Spontaneous action potential generation due to persistent sodium channel currents in simulated carotid body afferent fibers

β-Adrenoceptor blockade prevents carotid body hyperactivity and elevated vascular sympathetic nerve density induced by chronic intermittent hypoxia

Carotid body (CB) hyperactivity promotes hypertension in response to chronic intermittent hypoxia (CIH). The plasma concentration of adrenaline is reported to be elevated in CIH and our previous work suggests that adrenaline directly activates the CB. However, a role for chronic adrenergic stimulation in mediating CB hyperactivity is currently unknown. This study evaluated whether beta-blocker treatment with propranolol (Prop) prevented the development of CB hyperactivity, vascular sympathetic nerve growth and hypertension caused by CIH. Adult male Wistar rats were assigned into 1 of 4 groups: Control (N), N + Prop, CIH and CIH + Prop. The CIH paradigm consisted of 8 cycles h-1, 8 h day-1, for 3 weeks. Propranolol was administered via drinking water to achieve a dose of 40 mg kg-1 day-1. Immunohistochemistry revealed the presence of both β1 and β2-adrenoceptor subtypes on the CB type I cell. CIH caused a 2-3-fold elevation in basal CB single-fibre chemoafferent activity and this was prevented by chronic propranolol treatment. Chemoafferent responses to hypoxia and mitochondrial inhibitors were attenuated by propranolol, an effect that was greater in CIH animals. Propranolol decreased respiratory frequency in normoxia and hypoxia in N and CIH. Propranolol also abolished the CIH mediated increase in vascular sympathetic nerve density. Arterial blood pressure was reduced in propranolol groups during hypoxia. Propranolol exaggerated the fall in blood pressure in most (6/7) CIH animals during hypoxia, suggestive of reduced sympathetic tone. These findings therefore identify new roles for β-adrenergic stimulation in evoking CB hyperactivity, sympathetic vascular hyperinnervation and altered blood pressure control in response to CIH.

A painful neuropathy-associated Nav1.7 mutant leads to time-dependent degeneration of small-diameter axons associated with intracellular Ca2+ dysregulation and decrease in ATP levels

Small fiber neuropathy is a painful sensory nervous system disorder characterized by damage to unmyelinated C- and thinly myelinated Aδ- nerve fibers, clinically manifested by burning pain in the distal extremities and dysautonomia. The clinical onset in adulthood suggests a time-dependent process. The mechanisms that underlie nerve fiber injury in small fiber neuropathy are incompletely understood, although roles for energetic stress have been suggested. In the present study, we report time-dependent degeneration of neurites from dorsal root ganglia neurons in culture expressing small fiber neuropathy-associated G856D mutant Nav1.7 channels and demonstrate a time-dependent increase in intracellular calcium levels [Ca2+]i and reactive oxygen species, together with a decrease in ATP levels. Together with a previous clinical report of burning pain in the feet and hands associated with reduced levels of Na+/K+-ATPase in humans with high altitude sickness, the present results link energetic stress and reactive oxygen species production with the development of a painful neuropathy that preferentially affects small-diameter axons.

Sodium channels contribute to degeneration of dorsal root ganglion neurites induced by mitochondrial dysfunction in an in vitro model of axonal injury

Axonal degeneration occurs in multiple neurodegenerative disorders of the central and peripheral nervous system. Although the underlying molecular pathways leading to axonal degeneration are incompletely understood, accumulating evidence suggests contributions of impaired mitochondrial function, disrupted axonal transport, and/or dysfunctional intracellular Ca(2+)-homeostasis in the injurious cascade associated with axonal degeneration. Utilizing an in vitro model of axonal degeneration, we studied a subset of mouse peripheral sensory neurons in which neurites were exposed selectively to conditions associated with the pathogenesis of axonal neuropathies in vivo. Rotenone-induced mitochondrial dysfunction resulted in neurite degeneration accompanied by reduced ATP levels and increased ROS levels in neurites. Blockade of voltage-gated sodium channels with TTX and reverse (Ca(2+)-importing) mode of the sodium-calcium exchanger (NCX) with KB-R7943 partially protected rotenone-treated neurites from degeneration, suggesting a contribution of sodium channels and reverse NCX activity to the degeneration of neurites resulting from impaired mitochondrial function. Pharmacological inhibition of the Na(+)/K(+)-ATPase with ouabain induced neurite degeneration, which was attenuated by TTX and KB-R7943, supporting a contribution of sodium channels in axonal degenerative pathways accompanying impaired Na(+)/K(+)-ATPase activity. Conversely, oxidant stress (H2O2)-induced neurite degeneration was not attenuated by TTX. Our results demonstrate that both energetic and oxidative stress targeted selectively to neurites induces neurite degeneration and that blockade of sodium channels and of reverse NCX activity blockade partially protects neurites from injury due to energetic stress, but not from oxidative stress induced by H2O2.

