Electrical Stimulation Of Dorsal Root Research Articles

MODULATION OF THE MONOSYNAPTIC REFLEX POTENTIALSIN THE DECEREBRATED RATS UNDER THE INFLUENCE OF HYDROXYTRYPTOPHAN.

We studied the serotonin effect on monosynaptic reflex potentials (MSR) of spinal motorneurons in the decerebrated rats in control and after intraperitoneal administration of serotonin precursor – 5-Hydroxytryptophan (5-HTP). MSR of motorneurons in the lumbar spinal cord were registered using electrical stimulation of dorsal root of the 5th lumbar section. During stimulation physiological saline or 5-hydroxytryptophan was injected intraperitoneally. In comparison with average amplitude of the control MSR there were registered significant increase in amplitudes of the MSR (169% and +172%, P <0,001) in animals with injection 5-HTP. These data suggest that serotonin release after 5-HTP administration leads to activation of motorneurons in the lumbar spinal cord. The mechanism of this activation may be related to the weakening of the inhibitory control of interneurons in the transmission pathways of the excitatory influences from muscle afferent to motorneurons and to the postural (antigravity) reflex reactions which necessary for the initiation of locomotion.

Evidence for the existence of nociceptors in rat thoracolumbar fascia

Recently, the existence of nociceptive fibers in fascia tissue has attracted much interest. Fascia can be a source of pain in several disorders such as fasciitis and non-specific low back pain. However, little is known about the properties of fascia nociceptors and possible changes of the fascia innervation by nociceptors under pathological circumstances. In this histologic study, the density of presumably nociceptive fibers and free nerve endings was determined in the three layers of the rat TLF: inner layer (IL, covering the multifidus muscle), middle layer (ML) and outer layer (OL). As markers for nociceptive fibers, antibodies to the neuropeptides CGRP and SP as well as to the transient receptor potential vanilloid 1 (TRPV1) were used. As a pathological state, inflammation of the TLF was induced with injection of complete Freund's adjuvant. The density of CGRP- and SP-positive fibers was significantly increased in the inner and outer layer of the inflamed fascia. In the thick middle layer, no inflammation-induced change occurred. In additional experiments, a neurogenic inflammation was induced in the fascia by electrical stimulation of dorsal roots. In these experiments, plasma extravasation was visible in the TLF, which is clear functional evidence for the existence of fascia nociceptors. The presence of nociceptors in the TLF and the increased density of presumably nociceptive fibers under chronic painful circumstances may explain the pain from a pathologically altered fascia. The fascia nociceptors probably contribute also to the pain in non-specific low back pain.

Minocycline inhibits neurogenic inflammation by blocking the effects of tumor necrosis factor-α.

It has been well established that neurogenic inflammation is one of the major pathological processes underlying inflammatory pain, but there are few effective anti-inflammatory drugs to alleviate such pain. The present study shows that minocycline, a widely used glial activation inhibitor, is effective in reducing neurogenic inflammation. Patch-clamp recordings showed that small sized dorsal root ganglion (DRG) neurons were dramatically excited following intradermal capsaicin injection in the rat hind paw, evidenced by decreased rheobase and membrane threshold. Pretreatment with minocycline (30mg/kg for 1day, intraperitoneal injection) blocked the increased neuronal excitability. Western blot and immunostaining of DRG revealed the activation of satellite glial cells (SGCs) following capsaicin injection. The up-regulation of glial fibrillary acidic protein (GFAP) was significantly inhibited by minocycline pre-administration. Measurement of tumor necrosis factor α (TNF-α) and its receptor, TNF-α receptor 1 (TNFR1), showed that minocycline mainly blocked the up-regulation of TNF-α in SGCs and TNFR1s in neurons following capsaicin injection. The pivotal role of TNF-α in neurogenic inflammation was further supported by the findings that incubation DRG with TNF-α mimicked the increased excitability of DRG neurons induced by capsaicin injection, and that TNF-α application enhanced cutaneous vasodilation in the hind paws induced by antidromic electrical stimulation of dorsal roots. Based on these results, we propose that minocycline is a potential therapeutic drug that can reduce neuronal excitability and neurogenic inflammation by working on SGCs to inhibit the expression of TNF-α.

