Primary Afferent Hyperpolarization Research Articles

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents.

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Patterns of primary afferent depolarization of segmental and ascending intraspinal collaterals of single joint afferents in the cat

We have examined in the anesthetized cat the threshold changes produced by sensory and supraspinal stimuli on intraspinal collaterals of single afferents from the posterior articular nerve (PAN). Forty-eight fibers were tested in the L3 segment, in or close to Clarke's column, and 70 fibers in the L6-L7 segments within the intermediate zone. Of these, 15 pairs of L3 and L6-L7 collaterals were from the same afferent. Antidromically activated fibers had conduction velocities between 23 and 74 m/s and peripheral thresholds between 1.1 and 4.7 times the threshold of the most excitable fibers (xT), most of them below 3 xT. PAN afferents were strongly depolarized by stimulation of muscle afferents and by cutaneous afferents, as well as by stimulation of the bulbar reticular formation and the midline raphe nuclei. Stimulation of muscle nerves (posterior biceps and semitendinosus, quadriceps) produced a larger PAD (primary afferent depolarization) in the L6-L7 than in the L3 terminations. Group II were more effective than group I muscle afferents. As with group I muscle afferents, the PAD elicited in PAN afferents by stimulation of muscle nerves could be inhibited by conditioning stimulation of cutaneous afferents. Stimulation of the cutaneous sural and superficial peroneal nerves increased the threshold of few terminations (i.e., produced primary afferent hyperpolarization, PAH) and reduced the threshold of many others, particularly of those tested in the L6-L7 segments. Yet, there was a substantial number of terminals where these conditioning stimuli had minor or no effects. Autogenetic stimulation of the PAN with trains of pulses increased the intraspinal threshold in 46% and reduced the threshold in 26% of fibers tested in the L6-L7 segments (no tests were made with trains of pulses on fibers ending in L3). These observations indicate that PAN afferents have a rather small autogenetic PAD, particularly if this is compared with the effects of heterogenetic stimulation. Therefore, the depression of the PAN intraspinal fields produced by autogenetic stimulation described by Rudomin et al. (Exp Brain Res DOI 10.1007/s00221-006-0600-x, 2006) may be ascribed to other mechanisms besides a GABAa PAD. It is suggested that the small or no autogenetic PAD displayed by the examined joint afferents prevents presynaptic filtering of their synaptic actions and preserves the original information generated in the periphery. This could be important for proper adjustment of limb position.

Sensory integration in presynaptic inhibitory pathways during fictive locomotion in the cat.

The aim of this study is to understand how sensory inputs of different modalities are integrated into spinal cord pathways controlling presynaptic inhibition during locomotion. Primary afferent depolarization (PAD), an estimate of presynaptic inhibition, was recorded intra-axonally in group I afferents (n = 31) from seven hindlimb muscles in L(6)-S(1) segments during fictive locomotion in the decerebrate cat. PADs were evoked by stimulating alternatively low-threshold afferents from a flexor nerve, a cutaneous nerve and a combination of both. The fictive step cycle was divided in five bins and PADs were averaged in each bin and their amplitude compared. PADs evoked by muscle stimuli alone showed a significant phase-dependent modulation in 20/31 group I afferents. In 12/20 afferents, the cutaneous stimuli alone evoked a phase-dependent modulation of primary afferent hyperpolarization (PAH, n = 9) or of PADs (n = 3). Combining the two sensory modalities showed that cutaneous volleys could significantly modify the amplitude of PADs evoked by muscle stimuli in at least one part (bin) of the step cycle in 17/31 (55%) of group I afferents. The most common effect (13/17) was a decrease in the PAD amplitude by 35% on average, whereas it was increased by 17% on average in the others (4/17). Moreover, in 8/13 afferents, the PAD reduction was obtained in 4/5 bins i.e., for most of the duration of the step cycle. These effects were seen in group I afferents from all seven muscles. On the other hand, we found that different cutaneous nerves had quite different efficacy; the superficial peroneal (SP) being the most efficient (85% of trials) followed by Saphenous (60%) and caudal sural (44%) nerves. The results indicate that cutaneous interneurons may act, in part, by modulating the transmission in PAD pathways activated by group I muscle afferents. We conclude that cutaneous input, especially from the skin area on the dorsum of the paw (SP), could subtract presynaptic inhibition in some group I afferents during perturbations of stepping (e.g., hitting an obstacle) and could thus adjust the influence of proprioceptive feedback onto motoneuronal excitability.

