The contribution of increases in extracellular potassium to primary afferent depolarization in the bullfrog spinal cord

To assess the extent to which depolarization by accumulated K+ contributes to the generation of primary afferent depolarization (PAD), the isolated bullfrog spinal cord was superfused with K+-rich Ringer solutions and the resultant dorsal root depolarizations were recorded extracellularly. Action potential blockade (with tetrodotoxin) did not reduce the K+-induced depolarization of primary afferents, indicating that the depolarization was generated locally in the region around the afferents. In this respect superfusion with K+-rich solutions adequately models the localized K+ accumulation which occurs physiologically during afferent activity. K+-induced depolarizations were decreased in the presence of 20 mM Mg2+; this effect was due to a direct decrease in the membrane response to K+ and not to blockade of K+-induced transmitter release onto primary afferents. The depolarization caused by a K+ concentration comparable to a maximum estimate of the K+ accumulating around afferent terminals following a single afferent volley was found to account for no more than about one-third of the DRP height. However, higher K+ levels, comparable to those resulting from high frequency afferent stimulation, caused large depolarizations of primary afferents, sometimes greater than the DRP amplitude. Therefore, K+-induced depolarization may contribute more significantly to PAD evoked by high frequency afferent activity.     

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.     

The effect of intravenously administered gamma-aminobutyric acid on afferent fiber polarization.

(1) The effect of intravenously administered gamma-aminobutyric acid (GABA) on afferent fiber polarization in the feline spinal cord was ascertained from fluctuations induced in the DC level of dorsal root filaments. (2) A dose-related depolarization of the filament, with a concomitant reduction in the magnitude of the dorsal root potential, was observed after 50 and 100 mg/kg GABA. (3) GABA also depolarized filaments of preparations in which interneuronal activity was suppressed by pretreatment with tetrodotoxin. Since the magnitude of the depolarization induced in these preparations was equal to that observed in nonpretreated animals, it is likely that the depolarization in the latter preparations reflects a direct effect on afferent terminals or fibers rather than an action on interneurons. (4) GABA failed to depolarize filaments in animals pretreated with bicuculline. This suggests that intravenously administered GABA interacted with receptors that are identical with or similar to those involved in neurally evoked primary afferent depolarization (PAD). (5) The direct depolarization of afferent fibers by intravenous GABA and the blockade thereof by bicuculline are characteristics compatible with those of the endogenous axo-axonic transmitter operating in pathways mediating neurally evoked PAD. These data, therefore, support the involvement of GABA at this synapse in the mammalian spinal cord.     

Depolarization of primary afferents evoked by cyclically modulated natural activity of proprioceptors of the hindlimb of the cat

The primary afferent depolarization (PAD) evoked during passive sinusoidal movements of a hindlimb in the ankle joint was investigated in decerebrated cats. The frequency of movements varied within 0.14-5.0 Hz, the amplitude of the joint angle with respect to the axis of the tibia changed from 90 degrees to 130 degrees. The dorsal root potential (DRP) negativity increased both during flexion and during extension of the joint. The amplitude of the evoked DRPs was about 50-100 mV. A strong negative correlation was observed between the latency and rise time of the DRP and the frequency of the joint angle changes. During flexion the latency changed from 650 ms at 0.14-0.16 Hz frequency to 100-110 ms at 2.0 Hz and higher frequencies; during extension at the same frequencies the latency changed from 300 ms to 80-85 ms. The latency and rise time became minimal at 2.0 Hz frequency and practically did not change during the further increase of the oscillation frequency. The cord dorsum potential (CDP) evoked by the cutaneous nerve stimulation was recorded in parallel with the DRP. Periodical changes of the N-component of the CDP were in the opposite phase to changes of the DRP. Mechanisms of the observed changes of the PAD and functional significance of these changes during rhythmical motor acts are discussed.     

