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Multiple sites for the initiation of action potentials in neurons of the inferior mesenteric ganglion of the guinea-pig.

Responses to nerve stimulation were recorded with intracellular microelectrodes from neurons in the inferior mesenteric ganglion of the guinea-pig. An event was recorded which was subthreshold for a somatic action potential but which had a short rise time and a rapid initial repolarization that gave the event a small but well-defined peak. This event was termed a "partial spike." During repeated stimulation of the same nerve trunk, the fluctuations in the amplitude of the partial spike were small compared to those of evoked synaptic potentials. Stimulation of different nerve trunks evoked partial spikes of different amplitudes. When different nerve trunks were stimulated at short intervals between stimuli, one partial spike could occlude another partial spike. Antidromic responses could not be blocked by a preceding partial spike. This suggests that partial spikes are not initial segment spikes. Tubocurarine reversibly abolished partial spikes which is indicative of a synaptic origin. It is concluded that partial spikes result from action potentials initiated by synaptic potentials in the dendrites and which fail to generate somatic action potentials.

Action Potentials↗

1S,3R-ACPD dose dependently induces a slow onset potentiation in the dentate gyrus in vivo.

It has been demonstrated by others in vitro that application of 1S,3R-aminocyclopentane-2,3-dicarboxylic acid (ACPD) facilitates long-term potentiation and triggers a slow-onset potentiation in the hippocampus. This study examined the effect of ACPD in the dentate gyrus when applied in vivo. Weak tetanisation produced a short-term potentiation of field excitatory post-synaptic potential (EPSP) and population spike. A similar response was seen upon application of ACPD (40 microM in 5 microl vehicle) via the lateral cerebral ventricle 30 min prior to tetanus, whereas ACPD (80 microM/5 microl) facilitated short-term potentiation into long-term potentiation. (R,S)-alpha-Methyl-4-carboxyphenyl-glycine (MCPG 200 mM/5 microl), completely inhibited this effect. ACPD had no effect on baseline recordings at 40 and 80 microM/5 miccrol, however 4 mM/5 microl ACPD induced a slow-onset potentiation of field EPSP and population spike which was maintained for over 4 h. MCPG or D-2-amino-5-phosphonopentanoate (AP5 20 mM/5 microl) applied prior to ACPD completely inhibited this effect. These results suggest that previously reported in vitro effects of ACPD in the CA1 region, also occur in the dentate gyrus in vivo. Furthermore, they confirm that activation of mGlu receptors by ACPD in vivo facilitates long-term potentiation, and indicate that in the dentate gyrus, ACPD-induced slow-onset potentiation is NMDA receptor-dependent.

Action Potentials↗

Learning-induced long-term synaptic modifications in the olfactory cortex.

The idea that memory is manifested at the cellular level by enhancement of synaptic connections between simultaneously activated neurons has been suggested half a century ago by Hebb, and is widely accepted since. Much effort is done to describe such enhancement and reveal the underlying mechanisms. Learning-induced synaptic modifications were studied in the last decade with in-vitro brain slices preparations. Several forms of long-term enhancement of synaptic connections between layer II pyramidal neurons in the piriform cortex accompany olfactory learning. Such modifications were described also in other brain areas, following other training paradigms. Post-synaptic enhancement of synaptic transmission is indicated by reduced rise time of (post synaptic potentials) PSPs and formation of new synaptic connections is indicated by increased spine density along dendrites of these neurons. Enhanced synaptic release is indicated by reduced paired-pulse facilitation. In slices from trained rats predisposition for long-term potentiation is decreased and predisposition for long-term depression is increased. These modifications are attributed to olfactory-discrimination rule learning, rather than to memories for specific odors, and may be subsequent to intrinsic modifications in pyramidal neurons that create favorable conditions for activity-dependent synaptic enhancement.

Animals↗

Neural coding: hybrid analog and digital signalling in axons.

Mammalian axons are thought to act as digital signaling devices, conveying information only by the timing and rate of all-or-none action potentials. Two recent studies now show that synaptic potentials can also spread far down the axon and influence action potential-triggered transmitter release in a graded, 'analog' manner. Axons thus encode information both about subthreshold and suprathreshold synaptic activity.

