Junctional mechanisms at group Ia synapses

Reduced sensory synaptic excitation impairs motor neuron function via Kv2.1 in spinal muscular atrophy

Behavioral deficits in neurodegenerative diseases are often attributed to the selective dysfunction of vulnerable neurons via cell-autonomous mechanisms. Although vulnerable neurons are embedded in neuronal circuits, the contribution of their synaptic partners to the disease process is largely unknown. Here, we show that in a mouse model of spinal muscular atrophy (SMA), a reduction in proprioceptive synaptic drive leads to motor neuron dysfunction and motor behavior impairments. In SMA mice or after the blockade of proprioceptive synaptic transmission we observed a decrease in the motor neuron firing which could be explained by the reduction in the expression of the potassium channel Kv2.1 at the surface of motor neurons. Increasing neuronal activity pharmacologically by chronic exposure in vivo led to a normalization of Kv2.1 expression and an improvement in motor function. Our results demonstrate a key role of excitatory synaptic drive in shaping the function of motor neurons during development and the contribution of its disruption to a neurodegenerative disease.

Normal distribution of VGLUT1 synapses on spinal motoneuron dendrites and their reorganization after nerve injury

Peripheral nerve injury induces permanent alterations in spinal cord circuitries that are not reversed by regeneration. Nerve injury provokes the loss of many proprioceptive IA afferent synapses (VGLUT1-IR boutons) from motoneurons, the reduction of IA EPSPs in motoneurons, and the disappearance of stretch reflexes. After motor and sensory axons successfully reinnervate muscle, lost IA VGLUT1 synapses are not re-established and the stretch reflex does not recover; however, electrically evoked EPSPs do recover. The reasons why remaining IA synapses can evoke EPSPs on motoneurons, but fail to transmit useful stretch signals are unknown. To better understand changes in the organization of VGLUT1 IA synapses that might influence their input strength, we analyzed their distribution over the entire dendritic arbor of motoneurons before and after nerve injury. Adult rats underwent complete tibial nerve transection followed by microsurgical reattachment and 1 year later motoneurons were intracellularly recorded and filled with neurobiotin to map the distribution of VGLUT1 synapses along their dendrites. We found in control motoneurons an average of 911 VGLUT1 synapses; ~62% of them were lost after injury. In controls, VGLUT1 synapses were focused to proximal dendrites where they were grouped in tight clusters. After injury, most synaptic loses occurred in the proximal dendrites and remaining synapses were declustered, smaller, and uniformly distributed throughout the dendritic arbor. We conclude that this loss and reorganization renders IA afferent synapses incompetent for efficient motoneuron synaptic depolarization in response to natural stretch, while still capable of eliciting EPSPs when synchronously fired by electrical volleys.

Quantitative analysis of synaptic contacts made between functionally identified oralis neurons and trigeminal motoneurons in cats

A previous study revealed that rostrodorsomedial oralis (Vo.r) neurons synapsing on trigeminal motoneurons use GABA and/or glycine as neurotransmitters. To determine the number and spatial distribution of contacts, injections of biotinamide and horseradish peroxidase were made into a Vo.r neuron and an alpha-motoneuron in the jaw-closing (JC) and jaw-opening (JO) motor nucleus, respectively, in 39 cats. All Vo.r neurons responded to low-threshold mechanical stimulation of the oral tissues. Single Vo.r neurons terminating in the JC nucleus (Vo.r-dl neurons; n = 5) issued, on average, 10 times more boutons than Vo.r neurons terminating in the JO nucleus (Vo.r-vm neurons; n = 5; 4437 vs 445). The Vo.r-dl neuron-JC alpha-motoneuron pairs (n = 4) made contacts on either the soma-dendritic compartment or dendrites, and the Vo.r-vm neuron-JO motoneuron pairs (n = 2) made contacts on dendrites, with a range of two to seven contacts. In five of the six pairs, individual or groups of two to three terminals contacted different dendritic branches of a postsynaptic cell. The Vo.r-dl neurons innervated a greater number of counter-stained motoneuronal somata than did the Vo.r-vm neurons (216 vs 26). Total number of contacts per Vo.r neuron was higher for the Vo.r-dl than Vo.r-vm neurons (786 vs 72). The present study demonstrates that axonal branches of Vo.r neurons are divided into two types with different innervation domains on the postsynaptic neuron and that they are highly divergent. The overall effect exerted by these neurons is predicted to be much greater within the JC than JO motoneuron pool.

Functional analysis of the sensory motor pathway of resistance reflex in crayfish. II. Integration Of sensory inputs in motor neurons

The in vitro preparation of the fifth thoracic ganglion of the crayfish was used to analyze the connections supporting the monosynaptic reflex responses recorded from the depressor motor neurons (Dep MNs). Dep MNs are directly connected by the release-sensitive afferents from a proprioceptor, the coxo-basipodite chordotonal organ (CBCO), which is released by upward movements of the leg. Sine-wave movements, applied to the CBCO strand from the most released position, allowed us to stimulate the greatest part of release-sensitive CBCO fibers. Systematic intracellular recordings from all Dep MNs performed in high divalent cation saline allowed us to determine the connections between CBCO afferents and their postsynaptic Dep MNs: it highlighted the sequential activation of the different Dep MNs involved in the monosynaptic reflex. The convergence of different sensory afferents onto a given Dep MN, and the divergence of a given sensory afferent onto several Dep MNs illustrates the complexity of the sensory-motor reflex loops involved in the control of locomotion and posture. Electrophysiological experiments and simulations were performed to analyze the mechanisms by which Dep MNs integrate the large amount of sensory input that they receive. Paired intracellular recording experiments demonstrated that postsynaptic response shapes characteristic of both phasic and phaso-tonic afferents could be induced by varying the presynaptic firing frequency, whatever the postsynaptic Dep MN. Compartment model simulations were used to analyze the role of the sensory-motor synapse characteristics in the summation properties of postsynaptic MN. They demonstrated the importance of the postsynaptic compartment geometry, because large postsynaptic compartments allowed to generate greater excitatory postsynaptic potential (EPSP) summations than small ones. The results presented show that velocity information is the most effective to elicit large compound EPSPs in MNs. We therefore suggest that the negative feedback reflex is mainly based on the detection of leg movements.

