Amplitude Of Excitatory Junction Potentials Research Articles

Effects of notexin on neuromuscular transmission in the guinea pig vas deferens

J. B. Harris and A.-M. Surprenant. Effects of notexin on neuromuscular transmission in the guinea pig vas deferens. Toxicon 28, 393–401, 1990.—The effects of notexin on neuromuscular transmission in the isolated vas deferens of the guinea pig have been investigated using intracellular recording techniques. The toxin had no effect on the resting membrane potential, the amplitude or frequency of spontaneous excitatory junctional potentials or on the time constant of the decay of the evoked excitatory junctional potential in the smooth muscle cells. The amplitude of the evoked excitatory junctional potential was selectively reduced in the presence of notexin. The reduction in amplitude lasted for 20–30 min. Recovery occurred despite the continued presence of the toxin. Once recovery had been achieved the preparation remained insensitive to further exposure to the toxin. Although the amplitude of excitatory junctional potentials recovered in the continued presence of notexin, facilitation remained depressed. Action potentials, when generated, were indistinguishable from those in control tissues. It is concluded that notexin is a presynaptic inhibitor of transmission in the guinea pig vas deferens.

Evidence for reduced presynaptic Ca2+ entry in a lobster neuromuscular junction at high pressure.

1. Previous studies have shown that hyperbaric pressure depresses synaptic transmission and have suggested that the effect is primarily on transmitter release. The present study analysed the effects of pressure at a crustacean neuromuscular junction. Changes in pressure were compared to changes in extracellular calcium concentration [Ca2+]o with respect to effects on excitatory junction potential (EJP) amplitude, time course, facilitation and potentiation. 2. The effects of 10.1 MPa pressure on EJP amplitude, facilitation and potentiation, but not time course, were mimicked by reducing [Ca2+]o to approximately one-half the normal level. 3. The effects of pressure and the interaction between compression and calcium concentration were analysed in terms of a model of transmitter release. The model assumes that release is dependent on internal calcium concentration, as modulated by both influx and removal processes; that calcium influx is a saturating function of [Ca2+]o; and that release and removal are saturating functions of [Ca2+]i. 4. The results were consistent with the hypothesis that increased pressure acts primarily to reduce calcium influx into the nerve terminal.

Neuromuscular transmission in larval tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae)

The morphology, cable electric properties and synaptic responses of skeletal motor units in fourth-instar Heliothis virescens are described. The ventral internal lateral (VIL 1) muscle has an input impedance of approx. 150 kΩ and a length constant that is 60% of the mean fibre length. Significant electrotonic coupling, indicated by a coupling coefficient of approx. 0.4, is evident between VIL 1 in adjacent segments. Each VIL 1 is innervated by a single “fast” excitatory motor neurone which generates a composite neuromuscular response consisting of an excitatory junctional potential (EJP) and overshooting action potential. The neuromuscular response is rapidly desensitized when exposed to bath-applied glutamate and related analogues. The order of effectiveness is quisqualate > glutamate > aspartate. The EJP amplitude is reduced and the action potential abolished in low calcium (0.75 mM) saline. Quantum content measured under normal conditions is approx. 17, but drops to 3 when extracellular calcium is reduced to 0.75 mM. MEJC durations are unusually long in physiological saline and are further prolonged to tens of milliseconds when extracellular calcium is lowered, suggesting a pronounced influence of calcium on transmitter-activated channel kinetics.

Synaptic plasticity and humoral modulation of neuromuscular transmission in the lobster claw opener during the molt cycle

1. Neuromuscular properties of the lobster dactyl opener were studied at different stages of the molt cycle. 2. Excitatory junctional potentials (EJPs) were found to be significantly larger in hard-shelled premolt lobsters than in soft-shelled postmolt animals. 3. Inhibitory junctional potentials (IJPs) were larger in postmolt claw preparations than in premolt ones. 4. Excitatory transmission was also monitored during superfusion with plasma samples obtained from lobsters in vartious stages of the molt cycle (postmolt = AB; intennolt = C; premolt = D 2). 5. D 2 plasma increased EJP amplitude by an average of 28% relative to baseline levels (in saline), while AB plasma reduced EJP size by an average of 6%. 6. Premolt and postmolt plasma produced opposite effects on quantal content ( m); D 2; plasma increased m by a mean of 39%, whereas AB plasma caused a mean reduction by 10%, which suggests that humoral factors may act presynaptically to alter transmitter release. 7. These results provide evidence of neuromuscular plasticity in the opener muscle during the molt cycle, and are consistent with changes in claw-opening behavior seen during this cycle.

