Presynaptic Inhibition Of Primary Afferents Research Articles

Requirement of neuronal connexin36 in pathways mediating presynaptic inhibition of primary afferents in functionally mature mouse spinal cord

Electrical synapses formed by gap junctions containing connexin36 (Cx36) promote synchronous activity of interneurones in many regions of mammalian brain; however, there is limited information on the role of electrical synapses in spinal neuronal networks. Here we show that Cx36 is widely distributed in the spinal cord and is involved in mechanisms that govern presynaptic inhibition of primary afferent terminals. Electrophysiological recordings were made in spinal cord preparations from 8- to 11-day-old wild-type and Cx36 knockout mice. Several features associated with presynaptic inhibition evoked by conditioning stimulation of low threshold hindlimb afferents were substantially compromised in Cx36 knockout mice. Dorsal root potentials (DRPs) evoked by low intensity stimulation of sensory afferents were reduced in amplitude by 79% and in duration by 67% in Cx36 knockouts. DRPs were similarly affected in wild-types by bath application of gap junction blockers. Consistent with presynaptic inhibition of group Ia muscle spindle afferent terminals on motoneurones described in adult cats, conditioning stimulation of an adjacent dorsal root evoked a long duration inhibition of monosynaptic reflexes recorded from the ventral root in wild-type mice, and this inhibition was antagonized by bicuculline. The same conditioning stimulation failed to inhibit monosynaptic reflexes in Cx36 knockout mice. Immunofluorescence labelling for Cx36 was found throughout the dorsal and ventral horns of the spinal cord of juvenile mice and persisted in mature animals. In deep dorsal horn laminae, where interneurones involved in presynaptic inhibition of large diameter muscle afferents are located, cells were extensively dye-coupled following intracellular neurobiotin injection. Coupled cells displayed Cx36-positive puncta along their processes. Our results indicate that gap junctions formed by Cx36 in spinal cord are required for maintenance of presynaptic inhibition, including the regulation of transmission from Ia muscle spindle afferents. In addition to a role in presynaptic inhibition in juvenile animals, the persistence of Cx36 expression among spinal neuronal populations in the adult mouse suggests that the contribution of electrical synapses to integrative processes in fully mature spinal cord may be as diverse as that found in other areas of the CNS.

Presynaptic inhibition of primary afferents by depolarization: observations supporting nontraditional mechanisms

Primary afferent neurotransmission is the fundamental first step in the central processing of sensory stimuli and is controlled by pre- and postsynaptic inhibitory mechanisms. Presynaptic inhibition (PSI) is probably the more powerful form of inhibitory control in all primary afferent fibers. A major mechanism producing afferent PSI is via a channel-mediated depolarization of their intraspinal terminals, which can be recorded extracellularly as a dorsal root potential (DRP). Based on measures of DRP latency it has been inferred that this primary afferent depolarization (PAD) of low-threshold afferents is mediated by minimally trisynaptic pathways with pharmacologically identified GABAergic interneurons forming last-order axo-axonic synapses onto afferent terminals. There is still no "squeaky clean" evidence of this organization. This paper describes recent and historical work that supports the existence of PAD occurring by more direct pathways and with a complex pharmacology that questions the proprietary role of GABA and GABA(A) receptors in this process. Cholinergic transmission in particular may contribute significantly to PAD, including via direct release from primary afferents.

The contributions of program and afferent factors to the control of human walking

The relative contributions of program and afferent factors to the control of human locomotion were studied using physical modeling of abnormalities of the biomechanical structure of walking and simultaneous recording of the parameters of movement and muscle electrical activity. During walking, three zones of activity were observed in electromyograms: Mx (maximum-amplitude), Me (medium-amplitude), and L (low-amplitude) zones. The Mx and Me zones of activity were related to the period of programmed excitation of the neuronal sspinal cord structures, and the L zone, to the period of programmed inhibition. The afferent influences were effective only in zones Mx and Me, where they amplified, attenuated, or transferred the muscle activity by means of proprioceptive reflexes. Amplified activity was a result of afferent inflow from the muscle spindles and was observed in the cases of muscle strain and a considerable load on muscles. Attenuated activity was caused by a limited input from the Ia, Ib, and II afferents (during joint immobilization and “switching off” of the receptor field of the ankle joint and foot). The drastic decrease in the muscle activity upon the exclusion of the joint from locomotion was presumably caused by presynaptic inhibition of primary afferents, which was induced by supraspinal structures.