Action Potentials↗

The effects of ethanol on pre-synaptic components of synaptic transmission in a model glutamatergic synapse: the crayfish neuromuscular junction.

We have elucidated some of the mechanisms by which ethanol (EtOH) reduces synaptic efficacy at model glutamatergic synapses. The crayfish phasic and tonic neuromuscular junctions are superb models for directly assessing the effects of EtOH on pre-synaptic components of synaptic transmission. The ability to perform quantal analysis of synaptic transmission has allowed us to assess pre-synaptic alterations of release. Using this system, we report that the application of EtOH, within a range observed in intoxicated humans (44 and 88 mM), resulted in a diminution of excitatory post-synaptic potentials (EPSP) amplitudes. Additionally, using focal macro-patch recordings, quantal synaptic currents were recorded to assess the pre-synaptic component as potential target sites for EtOH's action. At the tonic neuromuscular junctions, EtOH (88 mM) reduced the probability of release (p), and in some cases, reduced the number of the release sites (n), but did not alter facilitation index nor did it affect the latency of vesicular release. At the phasic neuromuscular junction, a reduction in synaptic charge occurred during the presence of EtOH. Thus, the observed decrease in synaptic strength is at least partially attributable to a pre-synaptic alteration, specifically the release of fewer vesicles.

Action Potentials↗

Activation of p75NTR by proBDNF facilitates hippocampal long-term depression.

Pro- and mature brain-derived neurotrophic factor (BDNF) activate two distinct receptors: p75 neurotrophin receptor (p75(NTR)) and TrkB. Mature BDNF facilitates hippocampal synaptic potentiation through TrkB. Here we report that proBDNF, by activating p75(NTR), facilitates hippocampal long-term depression (LTD). Electron microscopy showed that p75(NTR) localized in dendritic spines, in addition to afferent terminals, of CA1 neurons. Deletion of p75(NTR) in mice selectively impaired the NMDA receptor-dependent LTD, without affecting other forms of synaptic plasticity. p75(NTR-/-) mice also showed a decrease in the expression of NR2B, an NMDA receptor subunit uniquely involved in LTD. Activation of p75(NTR) by proBDNF enhanced NR2B-dependent LTD and NR2B-mediated synaptic currents. These results show a crucial role for proBDNF-p75(NTR) signaling in LTD and its potential mechanism, and together with the finding that mature BDNF promotes synaptic potentiation, suggest a bidirectional regulation of synaptic plasticity by proBDNF and mature BDNF.

Animals↗

Possible mechanisms of enkephalin action on hippocampal CA1 pyramidal neurons.

(1) Intracellular and extracellular recordings were made from CA1 pyramidal neurons in an in vitro rat hippocampal slice preparation, while [D-Ala2, D-Leu5]enkephalin (DADL) was applied by perfusion at a known concentration (1 to 5 X 10-7 M), in a small droplet, or by iontophoresis into the cellular and dendritic layers of the slice. The effects of DADL on synaptic potentials and membrane properties were studied in an effort to determine the mechanisms underlying its epileptogenic action in the hippocampus. (2) DADL increased the size and often the duration of excitatory postsynaptic potentials (EPSPs) generated on either the apical or basal dendrites; this resulted in an increased discharge probability for a constant orthodromic stimulus. Extracellular field potential recordings showed a larger population spike for a given size field EPSP. These effects of DADL could be reversed substantially by perfusion with naloxone (1 to 5 X 10-7 M) and appeared qualitatively different from the epileptiform actions of penicillin. (3) DADL did not appear to increase the intrinsic excitability of the soma membrane, since membrane potential, input resistance, spike threshold, and antidromic field potentials all were unchanged. In addition, the shape of the membrane charging curve during hyperpolarizing current injection was not changed noticeably by DADL. (4) At the concentrations tested, DADL did not attenuate recurrent inhibition in the CA1 region, as evaluated by comparing in the absence and presence of DADL: (a) antidromically evoked recurrent inhibitory postsynaptic potentials (IPSPs) and their dependence of membrane potential, (b) the reduction of a synaptically driven population spike by a prior antidromic volley, (c) iontophoretic GABA (gamma-aminobutyric acid) responses. Similarly, IPSPs evoked by orthodromic stimulation appeared either unaffected or occasionally enhanced by DADL. (5) By iontophoretic mapping, it was shown that the DADL-sensitive sites are limited to stratum oriens and stratum pyramidale. Local application of DADL into stratum radiatum was relatively ineffective in enhancing the efficacy of synapses located in this region. (6) The dendritic input-output relationship between the presynaptic fiber volley and the field EPSP was not changed by DADL. This finding and the results of the iontophoretic mapping experiments suggest that increased excitatory transmitter release was not involved. (7) The data are consistent with the proposal that DADL selectively attenuates a dendritic IPSP which is virtually invisible to the soma, although the possibility cannot be ruled out that DADL may, in addition, act to enhance the responsiveness of pyramidal dendritic membrane to excitatory synaptic activation.

