Frequency-dependent synaptic depression and the balance of excitation and inhibition in the neocortex

The stability of cortical neuron activity in vivo suggests that the firing rates of both excitatory and inhibitory neurons are dynamically adjusted. Using dual recordings from excitatory pyramidal neurons and inhibitory fast-spiking neurons in neocortical slices, we report that sustained activation by trains of several hundred presynaptic spikes resulted in much stronger depression of synaptic currents at excitatory synapses than at inhibitory ones. The steady-state synaptic depression was frequency dependent and reflected presynaptic function. These results suggest that inhibitory terminals of fast-spiking cells are better equipped to support prolonged transmitter release at a high frequency compared with excitatory ones. This difference in frequency-dependent depression could produce a relative increase in the impact of inhibition during periods of high global activity and promote the stability of cortical circuits.     

Activity- and BDNF-induced plasticity of miniature synaptic currents in ES cell-derived neurons integrated in a neocortical network

In vitro differentiated embryonic stem (ES) cells have been proposed as potential donor cells for cell replacement therapies of neurodegenerative diseases. The functional synaptic integration of such cells appears conceivable because ES cell-derived neurons are well known to establish excitatory and inhibitory synapses. However, long-term synaptic plasticity, a prerequisite of memory formation, has not yet been demonstrated at these synapses. After in vitro differentiation and purification by immunoisolation, we co-cultured ES cell-derived neurons with neocortical explants, which strongly innervated the ES cell-derived target neurons. ES cell-derived neurons exhibited action potential firing similar to primary cultured neocortical neurons. The formation of glutamatergic synapses was indicated by AMPA receptor-mediated miniature excitatory postsynaptic currents (AMPA mEPSCs). In addition, a N-methyl-D-aspartate receptor-mediated, D-2-amino-5-phosphonopentanoic acid-sensitive mEPSC component was observed. We first studied activity-dependent homeostatic plasticity (synaptic scaling) of mEPSCs at glutamatergic synapses. Chronic blockade of action potential activity by TTX resulted in an increase in the amplitudes of AMPA mEPSCs. This indicates that ES cell-derived neurons are capable of a homeostatic regulation of postsynaptic AMPA receptors. In addition, we investigated neurotrophin-induced synaptic plasticity of mEPSCs at glutamatergic synapses. Chronic addition of brain-derived neurotrophic factor (BDNF; 100 ng/ml) to the culture medium resulted in an increase in both the frequency and the amplitudes of AMPA mEPSCs. These results suggest that BDNF induces the formation and/or the functional maturation of presynaptic release sites in parallel with an upregulation of postsynaptic AMPA receptors. Thus BDNF represents a potential co-factor that could improve functional synaptic integration of ES cell-derived neurons into neocortical networks.     

Suprachiasmatic nucleus communicates with anterior thalamic paraventricular nucleus neurons via rapid glutamatergic and gabaergic neurotransmission: state-dependent response patterns observed in vitro

The hypothalamic suprachiasmatic nucleus uniquely projects to the midline thalamic paraventricular nucleus. To characterize this projection, patch clamp techniques applied in acute rat brain slice preparations examined responses of anterior thalamic paraventricular nucleus neurons to focal suprachiasmatic nucleus stimulation. Whole cell recordings from slices obtained during daytime (n=40) revealed neurons with a mean membrane potential of -66+/-1.2 mV, input conductance of 1.5+/-0.1 nS and state-dependent tonic or burst firing patterns. Electrical stimulation (one or four pulses) in suprachiasmatic nucleus elicited monosynaptic excitatory postsynaptic potentials (mean latency of 12.6+/-0.6 ms; n=12), featuring both AMPA and N-methyl-D-aspartate-glutamate receptor-mediated components, and monosynaptic bicuculline-sensitive inhibitory postsynaptic potentials (mean latency of 16.6+/-0.6 ms; n=7) reversing polarity at -72+/-2.6 mV, close to the chloride equilibrium potential. Glutamate microstimulation of suprachiasmatic nucleus also elicited transient increases in spontaneous excitatory or inhibitory postsynaptic currents in anterior thalamic paraventricular neurons. Recordings from rats under reverse light/dark conditions (n=22) yielded essentially similar responses to electrical stimulation. At depolarized membrane potentials, suprachiasmatic nucleus-evoked excitatory postsynaptic potentials triggered single action potentials, while evoked inhibitory postsynaptic potentials elicited a silent period in ongoing tonic firing. By contrast, after manual adjustment of membrane potentials to hyperpolarized levels, neuronal response to the same "excitatory" stimulus was a low threshold spike and superimposed burst firing, while responses to "inhibitory" stimuli paradoxically elicited excitatory rebound low threshold spikes and burst firing. These data support the existence of glutamatergic and GABAergic efferents from the suprachiasmatic nucleus to its target neurons. Additionally, in thalamic paraventricular nucleus neurons, responses to activation of their suprachiasmatic afferents may vary in accordance with their membrane potential-dependent intrinsic properties, a characteristic typical of thalamocortical neurons.     

