Positive Dorsal Root Potentials Research Articles

Presynaptic facilitatory action of locus coeruleus stimulation upon hindlimb sensory impulse transmission in decerebrate cats.

This study sought to delineate the presynaptic role of the locus coeruleus (LC) on hindlimb primary afferent terminals. Changes in presynaptic function in response to LC stimulation were assessed by measuring the dorsal root potential (DRP), interaction of LC- and peripherally-evoked DRPs, and intraspinal afferent terminal excitability. LC stimulation in unanesthetized, decerebrate cats produced a sequence of early and late positive DRPs succeeded by a small-sized negative DRP. Conditioning the negative DRPs elicited from individual hindlimb nerve branches with LC stimuli led to a decrease in test DRPs. Similarly, there was a predominant decrease in excitability in both large muscle and cutaneous afferent terminals. These data suggest a presynaptic role of the LC in augmenting afferent impulse transmission, presumably through inhibition of tonically active interneurons having axoaxonic contacts on primary afferents; functionally, presynaptic facilitation.

Effect of Lissauer tract stimulation on activity in dorsal roots and in ventral roots

In decerebrate spinal cats, microelectrode stimulation of the lumbar lateral Lissauer tract (LLT), which does not spread to the dorsal roots, produces a volley in the tract that is conducted at 0.5 to 2.5 m/s. In 38 cats, such a volley in the LLT produced a negative dorsal root potential (DRP) beginning at 15 to 20 ms and lasting 30 to 40 ms, followed by a positive DRP lasting 90 to 140 ms. In seven cats, LLT stimulation produced a positive DRP alone beginning at 30 to 40 ms and lasting 80 to 110 ms. The LLT-evoked DRP was limited to the dorsal rootlets located within a few millimeters on either side of the stimulus point, unlike the dorsal root-evoked DRP. This local distribution parallels the limited spread of the LLT-evoked compound action potential. The LLT-evoked DRP occluded the dorsal root-evoked DRP. Conditioning stimuli applied to the LLT produced early facilitation and late inhibition of the monosynaptic ventral root reflex. The polysynaptic reflexes were inhibited for as long as 150 ms. With higher stimulus intensities, the polysynaptic reflexes were first inhibited and then facilitated for 150 to 1000 ms. Barbiturates first prolonged and then attenuated the LLT-evoked DRP. Picrotoxin reduced the LLT-evoked DRP but failed to have any effect on the facilitation or inhibition of the ventral root responses. Naloxone produced an enhancement of dorsal root-evoked polysynaptic reflexes. Although naloxone had no effect on the LLT facilitation of the monosynaptic reflex, it antagonized the LLT inhibition of the polysynaptic reflex.

The effect of diazepam and picrotoxin on brainstem evoked dorsal root potentials

Diazepam has been demonstrated to have the effect of enhancing presynaptic inhibition or facilitation from supraspinal sites in cats as determined by increased negative or positive dorsal root potentials and inhibition or facilitation of the monosynaptic reflex. Experiments were performed on cats made decerebrate at the precollicular level. Dorsal root potentials were recorded from the most caudal rootlet of L6 or L7 and the monosynaptic reflex was recorded from the ventral root of L7. Stimulation was either a single shock to the gastrocnemius-soleus or common peroneal nerve or a 20msec train at 500 Hz to sites in the brainstem reticular formation. The initial negative dorsal root potentials were enhanced by diazepam regardless of origin. Diazepam also enhanced positive dorsal root potentials from the pontine reticular formation. Some pure positive dorsal root potentials from the caudal medullary reticular formation were depressed by diazepam. Picrotoxin was always found, including when diazepam depressed dorsal root potentials, to have the antagonistic action of diazepam; i.e., usually depressing the negative or positive supraspinal dorsal root potential.

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.

Positive dorsal root potentials evoked by stimulation of the brain stem reticular formation

Positive dorsal root potentials evoked by stimulation of the brain stem reticular formation

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.

Properties and distribution of peripherally evoked presynaptic hyperpolarization in cat lumbar spinal cord.

1. The action of peripheral nerve volleys on the polarization of presynaptic terminals of inactive sensory fibres in cat lumbar spinal cord has been investigated by recording (a) the dorsal root potential (DRP), (b) intracellular changes in polarization of single preterminal axons (PAD or PAH), and (c) changes in excitability of populations of preterminal axons.2. Presynaptic hyperpolarization (positive DRP-PAH) can be evoked by stimulation of muscle group III afferents as well as by volleys in cutaneous Abeta, Adelta and C afferents. These volleys can also produce presynaptic depolarization (negative DRP-PAD).3. The positive DRP is observed in the decerebrate state and increases in amplitude following spinalization.4. Picrotoxin blocks the positive DRP at the same dosages required to block the negative DRP. Test negative DRPs are depressed during a conditioning positive DRP. These results are used to support earlier suggestions that the positive DRP results from inhibition of interneurones mediating the negative DRP.5. Trains of group III stimuli at 20/sec evoke a steady positive DRP. Trains of the same intensity at 200/sec evoke a phasic negative DRP. This frequency dependence is observed for PAD and PAH in single sensory axons.6. The DRPs recorded from different dorsal root filaments in response to a given stimulus vary widely in the ratio of negative to positive DRP.7. Intracellular recording from single axons reveals that the same stimuli evoke widely varying ratios of PAD and PAH.8. Stimulation of FRA evokes PAH > PAD in PBST group I afferents, PAD > PAH in sural A fibres and intermediate effects in G-S group I units.9. It is suggested that activation of flexor reflex afferents may selectively potentiate the synaptic efficacy of large muscle afferents mediating the flexor reflex rather than large skin afferents or large afferents from extensor muscles.

