Functional properties of regenerating sinus nerve fibres in the rabbit

Voltage-gated Na(+) channels in chemoreceptor afferent neurons--potential roles and changes with development

Carotid body chemoreceptors increase their action potential (AP) activity in response to a decrease in arterial oxygen tension and this response increases in the post-natal period. The initial transduction site is likely the glomus cell which responds to hypoxia with an increase in intracellular calcium and secretion of multiple neurotransmitters. Translation of this secretion to AP spiking levels is determined by the excitability of the afferent nerve terminals that is largely determined by the voltage-dependence of activation of Na(+) channels. In this review, we examine the biophysical characteristics of Na(+) channels present at the soma of chemoreceptor afferent neurons with the assumption that similar channels are present at nerve terminals. The voltage dependence of this current is consistent with a single Na(+) channel isoform with activation around the resting potential and with about 60-70% of channels in the inactive state around the resting potential. Channel openings, due to transitions from inactive/open or closed/open states, may serve to amplify external depolarizing events or generate, by themselves, APs. Over the first two post-natal weeks, the Na(+) channel activation voltage shifts to more negative potentials, thus enhancing the amplifying action of Na(+) channels on depolarization events and increasing membrane noise generated by channel transitions. This may be a significant contributor to maturation of chemoreceptor activity in the post-natal period.

Regeneration of baroafferents after implantation into different vessels

Regeneration of peripheral nerves involves an essential contribution by surrounding tissues. This study focuses on the role of the target tissue on the regeneration of afferent peripheral nerves. We hypothesized that nerves implanted into the appropriate target tissue regain their function, whereas they degenerate when implanted into a different tissue. Therefore, aortic nerves of rabbits were transected and implanted into arteries or veins, and their function and structure was reevaluated after 1.5, 3, and 10 months. In a subset of animals, the nerves were again severed and implanted into the other vessel. Twelve of 18 nerves implanted into arteries regained typical neurophysiological activity, but none of those implanted into veins. Two times even baroreflexes were elicited through the newly built nerve endings. The structure of the nerve endings implanted into arteries resembled baroreceptors, whereas no fiber growth was detected in veins. Morphometrically, the fiber number and diameter increased over the observed time period after implantation into arteries. Nerves implanted into veins, transected after 3 months, and then implanted into arteries also regained neurophysiological activity. Again, they rebuilt baroreceptors and significantly increased their fiber number and diameter. In conclusion, when severed baroafferents are implanted into arteries, they regenerate new baroreceptors and restore the normal myelination and fiber size of the nerve over time, whereas veins seem to inhibit nerve fiber sprouting and regeneration of severed fibers.

An important functional role of persistent Na+ current in carotid body hypoxia transduction

Systemic hypoxia in mammals is sensed and transduced by the carotid body into increased action potential (AP) frequency on the sinus nerve, resulting in increased ventilation. The mechanism of hypoxia transduction is not resolved, but previous work suggested that fast Na(+) channels play an important role in determining the rate and timing of APs (Donnelly, DF, Panisello JM, and Boggs D. J Physiol. 511: 301-311, 1998). We speculated that Na(+) channel activity between APs, termed persistent Na(+) current (I(NaP)), is responsible for AP generation that and riluzole and phenytoin, which inhibit this current, would impair organ function. Using whole cell patch clamp recording of intact petrosal neurons with projections to the carotid body, we demonstrated that I(NaP) is present in chemoreceptor afferent neurons and is inhibited by riluzole. Furthermore, discharge frequencies of single-unit, chemoreceptor activity, in vitro, during normoxia (Po(2) 150 Torr) and during acute hypoxia (Po(2) 90 Torr) were significantly reduced by riluzole concentrations at or above 5 microM, and by phenytoin at 100 microM, without significant affect on nerve conduction time, AP magnitude (inferred from extracellular field), and AP duration. The effect of both drugs appeared solely postsynaptic because hypoxia-induced catecholamine release in the carotid body was not altered by either drug. The respiratory response of unanesthetized, unrestrained 2-wk-old rats to acute hypoxia (12% inspired O(2) fraction), which was measured with whole body plethysmography, was significantly reduced after treatment with riluzole (2 mg/kg ip) and phenytoin (20 mg/kg ip). We conclude that I(NaP) is present in chemoreceptor afferent neurons and serves an important role in peripheral chemoreceptor function and, hence, in the ventilatory response to hypoxia.

