Increase of blood flow in skin and spinal cord following activation of small diameter primary afferents

Activation of unmyelinated primary afferents produces vasodilatation and plasma extravasation in the skin. Here, using the laser Doppler technique to measure changes in blood flow and the Evans blue technique for quantification of plasma extravasation, we have asked whether the stimulation of C-fibre precipitates the same phenomena in the spinal cord. Our results show that there is an increase of blood flow, but no extravasation in the ipsilateral lumbar enlargement of the spinal cord following supramaximal electrical stimulation of the sciatic nerve. The blood flow increases were small and short-lived compared with those seen in skin, and could be completely explained by concomitant blood pressure changes. Hence, whilst the same substances are apparently released from the peripheral and central terminals of primary afferent fibres, their ability to produce vasodilatation and extravasation is absent or severely restricted in the spinal cord.     

The effects of electrical stimulation of A and C visceral afferent fibres on the excitability of viscerosomatic neurones in the thoracic spinal cord of the cat

Single unit electrical activity has been recorded from viscerosomatic neurons in the lower thoracic spinal cord of decerebrate spinalized cats. The responses of the cells to electrical stimulation of afferent fibres in the splanchnic (SPLN) nerve and the effects of repetitive stimulation of somatic and visceral afferent C-fibres have been studied. Four groups of viscerosomatic neurones could be distinguished according to the type of visceral afferent input of the cells: (1) A-only cells (32.9%), driven only by stimulation of A delta afferent fibres in the SPLN nerve; (2) C-only cells (3%), driven only by stimulation of C afferent fibres in the SPLN nerve; (3) A + C cells (45.7%), driven by both A delta and C afferent fibres in the SPLN nerve; and (4) A + C? cells (18.6%), driven by A delta visceral afferents and showing signs of responsiveness to C-fibres though lacking a distinct response volley to visceral C-fibre activation. Two cells of the A + C group and located in lamina I of the dorsal horn responded to SPLN nerve stimulation in a manner consistent with the afferent fibre composition of the nerve, that is, showed evidence of strong monosynaptic links with SPLN afferent C-fibres and weaker responses to SPLN A delta afferents. Excitability changes of viscerosomatic neurones ('wind up', 'wind down' and changes in background activity) were also observed in the majority of neurones following electrical stimulation of somatic and of visceral afferent C-fibres.(ABSTRACT TRUNCATED AT 250 WORDS)     

Distribution of small-diameter primary afferents containing oligosaccharide residues recognized by monoclonal antibody LA4 in the dorsal horn of the cat

Small-diameter primary afferents expressing oligosaccharide residues recognized by the monoclonal antibody LA4 constitute a newly described primary afferent population in the cat. LA4-immunoreactive primary afferents are found mainly in lamina II (LII) at all levels of the cat spinal cord. However, within LII immunoreactive fibers are most concentrated in outer LII and practically absent from the inner third of the lamina. In addition at sacral levels, a lateral bundle of immunoreactive fibers enters laminae V–VII. The distribution pattern and the type of oligosaccharide expressed by LA4-immunoreactive fibers suggest that they may be homologous to primary afferents shown in the rat to contain fluoride-resistant acid phosphatase activity. However, in the rat spinal cord the whole of inner LII is the main projection area of LA4-immunoreactive fibers (Alvarez et al.,J. Neurocytol., 18 (1989) 611–629). This difference in the location of primary afferents within LII may reflect a species difference in the physiology of LII inner.     