A presynaptic mechanism evoked from brain stem reticular formation in the lumbar cord and its temporal significance

Summary (1) In the decerebrate cat, stimulation of a large spectrum of sites in the brain stem reticular formation (RF) evoked negative and postive dorsal root potentials (DRPs) in the lumbar spinal cord. (2) Both negative and postive DRPs are developed as a function of the stimulus intensity delivered to the reticular sites, although the relationship is not linear. (3) There is a well correlated temporal course of the negativity of DRP, the inhibitory phase of monosynaptic reflexes (MSRs), as well as the increased excitability of intraspinal Ia terminals revealing the presence of primary afferent depolarization (PAD), under the influence of the same RF stimulation. These evidences thus point to the existence of a presynaptic inhibitory mechanism exerted from the RF onto the lumbar cord. (4) Concomitant positive DRP and facilitation of MSR on a parallel time basis, the latter of which overrides a preceding inhibition which corresponds in time to the negative wave, has been shown. This is interpreted as the reflection of a hyperpolarization of the primary afferent terminals (PAH) underlying a presynaptic facilitation. (5) It is concluded that at least two separate groups of reticular neurons are exerting their effects, one initiating PAD and the other PAH on the primary afferent terminals in the lumbar cord. These two would be modulating sensory inputs at the presynaptic level through a competitive process.

Cerebellar Control of the Vestibular Pathways to Spinal Motoneurons and Primary Afferents

Publisher Summary This chapter shows that stimulation of the fastigial nucleus produces not only contraction of the extensor muscles, but also negative dorsal root potentials (DRPs) in the lumbar cord. These DRPs are because of depolarization of the terminal arborizations of the primary afferents and involve the group Ia pathway from both extensor and flexor muscles and the cutaneous afferents. The fastigial influence on the extensor muscles is mediated via the lateral vestibular nucleus, which is known to exert a facilitatory influence on the extensor motoneurons. On the other hand, the medial vestibular nucleus is in the transmission of the fastigial influence on primary afferents. Stimulation of the vermal cortex of the cerebellar anterior lobe with the same intensities, which inhibit the decerebrate rigidity, may evoke positive DRPs in the lumbar cord. The same stimulation is also able to suppress the depolarization in the group Ia muscle and the cutaneous afferents elicited by stimulation of the VIII nerve. It appears that neurons in the lateral and the medial vestibular nuclei influence the motoneurons and the primary afferents respectively, therefore, it is concluded that the cerebellar inhibitory control of the vestibular reflex path to motoneurons is exerted through the lateral vestibular nucleus, while the cerebellar inhibitory control of the vestibular reflex path to primary afferents is exerted through the medial vestibular nucleus.

Primary afferent depolarization and flexion reflexes produced by radiant heat stimulation of the skin.

1. The reflex effects of pulses of intense radiant heat applied to the skin of the central plantar pad have been studied in unanaesthetized (decerebrate) spinal cats.2. Pad heat pulses produced flexion of the ipsilateral hind limb and increased ipsilateral flexor monosynaptic reflexes, due to post-synaptic excitation of flexor alpha motoneurones. These effects were accompanied by reduction of extensor monosynaptic reflexes and post-synaptic inhibition of extensor motoneurones.3. Ipsilateral (and contralateral) pad heat pulses consistently evoked negative dorsal root potentials (DRPs) as well as increased excitability of both cutaneous and group Ib muscle afferent terminals. The excitability of group Ia afferents was sometimes also increased during pad heat pulses, but to a lesser extent.4. Pad heat pulses produced negative DRPs in preparations in which positive DRP components could be demonstrated following electrical stimulation of both skin and muscle nerves.5. The motor and primary afferent effects of heat pulses always accompanied one another, beginning after the pad surface temperature had reached rather high levels (usually 48-55 degrees C).6. Negative DRPs increased excitability of cutaneous and group Ib afferents, and motoneurone activation produced by pad heat pulses was essentially unmodified when conduction in large myelinated afferents from the central plantar pad was blocked by cooling the posterior tibial nerve trunk.7. It is concluded that adequate noxious activation of cutaneous afferents of small diameter produces primary afferent depolarization in a variety of large diameter afferent fibres, as well as post-synaptic effects in alpha motoneurones.

Positive dorsal root potentials produced by stimulation of small diameter muscle afferents

Positive dorsal root potentials produced by stimulation of small diameter muscle afferents

RESPONSES OF SINGLE DORSAL CORD CELLS TO PERIPHERAL CUTANEOUS UNMYELINATED FIBRES.

IN the dorsal part of the dorsal horn there is a lamina of cells which respond to cutaneous stimulation and send their axons into the dorsolateral tract. In previous investigations1 it was apparent that many different types of A fibres converged on these cells. It is, therefore, interesting to see whether C fibres also affect their firing. In a recent investigation2 it was found that an afferent volley in the unmyelinated fibres led to a positive dorsal root potential as opposed to the well-known negative dorsal potential which is elicited by the large myelinated fibres. It was suggested that C fibres led to presynaptic hyperpolarization which would produce facilitation as contrasted with the presynaptic inhibitory effects of the A fibres. An investigation of the ventral root reflex (VRR, ref. 2) showed no late component which could be attributed to C fibres; however, a tetanus in the C's was found to potentiate the VRR elicited by the A fibres in the same peripheral nerve. It was also known that a large stimulus to a peripheral nerve led to late discharges in various midbrain and forebrain structures3. With these factors in mind an investigation of the response of cells in the dorsolateral tract of the cat to C fibres was carried out.