Role of group-II and -III metabotropic glutamate receptors in the modulation of miniature synaptic activity in frog spinal-cord motoneurons

Results of the present work demonstrate the pronounced modulating effects mediated by group-II and-III metabotropic glutamate receptors (mGluRs) on miniature postsynaptic potentials (mPSPs) of frog spinal motoneurons. The character of the effects of the group-II and-III mGluRs ligands, i.e., changes in the mPSPs frequency and the absence of significant changes in their amplitude, indicates the presynaptic mechanism of the modulation due to a change of the process of transmitter release. The application of ethylglutamate (EGLU) and methylaminophosphobutyrate (MAP4), which are selective antagonists of group-II and-III mGluRs, increased frequency of mPSPs by an average of 52.8 ± 30.2% (in four out of six motoneurons) and by 54.7 ± 23.7% (in all 7 motoneurons), respectively. The application of group-III mGluRs agonist L-aminophosphobutyrate (L-AP4) decreased the mPSP frequency by 21.8 ± 5.2% in three out of five motoneurons. The efficiency of the use of an antagonist and the comparatively low efficiency of the agonist suggest that presynaptic mGluRs are tonically activated during motoneuronal synapses. The absence of a group-II mGluR antagonist effect in some motoneurons appears to be explained by the specific localization of group-II mGluRs in the preterminal area distant from the transmitter release site. The modulation of pharmacologically isolated inhibitory miniature activity and its glycine and GABAergic fractions due to the group-III mGluRs-mediated heteroreceptor was investigated. The MAP4 application was shown to increase the glycine-mediated mIPSPs frequency to a greater degree than the GABA-mediated mIPSPs frequency, as their modulations were equal to an average of 97.6 ± 20.7% (n = 7) and 54.6 ± 20.8% (n = 5), respectively. This difference might possibly be due to the segregation of the postsynaptic glycine and GABA_A receptors. The study of the convergence of the modulating effects of the presynaptic mGluRs and metabotropic GABA_B receptors has shown that, under the condition of the blockage of the tonically active GABA_B receptor by phaclofen, the application of the group-III mGluR agonist L-AP4 produces the typical effect, which was completely eliminated by subsequent application of the group-III mGluRs antagonist MAP4. This result agrees with the point of view regarding the independence of effects mediated by GABA_B receptors and group-III mGluRse.     

The role of group II and III metabotropic glutamate receptors in modulation of miniature synaptic activity in frog spinal cord motoneurons

The results of present work demonstrate significant modulating effects mediated by group II and III mGluRs on miniature postsynaptic potentials (mPSP) of the frog spinal motoneurons. The mode of group II and III mGluRs ligands influences, i. e. the changes in the mPSPs average frequency without significant changes in their average amplitude, suggests the presynaptic mechanism of modulation by the change in transmitter release. Selective antagonists of group II and III mGluRs (EGLU and MAP4) increased the average frequency of mPSPs by 52.8 +/- 30.2% (in 4 of 6 motoneurons) and by 54.7 +/- 23.7% (in all 7 motoneurons), respectively. Application of the group III mGluRs agonist LAP4 decreased the mPSPs frequency by 21.8 +/- 5.2% in 3 of 5 motoneurons. The efficiency of the antagonist usage and comparative low efficiency of the agonist suggest that presynaptic mGluRs at motoneuronal synapses under normal condition possess some level of tonic activity. The lack of group II mGluR antagonist effect on some motoneurons appears to be explained by specific localization of the group II mGluRs in preterminal area which is distant from the transmitter release site. The hetero-receptor modulation of pharmacologically isolated inhibitory miniature activity and its glycine- and GABAergic fractions by group III mGluRs was investigated. MAP4 application has been shown to increase the glycine-mediated mlPSPs frequency more than GABA-mediated mlPSPs frequency: in average by 97.6 +/- 20.7% (n = 7) and 54.6 +/- 20.8% (n = 5), respectively. This difference may be due to the segregation of the postsynaptic glycine- and GABA-receptors. The preliminary examination of the convergence of the presynaptic mGluRs and metabotropic GABA(B) receptors influences on GABA-mediated IPSPs was undertaken. It has been shown that presynaptic GABA(B) receptors are tonically active under normal condition. Under condition of GABA(B) receptor blockage by phaclofen, the application of group III mGluR agonist L-AP4 elicited typical effect which was completely taken off by subsequent application of the group III mGluRs antagonist MAP4. This result is in accordance with the assumption that the effects mediated by GABA(B) receptors and mGluRs are independent.     