Chemoreceptor discharges and cytochrome redox changes of the rat carotid body: role of heme ligands

In superfused in vitro rat carotid body, we recorded chemoreceptor discharges and the redox state of cytochromes simultaneously to identify the primary oxygen-sensing protein controlling transmitter release and electrical activity of the carotid sinus nerve. These parameters were tested under the influence of heme ligands such as oxygen, cyanide, 4-(2-aminoethyl)-benzenesulfonyl fluoride, and CO. During stimulation, there was an initial increase in discharge frequency followed by a decline or suppression of activity. Photometric changes lagged and were maintained as nerve activity decreased. Reducing mitochondrial cytochromes by cyanide or prolonged severe hypoxia, suppressed the chemoreceptor discharge. 4-(2-Aminoethyl)-benzenesulfonyl fluoride, a specific inhibitor of the phagocytic cytochrome b(558), also silenced the chemoreceptors after an initial excitation. CO increased the chemoreceptor discharge under normoxia, an effect inhibited by light, when the cytochromes were not reduced. When the discharges were depressed by severe hypoxia, exposure to light excited the chemoreceptors and the cytochromes were reduced. The rapidity of the chemosensory responses to light and lack of effect on dopamine release from type I cells led us to hypothesize that carotid body type I cells and the apposed nerve endings use different mechanisms for oxygen sensing: the nerve endings generate action potentials in association with membrane heme proteins whereas cytosolic heme proteins signal the redox state, releasing modulators or transmitters from type I cells.

Redox-dependent binding of CO to heme protein controls P(O2)-sensitive chemoreceptor discharge of the rat carotid body

Simultaneous recordings of chemoreceptor discharge and redox state of cytochromes have been carried out on the rat carotid body in vitro under the influence of carbon monoxide (CO) in order to identify the primary oxygen sensor protein controlling transmitter release and electrical activity. CO excites in a photolabile manner chemoreceptor discharge under normoxic conditions and inhibits under hypoxic conditions probably by binding to heme proteins. We hypothesize that type I cells and adjacent nerve endings of the carotid body tissue have a different apparatus with oxygen sensing heme proteins to cooperate for the generation of peripheral chemoreceptor response. Transmitter release from type I cells might be established in a redox dependent manner whereas membrane potential of nerve endings might be controlled by a heme coupled to ion channels.

Studies on the regenerated carotid sinus nerve of the rabbit

1. The central end of the distally cut left carotid sinus nerve was sutured to the tunica media of the external carotid artery, 1 cm cranial to the carotid bifurcation, in nineteen rabbits. The carotid body was removed in fourteen of these rabbits but left in situ in the remaining five. After 56-165 days of recovery a neuroma was identified at the site of the suture. Ventilatory reflexes mediated by both sinus nerves were tested and afferent activity recorded from the regenerated nerve. 2. Ventilatory reflex responses to hypoxia and sodium cyanide were abolished on sectioning the right sinus nerve, whilst the hypercapnic response was maintained. 3. Electrical stimulation of the regenerated sinus nerve caused hypotension and hyperventilation. These responses were attenuated compared to stimulation of the right sinus nerve. 4. A level of afferent activity equivalent to that found in non-regeneration experiments was recorded from all regenerated sinus nerves. Whole-nerve afferent activity was modulated by changes in carotid sinus blood pressure but not by changes in Pa,O2, Pa,CO2 (arterial O2 and CO2 pressures) or intracarotid injection of sodium cyanide. 5. A minimum of thirty single afferent fibres was identified in each experiment, the vast majority of which were mechanoreceptors. In only nine experiments were chemoreceptor fibres found and only twelve chemoreceptor fibres (1.7% of total) were identified in these nine experiments. In ten experiments no chemoreceptor fibres could be found. Leaving the carotid body in situ increased the incidence of chemoreceptive preparations. A small number of fibres unresponsive to mechanical stimulation and asphyxia was also identified. 6. The responses of regenerated chemoreceptor fibres to physiological and pharmacological stimuli were generally similar to those found in control carotid body preparations. Fibres unresponsive to mechanical stimulation and asphyxia did not respond to sodium cyanide, dopamine or isoprenaline; some of these fibres were excited by nicotine. 7. The receptive fields of mechanosensitive fibres were localized on or up to 2 cm away from the neuroma. Surface application of 20-40 microliters sodium cyanide (200 micrograms ml-1) was used to localize the receptive fields of seven of the twelve chemoreceptor fibres. All seven were localized to the site of the carotid body. 8. The neuroma and site of the carotid body were examined under light and electron microscopy. Glomus tissue was absent from the neuroma but was found at the site of the carotid body. 9. In conclusion, recovery of chemoreceptor function after carotid sinus nerve section appears to be associated with reinnervation of glomus tissue.

