Differential distribution and function of hyperpolarization-activated channels in sensory neurons and mechanosensitive fibers

Angiotensin II-NADPH oxidase-derived superoxide mediates diabetes-attenuated cell excitability of aortic baroreceptor neurons

Overactivation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is involved in diabetes-depressed excitability of aortic baroreceptor neurons in nodose ganglia. This involvement links to the autonomic dysfunction associated with high morbidity and mortality in diabetic patients. The present study examined the effects of an angiotensin II type I receptor (AT(1)R) antagonist (losartan), a NADPH oxidase inhibitor (apocynin), and a superoxide dismutase mimetic (tempol) on the enhanced HCN currents and attenuated cell excitability in diabetic nodose neurons. In sham and streptozotocin-induced type 1 diabetic rats, HCN currents and cell excitability of aortic baroreceptor neurons were recorded by the whole cell patch-clamp technique. The angiotensin II level in nodose ganglia from diabetic rats was higher than that from sham rats (101.6 ± 4.8 vs. 38.9 ± 4.2 pg/mg protein, P < 0.05). Single-cell RT-PCR, Western blot, immunofluorescence staining, and chemiluminescence data showed that mRNA and protein expression of AT(1)R, protein expression of NADPH oxidase components, and superoxide production in nodose neurons were increased in diabetic rats compared with those from sham rats. HCN current density was higher and cell excitability was lower in aortic baroreceptor neurons from diabetic rats than that from sham rats. Losartan (1 μM), apocynin (100 μM), and tempol (1 mM) normalized the enhanced HCN current density and increased the cell excitability in the aortic baroreceptor neurons of diabetic rats. These findings suggest that endogenous angiotensin II-NADPH oxidase-superoxide signaling contributes to the enhanced HCN currents and the depressed cell excitation in the aortic baroreceptor neurons of diabetic rats.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

Postnatal maturation of the hyperpolarization-activated cation current, I(h), in trigeminal sensory neurons

Hyperpolarization-activated inward currents (I(h)) contribute to neuronal excitability in sensory neurons. Four subtypes of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate I(h), with different activation kinetics and cAMP sensitivities. The aim of the present study was to examine the postnatal development of I(h) and HCN channel subunits in trigeminal ganglion (TG) neurons. I(h) was investigated in acutely dissociated TG neurons from rats aged between postnatal day (P)1 and P35 with whole cell patch-clamp electrophysiology. In voltage-clamp studies, I(h) was activated by a series of hyperpolarizing voltage steps from -40 mV to -120 mV in -10-mV increments. Tail currents from a common voltage step (-100 mV) were used to determine I(h) voltage dependence. I(h) activation was faster in older rats and occurred at more depolarized potentials; the half-maximal activation voltage (V(1/2)) changed from -89.4 mV (P1) to -81.6 mV (P35). In current-clamp studies, blocking I(h) with ZD7288 caused membrane hyperpolarization and increases in action potential half-duration at all postnatal ages examined. ZD7288 also reduced the action potential firing frequency in multiple-firing neurons. Western blot analysis of the TG detected immunoreactive bands corresponding to all HCN subtypes. HCN1 and HCN2 band density increased with postnatal age, whereas the low-intensity HCN3 and moderate-intensity HCN4 bands were not changed. This study suggests that functional I(h) are activated in rat trigeminal sensory neurons from P1 during postnatal development, have an increasing role with age, and modify neuronal excitability.

Kv1.3 channels regulate synaptic transmission in the nucleus of solitary tract

The voltage-gated K(+) channel Kv1.3 has been reported to regulate transmitter release in select central and peripheral neurons. In this study, we evaluated its role at the synapse between visceral sensory afferents and secondary neurons in the nucleus of the solitary tract (NTS). We identified mRNA and protein for Kv1.3 in rat nodose ganglia using RT-PCR and Western blot analysis. In immunohistochemical experiments, anti-Kv1.3 immunoreactivity was very strong in internal organelles in the soma of nodose neurons with a weaker distribution near the plasma membrane. Anti-Kv1.3 was also identified in the axonal branches that project centrally, including their presynaptic terminals in the medial and commissural NTS. In current-clamp experiments, margatoxin (MgTx), a high-affinity blocker of Kv1.3, produced an increase in action potential duration in C-type but not A- or Ah-type neurons. To evaluate the role of Kv1.3 at the presynaptic terminal, we examined the effect of MgTx on tract evoked monosynaptic excitatory postsynaptic currents (EPSCs) in brain slices of the NTS. MgTx increased the amplitude of evoked EPSCs in a subset of neurons, with the major increase occurring during the first stimuli in a 20-Hz train. These data, together with the results from somal recordings, support the hypothesis that Kv1.3 regulates the duration of the action potential in the presynaptic terminal of C fibers, limiting transmitter release to the postsynaptic cell.

HCN channels in behavior and neurological disease: too hyper or not active enough?

The roles of cells within the nervous system are based on their properties of excitability, which are in part governed by voltage-gated ion channels. HCN channels underlie the hyperpolarization-activated current, I(h), an important regulator of excitability and rhythmicity through control of basic membrane properties. I(h) is present in multiple neuronal types and regions of the central nervous system, and changes in I(h) alter cellular input-output properties and neuronal circuitry important for behavior such as learning and memory. Furthermore, the pathophysiology of neurological diseases of both the central and peripheral nervous system involves defects in excitability, rhythmicity, and signaling, and animal models of many of these disorders have implicated changes in HCN channels and I(h) as critical for pathogenesis. In this review, we focus on recent research elucidating the role of HCN channels and I(h) in behavior and disease. These studies have utilized knockout mice as well as animal models of disease to examine how I(h) may be important in regulating learning and memory, sleep, and consciousness, as well as how misregulation of I(h) may contribute to epilepsy, chronic pain, and other neurological disorders. This review will help guide future studies aimed at further understanding the function of this unique conductance in both health and disease of the mammalian brain.