Effect of ammonium acetate on depolarization processes in the central terminals of primary afferents]

Experiments on cats determined that ammonium acetate injected intravenously (2-4 mM/kg) supressed the processes of primary afferent depolarization (PAD) which are thought to be responsible for the presynaptic inhibition of spinal reflexes. The supression was transient and proceeded in paralle to depression of postsynaptic inhibition of monosynaptic reflexes. Ammonium acetate slightly decreased the amplitude of the negative postsynaptic potentials recorded form the dorsal surface of lumbar cord in response to stimulation of hind limb afferent nerves and increased polysynaptic reflex discharges in appropriate ventral roots. These findings make it unlikely that the ammonium depression of PAD is a result of impairment of interneuronal activity. A suggestion is made that ammonium depression of PAD results from diminition of the EMF for synaptic currents producing PAD.     

The role of 5-HT, GABA and opioid peptides in presynaptic inhibition of tooth pulp input from the medial brainstem

In decerebrate or chloralose-anaesthetized cats electrical stimulation in the spinal trigeminal nucleus evoked antidromic responses in the mandibular canine tooth pulp. Conditioning stimulation in nucleus raphe magnus (NRM) and in the adjacent contralateral medullary reticular formation, nucleus reticularis gigantocellularis (NRGC) and nucleus reticularis magnocellularis (NRMC), produced a decrease in the threshold for the antidromic responses in a proportion of the tooth pulp inputs. This was interpreted as being due to depolarization of the tooth pulp afferent terminals, reflecting presynaptic inhibition. The primary afferent depolarization (PAD) of tooth pulp afferent terminals by NRM stimulation could be selectively blocked by bicuculline applied intravenously or by iontophoresis in the terminal region. Intravenous naloxone, cinanserin and methysergide had no effect on the PAD evoked from NRM, NRGC or NRMC. Thus NRM appears to exert presynaptic inhibitory control of A delta tooth pulp input to the spinal trigeminal nucleus via GABA-containing neurones.     

Intra-axonal recordings in rat dorsal column axons: membrane hyperpolarization and decreased excitability precede the primary afferent depolarization

Intra-axonal recordings were obtained in the dorsal columns of the rat lumbosacral spinal cord. Dorsal root or dorsal column stimulation at levels subthreshold for the impaled axon elicited a prolonged depolarization corresponding to the primary afferent depolarization (PAD). The depolarization was preceded by a brief hyperpolarizing potential during which excitability was decreased. The hyperpolarization corresponds temporally to the extracellularly recorded DRP IV component of the dorsal root potential described by Lloyd and McIntyre, and may represent the intracellular correlate of this potential. Possible mechanisms for this hyperpolarization include electrical interactions between neuronal elements, a biphasic GABA response, or attenuation of background afferent axonal depolarization.     

Phase-dependent modulation of dorsal root potentials evoked by peripheral nerve stimulation during fictive locomotion in the cat

To help elucidate the role of presynaptic mechanisms in the control of locomotor movements, the transmission of PAD pathways was investigated by recording dorsal root potentials (DRPs) evoked by electrical stimulation of cutaneous and muscle nerves of both hindlimbs at various phases of the fictive step cycle. Fictive locomotion occurred spontaneously in decorticate cats or by stimulating the mesencephalic locomotor region (MLR) as well as in low spinal cats injected with nialamide and L-DOPA. Evoked DRPs were superimposed on a fluctuating DRP accompanying the fictive locomotor rhythm (locomotor DRP) which typically consisted of two peaks of depolarization per cycle, the largest peak occurring during the flexor phase. The amplitude of evoked DRPs was substantially modulated throughout the locomotor cycle and followed a similar modulation pattern for all stimulated nerves whether ipsilateral (i-) or contralateral (co-). The amplitude of evoked DRPs decreased at the beginning of the flexor phase, dropped to a minimum later in the flexor phase and then increased during the extensor phase where it became maximum. Results were comparable in decorticate and spinal preparations and for L6 and L7 rootlets with cutaneous and muscle nerve stimulation. It is noteworthy that the modulation pattern for a given rootlet was similar for i- and co- stimulation, even though the bilateral locomotor DRPs fluctuate out-of-phase with each other, subjecting the stimulated fibres to opposite presynaptic polarization changes. This suggests that the modulation may depend more on the presynaptic mechanisms of the receiving fibres than on those of the stimulated fibres. These results demonstrate that the transmission in spinal pathways involved in primary afferent depolarization (PAD) is phasically modulated by the activity in the spinal locomotor network. It is further suggested that the presynaptic inhibition associated with PAD evoked by movement-related sensory feedback during real locomotion could be modulated in a similar way.     

