H.-J. Häbler

Activation of unmyelinated afferent fibres by mechanical stimuli and inflammation of the urinary bladder in the cat.

1. We examined the functional properties of unmyelinated primary afferent neurones innervating the pelvic viscera in twenty‐five anaesthetized cats. The axons were isolated from the intact dorsal root and the intact or chronically de‐efferented ventral root of the segment S2. All units were electrically identified with electrical stimulation of the pelvic nerve. 2. The responses of the neurones were studied with natural stimulation of the urinary bladder using innocuous and noxious increases of intravesical pressure and at the onset of an acute artificial inflammation induced by intraluminal injection of mustard or turpentine oil. 3. Out of 297 unmyelinated afferent units isolated from the dorsal root, seven were excited by an increase of the intravesical pressure during contractions and distension of the urinary bladder. These units were silent when the bladder was empty and had thresholds of 30‐50 mmHg which are presumed to be noxious. Further increases of the intravesical pressure were accurately encoded by the discharge rate of the fibres. Out of sixty‐eight unmyelinated afferent units isolated from the ventral root none was activated by these stimuli. 4. Intraluminal injection of mustard oil excited mechanosensitive units at short latency. The discharge was not closely related to changes of the intravesical pressure and the units displayed on‐going activity after the irritant had been removed. This observation suggests that the units had also chemosensitive properties and that the receptive endings were located in the bladder wall. 5. In sixteen cats ninety‐five afferent fibres that were not activated by noxious mechanical stimuli of the urinary bladder were systematically tested with intraluminal injections of mustard oil. This excited 7/67 dorsal root units and 4/28 ventral root units with short latency. Intraluminal application of turpentine oil, tested on twenty‐six afferents in four animals, did not produce a rapid excitation. 6. Following the induction of an inflammation some previously non‐mechanosensitive units started to respond to changes of intravesical pressure in the biologically relevant pressure range of the urinary bladder. 7. In conclusion, a small subpopulation (2.4%) of unmyelinated visceral afferents responds to high, presumably noxious, intravesical pressure and intraluminal application of chemical irritants. Acute inflammation excites a larger proportion of afferents (9.5%) that are not activated by acute noxious mechanical stimulation of the normal urinary bladder. In the inflamed bladder some previously non‐mechanosensitive units started to respond to increases of intravesical pressure. These novel types of chemosensitive receptors may contribute considerably to the pathogenesis of visceral pain states.

Spontaneous activity of axotomized afferent neurons after L5 spinal nerve injury in rats

Abstract After mechanical injury of a peripheral nerve some axotomized afferent neurons develop spontaneous activity, which is thought to trigger abnormal pain behavior in rats and neuropathic pain in humans. Here, we analysed the ectopic activity in axotomized afferent fibers recorded from the L5 dorsal root in different time periods after L5 spinal nerve lesion and the effects of sympathectomy on it. The following results were obtained: (1) Up to 6 hours after spinal nerve transection there was almost no spontaneous activity in axotomized afferents, except short‐lasting injury discharges at the time of transection; (2) Three to 8 days following spinal nerve lesion, the rate of spontaneous activity was 7.3±7.7 imp/s (mean±SD, median 5.0 imp/s, n=204); 41.6% of the spontaneously active afferent neurons exhibited a bursting pattern with interspike intervals of 32.4±18.3 ms; (3) Twenty to 53 days after nerve lesion the rate of spontaneous activity had decreased significantly to 3.4±4.3 imp/s (median 2.6 imp/s, n=120). The frequency of bursting and non‐bursting neurons remained roughly the same; (4) In sympathectomized rats, 15–45 days following spinal nerve lesion, the mean discharge rate was 3.8±4.3 imp/s (median 2.3 imp/s, n=255). However, the percentage of bursting neurons and the intraburst frequency decreased significantly; (5) Spontaneous activity occurred in afferent A‐fibers but not in afferent C‐fibers. These results suggest that ectopic activity in axotomized afferent neurons develops within the first days after L5 spinal nerve lesion, decreases with time and is only marginally dependent on the sympathetic innervation. There was a positive correlation between this ectopic activity and the allodynia‐like behavior in spinal nerve‐lesioned rats.