Presynaptic spike broadening reduces junctional potential amplitude

Presynaptic modulation of action potential duration may regulate synaptic transmission in both vertebrates and invertebrates. Such synaptic plasticity is brought about by modifications to membrane currents at presynaptic release sites, which, in turn, lead to changes in the concentration of cytosolic calcium available for mediating transmitter release. The 'primitive' neuromuscular junction of the jellyfish Polyorchis penicillatus is a useful model of presynaptic modulation. In this study, we show that the durations of action potentials in the motor neurons of this jellyfish are negatively correlated with the amplitude of excitatory junctional potentials. We present data from in vitro voltage-clamp experiments showing that short duration voltage spikes, which elicit large excitatory junctional potentials in vivo, produce larger and briefer calcium currents than do long duration action potentials, which elicit small excitatory junctional potentials.

Cyclic Nucleotide Regulation of Neurotransmission in Guinea Pig Mesenteric Artery

The possible involvement of three different second-messenger systems, namely cyclic AMP/protein kinase (PK)-A, cyclic GMP/PK-G, and diacylglycerol (DG)/PK-C systems, in the perivascular nerve terminals of guinea pig mesenteric artery was examined by intracellular microelectrode recording. Excitatory junction potentials (EJPs) were evoked by perivascular nerve stimulation. Isoproterenol (0.1 microM) enhanced the EJP amplitude without modifying the passive membrane properties of the vascular smooth muscle (VSM) cells. The facilitatory effect of isoproterenol on EJP amplitude was completely abolished by beta-adrenergic blockade (0.3 microM propranolol). Forskolin (activator of adenylate cyclase) also augmented the EJP amplitude in a concentration-dependent manner (EC50 congruent to 10 microM), without affecting the passive membrane properties of the VSM cells. In addition, forskolin (1-10 mM) markedly potentiated the isoproterenol-induced stimulation of EJP amplitude (EC50 congruent to 2 microM). A permeant analogue of cyclic AMP, 8-bromo-cyclic AMP (0.1 and 1 mM), enhanced the EJP amplitude, thus mimicking the effects of isoproterenol and forskolin. 8-Bromo-cyclic AMP had no effect on the resting potential or current-voltage relationship of the VSM cells, thus suggesting that the membrane properties of the VSM cells were not altered. 8-Bromo-cyclic GMP (1 mM) also augmented the EJP amplitude, but its facilitatory effect was weaker than that of 8-bromo-cyclic AMP. 8-Bromo-cyclic GMP hyperpolarized the VSM membrane by 4 mV and decreased the input resistance, presumably due to an increase in K+ conductance. Phorbol-12-myristate-13-acetate (PMA, 30-300 nM), a direct activator of PK-C, significantly enhanced the EJP amplitude after 40 min in a concentration-dependent manner, without affecting the resting potential of the VSM cells. From these results, we suggest that cyclic AMP/PK-A, cyclic GMP/PK-G, and DG/PK-C systems might be involved in regulation of the release of neurotransmitter in the perivascular nerve terminals. However, the possibility of some action on the postsynaptic VSM cell cannot be excluded.

Pertussis toxin effects on transmitter release from perivascular nerve terminals

Pertussis toxin (PT) was used to investigate the possible involvement of an inhibitory GTP-binding protein (G protein) in neuromuscular transmission at the sympathetic nerve terminal of guinea pig mesenteric artery. When the intracellular microelectrode technique was used, the amount of transmitter released was evaluated by recording the amplitude of excitatory junction potentials (EJPs) in the vascular smooth muscle (VSM) cells. EJPs were evoked by perivascular nerve stimulation with brief square pulses. The amplitude of EJPs was depressed by exogenously applied norepinephrine (NE) (greater than or equal to 10(-7) M) or histamine (greater than or equal to 10(-7) M). However, these effects of NE and histamine on EJP amplitude were almost completely blocked in PT-pretreated VSM cells (2 x 10(-7) g/ml PT, 24 h at 21 degrees C). These effects of PT pretreatment were time and temperature dependent. In contrast, there was no significant change in the resting membrane potential (-70.1 +/- 2.1 mV) or input resistance (11.9 +/- 0.4 M omega) in the VSM cells after PT pretreatment. These results suggest that the effects of NE and histamine on EJP amplitude may be mediated by PT-sensitive G proteins in the presynaptic nerve terminal.

