Regulation of neuronal plasticity and fear by a dynamic change in PAR1-G protein coupling in the amygdala.

Fear memories are acquired through neuronal plasticity, an orchestrated sequence of events regulated at circuit and cellular levels. The conventional model of fear acquisition assumes unimodal (for example, excitatory or inhibitory) roles of modulatory receptors in controlling neuronal activity and learning. Contrary to this view, we show that protease-activated receptor-1 (PAR1) promotes contrasting neuronal responses depending on the emotional status of an animal by a dynamic shift between distinct G protein-coupling partners. In the basolateral amygdala of fear-naive mice PAR1 couples to Gαq/11 and Gαo proteins, while after fear conditioning coupling to Gαo increases. Concurrently, stimulation of PAR1 before conditioning enhanced, but afterwards it inhibited firing of basal amygdala neurons. An initial impairment of the long-term potentiation (LTP) in PAR1-deficient mice was transformed into an increase in LTP and enhancement of fear after conditioning. These effects correlated with more frequent 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) receptor-mediated miniature post synaptic events and increased neuronal excitability. Our findings point to experience-specific shifts in PAR1-G protein coupling in the amygdala as a novel mechanism regulating neuronal excitability and fear.

P/Q-type calcium channel ablation in a mice glycinergic synapse mediated by multiple types of Ca²+ channels alters transmitter release and short term plasticity.

Ca(v)2.1 channels (P/Q-type) play a prominent role in controlling neurotransmitter release. Transgenic mice in which the α1A pore-forming subunit of Ca(v)2.1 channels is ablated (KO) provide a powerful tool to study Ca(v)2.1 function in synaptic transmission in vivo. Whole-cell patch clamp was used to measure inhibitory glycinergic postsynaptic currents (IPSCs) from the lateral superior olive (LSO). Comparing wild-type (WT) and KO mice, we investigated the relevance of P/Q-type calcium channels at a glycinergic synapse mediated by multiple types of Ca(2+) channels, in opposition to synapses where only this type of Ca(2+) channels are in charge of transmitter release. We found that in KO mice, N-type and L-type Ca(2+) channels control synaptic transmission, resulting in a functional but reduced glycinergic transmitter release. Pair pulse facilitation of synaptic currents is retained in KO mice, even when synaptic transmission is driven by either N or L-type calcium channels alone, in contrast with lack of this phenomenon in other synapses which are exclusively mediated by P/Q-type channels. Thus, pointing a difference between P/Q- and N-type channels present in single or multiple types of calcium channels driven synapses. Significant alterations in short-term synaptic plasticity were observed. KO mice exhibited a stronger short term depression (STD) of IPSCs during repetitive stimulation at high frequency and recovered with a larger time constant compared to WT mice. Finally, transmitter release at the LSO synapse from KO mice was strongly modulated by presynaptic GTP-binding protein-coupled receptor γ-aminobutyric acid type B (GABA(B)).

Acceleration of AMPA receptor kinetics underlies temperature-dependent changes in synaptic strength at the rat calyx of Held.

It is well established that synaptic transmission declines at temperatures below physiological, but many in vitro studies are conducted at lower temperatures. Recent evidence suggests that temperature-dependent changes in presynaptic mechanisms remain in overall equilibrium and have little effect on transmitter release at low transmission frequencies. Our objective was to examine the postsynaptic effects of temperature. Whole-cell patch-clamp recordings from principal neurons in the medial nucleus of the trapezoid body showed that a rise from 25 degrees C to 35 degrees C increased miniature EPSC (mEPSC) amplitude from -33 +/- 2.3 to -46 +/- 5.7 pA (n=6) and accelerated mEPSC kinetics. Evoked EPSC amplitude increased from -3.14 +/- 0.59 to -4.15 +/- 0.73 nA with the fast decay time constant accelerating from 0.75 +/- 0.09 ms at 25 degrees C to 0.56 +/- 0.08 ms at 35 degrees C. Direct application of glutamate produced currents which similarly increased in amplitude from -0.76 +/- 0.10 nA at 25 degrees C to -1.11 +/- 0.19 nA 35 degrees C. Kinetic modelling of fast AMPA receptors showed that a temperature-dependent scaling of all reaction rate constants by a single multiplicative factor (Q10=2.4) drives AMPA channels with multiple subconductances into the higher-conducting states at higher temperature. Furthermore, Monte Carlo simulation and deconvolution analysis of transmission at the calyx of Held showed that this acceleration of the receptor kinetics explained the temperature dependence of both the mEPSC and evoked EPSC. We propose that acceleration in postsynaptic AMPA receptor kinetics, rather than altered presynaptic release, is the primary mechanism by which temperature changes alter synaptic responses at low frequencies.

