl2 (0.1-2.0 mM) had no significant effect (DeltaGN = 128 +/- 14%, n = 8). The hypoxic response was also abolished in low [Ca+2]o (DeltaGN = 25 +/- 16%, DeltaIBH = -6 +/- 8 pA, n = 13), but was unaffected by recording with electrodes containing EGTA (10 mM), BAPTA (10-30 mM), Cs+, or Cl-, as well as in the presence of external tetraethylammonium and 4-aminopyridine. Furthermore, preincubation of the slices with botulinum toxin A (100 nM), which is known to reduce Ca2+-dependent transmitter release, blocked the hypoxic response (DeltaGN = -3 +/- 15%, DeltaIBH = 10 +/- 5 pA, n = 4). We suggest that a positive shift in the voltage-dependence of Ih and a change in its activation kinetics, which transforms it into a fast activating current, may be responsible for the hypoxia-induced changes in GN and IBH, probably via an increase in Ca+2-dependent transmitter release.

Identification of a gene encoding a hyperpolarization-activated pacemaker channel of brain

The generation of pacemaker activity in heart and brain is mediated by hyperpolarization-activated cation channels that are directly regulated by cyclic nucleotides. We previously cloned a novel member of the voltage-gated K channel family from mouse brain (mBCNG-1) that contained a carboxy-terminal cyclic nucleotide-binding domain (Santoro et al., 1997) and hence proposed it to be a candidate gene for pacemaker channels. Heterologous expression of mBCNG-1 demonstrates that it does indeed code for a channel with properties indistinguishable from pacemaker channels in brain and similar to those in heart. Three additional mouse genes and two human genes closely related to mBCNG-1 display unique patterns of mRNA expression in different tissues, including brain and heart, demonstrating that these channels constitute a widely expressed gene family.

The role of IP prostanoid receptors in inflammatory pain

Prostanoid receptor-mediated sensitization of sensory nerve fibres is a key contributor to the generation of hyperalgesia. It is generally thought that prostaglandin (PG) E2 is the principal pro-inflammatory prostanoid. Consequently, prostanoid EP receptors on sensory neurones have been identified as potential therapeutic targets. However, IP prostanoid receptors are also present on sensory neurones, and recent data from transgenic mice lacking the IP receptor demonstrate its importance in the induction of oedema and pain behaviour. PGI2, the primary endogenous agonist for the IP receptor, is rapidly produced following tissue injury or inflammation; thus, it may be of equal, or greater, importance than PGE2 during episodes of inflammatory pain. In this review, Keith Bley, John Hunter, Richard Eglen and Jacqueline Smith compare the roles of EP and IP receptors in nociception and suggest that the IP receptor constitutes a novel target for anti-nociceptive agents.

Periodicity of thalamic synchronized oscillations: the role of Ca2+-mediated upregulation of Ih

Thalamocortical networks can generate both normal and abnormal patterns of synchronized network activity, such as spindle waves and spike-and-wave seizures. These periods of synchronized discharge are often separated by a silent, refractory phase of between 5 and 20 s. In vitro investigations have demonstrated that this refractory period is due in large part to the persistent activation of the hyperpolarization-activated cation current Ih in thalamocortical cells. Here, we show that increases in [Ca2+]i due to rebound Ca2+ bursts result in persistent activation of Ih resulting from a positive shift in the activation curve of this current. The dynamical upregulation and persistent activation of Ih is the critical determinant of the time course of the refractory period. These findings demonstrate that periodicity in neural network oscillations may be generated through an interaction between the electrophysiological properties and intracellular signaling pathways of the constituent neurons.