Endocannabinoid-mediated short-term suppression of excitatory synaptic transmission to medium spiny neurons in the striatum

Medium spiny neurons in the dorsal striatum receive glutamatergic excitatory synaptic inputs from the cerebral cortex. These synapses undergo long-term depression that requires release of endocannabinoids from medium spiny neurons and activation of cannabinoid CB1 receptors. However, it remains unclear how cortico-striatal synapses exhibit endocannabinoid-mediated short-term suppression, which has been found in various brain regions including the hippocampus and cerebellum. Endocannabinoids are released from postsynaptic neurons by strong depolarization and resultant Ca2+ elevation or activation of postsynaptic Gq/11-coupled receptors such as group I metabotropic glutamate receptors (mGluRs) and M1/M3 muscarinic acetylcholine receptors. Moreover, endocannabioids are effectively released when weak depolarization is combined with Gq/11-coupled receptor activation. We found that muscarinic activation induced transient suppression of excitatory synaptic transmission to medium spiny neurons, which was independent of retrograde endocannabinoid signaling but was mediated directly by presynaptic muscarinic receptors. Neither postsynaptic depolarization alone nor depolarization and muscarinic activation caused suppression of cortico-striatal synapses. In contrast, activation of group I mGluRs readily suppressed cortico-striatal excitatory synaptic transmission. Furthermore, postsynaptic depolarization induced clear suppression when combined with group I mGluR activation. These results indicate that group I mGluRs but not muscarinic receptors contribute to endocannabinoid-mediated short-term suppression of cortico-striatal excitatory synaptic transmission.     

Activity-dependent depression of local excitatory connections in the CA1 region of mouse hippocampus

The existence of recurrent excitatory synapses between pyramidal cells in the hippocampal CA1 region has been known for some time yet little is known about activity-dependent forms of plasticity at these synapses. Here we demonstrate that under certain experimental conditions, Schaffer collateral/commissural fiber stimulation can elicit robust polysynaptic excitatory postsynaptic potentials due to recurrent synaptic inputs onto CA1 pyramidal cells. In contrast to CA3 pyramidal cell inputs, recurrent synapses onto CA1 pyramidal cells exhibited robust paired-pulse depression and a sustained, but rapidly reversible, depression in response to low-frequency trains of Schaffer collateral fiber stimulation. Blocking GABA(B) receptors abolished paired-pulse depression but had little effect on low-frequency stimulation (LFS)-induced depression. Instead, LFS-induced depression was significantly attenuated by an inhibitor of A1 type adenosine receptors. Blocking the postsynaptic effects of GABA(B) and A1 receptor activation on CA1 pyramidal cell excitability with an inhibitor of G-protein-activated inwardly rectifying potassium channels had no effect on either paired-pulse depression or LFS-induced depression. Thus activation of presynaptic GABA(B) and adenosine receptors appears to have an important role in activity-dependent depression at recurrent synapses. Together, our results indicate that CA3-CA1 and CA1-CA1 synapses exhibit strikingly different forms of short-term synaptic plasticity and suggest that activity-dependent changes in recurrent synaptic transmission can transform the CA1 region from a sparsely connected recurrent network into a predominantly feedforward circuit.     

