How can exocytosis account for the actual properties of miniature synaptic signals?

Age- and gender-related differences in GABAA receptor-mediated postsynaptic currents in GABAergic neurons of the substantia nigra reticulata in the rat

The responsiveness of the rat anterior substantia nigra pars reticulata (SNR) GABAergic neurons to GABA(A)ergic drugs changes with age and gender, altering its role in seizure control. To determine whether maturational and gender-specific differences in the properties of spontaneous GABA(A)Rs-mediated inhibitory postsynaptic currents (sIPSCs) underlie these events, we studied sIPSCs at baseline and after application of the alpha1 GABA(A)Rs subunit selective agonist zolpidem, at postnatal days (PN) 5-9, PN12-15, and PN28-32. Results were correlated with the alpha1 and alpha 3 GABA(A)Rs subunit immunoreactivity (-ir) at PN5, PN15, and PN30, using immunochemistry. The mean frequency, amplitude and charge transfer increased whereas the 10-90% rise time and decay time accelerated with age in both genders. The faster sIPSC kinetics in older rats were paralleled by increased alpha1-ir and decreased alpha 3-ir. At PN5-9, males had more robust sIPSCs (frequency, amplitude, charge carried per event and charge transfer) than females. At PN28-32, males exhibited higher amplitudes and faster kinetics than females. The zolpidem-induced increase of decay times, amplitude and charge transfer and alpha1-ir expression were the lowest in PN5-9 males but increased with age, in both genders. Our findings demonstrate that alterations in GABA(A)Rs subunit expression partially underlie age- and gender-specific sIPSC changes in SNR neurons. However, the observation of gender differences in sIPSC kinetics that cannot be attributed to changes in perisomatic alpha1 expression suggests the existence of additional gender-specific factors that control the sIPSC kinetics in rat SNR.

Presynaptic quantal plasticity: Katz's original hypothesis revisited

Changes in the amplitudes of signals conveyed at synaptic contacts between neurons underlie many brain functions and pathologies. Here we review the possible determinants of the amplitude and plasticity of the elementary postsynaptic signal, the miniature. In the absence of a definite understanding of the molecular mechanism releasing transmitters, we investigated a possible alternative interpretation. Classically, both the quantal theory and the vesicle theory predict that the amount of transmitter producing a miniature is determined presynaptically prior to release and that rapid changes in miniature amplitude reflect essentially postsynaptic alterations. However, recent data indicates that short-term and long-lasting changes in miniature amplitude are in large part due to changes in the amount of transmitter in individual released packets that show no evidence of preformation. Current representations of transmitter release derive from basic properties of neuromuscular transmission and endocrine secretion. Reexamination of overlooked properties of these two systems indicate that the amplitude of miniatures may depend as much, if not more, on the Ca(2+) signals in the presynaptic terminal than on the number of postsynaptic receptors available or on vesicle's contents. Rapid recycling of transmitter and its possible adsorption at plasma and vesicle lumenal membrane surfaces suggest that exocytosis may reflect membrane traffic rather than actual transmitter release. This led us to reconsider the disregarded hypothesis introduced by Fatt and Katz (1952; J Physiol 117:109-128) that the excitability of the release site may account for the "quantal effect" in fast synaptic transmission. In this case, changes in excitability of release sites would contribute to the presynaptic quantal plasticity that is often recorded.

Synaptic inhibition of pyramidal cells evoked by different interneuronal subtypes in layer v of rat visual cortex

Properties of GABA(A) receptor-mediated unitary inhibitory postsynaptic currents (uIPSCs) in pyramidal (P) cells, evoked by fast spiking (FS) and low-threshold spike (LTS) subtypes of interneurons in layer V of rat visual cortex slices were examined using dual whole cell recordings. uIPSCs evoked by FS cells were larger and faster rising than those evoked by LTS cells, consistent with the known primary projections of FS and LTS cell axons to perisomatic and distal dendritic areas of layer V pyramidal cells, respectively, and the resulting electrotonic attenuation for LTS-P synaptic events. Unexpectedly, the decay time constants for LTS-P and FS-P uIPSCs were not significantly different. Modeling results were consistent with differences in the underlying GABA(A) receptor-mediated conductance at LTS-P and FS-P synapses. Paired-pulse depression (PPD), present at both synapses, was associated with an increase in failure rate and a decrease in coefficient of variation, indicating that presynaptic mechanisms were involved. Furthermore, the second and first uIPSC amplitudes during PPD were not inversely correlated, suggesting that PPD at both synapses is independent of previous release and might not result from depletion of the releasable pool of synaptic vesicles. Short, 20-Hz trains of action potentials in presynaptic interneurons evoked trains of uIPSCs with exponentially decreasing amplitudes at both FS-P and LTS-P synapses. FS-P uIPSC amplitudes declined more slowly than those of LTS-P uIPSCs. Thus FS and LTS cells, with their differences in firing properties, synaptic connectivity with layer V P cells, and short-term synaptic dynamics, might play distinct roles in regulating the input-output relationship of the P cells.