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.     

Cortico-pyramidal and cortico-extrapyramidal synaptic influences on lumbar motoneurons in monkeys

Postsynaptic potentials evoked by stimulation of the motor cortex or pyramids before and after acute pyramidotomy were investigated in the lumbar motoneurons of monkeys. In response to activation of fibers of the pyramidal tract monosynaptic EPSPs predominated in motoneurons innervating the distal muscles of the hind limbs. Monosynaptic EPSPs in the motoneurons of the distal muscles had a significantly higher amplitude and could be evoked by weaker stimuli than EPSPs in the motoneurons of the proximal muscles. Cortico-motoneuronal EPSPs in the motoneurons of the distal muscles had a less marked frequency potentiation than EPSPs with monosynaptic segmental delay in the motoneurons of the proximal muscles. Cortico-extrapyramidal synaptic responses appeared in the pyramidotomized monkeys during intensive repetitive stimulation of the motor cortex in motoneurons of both distal and proximal muscles. These effects, transmitted by descending projections of the brain stem, may be responsible for the partial preservation of cortical motor control after pyramidotomy.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 587–596, November–December, 1972.     

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus.

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.     

Rubrospinal synaptic influences on the lumbar motoneurons of the rat

Intracellular recording was employed in experiments on rats with the nervous system intact and after acute pyramidotomy to study the postsynaptic effects produced in the lumbar motoneurons on stimulation of the nucleus ruber. Stimulation of this nucleus with single stimuli and with a short series of stimuli caused excitatory and inhibitory postsynaptic potentials (EPSP and IPSP) to develop in the motoneurons. Most of the EPSP recorded were disynaptic, but response development involved a monosynaptic segmental delay in five of the 124 cells that exhibited EPSP. A capacity for high-frequency potentiation was a characteristic feature of the disynaptic excitatory and inhibitory effects. Transmembrane polarization of the motoneurons had a marked influence on the amplitude of the disynaptic EPSP and IPSP. The properties of the disynaptic rubrospinal influences were similar to those described for the cat.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 266–273, May–June, 1971.     

Ultrastructure and synaptic architecture of spinal motoneurons in the frog (Rana catesbeiana)

An electron-microscopic analysis of spinal motoneurons and their synapses was carried out in a frog (Rana catesbeiana). Six different types of boutons (S, F, M, P, C and GS) have been identified. Their distribution on spinal motoneuron somata and proximal dendrites is described. The mean linear percentage of the surface area covered by boutons is 26.1 +/- 1.9%. S-type boutons are preferentially concentrated on the soma and proximal dendrites. The relative number of S-type boutons (58.7%) was greater (p less than 0.01) than that of F-type boutons (41.3%). This is in contrast to mammalian spinal motoneurons where F-type boutons are much more numerous on the soma than S-type boutons. F-type boutons are randomly distributed and the average ratio of S:F-type boutons is 20:14 (S:F ratio = 1.4). In contrast, M-type boutons synapse exclusively on the distal part of the dorsal dendrites and are restricted to the intermediate zone or to the dorsal horn. P-type boutons form synapses upon the large M-type boutons. The polarity of these axoaxonic synapses is always from P to M. Similarities and differences between the synaptology of frog and mammalian spinal motoneurons are discussed.     