Modification of primary afferent depolarization in cat group Ia afferents following high frequency intra-axonal tetanization of individual afferents

Intra-axonal tetanization of a single, functionally-identified group Ia afferent from the triceps surae muscle in the anesthetized cat produces marked enhancement and slowing of the primary afferent depolarization (PAD) generated in the Ia afferent by volleys in flexor muscle group Ia afferents, plus a pronounced transmembrane hyperpolarizing undershoot (HPU) which disappears more rapidly than the enhanced PAD. These alterations are qualitatively and quantitatively similar to those found after conditioning tetani are applied to the whole muscle nerve. The occurrence of these PAD changes after intra-axonal tetanization of a single group Ia afferent appears to rule out the participation of non-specific alterations in extracellular ionic concentrations or activation of polysynaptic pathways in their genesis.     

Quantitative characteristics of inhibition in the cuneate nucleus of the cat

Several properties of inhibition in the cuneate nucleus were investigated as an aid in deriving cuneate circuitry. Inhibition was examined by measuring changes in primary afferent terminal depolarization (PAD) and in the size of evoked medial lemniscal activity. Inhibition was produced by stimulation of several peripheral and central sites. Quantitative measurements were made of the inhibitory time course and degree of facilitation to conditioning trains of different lengths. The data suggest a dual organization of inhibition, dependent upon the conditioning site and characterized by the inhibitory time course. Additional information was derived from PAD measurements following simultaneous stimulation of two conditioning sites and from an investigation of a cortico-cuneate feedback loop. The proposed two inhibitory systems appear to converge for simultaneous activation did not produce simple summation of PAD. These results were substantiated when PAD was measured from single, identified cutaneous fibers. The time course of single fiber PAD was similar to that in a whole nerve preparation. Single fibers could be depolarized by conditioning several sites and with no apparent fiber class dependence. Several models of inhibitory circuitry are presented, and the features that must be taken into account are discussed.     

Differences in the excitability of two populations of trigeminal primary afferent central terminals

The excitability of lingual and inferior dental nerve terminals in the trigeminal main sensory nucleus was increased following a conditioning stimulus delivered to the same nerves of the contralateral side. The lingual nerve primary afferent depolarization (PAD) began at a shorter CS-TS interval (CS, conditioning stimulus; TS, test stimulus), and reached a maximum level of depolarization sooner, than did the PAD evoked in the inferior dental nerve. This difference in the time course of the PAD was not dependent on the site of the CS;i.e., the same time course was observed for the lingual nerve terminals whether the CS was delivered to either the contralateral lingual, or inferior dental nerve.The effect of the contralateral CS was also tested on the ipsilateral lingual-digastric and inferior dental-digastric reflexes in order to determine if the PAD observed in the ipsilateral nerve terminals would be reflected in similar changes in reflexes mediated by those nerves. It was found that both digastric reflexes were inhibited by the CS. The time course of the inhibition showed similar characteristics to that of the previously discussed PAD;i.e., the onset of the lingual-digastric reflex inhibition began a shorter CS-TS interval and reached maximum effectiveness sooner than did the inhibition of the inferior dental-digastric reflex. The possible significance of the results in relation to the role of presynaptic inhibitory mechanisms in the reflex control of jaw movement is discussed.     