Regulation of voltage-dependent excitatory responses to α,β-methylene ATP, ATP and non-adrenergic nerve stimulation by dihydropyridines in the guinea-pig vas deferens

Simultaneous recordings of mechanical and intracellular electrical activity were obtained from the guinea-pig vas deferens. where nerve stimulation. ATP and the stable nucleotide analogue α,β-methylene ATP elicited excitatory responses. Excitatory junction potentials and action potentials were elicited by low-frequency (trains of pulses, generally ⩽ 2 Hz) field stimulation. α,β-Methylene ATP and ATP elicited only concentration-dependent depolarizations at low concentrations, while higher concen- trations elicited a superimposed action potential discharge which was accompanied by mechanical contraction. The voltage threshold at which action potential discharge was initiated by these three stimuli was about —45 mV (resting membrane potential averaged −66mV). Action potential discharges and contractile responses were antagonized by nifedipine and augmented by Bay K 8644 at concentrations (1 and 0.5μM. respectively) which exhibited only small effects on either excitatory junction potential amplitudes or nucleotide-induced depolarizations. Bay K 8644 enhanced and nifedipine antagonized the repolarization (rectification) phase of action potential discharge elicited by nerve stimulation and drugs; after-hyperpolarizations were prominent in the presence of Bay K 8644 (0.1–5 μM). Excitatory junction potentials were antagonized after exposure to α,β(-methylene ATP. This antagonistic effect of α,β(-methylene ATP was also observed following depolarizations elicited in the absence and presence of nifedipine (1μM). Noradrenaline was approximately 50–100 times less potent than α,β-methylene ATP in eliciting action potential discharge and contraction. It was only when a high concentration of noradrenaline was used (about 60–100μM) that the noradrenaline-induced depolarization attained the voltage threshold for action potential initiation. These results illustrate the similarity of the electrical components which underlie excitation by nerve simulation and adenine nucleotides in the vas deferens. and demonstrate the ability of dihydropyridines to regulate voltage-dependent events associated with both the generation and inactivation of muscle action potentials. These are probably voltage-dependent calcium currents and calcium-activated potassium currents, respectively. Neither excitatory junction potentials nor the mechanism of desensitization of the ATP purinoceptor by α,β-methylene ATP involve voltage-dependent calcium channels.

Unpaired Median Neurones in a Lepidopteran Larva (Antheraea Pernyi): II. Peripheral Effects and Pharmacology

ABSTRACT Two unpaired median cells (MCI and MC2) had a temporal pattern of firing that correlated with phasic muscular activity in preparations of larval Antheraea pernyi, and previous work has indicated that the axons of median cells are associated with nerve trunks innervating blocks of muscle. In spite of this, action potentials in median cells were not found to have any one-for-one effects on either the tension or the electrical activity of somatic muscle fibres. However, bursts of action potentials in MC2 were shown to modulate both tension production and electrophysiological activity of a number of motor units. These effects consisted of an increase in twitch tension, a relaxation of basal resting tension, an increase in relaxation rate following contractions, a hyperpolarization of some muscle fibres and an increase in amplitude of excitatory junction potentials. The relative potency of these different effects varied between fast and slow muscles. All of these effects were mimicked by the application of octopamine and synephrine, and in higher concentrations by a number of other biogenic amines and adrenergic agonists. The possibility that the effects of median cell activity were mediated by the release of endogenous octopamine was supported by the observation that phentolamine (10−5moll-1) blocked the effects of both MC2 impulses and the application of exogenous octopamine, whereas propanolol affected neither set of responses. This observation also indicated a pharmacological similarity with a number of other octopamine-sensitive insect tissue preparations. MCI had similar effects to MC2 on the electrical activity of a number of muscles, suggesting that these two cells play a similar role. These observations provide strong evidence for the presence of an identifiable octopaminergic system of neurones, similar to the dorsal unpaired median (DUM) neurones that have been extensively studied in the locust.