Intense synaptic activity enhances temporal resolution in spinal motoneurons

In neurons, spike timing is determined by integration of synaptic potentials in delicate concert with intrinsic properties. Although the integration time is functionally crucial, it remains elusive during network activity. While mechanisms of rapid processing are well documented in sensory systems, agility in motor systems has received little attention. Here we analyze how intense synaptic activity affects integration time in spinal motoneurons during functional motor activity and report a 10-fold decrease. As a result, action potentials can only be predicted from the membrane potential within 10 ms of their occurrence and detected for less than 10 ms after their occurrence. Being shorter than the average inter-spike interval, the AHP has little effect on integration time and spike timing, which instead is entirely determined by fluctuations in membrane potential caused by the barrage of inhibitory and excitatory synaptic activity. By shortening the effective integration time, this intense synaptic input may serve to facilitate the generation of rapid changes in movements.     

Cholinergic modulation of the synaptic transmission in the frog spinal cord

It was found during experiments on isolated frog spinal cord involving extracellular recording from the dorsal roots (sucrose bridging) and intracellular recording from motoneurons by microelectrodes that 10 mM of the M-cholinomimetic arecoline produces motoneuronal depolarization which is matched by depolarizing electronic ventral root potentials and a rise in motoneuronal input resistance. Arecoline changes synaptic transmission by increasing the amplitude of postsynaptic potentials during intracellular recording and that of motoneuronal reflex discharges in the ventral roots but reduces the duration of dorsal root potentials. In the presence of arecoline, L-glutamate-induced motoneuronal response increases. Facilitation of synaptic transmission produced by arecoline in the spinal cord is bound up with cholinergic M₂- activation, since it is suppressed by atropine but not by low concentrations of pirenzipine; it is also coupled with a reduction in adenylcyclase activity. When motoneuronal postsynaptic response has been suppressed, as in the case of surplus calcium or theophylline, arecoline produces an inhibitory effect on the amplitude of motoneuronal monosynaptic reflex discharges which is suppressed by pirenzipine at a concentration of 1×10^–7 M. This would indicate the presence at the primary afferent terminals of presynaptic cholinergic M₁ receptors which mediate its inhibition of impulses of transmitter release. This effect is independent of changes in cyclic nucleotide concentration.A. M. Gorkii Medical Institute, Donetsk. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 399–405, May–June, 1987.     

Presynaptic activity and CaMKII modulate retrograde semaphorin signaling and synaptic refinement

Establishing synaptic connections often involves the activity-dependent withdrawal of off-target contacts. We describe an in vivo role for temporally patterned electrical activity, voltage-gated calcium channels, and CaMKII in modulating the response of Drosophila motoneurons to the chemorepellent Sema-2a during synaptic refinement. Mutations affecting the Sema-2a ligand, the plexin B receptor (plexB), the voltage-gated Ca(v)2.1 calcium channel (cac), or the voltage-gated Na(v)1 sodium channel (mle(nap-ts);tipE) each result in ectopic neuromuscular contacts. Sema-2a interacts genetically with both of the channel mutations. The cac phenotype is enhanced by the Sema-2a mutation and is suppressed by either plexB overexpression or patterned, low-frequency (0.01 Hz) bouts of electrical activity in the embryo. The calcium-dependent suppression of ectopic contacts also depends on the downstream activation of CaMKII. These results indicate a role for patterned electrical activity and presynaptic calcium signaling, acting through CaMKII, in modulating a retrograde signal during the refinement of synaptic connections.     