Animals↗

Synaptically recruited apical currents are required to initiate axonal and apical spikes in hippocampal pyramidal cells: modulation by inhibition.

Dendritic voltage-dependent currents and inhibition modulate the information flow between synaptic and decision areas. Subthreshold and spike currents are sequentially recruited by synaptic potentials in the apical shaft of pyramidal cells, which may also decide cell output. We studied the global role of proximal apical recruited currents on cell output in vitro and in the anesthetized rat after local blockade of Na+ currents in the axon initial segment (AIS) or the proximal apical shaft and their modulation by inhibition. Microejection of TTX, field potentials, and intrasomatic and intradendritic recordings were employed. Dendritic population spikes (PSs) were much smaller in vitro, but the gross relations between synaptic and active currents are similar to in vivo. Activation of Schaffer collaterals triggered PSs and action potentials (APs) in the apical shaft that fully propagated to the axon. However, the specific blockade of proximal Na+ currents avoided cell firing, although antidromic PSs and APs readily invaded somata. The somatic depolarization of subthreshold excitatory postsynaptic potentials (EPSPs) also decreased to about 50%. These results were not due to decreased excitatory input by TTX. However, when GABA(A) inhibition was locally removed, Schaffer synaptic currents skipped the proximal dendrite and fired somatic PSs, although initiated at the AIS. It is concluded that apical currents recruited en passant by Schaffer synaptic potentials in the apical shaft constitute a necessary amplifier for this input to cause output decision. Local inhibition decides when and where an AP will initiate, constituting an efficient mechanism to discriminate and weight different inputs.

Action Potentials↗

Intracellular calcium changes associated with cholinergic nicotinic receptor activation in cultured myenteric plexus neurones.

Intracellular free calcium concentration ([Ca2+]i) was measured in cultured explants of myenteric plexus neurones by using the fluorescent calcium indicator Indol in combination with patch-clamp techniques. The basal [Ca2+]i was 94 nM and spontaneous oscillations in the internal free calcium concentration were recorded. These oscillations were associated with bursts of action potentials triggered by spontaneous nicotinic excitatory synaptic potentials. Under voltage clamp conditions, application of the selective nicotinic agonist m-hydroxyphenylpropyl-trimethylammonium iodide (10 microM) induced an inward current and increased the intracellular free calcium concentration. We conclude that cholinergic synaptic excitatory activity provide a regular calcium entry in myenteric neurone and suggest that the nicotinic channel might be significantly permeable to calcium.

Animals↗

Effects of kynurenate on root potentials evoked by synaptic activity and amino acids in the frog spinal cord.

