Amplitude Of Miniature Potentials Research Articles

Second Messengers in the Presynaptic Regulation of Glycinergic Synapses in Frog Motoneurons

The possible pathways mediating the inhibitory influences of two types of presynaptic metabotropic receptors – group III metabotropic glutamate receptors (mGluRIII) and GABAB receptors (GABABR) – on miniature glycinergic activity in motoneurons in the isolated spinal cord of the frog Rana ridibunda were studied. The glycinergic component is dominant in the inhibitory miniature activity of motoneurons [3]. Baclofen and LAP4, which are selective agonists of GABABR and mGluRIII, inhibited miniature glycinergic activity, decreasing frequency by 48.0 ± 5.6 (n = 8) and 48.5 ± 8.6% (n = 5), respectively. The amplitude of miniature potentials did not change significantly, which is evidence for a presynaptic mechanism of modulation of synaptic transmission. Studies of the stages in the mediation of the influences of the metabotropic receptors studied here on the process of glycine release from presynaptic terminals were performed by applying their selective agonists in conditions of previous selective blockade of the components mediating modulation of glycinergic synaptic transmission. The effects of mGluRIII were shown to be mediated tghe feedback via adenylate cyclase (AC), the next stage being mediated by protein kinase A (PKA), as their selective blockade virtually eliminated by effect of activation of mGluRIII. The effects of GABABR on miniature glycinergic activity did not involve AC activity, as its blocker SQ22536 did not alter the inhibitory effect of baclofen. The effects of GABABR were mediated by negative feedback via phospholipase C (PLC), as its prior exclusion (inhibitor U73122) prevented the effect of application of baclofen. However, the next possible stage in this pathway – protein kinase C (PKC) – did not have a role in mediating the influences of GABABR, as the PKC blocker (GF109302X) had no influences on the effect of baclofen. The common component in mediating the effects of mGluRIII and GABABR on miniature glycinergic activity may consist of inositol triphosphate receptors (IP3R), which regulate calcium release from intraterminal depots and, according to published data, are linked with both PKA or cAMP and with PLC. In our experiments, 2-APB, an IP3R antagonist, decreased miniature glycinergic activity frequency by 26 ± 8% (n = 7) and prevented the inhibitory effects of application of both mGluRIII and GABABR agonists. This stage of mediation of the influences of mGluRIII and GABABR appears to involve collision and interaction between these (crosstalk), as demonstrated by our previous studies [2].

Analysis of factors restricting the use of binomial statistics for studying transmitter release in neuromuscular junctions

Binomial parameters of transmitter secretion were calculated on the basis of analysis of synaptic potentials in the frog sartorius muscle. Negative values of the parameter p were found in some synapses. This happened most often in low Ca2+ concentrations and with low amplitude of miniature end-plate potentials. The results were interpreted in terms of a model assuming spatial heterogeneity of probability of transmitter quantum release at different release points. Simulation of transmitter secretion by computer showed that the appearance of negative values of the parameter p and incorrect estimates of n experimentally are connected with the form of distribution of probability of transmitter quantum release in the synapse and with the amplitude of miniature potentials.

Effects of pharmacological drugs on miniature end-plate potentials

The action of cholinesterase and of some pharmacological drugs on miniature end-plate potentials was investigated. If the quantum of acetylcholine spontaneously released from nervous endings and producing a single miniature potential consisted of only one molecule, cholinesterase could destroy it completely. This would cause the decrease of miniature potential frequency, but would not act on its amplitude. Observed changes in the miniature potential amplitude, caused by cholinesterase, indicate the multimolecular composition of the acetylcholine quantum. The table presents data of the action of atropine, diplacine, procaine, caffeine, urethane and phosphacol on the parameters of miniature potentials. L'action de la cholinesterase et de quelques substances pharmacologiques sur les potentiels miniatures des plaques terminales neuromusculaires est examinée. Si le quantum d'acétylcholine libéréspontanément par les terminaisons nerveusesétait monomoléculaire, la cholinestérase pourrait la détruire totalement. Il en résulterait une diminution de la fréquence des potentiels miniatures, sans effet sur l'amplitude. Les changements d'amplitude des potentiels miniatures, engendrés par la cholinestérase, indiqueraient donc une composition multimoléculaire du quantum d'acétylcholine. Un tableau synoptique réunit les données concernant l'effet de l'atropine, de la diplacine, de la procaine, de la caféine, de l'uréthane et de la phosphacole sur les paramètres des potentiels miniatures. Es wurde der Einfluss der Cholinestérase und gewisse pharmacologische Stoffen an die Miniaturpotentialen der Endplatte untersucht. Wenn das aus der Nervenbeendung spontan freilassendes und ein einzelnen Miniaturpotential schaffendes Acetylcholinquantum nur aus einer Molecule besteht, möge die Cholinesterase völlig zerstören. Es würde zu der Verminderung der Frequenz der Miniaturpotentialen führen, aber die Amplitude worden nicht beeinflüssen. Die beobachteten Anderungen der Amplituden der Miniaturpotentialen, die von der Cholinesterase erregt sind, zeigen dass das Acetylcholinquantum aus vielen Moleculen besteht. In der Tabelle sind Datenüber den Einfluss Atropin, Dyplacin, Procain, Coffein, Urethan und Phosphacol auf die Parameter, der Miniaturpotentialen dargestellt.