Postsynaptic metabolic monoamine modulation of effects mediated by NMDA-glutamate receptors in frog spinal motoneurons

We investigated the effect of monoamines and their agonists and antagonists upon responses of motoneurons medicated by NMDA-glutamate receptors. It was found that adrenalin strengthens the responses studied through activation of -and -adrenoreceptors. This effect of adrenalin is suppressed by an -adrenoblocker (phentolamine) and a -adrenoblocker (propranolol); it is restored by an -adrenomimetic (phenylephrine hydrochloride) and a -adrenomimetic (neoepinephrine). In 66% of the cases, dopamine inhibited motoneurons responses; in 34% of the cases, it strengthened such responses. In the presence of a selective blocker of D₂-dopamine receptors (sulpiride), it only strengthened motoneuron responses. Serotonin strengthens responses of motoneurons; its action is suppressed by a blocker of type-II serotonin receptors (dezeril). A selective agonist of subtype-1A serotonin receptors, campirone, suppresses motoneuron responses. The effect of campirone is weakened by propranolol. The strengthening of NMDA-responses of motoneurons evoked by neoepinephrine and dopamine in the presence of sulpiride is weakened by a blocker of cAMP-dependent protein kinase, tolbutamide, and the strengthening of responses by phenylephrine hydrochloride and serotonin is weakened by an inhibitor of calmodulin, trifluoroperazine.M. Gorky Medical Institute, Ukrainian Ministry of Public Health, Donetsk. Translated from Neurofiziologiya, Vol. 23, No. 6, pp. 683–690, November–December, 1991.     

Peptidergic modulation of the hippocampus synaptic activity

Effects of two newly synthesized nootropic and anxiolytic dipeptides: Noopept and Selank on inhibitory synaptic transmission in hippocampal CA1 pyramidal cells were investigated using patch-clamp technique in whole-cell configuration. Bath application of Noopept (1 microM) or Selank (2 microM) significantly increased the frequency of spike-dependent spontaneous m1PSCs, whereas spike-independent mlPSCs remained unchanged. It was suggested that both peptides mediated their effect sue to activation of inhibitory interneurons terminating on CA1 pyramidal cells. Results of current clamp recording of inhibitory interneurons residing in stratum radiatum confirmed this suggestion, at least for Noonent.     

Presynaptic calcium stores and synaptic transmission

Following the gradual recognition of the importance of intracellular calcium stores for somatodendritic signaling in the mammalian brain, recent reports have also indicated a significant role of presynaptic calcium stores. Ryanodine-sensitive stores generate local, random calcium signals that shape spontaneous transmitter release. They amplify spike-driven calcium signals in presynaptic terminals, and consequently enhance the efficacy of transmitter release. They appear to be recruited by an association with certain types of calcium-permeant ion channels, and they induce specific forms of synaptic plasticity. Recent research also indicates a role of inositoltrisphosphate-sensitive presynaptic calcium stores in synaptic plasticity.     

Presynaptic strontium dynamics and synaptic transmission

Strontium can replace calcium in triggering neurotransmitter release, although peak release is reduced and the duration of release is prolonged. Strontium has therefore become useful in probing release, but its mechanism of action is not well understood. Here we study the action of strontium at the granule cell to Purkinje cell synapse in mouse cerebellar slices. Presynaptic residual strontium levels were monitored with fluorescent indicators, which all responded to strontium (fura-2, calcium orange, fura-2FF, magnesium green, and mag-fura-5). When calcium was replaced by equimolar concentrations of strontium in the external bath, strontium and calcium both entered presynaptic terminals. Contaminating calcium was eliminated by including EGTA in the extracellular bath, or by loading parallel fibers with EGTA, enabling the actions of strontium to be studied in isolation. After a single stimulus, strontium reached higher peak free levels than did calcium (approximately 1.7 times greater), and decayed more slowly (half-decay time 189 ms for strontium and 32 ms for calcium). These differences in calcium and strontium dynamics are likely a consequence of greater strontium permeability through calcium channels, lower affinity of the endogenous buffer for strontium, and less efficient extrusion of strontium. Measurements of presynaptic divalent levels help to explain properties of release evoked by strontium. Parallel fiber synaptic currents triggered by strontium are smaller in amplitude and longer in duration than those triggered by calcium. In both calcium and strontium, release consists of two components, one more steeply dependent on divalent levels than the other. Strontium drives both components less effectively than does calcium, suggesting that the affinities of the sensors involved in both phases of release are lower for strontium than for calcium. Thus, the larger and slower strontium transients account for the prominent slow component of release triggered by strontium.     