Horseradish peroxidase study of the spatial and electrotonic distribution of group Ia synapses on type-identified ankle extensor motoneurons in the cat

Eight functionally identified group Ia muscle afferents from triceps surae or plantaris muscles were labeled intraaxonally with horseradish peroxidase (HRP) in seven adult cats. Subsequently, HRP was injected into two to six homonymous or heteronymous alpha-motoneurons per animal (total = 22), each identified by motor unit type and located near the site of afferent injection. The complete trajectories of labeled afferents were reconstructed, and putative synaptic contacts on HRP-labeled motoneurons were identified at high magnification. Dendritic paths from each contact were also mapped and measured. A total of 24 contact systems (the combination of a group Ia afferent and a postsynaptic motoneuron) were reconstructed, of which 17 were homonymous, and seven were heteronymous. Overall, homonymous contact systems had an average of 9.6 boutons, whereas heteronymous contact systems had an average of 5.9 boutons. The average number of boutons found on type S motoneurons in homonymous contact systems was smaller (6.4, range 3-17) than in systems involving types FF or FR motoneurons (FF: 10.4, range 4-18; FR: 11.3, range 4-32). Neither of these differences were statistically significant. In contrast to earlier reports, a majority (15/24) of contact systems included more than one collateral from the same Ia afferent. The complexity (number of branch points) in the arborization pathway leading to each contact (overall mean 8.4 +/- 3.3) was virtually identical in all contact systems, irrespective of the type of postsynaptic motoneuron. The three-dimensional distribution of group Ia contacts was not coextensive with the radially organized dendrites of motoneurons: Dendrites oriented in the ventromedial to dorsolateral axis had the fewest (8%) contacts, whereas rostrocaudal dendrites had the most (63%) contacts. Nevertheless, contacts were widely distributed on the motoneuron surface, with few on and near the soma (< or = 200 microns radial distance from the soma) or on the most distal parts of the tree (> or = 1,000 microns). The boutons in individual contact systems also showed wide spatial and estimated electrotonic distributions; only 3/24 systems had all contact located within a restricted spatial/electrotonic region. The relations between these anatomical results and existing electrophysiological data on group Ia synaptic potentials are discussed.

Morphology and electrophysiology of superior laryngeal nerve afferents and postsynaptic neurons in the medulla oblongata of the cat

Intra-axonal recordings were made from 24 afferent fibres of the superior laryngeal nerve in and around the nucleus tractus solitarius, in 26 pentobarbitone-anaesthetized cats. Conduction velocity ranged from 15 to 38 m/s. Four afferents were injected with horseradish peroxidase. They showed dense terminal arborization in the region of the ventral and ventrolateral subnuclei of the nucleus tractus solitarius, both rostral and caudal to the obex. Six other intra-axonal recordings were thought to originate from axons of neurons postsynaptic to superior laryngeal afferents; one of these was injected with horseradish peroxidase and showed a similar arborization pattern to that of the afferent axons. In the same region, intracellular recordings were made from 124 neurons which responded to superior laryngeal nerve stimulation with excitatory postsynaptic potentials (mean latency 2.7 +/- 1.0 ms). Ninety-nine of these neurons were thought to receive a monosynaptic input. The stimulation threshold evoking these responses was similar to that which inhibited phrenic nerve discharge. Eleven of the monosynaptically excited neurons were injected with horseradish peroxidase. They had fusiform or stellate somata and simple dendritic trees, radiating mainly in the transverse plane. In one experiment, in which both a superior laryngeal nerve afferent fibre and a neuron were labelled, afferent terminal varicosities were found in close apposition with the postsynaptic membrane of the injected neuron. Four of 14 (29%) tested neurons could be antidromically activated from the C3 spinal segment. The stimulus thresholds and onset latencies of the responses of superior laryngeal nerve afferents and medullary neurons to stimulation of the superior laryngeal nerve are consistent with their involvement in the reflex inhibition of respiratory neurons evoked by superior laryngeal nerve stimulation.

In vitro studies of prolonged synaptic depression in the neonatal rat spinal cord