Plasticity of hyperpolarization-activated and cyclic nucleotid-gated cation channel subunit 2 expression in the spinal dorsal horn in inflammatory pain

A great deal of experimental evidence has already been accumulated that hyperpolarization-activated and cyclic nucleotide-gated cation channels (HCN) expressed by peripheral nerve fibers contribute to the initiation of nerve activities leading to pain. Complementing these findings, we have recently demonstrated that HCN subunit 2 (HCN2) channel protein is also widely expressed by axon terminals of substance P (SP)-containing peptidergic nociceptive primary afferents in laminae I-IIo of the spinal dorsal horn, and postulated that they may play a role in spinal pain processing. In the present study, we investigated how the expression of HCN2 ion channels in the spinal dorsal horn may change in inflammatory pain evoked by unilateral injection of complete Freund's adjuvant (CFA) into the hind paw of rats. We found that 3 days after CFA injection, when the nociceptive responsiveness of the inflamed hind paw had substantially increased, the numbers of HCN2-immunolabeled axon terminals were also significantly augmented in laminae I-IIo of the spinal dorsal horn ipsilateral to the site of CFA injection. The elevation of HCN2 immunoreactivity was paralleled by an increase in SP immunoreactivity. In addition, similarly to control animals, the co-localization between HCN2 and SP immunoreactivity was remarkably high, suggesting that central axon terminals of nociceptive primary afferents that increased their SP expression in response to CFA injection into the hind paw also increased their HCN2 expression. The results indicate that HCN2 ion channel mechanisms may play a role in SP-mediated spinal pain processing not only in naive animals but also in chronic inflammatory pain.

Facilitation of Ih channels by P2Y1 receptors activation in Mesencephalic trigeminal neurons

P2Y(1) receptors, a subset of G-protein coupled receptors, have been shown to participate in sensory transduction in the periphery nervous system. However, little is known about their sensory function in the central nervous system. Here, by using immunohistochemistry, we showed that P2Y(1) receptors are predominantly localized in the somata of Mesencephalic trigeminal neurons (Mes V neurons), the primary sensory neurons in brainstem. Whole-cell voltage-clamp recording revealed that ADP-beta-S, a P2Y receptor agonist, enhanced the activity of hyperpolarization-activated cation channels (Ih channels) in Mes V neurons and that the activity-enhancing effect of ADP-beta-S could be blocked by a specific P2Y(1) receptor antagonist, MRS 2179. Taken together, these results suggested a possible role of P2Y(1) receptors in the information transduction of central sensory neurons through regulating Ih channel activities.

Properties of low-threshold motor axons in the human median nerve

This study investigated the excitability and accommodative properties of low-threshold human motor axons to test whether these motor axons have greater expression of the persistent Na(+) conductance, I(NaP). Computer-controlled threshold tracking was used to study 22 single motor units and the data were compared with compound motor potentials of various amplitudes recorded in the same experimental session. Detailed comparisons were made between the single units and compound potentials that were 40% or 5% of maximal amplitude, the former because this is the compound potential size used in most threshold tracking studies of axonal excitability, the latter because this is the compound potential most likely to be composed entirely of motor axons with low thresholds to electrical recruitment. Measurements were made of the strength-duration relationship, threshold electrotonus, current-voltage relationship, recovery cycle and latent addition. The findings did not support a difference in I(NaP). Instead they pointed to greater activity of the hyperpolarization-activated inwardly rectifying current (I(h)) as the basis for low threshold to electrical recruitment in human motor axons. Computer modelling confirmed this finding, with a doubling of the hyperpolarization-activated conductance proving the best single parameter adjustment to fit the experimental data. We suggest that the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel(s) expressed on human motor axons may be active at rest and contribute to resting membrane potential.

Functional impact of the hyperpolarization-activated current on the excitability of myelinated A-type vagal afferent neurons in the rat

1. The hyperpolarization-induced, cation-selective current I(h) is widely observed in peripheral sensory neurons of the vagal and dorsal root ganglia, but the peak magnitude and voltage- and time-dependent properties of this current vary widely across afferent fibre type. 2. Using patch clamp investigations of rat isolated vagal ganglion neurons (VGN) identified as myelinated A-type afferents, we established a compendium of functional correlates between changes in membrane potential and the dynamic discharge properties of these sensory neurons as a result of the controlled recruitment of I(h) using the current clamp technique. 3. Two robust responses were observed in response to hyperpolarizing step currents: (i) upon initiation of the negative step current, there was a rapid hyperpolarization of membrane potential followed by a depolarizing voltage sag (DVS) towards a plateau in membrane potential as a result of steady state recruitment of I(h); and (ii) upon termination of the negative step current, there was a rapid return to the pretest resting membrane potential that often led to spontaneous action potential discharge. These data were strongly correlated (r(2) > 0.9) with a broad compendium of dynamic discharge characteristics in these A-type VGN. 4. In response to depolarizing step currents of increasing magnitude, the discharge frequency of the A-type VGN responded with increases in the rate of sustained repetitive discharge. Upon termination of the depolarizing step current, there was a post-excitatory membrane hyperpolarization of a magnitude that was strongly correlated with action potential discharge rate (r(2) > 0.9). 5. Application of the selective hyperpolarization-activated cyclic nucleotide gated (HCN) channel blockers ZD7288 (10 micromol/L) or CsCl (1.0 mmol/L) abolished I(h) and all of the aforementioned functional correlates. In addition to reducing the excitability of the A-type VGN to step depolarizing currents. 6. Because there is increasing evidence that the HCN channel current may represent a valid target for pharmacological intervention, the quantitative relationships described in the present study could potentially help guide the molecular and/or chemical modification of HCN channel gating properties to effect a particular outcome in VGN discharge properties, ideally well beyond merely selective blockade of a particular HCN channel subtype.

Angiotensin II enhances hyperpolarization-activated currents in rat aortic baroreceptor neurons: involvement of superoxide

As an endogenous physiologically active peptide, angiotensin (ANG) II plays an important role in the maintenance of blood pressure. In the arterial baroreceptor reflex (a pivotal regulator of blood pressure), aortic baroreceptor (AB) neurons located in the nodose ganglia (NG) are a primary afferent limb of the baroreflex. Hyperpolarization-activated currents ( Ih) in the AB neurons contribute to the excitability of the AB neurons. Therefore, the present study was to measure the modulating effect of ANG II on the Ih in the primary AB neurons isolated from rats. Data from immunofluorescent and Western blot analyses showed that protein of AT1 and AT2 receptors was expressed in the nodose neurons. In the whole cell patch-clamp recording, ANG II concentration dependently enhanced the Ih density in the AB neurons (100 nM ANG II-induced 53.8 ± 3.8% increase for A-type AB neurons and 30.4 ± 7.7% increase for C-type AB neurons at test pulse −140 mV, P < 0.05). ANG II also decreased membrane excitability in the AB neurons. AT1 receptor antagonist (1 μM losartan) but not AT2 receptor antagonist (1 μM PD-123,319) totally abolished the effect of ANG II on the Ih and neuronal excitability. In addition, NADPH oxidase inhibitor (100 μM apocynin) and superoxide scavenger (1 mM tempol) also significantly blunted the ANG II-induced increase of the Ih and decrease of the membrane excitability in the AB neurons. Furthermore, losartan, apocynin, or tempol significantly attenuated the superoxide overproduction in the NG tissues induced by ANG II. These results suggest that ANG II-NADPH oxidase-superoxide signaling can activate the Ih and subsequently decrease the membrane excitability of rat AB neurons.