Detecting synaptic connections in the medial nucleus of the trapezoid body using calcium imaging.

The study of synaptic transmission in brain slices generally entails the patch-clamping of postsynaptic neurones and stimulation of identified presynaptic axons using a remote electrical stimulating electrode. Although patch recording from postsynaptic neurones is routine, many presynaptic axons take tortuous turns and are severed in the slicing procedure, blocking propagation of the action potential to the synaptic terminal and preventing synaptic stimulation. Here we demonstrate a method of using calcium imaging to select postsynaptic cells with functional synaptic inputs prior to patch-clamp recording. We have used this method for exploring transmission in the auditory brainstem at the medial nucleus of the trapezoid body neurones, which are innervated by axons from the contralateral cochlear nucleus. Brainstem slices were briefly loaded with the calcium indicator fura-2 AM and stimulated with an electrode placed on the midline. Electrical stimulation caused a rise in intracellular calcium concentration in those postsynaptic neurones with active synaptic connections. Since <10% of the medial nucleus of the trapezoid body neurones retain viable synaptic inputs following the slicing procedure, preselecting those cells with active synapses dramatically increased our recording success. This detection method will greatly ease the study of synaptic responses in brain areas where suprathreshold synaptic inputs occur but connectivity is sparse.

Calcium channels triggering transmitter release in the rat medial superior olive.

We used whole cell voltage clamp recordings from neurones in rat auditory brainstem slices to study the Ca(2+) channel types involved in triggering synaptic glutamate and glycine release in the medial superior olivary nucleus. Glutamate release from the anterior ventral cochlear (aVCN) bushy neurone synapse did not involve L-type Ca(2+) channels (alpha(1C-D); Ca(V)1.2-1.3), but was mediated with similar efficacies by both N-type (alpha(1B); Ca(V)2.2) and the P/Q-type Ca(2+) channels (alpha(1A); Ca(V)2.1). Glycine release from the medial nucleus of the trapezoid body (MNTB) synapse was mediated predominantly by P/Q-type Ca(2+) channels, but with a significant contribution from N-type Ca(2+) channels. Combined application of the P/Q- and N-type Ca(2+) channel toxins, omega-agatoxin IVA and omega-conotoxin GVIA, left a very small remnant of both the inhibitory and excitatory postsynaptic currents, probably reflecting a minimal contribution of R-type Ca(2+) channels (alpha(1E); Ca(V)2.3) to transmitter release. In contrast with aVCN bushy neurones, MNTB somata lacked both T- (alpha(1G-I); Ca(V)3.1-3.3) and L-type channels, but expressed a higher proportion of P/Q-type Ca(2+) channels.

Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem.

1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.

Characterisation of inhibitory and excitatory postsynaptic currents of the rat medial superior olive.