1. Synaptic transmission between dorsal root afferents and alpha-motoneurones was studied in the in vitro hemisected spinal cord preparation isolated from neonatal rats. 2. Repetitive stimulation of the dorsal roots depressed the monosynaptic reflex recorded from the homologous ventral roots. The depression developed within the first five to six pulses in a stimulus train and stabilized at a plateau-like level for many seconds of stimulation. 3. The magnitude of the reflex depression depended on the stimulation interval and was capable of reducing the reflex to 17% of its undepressed control during 5 Hz stimulus trains. Complete recovery from depression was obtained at stimulation intervals greater than or equal to 30 s. 4. Monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded intracellularly after reduction of the activity in polysynaptic pathways by addition of mephenesin to the bathing media. These EPSPs exhibited a prolonged, frequency-dependent synaptic depression. The depression reduced the amplitude of the EPSP to 25% of the undepressed control during 5 Hz stimulus trains, and was alleviated completely at stimulus interval greater than or equal to 60 s. 5. The prolonged EPSP depression was not altered by blockade of glycinergic and type-A gamma-aminobutyric acid (GABAA-ergic) receptors underlying postsynaptic inhibition in the spinal cord. Injection of current steps to motoneurones before and during the prolonged depression revealed similar values of the membrane time constant and input resistance. These excluded changes in the passive properties of the motoneurone membrane as an explanation for the observed synaptic depression. 6. Extracellular recordings of terminal potentials and their accompanying synaptic fields from motor nuclei in the ventrolateral cord revealed that the frequency-dependent depression in the synaptic fields was not preceded by any detectable changes in the amplitude or the shape of the terminal potential, suggesting that the depression cannot be attributed to impairment of action potential invasion to the afferent terminals. 7. Reduction of the basic level of transmitter release in the spinal cord by increasing the Mg2+/Ca2+ ratio of the bathing solution or by application of 2 microM of L(-)baclofen markedly diminished the synaptic potential depression at all the stimulation intervals tested in this study. Recovery from depression was evident for stimulation intervals greater than or equal to 5 s. Under these conditions, short tetanic trains (5 pulses at 25 Hz) revealed a substantial facilitation and potentiation of the EPSPs. 8. We suggest that prolonged depression of synaptic potentials in the neonatal rat reflects decreased transmitter output from the activated afferent terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium facilitation of group Ia EPSPs evoked in cat spinal motoneurones in vivo

The extracellular environment of motoneurones in the cat spinal cord in vivo was altered by means of local perfusion of the central canal. Intracellular recordings were made to determine the effects of raised extracellular Ca2+ or Mg2+ concentration on the monosynaptic afferent excitatory postsynaptic potential (EPSP). Raised extracellular Mg2+ concentration reversibly reduced the EPSP amplitude, whereas raised extracellular Ca2+ concentration produced extremely large increases in the monosynaptic EPSP amplitude, up to almost an order of magnitude. In some cases, a reduction in amplitude of the EPSP and a delay in its onset were also observed, following raised extracellular Ca2+ concentration. This effect was thought to be due to a divalent cation block of the presynaptic action potential. A major conclusion from this study is that group Ia afferent terminals have a much greater transmitter release capacity than suggested by previous studies at this connection.

"Upstream" regulation of the release probability in sympathetic nerve varicosities

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

Synaptic potentials evoked in cat dorsal spinocerebellar tract neurones by impulses in single group I muscle afferents

1. Excitatory postsynaptic potentials (EPSPs) evoked by impulses in single group I muscle afferents were recorded intracellularly in dorsal spinocerebellar tract (DSCT) neurones in the spinal cords of anaesthetized cats. 2. In the same experiments, electrotonic membrane properties of DSCT neurones were measured using the voltage response of each cell to a brief intracellular current pulse. 3. Single group I fibre EPSPs were found to exhibit a large range of amplitudes, from 210 microV to 3.4 mV. All of these EPSPs exhibited uniformly rapid rise times, in contrast to the wide range of time courses exhibited by group I a EPSPs recorded in motoneurones. 4. Electrotonic analysis of DSCT neurones indicated that the time constants of these cells ranged from 5.9 to 18.2 ms, with an average value of 10.9 ms. 5. Current pulse responses of the majority (approximately three-quarters) of DSCT neurones were well described by a simple cable model. Equivalent dendritic cable lengths were calculated for DSCT neurones and found to have an average value of 1.0 space constants, which is considerably less than that calculated for motoneurones. 6. Application of the simple cable model of DSCT neurones demonstrated that the rapid rise-times of single group I EPSPs can be explained by a substantial somatic input to these cells. However, in addition to this strong somatic component, there may also be a contribution from dendritic synapses which prolong the initial decay phase of these EPSPs. The final decay of single fibre EPSPs in DSCT neurones is explained simply by the passive membrane time constant of these cells.

Activation of type B gamma-aminobutyric acid receptors in the intact mammalian spinal cord mimics the effects of reduced presynaptic Ca2+ influx

Intracellular recordings from mammalian spinal motoneurons in vivo show that the type B gamma-aminobutyric acid receptor agonist, L-(-)-baclofen, when administered systemically to pentobarbital-anesthetized or decerebrate unanesthetized cats decreases the amplitude of monosynaptic group Ia excitatory postsynaptic potentials (EPSPs), markedly increases tetanic and posttetanic potentiation, and reduces or abolishes synaptic depression during high-frequency synaptic activation and in the posttetanic period. These changes occur without detectable alteration in motoneuron input resistance, EPSP shape, or the invasion of action potentials into the intraspinal group Ia terminal arborizations. The baclofen-induced effects are qualitatively similar to those observed in more accessible synaptic systems when presynaptic Ca2+ influx and, concomitantly, transmitter release are reduced. Based on these and other recent findings regarding the mechanism of action of baclofen and the distribution of its receptors in the spinal cord, we suggest that L-(-)-baclofen modifies frequency modulation of Ia synaptic transmission by reducing presynaptic Ca2+ influx and the concomitant level of transmitter release from Ia afferent terminals. The drug appears to be a useful tool in studies of the ionic mechanisms that control the release of transmitter and its frequency modulation at inaccessible mammalian synapses.

Differing actions of convulsant and nonconvulsant barbiturates: an electrophysiological study in the isolated spinal cord of the rat

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono- and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 microM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 microM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (-)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(-)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.