Substance P regulates Ih via a NK-1 receptor in vagal sensory neurons of the ferret

Substance P (SP) hyperpolarizes approximately 80% of ferret vagal sensory neurons (nodose ganglion neurons) via NK-1 receptor-mediated activation of a potassium current (IK). A depolarizing current activated by membrane hyperpolarization could minimize the SP-induced hyperpolarization. Such a current exists in 65% of the nodose neurons (n = 264). In this study, we examine this current and how it can interact with SP-induced membrane hyperpolarizations. This slowly developing, noninactivating inward current, designated Ih, was activated maximally at about -120 mV and had a reversal potential value of -23 +/- 4.4 mV (n = 4). The time course of activation followed voltage-dependent, monoexponential kinetics. Steady-state activation curves derived from tail current analysis were well fit by a Boltzmann equation yielding a half-activation potential (V1/2) of-77 +/- 1.5 mV and a ks value of 18 +/- 0.5 (n = 8). In the presence of 1 mM cesium, the current was completely abolished. These parameters are consistent with those derived for Ih in other neurons. Substance P (200 nM) reduced the magnitude of Ih elicited by membrane hyperpolarizations to about -110 mV but did not affect the magnitude of Ih elicited by hyperpolarizations to more negative potentials. Tail current analysis revealed that this effect was the result of a SP-induced shift of the Ih activation curve to more negative membrane potentials. The V1/2 value for Ih was shifted by -20 +/- 1.4 mV in the presence of SP with no change in ks (18 +/- 0.7; n = 5). The SP effect on Ih, like its effect on IK, was blocked reversibly by 10 nM CP99,994, a NK-1 antagonist, and was mimicked by the NK-1 agonist Ac-[Arg6, Sar9, Met(O2)11]SP(6-11) (ASMSP; 200 nM). Ih was not affected by NK-2 or NK-3 selective agonists (n = 4 for each) nor was the effect of SP on Ih reduced by an NK-2 antagonist (n = 4). These results show that SP activates a NK-1 receptor coupled to the Ih channel. Thus NK-1 receptor activation in ferret vagal afferents not only leads to membrane hyperpolarization but it also can enhance synergistically this inhibitory effect by decreasing Ih.

Threshold tracking techniques in the study of human peripheral nerve

Conventional electrophysiological tests of nerve function focus on the number of conducting fibers and their conduction velocity. These tests are sensitive to the integrity of the myelin sheath, but provide little information about the axonal membrane. Threshold tracking techniques, in contrast, test nerve excitability, which depends on the membrane properties of the axons at the site of stimulation. These methods are sensitive to membrane potential, and to changes in membrane potential caused by activation of ion channels and electrogenic ion pumps, including those under the myelin sheath. This review describes the range of threshold tracking techniques that have been developed for the study of human nerves in vivo: resting threshold is compared with the threshold altered by a change in environment (e.g., ischemia), by a preceding single impulse (e.g., refractoriness, superexcitability) or impulse train, or by a subthreshold current (e.g., threshold electrotonus). Few clinical studies have been reported so far, mainly in diabetic neuropathy and motor neuron disease. Threshold measurements seem well suited for studies of metabolic and toxic neuropathies but insensitive to demyelination. Until suitable equipment becomes more widely available, their full potential is unlikely to be realized.

Lack of regulation by intracellular Ca2+ of the hyperpolarization-activated cation current in rat thalamic neurones

1. The regulation of the hyperpolarization-activated cation current, Ih, in thalamocortical neurones by intracellular calcium ions has been implemented in a number of mathematical models on the waxing and waning behaviour of synchronized rhythmic activity in thalamocortical circuits. In the present study, the Ca2+ dependence of Ih in thalamocortical neurones was experimentally investigated by combining Ca2+ imaging and patch-clamp techniques in the ventrobasal thalamic complex (VB) in vitro. 2. Properties of Ih were analysed before and during rhythmic stimulation of Ca2+ entry by trains of depolarizing voltage pulses. Despite a significant increase in intracellular Ca2+ concentration ([Ca2+]i) from resting levels of 74 +/- 23 nM to 251 +/- 78 nM upon rhythmic stimulation, significant differences in the voltage dependence of Ih activation did not occur (half-maximal activation at -86.4 +/- 1.3 mV vs. -85.2 +/- 2.9 mV; slope of the activation curve, 11.2 +/- 2.4 mV vs. 12.5 +/- 2.5 mV). Recording of Ih with predefined values of [Ca2+]i (13.2 nM or 10.01 microM in the patch pipette) revealed no significant differences in the activation curve or the fully activated I-V relationship of Ih. 3. In comparison, stimulation of the intracellular cyclic adenosine monophosphate (cAMP) pathway induced a significantly positive shift in Ih voltage dependence of +5.1 +/- 1.9 mV, with no alteration in the fully activated I-V relationship. 4. These data argue against a direct regulation of Ih by intracellular Ca2+, and particularly do not support a primary role of Ca(2+)-dependent modulation of the Ih channels in the waxing and waning of sleep spindle oscillations in thalamocortical neurones.