Myelinated primary afferents of the sacral spinal cord responding to slow filling and distension of the cat urinary bladder.

1. A total of sixty‐five sacral afferent neurones with myelinated fibres supplying the urinary bladder was recorded from the sacral roots S2 in anaesthetized cats. All afferent units were identified with electrical stimulation of the pelvic nerve. The discharge properties were quantitatively evaluated using slow filling at rates of 1‐2 ml min‐1 and isotonic distension to preset pressure levels. Eight afferents were studied prior to and after acute sacral de‐efferentation of the urinary bladder. 2. All afferent units were silent when the bladder was empty and responded in a graded manner to an increase of intravesical pressure. During slow filling the level of afferent activity correlated closely with the level of the intravesical pressure. All afferents behaved like slowly adapting mechanoreceptors with both a dynamic and static component of their discharge. With the exception of two units the intraluminal pressure threshold was below 25 mmHg. Thus virtually all myelinated afferents respond in the pressure range that is reached during a non‐painful micturition cycle. 3. The stimulus‐response functions of the afferents were similar regardless of whether intravesical pressure was increased by slow filling or by distension. However, during slow filling stimulation response functions often exhibited steeper slopes between 5 and 25 mmHg indicating that relatively small changes of intravesical pressure result in large changes of afferent activity. Nevertheless, all units displayed monotonically increasing stimulus response functions throughout the innocuous and noxious pressure level. 4. The stimulus‐response functions of the afferent neurones did not change after acute de‐efferentation of the urinary bladder, although the rapid phasic fluctuations of afferent activity that are produced by small contractions of the urinary bladder under normal conditions largely disappeared. This means that contractions and distension activate the afferent endings by a common mechanism. 5. It is concluded that the myelinated sacral afferents of the urinary bladder form a homogeneous population which encodes all information necessary for the normal regulation of this organ. Furthermore, this set of afferents mediates all sensations which may reach consciousness within a normal micturition cycle.

Activation of unmyelinated afferents in chronically lesioned nerves by adrenaline and excitation of sympathetic efferents in the cat

Neuroma-in-continuity was experimentally produced in cats by suturing the proximal stump of a transected hindlimb cutaneous nerve to the distal stump of the transected tibial nerve. Eleven to 20 months afterwards single non-myelinated fibres were identified by electrical recording from filaments of the nerves proximally from the neuroma. When tested with repetitive electrical stimulation of the lumbar sympathetic trunk (LST) or i.v. injections of adrenaline, C-fibres in 14 out of 30 filaments were excited by at least one of these stimuli (12 by adrenaline, 7 by stimulation of LST). The fibres responding to LST stimulation were already substantially excited by low frequency stimulation (1-2 Hz). During systemic hypoxia known to result in reflex increases of efferent sympathetic activity responses were elicited in 4 out of 6 C-fibres. Thus, stimuli simulating physiological conditions of sympathetic activity can excite afferent C-fibres in a chronically lesioned nerve.

SILENT AFFERENTS : A SEPARATE CLASS OF PRIMARY AFFERENTS ?

1. In recent years, fine sensory nerve fibres have been detected that are not excited by physiological stimuli, even at potentially tissue damaging intensities. These silent afferents are known to supply knee joint, skin and viscera; in the last case, silent afferents seem to be particularly numerous.

A novel type of unmyelinated chemosensitive nociceptor in the acutely inflamed urinary bladder

Single primary afferents supplying the cat urinary bladder were electrophysiologically recorded in the sacral dorsal roots. Under normal conditions, afferents were not spontaneously active when the bladder was empty and innocuous increases of intravesical pressure excited mainly thin myelinated, lowthreshold mechanoreceptors. Less than 2.5% of all unmyelinated visceral afferents responded to a mechanical stimulus, but all had high, presumably noxious thresholds. During an acute inflammation induced with intravesical injections of 2.5% mustard oil or 50–100% turpentine oil both populations developed resting activity and changed their mechanosensitive properties. Out of the many unmyelinated afferents without appreciable mechanosensitivity, an entirely new subpopulation was activated by chemical irritants during an acute inflammation. Subsequently, some of these chemosensitive receptors started to display new mechanosensitive properties. This novel population of sensory neurones may contribute considerably to the pathogenesis of visceral pain states including reflex disturbances of bladder motility.