The medial superior olive (MSO) is part of the binaural auditory pathway, receiving excitatory projections from both cochlear nuclei and an inhibitory input from the ipsilateral medial nucleus of the trapezoid body (MNTB). We characterised the excitatory and inhibitory synaptic currents of MSO neurones in 3- to 14-day-old rats using whole-cell patch-clamp methods in a brain slice preparation.A dual component EPSC was mediated by AMPA and NMDA receptors. The AMPA receptor-mediated EPSC decayed with a time constant of 1.99+/-0.16 ms (n = 8). Following blockade of glutamate receptors, a monosynaptic strychnine-sensitive response was evoked on stimulation of the MNTB, indicative of a glycine receptor-mediated IPSC. GABAA receptors contributed to IPSCs in rats under 6 days old (bicuculline blocked 30% of the IPSC). In older rats little or no bicuculline-sensitive component was detectable, except in the presence of flunitrazepam. These glycinergic IPSCs showed a reversal potential that varied with changes in [Cl-]i, as predicted by the Nernst equation. The IPSC exhibited two developmentally relevant changes. (i) At around postnatal day 6, the GABAA receptor-mediated component declined, leaving a predominant glycine-mediated IPSC. The isolated glycinergic IPSC decayed with time constants of 7.8+/-0.3 and 38.3+/-1.7 ms, with the slower component contributing 7.8+/-0.6% of the peak amplitude (n = 121, 3-11 days old, -70 mV, 25 deg C). (ii) After day 11 the IPSC fast decay accelerated to 3.9+/-0.3 ms (n = 12) and the magnitude of the slow component declined to less than 1%. Spontaneous miniature glycinergic IPSCs (mIPSCs) were variable in amplitude and were of large conductance (1.83+/-0.19 nS, n = 8). The amplitude was unchanged on lowering [Ca2+]o. The time course of evoked and spontaneous miniature glycinergic IPSCs were compared. The 10-90% rise times were 0.7 and 0.6 ms, respectively. The evoked IPSC decayed with a fast time constant of 7.2+/-0.7 ms, while the mIPSC decayed with a fast time constant of 5.3+/-0.4 ms in the same seven cells.The glycinergic IPSC decay was voltage dependent with an e-fold change over 118 mV. The temperature dependence of the IPSC decay indicated a Q10 value of 2. Picrotoxin and cyanotriphenylborate had little or no effect on IPSCs from 6- to 14-day-old animals, implying homomeric channels are rare. We conclude that the MSO receives excitatory inputs mediated by AMPA and NMDA receptors and a strong glycinergic IPSC which has a significant contribution from GABAA receptors in neonatal rats. Functionally, the IPSC could increase membrane conductance during the decay of binaural glutamatergic EPSCs, thus refining coincidence detection and interaural timing differences.

Possible modulatory role of voltage-activated Ca(2+) currents determining the membrane properties of isolated pyramidal neurones of the rat dorsal cochlear nucleus.

Voltage-activated Ca(2+) currents have been studied in pyramidal cells isolated enzymatically from the dorsal cochlear nuclei of 6-11-day-old Wistar rats, using whole-cell voltage-clamp. From hyperpolarized membrane potentials, the neurones exhibited a T-type Ca(2+) current on depolarizations positive to -90 mV (the maximum occurred at about -40 mV). The magnitude of the T-current varied considerably from cell to cell (-56 to -852 pA) while its steady-state inactivation was consistent (E(50)=-88.2+/-1.7 mV, s=-6. 0+/-0.4 mV). The maximum of high-voltage activated (HVA) Ca(2+) currents was observed at about -15 mV. At a membrane potential of -10 mV the L-type Ca(2+) channel blocker nifedipine (10 microM) inhibited approximately 60% of the HVA current, the N-type channel inhibitor omega-Conotoxin GVIA (2 microM) reduced the current by 25% while the P/Q-type channel blocker omega-Agatoxin IVA (200 nM) blocked a further 10%. The presence of the N- and P/Q-type Ca(2+) channels was confirmed by immunochemical methods. The metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1, 3-dicarboxylic acid (200 microM) depressed the HVA current in every cell studied (a block of approximately 7% on an average). The GABA(B) receptor agonist baclofen (100 microM) reversibly inhibited 25% of the HVA current. Simultaneous application of omega-Conotoxin GVIA and baclofen suggested that this inhibition could be attributed to the nearly complete blockade of the N-type channels. Possible physiological functions of the voltage-activated Ca(2+) currents reported in this work are discussed.