The study of the role of the GABA A and GABA B receptors in presynaptic inhibition of primary afferents of spinal cord of the frog Rana ridibunda

The role of GABAA- and GABAB-receptors in presynaptic inhibition of primary afferent fibers was studied on an isolated preparation of the spinal cord of the frog Rana ridibunda. It is shown that the inhibitory effect of GABA on synaptic transmission from afferent fiber to motoneuron is caused by activation of both GABAA- and GABAB-receptors. A temporal correlation (± 5 min) was shown between the blocking action of bicuculline (a specific antagonist of GABAA-receptors) on primary afferent fiber depolarization (PAD) and its potentiating effect on the excitatory postsynaptic potential (EPSP) at parallel intracellular recording of EPSP in motoneuron and PAD in axons of the dorsal root. As a basis of this correlation, the single GABAA-receptor mechanism is discussed, which mediates the effect of bicuculline on PAD and EPSP. When a specific agonist of GABAB-receptor, baclofen, and an antagonist of GABAB-receptor, 2(OH)-saclofen, were applied, the obtained data indicated an involvement of GABAB-receptors in inhibition of synaptic transmission from afferent fibers to the motoneuron. Analysis of parameters of the unitary synaptic responses recorded in the control experiments and of their changes under the effect of (− )-baclofen indicates that the inhibitory action caused by activation of GABAB-receptors develops at the presynaptic level.

Antidromic discharges of dorsal root afferents in the neonatal rat

Presynaptic inhibition of primary afferents can be evoked from at least three sources in the adult animal: 1) by stimulation of several supraspinal structures; 2) by spinal reflex action from sensory inputs; or 3) by the activity of spinal locomotor networks.The depolarisation in the intraspinal afferent terminals which is due, at least partly, to the activation of GABA A receptors may be large enough to reach firing threshold and evoke action potentials that are antidromically conducted into peripheral nerves. Little is known about the development of presynaptic inhibition and its supraspinal control during ontogeny. This article, reviewing recent experiments performed on the in vitro brainstem/spinal cord preparation of the neonatal rat, demonstrates that a similar organisation is present,to some extent, in the new-born rat. A spontaneous activity consisting of antidromic discharges can be recorded from lumbar dorsal roots. The discharges are generated by the underlying afferent terminal depolarizations reaching firing threshold. The number of antidromic action potentials increases significantly in saline solution with chloride concentration reduced to 50% of control. Bath application of the GABA A receptor antagonist, bicuculline (5–10 μM) blocks the antidromic discharges almost completely. Dorsal root discharges are therefore triggered by chloride-dependent GABA A receptor-mediated mechanisms; 1) activation of descending pathways by stimulation delivered to the ventral funiculus (VF) of the spinal cord at the C1 level; 2) activation of sensory inputs by stimulation of a neighbouring dorsal root; or 3) pharmacological activation of the central pattern generators for locomotion evokes antidromic discharges in dorsal roots. VF stimulation also inhibited the response to dorsal root stimulation. The time course of this inhibition overlapped with that of the dorsal root discharge suggesting that part of the inhibition of the monosynaptic reflex may be exerted at a presynaptic level. The existence of GABA A receptor-independent mechanisms and the roles of the antidromic discharges in the neonatal rat are discussed.