Restoration of hypoxic respiratory responses in the awake rat after carotid body denervation by sinus nerve section

The restoration of ventilatory responses to hypoxia after carotid body denervation was studied in twenty-eight awake rats. The respiratory depression seen in moderate hypoxia (partial pressure of inspired O2, PI,O2, 80-100 mmHg) 3 days after bilateral carotid sinus nerve section disappeared by day 10. By day 17 respiratory stimulation occurred at all levels of PI,O2 below 125 mmHg. The largest restored response, in severe hypoxia (PI,O2 50-60 mmHg), was approximately 55% of the pre-denervation response. The response showed little further change from day 17 to day 192. A comparison of the effect of bilateral section of the glossopharyngeal nerve and of the abdominal vagus 1 and 28 days after carotid sinus nerve section demonstrated that the restoration of hypoxic response resulted in part from an enhanced effect of the inputs from the secondary glomus tissue served by these nerves. A comparison of the effect of bilateral section of glossopharyngeal, abdominal vagal and aortic depressor nerves 1 and 28 days after carotid sinus nerve section demonstrated an increase of a residual hypoxic response which must result either from inputs from unidentified peripheral chemoreceptors or from central mechanisms. Bilateral sectioning of the aortic depressor nerves produced no additional effect on restored responses to sectioning glossopharyngeal and abdominal vagal nerves, providing further evidence against significant aortic body function in the rat. The studies support the hypothesis that central neural reorganization provides compensation for loss of carotid body function by enhancement of effects of normally subsidiary inputs.

Cat carotid bodies reinnervated by normal or foreign nerves

The carotid body of the cat was reinnervated with either its native nerve, the carotid sinus nerve (CSN, re-anastomosis), or a foreign nerve, the lingual branch of the IXth cranial nerve (LN, cross-anastomosis). In both types of preparations, regenerating axons from the LN or CSN readily penetrated carotid body parenchymal tissue, as demonstrated by axoplasmic transport of radiolabeled material from the petrosal (sensory) ganglion. Electron microscopy revealed nearly normal fiber invasion into lobules of glomus (type I) and sustentacular (type II) cells following reinnervation by either the foreign or native nerve. However, while the regenerated CSN fibers formed a normal complement of specialized axon terminals in contact with type I cells, the incidence of such terminals in LN reinnervated carotid bodies was reduced by over 90% (2-19 months survival time). This low incidence of specialized LN endings was correlated with reductions in the magnitude of the chemosensory discharge elicited in these preparations by asphyxia, NaCN or acetylcholine. These data suggest that chemosensitivity depends upon intimate association between glomus cells and afferent nerve endings; and that the ability to form such contacts may reside in particular axons whose incidence is higher in the CSN than in the LN.