Excitability changes in sacral afferents innervating the urethra, perineum and hindlimb skin of the cat during micturition.

1. Excitability changes in afferents innervating the urethra, perineum and hindlimb were measured in decerebrated cats during micturition and in response to stimulation of lumbosacral afferents. Increases in excitability were interpreted as primary afferent depolarization (PAD) and decreases as primary afferent hyperpolarization. 2. Excitability increases were observed in 11 of 19 urethral pudendal afferents during micturition. Four of these 11 afferents showed an excitability increase during voiding. Seven of these showed a biphasic change with a decrease in excitability when sphincter activity resumed at the end of the void. Three of 19 afferents showed an excitability decrease during micturition and no change was detected in five afferents. 3. During micturition, the peak amplitude of urethral afferent-evoked excitatory postsynaptic potentials in seven of eight sphincter motoneurones was diminished to a mean of 36% of control values. 4. Eighty per cent of hindlimb cutaneous afferents and 50% of dorsal penile/clitoral and superficial perineal nerve afferents in the sacral cord showed increased excitability during voiding. No excitability increases were measured in 13 hindlimb cutaneous fibres examined in the lumbar segments. 5. PAD was observed in sacral urethral, perineal and hindlimb cutaneous afferents in response to electrical stimulation of other perineal, urethral, hindlimb cutaneous and group II muscle afferents. 6. It is concluded that control of transmission from urethral afferents by the micturition circuitry is different to that by sensory transmission from hindlimb and perineal regions during micturition. We hypothesize that more than one population of sacral PAD-mediating interneurones is involved.

Long-latency, nonreciprocal reflex responses of antagonistic hind limb muscles after cutaneous nerve stimulation in the cat

The time course of changes in monosynaptic reflex amplitude, after conditioning from both ipsi- and contralateral sural nerves at different stimulus strengths, was studied on two antagonistic motoneuronal pools acting on ankle muscles in spinal cats. Attention was focused on late effects, namely those appearing after a dely of more than 30 ms from the cutaneous stimulus. With low-threshold afferent activation, at conditioning-test intervals to 30 ms, the ipsilateral extensor monosynaptic reflex, recorded from the proximal stump of L7 to S1 ventral roots, showed marked inhibition; at longer intervals, a late facilitation period (LFP) lasting to 100 ms was observed. Increasing stimulus strength did not modify the time course of reflex excitability, but might enhance the amount of the facilitatory effecct. On the flexor monosynaptic reflex, sural conditioning induced, after the expected early facilitation, a second facilitatory period, starting at about 30 ms and recovering at about 130 ms. The excitability of antagonistic contralateral motoneuronal pools was also influenced, showing again a LFP with the same time course. The LFP was present after stimulation of the sural and saphenous nerves and was absent after stimulation of a muscle nerve. These late, long-lasting and nonreciprocal facilitatory effects on flexor and extensor ipsilateral motoneurons were quite distinct from the early reciprocal responses, and were evoked by large cutaneous fibers. An interpretation is put forward in light of the primary afferent hyperpolarization of Ia afferent terminals. A correlation is tentatively proposed with the mechanism subserving the stumbling corrective reaction.