The effects of kynurenate (Kyn) on synaptic- and excitatory amino acid-mediated responses in isolated, hemisected spinal cords of frog were examined. Kyn (0.5 mM) rapidly and reversibly blocked greater than 90% of the synaptically mediated ventral root potential (VRP) produced by stimulation of the dorsal root. Spontaneous activities recorded from both ventral and dorsal roots were also reversibly blocked by Kyn. However, Kyn had no effect on action potentials or excitability per se, nor was it a general inhibitor of synaptic transmission since Kyn concentrations as high as 2.5 mM had no effect on synaptically mediated dorsal root potentials produced by stimulation of the ventral root. In addition, Kyn had no effect on synaptic transmission in sympathetic ganglia of frog. Although Kyn (2.5 mM) by itself produced no ventral root response in spinal cords treated with tetrodotoxin, it antagonized those induced by the excitatory amino acids N-methyl-D,L-aspartate, quisqualate, kainate, aspartate, and glutamate. The ventral root responses to all concentrations of quisqualate tested were depressed by 2.5 mM Kyn. In addition, when Kyn was washed out, the rate of recovery from Kyn block was accelerated by the presence of quisqualate. These results indicate that quisqualate and Kyn compete for common binding sites. However, low concentrations of Kyn (e.g. 0.1 mM) potentiated the peak of the response to saturating concentrations of quisqualate by as much as 30%. The durations of the potentiated quisqualate responses were significantly shorter than the control responses. Thus, Kyn does not act simply as a competitive inhibitor of quisqualate.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗

Distribution of vestibulospinal synaptic input to cat triceps surae motoneurons.

We applied supramaximal, repetitive stimulation to the lateral vestibular nucleus (Deiters' nucleus, DN) at 200 Hz to evoke stead-state synaptic potentials in ipsilateral triceps surae motoneurons of the cat. The effective synaptic currents underlying these potentials were measured using a modified voltage-clamp technique. The steady-state effective synaptic currents evoked by activating DN were generally small and depolarizing (mean 2.5 +/- 2.6 nA). DN stimulation generated hyperpolarizing synaptic currents in 2 of the 34 triceps motoneurons studied. The effective synaptic currents from DN tended to be larger in putative type F motoneurons than in putative type S cells (type F mean 3.0 +/- 3.1 nA; type S mean 1.8 +/- 1.0 nA). There was a statistically significant difference between the inputs to putative type FF and putative type S motoneurons (mean difference 2.8 nA, t = 2.87, P < 0.01). The synaptic input from DN to medial gastrocnemius motoneurons had approximately the same amplitude as that from homonymous Ia afferent fibers. However, the distribution of DN input with respect to putative motor unit type was the opposite of that previously reported for Ia afferent input. Thus, the synaptic input from DN might act to compress the range of recruitment thresholds within the motoneuron pool and thereby increase the gain of its input-output function.

Animals↗

Transient and sustained types of long-term potentiation in the CA1 area of the rat hippocampus.

Synaptic potentiation induced by high frequency stimulation was investigated by recording field excitatory postsynaptic potentials (f-EPSPs) in rat hippocampal slices. Potentiation consisted of a transient period of decaying f-EPSPs (short-term potentiation, STP) that led to a plateau of continuously potentiated f-EPSPs (long-term potentiation, LTP). Here we show that a previously unknown type of transient, use-dependent, long-lasting potentiation (t-LTP) can account for STP. t-LTP could be stored for more than 6 h and its decay was caused by synaptic activation. Both the expression and the decay of t-LTP were input specific. t-LTP was induced differently from conventional LTP in that the amplitude of t-LTP was dependent upon the stimulation frequency, whereas the magnitude of LTP was dependent on the number of stimuli in the induction train. The decay of t-LTP could not be prevented by the blockage of glutamate receptors, but was prevented by the blockage of stimulus-evoked neurotransmitter release, suggesting that t-LTP is expressed presynaptically. Paired-pulse stimulation experiments showed that the decay of t-LTP was mediated by a decrease in the probability of neurotransmitter release. The decline of t-LTP could be prolonged by the activation of NMDA receptors. Hence, both single and paired-pulse stimuli prolonged the decline of the t-LTP. This decline could be prevented by high frequency burst stimulation (200 Hz). We conclude that t-LTP allows dynamic modulation of synaptic transmission by providing not only spatial association but also temporal convergence between synaptic inputs. Therefore, t-LTP might be a substrate for the encoding of synaptic memory.

2-Amino-5-phosphonovalerate↗

Electrophysiologic studies on the pallido- and cerebellothalamic projections in squirrel monkeys (Saimiri sciureus).