Nuclear Calcium Signaling in Spinal Neurons Drives a Genomic Program Required for Persistent Inflammatory Pain

Persistent pain induced by noxious stimuli is characterized by the transition from normosensitivity to hypersensitivity. Underlying mechanisms are not well understood, although gene expression is considered important. Here, we show that persistent nociceptive-like activity triggers calcium transients in neuronal nuclei within the superficial spinal dorsal horn, and that nuclear calcium is necessary for the development of long-term inflammatory hypersensitivity. Using a nucleus-specific calcium signal perturbation strategy in vivo complemented by gene profiling, bioinformatics, and functional analyses, we discovered a pain-associated, nuclear calcium-regulated gene program in spinal excitatory neurons. This includes C1q, a modulator of synaptic spine morphogenesis, which we found to contribute to activity-dependent spine remodelling on spinal neurons in a manner functionally associated with inflammatory hypersensitivity. Thus, nuclear calcium integrates synapse-to-nucleus communication following noxious stimulation and controls a spinal genomic response that mediates the transition between acute and long-term nociceptive sensitization by modulating functional and structural plasticity.

An ex vivo preparation of mature mice spinal cord to study synaptic transmission on motoneurons

Mammalian spinal cord motoneurons are highly susceptible to chemical and mechanical disturbances, which imposes substantial difficulties for electrophysiological investigation in acute in vitro preparations. The aim of the present study was to establish an isolated spinal cord preparation from adult mice and to examine the synaptic activities of motoneurons in vitro. We removed the lumbo-sacral cord from the vertebral canal by hydraulic extrusion and maintained the isolated cord in vitro for extracellular recordings. Population spikes of motoneurons were evoked by electrical stimulation of dorsal roots (orthodromic) or ventral roots (antidromic) and these evoked responses could be continuously monitored for 5–6 h. The orthodromic population spikes were reversibly suppressed by the AMPA/kainate receptor antagonist 2,3-dihyro-6-nitro-7-sulfamoylbenzo quinoxaline (NBQX, 10 μM) but they persisted in the presence of the NMDA receptor antagonist d(−)-2-amino-5-phosphonovaleric acid (AP5, 50 μM). The antidromic population spikes exhibited evident paired pulse inhibition when evoked at inter-stimulus intervals of £ 6 ms. Histological examination revealed that basic morphological features of the lumbo-sacral motoneurons were preserved after 3–4 h of in vitro maintenance. This in vitro preparation is ideally suited for the electrophysiological study of synaptic transmission on adult mouse spinal motoneurons.

Brain-Derived Neurotrophic Factor Is Released in the Dorsal Horn by Distinctive Patterns of Afferent Fiber Stimulation

Brain-derived neurotrophic factor (BDNF) is synthesized by small neuron cell bodies in the dorsal root ganglia (DRG) and is anterogradely transported to primary afferent terminals in the dorsal horn where it is involved in the modulation of painful stimuli. Here we show that BDNF is released in the rat isolated dorsal horn after chemical stimulation by capsaicin or electrical stimulation of dorsal roots. Capsaicin superfusion (1-100 microm) induced a dose-dependent release of BDNF, measured using ELISA. The highest dose of capsaicin also induced a depletion of BDNF protein in the dorsal horn. BDNF release was also seen after electrical stimulation of the dorsal roots at C-fiber strength. This release was encoded by specific patterns of afferent fiber stimulation. Neither continuous low-frequency (480 pulses, 1 Hz) nor tetanic high-frequency (300 pulses in 3 trains, 100 Hz) stimulation evoked release of BDNF, although substance P (SP) release was observed under both of these conditions. However, BDNF was released after short bursts of high-frequency stimulation (300 pulses in 75 trains, 100 Hz) along with SP and glutamate. The NMDA antagonist d-AP-5 inhibited electrically evoked BDNF release. BDNF release was also measured after systemic or intrathecal NGF treatment. This upregulated BDNF content in the DRG and increased the capsaicin-evoked release of BDNF. Similarly, the amount of BDNF released by burst stimulation was increased after NGF treatment. This activity-dependent release continued to be encoded solely by this stimulation pattern. These experiments demonstrate that BDNF release in the dorsal horn is encoded by specific patterns of afferent fiber stimulation and is mediated by NMDA receptor activation.