GABAergic terminals are presynaptic to primary afferent terminals in the substantia gelatinosa of the rat spinal cord

Multiple, dorsal rhizotomies were performed unilaterally at lumbar levels L1–L4 in adult rats. Following 24–48 h degeneration periods and fixation by intracardiac perfusions, spinal cord were removed and transversely cut into 150 μm thick sections. These sections were incubated in immunocytochemical reagents for the peroxidase-labeling of glutamic acid decarboxylase (GAD), the enzyme that synthesizes the neurotransmitter γ-aminobutyric acid (GABA). The sections were then prepared for electron microscopic examination, while other sections were processed for light microscopic, GAD immunocytochemistry and for Fink-Heimer staining of degenerating axons and axon terminals. Thirty-six hours following dorsal rhizotomies, the sections that were prepared for the light microscopic study of terminal degeneration showed large numbers of degenerating profiles in the ipsilateral substantia gelatinosa while degenerating profiles were virtually absent contralaterally. In electron microscopic preparations, degenerating primary afferent terminals were commonly observed at the centers of rosettes where they formed synaptic contacts with other axon terminals and with surrounding dendrites. Several types of synaptic relationships were observed in the rosettes which involved both GAD-positive and degenerating primary afferent terminals. Such synaptic relationships included those in which: (a) a single GAD-positive terminal was presynaptic to the central, primary afferent terminal, (b) two different GAD-positive terminals formed synapses with opposite sides of the same central, primary afferent terminal and were also closely apposed to the surrounding dendrites of the rosette, and (c) a GAD-positive terminal was presynaptic to a primary afferent terminal and both types of terminals were presynaptic to the same dendrite of the rosette. The synaptic relationships described in this study are discussed with respect to their possible functional roles in such GABA-mediated phenomena as: (a) primary afferent depolarization, (b) the dorsal root reflex and (c) primary afferent hyperpolarization. Our observations support the concept that GABAergic axon terminals are involved in the synaptic circuits which produce presynaptic inhibition and presynaptic facilitation of the primary afferent input to the dorsal spinal cord. Collectively the observed synaptic relationships could provide a morphological substrate that is compatible with an inhibitory surround system in the substantia gelatinosa.

Excitability changes of terminal arborizations of single Ia and Ib afferent fibers produced by muscle and cutaneous conditioning volleys.

1. In cats anesthetized with sodium pentobarbital, recordings were made from dorsal root ganglion (DRG) cells having a peripheral process in the gastrocnemius-soleus (GS) nerve. The GS nerve was left in continuity with the muscle to allow identification of group Ia and Ib fibers by responses of the receptors to muscle stretch and contraction. The central processes of the DRG cells were activated antidromically by stimulation within the spinal cord so that changes in the excitability of the fibers could be examined following conditioning volleys in muscle and cutaneous nerves. 2. Recordings were made from 44 DRG cells. Of these, 15 were group Ia and 9 group Ib afferents of the GS nerve. 3. Of 15 Ia fibers, 12 were activated antidromically by stimulation in the motor nucleus, but no Ib fibers were discharged by such stimulation. Ib fibers could be antidromically activated by stimulation in the intermediate nucleus. 4. The central processes of the Ia DRG cells had slower conduction velocities than did the peripheral processes. 5. The thresholds to electrical stimulation of the peripheral processes of Ia and Ib fibers of the GS nerve showed considerable overlap. 6. All of the Ia DRG cells tested showed an increased excitability following conditioning volleys in the biceps-semitendinosus (BST) nerve. The increase in excitability was produced by the largest fibers of the BST nerve. 7. Stimulation of the sural (SU) or superficial peroneal (SP) cutaneous nerves also increased the excitability of some Ia fibers. However, other Ia fibers were unaffected, and in two cases the excitability was reduced. 8. The excitability of group Ib fibers was increased by conditioning volleys in the BST, SU, or SP nerves. 9. It is concluded that cutaneous volleys produce a mixture of primary afferent depolarization and primary afferent hyperpolarization in Ia fibers of anesthetized cats. Such converse actions probably cancel in excitability tests using population responses. 10. The excitability of single Ia fibers is not stationary in excitability presumably reflect slow alterations within the central nervous system, perhaps related to spontaneous alterations in the level of tonically maintained primary afferent depolarization.