Contrasting Ca2+ channel subtypes at cell bodies and synaptic terminals of rat anterioventral cochlear bushy neurones.

1. Whole-cell patch clamp recordings were made from bushy cells of the anterioventral cochlear nucleus (aVCN) and their synaptic terminals (calyx of Held) in the medial nucleus of the trapezoid body (MNTB). 2. Both high voltage-activated (HVA) and low voltage-activated (LVA) calcium currents were present in acutely dissociated aVCN neurones and in identified bushy neurones from a cochlear nucleus slice. 3. The transient LVA calcium current activated rapidly on depolarization (half-activation, -59 mV) and inactivated during maintained depolarization (half-inactivation, -89 mV). This T-type current was observed in somatic recordings but was absent from presynaptic terminals. 4. On the basis of their pharmacological sensitivity, P/Q-type Ca2+ channels accounted for only 6 % of the somatic HVA, while L-, N- and R-type Ca2+ channels each accounted for around one-third of the somatic calcium current. 5. The divalent permeabilities of these native calcium channels were compared. The Ba2+/Ca2+ conductance ratios of the somatic HVA and LVA channels were 1.4 and 0.7, respectively. The conductance ratio of the presynaptic HVA current was 0.9, significantly lower that that of the somatic HVA current. 6. We conclude that LVA currents are expressed in the bushy cell body, but are not localized to the excitatory synaptic terminal. All of the HVA current subtypes are expressed in bushy cells, but there is a strong polarity to their localization; P-type contribute little to somatic currents but predominate at the synaptic terminal; L-, N- and R-types dominate at the soma, but contribute negligibly to calcium currents in the terminal.

Facilitation of the presynaptic calcium current at an auditory synapse in rat brainstem.

1. The presynaptic calcium current (IpCa) was recorded from the calyx of Held in rat brainstem slices using the whole-cell patch clamp technique. 2. Tetanic activation of IpCa by 1 ms depolarizing voltage steps markedly enhanced the amplitude of IpCa. Using a paired pulse protocol, the second (test) response was facilitated with inter-pulse intervals of less than 100 ms. The facilitation was greater at shorter intervals and was maximal (about 20%) at intervals of 5-10 ms. 3. When the test pulse duration was extended, the facilitation was revealed as an increased rate of IpCa activation. From the current-voltage relationship measured at 1 ms from onset, facilitation could be described by a shift in the half-activation voltage of about -4 mV. 4. IpCa facilitation was not attenuated when guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS) or guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS) was included in the patch pipette, suggesting that G-proteins are not involved in this phenomenon. 5. On reducing [Ca2+]o, the magnitude of facilitation diminished proportionally to the amplitude of IpCa. Replacement of [Ca2+]o by Ba2+ or Na+, or buffering of [Ca2+]i with EGTA or BAPTA attenuated IpCa facilitation. 6. We conclude that repetitive presynaptic activity can facilitate the presynaptic Ca2+ current through a Ca2+-dependent mechanism. This mechanism would be complementary to the action of residual Ca2+ on the exocytotic machinery in producing activity-dependent facilitation of synaptic responses.

Contribution of the Kv3.1 potassium channel to high-frequency firing in mouse auditory neurones.