Selective depression of dorsal root-evoked high threshold synaptic excitation by the selective κ opioid receptor agonist enadoline in the neonatal rat hemisected spinal cord in vitro

The present study aimed to compare the actions of the selective κ opioid receptor agonist enadoline (CI-977) with morphine in order to see if there is a heterogeneity of opioid receptors between spinal reflex pathways. High (C- and A-fibre evoked activity) and low (A-fibres only) intensity electrical stimulation of dorsal roots in the neonatal rat hemisected spinal cord preparation in vitro was used to distinguish between synaptic activity measured in the corresponding ventral root. Enadoline selectively depressed the high intensity-evoked EPSP with an EC 50 of 7.6 nM ( n=7), contrasting with our previous finding in this preparation that morphine is an equipotent depressant of A- and C-fibre-mediated synaptic responses. The depressant effects of enadoline and morphine were reversed by naloxone giving apparent K d values of 14±3 nM ( n=4) for enadoline-induced and 4.2±1 nM ( n=4) for morphine-induced depression. These data suggest that activation of κ opioid receptors has a selective depressant action on C-fibre-mediated synaptic activity. Such a functional difference mediated at a subclass of opioid receptors has not been previously observed in an in vitro spinal preparation.

Antagonism of mGlu Receptors and Potentiation of EPSCs at Rat Spinal Motoneurones In Vitro

The patch-clamp technique has been used to record synaptic responses, elicited by electrical stimulation of dorsal roots, in 28 single motoneurones of in vitro spinal cord preparations from neonate (P5 to P8) rats. The effects of ( RS)-α-methyl-4-phosphonophenylglycine (MPPG) (200 μM), a potent antagonist at l-2-amino-4-phosphonobutanoate (AP4)-sensitive receptors, and ( RS)-α-methyl-4-carboxyphenylglycine (MCPG) (500 μM), which is a less selective antagonist of mGluRs, were tested on EPSCs alone and as antagonists of AP4-induced depression of EPSCs. The ec 50 for depression of EPSCs by AP4 (1.16 ± 0.12 μM, n = 8) was increased to 18.9 ± 0.7 μM ( n = 6) by MPPG. MCPG (500 μM) had no significant effect on the depressant potency of AP4. Under control conditions, EPSCs had mean peak amplitudes of 983 pA ± 64 SEM and mean charge transferred of 306 ± 37 pC ( n = 28). These values were increased significantly ( p < 0.05) to 1168 ± 68 pA and 363 ± 39 pC by MPPG ( n = 6), and 1150 ± 54 pA and 358 ± 33 pC ( n = 6) by MCPG. There was no significant difference between the enhancement of the initial peak of the EPSCs (mean latency from stimulus artifact 5.9 ± 0.3 ms) and later components, suggesting mGluRs to be present on primary afferent terminals presynaptic to motoneurones as well as in pathways via interneurones. These results are consistent with the presence of at least two types of presynaptic mGluR that modulate release of glutamate in segmental pathways convergent onto motoneurones. These receptors appear to be activated by interstitial glutamate tonically present in the present preparations. © 1997 Elsevier Science Ltd. All rights reserved.

Metabotropic glutamate receptor antagonists, like GABA B antagonists, potentiate dorsal root-evoked excitatory synaptic transmission at neonatal rat spinal motoneurons in vitro