Dorsal root section elicits signs of neuropathic pain rather than reversing them in rats with L5 spinal nerve injury

&NA; Mechanical allodynia‐ and hyperalgesia‐like behavior which develops in rats after L5 spinal nerve lesion has been suggested to be due to ectopic activity in the lesioned afferent neurons originating at the lesion site and/or in the dorsal root ganglion because it is eliminated by section of the dorsal root. Here we reevaluated the effect of a dorsal rhizotomy in rats after L5 spinal nerve lesion. Using calibrated von Frey hairs, paw withdrawal threshold to single stimuli and paw withdrawal incidence to repetitive stimulation were tested before and after nerve section. Neuropathic pain behavior of similar time course and magnitude also developed after cutting the L5 dorsal root, and L5 spinal nerve lesion‐induced abnormal behavior could not be reversed by dorsal rhizotomy. The neuropathic pain behavior elicited by dorsal root section also developed when impulse conduction in the dorsal root axons was blocked during rhizotomy by a local anesthetic, i.e. when the immediate injury discharge was prevented from reaching the spinal cord. These results challenge the widely accepted idea that neuropathic pain behavior developing after spinal nerve lesion is dependent on ectopic activity in the lesioned afferent neurons. However, the present results do not rule out the possibility that after the two nerve lesions the mechanisms generating neuropathic pain behavior are different. After dorsal rhizotomy neuropathic pain behavior may be related to deafferentation whereas after spinal nerve lesion it may be caused by ectopic activity.

Interaction of sympathetic vasoconstriction and antidromic vasodilatation in the control of skin blood flow

Abstract We studied the interaction between the vasoconstriction evoked by postganglionic sympathetic neurones (sympathetic vasoconstriction) and the vasodilatation mediated by small-diameter afferent neurones (antidromic vasodilatation) in hairless skin of anaesthetized rats kept under controlled conditions. In all animals both the lumbar sympathetic trunk (LST) and the ipsilateral dorsal root (DR) L5 were surgically exposed, sectioned and electrically stimulated using different protocols. This experimental approach results in the exclusive and selective activation of sympathetic efferents and primary afferents respectively. Blood flow responses were measured using laser Doppler flowmetry. Sectioning the LST resulted in a pronounced increase in cutaneous blood flow by 112±15% (mean±SEM, n=25) indicating that ongoing sympathetic vasoconstrictor activity had been abolished. When a brief antidromic vasodilatation was produced by DR stimulation with 10–15 pulses at 1 Hz with C-fibre intensity during a sustained sympathetic vasoconstriction, peak blood flow reached preconstriction levels at LST stimulation frequencies of ≤3 Hz. By contrast, antidromic vasodilatation was reduced at sympathetic stimulation frequencies of ≥5 Hz and absent when stimulating the LST with 20 Hz. A similar response characteristic was obtained when LST and DR stimulation were started simultaneously. Continuous DR stimulation with 0.1 Hz evoked a substantial increase in cutaneous blood flow by 38±10% (mean±SEM, n=8) to a new baseline level. When sympathetic vasoconstriction was elicited on this background DR stimulation, the responses were smaller at all sympathetic frequencies. However, the maximum decrease in blood flow was significantly smaller than the controls at LST stimulation with ≤3 Hz but not at higher frequencies. We conclude that sympathetic vasoconstriction and antidromic vasodilatation are competitive influences in the control of cutaneous blood flow. At low levels of cutaneous sympathetic vasoconstrictor activity, which probably prevail under resting conditions in the absence of cold stress, antidromic vasodilatation overrides sympathetic vasoconstriction. At high levels of cutaneous sympathetic activity, which may be reached in normal life under the conditions of severe cold, sympathetic vasoconstriction can suppress antidromic vasodilatation almost totally.

Classification of preganglionic neurones projecting into the cat cervical sympathetic trunk.