1. Using a combination of patch-clamp, in situ hybridization and computer simulation techniques, we have analysed the contribution of potassium channels to the ability of a subset of mouse auditory neurones to fire at high frequencies. 2. Voltage-clamp recordings from the principal neurones of the medial nucleus of the trapezoid body (MNTB) revealed a low-threshold dendrotoxin (DTX)-sensitive current (ILT) and a high-threshold DTX-insensitive current (IHT). 3. IHT displayed rapid activation and deactivation kinetics, and was selectively blocked by a low concentration of tetraethylammonium (TEA; 1 mM). 4. The physiological and pharmacological properties of IHT very closely matched those of the Shaw family potassium channel Kv3.1 stably expressed in a CHO cell line. 5. An mRNA probe corresponding to the C-terminus of the Kv3.1 channel strongly labelled MNTB neurones, suggesting that this channel is expressed in these neurones. 6. TEA did not alter the ability of MNTB neurones to follow stimulation up to 200 Hz, but specifically reduced their ability to follow higher frequency impulses. 7. A computer simulation, using a model cell in which an outward current with the kinetics and voltage dependence of the Kv3.1 channel was incorporated, also confirmed that the Kv3.1- like current is essential for cells to respond to a sustained train of high-frequency stimuli. 8. We conclude that in mouse MNTB neurones the Kv3.1 channel contributes to the ability of these cells to lock their firing to high-frequency inputs.

Inactivation of presynaptic calcium current contributes to synaptic depression at a fast central synapse.

Voltage-gated calcium channels are well characterized at neuronal somata but less thoroughly understood at the presynaptic terminal where they trigger transmitter release. In order to elucidate how the intrinsic properties of presynaptic calcium channels influence synaptic function, we have made direct recordings of the presynaptic calcium current (I(pCa)) in a brainstem giant synapse called the calyx of Held. The current was pharmacologically classified as P-type and exhibited marked inactivation. The inactivation was largely dependent upon the inward calcium current magnitude rather than the membrane potential, displayed little selectivity between divalent charge carriers (Ca2+, Ba2+ and Sr+), and exhibited slow recovery. Simultaneous pre- and postsynaptic whole-cell recording revealed that I(pCa) inactivation predominantly contributes to posttetanic depression of EPSCs. Thus, because of its slow recovery, I(pCa) inactivation underlies this short-term synaptic plasticity.

Synaptic transmission: well-placed modulators.

Metabotropic glutamate receptors are involved in the modulation of synaptic transmission; their localization in perisynaptic areas would appear to limit their activation by endogenous glutamate, but recent reports suggest that this strategic placement allows use-dependent activation of these synaptic modulators.

Membrane currents influencing action potential latency in granule neurons of the rat cochlear nucleus.

Granule cells are the most numerous neurons in the cochlear nucleus, but, because of their small size, little information on their membrane properties and ionic currents is available. We used an in vitro slice preparation of the rat ventral cochlear nucleus to make whole-cell recordings from these cells. Under current clamp, some granule neurons fired spontaneous action potentials and all generated a train of action potentials on depolarization (threshold current, 10-35 pA). Hyperpolarization increased the latency to the first action potential evoked during a subsequent depolarization. We examined which voltage-gated currents might underlie this latency shift. In addition to a fast inward Na+ current, depolarization activated two outward potassium currents. A transient current was rapidly inactivated by membrane potentials positive to -60 mV, while a second, more slowly inactivating current was observed following the decay of the transient current. No hyperpolarization-activated conductances were observed in these cells. Modelling of the currents suggests that removal of inactivation on hyperpolarization accounts for the increased action potential latency in granule cells. Such a mechanism could account for the 'pauser'-type firing patterns of the fusiform cells which receive a prominent projection from the granule cells in the dorsal cochlear nucleus.

Presynaptic calcium current modulation by a metabotropic glutamate receptor.

Metabotropic glutamate receptors (mGluRs) regulate transmitter release at mammalian central synapses. However, because of the difficulty of recording from mammalian presynaptic terminals, the mechanism underlying mGluR-mediated presynaptic inhibition is not known. Here, simultaneous recordings from a giant presynaptic terminal, the calyx of Held, and its postsynaptic target in the medial nucleus of the trapezoid body were obtained in rat brainstem slices. Agonists of mGluRs suppressed a high voltage-activated P/Q-type calcium conductance in the presynaptic terminal, thereby inhibiting transmitter release at this glutamatergic synapse. Because several forms of presynaptic modulation and plasticity are mediated by mGluRs, this identification of a target ion channel is a first step toward elucidation of their molecular mechanism.