Some characteristics of baclofen-evoked responses of primary afferents in frog spinal cord

Baclofen has been shown to be a selective agonist for a subclass of GABA receptors (GABAB) in many regions of the vertebrate nervous system. On the intraspinal terminals of dorsal roots (DRT), it evokes a pure hyperpolarizing response. We have previously shown that the response of DRT to GABA and some of its analogs (e.g. kojic amine) in isolated frog spinal cord is dual in nature, consisting of a bicuculline-sensitive depolarizing component and a bicuculline-resistant hyperpolarizing component. Under the working hypothesis that the hyperpolarizing component of the GABA-evoked response is mediated by the activation of GABAB receptors, we have examined, using the sucrose gap technique, some characteristics of the response of DRT to baclofen. We have found that this response is stereospecific (L-baclofen being about 100 times more potent than D-baclofen), dependent on [K]o (response amplitude inversely related to [K]o), blocked by barium (0.5 mM causing a reduction of the response amplitude to 37% of control), and is not significantly affected by 4-aminopyridine, nor by inorganic calcium channel blockers (manganese, cobalt, cadmium). Some proposed GABAB antagonists (delta-aminovaleric acid, delta-aminolaevulinic acid, phaclofen) are also rather ineffective at blocking it. These results are therefore consistent with the notion that the baclofen-evoked response of DRT is mediated by an increase in conductance to potassium ions.     

Fusimotor reflexes to antagonistic muscles simultaneously assessed by multi-afferent recordings from muscle spindle afferents

Several single agonist/antagonist primary muscle spindle afferents were simultaneously recorded in chloralose anaesthetized cats. It was shown that their dynamic and static sensitivity to sinusoidal muscle stretches could be increased or decreased via the fusimotor system by extension and flexion of the contralateral hind limb as well as by stretch of ipsilateral muscles and stimulation of ipsilateral skin nerves. The results seem to support the hypothesis that the primary muscle spindle afferents convey complex multisensory messages to the central nervous system (CNS).     

The effects of antidromic discharges on orthodromic firing of primary afferents in the cat

This study investigated the effects of antidromically conducted nerve impulses on the transmission of orthodromic volleys in primary afferents of the hindlimb in decerebrated paralyzed cats. Two protocols were used: (A) Single skin and muscle afferents (N=20) isolated from the distal part of cut dorsal rootlets (L7-S1) were recorded while stimulation was applied more caudally. The results showed that during the trains of three to 20 stimuli, the orthodromic firing frequency decreased or ceased, depending on the frequency of stimulation. Remarkably, subsequent to these trains, the occurrence of orthodromic spikes could be delayed for hundreds of ms (15/20 afferents) and sometimes stopped for several seconds (10/20 afferents). Longer stimulation trains, simulating antidromic bursts reported during locomotion, caused a progressive decrease, and a slow recovery of, orthodromic firing frequency (7/20 afferents), indicating a cumulative long-lasting depressing effect from successive bursts. (B) Identified stretch-sensitive muscle afferents were recorded intra-axonally and antidromic spikes were evoked by the injection of square pulses of current through the micropipette. In this case, one to three antidromic spikes were sufficient to delay the occurrence of the next orthodromic spike by more than one control inter-spike interval. If the control inter-spike interval was decreased by stretching the muscle, the delay evoked by antidromic spikes decreased proportionally. Overall, these findings suggest that antidromic activity could alter the mechanisms underlying spike generation in peripheral sensory receptors and modify the orthodromic discharges of afferents during locomotion.     

Naloxone effects on the blood pressure response induced by thin-fiber muscular afferents

Naloxone effects on the blood pressure level and on the blood pressure responses induced by thin-fiber muscular afferent stimulation were studied in anesthetized, bilaterally vagotomized and carotid sinus nerve-denervated dogs under artificial ventilation. Repetitive pulses of 8 Hz with various intensities were applied to the gastrocnemius nerve for 1 min while monitoring the compound action potentials. The mean arterial pressure significantly(P < 0.001) rose by10.95 ± 1.78mmHg (mean ± S.E.) about 5 min after a naloxone injection. Compared with the reflexive response in the control period, the depressor effect significantly decreased by3.80 ± 1.06mHg and the pressor effect significantly increased by3.63 ± 0.73mHg for 30 min after the injection of naloxone. No correlation was found between naloxone effects on the blood pressure level and on the reflex response, indicating an involvement of different mechanisms with these naloxone effects. We suggest that endogenous opiates might participate in the regulation of the blood pressure level, as well as of the blood pressure responses caused by thin-fiber muscular afferents.     