Diabetes alters protein expression of hyperpolarization-activated cyclic nucleotide-gated channel subunits in rat nodose ganglion cells

Vagal afferent neurons, serving as the primary afferent limb of the parasympathetic reflex, could be involved in diabetic autonomic neuropathy. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed in the vagal afferent neurons and play an important role in determining cell membrane excitation. In the present study, the protein expression and the electrophysiological characteristics of HCN channels were investigated in nodose ganglion (NG) afferent neurons (A-fiber and C-fiber neurons) from sham and streptozotocin (STZ)-induced diabetic rats. In the sham NG, HCN1, HCN3, and HCN4 were expressed in the A-fiber neurons; and HCN2, HCN3, and HCN4 were expressed in the C-fiber neurons. Compared to the sham NG neurons, diabetes induced the expression of HCN2 in the A-fiber neurons besides overexpression of HCN1 and HCN3; and enhanced the expression of HCN2 and HCN3 in C-fiber neurons. In addition, whole-cell patch-clamp data revealed diabetes also increased HCN currents in A-fiber and C-fiber neurons. However, we found that diabetes did not alter the total nodose afferent neuron number and the ratio of A-fiber/C-fiber neurons. These results indicate that diabetes induces the overexpression of HCN channels and the electrophysiological changes of HCN currents in the A- and C-fiber nodose neurons, which might contribute to the diabetes-induced alteration of cell excitability in the vagal afferent neurons.

Inflammation-induced increase in hyperpolarization-activated, cyclic nucleotide-gated channel protein in trigeminal ganglion neurons and the effect of buprenorphine

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are active at resting membrane potential and thus contribute to neuronal excitability. Their increased activity has recently been demonstrated in models of nerve injury-induced pain. The major aim of the current study was to investigate altered HCN channel protein expression in trigeminal sensory neurons following inflammation of the dura. HCN1 and HCN2 channel immunoreactivity was observed on the membranes of medium- to large-sized trigeminal ganglion neurons with 76% and 85% of HCN1 and HCN2 expressing neurons also containing the 200 kDa neurofilament protein (associated with myelinated fibers). Western immunoblots of lysates from rat trigeminal ganglia also showed bands with appropriate molecular weights for HCN1 and HCN2. Three days after application of complete Freund's adjuvant (CFA) to the dura mater, Western blot band densities were significantly increased; compared to control, to 166% for HCN1 and 284% for HCN2 channel protein. The band densities were normalized against alpha-actin. In addition, the number of retrogradely labeled neurons from the dura expressing HCN1 and HCN2 was significantly increased to 247% (HCN1) and 171% (HCN2), three days after inflammation. When the opioid receptor partial agonist, buprenorphine, was given systemically, immediately after CFA, the inflammation-induced increase in HCN protein expression in both Western blot and immunohistochemical experiments was not observed. These results suggest that HCN1 and HCN2 are involved in inflammation-induced sensory neuron hyperexcitability, and indicate that an opioid receptor agonist can reverse the protein upregulation.

Characteristics and physiological role of hyperpolarization activated currents in mouse cold thermoreceptors

Hyperpolarization-activated currents (I(h)) are mediated by the expression of combinations of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits (HCN1-4). These cation currents are key regulators of cellular excitability in the heart and many neurons in the nervous system. Subunit composition determines the gating properties and cAMP sensitivity of native I(h) currents. We investigated the functional properties of I(h) in adult mouse cold thermoreceptor neurons from the trigeminal ganglion, identified by their high sensitivity to moderate cooling and responsiveness to menthol. All cultured cold-sensitive (CS) neurons expressed a fast activating I(h), which was fully blocked by extracellular Cs(+) or ZD7288 and had biophysical properties consistent with those of heteromeric HCN1-HCN2 channels. In CS neurons from HCN1(-/-) animals, I(h) was greatly reduced but not abolished. We find that I(h) activity is not essential for the transduction of cold stimuli in CS neurons. Nevertheless, I(h) has the potential to shape the excitability of CS neurons. First, I(h) blockade caused a membrane hyperpolarization in CS neurons of about 5 mV. Furthermore, impedance power analysis showed that all CS neurons had a prominent subthreshold membrane resonance in the 5-7 Hz range, completely abolished upon blockade of I(h) and absent in HCN1 null mice. This frequency range matches the spontaneous firing frequency of cold thermoreceptor terminals in vivo. Behavioural responses to cooling were reduced in HCN1 null mice and after peripheral pharmacological blockade of I(h) with ZD7288, suggesting that I(h) plays an important role in peripheral sensitivity to cold.

Regulation of firing frequency in nociceptive neurons by pro-inflammatory mediators

Nociceptive neurons generate trains of action potentials in response to painful stimuli, and the frequency of firing signals the intensity of the pain. Pro-inflammatory mediators such as prostaglandin E2 (PGE2) enhance the sensation of pain by increasing the frequency of action potential firing in response to a given level of painful stimulus. The mechanism by which the firing frequency is enhanced is discussed in the present review. One hypothesis proposes that the threshold for action potential initiation is lowered because the activation curve of a nociceptor-specific voltage-activated Na current, Na(V)1.8, is shifted to more negative values by PGE2. Recent measurements in our lab show, however, that the action potential threshold in fact changes little when AP firing is accelerated by PGE2. The enhanced firing is, however, abolished by a blocker of an inward current activated by hyperpolarisation, called I(h). The voltage sensitivity of I(h) shifts in the positive direction in small nociceptive neurons when they are exposed to pro-inflammatory mediators, such as PGE2, which activate adenylate cyclase and therefore increase levels of cAMP. By this mechanism the inward current between the resting membrane potential and the threshold for firing of action potentials is enhanced, and the rate of depolarisation in the interval between action potentials is therefore increased. We conclude that the major mechanism responsible for increasing action potential firing following tissue damage or metabolic stress is the hyperpolarisation-activated inward current, I(h), and that other mechanisms play at most a minor role.