Light microscopic and ultrastructural analysis of GABA‐immunoreactive profiles in the monkey spinal cord

It is hypothesized that terminals containing gamma-aminobutyric acid (GABA) participate in presynaptic inhibition of primary afferents. To date, few convincing GABA-immunoreactive (GABA-IR) axo-axonic synapses have been demonstrated in support of this theory. The goal of this study is to document the relationship between GABA-IR profiles and central terminals in glomerular complexes in lumbar cord of the monkey (Macaca fascicularis). In addition, the relationship between GABA-IR profiles and other neural elements are analyzed in order to better understand the processing of sensory input in the spinal cord. GABA-IR cell bodies were present in Lissauer's tract (LT) and in all laminae in the spinal gray matter except lamina IX. GABA-IR fibers and terminals were heavily concentrated in LT; laminae I, II, and III; and present in moderate concentration in the deeper laminae of the dorsal horn, ventral horn (especially in association with presumed motor neurons), and lamina X. Electron microscopic analysis confined to LT and laminae I, II, and III demonstrated GABA-IR cell bodies, dendrites, and myelinated and unmyelinated fibers. GABA-IR cell bodies received sparse synaptic input, some of which was immunoreactive for GABA. The majority of the synaptic input to GABA-IR neurons occurred at the dendritic level. Furthermore, the presence of numerous vesicle-containing GABA-IR dendrites making synaptic interactions indicated that GABA-IR dendrites also provided a major site of output. Two consistent arrangements were observed in laminae I-III concerning vesicle-containing GABA-IR dendrites: 1) they were often postsynaptic to central terminals and 2) they participated in reciprocal synapses. The majority of GABA-IR axon terminals observed contained round clear vesicles and varying numbers of dense core vesicles. Only on rare occasions were GABA-IR terminals with flattened vesicles observed. GABA-IR terminals were not observed as presynaptic elements in axo-axonic synapses; however, on some occasions, GABA-IR profiles presumed to be axon terminals were observed postsynaptic to large glomerular type terminals. Our findings suggest that a frequent synaptic arrangement exists in which primary afferent terminals relay sensory information into a GABAergic system for further processing. Furthermore, GABA-IR dendrites appear to be the major source of input and output for this inhibitory system. The implications of this GABAergic neurocircuitry are discussed in relation to the processing of sensory input in the superficial dorsal horn and in terms of mechanisms of primary afferent depolarization (PAD).

GABA distribution in a pain-modulating zone of trigeminal subnucleus interpolaris.

A recent model for control of spinal and medullary nociceptive neurons (Basbaum and Fields, 1984) incorporates a gamma-aminobutyric acid-ergic (GABA-ergic) cell into this circuitry and indicates that such elements could act as one substrate for presynaptic inhibition of primary afferents. This concept is supported by a variety of pharmacological and electrophysiological studies. We therefore examined the distribution of GABA-ergic activity in trigeminal subnucleus interpolaris (Vi) by focusing on the types of cells, together with dendritic and synaptic profiles, that are immunocytochemically labeled with an antiserum against glutamic acid decarboxylase (GAD). GAD occurred throughout Vi but was most concentrated in the ventrolateral quadrant and interstitial nucleus. It was localized to groups of small neurons with two to three primary dendrites, and within numerous punctate profiles suggestive of synaptic elements. Electron microscopy revealed labeled dendrites, some of which were postsynaptic to scalloped terminals of presumptive primary afferents. Other labeled dendritic elements, which were quite variable in size, engaged both GAD-labeled and unlabeled synapses. Most GAD synapses displayed clear round vesicles and formed contacts with unlabeled perikarya and a variety of dendritic processes. Numerous GAD-positive synapses were also incorporated into axoaxonic clusters, in which the GAD element was presynaptic to scalloped terminals. Others engaged in serial arrays with other unlabeled terminals, which, in turn, were presynaptic to dendrites. Occasionally, GAD synapses formed contacts with GAD-positive dendrites. These data show that GABA is localized to a variety of neuronal elements in ventrolateral Vi and the interstitial nucleus. These occur in spatial arrangements providing an anatomical substrate for postsynaptic modulation of activity in this area. GABA terminals also appear to be involved in a presynaptic inhibitory mechanism, which may, in some instances, affect transmission in primary afferents.