A role for short-term synaptic facilitation and depression in the processing of intensity information in the auditory brain stem

The nature of the synaptic connection from the auditory nerve onto the cochlear nucleus neurons has a profound impact on how sound information is transmitted. Short-term synaptic plasticity, by dynamically modulating synaptic strength, filters information contained in the firing patterns. In the sound-localization circuits of the brain stem, the synapses of the timing pathway are characterized by strong short-term depression. We investigated the short-term synaptic plasticity of the inputs to the bird's cochlear nucleus angularis (NA), which encodes intensity information, by using chick embryonic brain slices and trains of electrical stimulation. These excitatory inputs expressed a mixture of short-term facilitation and depression, unlike those in the timing nuclei that only depressed. Facilitation and depression at NA synapses were balanced such that postsynaptic response amplitude was often maintained throughout the train at high firing rates (>100 Hz). The steady-state input rate relationship of the balanced synapses linearly conveyed rate information and therefore transmits intensity information encoded as a rate code in the nerve. A quantitative model of synaptic transmission could account for the plasticity by including facilitation of release (with a time constant of approximately 40 ms), and a two-step recovery from depression (with one slow time constant of approximately 8 s, and one fast time constant of approximately 20 ms). A simulation using the model fit to NA synapses and auditory nerve spike trains from recordings in vivo confirmed that these synapses can convey intensity information contained in natural train inputs.     

Coincident glutamatergic and cholinergic inputs transiently depress glutamate release at rat schaffer collateral synapses

The mammalian hippocampus, together with subcortical and cortical areas, is responsible for some forms of learning and memory. Proper hippocampal function depends on the highly dynamic nature of its circuitry, including the ability of synapses to change their strength for brief to long periods of time. In this study, we focused on a transient depression of glutamatergic synaptic transmission at Schaffer collateral synapses in acute hippocampal slices. The depression of evoked excitatory postsynaptic current (EPSC) amplitudes, herein called transient depression, follows brief trains of synaptic stimulation in stratum radiatum of CA1 and lasts for 2-3 min. Depression results from a decrease in presynaptic glutamate release, as NMDA-receptor-mediated EPSCs and composite EPSCs are depressed similarly and depression is accompanied by an increase in the paired-pulse ratio. Transient depression is prevented by blockade of metabotropic glutamate and acetylcholine receptors, presumably located presynaptically. These two receptor types--acting together--cause depression. Blockade of a single receptor type necessitates significantly stronger conditioning trains for triggering depression. Addition of an acetylcholinesterase inhibitor enables depression from previously insufficient conditioning trains. Furthermore, a strong coincident, but not causal, relationship existed between presynaptic depression and postsynaptic internal Ca(2+) release, emphasizing the potential importance of functional interactions between presynaptic and postsynaptic effects of convergent cholinergic and glutamatergic inputs to CA1. These convergent afferents, one intrinsic to the hippocampus and the other likely originating in the medial septum, may regulate CA1 network activity, the induction of long-term synaptic plasticity, and ultimately hippocampal function.     

Differential activity-dependent, homeostatic plasticity of two neocortical inhibitory circuits

Chronic changes in neuronal activity homeostatically regulate excitatory circuitry. However, little is known about how activity regulates inhibitory circuits or specific inhibitory neuron types. Here, we examined the activity-dependent regulation of two neocortical inhibitory circuits—parvalbumin-positive (Parv+) and somatostatin-positive (Som+)—using paired recordings of synaptically coupled neurons. Action potentials were blocked for 5 days in slice culture, and unitary synaptic connections among inhibitory/excitatory neuron pairs were examined. Chronic activity blockade caused similar and distinct changes between the two inhibitory circuits. First, increases in intrinsic membrane excitability and excitatory synaptic drive in both inhibitory subtypes were consistent with the homeostatic regulation of firing rate of these neurons. On the other hand, inhibitory synapses originating from these two subtypes were differentially regulated by activity blockade. Parv+ unitary inhibitory postsynaptic current (uIPSC) strength was decreased while Som+ uIPSC strength was unchanged. Using short-duration stimulus trains, short-term plasticity for both unitary excitatory postsynaptic current (uEPSCs) and uIPSCs was unchanged in Parv+ circuitry while distinctively altered in Som+ circuitry—uEPSCs became less facilitating and uIPSCs became more depressing. In the context of recurrent inhibition, these changes would result in a frequency-dependent shift in the relative influence of each circuit. The functional changes at both types of inhibitory connections appear to be mediated by increases in presynaptic release probability and decreases in synapse number. Interestingly, these opposing changes result in decreased Parv+-mediated uIPSCs but balance out to maintain normal Som+-mediated uIPSCs. In summary, these results reveal that inhibitory circuitry is not uniformly regulated by activity levels and may provide insight into the mechanisms of both normal and pathological neocortical plasticity.     