Thalamic projections of the pallidum and the deep cerebellar nuclei were studied by unitary recordings as well as field potential analysis in the thalamus of squirrel monkeys (Saimiri Sciureus) under sodium pentobarbital anesthesia. Stimulation of the pallidum produced a positive field potential preceded by incoming afferent fiber volleys in the thalamus. Spontaneous discharges of thalamic neurons were suppressed during this positive potential corresponded to that of the hyperpolarizing potential. The hyperpolarization was presumed to be a monosynaptic inhibitory post synaptic potential by the short synaptic delay (about 0.5--0.7 ms) and responsiveness to high frequency stimulation (over 150 Hz). The positive field potential on stimulation of the external pallidal segment was distributed in L.po (VA) and the reticular thalamic nucleus around L.po, whereas that on stimulation of the internal segment was in V.o.a (the anterior basal part of VL) and in Z.o (upper part of VL). The projection of the external segment appeared to be less dense than that of the internal segment. The projection of deep cerebellar nuclei was situated in V.o.a, V.o.p (posterior part of basal part of VL), V.o.i (VLm), the intralaminar nucleus (CL), and some part of V. im (the rostral part of VPLo). Projections of the interpositus and dentate nuclei were distributed in a more anterior part than those of the fastigial nucleus. A certain topographical arrangement of the projections of these three nuclei was found in V.o.p, V.o.i and V.im. No significant overlap was detected between pallidum and the deep cerebellar nuclei within the thalamus.

Animals↗

The optically determined size of exo/endo cycling vesicle pool correlates with the quantal content at the neuromuscular junction of Drosophila larvae.

According to the current theory of synaptic transmission, the amplitude of evoked synaptic potentials correlates with the number of synaptic vesicles released at the presynaptic terminals. Synaptic vesicles in presynaptic boutons constitute two distinct pools, namely, exo/endo cycling and reserve pools (). We defined the vesicles that were endocytosed and exocytosed during high K+ stimulation as the exo/endo cycling vesicle pool. To determine the role of exo/endo cycling vesicle pool in synaptic transmission, we estimated the quantal content electrophysiologically, whereas the pool size was determined optically using fluorescent dye FM1-43. We then manipulated the size of the pool with following treatments. First, to change the state of boutons of nerve terminals, motoneuronal axons were severed. With this treatment, the size of exo/endo cycling vesicle pool decreased together with the quantal content. Second, we promoted the FM1-43 uptake using cyclosporin A, which inhibits calcineurin activities and enhances endocytosis. Cyclosporin A increased the total uptake of FM1-43, but neither the size of exo/endo cycling vesicle pool nor the quantal content changed. Third, we increased the size of exo/endo cycling vesicle pool by forskolin, which enhances synaptic transmission. The forskolin treatment increased both the size of exo/endo cycling vesicle pool and the quantal content. Thus, we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43 fluorescence by boutons. The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles.

Animals↗

Synaptic AMPA receptor exchange maintains bidirectional plasticity.

Activity-dependent synaptic delivery of GluR1-, GluR2L-, and GluR4-containing AMPA receptors (-Rs) and removal of GluR2-containing AMPA-Rs mediate synaptic potentiation and depression, respectively. The obvious puzzle is how synapses maintain the capacity for bidirectional plasticity if different AMPA-Rs are utilized for potentiation and depression. Here, we show that synaptic AMPA-R exchange is essential for maintaining the capacity for bidirectional plasticity. The exchange process consists of activity-independent synaptic removal of GluR1-, GluR2L-, or GluR4-containing AMPA-Rs and refilling with GluR2-containing AMPA-Rs at hippocampal and cortical synapses in vitro and in intact brains. In GluR1 and GluR2 knockout mice, initiation or completion of synaptic AMPA-R exchange is compromised, respectively. The complementary AMPA-R removal and refilling events in the exchange process ultimately maintain synaptic strength unchanged, but their long rate time constants ( approximately 15-18 hr) render transmission temporarily depressed in the middle of the exchange. These results suggest that the previously hypothesized "slot" proteins, rather than AMPA-Rs, code and maintain transmission efficacy at central synapses.