The role of γ-aminobutyric acid as a mediator of positive dorsal root potentials

Summary (1) A diphasic response consisting of a negative-, followed by a positive-going dorsal root potential (DRP) was evoked in L6 or L7 dorsal rootlets of unanesthetized spinal cats by 10 times group I threshold stimulation of the gastrocnemius-soleus muscle nerve. (2) The administration of 2.5 mg/kg bicuculine reversibly abolished, within 30 sec, first the negative and then the positive DRP. The time course for the recovery of the negative DRP was longer than that of the positive DRP. (3) Smaller doses of bicuculline (0.2–1.0 mg/kg) were usually much more effective against the negative than the positive DRP in terms of both magnitude and duration of suppression. (4) A selective action against the negative DRP was also observed, but with less consistency, after picrotoxin (3.0 mg/kg) administration. (5) Semicarbazide (200 mg/kg) induced a gradual diminution and often abolition of both DRPs over the course of 3 h. A selective reduction of the negative DRP occured after 60 min. (6) Suppression of the positive DRP by agents which block the receptor attachment (bicuculline and picrotoxin) and synthesis (semicarbazide) of GABA suggests that GABA is a transmitter in the pathway(s) mediating primary afferent hyperpolarization. (7) The possibility that GABA synapses occur in each of two separate pathways leading to the afferent terminal is discussed. According to this model, the positive DRP represents the GABA-mediated inhibition of a tonic non-GABA-mediated PAD; the negative DRP is conducted through a separate pathway terminating in a GABA-mediated axo-axonic synapse. The differential susceptibility would reflect, according to this model, anatomical or physiological differences between the GABA synapses of the respective pathways.

Primary afferent hyperpolarization and presynaptic facilitation of Ia afferent terminals induced by large cutaneous fibers.

Primary afferent hyperpolarization and presynaptic facilitation of Ia afferent terminals induced by large cutaneous fibers.

Primary afferent hyperpolarization without postsynaptic potentiation

Tetanization of a cutaneous afferent pathway does not lead to potentiation of postsynaptic activity but it produces a decrease in excitability of the pathway's central terminals and an increase in the threshold of the dorsal root reflexes evoked in the same fibers by stimulation of adjacent pathways. These two phenomena are presumably indicative of hyperpolarization of the central terminals. Possible explanations for the lack of post-tetanic potentiation in the presence of presynaptic hyperpolarization are discussed.

Hyperpolarization of trigeminal primary afferents evoked by stimulation of lumbar sympathetic trunk and peripheral nerves

The gate control theory has emphasized primary afferent hyperpolarization (PAH) as a specific determinant in the perception of a stimulus as painful. Previous studies which indicated that primary afferent depolarization (PAD) could be elicited in the trigeminal system by nonsegmental inputs suggested that this might also be the case for primary afferent hyperpolarization. Decreased excitability of trigeminal primary afferent fibers, an indirect reflection of PAH, was elicited by stimulation of the lumbar sympathetic trunk, sural, and sciatic nerves in cats. This PAH has a magnitude and time course similar to that previously demonstrated in the trigeminal system by segmental noxious inputs. We postulate that PAH is a general phenomenon related to the control of information transmission across afferent synapses rather than a phenomenon specifically related to the perception of various stimuli as painful.