Recordings of whole-cell synaptic current responses elicited by electrical stimulation of dorsal roots were made from motoneurons, identified by antidromic invasion, in isolated spinal cord preparations from five- to eight-day-old Wistar rats. Supramaximal electrical stimulation of the dorsal root evoked complex excitatory postsynaptic currents with mean latencies (±S.E.M.) of 6.1±0.26 ms, peak amplitude of −650±47 pA and duration of 4.30±0.46 s ( n=34). All phases of excitatory postsynaptic currents were potentiated to approximately 20% above control levels in the presence of the metabotropic glutamate receptor antagonists S-2-amino-2-methyl-4-phosphonobutanoate (MAP4; 200 μM; n=15) and 2 S,1′ S,2′ S-2-methyl-2-(carboxycyclopropyl)glycine (MCCG; 200 μM; n=9). A similar level of potentiation was produced by the GABA B receptor antagonist 3- N[1-( S)-(3,4-dichlorophenyl)ethyl]amino-2-( S)-hydroxypropyl- P-benzyl-phosphinic acid (CGP55845; 200 nM; n=5). MAP4 (200 μM) produced a six-fold rightward shift in the concentration–effect plot for the depressant action of the metabotropic glutamate receptor agonist S-2-amino-4-phosphonobutanoate ( L-AP4), whereas CGP55845 produced no significant change in the potency of l-AP4. MAP4 did not antagonize the depressant actions of baclofen ( n=8), 1 S,3 S-1-aminocyclopentane-1,3-dicarboxylate ( n=4) or 2 S, 1′ S, 2′ S-2-(carboxycyclopropyle) glycine ( n=4). The metabotropic glutamate receptor antagonists produced no change in the holding current of any of the neurons, indicating that they had no significant postsynaptic excitatory actions. These results are the first to indicate a possible physiological role for metabotropic glutamate receptors in the spinal cord. Like GABA B receptors, they control glutamatergic synaptic transmission in the segmental spinal pathway to motoneurons. This is likely to be a presynaptic control mechanism.

Sumatriptan inhibits the release of CGRP and substance P from the rat spinal cord.

The possible presynaptic action of the anti-migraine drug sumatriptan on primary afferent fibres containing substance P and/or calcitonin gene-related peptide was investigated on superfused rat horizontal spinal cord slices with attached dorsal roots. Electrical stimulation of dorsal roots triggered a significant overflow of both peptides; this could be reduced by sumatriptan in a concentration-dependent manner. As expected from the involvement of 5-HT1B/1.D beta receptors, methiothepin, (-)tertatolol and GR 127,935, but not WAY 100,635, prevented the inhibitors effect of sumatriptan. These data support the idea that the anti-migraine action of sumatriptan may involve, at least in part, a presynaptic inhibitory control of nociceptive (trigeminovascular) substance P- and/or calcitonin gene-related peptide-containing sensory fibres.

Antagonism of baclofen-induced depression of whole-cell synaptic currents in spinal dorsal horn neurones by the potent GABA B antagonist CGP55845

The potencies of two GABA B receptor antagonists P-[3-aminopropyl]- P-diethoxymethyl-phosphinic acid (CGP35348) and the novel compound 3- N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl- P-benzyl-phosphinic acid (CGP55845) have been compared in an in vitro spinal cord preparation. They have been tested as antagonists of baclofen-induced depression of EPSCs of patch-clamped dorsal horn neurones following electrical stimulation of dorsal roots. Mean EC 50 values for the depressant action of baclofen were increased by 50- and 140-fold respectively in the presence of CGP35348 (200 μM) ( n = 5) and CGP55845 (100 nM) ( n = 4). This potency of CGP55845 is > 1000-fold higher than that reported previously for other GABA B receptor antagonists.

Synaptic activation of dorsal horn neurons by selective C-fibre excitation with capsaicin in the mouse spinal cord in vitro