Peripheral pain mechanisms

Our understanding of the cellular and molecular bases of transduction of painful stimuli has burgeoned in the past year, mainly as a result of studies on isolated sensory neurones in culture. The ion channels underlying neuronal responses to noxious heat, to protons and to ATP have recently been characterized. The typical increase in nociceptor sensitivity produced by tissue damage has been found to be mediated by at least two distinct mechanisms. In the first, bradykinin augments the current activated by heat through a mechanism that involves activation of protein kinase C. In a second sensitization mechanism, prostaglandin E2 alters the voltage threshold of several ion channels, including a novel tetrodotoxin-insensitive Na+ channel, in such a way that initiation of action potentials is facilitated.

Hyperpolarization-activated current (Ih) in primary auditory neurons

A hyperpolarization-activated current (termed I[h]) is believed to provide a pacemaker depolarization in sinoatrial node cells and in some central and peripheral neurons. In the present study, we examined if such an inward cation current exists in primary auditory neurons using the whole-cell patch-clamp technique. A large inward, non-inactivating current was seen during hyperpolarizing steps negative to the resting potential. A depolarizing sag occurred during hyperpolarizing current injection, and upon termination of the current injection there was an overshoot, or a rebound firing. A low concentration of Cs+, but not Ba2+, reversibly blocked the inward current and depolarizing sag. The activation of the current showed voltage dependence with half-activation occurring at -101 +/- 1 mV. The time course of I(h) activation was fitted by double exponential function and was voltage-dependent (time constants: tau1 and tau2 = 480 and 3125 ms at -100 mV, and 66 and 404 ms at -160 mV). The reversal potential of the current was -36 mV measured from tail currents. The conductance of the current was decreased in Na+-free solution, and increased in high K+ solution. Increases in the levels of intracellular cAMP or cGMP enhanced the current. The results suggest that there exists a hyperpolarization-activated inward cation current in mammalian primary auditory neurons. This current may provide a depolarizing current during the membrane hyperpolarization following each firing of the primary auditory nerve.

Modulation of IH by 5-HT in neonatal rat motoneurones in vitro: mediation through a phosphorylation independent action of cAMP

The depolarization of adult and neonatal rat facial and spinal motoneurones by 5-hydroxytryptamine (5-HT) in part involves an enhancement of the hyperpolarization-activated, inward-rectifier, IH. Under experimental conditions which promote this action, 5-HT evokes an inward current which can be mimicked by intracellularly applied adenosine 3',5'-cyclic monophosphate (cAMP) and potentiated by the cAMP-specific phosphodiesterase inhibitor Ro 20-1724. In this study, we show that this action of 5-HT can be blocked by the adenylyl cyclase inhibitors 2'3'-dideoxyadenosine (2',3'-DDA). 5'-adenylimidodiphosphate (AMP-PNP) and SQ-22536 (9-(tetrahydro-2-furyl)adenine), but not by external or internal application of the protein kinase inhibitors H-7, staurosporine and chelerythrine. The most recently cloned 5-HT receptor subtypes, 5-HT4, 5-HT6 and 5-HT7, can all stimulate adenylyl cyclase when activated. In the presence of internal GTP-gamma-S, 5-HT irreversibly enhanced IH. The 5-HT-induced inward current could be reversibly blocked by methysergide, but not by the 5-HT4 receptor antagonist GR-113808A, the 5-HT6 and 5-HT7 antagonist clozapine and the 5-HT1A antagonist WAY-100365. 5-Methoxytryptamine (5-MeOT) and 5-carboxamidotryptamine (5-CT) mimicked the action of 5-HT with a rank order of potency of 5-HT = 5MeOT > 5-CT. Surprisingly, 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH DPAT), a 5-HT1A and 5-HT7 agonist was inactive on facial motoneurones unlike its reported agonist action on spinal motoneurones. It is proposed that cAMP produced by 5-HT-mediated stimulation of adenylyl cyclase acts in a phosphorylation-independent manner, possibly directly, on the IH channel. The 5-HT receptor subtype mediating this response cannot be correlated with any of the classified 5-HT receptor subtypes that stimulate adenylyl cyclase.