Presynaptic inhibition of identified wind-sensitive afferents in the cercal system of the locust

The paired cerci located at the tip of the locust abdomen bear a large number of wind-sensitive filiform hairs, each of which sends an axon via the cercal nerve into the terminal ganglion of the CNS. The filiform afferents fire bursts of action potentials when their hairs are displaced by wind or mechanical stimuli. Filiform axon terminals in the CNS are depolarized concomitantly with the discharge of another type of unit (a primary afferent-depolarizing, or PAD, unit) recorded in the cercal nerve. The instantaneous spike frequency of PAD unit discharges matches the evoked depolarization very closely, and during such depolarizations spike amplitudes in the filiform afferent terminals are reduced by up to 55%. Depolarizing current pulses injected into the axonal terminals of an identified filiform afferent evoke spikes that are blocked by the PAD unit, probably via an intercalated interneuron. The PAD unit makes a monosynaptic connection with only one of the 4 giant interneurons (GIN 2) on each side of the terminal ganglion, and indirect connections with 2 others. Depolarizing current pulses injected into the neuropilar segments of GINs evoke fewer spikes when the PAD unit is active, consistent with the PAD unit's mediation of conductance changes in postsynaptic cells. Iontophoretic injection of Lucifer yellow shows the PAD unit to be an afferent with axon terminals overlapping those of filiform afferents and posteriorly directed branches of interneurons such as GIN 2 in the CNS. Passive movements of a cercus, monitored with a position transducer, show that the PAD unit fires discrete bursts during cercal displacement. The PAD unit most probably has its soma and dendrites in tissue spanning the cercal base. By responding to cercal movements sufficient to also activate filiform hairs, and by mediating conductance changes in both the presynaptic terminals of filiform afferents and the postsynaptic membranes of interneurons, the PAD unit desensitizes a pathway to movement-generated afferent input, and ensures that the locust remains sensitive to external wind stimuli.     

The effects of naloxone, morphine and methionine enkephalinamide on Ia afferent terminations in the cat spinal cord

Naloxone, morphine, Met5-enkephalinamide (MENKA) and procaine were administered microelectrophoretically near extracellularly stimulated extensor muscle group Ia afferent fibres and terminations in the lumbar spinal cord of cats anaesthetized with pentobarbitone sodium. Observations were made of effects on the electrical threshold, on the depolarizing action of GABA or piperidine-4-sulphonate (P4S), and on bicuculline-sensitive primary afferent depolarization (PAD) generated by tetanic stimulation of flexor muscle low threshold afferents. All 4 agents reversibly elevated the threshold of Ia fibres in the dorsal column and Ia terminations in the ventral horn. The depolarizations of terminations by GABA or P4S were also reduced, an effect, which for all except MENKA, probably accounted for a concomitant reduction in PAD. In the absence of a consistent effect on either threshold or depolarization by GABAmimetics, MENKA reversibly diminished PAD, an action blocked by naloxone. Intravenously administered naloxone, in doses known to enhance spinal monosynaptic excitation in the cat, had no effect on GABAergic PAD and little or no effect on Ia termination threshold. The results are discussed in relation to a naloxone-sensitive effect of MENKA which reduces transmitter release from GABAergic axo-axonic synapses on Ia terminals, but which does not account for the enhancement of spinal reflexes by naloxone.     

Changes in the intensity of integral afferent inflow from limb receptors and the level of polarization of primary afferent endings in the decerebrate cat during scratching

The experiments performed on decerebrated cats have shown that afferent activity during scratching consisted of two components--tonic and periodic phasic ones. The first component was determined by the limb position, the second was closely related to the amplitude and velocity of joint angle changes. Maximum of integral afferent activity in the cycle coincided with the scratching jerk phase. These two components of afferent activity evoked corresponding depolarization changes in primary afferent terminals and these changes added to those elicited by central generator. Statistical correlations between the aforementioned parameters were studied. The afferent control mechanisms of scratching generator are under discussion.     

Depression of afferent-induced primary afferent depolarization at the lumbar spinal cord following concussive head injury

Afferent-induced primary afferent depolarization (PAD) was depressed for 2-5 min following concussive head injury in the cat, as assessed by dorsal root potentials and augmentation of antidromic dorsal root potentials, both evoked by stimulation of adjacent dorsal roots. These changes in PAD were abolished by spinal cord transection but not affected by midpontine transection. Spontaneous dorsal root potentials, resting amplitudes of antidromic dorsal root potentials and reductions of antidromic dorsal root potentials following tetanic root stimulation were not substantially altered by injury. These findings suggest that concussive head injury depresses spinal interneuronal transmission by neurally mediated processes involving the bulbar brainstem.     