Low doses (0.2–0.8 μM) of capsaicin were used to achieve selective excitation of C-fibres and the consequent synaptic activation of dorsal horn neurons (laminae I–VI) in the spinal cord of the 12–20-day-old mouse, maintained in vitro. Most dorsal horn cells were activated by application of capsaicin to dorsal root ganglia. The response consisted of a long-lasting membrane depolarization with increased regenerative (synaptic) activity in 79% of the cells, and in a further 7% only an increased synaptic activity was evoked. These effects of capsaicin were completely blocked by removing extracellular calcium ions from the superfusate to the spinal cord, or by the addition of 1 μM tetrodotoxin, suggesting a presynaptic origin of the capsaicin action. Only 67% of cells excited by capsaicin were sensitive to exogenous substance P. The excitatory amino acid antagonists, kynurenic acid (50–100 μM) or (−)-2-amino-5-phosphonovaleric acid (10–20 μM) completely blocked the capsaicin-evoked response in deep dorsal horn cells, indicating the involvement of excitatory amino acid receptors in the synaptic pathway. However, in superficial dorsal horn neurons these antagonists attenuated, but never completely abolished, the capsaicin-evoked depolarization. The kynurenic acid-resistant component of the capsaicin-evoked excitation in superficial dorsal horn cells suggests the involvement of non-amino acid excitatory transmitters—possibly neuropeptides—in the synaptic transmission. Activation of primary afferents by high-intensity electrical stimulation of the dorsal roots induced a prolonged (0.5–3 s) postsynaptic excitation in the majority of deep dorsal horn cells. The duration of the synaptic response was significantly reduced by (−)-2-amino-5-phosphonovaleric acid. Following repeated application of capsaicin, desensitization of the capsaicin-evoked synaptic activation of dorsal horn cells occurred. This effect was paralleled with the loss of the prolonged (−)-2-amino-5-phosphonovaleric acid-sensitive phase of the excitatory postsynaptic potential evoked by the high-intensity electrical stimulation of dorsal roots. This observation suggested that activation of the N-methyl- d-aspartate receptors in the dorsal horn can be activated by small-calibre capsaicin-sensitive fibres. In summary, our data suggest that the selective activation of C-fibre afferents with capsaicin produces synaptic activity in the dorsal horn which has a strong excitatory amino acid component as well as a non-excitatory amino acid, possibly peptidergic, component.

APV-sensitive dorsal root afferent transmission to neonate rat sympathetic preganglionic neurons in vitro

1. Intracellular recordings were made from antidromically identified sympathetic preganglionic neurons (SPNs) in transverse thoracolumbar spinal cord slices from neonate (12- to 22-day-old) rats. 2. Electrical stimulation of dorsal roots or dorsal root entry zone elicited in SPNs an excitatory postsynaptic potential (EPSP) or multiple EPSPs of varying latencies. The EPSP could be graded by varying the stimulus intensity and, on reaching the threshold, discharged an action potential. 3. The dorsal root-evoked EPSPs had a mean synaptic latency of 2.6 ms (range: 1.2-11 ms), suggesting a polysynaptic pathway. The EPSPs were characteristically slow in onset with a mean rise time and half-decay time of 8.3 and 23 ms, respectively. 4. At the resting membrane potential of -50 to -60 mV, the amplitude of EPSPs recorded in normal (1.3 mM Mg2+) Krebs solution was reduced by membrane hyperpolarization or depolarization. In Mg2(+)-free solution, EPSPs were potentiated and reached threshold for spike discharge. 5. The EPSPs were suppressed by the nonselective glutamate receptor antagonist kynurenic acid (0.1-0.5 mM) and by the N-methyl-D-aspartate (NMDA) receptor antagonists D-2-amino-5-phosphonovaleric acid (APV; 1-10 microM) and ketamine (5-10 microM), but not by the quisqualate (QA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1-10 microM). The latter depressed the EPSPs elicited by stimulation of lateral funiculus in the same SPNs. 6. NMDA applied by pressure elicited a depolarization in the SPNs. In normal Krebs solution the response was voltage dependent with the peak amplitude occurring around -60 mV; conditioning depolarization or hyperpolarization diminished the response.(ABSTRACT TRUNCATED AT 250 WORDS)

An in vitro mature spinal cord preparation from the rat

The preparation of an isolated hemisected spinal cord preparation, maintained in vitro, from mature (180–300 g body weight) rats is described. Sacral and coccygeal segments (S2-Co1) gave consistent ventral root reflexes (DR-VRP) from electrical stimulation of dorsal roots. The mean latency and amplitude of the fastest component in the ventral root reflex, at 25°C, were 1.6 msec ± 0.4 SE mean and 8.2 mV ± 0.9 SE mean, respectively (28 preparations). This component was resistant to the NMDA antagonist, 2-amino-5-phosphonopentanoate (AP5) but was depressed markedly by kynurenate. A slow component of the ventral root reflex, which was sensitive to AP5 was enhanced and spontaneous AP5-sensitive synaptic potentials sensitive to AP5 appeared in the absence of magnesium ions. The excitant amino acids l-aspartate, l-glutamate, N- methyl- d-aspartate (NMDA), kainate and quisqualate produced dose-dependent depolarizing responses in the ventral roots. The relative depolarizing potencies ± SE mean (N) of NMDA, kainate and quisqualate, relative to l-glutamate = 1, were 96 ± 30 (6), 234 ± 57 (6) and 145 ± 40 (5), respectively. These properties, apart from reduced latency of synaptic responses, are similar to those described previously for preparations from immature animals. However, it will be easier with the mature preparation to selective activate high and low threshold primary afferents.