Membrane properties and electrogenesis in the distal axons of small dorsal root ganglion neurons in vitro

Although it is generally thought that sensory transduction occurs at or close to peripheral nerve endings, with action potentials subsequently propagating along the axons of dorsal root ganglia (DRG) neurons toward the central nervous system, the small diameter of nociceptive axons and their endings have made it difficult to estimate their membrane properties and electrogenic characteristics. Even the resting potentials of nociceptive axons are unknown. In this study, we developed the capability to record directly with patch-clamp electrodes from the small-diameter distal axons of DRG neurons in vitro. We showed using current-clamp recordings that 1) these sensory axons have a resting potential of −60.2 ± 1 mV; 2) both tetrodotoxin (TTX)-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ channels are present and available for activation at resting potential, at densities that can support action potential electrogenesis in these axons; 3) TTX-sensitive channels contribute to the amplification of small depolarizations that are subthreshold with respect to the action potential in these axons; 4) TTX-R channels can support the production of action potentials in these axons; and 5) these TTX-R channels can produce repetitive firing, even at depolarized membrane potentials where TTX-S channels are inactivated. Finally, using voltage-clamp recordings with an action potential as the command, we confirmed the presence of both TTX-S and TTX-R channels, which are activated sequentially during action potential in these axons. These results provide direct evidence for the presence of TTX-S and TTX-R Na+ channels that are functionally available at resting potential and contribute to electrogenesis in small-diameter afferent axons.

Small nerve fibres, small hands and small feet: a new syndrome of pain, dysautonomia and acromesomelia in a kindred with a novel NaV1.7 mutation

The Na(V)1.7 sodium channel is preferentially expressed within dorsal root ganglion and sympathetic ganglion neurons and their small-diameter peripheral axons. Gain-of-function variants of Na(V)1.7 have recently been described in patients with painful small fibre neuropathy and no other apparent cause. Here, we describe a novel syndrome of pain, dysautonomia, small hands and small feet in a kindred carrying a novel Na(V)1.7 mutation. A 35-year-old male presented with erythema and burning pain in the hands since early childhood, later disseminating to the feet, cheeks and ears. He also experienced progressive muscle cramps, profound sweating, bowel disturbances (diarrhoea or constipation), episodic dry eyes and mouth, hot flashes, and erectile dysfunction. Neurological examination was normal. Physical examination was remarkable in revealing small hands and feet (acromesomelia). Blood examination and nerve conduction studies were unremarkable. Intra-epidermal nerve fibre density was significantly reduced compared to age- and sex-matched normative values. The patient's brother and father reported similar complaints including distal extremity redness and pain, and demonstrated comparable distal limb under-development. Quantitative sensory testing revealed impaired warmth sensation in the proband, father and brother. Genetic analysis revealed a novel missense mutation in the SCN9A gene encoding sodium channel Na(V)1.7 (G856D; c.2567G > A) in all three affected subjects, but not in unaffected family members. Functional analysis demonstrated that the mutation hyperpolarizes (-9.3 mV) channel activation, depolarizes (+6.2 mV) steady-state fast-inactivation, slows deactivation and enhances persistent current and the response to slow ramp stimuli by 10- to 11-fold compared with wild-type Na(V)1.7 channels. Current-clamp analysis of dorsal root ganglion neurons transfected with G856D mutant channels demonstrated depolarized resting potential, reduced current threshold, increased repetitive firing in response to suprathreshold stimulation and increased spontaneous firing. Our results demonstrate that the G856D mutation produces DRG neuron hyperexcitability which underlies pain in this kindred, and suggest that small peripheral nerve fibre dysfunction due to this mutation may have contributed to distal limb under-development in this novel syndrome.

Physiological interactions between Nav1.7 and Nav1.8 sodium channels: a computer simulation study