Effects of ischemia on the function and structure of the cat carotid body

Interrupting the blood supply to the carotid body by ligating arteries of its vascular peduncle altered the chemoreceptive properties of the carotid nerve and produced structural changes in parenchymal (glomus and sustentacular) cells. The onset of ischemia was marked by an increase in the discharge of both A (myelinated) and C (unmyelinated) sensory fibers followed by depression and finally by receptor silence. The discharge of A-fibers disappeared after 30-50 min and that of C-fibers after 60-90 min. During ischemia of 15-60 min duration the threshold to pharmacological (NaCN, ACh) and 'natural' (hypoxic) stimuli progressively increased and was accompanied by reversible changes in the structure of parenchymal cells and nerve endings. Ischemia for 2 h or longer produced irreversible functional damage and disappearance of glomus and sustentacular cells from the carotid body. Following ischemic injury, nerve fibers regenerated and all responded to mechanical stimuli but only a few were stimulated by natural or pharmacological agents. Thus, parenchymal cells of the carotid body appear to be most important in transduction by allowing sensory fibers to respond to chemical stimuli.

Cytospecific properties of arterial chemosensory neurons

The carotid body of the cat was reinnervated either by the carotid branch or by the glossal branch of the IXth nerve and evaluated histologically and neurophysiologically. Regenerating foreign fibers re-established 90% fewer specialized terminals on glomus cells and displayed a greatly diminished chemosensory response, compared to axons of the regenerating carotid branch. Regenerating sensory neurons appear to develop chemosensitivity as a consequence of contact with glomus cells.

Adrenergic mechanisms and chemoreception in the carotid body of the cat and rabbit

1. The effect of beta-adrenergic and dopaminergic agonists and antagonists on the chemoreceptor response to graded hypoxia and hypercapnia was tested in nineteen cats and ten rabbits anaesthetized either with chloralose-urethane or pentobarbitone sodium, paralysed with pancuronium bromide and artificially ventilated.2. The inhibitory action of dopamine was confirmed. The inhibition following intra-arterial bolus injection was blocked by haloperidol; dopamine then excited and this excitation was blocked with propranolol. Adrenaline or noradrenaline caused a transient inhibition followed by a marked excitation. The inhibition was blocked with haloperidol and the excitation blocked with propranolol or metoprolol. Isoprenaline excited without inhibition and this was blocked with propranolol or metoprolol.3. A novel finding was that the chemoreceptor response to hypoxia was markedly reduced or even abolished with propranolol or metoprolol. The response was enhanced with a constant infusion of isoprenaline, adrenaline or noradrenaline in proportion to the degree of hypoxia, an effect mimicked by raising CO(2). The chemoreceptor response to hypoxia was similarly enhanced by haloperidol and depressed by a constant infusion of dopamine in proportion to the degree of hypoxia.4. The effect of these drugs on the chemoreceptor response to hypercapnia was less constant. In the majority of tests the aminergic agonists and antagonists caused a parallel shift of the CO(2) response curves in the same direction as the O(2) response curves and by amounts proportional to the degree of hypoxia. In some tests these drugs caused a change in the slope of the CO(2) response curves but only if P(a, O2) was less than 60 mmHg.5. One interpretation of these results is that hypoxia exerts a presynaptic action, causing the release of noradrenaline and dopamine from Type I cells, and that these substances act upon aminergic receptors on the sensory fibre, causing a change in potential and discharge frequency proportional to the rates of dopamine and noradrenaline release.6. An additional or alternative interpretation is that O(2) and CO(2) (the latter most probably acting on intracellular pH) alter the sensitivity of the aminergic receptors to their agonists.