Effects of calcium and magnesium on transmitter release at Ia synapses of rat spinal motoneurones in vitro

The lumbar spinal cord excised from neonatal rats was superfused with an oxygenated saline, and monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) were recorded from the lumbar motoneurones following stimulation of muscle nerves of the hind leg. The amplitude of monosynaptic e.p.s.p.s increased with an increase in the external Ca2+ concentration ([Ca2+]o). In [Ca2+]o of 0.5-2 mM, the relation between the e.p.s.p. amplitude and [Ca2+]o was linear with a slope of 1.6 on double logarithmic co-ordinates. An increase in [Mg2+]o in a range of 1-5 mM reduced the e.p.s.p. amplitude without affecting the slope of the relation between log e.p.s.p. amplitude and log [Ca2+]o, suggesting competitive interaction between Ca2+ and Mg2+. When [Ca2+]o was increased to 4-12 mM, the latency of monosynaptic e.p.s.p.s was prolonged, and the antidromic action potential failed to invade the cell body or initial segment of motoneurones. Under these conditions, the monosynaptic e.p.s.p.s were still present. It is concluded that at near-normal levels of [Ca2+]o the effects of Ca2+ and Mg2+ on the e.p.s.p. amplitude are accounted for entirely by their competitive interaction for the probability of transmitter release without altering the mode of impulse propagation at central terminals of the Group Ia sensory fibres. It is suggested that impulses of Ia sensory fibres normally invade their terminals without intermittent blocks at their terminal branch points.

Synaptic excitation in cultures of mouse spinal cord neurones: receptor pharmacology and behaviour of synaptic currents

Fast monosynaptic excitatory post-synaptic potentials between spinal cord neurones in cell culture (s.c.-s.c. e.p.s.p.s) were studied with current-clamp and two-electrode voltage-clamp methods. The reversal potential, response to acidic amino acid antagonists, and behaviour of the synaptic current were examined. The amplitude of the e.p.s.p. increased with membrane potential hyperpolarization and decreased with depolarization. The reversal potential of the e.p.s.p. was +3.8 +/- 2.5 mV (mean +/- S.E. of mean). The reversal potential for responses to ionophoretically applied L-glutamate and L-aspartate was also near 0 mV. The acidic amino acid antagonist, cis-2,3-piperidine dicarboxylic acid (PDA, 0.25-1.0 mM) reversibly antagonized the monosynaptic e.p.s.p.s as well as responses to kainate (KA) or quisqualate (QA). The selective N-methyl-D-aspartate antagonist, (+/-) 2-amino-5-phosphonovaleric acid (APV), had little effect on either the monosynaptic e.p.s.p.s or responses to QA or KA at concentrations that abolished responses to L-aspartate. Under voltage clamp, the peak synaptic current (e.p.s.c.) was linearly related to the membrane potential, increasing in amplitude with hyperpolarization and decreasing with depolarization from the resting potential. The decay of a somatic e.p.s.c. was well fitted by a single exponential function with a time constant of 0.6 ms at 25 degrees C. E.p.s.c.s which had proximal dendritic locations had decay time constants of 1-2 ms. The decay time constant was voltage-insensitive between -80 and +10 mV. We suggest that an acidic amino acid receptor other than that for NMDA mediates excitatory transmission at the s.c.-s.c. synapse; and that the underlying conductance mechanism is voltage insensitive with an estimated mean channel lifetime of less than 1 ms.

Electrophysiological properties of neonatal rat motoneurones studied in vitro

The electroresponsive properties of neonatal lumbar spinal motoneurones were studied using isolated, hemisected spinal cords from neonatal rats aged 3-12 days. The extracellular and intracellular responses to electrical stimulation of the ventral and dorsal root were studied as well as the intracellular response to current injection. Field potentials recorded in the lateral motor area following electrical stimulation of lumbar ventral roots had a triphasic positive-negative-positive wave form. The negative component did not return to the base line smoothly but exhibited a 'shoulder' where the negativity increased in duration. Following electrical stimulation of the dorsal root, presynaptic field potentials were recorded upon activation of the afferent axons as well as following synaptic activation of interneurones and motoneurones. The input resistances of neonatal motoneurones determined from the slope of current-voltage plots were high compared with the adult. The resistance decreased with age with a mean of 18.1 M omega for animals 3-5 days old, 8.8 M omega for animals 6-8 days old and 5.4 M omega for animals 9-11 days old. Values for the membrane time constant were similar to those in the adult with a mean of 4.5 ms. Action potentials elicited by ventral or dorsal root stimulation or by intracellular current injection were marked by a pronounced after-depolarization (a.d.p.) and an after-hyperpolarization (a.h.p.). The amplitude of the a.h.p. varied with that of the a.d.p. The amplitude of excitatory post-synaptic potentials (e.p.s.p.s) elicited by electrical stimulation of the dorsal root was affected by intracellular current injection. Two types of e.p.s.p.s were distinguished: those with a biphasic reversal (early phase first) and those in which the early phase was unaffected by inward current injection while the later phase was reversed. Unlike in the adult, the reversals could be achieved with low current levels and the amplitude of both types of e.p.s.p. was increased by inward current injection. Inhibitory post-synaptic potentials (i.p.s.p.s) were elicited by dorsal or ventral root stimulation. The amplitude of these i.p.s.p.s was diminished and reversed in sign with inward current injection and their amplitude was enhanced with outward current injection. Activation of neonatal motoneurones with long current pulses revealed that there is one steady-state firing range.(ABSTRACT TRUNCATED AT 400 WORDS)

Ia afferent excitation of motoneurones in the in vitro new-born rat spinal cord is selectively antagonized by kynurenate