Autoregulation of acetylcholine release from vagus nerve terminals through activation of muscarinic receptors in the dog trachea.

1. The effects of pirenzepine and gallamine on the membrane and contractile properties of smooth muscle cells and on excitatory neuro-effector transmission in the dog trachea were investigated by means of microelectrode, double sucrose gap and tension recording methods. 2. Pirenzepine (10(-7) M) and gallamine (10(-5) M) had no effect on the resting membrane potential or the input resistance of the smooth muscle cells. 3. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of twitch contractions evoked by field stimulation in the combined presence of indomethacin (10(-5) M) and propranolol (10(-6) M). At higher concentrations pirenzepine (10(-8) M) inhibited the twitch contractions in a dose-dependent manner. Both pirenzepine and gallamine in doses over 10(-7) and 10(-5) M, respectively, reduced muscle tone. 4. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by field stimulation (single or repetitive stimulation). However, a high concentration of pirenzepine (10(-8) M) reduced the amplitude of e.j.ps. In parallel with its action on e.j.ps, pirenzepine (over 10(-9) M) reduced the response of smooth muscle cells to acetylcholine (ACh), in a dose-dependent manner. Gallamine (5 X 10(-5) M) markedly enhanced the amplitude of e.j.ps but also reduced the response of muscle cells to ACh. 5. ACh (10(-10)-10(-9) M) inhibited twitch contractions evoked by field stimulation, with a slight increase of resting tension. 6. Gallamine enhanced the summation of e.j.ps during repetitive field stimulation at a high frequency (20 Hz), but was without effect on the depression phenomena of e.j.ps observed during double stimulus experiments at different time intervals (5-60 s). 7. These results indicate that both pirenzepine and gallamine have dual actions on pre- and post-junctional muscarinic receptors in dog tracheal tissue. At low concentrations both agents potentiate excitatory neuro-effector transmission, presumably due to enhancement of release of ACh from vagal nerve terminals through blockade of a negative auto-regulatory process activated by endogenous ACh. At higher concentrations, these agents inhibit the response of smooth muscle cells to ACh through post-junctional muscarinic receptors and relaxation of the muscle tissue occurs.

O-211 - Autoregulation of acetylcholine release from vagus nerve terminals through activation of muscarinic receptors in the dog trachea

1. The effects of pirenzepine and gallamine on the membrane and contractile properties of smooth muscle cells and on excitatory neuro-effector transmission in the dog trachea were investigated by means of microelectrode, double sucrose gap and tension recording methods. 2. Pirenzepine (10(-7) M) and gallamine (10(-5) M) had no effect on the resting membrane potential or the input resistance of the smooth muscle cells. 3. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of twitch contractions evoked by field stimulation in the combined presence of indomethacin (10(-5) M) and propranolol (10(-6) M). At higher concentrations pirenzepine (10(-8) M) inhibited the twitch contractions in a dose-dependent manner. Both pirenzepine and gallamine in doses over 10(-7) and 10(-5) M, respectively, reduced muscle tone. 4. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by field stimulation (single or repetitive stimulation). However, a high concentration of pirenzepine (10(-8) M) reduced the amplitude of e.j.ps. In parallel with its action on e.j.ps, pirenzepine (over 10(-9) M) reduced the response of smooth muscle cells to acetylcholine (ACh), in a dose-dependent manner. Gallamine (5 X 10(-5) M) markedly enhanced the amplitude of e.j.ps but also reduced the response of muscle cells to ACh. 5. ACh (10(-10)-10(-9) M) inhibited twitch contractions evoked by field stimulation, with a slight increase of resting tension. 6. Gallamine enhanced the summation of e.j.ps during repetitive field stimulation at a high frequency (20 Hz), but was without effect on the depression phenomena of e.j.ps observed during double stimulus experiments at different time intervals (5-60 s). 7. These results indicate that both pirenzepine and gallamine have dual actions on pre- and post-junctional muscarinic receptors in dog tracheal tissue. At low concentrations both agents potentiate excitatory neuro-effector transmission, presumably due to enhancement of release of ACh from vagal nerve terminals through blockade of a negative auto-regulatory process activated by endogenous ACh. At higher concentrations, these agents inhibit the response of smooth muscle cells to ACh through post-junctional muscarinic receptors and relaxation of the muscle tissue occurs.