1. The spontaneous and reflex activity patterns of 167 single preganglionic axons dissected from the cervical sympathetic trunk were examined in chloralose‐anaesthetized cats. Each neurone was classified into one of four major groups, on the basis of three principal criteria: the presence or absence of significant cardiac rhythmicity of the activity, the response to noxious stimulation of the skin, and the coupling of its activity to central inspiratory drive (phrenic nerve activity). Most neurones were also subjected to additional tests, which included carotid chemoreceptor stimulation, nasopharyngeal probing, systemic hypercapnia (ventilation with 8% CO2), hyperventilation, adrenaline‐induced blood pressure rises and retinal illumination. 2. Group I neurones (n = 69; 41%) showed significant cardiac rhythmicity, indicating strong baroreceptor control. Most (54/69) were excited by noxious stimuli, the rest being unaffected. Their activity showed variable degrees of excitatory coupling to the central inspiratory drive, and was enhanced by hypercapnia (35/39). Their responses to stimulation of arterial chemoreceptors (12/15) and nasopharyngeal receptors (24/35) were excitatory. 3. Group II neurones (n = 39; 23%) were inhibited by noxious stimulation of skin. With nine exceptions, they showed no significant cardiac rhythmicity, although they were weakly inhibited by an adrenaline‐induced blood pressure rise. Their coupling to central inspiratory drive was weak or absent, and their responses to hypercapnia and hyperventilation were variable. By contrast to other groups, they were inhibited by both chemoreceptor stimulation (9/10) and nasopharyngeal stimulation (17/18). 4. Group III neurones (n = 33; 20%) showed no significant cardiac rhythmicity, but their activity was closely coupled to central inspiratory drive. They were inhibited by hyperventilation (9/9) and excited by hypercapnia (20/21), but only fired during the central inspiratory phase and sometimes during late expiration. Their responses to noxious stimulation (28/33), chemoreceptor stimulation (8/11) and nasopharyngeal probing (24/24) were excitatory, but the induced activity was ‘gated’ by the respiratory cycle, occurring primarily during inspiration and avoiding the postinspiratory phase. 5. Group IV neurones (n = 26; 16%) showed no significant cardiac or respiratory related activity and were either excited (n = 22) or unaffected (n = 4) by noxious stimuli. One of the latter and three group II neurones were inhibited by retinal illumination; thirty‐one other neurones of all classes were unaffected. 6. Approximately 45% of thoracic sympathetic neurones were silent under the experimental conditions. About 25% of these could be recruited by systemic hypercapnia leaving 34% without spontaneous and reflex activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory modulation of the activity in sympathetic neurones supplying muscle, skin and pelvic organs in the cat.

1. The respiratory‐related modulation of activity in neurones of the lumbar sympathetic outflow to skeletal muscle, skin and pelvic organs was investigated in anaesthetized, paralysed and artificially ventilated cats, using single‐ and multi‐unit recordings. The activity of the neurones was analysed with respect to the phrenic nerve discharge under various experimental conditions. 2. Neurones tentatively classified as muscle vasoconstrictor and visceral vasoconstrictor neurones exhibited two activity peaks, one caused by baroreceptor unloading during the declining phase of the second order blood pressure waves and a respiratory drive‐dependent peak in parallel with inspiration. The two peaks were separated by depressions of activity in early inspiration and post‐inspiration. After cutting vagus and buffer nerves the activity peak during inspiration remained and was followed and sometimes preceded by a depression of activity. 3. The majority of the neurones tentatively classified as cutaneous vasoconstrictor neurones exhibited no respiratory modulation in their activity. Others exhibited an activity peak in expiration, an activity peak in inspiration, or a respiratory profile similar to that in muscle vasoconstrictor neurones. During increased respiratory drive (induced by hypercapnia) some neurones with unmodulated activity changed to an inspiratory or an expiratory pattern. Neurones discharging predominantly in inspiration projected preferentially to hairless skin. 4. Neurones which were tentatively classified as sudomotor neurones discharged predominantly in early expiration. 5. Some preganglionic neurones which were tentatively classified as motility‐regulating neurones discharged during expiration. The majority of these neurones disclosed no respiratory modulation of their activity. 6. The study shows that different types of neurone of the lumbar sympathetic system exhibit distinct patterns of respiratory modulation in their activity. We conclude that the type and degree of central coupling between respiratory system and sympathetic nervous system may vary according to the destination of the sympathetic neurones.