Role of primary afferents in spinal cord stimulation-induced vasodilation: characterization of fiber types

Selected patients with peripheral vascular disease can be treated with spinal cord stimulation (SCS) to improve blood flow in the limbs. However, the mechanisms producing these effects remain unclear. The present study was designed to investigate if SCS produces cutaneous vasodilation via antidromic activation of the unmyelinated C-fibers and/or the small myelinated fibers. SCS was applied to anesthetized rats with a ball electrode at the L2-L3 spinal level. In Protocol 1, effects of capsaicin were examined. Blood flow changes in the hindpaw induced by SCS were measured in the footpad with laser Doppler flowmeters. Topical application of capsaicin (1%) on the tibial nerve did not affect SCS-induced vasodilation at 30 and 60% of motor threshold (MT). However, the duration of vasodilation induced by SCS at 90% MT and at 10 times MT was significantly reduced after capsaicin application on the tibial nerve. In Protocol 2, antidromic compound action potentials (CAPs) of the tibial nerve were recorded in response to SCS. CAPs of the large and the small myelinated afferent fibers were observed in response to SCS at all intensities. However, even with SCS at ten times MT, CAPs of C-fibers could not be detected in the tibial nerve. In Protocol 3, antidromic CAPs of the dorsal root were measured in response to SCS. Antidromic CAPs of C-fibers in dorsal roots were evoked by SCS at >or=90% of MT. It is concluded that SCS-induced vasodilation at or=90% of MT may also involve antidromic activation of some unmyelinated C-fibers.     

Differing action potential shapes in rat dorsal root ganglion neurones related to their substance P and calcitonin gene-related peptide immunoreactivity

Intracellular electrophysiological recordings were made in vitro at 36.5°C from lumbar (L4 to L6) dorsal root ganglion neurones of 6–8-week-old female rats. Electrophysiological properties were recorded prior to intracellular injection with fluorescent dye. The following showed substance P-like immunoreactivity (SP-LI): 8/19 C-fibre cells, 6/26 Aδ cells, and 0/52 Aα/β cells. In C-fibre neurones, there were no significant differences in action potential (AP) characteristics between those with SP-LI and those without. In contrast, the Aδ neurones with SP-LI had significantly deeper and longer afterhyperpolarisations (AHPs), but their AP durations did not differ from those without SP-LI. Both SP-LI and calcitonin gene-related peptide-LI (CGRP-LI) were examined on 18 Aδ and 11 C cells. Most (7/8) neurones with SP-LI also showed CGRP-LI, but only 7/21 neurones without SP-LI showed CGRP-LI. One C cell showed SP-LI but no CGRP-LI. Neurones with neither peptide (−/−), with only CGRP (−/CGRP) or with both peptides (SP/CGRP) were compared (n = 28). The numbers in each group were, respectively, 5, 2, and 3 with C-fibres and 9, 5, and 4 with Aδ-fibres. The peptide content and AP shape were related in Aδ neurones. Most −/− Aδ neurones had short APs and short AHPs; most −/CGRP neurones had long APs and long AHPs, and SP/CGRP neurones had short APs with deep, long AHPs. There was a positive correlation between log₁₀ of the area under the AP (AP area) and log₁₀ AHP duration in Aδ neurones. All SP- and most CGRP-containing Aδ neurones had AP shapes similar to those previously described for nociceptive neurones. However, a few without peptide also showed such properties, raising the possibility that some nociceptive neurones did not express these peptides. J. Comp. Neurol. 388:541–549, 1997. © 1997 Wiley-Liss, Inc.     