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.     

Long duration ventral root potentials in the neonatal rat spinal cord in vitro; the effects of ionotropic and metabotropic excitatory amino acid receptor antagonists.

Long duration, primary afferent evoked ventral root potentials (VRP's) have been recorded in vitro from hemisected spinal cords prepared from 8-12-day-old rat pups. Single shock stimulation of a dorsal root at stimulus strengths sufficient to recruit C/group IV afferent fibres evoked a long duration (11.9 +/- 1.2 s) ipsilateral VRP in all preparations. This long duration VRP consisted of two components, (i) a slow wave, time to peak 137.0 +/- 5.1 ms, the amplitude of which was reduced to 8.7% of mean control value in the presence of the N-methyl-D-aspartate (NMDA) antagonist D-AP5 (40 microM), (ii) a prolonged wave with a time to peak of 2.0 +/- 0.2 s which was partially resistant to D-AP5 (40 microM). Both the slow and the prolonged waves were unaffected following superfusion with the metabotropic excitatory amino acid (EAA) receptor antagonist L-AP3 (100-200 microM). Low frequency (1-10 Hz) repetitive stimulation (20 s duration) of high threshold dorsal root afferents evoked a temporal summation of synaptic activity which generated a progressively depolarizing VRP. This cumulative VRP was graded with frequency of stimulation (0.89 +/- 0.13 to 1.25 +/- 0.19 mV). The cumulative VRP was followed by a post-stimulus depolarization which outlasted the period of repetitive stimulation by tens of seconds (47.6 +/- 8.4 to 91.2 +/- 19.9 s). In the presence of AP5 the amplitude of the cumulative VRP was depressed to 54.5 +/- 11.5% of control values when low frequency (1.0 Hz) stimulation was used. The proportion of the cumulative VRP resistant to D-AP5 increased as the frequency of stimulation was increased to 10 Hz. The decay time of the post-stimulus depolarization was unaffected by AP5. Neither the amplitude nor the post-stimulus depolarization of the cumulative VRP was affected by 200 microM L-AP3. It is suggested that both an AP5 sensitive and AP5 insensitive potential contribute to the long duration VRP evoked in the neonatal rat spinal cord following single shock high threshold afferent stimulation. Moreover, the AP5 insensitive prolonged depolarization is manifest following sustained low frequency stimuli and higher frequency inputs.     

Persistence of PAD and presynaptic inhibition of muscle spindle afferents after peripheral nerve crush

Two to twelve weeks after crushing a muscle nerve, still before the damaged afferents reinnervate the muscle receptors, conditioning stimulation of group I fibers from flexor muscles depolarizes the damaged afferents [M. Enriquez, I. Jimenez, P. Rudomin, Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush. Exp. Brain Res., 107 (1996), 405-420]. It is not known, however, if this primary afferent depolarization (PAD) is indeed related to presynaptic inhibition. We now show in the cat that 2-12 weeks after crushing the medial gastrocnemius nerve (MG), conditioning stimulation of group I fibers from flexors increases the excitability of the intraspinal terminals of both the intact lateral gastrocnemius plus soleus (LGS) and of the previously damaged MG fibers ending in the motor pool, because of PAD. The PAD is associated with the depression of the pre- and postsynaptic components of the extracellular field potentials (EFPs) evoked in the motor pool by stimulation of either the intact LGS or of the previously damaged MG nerves. These observations indicate, in contrast to what has been reported for crushed cutaneous afferents [K.W. Horch, J.W. Lisney, Changes in primary afferent depolarization of sensory neurones during peripheral nerve regeneration in the cat, J. Physiol., 313 (1981), 287-299], that shortly after damaging their peripheral axons, the synaptic efficacy of group I spindle afferents remains under central control. Presynaptic inhibitory mechanisms could be utilized to adjust the central actions of muscle afferents not fully recovered from peripheral lesions.