Slow depolarizing potentials recorded from glial cells in the rat superficial dorsal horn.

1. Intracellular potentials were recorded from inexcitable cells in the superficial dorsal horn of the rat lumbar spinal cord in vitro. 2. These cells had a large resting membrane potential (up to -90 mV) but when depolarized were unable to produce action potentials. The resting potential was highly dependent upon extracellular K+ concentrations ([K+]o), with a slope of about 45 mV for a 10-fold change in [K+]o. 3. Electrical stimulation of dorsal roots evoked a slow depolarizing potential lasting for many seconds in the inexcitable cells. The slow potentials were not accompanied by any appreciable change in input conductance, nor were their amplitudes dependent upon the membrane potential. 4. The morphology of the inexcitable cells marked by intracellular injection of a fluorescent dye, Lucifer Yellow, was consistent with their being glial cells. 5. [K+]o values were measured by K+-sensitive electrodes simultaneously with intracellular glial potentials. Changes in the K+-electrode potential occurred in parallel with slow depolarizing potentials following stimulation of dorsal roots. When the stimulus intensity was altered, changes in the magnitude of the K+-electrode response were correlated with those of the slow potentials. 6. Replacement of extracellular Ca2+ by Mg2+ abolished both the slow potentials and K+-electrode responses evoked by afferent stimulation, suggesting that impulses in primary afferent fibres do not directly contribute to the glial slow potentials. 7. It is concluded that the slow potentials in glial cells result from a transient increase in [K+]o at the superficial dorsal horn, which is induced by activation of neighbouring internuncial neurones.

Vasoactive intestinal polypeptide nullifies cytochrome reduction response to afferent activation of the perfused bullfrog brain

Redox states of cytochromes a(a 3), bandc+c 1 were continuously measured on the surface of the cerebral hemisphere of the perfused bullfrog brain with the aid of a scanning organ spectrophotometer. To correlate with changes of the redox state, electrical activity of the brain was simultaneously recorded in the vicinity of the tip of the light guide of the spectrophotometer. Electrical stimulation of the third dorsal root of the spinal cord induced parallel reduction of the cytochromes and burst activity in the brain. Intracerebroventricular (i.c.v.) infusion of vasoactive intestinal polypeptide (VIP; 30 pmol/100 g body weight) nullified the cytochrome reduction induced by the electrical stimulation of the dorsal root, but did not attenuate the burst activity. The i.c.v. infusion of VIP (30 pmol/100 g b. wt. per se had little if any influence upon both redox state and burst activity. The infusion of 1 nM VIP into the internal carotid arteries also nullified the cytochrome reduction response to the afferent electrical activation. The present results provide evidence that one of the physiological roles of the VIPergic neurons in the brain may be to balance the redox states of electron transfer systems in neurons.

Evidence supporting the indirect depolarization of primary afferent terminals in the frog by excitatory amino acids.