Role of the hyperpolarization-activated current Ih in somatosensory neurons

The hyperpolarization-activated current (I(h)) is an inward current activated by hyperpolarization from the resting potential and is an important modulator of action potential firing frequency in many excitable cells. Four hyperpolarization-activated, cyclic nucleotide-modulated subunits, HCN1-4, can form I(h) ion channels. In the present study we investigated the function of I(h) in primary somatosensory neurons. Neuronal firing in response to current injection was promoted by elevating intracellular cAMP levels and inhibited by blockers of I(h), suggesting that I(h) plays a critical role in modulating firing frequency. The properties of I(h) in three size classes of sensory neurons were next investigated. In large neurons I(h) was fast activating and insensitive to elevations in cAMP, consistent with expression of HCN1. I(h) was ablated in most large neurons in HCN1(-/-) mice. In small neurons a slower activating, cAMP-sensitive I(h) was observed, as expected for expression of HCN2 and/or HCN4. Consistent with this, I(h) in small neurons was unchanged in HCN1(-/-) mice. In a neuropathic pain model HCN1(-/-) mice exhibited substantially less cold allodynia than wild-type littermates, suggesting an important role for HCN1 in neuropathic pain. This work shows that I(h) is an important modulator of action potential generation in somatosensory neurons.

Brain-derived neurotrophic factor in arterial baroreceptor pathways: implications for activity-dependent plasticity at baroafferent synapses

Functional characteristics of the arterial baroreceptor reflex change throughout ontogenesis, including perinatal adjustments of the reflex gain and adult resetting during hypertension. However, the cellular mechanisms that underlie these functional changes are not completely understood. Here, we provide evidence that brain-derived neurotrophic factor (BDNF), a neurotrophin with a well-established role in activity-dependent neuronal plasticity, is abundantly expressed in vivo by a large subset of developing and adult rat baroreceptor afferents. Immunoreactivity to BDNF is present in the cell bodies of baroafferent neurons in the nodose ganglion, their central projections in the solitary tract, and terminal-like structures in the lower brainstem nucleus tractus solitarius. Using ELISA in situ combined with electrical field stimulation, we show that native BDNF is released from cultured newborn nodose ganglion neurons in response to patterns that mimic the in vivo activity of baroreceptor afferents. In particular, high-frequency bursting patterns of baroreceptor firing, which are known to evoke plastic changes at baroreceptor synapses, are significantly more effective at releasing BDNF than tonic patterns of the same average frequency. Together, our study indicates that BDNF expressed by first-order baroreceptor neurons is a likely mediator of both developmental and post-developmental modifications at first-order synapses in arterial baroreceptor pathways.

Distribution of voltage-gated potassium and hyperpolarization-activated channels in sensory afferent fibers in the rat carotid body

The chemosensory glomus cells of the carotid body (CB) detect changes in O2 tension. Carotid sinus nerve fibers, which originate from peripheral sensory neurons located within the petrosal ganglion, innervate the CB. Release of transmitter from glomus cells activates the sensory afferent fibers to transmit information to the nucleus of the solitary tract in the brainstem. The ion channels expressed within the sensory nerve terminals play an essential role in the ability of the terminal to initiate action potentials in response to transmitter-evoked depolarization. However, with a few exceptions, the identity of ion channels expressed in these peripheral nerve fibers is unknown. This study addresses the expression of voltage-gated channels in the sensory fibers with a focus on channels that set the resting membrane potential and regulate discharge patterns. By using immunohistochemistry and fluorescence confocal microscopy, potassium channel subunits and HCN (hyperpolarization-activated) family members were localized both in petrosal neurons that expressed tyrosine hydroxylase and in the CSN axons within the carotid body. Channels contributing to resting membrane potential, including HCN2 responsible in part for I(h) current and the KCNQ2 and KCNQ5 subunits thought to underlie the neuronal "M current," were identified in the sensory neurons and their axons innervating the carotid body. In addition, the results presented here demonstrate expression of several potassium channels that shape the action potential and the frequency of discharge, including Kv1.4, Kv1.5, Kv4.3, and K(Ca) (BK). The role of these channels should be considered in interpretation of the fiber discharge in response to perturbation of the carotid body environment.

Peripheral mechanisms I: plasticity of peripheral pathways

Cough plays a vital role in protecting the lower airways from inhaled irritants, pollutants, and infectious agents. The cough reflex exhibits remarkable plasticity, such that in the context of infectious or inflammatory respiratory diseases such as asthma, chronic bronchitis, and idiopathic pulmonary fibrosis the cough reflex can become dysregulated, leading to a chronic cough. A chronic, nonproductive (dry) cough can rob sufferers of quality of life. Plasticity of the cough reflex likely involves multiple intersecting pathways within the airways, the peripheral nerves that supply them, and the central nervous system. While further studies are needed to determine the presence and relevance of many of these specific pathways in cough associated with chronic respiratory disease, the last decade has yielded unprecedented insight into the molecular identity of the ion channels and associated proteins that initiate and conduct action potentials in the primary sensory nerves involved in reflexes such as cough. We now know, for instance, that members of the transient receptor potential superfamily of nonselective cation channels function as transducers that convert specific external stimuli into neuronal activation. We also know that certain Na+ and K+ channels play specialized roles in regulating action potential discharge in irritant-sensing afferent nerves. In this chapter, we summarize the available information regarding factors that may modulate afferent neuron function acutely, via posttranslational modifications and over the longer term through neurotrophin-dependent alterations of the transcriptional programs of adult sensory neurons.

Polyunsaturated fatty acid modulation of voltage-gated ion channels

Arachidonic acid (AA) was found to inhibit the function of whole-cell voltage-gated (VG) calcium currents nearly 16 years ago. There are now numerous examples demonstrating that AA and other polyunsaturated fatty acids (PUFAs) modulate the function of VG ion channels, primarily in neurons and muscle cells. We will review and extract some common features about the modulation by PUFAs of VG calcium, sodium, and potassium channels and discuss the impact of this modulation on the excitability of neurons and cardiac myocytes. We will describe the fatty acid nature of the membrane, how fatty acids become available to function as modulators of VG channels, and the physiologic importance of this type of modulation. We will review the evidence for molecular mechanisms and assess our current understanding of the structural basis for modulation. With guidance from research on the structure of fatty acid binding proteins, the role of lipids in gating mechanosensitive (MS) channels, and the impact of membrane lipid composition on membrane-embedded proteins, we will highlight some avenues for future investigations.