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4–5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5–100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca²⁺ control in the presynaptic terminal with a reduced Ca²⁺ influx and/or a higher Ca²⁺ buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.     

Changes in membrane properties of CA1 pyramidal neurons after transient forebrain ischemia in vivo

We have previously identified three distinct populations of CA1 pyramidal neurons after reperfusion based on differences in synaptic response, and named these late depolarizing postsynaptic potential neurons (enhanced synaptic transmission), non-late depolarizing postsynaptic potential and small excitatory postsynaptic neurons (depressed synaptic transmission). In the present study, spontaneous activity and membrane properties of CA1 neurons were examined up to 48 h following approximately 14 min ischemic depolarization using intracellular recording and staining techniques in vivo. In comparison with preischemic properties, the spontaneous firing rate and the spontaneous synaptic activity of CA1 neurons decreased significantly during reperfusion; spontaneous synaptic activity ceased completely 36-48 h after reperfusion, except for a low level of activity which persisted in non-late depolarizing postsynaptic potential neurons. Neuronal hyperactivity as indicated by increasing firing rate was never observed in the present study. The membrane input resistance and time constant decreased significantly in late depolarizing postsynaptic potential neurons at 24-48 h reperfusion. In contrast, similar changes were not observed in non-late depolarizing postsynaptic potential neurons. The rheobase, spike threshold and spike frequency adaptation in late depolarizing postsynaptic potential neurons increased progressively following reperfusion. Only a transient increase in rheobase and spike threshold was detected in non-late depolarizing postsynaptic potential neurons and spike frequency adaptation remained unchanged in these neurons. The amplitude of fast afterhyperpolarization increased in all neurons after reperfusion, with the smallest increment in non-late depolarizing postsynaptic potential neurons. Small excitatory postsynaptic potential neurons shared similar changes to those of late depolarizing postsynaptic potential neurons. These results suggest that the enhancement and depression of synaptic transmission following ischemia are probably due to changes in synaptic efficacy rather than changes in intrinsic membrane properties. The neurons with enhanced synaptic transmission following ischemia are probably the degenerating neurons, while the neurons with depressed synaptic transmission may survive the ischemic insult.     

Lesion-induced transient suppression of inhibitory function in rat neocortex in vitro

The structural and functional consequences of a local thermolesion were examined in rat neocortex with electrophysiological in vitro techniques and immunocytochemistry. Age-matched untreated and sham-operated animals served as controls and were analysed in the same way. The lesions consisted of a core of coagulated tissue 2–3 mm in diameter and reached ventrally into the deep cortical layers. After two days reactive astrocytes and after nine days a dense gliosis were observed in the immediate vicinity. Modifications in the intrinsic membrane characteristics and the synaptic network properties were investigated with intra- and extracellular recorcling techniques after survival times of one to eight days. Neurons recorded in the surrounding of lesions in neocortical slices revealed a significantly more depolarized resting membrane potential and a higher neuronal input resistance. In comparison to cells in control slices, maximal discharge rates to injection of depolarizing current pulses of neurons close to a focal lesion were not significantly altered and intrinsic burst firing was never observed. However, between postlesion days 1 and 5, neurons in the surroundings of lesions showed a transient increase in synaptic excitability. This hyperactivity was most clearly pronounced at a distance of 2–3 mm from the centre of the lesion (i.e. about 1–1.5 mm away from the lesion border) and characterized by long-duration field potential responses and multiphasic long-lasting excitatory postsynaptic potentials to orthodromic stimulation of the afferent input. This lesion-induced hyperexcitability was associated with a significant reduction in the peak conductance of the Cl⁻ -dependent fast inhibitory postsynaptic potential and the K⁺-dependent long-latency inhibitory postsynaptic potential, suggesting that the intracortical GABAergic system was functionally impaired. The decrease in synaptic inhibition was associated with prolonged N-methyl-d-aspartate receptor-mediated activity, which could be reversibly blocked byd-amino-phosphonovaleric acid. In addition, neurons recorded in the vicinity of the lesion responded to an orthodromic synaptic stimulus with a long-lasting burst.