Porocytosis: a new approach to synaptic function

We propose a new approach to address the question of how a single quantum of neurotransmitter is secreted from a presynaptic terminal whose clustered secretory vesicles are locally bathed in high levels of calcium ions [Proceedings of the Symposium on Bioelectrogenesis (1961) 297-309; The Physiology of Synapses (1964) Chapters 1, 4, 5, 6; How the Self Controls its Brain (1994) Chapters 1, 4, 5, 6; Science 256 (1992) 677-679]. This hypothesis, which we term 'porocytosis', posits that the post-synaptic quantal response results from transmitter secreted through an array of docked vesicle/secretory pore complexes. The transient increase in calcium ions, which results from the voltage activated calcium channels, stimulates the array of secretory pores to simultaneously flicker open to pulse transmitter. Porocytosis is consistent with the quantal nature of presynaptic secretion and transmission, and with available biochemical, morphological and physiological evidence. It explains the frequency dependency of quantal size as a function of the secretion process. It permits a signature amount of transmitter release for different frequencies allowing a given synapse to be employed in different behavioral responses. The porocytosis hypothesis permits fidelity of secretion and the seemingly apposed characteristic of synaptic plasticity. The dynamics inherent in an array insure a constant quantal size as a function of the number of units within the array. In this hypothesis, plasticity is a consequence of concurrent pre- and post-synaptic changes due to a change in array size. Changes in the number of docked vesicle-secretory pore complexes composing the array can explain facilitation, depletion, graded excitation-secretion and long term plasticity.

Surface-accessible GABA supports tonic and quantal synaptic transmission

Exocytosis is commonly viewed as the only secretory process able to account for quantal forms of fast synaptic transmission. However, the demonstrated variability and composite properties of miniature postsynaptic signals are not easily explained by all-or-none exocytotic discharge of transmitter in solution from inside vesicles. Recent studies of endocrine secretion have shown that hormone release does not coincide with exocytosis due to its trapping in the core matrix of the granule. Thus, we tested whether the synaptic transmitter GABA could also be held in a matrix before being released. Using confocal microscopy and flow cytometry of embryonic rat hippocampal neurons, we found a GABA immunoreaction at the surface of live cell bodies and growth cones that coincided spatially and quantitatively with the binding of tetanus toxin fragment C (TTFC). TTFC binds predominantly at membrane sites containing the trisialoglycosphingolipid GT1b. Using flow cytometry, GT1b-containing liposomes preincubated in 100 nM GABA exhibited the same relationship between GABA and TTFC surface binding as found on neurons and growth cones. Embryonic neurons differentiated in culture expressed initially a tonic, and after 3-5 days, transient, postsynaptic signals mediated by GABA acting at GABA(A) receptor/Cl(-) channels. A stream of saline applied to the neuronal surface rapidly and reversibly suppressed both tonic and transient signals. A brief application of the GABAmimetic isoguvacine immediately transformed both tonic and transient GABAergic signals into tonic and transient isoguvacinergic signals. These results and those in the literature are consistent with an immediately releasable compartment of transmitter accessible from the presynaptic surface.

Correlation of miniature synaptic activity and evoked release probability in cultures of cortical neurons

Spontaneous miniature synaptic activity is caused by action potential (AP)-independent release of transmitter vesicles and is regulated at the level of single synapses. In cultured cortical neurons we have used this spontaneous vesicle turnover to load the styryl dye FM1-43 into synapses with high rates of miniature synaptic activity. Automated selection procedures restricted analysis to synapses with sufficient levels of miniature activity-mediated FM1-43 uptake. After FM1-43 loading, vesicular FM1-43 release in response to AP stimulation was recorded at single synapses as a measure of release probability. We find that synapses with high rates of miniature activity possess significantly enhanced evoked release rates compared with a control population. Because the difference in release rates between the two populations is [Ca(2+)](o)-dependent, it is most likely caused by a difference in release probability. Within the subpopulation of synapses with high miniature activity, we find that the probabilities for miniature and AP-evoked release are correlated at single synaptic sites. Furthermore, the degree of miniature synaptic activity is correlated with the vesicle pool size. These findings suggest that both evoked and miniature vesicular release are regulated in parallel and that the frequency of miniature synaptic activity can be used as an indicator for evoked release efficacy.