Inhibition of withdrawal responses by pelvic nerve electrical stimulation

In urethane-anesthetized rats, the compound action potential of the pelvic nerve was found to consist of three different waves, two in the Aδ fiber and one in the C-fiber range of conduction velocity. Electrical stimulation of the pelvic nerve produced a complete inhibition of the withdrawal response to noxious foot pinch or foot compression. The electromyographic (EMG) activity of the contralateral posterior biceps muscle was used to record the withdrawal response. The withdrawal response inhibition was related to the duration and the frequency of electrical stimulation. Low (5–10 Hz) and high (100–300 Hz) frequencies were ineffective in inhibiting the withdrawal response, whereas intermediate frequencies (20–80 Hz) produced a complete inhibition of the withdrawal response. Short (300 ms) trains of stimulation inhibited the withdrawal response only during the stimulation period. Longer trains of stimulation (500 ms-10 s) produced long-lasting inhibition of the response to noxious stimulation. The inhibition persisted for up to 20 s after the end of electrical stimulation of the pelvic nerve. Aδ fiber stimulation was adequate to inhibit the withdrawal response in most (15 out of 17) of the animals. However, Aδ plus C-fiber stimulation was needed to inhibit the response to noxious stimulation in two animals. In addition to inhibiting the response to noxious stimulation, pelvic nerve electrical stimulation reflexively activated abdominal muscles. On the basis of present findings using electrical stimulation, it can be suggested that, in the rat, Aδ and C-visceral afferents of the pelvic nerve mediate the analgesic effect of vaginocervical probing pelvic and Aδ afferents the contraction of abdominal muscles in the fetus-expulsion reflex.     

The development of the relationship between dorsal root afferents and motoneurons in the larval bullfrog spinal cord

The relationship of dorsal root afferents to motoneuron somata and dendrites was studied by labelling dorsal and ventral roots of the tadpole lumbar enlargement with HRP at different stages of hindlimb development. Procedures were used which allowed for sequential light and electron microscopic analysis to determine whether close appositions between labelled elements represented synaptic contacts. Lateral motor column (LMC) motoneuron dendrites grow first into the lateral funiculus, and later begin arborizing within the spinal gray, concurrent with the arrival of developing dorsal root afferent fibers. Mature-appearing synaptic contacts between dorsal root afferents and motoneuron dendrites are established first on distal dendrites, and are observed on progressively more proximal dendrites as hindlimb development proceeds. Migrating motoneurons were also labelled in some animals. Distinct dorsal and ventral migratory pathways were noted; cells migrating dorsally were contacted by developing dorsal root afferents. Migrating motoneurons were associated with radially oriented processes, and were often closely apposed to other cells. The coincident development of dorsal root projections and the motoneuron dendrites which these fibers innervate in the adult, as well as the interaction between these two systems during cell migration, suggest that these two systems may be interdependent in establishing their normal relationship during development.     

Excitation of mouse motoneurones by GABA-mediated primary afferent depolarization

Observations on the reflex properties of a mouse spinal cord preparation in vitro are reported. The findings show that the synaptically evoked, GABA-mediated, discharge of action potentials in primary afferent fibres, monitored as the dorsal root reflex, may lead to the excitation of motoneurones. Subthreshold, bicuculline-sensitive increases in motoneuronal excitability, followed by prolonged inhibition, may be seen in preparations in which the delayed reflex is not seen. Thus, primary afferent depolarization may both increase motoneuronal excitability and also cause presynaptic inhibition of afferent input.     