Blunted excitability of aortic baroreceptor neurons in diabetic rats: involvement of hyperpolarization-activated channel

AIMS

Although dysfunction of arterial baroreflex occurs in human and animal models of type-1 diabetes (T1D), the mechanisms involved in the impairment of the baroreflex still remain unclear. The nodose ganglion (NG) contains the cell bodies of the aortic baroreceptor (AB) neurons. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed in AB neurons and play an important role in regulating the cell excitability. We investigated whether the excitability of AB neurons is depressed in streptozotocin (STZ)-induced T1D rats and whether HCN channels are involved in this depression.

METHODS AND RESULTS

Using the whole-cell patch clamp technique, we found that AB neuron excitability (action potential frequency at 50 pA current stimulation) in the T1D rats was lower than that in the sham rats (0.4 +/- 0.5 vs. 4.8 +/- 0.6 spikes/s, P < 0.05; AB neurons were identified by DiI staining). In addition, HCN current density in AB neurons from the T1D rats was bigger than that from the sham rats (60.2 +/- 6.1 vs. 30.7 +/- 4.9 pA/pF at test pulse -140 from holding potential -40 mV, P < 0.05). Furthermore, HCN channel blockers (5 mM cesium chloride and 100 microM ZD7288) significantly reduced HCN currents and increased action potential frequency of the AB neurons in sham and T1D rats. Immunofluorescent and western blot analyses demonstrated that the expression of HCN1 and HCN2 channel protein in the NG from the T1D rats was higher than that from the sham rats.

CONCLUSION

These results indicate that the HCN channels influence the excitability of AB neurons, and more importantly, contribute to the decreased excitability of AB neurons in T1D rats.

Ion channels in mammalian vestibular afferents may set regularity of firing

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Characteristics of HCN channels and their participation in neuropathic pain

Neuropathic pain is induced by the injury to nervous systems and characterized by hyperalgesia, allodynia and spontaneous pain. The underlying mechanisms include peripheral and central sensitization resulted from neuronal hyperexcitability. A number of ion channels are considered to contribute to the neuronal hyperexcitability. Here, we particularly concentrate on an interesting ion channel, hyperpolarization-activated cyclic nucleotide gated (HCN) channels. We overview its biophysical properties, physiological functions, followed by focusing on the current progress in the study of its role in the development of neuropathic pain. We attempt to provide a comprehensive review of the potential valuable target, HCN channels, in the treatment of neuropathic pain.

Hyperpolarization-activated current (I(h)) contributes to excitability of primary sensory neurons in rats

In various excitable tissues, the hyperpolarization-activated, cyclic nucleotide-gated current (I(h)) contributes to burst firing by depolarizing the membrane after a period of hyperpolarization. Alternatively, conductance through open channels I(h) channels of the resting membrane may impede excitability. Since primary sensory neurons of the dorsal root ganglion show both loss of I(h) and elevated excitability after peripheral axonal injury, we examined the contribution of I(h) to excitability of these neurons. We used a sharp electrode intracellular technique to record from neurons in nondissociated ganglia to avoid potential artefacts due to tissue dissociation and cytosolic dialysis. Neurons were categorized by conduction velocity. I(h) induced by hyperpolarizing voltage steps was completely blocked by ZD7288 (approximately 10 microM), which concurrently eliminated the depolarizing sag of transmembrane potential during hyperpolarizing current injection. I(h) was most prominent in rapidly conducting Aalpha/beta neurons, in which ZD7288 produced resting membrane hyperpolarization, slowed conduction velocity, prolonged action potential (AP) duration, and elevated input resistance. The rheobase current necessary to trigger an AP was elevated and repetitive firing was inhibited by ZD7288, indicating an excitatory influence of I(h). Less I(h) was evident in more slowly conducting Adelta neurons, resulting in diminished effects of ZD7288 on AP parameters. Repetitive firing in these neurons was also inhibited by ZD7288, and the peak frequency of AP transmission during tetanic bursts was diminished by ZD7288. Slowly conducting C-type neurons showed minimal I(h), and no effect of ZD7288 on excitability was seen. After spinal nerve ligation, axotomized neurons had less I(h) compared to control neurons and showed minimal effects of ZD7288 application. We conclude that I(h) supports sensory neuron excitability, and loss of I(h) is not a factor contributing to increased neuronal excitability after peripheral axonal injury.

The KCNQ/M-current modulates arterial baroreceptor function at the sensory terminal in rats

The ion channels responsible for the pattern and frequency of discharge in arterial baroreceptor terminals are, with few exceptions, unknown. In this study we examined the contribution of KCNQ potassium channels that underlie the M-current to the function of the arterial baroreceptors. Labelled aortic baroreceptor neurons, immunohistochemistry and an isolated aortic arch preparation were used to demonstrate the presence and function of KCNQ2, KCNQ3 and KCNQ5 channels in aortic baroreceptors. An activator (retigabine) and an inhibitor (XE991) of the M-current were used to establish a role for these channels in setting the resting membrane potential and in regulating the response to ramp increases in arterial pressure. Retigabine raised the threshold for activation of arterial baroreceptors and shifted the pressure-response curve to higher aortic pressures. XE991, on the other hand, produced an increase in excitability as shown by an increase in discharge at elevated pressures as compared to control. We propose that KCNQ2, KCNQ3 and KCNQ5 channels provide a hyperpolarizing influence to offset the previously described depolarizing influence of the HCN channels in baroreceptor neurons and their terminals.

Regional Specification of Threshold Sensitivity and Response Time in CBA/CaJ Mouse Spiral Ganglion Neurons

Previous studies of spiral ganglion neuron electrophysiology have shown that specific parameters differ according to cochlear location, with apical neurons being distinctly different from basal neurons. To align these features more precisely along the tonotopic axis of the cochlea, we developed a novel spiral ganglion culture system in which positional information is retained. Patch-clamp recordings made from neurons of known gangliotopic location revealed two basic firing pattern distributions. Membrane characteristics related to spike timing, such as accommodation, latency and onset tau, were distinctly heterogeneous, yet when averaged, they were distributed in a graded manner along the length of the cochlea. Action potential threshold levels also displayed a wide range, the averages of which were distributed nonmonotonically such that neurons with the greatest sensitivity were localized to the mid-regions of the ganglion. These studies shed new light on the complexity and sophistication of the intrinsic firing features of spiral ganglion neurons. Because timing-related elements are organized in an overall tonotopic manner, it is hypothesized that they contribute to aspects of frequency-dependent acoustic processing. On the other hand, the different distribution of threshold levels, with the greatest sensitivity in the middle region of the tonotopic map, suggests that this neuronal parameter is regulated differently and thus may contribute a distinct realm of auditory sensory processing.