Presynaptic inhibition of primary afferents.

This may sound like a dry topic, but it's not. T h e concept of presynaptic inhibition of primary afferents is appealing to theoreticians. primarily for clinical reasons. There is an overwhelming amount of evidence that inhibition of pain perception can and does occur in the nervous system. Clinical observations have documented many instances, such as battle injuries and sports injuries, where perception of an appropriate painful stimulus is effectively suppressed, sometimes for long periods of time. The dramatic selective effect of opiates, which mimic enkephalin neurotransmitters, on pain perception is further clinical evidence for specific neural mechanisms to inhibit pain. Various animal studies have documented that a variety of behavioral, pharmacologic, and CNS stimulation manipulations can produce profound and specific analgesic effects. Neuronal recording studies have documented that nociception (responsiveness to presumably painful stimulation) can be suppressed in secondary neurons in the dorsal horn of the spinal cord--the first relay after primary afferents--by many of the manipulatfons mentioned above. There are several ways that neural circuitry could be arranged to cause this inhibition. Two dominant themes in proposals for such circuitry are inhibitory effects on the spinal cord neurons themselves (postsynaptic inhibition), and inhibitory effects on primary afferents which prevent therri from activating secondary

The ultrastructure and synaptic architecture of phrenic motor neurons in the spinal cord of the adult rat.

Although light microscopic studies have analysed phrenic motor neurons in several different species, there has never been an ultrastructural investigation of identified phrenic motor neurons. In addition, electrophysiological studies have raised questions relating to the function of phrenic motor neurons which may be answered only by direct electron microscopic investigation. Thus, the present study was carried out to provide a detailed ultrastructural analysis of identified phrenic motor neurons. Phrenic motor neurons in the spinal cord of the rat were labelled by retrogradely transported horseradish peroxidase (HRP) after transecting the phrenic nerve in the neck and applying the enzyme directly to the central stump of the transected nerve. The results showed that the general ultrastructural characteristics of phrenic motor neurons were similar to those previously reported for other spinal motor neurons. However, phrenic primary dendrites appeared to be isolated from all other dendritic profiles in the neuropil. Primary dendrites were not fasciculated. Fasciculation occurred only among the more distal secondary and tertiary phrenic dendritic branches. Direct dendrodendritic or dendrosomatic apposition was rarely seen; gap junctions between directly apposing phrenic neuronal membranes were not observed. The membranes of adjacent phrenic neuronal profiles were most frequently separated by intervening sheaths of astroglial processes. Myelinated phrenic axons and a phrenic axon collateral were identified. The initial portion of the phrenic axon collateral was cone-shaped, lacked myelin, and thus resembled a miniature axon hillock. In one instance, a large accumulation of polyribosomes was observed within the hillock-like structure of a phrenic axon collateral. Eight morphological types of synaptic boutons, M, P, NFs, S, NFf, F, G and C were classified according to criteria used by previous investigators. Most of these endings (M, NFs, NFf, S and F) made synaptic contact with profiles of labelled phrenic somata and dendrites. F, NFf, and S boutons also terminated on phrenic axon hillocks. C and G boutons contacted exclusively phrenic somata and small calibre dendrites, respectively. P boutons established axo-axonic synaptic contacts with the M and NFs bouton. The morphological findings of the present study provide new data that may be related to phrenic synchronized output and presynaptic inhibition of primary afferents terminating on phrenic motor neurons.

5-Hydroxytryptamine reduces substabce P responses on dorsal horn interneurones: a possible interaction of neurotransmitters

Responses of dorsal horn neurons to iontophoretically administered substance P were attenuated by iontophoretically applied 5-hydroxytryptamine (5-HT). This occurred even when 5-HT alone had no effect on the excitability of the cell. 5-HT did not attenuated responses to iontophoretic application of glutamate. It has been suggested that 5-HT presynaptically inhibits the nociceptive afferents but direct application of 5-HT whilst recording the polarisation of primary afferents in vivo failed to confirm this postulate. These results cannot be explained by a presynaptic inhibition of primary afferents by 5-HT and it is suggested that 5-HT is either acting directly on the substance P receptor or on the receptor-excitation coupling mechanism.