Epinephrine- and norepinephrine-evoked potential changes of frog primary afferent terminals: Pharmacological characterization of α and β components

The effects of superfused epinephrine (E) and norepinephrine (NE) on the membrane potential of primary afferent fibers of the isolated frog spinal cord were studied by sucrose gap recordings from the dorsal root. In all preparations both E and NE, applied in concentrations ranging from 0.1 microM to 1.0 mM, produced a hyperpolarization of afferent terminals. In many instances this was followed by a slow depolarization and, in a small number of cords, a small depolarization preceded the increase in membrane potential. E- and NE-induced hyperpolarizations were blocked by the selective alpha 2-antagonists yohimbine and piperoxan, but not by the selective alpha 1-antagonists prazosin and corynanthine or by the beta-blockers propranolol and sotalol. The alpha 2-agonists clonidine, alpha-methylnorepinephrine and guanabenz also hyperpolarized terminals, causing a change in potential that was reduced by yohimbine and piperoxan. Taken together, these results suggest that alpha 2-receptors mediate the hyperpolarizing effects of E and NE. The beta-agonist isoproterenol evoked a slow depolarization similar to that produced by E and NE. The isoproterenol-depolarization was antagonized by propranolol. Sometimes, application of E and NE after superfusion with yohimbine produced only a depolarization of the dorsal root and this depolarization was sensitive to propranolol. It would appear therefore that the late depolarization seen after the application of E and NE is produced by activation of beta-receptors. In contrast, the alpha 1-agonist phenylephrine elicited a short latency, short duration depolarization similar to those seen preceding approximately 10% of the E- and NE-hyperpolarizations. Such short-latency depolarizations were blocked by prazosin and corynanthine. The major component of the response to both E and NE is indirectly mediated through a synaptic process: application of Mn2+, Mg2+, procaine or tetrodotoxin in concentrations sufficient to block synaptic transmission substantially reduced, but never eliminated, the actions of the catecholamines. Interneurons are probably involved because mephenesin, which reduces interneuronal transmission, significantly decreased the E and NE effects. Furthermore, interneurons which secrete excitatory amino acids and/or GABA may mediate the indirect effects of the catecholamines on afferent terminals because (-)baclofen and D.L-alpha-aminoadipate decrease, and picrotoxin and bicuculline increase, the dorsal root (DR) effects of E and NE.(ABSTRACT TRUNCATED AT 400 WORDS)     

The projection fields of spinal border cells in the cerebellar anterior lobe in the cat: an anterograde WGA-HRP study

The projection fields of spinal border cells (SBCs) in the cerebellar anterior lobe were studied using the anterograde transport of wheat germ agglutinin bound to horseradish peroxidase in the cat. Many anterogradely labeled terminals of the SBCs were seen in the apical to middle parts of sublobules IIb-Va on the side ipsilateral to the injection. In the mediolateral extent they were distributed densely in areas in the lateral part of the vermis (zone B) to the intermediate-lateral part of the lobules (zones C1-D). The present study suggests that the SBCs project, predominantly ipsilaterally, to specific parts of sublobules IIb-Va.     

The effects of small doses of barbiturate on the activity of primate nociceptive tract cells

The activity of neurons identified as or similar to spinothalamic tract cells was compared in anesthetized, intact monkeys and in unanesthetized, decerebrate or spinalized animals. Background activity and responses to A- and C-fiber volleys were smaller in decerebrate than in the other preparations, suggesting the presence of a tonic descending inhibition of nociceptive tract cells in the decerebrate state. Injections of small doses of sodium pentobarbital caused a slight increase in sensitivity of peripheral receptive fields to mechanical stimuli in some cells, and an increase in responses to squeezing the skin with forceps and to C-fiber volleys. Larger doses of pentobarbital reduced the responses to A- and C-fiber volleys. These observations suggest that the activity of nociceptive tract cells is well preserved in the presence of the doses of pentobarbital we routinely use and are consistent with the hyperalgesic state reported to be produced by small doses of barbiturate.     