Intracellular recordings from motoneurones in in vitro preparations of new-born rat spinal cord were used to study the sensitivity of the Ia excitatory post-synaptic potential (e.p.s.p.) to antagonists of excitatory amino acids, in order to test whether group Ia primary afferents release L-glutamate, or a similar compound, as a neurotransmitter. The Ia e.p.s.p. was isolated for study by using low intensity stimulation of individual muscle nerves and by the addition to the superfusate of high concentrations of divalent cations which suppressed polysynaptic inputs to the motoneurones. The pattern of convergence of group Ia afferents from homonymous, heteronymous and antagonist muscle nerves onto motoneurones in the new-born rat was similar to that reported in the adult cat spinal cord. Homonymous muscle nerve stimulation evoked the largest amplitude Ia e.p.s.p.s while heteronymous muscle nerve stimulation elicited smaller e.p.s.p.s or had no effect. Stimulation of antagonist muscle nerves resulted in inhibitory post-synaptic potentials (i.p.s.p.). Superfusion of the specific N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovalerate, did not inhibit the Ia e.p.s.p. but did suppress later, polysynaptic components of the response evoked from dorsal roots. Kynurenate was a potent inhibitor of the Ia e.p.s.p. The site of action of kynurenate was examined by observing its effect on synaptic depression and was found to be consistent with a post-synaptic mechanism. Kynurenate selectively blocked the depolarization of motoneurones elicited by L-glutamate and had no effect on the depolarization evoked by carbachol. The selectivity of action of kynurenate was further examined by comparing its effect on the recurrent i.p.s.p. evoked by ventral root stimulation with its effect on the Ia e.p.s.p. The recurrent i.p.s.p. was antagonized by strychnine and dihydro-beta-erythroidine while kynurenate, at a concentration which greatly reduced the Ia e.p.s.p., had no effect. These results suggest that stimulation of group Ia primary afferents evokes the release of L-glutamate, or a similar compound, which activates non-NMDA excitatory amino acid receptors on motoneurones which, in turn, mediate the Ia e.p.s.p.

Characterization of miniature inhibitory post-synaptic potentials in rat spinal motoneurones

Intracellular recordings were made from motoneurones in the isolated spinal cord of neonatal rats. After action potentials had been abolished by tetrodotoxin (TTX, 10(-6) g/ml), small (approximately 0.4 mV) depolarizing potentials occurred spontaneously in motoneurones at low frequencies (approximately 1.5 Hz). These potentials were detectable only after the intracellular Cl- concentration of motoneurones was raised by using KCl electrodes and most of them were blocked by strychnine, suggesting that they are inhibitory post-synaptic potentials (i.p.s.p.s). These spontaneous i.p.s.p.s under TTX are designated as 'miniature i.p.s.p.s' in order to distinguish them from i.p.s.p.s arising from spontaneous impulse activities of interneurones or afferent fibres. The miniature i.p.s.p.s were still observed after Ca2+ in saline was substituted by Mg2+ or Mn2+. In low Ca2+ and high Mg2+ saline, the amplitude distribution of miniature i.p.s.p.s was essentially the same as in normal saline. The frequency of miniature i.p.s.p.s increased when external Ca2+ concentration was raised. The frequency decreased to about 60% of the control when external Ca2+ was substituted by Mg2+ (2-4 mM), whereas it increased to more than 20-fold when substituted by Mn2+ (3-5 mM). When the external K+ concentration was raised, the frequency of miniature i.p.s.p.s under TTX increased non-linearly with the K+ concentration. The maximum slope in the relation between the log frequency and log K+ concentration was about 3.6. When the osmotic pressure was increased by adding sucrose, miniature i.p.s.p.s increased in frequency. The effect of osmotic pressure was relatively mild compared with that reported for the miniature end-plate potentials (e.p.p.s) in the frog. When the temperature was raised, the frequency of miniature i.p.s.p.s increased. The relation between frequency and temperature fitted approximately to a straight line in Arrhenius plot with a Q10 of about 2.6. These characteristics of the miniature i.p.s.p.s closely resemble those of the miniature e.p.p.s. It is concluded that the miniature i.p.s.p.s recorded in motoneurones are equivalent in nature to the miniature e.p.p.s in neuromuscular junctions, thus reflecting the spontaneous release of quantal packages of the inhibitory transmitter.

The ultrastructure of group Ia afferent fiber synapses in the lumbosacral spinal cord of the cat

Ia synapses in laminae VI and IX of the cat's spinal cord were examined in the electron microscope following iontophoretic injection of horseradish peroxidase (HRP) into single, identified, Ia afferent fibers from gastrocnemius muscles. Ia boutons contacting motoneuron dendrites in lamina IX contained spherical synaptic vesicles and generally contacted only one postsynaptic profile. The Ia boutons were often postsynaptic to smaller P-type axonal terminals. Consequently Ia boutons may be classified as S-boutons with axo-axonic contacts.