Membrane properties and synaptic inputs of suprachiasmatic nucleus neurons in rat brain slices

1. Whole-cell recordings were made from 390 neurons of the suprachiasmatic nucleus (SCN) in horizontal brain slices during different portions of the circadian day. The locomotor activity of the rats was measured prior to the preparation of brain slices to insure that each rat was entrained to a 12 h-12 h light-dark cycle. 2. The mean input conductance was 42% higher (1.58 nS) in neurons recorded near the subjective dawn than those (1.11 nS) recorded near the subjective dusk. The current required to hold the neurons at -60 mV also showed a circadian variation with a peak in the middle of the subjective day and a nadir in the middle of the subjective night. Analysis of the variations in the input conductance and the holding current at -60 mV suggested that at least two ion conductances are involved in the pacemaking of the circadian rhythms. 3. Voltage-clamped SCN neurons often had both outward and inward spontaneous postsynaptic currents. The outward currents were blocked by bicuculline but not by strychnine, and were identified as IPSCs mediated by GABAA receptors. The inward currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and were identified as EPSCs mediated by glutamate. Most spontaneous synaptic currents were miniature currents but action potential-dependent large events were seen more often in IPSCs than in EPSCs. 4. Stimulation of the optic nerve or chiasm usually evoked a monosynaptic EPSC which was mediated by both NMDA and non-NMDA receptors. In 13% of cells, optic nerve stimulation evoked an outward current or an inward current followed by an outward current; all the evoked currents were blocked by 4-aminophosphonovaleric acid (APV) and CNQX whereas the outward current only was blocked by bicuculline, suggesting involvement of an inhibitory interneuron. 5. SCN neurons sum the excitatory inputs from both optic nerves; on average each SCN cell receives innervation from at least 4.8 retinohypothalamic tract (RHT) axons. 6. Focal stimulation in the vicinity of the recorded neuron revealed that nearly all SCN neurons receive local or extranuclear GABAergic inputs operating via GABAA receptors. The EPSCs activated by such stimulation were not significantly different in amplitude and pharmacological properties from those induced by RHT stimulation. 7. One hundred and one neurons were labelled with neurobiotin during whole-cell recording. Based on the dendritic structures, four types of SCN neurons (monopolar, radial, simple bipolar and curly bipolar) were identified. The curly bipolar cells had a higher membrane conductance, holding current and hyperpolarization-activated current (Ih) amplitude than the other neuronal types. Radial neurons did not respond to optic nerve stimulation, which activated EPSCs in the other cell types.

Function of the hyperpolarization-activated inward rectification in nonmyelinated peripheral rat and human axons

The function of time-dependent, hyperpolarization-activated inward rectification was analyzed on compound potentials of nonmyelinated axons in the mammalian peripheral nervous system. Isolated rat vagus nerves and fascicles of biopsied human sural nerve were tested in a three-chambered, Vaseline-gap organ bath at 37 degrees C. Inward rectification was assessed by recording the effects of long-lasting hyperpolarizing currents on electrical excitability with the use of the method of threshold electrotonus (program QTRAC, copyright Institute of Neurology, London, UK) and by measuring activity-dependent changes in conduction velocity and membrane potential. Prominent time-dependent, cesium-sensitive inward rectification was revealed in rat vagus and human sural nerve by recording threshold electrotonus to 200-ms hyperpolarizing current pulses. A slowing of compound action potential conduction was observed during a gradual increase in the stimulation frequency from 0.1 to 3 Hz. Above a stimulation frequency of 0.3 Hz, this slowing of conduction was enhanced during bath application of 1 mM cesium. Cesium did not alter action potential waveforms during stimulation at frequencies < 1 Hz. Cesium-induced slowing in action potential conduction was correlated with membrane hyperpolarization. The hyperpolarization by cesium was stronger during higher stimulation frequencies and small in unstimulated nerves. These data show that a cesium-sensitive, time-dependent inward rectification in peripheral rat and human nonmyelinated nerve fibers limits the slowing in conduction seen in such axons at action potential frequencies higher than approximately 0.3 Hz.