Post-spike excitability indicates changes in membrane potential of isolated C-fibers

Recording of action potentials from single unmyelinated nerve fibers by microneurography is an important tool to investigate peripheral neural functions in human neuropathies. However, the interpretation of microneurography recordings can be difficult because axonal membrane potential is not revealed by this method. We tested the hypothesis that the recovery cycle of excitability after a single action potential is correlated with changes in the axonal membrane potential. To this end, we used the threshold tracking technique to study how different chemical mediators, with known effects on the membrane potential, influence the post-spike superexcitability of C-fiber compound action potentials in isolated rat sural and vagus nerves. We found that: (1) some chemical mediators (e.g., adenosine 5'-triphosphate) produce a reduction or loss of superexcitability together with increased axonal excitability, indicating membrane depolarization; (2) blockade of axonal hyperpolarization-activated (Ih) currents produces an enhancement of superexcitability together with a decreased excitability, indicating membrane hyperpolarization; and (3) application of calcium produces an increase in membrane threshold without an alteration in superexcitability, indicating a non-specific increase in surface charge and a change in the voltage-dependent activation of sodium channels. In addition, we demonstrated that membrane depolarization and hyperpolarization induce opposite post-spike latency shifts (changes in supernormality) in rat and human nerve segments. Thus, recordings of post-spike excitability and shifts in latency are sensitive techniques for detection of various types of neuromodulation, which are correlated with changes in membrane potential of unmyelinated peripheral axons and may help to understand observations obtained by microneurography in peripheral human neuropathies.

The role of pacemaker currents in neuropathic pain

Hyperpolarization-activated cation nonselective cyclic nucleotide-gated (HCN) channels mediate pacemaker currents that control basic rhythmic processes including heartbeat. Alterations in HCN channel expression or function have been described in both epilepsy and cardiac arrhythmias. Recent evidence suggests that pacemaker currents may also play an important role in ectopic neuronal activity that manifests as neuropathic pain. Pacemaker currents are subject to endogenous regulation by cyclic nucleotides, pH and perhaps phosphorylation. In addition, a number of neuromodulators with known roles in pain affect current density and kinetics. The pharmacology of a number of drugs that are commonly used to treat neuropathic pain includes effects on pacemaker currents. Altered pacemaker currents in injured tissues may be an important mechanism underlying neuropathic pain, and drugs that modulate these currents may offer new therapeutic options.

HCN-encoded pacemaker channels: from physiology and biophysics to bioengineering

The depolarizing membrane ionic current I(h) (also known as I(f), "f" for funny), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN1-4) channel gene family, was first discovered in the heart over 25 years ago. Later, I(h) was also found in neurons, retina, and taste buds. HCN channels structurally resemble voltage-gated K(+) (Kv) channels but the molecular features underlying their opposite gating behaviors (activation by hyperpolarization rather than depolarization) and non-selective permeation profiles (> or =25 times less selective for K(+) than Kv channels) remain largely unknown. Although I(h) has been functionally linked to biological processes from the autonomous beating of the heart to pain transmission, the underlying mechanistic actions remain largely inferential and, indeed, somewhat controversial due to the slow kinetics and negative operating voltage range relative to those of the bioelectrical events involved (e.g., cardiac pacing). This article reviews the current state of our knowledge in the structure-function properties of HCN channels in the context of their physiological functions and potential HCN-based therapies via bioengineering.

Role of peripheral hyperpolarization-activated cyclic nucleotide-modulated channel pacemaker channels in acute and chronic pain models in the rat

Hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels contribute to rhythmic spontaneous activity in the heart and CNS. Ectopic spontaneous neuronal activity has been implicated in the development and maintenance of acute and chronic hyperalgesia, allodynia and spontaneous pain. Previously, we documented that systemic administration of ZD7288, a specific blocker of pacemaker current (I(h)), decreased ectopic activity in dorsal root ganglion (DRG) and reversed tactile allodynia in spinal nerve ligated (SNL) rats [Chaplan SR, Guo HQ, Lee DH, Luo L, Liu C, Kuei C, Velumian AA, Butler MP, Brown SM, Dubin AE (2003) Neuronal hyperpolarization-activated pacemaker channels drive neuropathic pain. J Neurosci 23:1169-1178]. Spontaneous pain is the chief clinical manifestation of peripheral nerve injury; however, a role for I(h) in spontaneous pain has not been described. Here, in further rat studies, we report that systemic administration of ZD7288 reversed spontaneous pain induced by mild thermal injury (MTI) and tactile allodynia induced by SNL and MTI. In contrast, ZD7288 did not reduce thermal hyperalgesia. An important locus of action appears to be in the skin since intraplantar (local) administration of ZD7288 completely suppressed tactile allodynia arising from MTI and SNL and reduced spontaneous pain due to MTI. Immunohistochemical staining of plantar skin sections detected HCN1-HCN4 expression in mechanosensory structures (e.g., Meissner's corpuscles and Merkel cells). Collectively, these data suggest that expression and modulation of I(h) in the peripheral nervous system, including specialized sensory structures, may play a significant role in sensory processing and contribute to spontaneous pain and tactile allodynia.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Dual stretch responses of mHCN2 pacemaker channels: accelerated activation, accelerated deactivation

Mechanoelectric feedback in heart and smooth muscle is thought to depend on diverse channels that afford myocytes a mechanosensitive cation conductance. Voltage-gated channels (e.g., Kv1) are stretch sensitive, but the only voltage-gated channels that are cation permeant, the pacemaker or HCN (hyperpolarization-activated cyclic nucleotide-gated) channels, have not been tested. To assess if HCN channels could contribute to a mechanosensitive cation conductance, we recorded I(HCN) in cell-attached oocyte patches before, during, and after stretch for a range of voltage protocols. I(mHCN2) has voltage-dependent and instantaneous components; only the former was stretch sensitive. Stretch reversibly accelerated hyperpolarization-induced I(mHCN2) activation (likewise for I(spHCN)) and depolarization-induced deactivation. HCN channels (like Kv1 channels) undergo mode-switch transitions that render their activation midpoints voltage history dependent. The result, as seen from sawtooth clamp, is a pronounced hysteresis. During sawtooth clamp, stretch increased current magnitudes and altered the hysteresis pattern consistent with stretch-accelerated activation and deactivation. I(mHCN2) responses to step protocols indicated that at least two transitions were mechanosensitive: an unspecified rate-limiting transition along the hyperpolarization-driven path, mode I(closed)-->mode II(open), and depolarization-induced deactivation (from mode I(open) and/or from mode II(open)). How might this affect cardiac rhythmicity? Since hysteresis patterns and "on" and "off" I(HCN) responses all changed with stretch, predictions are difficult. For an empirical overview, we therefore clamped patches to cyclic action potential waveforms. During the diastolic potential of sinoatrial node cell and Purkinje fiber waveforms, net stretch effects were frequency dependent. Stretch-inhibited (SI) I(mHCN2) dominated at low frequencies and stretch-augmented (SA) I(mHCN2) was progressively more important as frequency increased. HCN channels might therefore contribute to either SI or SA cation conductances that in turn contribute to stretch arrhythmias and other mechanoelectric feedback phenomena.