1. Isolated hemisected spinal cords of the frog have been used to investigate the way excitant amino acids depolarize primary afferent fibres and terminals.2. GABA and excitant amino acids caused depolarization in dorsal roots. But dorsal roots sectioned at the point of exit from the spinal cord responded only to GABA.3. Prolonged application of kainate or N-methyl-D-aspartate to hemicords caused a depolarization of dorsal roots in association with an increased extra-cellular [K(+)]. The two effects decayed with similar time courses. The depolarization recorded from ventral roots was maintained in the presence of the excitants.4. Field potentials, elicited by electrical stimulation of ventral roots and recorded in the ventral horn of Mg blocked preparations, were abolished by prolonged treatment with kainate. Corresponding dorsal horn field potentials elicited by electrical stimulation of dorsal roots were resistant to the presence of kainate.5. Excitability of motoneurones or afferent terminals was measured from the amplitude of action potentials evoked by submaximal cathodal stimulation of ventral or dorsal horns and recorded in ventral or dorsal roots respectively. Prolonged application of kainate to Mg blocked preparations abolished the excitability of motoneurones within 5 min, but the excitability of primary afferent terminals was increased and maintained for several hours.6. These observations support the hypothesis that primary afferent terminals in the frog do not have receptors for excitatory amino acids and that depolarization of terminals induced by excitatory amino acids is mediated through release of K from other cells within the dorsal horn.

Mixed (chemical and electrical) synapses on frog spinal motoneurons.

Freeze-fracture replicas and ultrathin sections were used to characterize the gap junctions on the somata and large dendrites of frog motoneurons found earlier by Sonnhof et al. (1977). In freeze-fracture replicas one of the specific features of these relatively frequent gap junctions is the presence of circular regions of non-junctional membrane ("fenestrae") within areas of typical gap junction appearance displaying P-face particles or E-face pits. Such "fenestrated" gap junctions are mostly associated with membrane specializations indicative of the active zone of a chemical synapse (including vesicle attachment sites in non anaesthetized animals) to constitute mixed synapses. These findings could be verified in ultrathin sections, which revealed that the vesicles of the chemical component of the mixed synapses were spherical and agranular. Our results suggest that the mixed synapses are predominantly axo-somatic and axo-dendritic. The existence of dendro-dendritic gap junctions in the ventral horn region as described by Sotelo and Taxi (1970) was verified in ultrathin sections; they were rare, solely electrotonic in character, and probably represent the morphological basis for the VR-EPSP (Katz and Miledi, 1963; Kubota and Brookhart, 1963), i.e. electrotonic coupling between motoneurons of different spinal segments (Washizu, 1960). Electrotonic coupling can also be demonstrated between motoneurons and afferent fibers of the dorsal root and the lateral column. Electrotonic potentials recorded within motoneurons during electrical stimulation of dorsal root or lateral column precede the chemical postsynaptic potentials; after Mn2+-blockade of chemical synaptic transmission, the electrotonic component persists. Some fibers of these afferent pathways are therefore assumed to act monosynaptic on the motoneuron via mixed axo-somatic and axo-dendritic synapses.

Observations on stretch reflexes in lumbar back muscles of the cat.

Reflex responses to brief muscle stretch and to electrical stimulation of dorsal roots were studied in cat longissimus and iliocostalis muscles by myographic and electromyographic recordings in spinal and anesthetized preparations. Brief stretch applied simultaneously to both muscles, by pulling at an isolated segment of the iliac bone, elicited contractions only in the central region of longissimus, composed of slowly contracting fibers, the time to peak tension varying between 70 and 100 ms. No reflex responses were observed in the faster contracting parts of this muscle or in the iliocostalis under the stimulation conditions used. The total reflex time usually varied from 4.5 to 7 ms which is shown to correspond to intra-spinal conduction times, approximately between 2 and 5 ms. Only exceptionally was a central reflex delay corresponding to a monosynaptic transmission observed. Reflexes evoked by dorsal root simulation (L4, L5) have a central conduction time similar to those elicited by adequate stimulation. The possibility that the central pathway for stretch reflexes in longissimus may involve more than two neurons is considered. In decerebrate preparations the central region of longissimus displays a prominent tonic stretch reflex. With the longissimus in situ the magnitude of the reflex is highly dependent on the position of the lumbar spine relative to the pelvic girdle.

Operative treatment of chronic pain with electrical stimulation of dorsal root of spinal cord : T. Riechert, H. Kapp, J.-U. Karnik, C. L. Schmidt and U. Thoden, Ditsch. med. Wschr., 98 (1973) 1130–1131

Pain: September 1975 - Volume 1 - Issue 3 - p 308 doi: 10.1016/0304-3959(75)90056-1