Presynaptic inhibition: the mechanism of protection from habituation of the crayfish lateral giant fibre escape response.

1. Mechanism of protection from habituation of the lateral giant escape reflex of the crayfish was studied. Experiments were designed to determine whether presynaptic inhibition of primary afferents for the reflex occurs following escape command neurone firing, and if so, whether it could account for protection of the first synapse from depression. 2. Synaptic transmission between afferents and interneurone A of the escape reflex is strongly inhibited following giant fibre spikes. 3. Giant fibre firing results in post-synaptic inhibition of interneurone A. However, inhibition of afferent input to interneurone A consistently outlasts both i.p.s.p.s and post-synaptic conductance increases in the neurone; the inhibition, therefore, is probably not exclusively post-synaptic. 4. Giant fibre firing results in excitability changes in sensory afferent terminals as measured by the amplitude of antidromic compound action potentials to focal electrical stimuli applied in the region of afferent terminals in the last abdominal ganglion. The time course of this effect parallels those of protection and inhibition of the first synapse. 5. The magnitude and time course of protection and inhibition of transmission to interneurone A parallel each other closely. Both processes considerably outlast measurable signs of post-synaptic inhibition. 6. We conclude that following giant fibre activity the first synapse of the lateral giant reflex is presynaptically inhibited and the presynaptic inhibition is responsible for the protection effect described in the preceding paper.

Dorsal root potentials after C-fiber stimulation.

A pure volley from C-fibers, set up by electrical stimulation of a cutaneous nerve and subsequent selective blocking of the A-fibers, generates in the cat spinal cord a dorsal root potential (C-DRP). Its polarity is the same as that of the dorsal root potential elicited by stimulation of the A-fibers (A-DRP), thus probably providing presynaptic inhibition of primary afferents. The size of the C-DRP increases in proportion to the size of the C-volley. A preceding A-DRP reduces the C-DRP.

Phasic events indicating presynaptic inhibition of primary afferents to the spinal cord during desynchronized sleep.

Phasic events indicating presynaptic inhibition of primary afferents to the spinal cord during desynchronized sleep.F Baldissera, M G Cesa-Bianchi, and M ManciaF Baldissera, M G Cesa-Bianchi, and M ManciaPublished Online:01 Sep 1966https://doi.org/10.1152/jn.1966.29.5.871MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByStudies on the human spontaneous electromyelogram (EMyeloG). II. Patients with peripheral nerve, root and spinal cord disordersElectroencephalography and Clinical Neurophysiology, Vol. 55, No. 1Spinal reflexes and lateral geniculate nucleus activity during sleep: Quantitative relationshipsExperimental Neurology, Vol. 77, No. 1A caudal spinal source of cervical and forelimb inhibition during sleepExperimental Neurology, Vol. 33, No. 3Supraspinal influences on hindlimb reflex pathways during natural sleep in the kittenBrain Research, Vol. 33, No. 2Spinal eletrogram of freely moving cat: supraspinal and segmental influencesBrain Research, Vol. 22, No. 2Modifications of dorsal root potentials during cortical seizuresElectroencephalography and Clinical Neurophysiology, Vol. 29, No. 2Spinal electrogram of the cat. I. Study of origin by degeneration and ischemiaElectroencephalography and Clinical Neurophysiology, Vol. 25, No. 2An analysis of potential changes in the spinal cord during desynchronized sleepBrain Research, Vol. 6, No. 4A study of eye movements in sleeping newborn infantsBrain Research, Vol. 5, No. 4 More from this issue > Volume 29Issue 5September 1966Pages 871-7 https://doi.org/10.1152/jn.1966.29.5.871PubMed5970672History Published online 1 September 1966 Published in print 1 September 1966 Metrics