Electrophysiological properties of the mesenteric artery of the rabbit with perinephritis hypertension.

Membrane potential and tension were simultaneously recorded in ileal arteries isolated from rabbits with perinephritis hypertension and from sham operated controls. Stimulation of perivascular nerves with single pulses produced excitatory junction potentials, action potentials and contraction. Setting resting tension to levels equivalent to intraluminal pressures of 23, 70 and 114 mm Hg had no effect on the resting membrane potential, excitatory junction potential amplitude, action potential threshold or membrane time constant. The resting membrane potential in arteries from hypertensive rabbits was 3.7 mV less negative than in the shams. There were no significant changes in the action potential threshold or membrane time constant. It is suggested that the changes observed in the membrane potential may contribute to the increased responsiveness found in arteries from this model of hypertension.

The action of chlordimeform and demethylchlordimeform on the octopamine-sensitive proleg retractor muscle of Antheraea larvae

Octopamine induces dose-dependent increases in peak tetanic tension and peak relaxation rate in the proleg retractor muscle (γ) of Antheraea larvae. These responses are sometimes accompanied by an increase in excitatory junction potential amplitude which, when present, is also dose-dependent. The responses to 10 −8 M octopamine are reversibly blocked by the α-adrenergic antagonist, phentolamine, whilst the β-adrenergic antagonist, propranolol, is without effect. Elevation of cyclic AMP (cAMP) levels using the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and the adenylate cyclase activator, forskolin, mimics the mechanical effects of octopamine on muscle γ. These results are consistent with the hypothesis that the muscle γ receptors are octopaminergic and mediate their mechanical effects via increases in cAMP levels. Low concentrations (10 −8 M to 10 −11 M) of the octopamine agonists chlordimeform and demethylchlordimeform (DCDM) mimic the effects of octopamine in a dose-dependent manner. At 10 −4 M, the responses are attenuated compared to those produced by octopamine; the effects of DCDM decreasing with time. It is suggested that these peripheral modulatory effects produced with chlordimeform and DCDM may enhance the centrally induced proleg retractions seen in intact chlordimeform-treated larvae, and thus contribute to crop protection.

Distribution of excitatory junction potential amplitude in cat cerebral arteries: Examination of its quantal nature and modulation by opiates

We used scorpion venom to release small amounts of an excitatory neurotransmitter from adventitial nerves in cat left anterior descending cerebral artery. We used glass microelectrodes to measure and record postsynaptic electrical events of minimal amplitude. These events were similar to postsynaptic spontaneous and electrically evoked excitatory junction potentials (ejp's) seen in skeletal muscle. We performed a frequency analysis of the ejp amplitudes to determine if they fit a unimodal or multimodal distribution. We also investigated the effects of phentolamine, norepinephrine, hydromorphone, and morphine on ejp amplitude and frequency in the artery. Statistical analysis of the ejp frequency and amplitude revealed a multipeaked distribution with decreasing peaks. These results were similar to the distribution reported for acetylcholine release in skeletal muscle. The ejps were inhibited by phentolamine, which suggested that these events were adrenergically mediated. Norepinephrine and the opiates, hydromorphone and morphine, reduced the frequency and amplitude of the ejp's. The vessels also constricted to increasing doses of norepinephrine both under control conditions and in the presence of opiate. These results suggest that norepinephrine blocks the ejp's by a feedback mechanism at the presynaptic membrane and that endorphins and/or enkephalins, also acting at this presynaptic site, may modulate neurotransmission in the cerebral circulation.