Thermal dependence of serotonergic modulation of neural activity in the hamster

1. The modulatory effect of serotonin on CA1 pyramidal cells in the hamster (Mesocricetus auratus) hippocampus was examined over a range of temperatures. 2. Following repetitive Schaffer collateral/commissural stimulation, changes in the amplitude of population spikes (the synchronous firing of CA1 pyramidal cells) were recorded in the hamster, a hibernator. Amplitudes were measured after 10 microM serotonin was added to and then withdrawn from the perfusing medium with the temperature of the bath fixed at different temperatures. 3. Between 35 degrees C and 15 degrees C a depression in population spike amplitude of at least 10% was seen in 36 of 43 trials, with an average depression of 68%. No significant temperature dependence of the depressive effect was seen. 4. Following the removal of serotonin from the perfusate, the spike amplitude was enhanced over the same range of temperatures, averaging 33% higher than control values. The enhancement was most pronounced at 35 degrees C and 15 degrees C and smallest at 25 degrees C. 5. Thus, over the entire temperature range of 35 degrees C to 15 degrees C, serotonin exerted a dual modulatory effect on the spike amplitude, a depression followed by an enhancement. Serotonin's modulatory effects on pyramidal cell excitation persist over temperatures encountered as the hamster enters hibernation.     

Three types of miniature inhibitory potentiaes in the frog spinal cord motoneurons: the possibility of GABA and glycine co-release

Miniature inhibitory postsynaptic potentials (mlPSPs) were recorded from motoneurons of the frog isolated spinal cord after blocking action potentials and ionotropic glutamate receptors (TTX 1 mcm: CNQX 25 mcm, D-AP5 50 mcm). Three types of mlPSPs were selected by their time characteristics) fast, slow and mlPSPs with two decay time constants. We classified 8.7% of mlPSPs as dual-component, 64.5% as fast mlPSPs, and 26.8% as slow mlPSPs. The GABA(A)R blocker bicuculline (20 mcm) diminished the number of the slow and dual-component events while the number of mlPSP with a fast kinetics was increased. The GlyR antagonist strychnine (1 mcm) reduced the frequency of fast mlPSPs and increased this parameter of slow mlPSPs. These data suggest existence of three different mlPSP groups distinguished by their kinetics and sensitivity to receptor antagonists: fast events mediated by glycine, slow events mediated by GABA and dual-component mlPSPs corresponding to glycine and GABA co-release.     

Changes in acetylcholine concentration, miniature end-plate potentials and synaptic vesicles in frog neuromuscular preparations during lanthanum treatment

ACh content and synaptic ultrastructure were compared in neuromuscular preparations (sartorius muscle of Rana esculenta) incubated in control saline and in saline containing 1 mM LaCl3. ACh concentrations remained constant for 6 hr in control preparations. La3+ caused a 38% depletion of ACh within the first 30 min with subsequent recovery to 120% of control values within 3-4 hr. Recovery was prevented by hemicholinium-3. At 23 degrees C La3+ caused complete loss of synaptic vesicles: no depletion was seen at 4 degrees C. Initially MEPP frequency increased 300- to 700-fold (23 degrees C), then declined. Mean vesicle diameter did not change, but SD increased. As the frequency of MEPPs declined, the percentage of s-MEPPs greatly increased. La3+ had a postsynaptic effect which increased the amplitudes of both s-MEPPs and bell-MEPPs within a few seconds. The s-MEPP mean did not change during the course of La3+ treatment although the bell-MEPP mean usually decreased. How the decrease in synaptic vesicles, decrease in MEPP frequencies, and changes in ACh levels relate to changes in the percentage of different classes of quanta is discussed.