Activity-dependent slowing of conduction velocity in uninjured L4 C fibers increases after an L5 spinal nerve injury in the rat

Growing evidence suggests that uninjured afferents may play an important role in neuropathic pain following nerve injury. The excitability of nociceptive neurons in the L4 spinal nerve appears to be enhanced following an injury to the adjacent L5 spinal nerve. In this study, we investigated whether the action-potential conduction properties of unlesioned, unmyelinated fibers are also altered. A teased-fiber technique was used to record from single C fibers from the L4 spinal nerve of the rat in vitro. Repeated electrical stimulation of the tibial nerve was used to investigate activity-dependent slowing of conduction velocity. Twin pulse stimulation at a 50 ms interpulse interval allowed investigation of supranormal conduction velocity. Blinded experiments were performed 8-10 days after sham surgery and after an L5 spinal nerve ligation (L5 SNL). Activity-dependent slowing revealed two populations of C fibers, a "nociceptor" population with a large degree of activity-dependent slowing and a "non-nociceptor" population with a smaller degree of activity-dependent slowing. Both populations showed enhanced activity-dependent slowing of conduction velocity and enhanced supranormal conduction velocities in lesioned animals compared to sham animals. Activity-dependent slowing was also enhanced after an L5 SNL in the mouse. These alterations in conduction velocity may reflect changes in expression of ion channels responsible for the membrane excitability. These data provide additional evidence that a nerve injury leads to persistent alterations in the properties of adjacent uninjured, unmyelinated fibers.

Dependence of hyperpolarisation-activated cyclic nucleotide-gated channel activity on basal cyclic adenosine monophosphate production in spontaneously firing GH3 cells

The hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels play a distinct role in the control of membrane excitability in spontaneously active cardiac and neuronal cells. Here, we studied the expression and role of HCN channels in pacemaking activity, Ca(2+) signalling, and prolactin secretion in GH(3) immortalised pituitary cells. Reverse transcriptase-polymerase chain reaction analysis revealed the presence of mRNA transcripts for HCN2, HCN3 and HCN4 subunits in these cells. A hyperpolarisation of the membrane potential below - 60 mV elicited a slowly activating voltage-dependent inward current (I(h)) in the majority of tested cells, with a half-maximal activation voltage of -89.9 +/- 4.2 mV and with a time constant of 1.4 +/- 0.2 s at -120 mV. The bath application of 1 mM Cs(+), a commonly used inorganic blocker of I(h), and 100 microM ZD7288, a specific organic blocker of I(h), inhibited I(h) by 90 +/- 4.1% and 84.3 +/- 1.8%, respectively. Receptor- and nonreceptor-mediated activation of adenylyl and soluble guanylyl cyclase and the addition of a membrane permeable cyclic adenosine monophosphate (cAMP) analogue, 8-Br-cAMP, did not affect I(h). Inhibition of basal adenylyl cyclase activity, but not basal soluble guanylyl cyclase activity, led to a reduction in the peak amplitude and a leftward shift in the activation curve of I(h) by 23.7 mV. The inhibition of the current was reversed by stimulation of adenylyl cyclase with forskolin and by the addition of 8-Br-cAMP, but not 8-Br-cGMP. Application of Cs(+) had no significant effect on the resting membrane potential or electrical activity, whereas ZD7288 exhibited complex and I(h)-independent effects on spontaneous electrical activity, Ca(2+) signalling, and prolactin release. These results indicate that HCN channels in GH(3) cells are under tonic activation by basal level of cAMP and are not critical for spontaneous firing of action potentials.

Characterization of hyperpolarization-activated current (Ih) in dorsal root ganglion neurons innervating rat urinary bladder

Afferent pathways innervating the urinary bladder consist of myelinated Adelta-fibers and unmyelinated C-fibers. Normal voiding is dependent on mechanoceptive Adelta-fiber bladder afferents that respond to bladder distention. However, the mechanisms for controlling the excitability of Adelta-fiber bladder afferents are not fully understood. We therefore used whole cell patch-clamp techniques to investigate the properties of hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents (I(h)) in dorsal root ganglion (DRG) neurons innervating the urinary bladder of rats. The neurons were identified by axonal tracing with a fluorescent dye, Fast Blue, injected into the bladder wall. Hyperpolarizing voltage step pulses from -40 to -130 mV produced voltage- and time-dependent inward I(h) currents in bladder afferent neurons. The amplitude and current density of I(h) at a holding potential of -130 mV was significantly larger in medium-sized bladder afferent neurons (diameter: 37.8 +/- 0.3 microm), a small portion (19%) of which were sensitive to capsaicin (1 microM), than in uniformly capsaicin-sensitive small-sized (27.6 +/- 0.5 microm) bladder neurons. In medium-sized bladder neurons, a selective HCN channel inhibitor, ZD7288, dose-dependently inhibited I(h) currents. ZD7288 (10 microM) also increased the time constant of the slow depolarization phase of spike after-hyperpolarization from 91.8 to 233.0 ms. These results indicate that I(h) currents are predominantly expressed in medium-sized bladder afferent neurons innervating the bladder and that inhibition of I(h) currents delayed recovery from the spike after-hyperpolarization. Thus, it is assumed that I(h) currents could control excitability of mechanoceptive Adelta-fiber bladder afferent neurons, which are usually capsaicin-insensitive and larger in size than capsaicin-sensitive C-fiber bladder afferent neurons.