Acromelic acid, a novel excitatory amino acid from a poisonous mushroom: effects on the crayfish neuromuscular junction

A novel amino acid, acromelic acid, which is one of kainoids isolated from a poisonous mushroom, markedly depolarizes the crayfish opener muscle fiber in a dose-dependent manner, its potency being much greater than that of kainoids such as kainic or domoic acid. Moreover, acrolemic acid markedly potentiates the glutamate response, in spite of the fact that it reduces the quisqualate response in a dose-dependent manner. The amplitude of excitatory junctional potentials was slightly reduced by acromelic acid.

Effects of octopamine and forskolin on excitatory junction potentials of developing and adult moth muscle.

Intracellular recordings were made from the dorsal longitudinal muscle of Manduca sexta to determine the effects of development and octopamine on the excitatory junction potential (EJP) produced in response to electrical stimulation of the motor nerve. Observations were made on pharate moths during the last 3 days before eclosion and on adults. In saline, the highest values for EJP amplitude and maximum rate of rise and for resting membrane potential are reached on the nineteenth day of the pupal period, the day the animal ecloses; adult values are slightly lower. In animals of all ages tested, DL-octopamine (5 X 10(-6) M) increases EJP amplitude and maximum rate of rise. Increases in amplitude are greater in animals at stage day 17 and 18 than in animals at stage day 19 and adult. Octopamine has no effect on EJP rise time (onset to peak) or recovery time (peak of EJP to 70% recovery). Octopamine causes a hyperpolarization of about 6 mV. The results show that developmental changes in synapse properties are paralleled only in part by changes induced by octopamine. Both development and octopamine increase EJP amplitude and maximum rate of rise, and neither alter rise time. EJP recovery time changes with development but not in response to octopamine. Forskolin (10(-4) M) mimics the effects of octopamine on day 17 animals. EJP amplitude and maximum rate of rise are increased by forskolin, and rise time and recovery time are unaffected. Forskolin, like octopamine, causes a 6 mV hyperpolarization of the muscle fiber. These results suggest that octopaminergic modulation at the Manduca sexta dorsal longitudinal neuromuscular junction may be mediated by changes in intracellular levels of cyclic AMP.

Effects of octopamine on miniature excitatory junction potentials from developing and adult moth muscle.

Intracellular recordings of excitatory junction potentials (EJPs) and miniature EJPs (MEJPs) were made from the dorsal longitudinal muscle of Manduca sexta to determine the sites of action of octopamine. MEJPs increased in amplitude and frequency as the moth developed during the 3 days before eclosion. DL-Octopamine (5 X 10(-6) M) increased the amplitude of excitatory junction potentials in both immature moths (one day before eclosion) and adults. Octopamine (10(-5) M) also increased the amplitude and frequency of MEJPs from immature animals (one and two days before eclosion) but had the opposite effect on adults and pharate adults ready to eclose. Treatment with octopamine (10(-5) M) resulted in a decrease in input resistance and a hyperpolarization in both immature and adult muscle fibers. The results suggest that octopamine acts both presynaptically and postsynaptically but that the increase in the amplitude of the evoked response is due primarily to influences on presynaptic processes.

A new potent channel blocker: Effects on glutamate responses at the crayfish neuromuscular junction

The glutamate blocking action of 5-methyl-1-phenyl-2-(3-piperidinopropylamino)-hexane-1-ol (MLV-5860) was studied at the crayfish neuromuscular junction using electrophysiological techniques. The opener muscle of the dactyl in the first leg of the crayfish was used to examine the action of the drug on the glutamate response. MLV-5860 reduced the amplitude of repetitively-induced glutamate potentials in a dose-dependent manner at the crayfish neuromuscular junction and this reduction was time- and activity-dependent. The minimum effective concentration of MLV-5860 to reduce the glutamate response was estimated to be lower than 50 nM, and therefore MLV-5860 is the most powerful glutamate blocker known at the crayfish neuromuscular junction. Pretreatment of the muscle fiber with concanavalin A did not affect the action of MLV-5860. MLV-5860 reduced the amplitude of excitatory junctional potentials (EJPs) and increased the decay rate of extracellularly-recorded EJPs in a dose-dependent manner. Quisqualate responses were also reduced by this drug but the conductance increase of the muscle membrane induced by GABA was not affected. MLV-5860 did not cause a significant change in the input resistance of the opener muscle fiber at concentrations less than 10 μM. The action of the drug is possibly explained in part by the open channel block of the glutamate-activated ion channels. The forward rate constant for channel blockade was estimated from the difference between the decay rate constants of extracellular EJPs in the absence and presence of the drug and the estimated value was 6.5 ± 1.4 × 10 7M −1s −1.