Electrical stimulation of the anterior ethmoidal nerve produces the diving response

Stimulation of the upper respiratory tract usually produces apnea, but it can also produce a vagally mediated bradycardia and a sympathetically mediated increase in peripheral vascular resistance. This cardiorespiratory response, often called the diving response, is usually initiated by nasal stimulation. The purpose of this research was to investigate the anterior ethmoidal nerve (AEN) that innervates the nasal mucosa of muskrats (Ondatra zibethicus). Electrical stimulation of the AEN (typically 50 Hz, 100 micros and 500 microA) produced immediate and sustained bradycardia and cessation of respiration similar to that of the diving response. Heart rate (HR) significantly decreased from 264+/-18 to 121+/-8 bpm, with a concurrent 4.2+/-0.9 s apnea, during the 5 s stimulation period. BP decreased from 97.9+/-4.8 to 91.2+/-6.4 mmHg. Using estimations from (1) cross-sectional areas of AEN trigeminal ganglion cells labeled with WGA-HRP, and (2) electron microscopic analysis of the AEN, we found that approximately 65% of the AEN is composed of unmyelinated C-fibers. In addition, 72.4% of myelinated fibers from the nerves that innervate the nasal passages were of small diameter (<6 microm, presumably Adelta fibers). Thus, the AEN of the muskrat contains a high concentration of small diameter fibers (89.8%). We conclude that electrical stimulation of small diameter fibers within the AEN of muskrats can produce the cardiovascular and respiratory responses similar to that of the diving response.     

Differential decerebrate control of depolarization in the central terminals of cutaneous afferents in the sacral cord

Presynaptic depolarization of cutaneous afferents has been investigated in the sacral cord of decerebrate cats before and after spinal cord transection. In the decerebrate state the central terminals of caudal femoral cutaneous nerve are depolarized by ipsilateral volleys entering the cord via sacral and lumbar dorsal roots. A significant increase of depolarization occurring after severing the cord indicates that there is tonic decerebrate inhibition of presynaptic depolarization in terminals of caudal femoral cutaneous nerve. In contrast to this finding, presynaptic depolarization evoked in the central terminals of the pudendal nerve by ipsilateral volleys entering the cord through sacral and lumbar dorsal roots is not subjected to decerebrate inhibitory control. It is suggested that differential inhibitory control of depolarization in the central terminals of cutaneous nerves in the sacral cord is related to the intraspinal course of their fibres, to differences in the receptor types involved, and to the location of their innervation fields. In more than half of the decerebrate preparations stimulation of the central terminals of cutaneous afferents through microelectrodes evokes antidromic spikes appearing simultaneously in ipsi- and contralateral nerves. The time course of bilateral excitability changes is similar on both sides of the cord. It is assumed that presynaptic effects are transmitted to the contralateral side by collaterals of ipsilateral cutaneous afferents.     

Specific and potassium components in the depolarization of the la afferents in the spinal cord of the cat

In the cat spinal cord, primary afferent depolarization (PAD) of group Ia fibers of extensor muscles is produced by high-frequency stimulation (100 Hz) of group I muscle flexor afferents without significant increases in extracellular potassium. On the other hand, the PAD produced by stimulation of mixed and pure cutaneous nerves correlates well with increases in potassium ions. We conclude that the PAD produced by group I muscle afferents results from the activation of specific pathways making axo-axonic synapses with the Ia fiber terminals. The PAD of Ia fibers resulting from activation of cutaneous nerves involves instead unspecific accumulation of potassium ions.     

Specific and potassium components in the depolarization of the Ia afferents in the spinal cord of the cat

In the cat spinal cord, primary afferent depolarization (PAD) of group Ia fibers of extensor muscles is produced by high-frequency stimulation (100 Hz) of group I muscle flexor afferents without significant increases in extracellular potassium. On the other hand, the PAD produced by stimulation of mixed and pure cutaneous nerves correlates well with increases in potassium ions. We conclude that the PAD produced by group I muscle afferents results from the activation of specific pathways making axo-axonic synapses with the Ia fiber terminals. The PAD of Ia fibers resulting from activation of cutaneous nerves involves instead unspecific accumulation of potassium ions.