Two voltage-dependent K+ conductances with complementary functions in postsynaptic integration at a central auditory synapse.

The medial nucleus of the trapezoid body (MNTB) relays auditory information important for sound source localization. MNTB neurons faithfully preserve the temporal patterning of action potentials (APs) occurring in their single giant input synapse, even at high frequencies. The aim of this work was to examine the postsynaptic potassium conductances that shape the transfer of auditory information across this glutamatergic synapse. We used whole cell patch techniques to record from MNTB neurons in thin slices of rat brainstem. Two types of potassium conductance were found which had a strong influence on an MNTB neuron's postsynaptic response. A small low voltage threshold current, Id, limited the response during each EPSP to a single brief AP. Id was specifically blocked by dendrotoxin (DTX), resulting in additional APs during the tail end of the EPSP. Thus DTX degraded the temporal fidelity of synaptic transmission, since one presynaptic AP then led to several postsynaptic APs. A second conductance was a fast delayed rectifier with a high voltage activation threshold, that rapidly repolarised APs and thus facilitated high frequency AP responses. Together, these two conductances allow high frequency auditory information to be passed accurately across the MNTB relay synapse and separately, such conductances may perform analogous functions elsewhere in the nervous system.

Pre- and postsynaptic glutamate receptors at a giant excitatory synapse in rat auditory brainstem slices.

1. Whole-cell patch recordings were used to examine the EPSC generated by the calyx of Held in neurones of the medial nucleus of the trapezoid body (MNTB). Each neurone receives a somatic input from a single calyx (giant synapse). 2. A slow NMDA receptor-mediated EPSC peaked in 10 ms and decayed as a double exponential with time constants of 44 and 147 ms. A fast EPSC had a mean rise time of 356 microseconds (at 25 degrees C), while the decay was described by a double exponential with time constants of 0.70 and 3.43 ms. 3. Cyclothiazide slowed the decay of the fast EPSC, indicating that it is mediated by AMPA receptors. The slower time constant was slowed to a greater extent than the faster time constant. Cyclothiazide potentiated EPSC amplitude, partly by a presynaptic mechanism. 4. The metabotropic glutamate receptor (mGluR) agonists, 1S,3S-ACPD, 1S,3R-ACPD and L-2-amino-4-phosphonobutyrate (L-AP4) reversibly depressed EPSC amplitude. A dose-response curve for 1S,3S-ACPD gave an EC50 of 7 microM and a Hill coefficient of 1.2. 5. Analysis of the coefficient of variation ratio showed that the above mGluR agonists acted presynaptically to reduce the probability of transmitter release. Adenosine and baclofen also depressed transmission by a presynaptic mechanism. 6. alpha-Methyl-4-carboxyphenylglycine (MCPG; 0.5-1 mM) did not antagonize the effects of 1S,3S-ACPD, while high concentrations of L-2-amino-3-phosphonopropionic acid (L-AP3; 1 mM) and 4-carboxy-3-hydroxyphenyglycine (4C3HPG; 500 microM) depressed transmission. 7. There was a power relationship between [Ca2+]o and EPSC amplitude with co-operativity values ranging from 1.5 to 3.4. 8. The mechanism by which mGluRs modulate transmitter release appeared to be independent of presynaptic Ca2+ or K+ currents, since ACPD caused no change in the level of paired-pulse facilitation or the duration of the presynaptic action potential (observed by direct recording from the terminal), indicating that the presynaptic mGluR transduction mechanism may be coupled to part of the exocytotic machinery. 9. Our data are not consistent with the presence at the calyx of Held of any one known mGluR subtype. Comparison of the time course and pharmacology of the fast EPSC with data from cloned AMPA receptors is consistent with the idea that GluR-Do subunits dominate the postsynaptic channels.