Bicuculline and Picrotoxin Blockade of Positive Dorsal Root Potentials

MOUNTING evidence suggests that GABA functions as a transmitter in the pathways mediating presynaptic inhibition in the spinal cord1–10 and cuneate nucleus11–14. In spite of recent attempts to locate a GABA synapse in these multi-synaptic pathways at the axo-axonic terminal4,13,15, the clarification of GABA's role in this system awaits the elucidation of the complex circuits mediating afferent terminal polarization. This complexity may include such features as tonic activity of interneurones and convergence of phasic inputs from different fibre groups. For example, Mendell16, recording spinal cord dorsal root potentials (DRPs), has observed that stimulation of high threshold afferent fibres from muscle nerve generates a positive going DRP, reflecting primary afferent hyperpolarization. From studies of the interactions between positive and negative DRPs he has postulated that the positive DRP results from a phasic inhibition of tonic activity in the pathway for the negative DRP17. This implies that the axoaxonic synapse is common to the pathways for both the positive and negative DRPs. Thus, if the only GABA synapse in this system is located at the axo-axonic junction, antagonists of GABA should block DRPs of both polarities.

Excitability changes in trigeminal primary afferent fibres elicited by dental pulp stimulation in the cat

The effect of monopolar and bipolar dental pulp stimulation and mechanical facial stimulation on the level of excitability of trigeminal primary afferent fibres was studied. Mechanical facial stimulation resulted in increased excitability of trigeminal primary afferents. Monopolar dental pulp stimulation resulted in a period of increased excitability similar in amplitude and time course to that with mechanical stimulation. This excitability increase was reduced in amplitude, but not abolished, in both cases by dorsolateral medullotomy at the level of the obex. Bipolar dental pulp stimulation resulted in a brief early period of increased excitability of lower amplitude than that seen with monopolar stimulation. This was followed by a prolonged consistent period of decreased excitability. Monopolar dental pulp stimulation also elicited a late period of decreased excitability, but it was inconsistent and of much lower amplitude than that seen with bipolar stimulation. Dorsolateral medullotomy at the level of the obex abolished the excitability decrease seen with both types of dental pulp stimulation. We suggest that monopolar dental pulp stimulation activated large mechanoreceptive periodontal afferent fibres, in addition to pulp fibres, with resultant primary afferent depolarization. Bipolar stimulation apparently activated afferents which were confined almost exclusively to the pulp and resulted in primary afferent hyperpolarization. These results suggest that studies which employ electrical stimulation of dental pulp as a painful stimulus should use exclusively bipolar stimulation, whenever possible. Studies of the sensation or physiologic changes evoked by dental pulp stimulation are difficult to interpret if monopolar stimulation is used. For strictly clinical localization of teeth with pulp pathology, however, the monopolar algesiometers used at present are probably satisfactory.

The rubrospinal tract. 3. Effects on primary afferent terminals.

1. Effects evoked by stimulation of the red nucleus on primary afferent terminals in the lower lumbar segments of cats have been investigated by recording dorsal root potentials (DRPs) and by recording (intracellularly and by excitability measurements) the primary afferent depolarization (PAD) evoked in terminals of different afferent systems. Control experiments suggest that the effects are mediated by the rubrospinal tract. 2. Stimulation of the red nucleus evoked a large DRP and correspondingly there was a pronounced PAD in Ib and low threshold cutaneous afferents. A dual effect was found in Ia afferent terminals; sometimes a weak PAD was detected while in other cases there was dominating primary afferent hyperpolarization (PAH). 3. Rubrospinal volleys are found to facilitate transmission of DRPs evoked from Ia, Ib, cutaneous and high threshold muscle afferents, presumably by exerting an excitatory action on the interneurones mediating the effect from these afferents. Stimulation of the red nucleus may also inhibit transmission in the pathway mediating depolarization of Ia afferent terminals from Ia afferents, probably by activating a segmental pathway from the flexor reflex afferents from which the same effect is evoked. It is postulated that the PAH evoked in Ia afferents from the red nucleus is due to this inhibitory effect and caused by a removal of a tonic PAD in them. 4. The possible role in motor regulation of the rubral effects on primary afferent terminals is discussed in relation to the rubrospinal effects on reflex pathways to motoneurones.