Miniature EPSPs and sensory encoding in the primary afferents of the vestibular lagena of the toadfish, Opsanus tau

The synaptic activity transmitted from vestibular hair cells of the lagena to primary afferent neurons was recorded in vitro using sharp, intracellular microelectrodes. At rest, the activity was composed of miniature excitatory postsynaptic potentials (mEPSPs) at frequencies from 5 to 20/s and action potentials (APs) at frequencies betwen 0 and 10/s. mEPSPs recorded from a single fiber displayed a large variability. For mEPSPs not triggering APs, amplitudes exhibited an average coefficient of variance (CV) of 0.323 and rise times an average CV of 0.516. APs were only triggered by mEPSPs with larger amplitudes (estimated 4-6 mV) and/or steeper maximum rate of rise (10.9 mV/ms, +/- 3.7 SD, n=4 experiments) compared to (3.50 mV/ms, +/-0.07 SD, n=6 experiments) for nontriggering mEPSPs. The smallest mEPSPs showed a fast rise time (0.99 ms between 10% and 90% of peak amplitude) and limited variability across fibers (CV:0.18) confirming that they were not attenuated signals, but rather represented single-transmitter discharges (TDs). The mEPSP amplitude and rise-time relationship suggests that many mEPSPs represented several, rather than a single pulse of secretion of TDs. According to the estimated overall TD frequency, the coincidence of TDs contributing to the same mEPSP were not statistically independent, indicating a positive interaction between TDs that is reminiscent of the way subminiature signals group to form miniature signals at the neuromuscular junction. Depending on the duration and intensity of efferent stimulation, a complete block of AP initiation occurred either immediately or after a delay of a few seconds. Efferent stimulation did not significantly change AP threshold level, but abruptly decreased mEPSP frequency to a near-complete block that followed the block of APs. Maximum mEPSP rate of rise decreased during, and recovered progressively after, efferent stimulation. After termination of efferent stimulation, mEPSP amplitude did not recover instantly and for a few seconds the amplitude distribution of synaptic events showed fewer large-amplitude events than during the control period. This confirms that mEPSP amplitude and rate of rise properties, which are critical for triggering afferent APs, are modified by efferent activity. The depression of afferent AP firing during efferent stimulation corresponded to a decrease in mEPSP frequency and, to a lesser extent, a decrease in mEPSP amplitude and rate of rise, suggesting, a decrease in the level of interaction among TDs contibuting to a mEPSP.

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons

Metabotropic glutamate receptor enhancement of spontaneous IPSCs in neocortical interneurons. J. Neurophysiol. 78: 2287-2295, 1997. Using neocortical layer I neurons as a model for GABAergic interneurons, we have studied gamma-aminobutyric acid-A (GABAA) receptor-mediated spontaneous inhibitory postsynaptic currents (IPSCs) and modulation by metabotropic glutamate receptors (mGluRs). In the presence of 0.5 mu M tetrodotoxin (TTX) and ionotropic glutamate receptor antagonists and under symmetrical Cl- conditions, the mean amplitude of miniature IPSCs (mIPSCs) was approximately 50 pA at a holding potential of -70 mV with individual events ranging from 10 to 400 pA. Averaged mIPSCs had a 10-90% rise time of approximately 0.6 ms. The decay was double exponential. The fast component had a time constant of approximately 4 ms and comprised approximately 40% of the total amplitude. The slow component had a time constant of approximately 22 ms. The frequency of spontaneous IPSCs (sIPSCs), recorded in the absence of TTX, was increased by bath application of the mGluR agonist 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD; 10-100 mu M) or the group I mGluR selective agonist quisqualic acid (Quis; 0.5-1 mu M). Under identical conditions, mIPSCs were not affected. The kinetics of sIPSCs and mIPSCs were not altered by ACPD or Quis. Quis (1 mu M) induced an inward current of approximately 70 pA at a holding potential of -70 mV, whereas ACPD (40-200 mu M) induced a smaller inward current. This current was linear over the voltage range -70 to +30 mV and reversed polarity near 0 mV. In current-clamp recordings, both Quis and ACPD induced a depolarization and action potential firing in layer I and deeper layer interneurons. We conclude that neocortical layer I neurons receive GABAA receptor-mediated inhibitory synaptic inputs. Activation of mGluRs, possibly mGluR1 and/or mGluR5, causes an enhancement of inhibitory synaptic transmission by directly depolarizing corticalGABAergic interneurons through the opening of nonselective cation channels.

Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex

With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (-50 to -70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to -30 mV or by the use of a Mg2+-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of approximately 0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of approximately 1.2 ms and comprised approximately 80% of the total amplitude. A small slow component had a decay time constant of approximately 4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at -60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca2+ permeable.