Anesthetics, Local↗

Retrograde activation of presynaptic NMDA receptors enhances GABA release at cerebellar interneuron-Purkinje cell synapses.

Synaptic inhibition is a vital component in the control of cell excitability within the brain. Here we report a newly identified form of inhibitory synaptic plasticity, termed depolarization-induced potentiation of inhibition, in rodents. This mechanism strongly potentiated synaptic transmission from interneurons to Purkinje cells after the termination of depolarization-induced suppression of inhibition. It was triggered by an elevation of Ca(2+) in Purkinje cells and the subsequent retrograde activation of presynaptic NMDA receptors. These glutamate receptors promoted the spontaneous release of Ca(2+) from presynaptic ryanodine-sensitive Ca(2+) stores. Thus, NMDA receptor-mediated facilitation of transmission at this synapse provides a regulatory mechanism that can dynamically alter the synaptic efficacy at inhibitory synapses.

Animals↗

Glutamate receptor-mediated synaptic excitation in axons of the lamprey.

1. Spontaneous and evoked synaptic inputs were recorded in vitro in the axons of lamprey reticulospinal neurones. After isolation of the axon from its somata, synaptic inputs were recorded using microelectrode and whole-cell patch clamp recording techniques. 2. Single stimuli applied to the spinal cord elicited Ca(2+)-dependent synaptic potentials with short latencies in reticulospinal axons. These synaptic inputs are capable of summation and generate sufficient depolarization to raise the membrane potential beyond threshold to initiate action potentials. Action potential initiation in the absence of the cell body indicates that these axons show synaptic integration. 3. Both evoked and spontaneous responses comprise at least two components of synaptic drive: a slow component (rise time of 9.6 +/- 2.1 ms) with a reversal potential of -53 +/- 19 mV and a fast component (rise time as fast as 0.85 ms) with a reversal potential of 0.3 +/- 9.1 mV. The responses are Ca2+ dependent, and are blocked by the substitution of Ba2+ for Ca2+ in the saline solution. 4. The slow component of synaptic input was blocked by the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline (5 microM). The fast component was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroqinoxaline-2,3-dione (CNQX; 10 microM) in Ringer solution containing physiological concentrations of Mg2+. Following removal of Mg2+ from the superfusate a further excitatory component was identified that was blocked by application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoate (AP5; 100 microM). 5. Comparison of the kinetic properties and the voltage sensitivity of the isolated components of evoked and spontaneous synaptic activity indicate that these responses are mediated by similar synaptic inputs. 6. These results suggest that axons and presynaptic terminals receive excitatory and inhibitory ionotropic receptor-mediated inputs. Summation of these inputs is possible indicating that the axons act as sites of synaptic integration similar to the role previously attributed only to neuronal dendrites and somata.

Action Potentials↗

Intracellular electrophysiology of CA1 pyramidal neurones in slices of the kainic acid lesioned hippocampus of the rat.

Intracellular recordings were made from hippocampal CA1 pyramidal cells in slices where the CA3/CA4 region had been lesioned using intracerebroventricular kainic acid. In 55% of the cells studied orthodromic excitation evoked bursts of action potentials. This bursting activity was associated with a decrease in or loss of the early phase to the hyperpolarisation which normally follows orthodromically evoked action potentials. The recurrent inhibitory post-synaptic potential produced by antidromic activation of pyramidal cells was also reduced or absent. A late phase to the orthodromic hyperpolarisation was reduced in cells from lesioned slices. However, in normal slices treated with bicuculline this potential showed an apparent increase. The afterhyperpolarisation which follows a short current evoked burst of action potentials was reduced in bursting cells from lesioned slices. In addition, a silent period in the firing pattern produced by long depolarising current pulses was reduced or absent in these cells. These results together with observations made with bicuculline suggest that the bursting activity in lesioned slices is largely due to a loss of inhibition mediated by gamma-aminobutyric acid. It is proposed that the kainic acid-lesioned in vitro hippocampus may be a suitable preparation for studying the electrophysiology of temporal lobe epilepsy.

Adaptation, Physiological↗