Tracing of frog sensory-motor synapses by intracellular injection of horseradish peroxidase

Monosynaptically connected primary afferent fibres and motoneurones of the isolated spinal cord of the frog were injected with horseradish peroxidase (HRP). Six labelled afferent fibre-motoneurone pairs were reconstructed and subjected to detailed analysis. Frog motoneurones possess eight to twelve dendritic arrays displaying some dorso-ventral asymmetry. Dorsal dendrites exhibit a rostro-caudal extent of 1.7-2.6 mm (average 2.2 mm). Primary afferent fibres bifurcate in the dorsal funiculus. First-order collaterals emanate from the main ascending and descending branches, at an average distance of 407 micron. The average number of boutons per collateral is 670. To reach a contacting bouton the presynaptic spike must pass on average five bifurcations and then zero to twelve boutons en passant, attached to a single terminal collateral branch. The structural equivalent of the axon cylinder of the collateral tree roughly preserves cross-sectional area. The branch power ranged between 1.15 and 3.35 (average 2.06). Primary afferent fibres usually form clusters of contacting boutons (contact regions). Connexions between an afferent fibre and a motoneurone comprise from five to twenty-three contact regions (average 12.5). Each contact region contains one to twelve contacting boutons (average 3.3). In two of three experiments contacting boutons were found to be significantly larger than non-contacting boutons. The average diameter of the former was 2.6 micron (range 1.2-4.0). In five out of six cases more than one collateral belonging to the same fibre participated in the connexion with a given motoneurone. The average number of contacting boutons per motoneurone and collateral is 19.1. It was estimated that each collateral could supply not more than thirty-five motoneurones. This would be less than 8.5% of the motoneurones with their dendrites which cross the termination space of a single collateral. The average number of contacting boutons forming one primary motoneurone connexion was 41.5 (range 21-72).

Inhibitory miniature synaptic potentials in rat motoneurons

In the newborn rat spinal cord, spontaneous potentials were recorded, with KCl electrodes, from motoneurons in the presence of tetrodotoxin (10(-6) g ml-1) to abolish nerve impulses. These potentials occurred at low frequencies (less than 2 Hz), and their mean amplitude was a fraction of 1 mV. An increase of osmolarity with sucrose or an increase of extracellular K+, increased the frequency of miniature synaptic potentials. The amplitude of the spontaneous potentials was increased by intracellular injection of Cl-. Strychnine (2-25 microM) completely abolished the spontaneous potentials. It is suggested that these potentials are produced by the spontaneous release of packages of inhibitory transmitter at synapses on motoneurons.

Amplitude fluctuations in synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in spinal motoneurones (of anaesthetized cats) by impulses in single group Ia axons. The morphological details of the Ia axon-motoneurone connexion involved in generating each e.p.s.p. were subsequently reconstructed, as described by Redman & Walmsley (1983). The fluctuation pattern of the peak amplitude of each e.p.s.p. was determined using a deconvolution method, taking into account the recording noise. Four e.p.s.p.s were analysed. One did not fluctuate in amplitude; the others fluctuated between discrete amplitudes which were separated by quantal increments. The number of increments which must be added to produce the largest peak amplitude of each e.p.s.p. was always less than, or equal to, the number of synaptic boutons in the connexion at which the e.p.s.p. was generated. The results are consistent with the hypothesis that transmission occurs in an all-or-none manner at each synaptic bouton. Different boutons in the termination of a Ia axon with a motoneurone have different probabilities of releasing transmitter, and this probability is sometimes zero at low stimulation rates. The results support the idea that the discrete amplitudes of an evoked e.p.s.p. result from intermittent transmission, in an all-or-none manner, at some or all of the boutons in the termination.

The synaptic current evoked in cat spinal motoneurones by impulses in single group 1a axons

Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in motoneurones of anaesthetized cats by impulses in single group 1 a axons. E.p.s.p.s with a time course which indicated a somatic site of origin were voltage-clamped using a single micro-electrode clamp. Excitatory post-synaptic currents (e.p.s.c.s) were found to peak in less than 0.2 ms, and to decay with an exponential time course. The time constant of decay was usually in the range 0.3-0.4 ms (at 37 degrees C). At the resting membrane potential, an e.p.s.p. with a peak of 100 microV was generated by an average peak e.p.s.c. of 330 pA. This corresponded to an average peak conductance increase of 5 nS. The e.p.s.c. decreased with membrane depolarization, and reversed to become an outward current at a null potential of +4.6 +/- 2 mV (+/- S.E. of mean; n = 7). Membrane hyperpolarization caused the peak e.p.s.c. to increase and the time constant of decay of the e.p.s.c. to decrease. The total charge in the synaptic current did not increase with hyperpolarization. This observation can explain earlier observations which showed that the peak amplitude of the e.p.s.p. did not increase with hyperpolarization. The number of ion channels opened by transmitter release at a single somatic bouton was estimated to be in the range 40-240.

Electron microscopic observations on the synaptic contacts of group Ia muscle spindle afferents in the cat lumbosacral spinal cord

After intra-axonal injection of horseradish peroxidase (HRP) into afferent fibers originating from muscle spindle primary endings of the cat gastrocnemius, group Ia boutons located in the ventral horn of the spinal cord were identified and studied electron microscopically. The Ia boutons were invariably found to contain spherical synaptic vesicles (S-type boutons), and a number of them were also postsynaptic to smaller P-type boutons (large S-type boutons with axo-axonic contacts). None of the present Ia-boutons belonged to the previously described M-type. The vast majority of the studied boutons were considered to be located at less than 500 microns distance from the alpha-motoneuron soma. The results are discussed in relation to previous light and electron microscopic data.

Impulse patterns of Ia afferents and Ia-activated DSCT neurons during sinusoidal muscle stretch in cats

The cat gastrocnemius muscles of one hind leg were stretched sinusoidally with amplitudes between 10 micrometer and 2.5 mm and frequencies between 1 and 30 Hz. The stretch response of deefferented muscle spindle afferents and of Ia-activated cells within Clarke's column were investigated by means of extra-cellular recordings of action potentials. The responses to 20-50 cycles were displayed in impulse patterns (raster diagrams) of the responding action potentials. The impulse patterns of Ia afferents exhibited a high degree of phase-locking (regularity) on the stretch cycle of amplitudes of about 50 micrometers at 3 Hz and all higher amplitudes or frequencies. At comparable stretch parameters the regularity in Ia afferents was 4-6 times larger than in Ia-activated DSCT neurons. The regularity in the DSCT patterns increased with an increase in stretch frequency. The impulse patterns of DSCT cells exhibited a high negative correlation between successive interspike intervals (-0.4 to -0.6) at low stretch frequencies (less than 3 Hz), which decreased with an increase in stretch frequency.