Adrenergic mechanisms and chemoreception in the carotid body of the cat and rabbit

1. The effect of beta-adrenergic and dopaminergic agonists and antagonists on the chemoreceptor response to graded hypoxia and hypercapnia was tested in nineteen cats and ten rabbits anaesthetized either with chloralose-urethane or pentobarbitone sodium, paralysed with pancuronium bromide and artificially ventilated.2. The inhibitory action of dopamine was confirmed. The inhibition following intra-arterial bolus injection was blocked by haloperidol; dopamine then excited and this excitation was blocked with propranolol. Adrenaline or noradrenaline caused a transient inhibition followed by a marked excitation. The inhibition was blocked with haloperidol and the excitation blocked with propranolol or metoprolol. Isoprenaline excited without inhibition and this was blocked with propranolol or metoprolol.3. A novel finding was that the chemoreceptor response to hypoxia was markedly reduced or even abolished with propranolol or metoprolol. The response was enhanced with a constant infusion of isoprenaline, adrenaline or noradrenaline in proportion to the degree of hypoxia, an effect mimicked by raising CO(2). The chemoreceptor response to hypoxia was similarly enhanced by haloperidol and depressed by a constant infusion of dopamine in proportion to the degree of hypoxia.4. The effect of these drugs on the chemoreceptor response to hypercapnia was less constant. In the majority of tests the aminergic agonists and antagonists caused a parallel shift of the CO(2) response curves in the same direction as the O(2) response curves and by amounts proportional to the degree of hypoxia. In some tests these drugs caused a change in the slope of the CO(2) response curves but only if P(a, O2) was less than 60 mmHg.5. One interpretation of these results is that hypoxia exerts a presynaptic action, causing the release of noradrenaline and dopamine from Type I cells, and that these substances act upon aminergic receptors on the sensory fibre, causing a change in potential and discharge frequency proportional to the rates of dopamine and noradrenaline release.6. An additional or alternative interpretation is that O(2) and CO(2) (the latter most probably acting on intracellular pH) alter the sensitivity of the aminergic receptors to their agonists.

Regeneration of barosensitivity in the aortic nerve of cats when severed and transposed on various vessels in the neck

1. In cats, regeneration of baroreceptors was studied in the aortic nerve after its peripheral end had been severed and sutured into an adventitial pouch of the common carotid artery (ten cats) on the wall of the external jugular vein (two cats) and into a cervical muscle (one cat). 2. Three weeks to four months after the initial operation, typical pulse synchronous baroreceptor activity was found in the whole desheathed nerve remnant in three of thirteen animals. This activity could be abolished by section of the vago-sympathetic bundle distal to the implantation site. 3. In every cat, including the one with the nerve in cervical muscle, bursts of spikes could be repeatedly evoked in the whole nerve upon stroking or distortion of the neuroma formed at the site of implantation. 4. Additionally, single barosensitive fibres could be teased from the aortic nerves in seven of the ten cats whose nerves has been sutured into the arterial wall. 5. Finally, distension sensitive single afferents with predominantly phasic response characteristics were found in the nerves on jugular veins. 6. Transposed aortic nerves therefore, are apparently capable of reinnervating the originally severed endings in some cases while forming a mechanosensitive neuroma in every case. When in contact with a vessel wall some truly barosensitive endings can also develop which in most instances have a more phasic than tonic response to pressure changes.

Transduction mechanisms in carotid body: glomus cells, putative neurotransmitters, and nerve endings

Carotid body chemoreceptors are activated by low PO2, high PCO2, acidity, increased temperature, and tonicity. These receptors are important in homeostasis and mediate their reflex effects on the CNS through sensory discharges of the carotid (sinus) nerve. The receptor complex is formed by glomus (type I) cells and carotid nerve endings, which, morphologically, appear to form a sensory synapse. The junction between glomus cells and nerve endings is enveloped by processes of sustentacular (type II) cells. The mechanisms of chemoreceptor transduction are complex; there is no agreement about the identity of the primary receptor element (glomus cell or nerve terminal) or what mechanisms are responsible for the onset of the sensory discharge in the carotid nerve. There is increasing evidence that integrity of the glomus cell is essential for normal transduction and that the receptor synapse described by morphologists may be functionally active. There is no conclusive evidence, however, that the glomus cell is the primary site of sensory transduction. Stimuli act on the glomus cell to release "transmitter" and/or "modulator" substances; but it is unknown if the released chemicals are directly responsible for the accompanying change in sensory impulse frequency or merely modify an already ongoing discharge. Interactions between glomus cells and nerves may be complicated enough to make it very difficult to resolve this question.