Is synaptic facilitation caused by presynaptic spike broadening?

Impairment of memory consolidation by galanin correlates with in vivo inhibition of both LTP and CREB phosphorylation

Changes in the state of CREB phosphorylation and in LTP in the hippocampus have been associated with learning and memory. Here we show that galanin, the neuropeptide released in the hippocampal formation from cholinergic and noradrenergic fibers, that has been shown to produce impairments in memory consolidation in the Morris water maze task inhibits both LTP and CREB phosphorylation in the rat hippocampus in vivo. While there are many transmitters regulating CREB phosphorylation none has been shown to suppress behaviorally-induced hippocampal CREB phosphorylation as potently as galanin. The in vivo inhibition of dentate gyrus-LTP and of CREB phosphorylation by the agonist occupancy of GalR1 and GalR2-type galanin receptors provides strong in vivo cellular and molecular correlates to galanin-induced learning deficits and designates galanin as a major regulator of the memory consolidation process.

Repolarization of the presynaptic action potential and short-term synaptic plasticity in the chick ciliary ganglion

Stimulation-induced increases in synaptic efficacy have been described as being composed of multiple independent processes that arise from the activation of distinct mechanisms at the presynaptic terminal. In the chick ciliary ganglion, four components of short-term synaptic plasticity have been described: F1 and F2 components of facilitation, augmentation, and potentiation. In the present study, intracellular recording from the presynaptic calyciform nerve terminal of the chick ciliary ganglion revealed that the late repolarization and afterhypolarization (AHP) phases of the presynaptic action potential are affected by repetitive stimulation and that the time course of these effects parallel that of facilitation. The effects of these changes in the presynaptic action potential time course on calcium influx were tested by using the recorded action potential waveforms as voltage command stimuli during whole-cell patch-clamp recordings from acutely isolated chick ciliary ganglion neurons. The "facilitated" action potential waveform (slowed repolarization, decreased AHP amplitude) evoked calcium current with slightly but significantly greater total calcium influx. Taken together, these results are consistent with the hypothesis that activity-dependent changes in the presynaptic action potential are one of several mechanisms contributing to the facilitation phase of stimulation-induced increases in transmitter release in this preparation.

Synaptic plasticity at crayfish neuromuscular junctions: facilitation and augmentation

Simultaneous intracellular recordings from presynaptic nerve terminals and postsynaptic muscle fibers were used to investigate the extent to which changes in presynaptic voltage may contribute to short-term facilitation and augmentation of transmitter release at neuromuscular junctions of the crayfish (Procambarus simulans) opener muscle. Presynaptic nerve terminals have an average resting membrane potential of about -80 mV, single action potentials have an average foot-to-peak amplitude of about 98 mV, and single action potentials are followed by a depolarizing after potential (DAP) of about 10 mV. During stimulus trains of 9-16 impulses at 100 Hz, amplitudes of excitatory postsynaptic potentials (EPSPs) continuously facilitate up to 100-fold. This dramatic facilitation is associated with only slight increases in the peak voltage and duration of APs for the first 2-4 pulses in such a stimulus train. Foot-to-peak total amplitude of APs usually decreases after the first pulse in a stimulus train. The data strongly suggest that short-term facilitation is not due to changes in the amplitude or duration of APs invading the presynaptic terminal. Upon cessation of a longer stimulus train, the presynaptic terminal exhibits a hyperpolarizing after potential (HAP) up to 16 mV in amplitude depending upon the frequency (10-100 Hz) and duration (1-10 sec) of the tetanic stimulation. This post-tetanic HAP decays with a time constant of 10-20 sec, which is approximately equal to the third time constant of decay in EPSP amplitude (augmentation) following tetanic stimulation. Hence, presynaptic voltage changes and/or processes associated with these voltage changes (e.g., accumulation of ions, changes in ionic conductances, etc.) may be partly responsible for augmentation of EPSP amplitudes.

Synaptic plasticity at the crayfish opener neuromuscular preparation

The crayfish opener neuromuscular preparation exhibits most of the plasticities yet described for any synapse, including facilitation, long-term potentiation, presynaptic inhibition, and modulation. Since the presynaptic terminals and postsynaptic muscle fibers can both be intracellularly penetrated, one can now more easily examine the cellular/molecular bases for these plasticities. Data from such studies suggest that facilitation may be influenced by something other than residual free calcium and that presynaptic inhibition is produced by a conductance increase to chloride in the terminals of the excitor axon. Several drugs (ethanol, pentobarbital) have significant effects on these synaptic plasticities over concentration ranges which produce obvious behavioral effects in crayfish and mammals. Hence, this preparation should be a useful model system to determine cellular/molecular bases for various synaptic plasticities and the effects of drugs on these plasticities.

Facilitation and the conduction of the nerve action potential at the frog neuromuscular junction

We have measured the time intervals (latencies) between the delivery of paired stimuli and the appearance of the extracellularly recorded nerve terminal action potentials (n.a.p.s) and end-plate currents (e.p.c.s) at different temperatures in the sciatic nerve, sartorius muscle preparation of the frog. When the interstimulus interval was 70 ms, the n.a.p. and e.p.c. latencies following the second stimulus of the pair were shorter than following the first at 10 degrees C and 15 degrees C, and facilitation of the e.p.c. was observed. The shortened latency was also observed at 20 degrees C by Barton and Cohen (1982), who suggested that this may be related to facilitation. At 5 degrees C, however, the n.a.p. and e.p.c. latencies following the second stimulus were longer than following the first, and facilitation was still observed. When a 30 ms interstimulus interval was used, the changes in latency were even greater at the temperatures studied. The changes in e.p.c. latency were due to changes in the conduction velocity of the n.a.p., not changes in synaptic delay. At all temperatures and interstimulus intervals studied, the larger amplitude n.a.p.s were conducted more slowly, however, facilitation was always observed. We conclude that changes in the amplitude and conduction velocity of the nerve action potential do not obviously affect facilitation at the frog neuromuscular junction.

Changes in miniature endplate potential frequency during repetitive nerve stimulation in the presence of Ca2+, Ba2+, and Sr2+ at the frog neuromuscular junction

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation-induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation-induced changes in MEPP frequency and EPP amplitude have similar time-courses.