Post-tetanic potentiation and facilitation of synaptic potentials evoked in cat spinal motoneurones

1. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in spinal alpha-motoneurones of the cat by impulses in single group Ia nerve fibres. 2. The average peak amplitude of some of these e.p.s.p.s was increased by a conditioning tetanus. The maximum increase observed was 54% of the control amplitude. 3. The average peak amplitude of some e.p.s.p.s was increased by a single conditioning stimulus which preceded the test stimulus by 1 or 2 msec. The maximum increase observed was 28% of the control amplitude. 4. The ability of e.p.s.p.s to potentiate following a tetanus was correlated with their ability to be facilitated by a single conditioning stimulus. 5. If an e.p.s.p. could be facilitated prior to a tetanus, the amount of facilitation was reduced after the tetanus, with all facilitation being abolished when post-tetanic potentiation was maximal. 6. The fluctuations of an e.p.s.p. were analysed before and after a tetanus. The peak amplitudes that an e.p.s.p. fluctuated between while potentiated did not gradually diminish as the effect of the tetanus disappeared. Post-tetanic potentiation, when it occurred, was accompanied by a decrease in the probability of occurrence of components with smaller peak amplitudes and an increase in the probability of occurrence of components with larger peak amplitudes. 7. These results are consistent with the suggestion that the magnitude of the synaptic potential generated at a single bouton does not vary from trial to trial (Jack, Redman & Wong, 1981a). Nor does the amplitude of this potential vary following a single conditioning stimulus or a tetanus. Post-tetanic potentiation and facilitation result from a decrease in the probability of failure to release transmitter following the conditioning stimuli.

Modifications to synaptic transmission at group Ia synapses on cat spinal motoneurones by 4-aminopyridine

1. The average amplitude of e.p.s.p.s evoked in cat spinal motoneurones by impulses in single group Ia afferents usually increased following the intravenous injection of 4-aminopyridine (4-AP). Most of this increase occurred over the first 30 min following injection of 4-AP. 2. The increase in the average amplitude following 4-AP occurred by a reduction in the probability of occurrence of component e.p.s.p.s with smaller peak amplitudes, and an increase in the probability of occurrence of component e.p.s.p.s with larger peak amplitudes. There was no evidence that the discrete amplitudes of components after 4-AP were a result of graded increases of the discrete amplitudes before 4-AP. 3. The interpretation suggested for these results is that each component e.p.s.p. is generated by transmission at a different combination of boutons. At each of these boutons sufficient transmitter is released to saturate all available receptors. The effect of 4-AP is to decrease the probability of failure to release transmitter at each bouton, including some boutons which, before 4-AP, did not release transmitter.

The components of synaptic potentials evoked in cat spinal motoneurones by impulses in single group Ia afferents

1. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in cat spinal motoneurones by impulses in single group Ia afferent fibres. The probability density of the fluctuations in peak amplitude of each e.p.s.p. was calculated from the recorded peak amplitude and the probability density of the recording noise. 2. Most e.p.s.p.s fluctuated between different components (i.e. individual e.p.s.p.s of a particular discrete amplitude) with peak amplitudes which were integer multiples of the increment between successive components. The average peak amplitude of this incremental e.p.s.p. was about 90 microV for e.p.s.p.s generated at or near the soma. 3. In general, the probability density of the peak amplitude could not be described using Poisson or binomial distributions. 4. For many e.p.s.p.s the complete time course of each component could be calculated. There was no variability in the amplitude of these components nor in their latency of onset. For some e.p.s.p.s there were differences in the latency and time course of the components. 5. The increments between successive components of e.p.s.p. generated proximally were no larger (at the soma) than the corresponding increments for e.p.s.p.s generated at more distal dendritic sites. 6. These results and those from subsequent papers (Jack, Redman & Wong, 1981; Hirst, Redman & Wong, 1981) reinforce earlier suggestions that each bouton behaves in an all-or-nothing manner with respect to post-synaptic effect, and the probability of failure varies at different boutons arising from the same afferent.

Postsynaptic control of lumbar motoneuron excitability during active sleep in the chronic cat

A correlated intracellular and extracellular study of lumbar motoneuron excitability during sleep and wakefulness was performed in the chronic, unanesthetized, undrugged, normally respiring cat. Experiments were designed to reveal the extent to which hypotonia during active sleep in mammals is dependent on postsynaptic inhibition of somatic motoneurons. Variations in the antidromic field potential, antidromic and orthodromic spike, EPSP, membrane input resistance and rheobasic current were studied. No change in motoneuron excitability occurred when quiet wakefulness was compared to quiet sleep. A decrease in excitability was present, due to postsynaptic inhibition, during active sleep. Further phasic decreases in excitability, also due to postsynaptic inhibition, occurred during active sleep in conjunction with clusters of rapid eye movements.