Capsaicin-resistant arterial baroreceptors

BACKGROUND

Aortic baroreceptors (BRs) comprise a class of cranial afferents arising from major arteries closest to the heart whose axons form the aortic depressor nerve. BRs are mechanoreceptors that are largely devoted to cardiovascular autonomic reflexes. Such cranial afferents have either lightly myelinated (A-type) or non-myelinated (C-type) axons and share remarkable cellular similarities to spinal primary afferent neurons. Our goal was to test whether vanilloid receptor (TRPV1) agonists, capsaicin (CAP) and resiniferatoxin (RTX), altered the pressure-discharge properties of peripheral aortic BRs.

RESULTS

Periaxonal application of 1 microM CAP decreased the amplitude of the C-wave in the compound action potential conducting at <1 m/sec along the aortic depressor nerve. 10 microM CAP eliminated the C-wave while leaving intact the A-wave conducting in the A-delta range (<12 m/sec). These whole nerve results suggest that TRPV1 receptors are expressed along the axons of C- but not A-conducting BR axons. In an aortic arch--aortic nerve preparation, intralumenal perfusion with 1 microM CAP had no effect on the pressure-discharge relations of regularly discharging, single fiber BRs (A-type)--including the pressure threshold, sensitivity, frequency at threshold, or maximum discharge frequency (n = 8, p > 0.50) but completely inhibited discharge of an irregularly discharging BR (C-type). CAP at high concentrations (10-100 microM) depressed BR sensitivity in regularly discharging BRs, an effect attributed to non-specific actions. RTX (< or = 10 microM) did not affect the discharge properties of regularly discharging BRs (n = 7, p > 0.18). A CAP-sensitive BR had significantly lower discharge regularity expressed as the coefficient of variation than the CAP-resistant fibers (p < 0.002).

CONCLUSION

We conclude that functional TRPV1 channels are present in C-type but not A-type (A-delta) myelinated aortic arch BRs. CAP has nonspecific inhibitory actions that are unlikely to be related to TRV1 binding since such effects were absent with the highly specific TRPV1 agonist RTX. Thus, CAP must be used with caution at very high concentrations.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

HCN1 ion channel immunoreactivity in spinal cord and medulla oblongata

Hyperpolarization-activated cyclic nucleotide-gated (HCN) non-selective cation channels in neurons carry currents proposed to perform diverse functions, including the hyperpolarization activated Ih current. The 4 HCN subunits have unique but overlapping patterns of expression in the CNS. Here, we examined the distribution of HCN1 channel subunits in the brainstem and spinal cord using immunohistochemistry. At all levels of the spinal cord dorsal horn, HCN1 immunoreactivity (HCN1-IR) was predominantly absent from laminae I and II, while a dense band of punctate labeling was visible in lamina III. Labeled neurons were identified in close vicinity to the central canal, in the lateral spinal nucleus, in the ventral horn and occasionally in lamina II and III. Those in the ventral horn were identified as alpha motor neurons using retrograde tracing and/or double or triple immunostaining with neuronal markers neurofilament 200 (NF200) and choline acetyltransferase. HCN1-IR neurons in the brainstem included neurons in sensory pathways such as the dorsal column nuclei, the area postrema, the spinal trigeminal nucleus as well as identified motor neurons in motor nuclei. In the nucleus ambiguus, a mixed visceral/motor nucleus, HCN1-IR was present only in NF200-IR cells, suggesting that it is expressed in motor but not autonomic preganglionic neurons. HCN1-IR motor neurons in the nucleus ambiguus also expressed the neurokinin 1 receptor and were labeled retrogradely from the larnyx. At the light microscopic level, the NTS and inferior olive contained punctate labeling, which ultrastructural examination revealed to be present in predominantly synaptic terminals or dendrites respectively. These data therefore described the first localization of the HCN1 subunit in the spinal cord and extend previous reports from the brainstem.

Most neurons in the nucleus tractus solitarii do not send collateral projections to multiple autonomic targets in the rat brain

The nucleus tractus solitarii (NTS) receives primary visceral afferents and sends projections to other autonomic nuclei at all levels of the neuroaxis. However, it is unknown if distinct populations of NTS neurons project to individual autonomic targets or if individual neurons in the NTS project to multiple autonomic targets. Understanding the basic circuitry of visceral reflex pathways is essential for the analyses of functional central autonomic networks. We examined projections from the NTS to autonomic targets within the hypothalamus (paraventricular nucleus, PVN), pons (parabrachial nucleus, PB), and medulla (caudal ventrolateral medulla, CVL) using retrograde tracing and immunohistochemistry. Dual retrograde tracer microinjections were made into pairs of targets (PVN + CVL; PVN + PB; PB + CVL), and the pattern of retrograde labeling was examined within NTS. The extent of collateralization, seen as dual retrogradely labeled neurons, was negligible for combined PVN and CVL injections and increased for injections combining PB with either PVN or CVL, but the majority of NTS neurons project to only one autonomic target. Immunohistochemistry for tyrosine hydroxylase (TH) was used to examine the pattern of TH-immunoreactivity (TH-ir) within retrogradely labeled NTS neurons. TH-ir was seen predominantly in projections to PVN, to a lesser degree in projections to PB, and was largely absent from projections to CVL. The percentage of dual retrogradely labeled neurons displaying TH-ir corresponded to the target displaying the most TH-ir, and TH-ir was not predictive of collateralization. Together, these results indicate that NTS neurons project to individual autonomic targets in the brain.

Functional Arterial Baroreflex Attenuates the Effects of Antihypertensive Drugs in Conscious Rats

The present work was designed to observe the influences of arterial baroreflex (ABR) function on cardiovascular effects produced by four routinely used antihypertensive drugs in conscious rats. A low ABR model was obtained by the performance of sinoaortic denervation (SAD). The doses of the four drugs were as follows: nifedipine (1.5, 3.0 mg/kg), captopril (50, 100 mg/kg), atenolol (10, 20 mg/kg), and hydrochlorothiazide (20, 40 mg/kg). They were administered via an intra-gastric catheter. Compared with sham-operated rats, SAD significantly increased blood pressure variability about 2 times without modification of blood pressure level. The decrease in blood pressure level induced by the four tested drugs was larger in SAD rats than in sham-operated rats, which decreased to about 10 mmHg. Pulse interval was not changed by the treatment of captopril, but prolonged by atenolol in both sham-operated and SAD rats. In sham-operated groups, treatment of both nifedipine and hydrochlorothiazide decreased pulse interval. Whereas in sinoaortic denervated ones, this tachycardia was prevented. Among the four tested drugs, it was found that only nifedipine and atenolol significantly decreased blood pressure variability in SAD rats. It can be concluded that arterial baroreflex function was able to attenuate the hypotensive effects produced by antihypertensive drugs in conscious rats.