We have examined the question of how the level of expression of sodium channel Nav1.8 affects the function of dorsal root ganglion (DRG) neurons that also express Nav1.7 channels and, conversely, how the level of expression of sodium channel Nav1.7 affects the function of DRG neurons that also express Nav1.8, using computer simulations. Our results demonstrate several previously undescribed effects of expression of Nav1.7: 1) at potentials more negative than −50 mV, increasing Nav1.7 expression reduces current threshold. 2) Nav1.7 reduces, but does not eliminate, the dependence of action potential (AP) threshold on membrane potential. 3) In cells that express Nav1.8, the presence of Nav1.7 results in larger amplitude subthreshold oscillations and increases the frequency of repetitive firing. Our results also demonstrate multiple effects of expression of Nav1.8: 1) dependence of current threshold on membrane potential is eliminated or reversed by expression of Nav1.8 at ≥50% of normal values. 2) Expression of Nav1.8 alone, in the absence of Nav1.7, can support subthreshold oscillation. 3) Nav1.8 is required for generation of overshooting APs, and its expression results in a prolonged AP with an inflection of the falling phase. 4) Increasing levels of expression of Nav1.8 result in a reduction in the voltage threshold for AP generation. 5) Increasing levels of expression of Nav1.8 result in an attenuation of Nav1.7 current during activity evoked by sustained depolarization due, at least in part, to accumulation of fast inactivation by Nav1.7 following the first AP. These results indicate that changes in the level of expression of Nav1.7 and Nav1.8 may provide a regulatory mechanism that tunes the excitability of small DRG neurons.

Developmental changes in the magnitude and activation characteristics of Na(+) currents of petrosal neurons projecting to the carotid body

Carotid bodies mediate hypoxia sensing for the respiratory system and increase their sensitivity in the post-natal period. The present study examined the characteristics and developmental change of fast Na(+) currents of chemoreceptor afferent neurons. Rat carotid bodies (P2-P19) were harvested intact with the petrosal ganglia and whole-cell recordings obtained from petrosal somas whose axons projected to the carotid body. The magnitude of Na(+) current increased in the post-natal period in parallel with increased conduction velocity and somal size. Voltage-dependence of activation significantly shifted towards negative potentials but no significant change occurred in the voltage dependence of inactivation or the slope factors for activation or inactivation. The leftward shift in activation increased slowly or non-inactivating currents around resting potential which increases afferent neuron excitability, a result confirmed in current clamp recordings. These results suggest that a developmental shift in Na(+) current activation plays a role in chemoreceptor maturation by enhancing excitability of the afferent neuron.

Sodium-calcium exchanger and multiple sodium channel isoforms in intra-epidermal nerve terminals

Background

Nociception requires transduction and impulse electrogenesis in nerve fibers which innervate the body surface, including the skin. However, the molecular substrates for transduction and action potential initiation in nociceptors are incompletely understood. In this study, we examined the expression and distribution of Na⁺/Ca²⁺ exchanger (NCX) and voltage-gated sodium channel isoforms in intra-epidermal free nerve terminals.

Results

Small diameter DRG neurons exhibited robust NCX2, but not NCX1 or NCX3 immunolabeling, and virtually all PGP 9.5-positive intra-epidermal free nerve terminals displayed NCX2 immunoreactivity. Sodium channel Na_V1.1 was not detectable in free nerve endings. In contrast, the majority of nerve terminals displayed detectable levels of expression of Na_V1.6, Na_V1.7, Na_V1.8 and Na_V1.9. Sodium channel immunoreactivity in the free nerve endings extended from the dermal boundary to the terminal tip. A similar pattern of NCX and sodium channel immunolabeling was observed in DRG neurons in vitro.

Conclusions

NCX2, as well as Na_V1.6, Na_V1.7, Na_V1.8 and Na_V1.9, are present in most intra-epidermal free nerve endings. The presence of NCX2, together with multiple sodium channel isoforms, in free nerve endings may have important functional implications.

Oxygen sensing in the brain--invited article

Carotid body arterial chemoreceptors are essential for a normal hypoxic ventilatory response (HVR) and ventilatory acclimatization to hypoxia (VAH). However, recent results show that O(2)-sensing in the brain is involved in these responses also. O(2)-sensing in the rostral ventrolateral medulla, the posterior hypothalamus, the pre-Bötzinger complex and the nucleus tractus solitarius contribute to the acute HVR. Chronic hypoxia causes plasticity in the brain that contributes to VAH and represents another time domain of central O(2)-sensing. The cellular and molecular mechanisms of acute O(2)-sensing in the brain remain to be determined but they appear to involve O(2)-sensitive ion channels and heme oxygenase-2, which acts by a different mechanism than has been described for the carotid body. It is not known if plasticity in such mechanisms of acute central O(2)-sensitivity contributes to VAH. However, O(2)-sensitive changes in gene expression in the brain do contribute to VAH and demonstrate another mechanism of O(2)-sensing that is important for ventilatory control. This time domain of O(2)-sensing in the brain involves gene expression under the control of hypoxia inducible factor-1+/- (HIF-1+/- and potentially several HIF-1+/- targets, such as erythropoietin, endothelin-1, heme oxygenase and tyrosine hydroxylase.