Selective effects of (-)-baclofen on spinal synaptic transmission in the cat

When ejected microelectrophoretically near spinal interneurones of cats anaesthetised with pentobarbitone and under conditions where postsynaptic excitability was maintained artificially at a constant level, (-), but not (+), -baclofen selectively reduced monosynaptic excitation by impulses in low threshold muscle (Ia and Ib) and cutaneous (Aalpha) afferents. Polysynaptic excitation of interneurones and Renshaw cells by impulses in higher threshold afferents was less affected, and baclofen had little or no effect on the cholinergic monosynaptic excitation of Renshaw cells. Glycinergic and gabergic inhibitions of spinal neurones were relatively insensitive to baclofen. These stereospecific actions of baclofen, produced by either a reduction in the release of excitatory transmitter or postsynaptic antagonism, suggest that Ia, Ib, and Aalpha afferents may release the same excitatory transmitter which differs from that of spinal excitatory interneurones. Microelectrophoretic (-), but not (+), -baclofen also reduced primary afferent depolarization of ventral horn Ia extensor afferent terminations produced by impulses in low threshold flexor afferents, without altering either the electrical excitability of the terminations or their depolarization by electrophoretic GABA or L-glutamate. This stereospecific action of baclofen is interpreted as a reduction in the release of GABA at depolarizing axo-axonic synapses on Ia terminals.

Dual mode of junctional transmission at synapses between single primary afferent fibres and motoneurones in the amphibian

1. The isolated hemisected frog spinal cord has been used to examine the effects of changes in ionic composition of perfusing medium on the intracellularly recorded e.p.s.p.s produced in single motoneurones by direct stimulation of individual dorsal root fibres through a separate intracellular micro-electrode.2. The monosynaptic single-fibre e.p.s.p.s usually reveal two distinct components, early and late. The early component is resistant to replacement of external Ca(2+) by Mn(2+) whereas the later is reversibly abolished. It is concluded that the junction between primary afferent fibre and motoneurone provides joint electrical and chemical transmission.3. The average amplitude of the unitary electrical e.p.s.p. varies at different junctions from 25 to 430 muV, mean 124 +/- 17 muV (n = 50). It is relatively stable and changes its amplitude only with changes in the height of presynaptic spike. The individual amplitudes observed in a given cell usually have a normal distribution suggesting that any variability in electrical response is entirely due to noise.4. The average amplitude of the single-fibre chemically mediated e.p.s.p. varies from less than 20 muV to 1.7 mV, mean 222 +/- 33 muV (n = 71). A positive correlation was found between the amplitudes of chemical and electrical e.p.s.p.s recorded from different motoneurones.5. Chemical e.p.s.p.s evoked by consecutive impulses in a single presynaptic fibre show statistical fluctuations in amplitude. The fluctuations occur in quantal steps in a manner described by binomial or Poisson statistics. Only in a few cases the deviation from stochastic distribution can be attributed to variable invasion of nerve impulses into the terminal region.6. The amplitude of the unit e.p.s.p. varies between 33 and 104 muV, mean 66.4 +/- 4.3 muV (n = 19). The mean quantum content (m) varies from less than 1 to more than 10. The m is reversibly reduced by Ca(2+) lack and by Mn(2+) until the chemically mediated response fluctuates according to a Poisson distribution with the unit e.p.s.p. equivalent to the single quantum of transmitter.7. When paired or repetitive stimuli are applied to the presynaptic fibre the facilitation of the chemically mediated unitary e.p.s.p. can be observed as well as the post-tetanic potentiation. The amplitude of the electrical e.p.s.p. remains unchanged during the period of post-tetanic potentiation, suggesting that the latter is attributable to some change restricted to the specific presynaptic mechanism responsible for the transmitter release but not to changes in presynaptic spike height.

The recovery of a random variable from a noisy record with application to the study of fluctuations in synaptic potentials

Analysis of fluctuations in the amplitude of evoked synaptic potentials can be severely handicapped by the presence of spontaneous synaptic potentials and recording noise. A numerical procedure has been described whereby it is possible to remove some of the masking effects of this noise from the underlying distribution of the fluctuating synaptic potentials. it is not necessary to make an initial assumption about the type of distribution which will best describe the fluctuations. To use this technique, it is necessary to measure the histograms which approximate the probability densities of both the noise, and the noisy evoked potential. It is also necessary to assume that the statistical mechanisms generating the noise are independent of those mechanisms which cause the fluctuation in synaptic transmission, and that the noise and the evoked potentials add linearly. The statistical reliability of the technique depends upon the amount of noise present, and the sample size. Problems of resolution which arise from finite sampling and high noise levels are discussed.

Feedback control of noradrenaline release as a function of noradrenaline concentration in the synaptic cleft in cortical slices of the rat

Cortical slices incubated with [3H]noradrenaline (NA) were used to study quantitatively the infelucne of the NA concentration in the synaptic cleft on electrically induced release of [3H]NA from adrenergic nerve terminals. Stimulation-induced [3H]-overflow was regarded to be proportional to the NA concentration in the synaptic cleft. High concentrations of piperoxan or clonidine were used to block, or maximally stimulate, respectively, the presynaptic alpha-receptors and thus to eliminate feedback control. These two extreme conditions were thought to delineate maximal feedback range. With alpha-receptors not artifically manipulated, [3H]overflow increased with stimulus intensity, yet not in proportion to the [3H]-overflow when the presynaptic alpha-receptors were blocked, demonstrating increasing feedback inhibition with increasing NA concentrations in the synaptic cleft. Feedback inhibition of NA release was shown to depend in an exponential fashion on NA concentration in the synaptic cleft. The slope of the regression line indicated that maximal inhibition of NA release occurred with NA concentrations in the synaptic cleft 65,000--700,000 times higher than threshold concentration. Data from experiments with different stimulus rates also supported the notion of feedback control of NA release.