Differential effect of stimulation strength in mouse vas deferens on inhibition of neuroeffector transmission by receptor type selective opioids.

In the mouse isolated vas deferens the amplitude of excitatory junction potentials (e.j.p.s) recorded intracellularly from smooth muscle cells varied with the strength of stimulation. Receptor type selective opioids were tested in this preparation. The mu-agonist normorphine (2,000 nmol/l) reduced the amplitude of e.j.p.s and shifted the stimulus-response curve in a parallel way to the right. By contrast, the kappa-agonist U-50488 (1,000 nmol/l) and the delta-agonist [D-Ala2,D-Leu5]-enkephalin (2 nmol/l) caused a non-parallel shift of the curve. In addition, opioids having a lower selectivity for one type of receptor were also used. The preferential kappa-agonists ethylketocyclazocine (40 nmol/l) and dynorphin A1-13 (100 nmol/l) produced parallel and non-parallel shifts, respectively. Thus, normorphine and ethylketocyclazocine were more effective in depressing e.j.p.s evoked by low intensities of stimulation than those evoked by high intensities of stimulation. U-50488, dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin caused an equal depression of e.j.p.s evoked by either intensity of stimulation. The preferential mu- and delta-antagonists naloxone (1,000 nmol/l) and ICI 154129 (10,000 nmol/l), reversed the action of the respective agonists normorphine and [D-Ala2, D-Leu5]-enkephalin. In addition, ICI 154129 (10,000 nmol/l) reversed the action of dynorphin A1-13, as well. The preferential kappa-antagonist MR-2266 (1,000 nmol/l) prevented the effect of both ethylketocyclazocine and U-50488. It is concluded that under the conditions of these experiments normorphine and ethylketocyclazocine acted at mu-, U-50488 at kappa-, and dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin at delta-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative comparison of the structural features of slow and fast neuromuscular junctions in Manduca

The multiterminal slow and fast neuromuscular junctions of the moth Manduca sexta were compared using scanning, thin-section, and freeze-fracture techniques to see what structural features might underlie their functional differences. Slow neuromuscular junctions, here formed on tonic muscle fibers, produce a facilitating e.j.p. the amplitude of which is 1/5 to 1/3 the size of a fast excitatory junction potential (EJP) and the duration of which is nearly four times longer. A slow junction consists of a single terminal branch that is shorter in length than either of the pair of branches that a fast junction forms close together on the muscle fiber. Within the junction, slow nerve terminals exhibit longer, more frequent constrictions and are very varicose compared with fast. Since fast larval junctions on tonic muscle fibers are also varicose (Schaner and Rheuben, 1985), this is unlikely to represent an intrinsic property of the nerve. However, calculations of the length constants of the varicose versus nonvaricose shapes indicate that the effect of passive cable properties on normal functioning may act to limit the length of the slow terminals more than that of fast. Even though the varicose shape can be predicted to prolong the time course of the EJP, calculations show that, at the measured length, this would not explain the very long EJP that is observed. Within the neuromuscular junctions, the synapses are characterized on the muscle membrane by a patch of densely packed particles on the external leaflet and on the nerve membrane by a single linear active zone. The total number of synapses per slow junction is about 1/3 that of fast junctions. There is a weak correlation between average area of the individual postsynaptic particle patches and cross-sectional area of the muscle fibers that transcends nerve and muscle fiber types. The average lengths of active zones from the two types do not differ significantly. However, the number of particles per active zone in slow junctions is about 55% of the number in fast active zones. Chemically fixed slow nerve terminals have a greater density of synaptic vesicles remaining than do fast. If a proportion of the active zone particles represent structures directly involved in the probability of transmitter release, such as Ca++ channels, then the latter two characteristics may jointly reflect differences in capability to release and mobilize transmitter that would partly explain the different EJP amplitude and facilitation properties.