The lesion-induced disturbance in the balance between the excitatory and inhibitory system may not only have profound influences on the mechanisms of intracortical information processing, but may also lead to the expression of epileptiform activity and long-term functional deficits.     

Presynaptic efficacy directs normalization of synaptic strength in layer 2/3 rat neocortex after paired activity

Paired neuronal activity is known to induce changes in synaptic strength that result in the synapse in question having different properties to unmodified synapses. Here we show that in layer 2/3 excitatory connections in young adult rat cortex paired activity acts to normalize the strength and quantal parameters of connections. Paired action potential firing produces long-term potentiation in only a third of connections, whereas a third remain with their amplitude unchanged and a third exhibit long-term depression. Furthermore, the direction of plasticity can be predicted by the initial strength of the connection: weak connections potentiate and strong connections depress. A quantal analysis reveals that changes in synaptic efficacy were predominantly presynaptic in locus and that the key determinant of the direction and magnitude of synaptic modification was the initial release probability (P(r)) of the synapse, which correlated inversely with change in P(r) after pairing. Furthermore, distal synapses also exhibited larger potentiations including postsynaptic increases in efficacy, whereas more proximal inputs did not. This may represent a means by which distal synapses preferentially increase their efficacy to achieve equal weighting at the soma. Paired activity thus acts to normalize synaptic strength, by both pre- and postsynaptic mechanisms.     

Equilibrium potential of GABA(A) current and implications for rebound burst firing in rat subthalamic neurons in vitro

Reciprocally connected glutamatergic subthalamic and GABAergic globus pallidus neurons have recently been proposed to act as a generator of low-frequency oscillatory activity in Parkinson's disease. To determine whether GABA(A) receptor-mediated synaptic potentials could theoretically generate rebound burst firing in subthalamic neurons, a feature that is central to the proposed oscillatory mechanism, we determined the equilibrium potential of GABA(A) current (E(GABA(A))) and the degree of hyperpolarization required for rebound firing using perforated-patch recording. In the majority of neurons that fired rebounds, E(GABA(A)) was equal to or more hyperpolarized than the hyperpolarization required for rebound burst firing. These data suggest that synchronous activity of pallidal inputs could underlie rhythmic bursting activity of subthalamic neurons in Parkinson's disease.     

Evidence for an ephaptic feedback in cortical synapses: postsynaptic hyperpolarization alters the number of response failures and quantal content.

The amplitude of excitatory postsynaptic potentials and currents increases with membrane potential hyperpolarization. This has been attributed to an increase in the driving force when the membrane potential deviates from the equilibrium potential of the respective ions. Here we report that in a subset of neocortical and hippocampal synapses, postsynaptic hyperpolarization affects traditional measures of transmitter release: the number of failures, coefficient of variation of response amplitudes, and quantal content, suggesting increased presynaptic release. The result is compatible with the hypothesis of Byzov on the existence of electrical (or "ephaptic") linking in purely chemical synapses. The linking, although negligible at neuromuscular junctions, could be functionally significant in influencing transmitter release at synapses with high resistance along the synaptic cleft. Our findings necessitate reconsideration of classical amplitude-voltage relations for such synapses. Thus, synaptic strength may be enhanced by hyperpolarization of the postsynaptic membrane potential. The positive ephaptic feedback could account for "all-or-none" excitatory postsynaptic potentials at some cortical synapses, large evoked and spontaneous multiquantal events and a high efficacy of large "perforated" synapses whose number increases following behavioural learning or the induction of long-term potentiation.     

Short-term plasticity of extrinsic excitatory inputs to neocortical layer 1

Previous studies have shown that different pyramidal cell inputs vary in the short-term plasticity expressed when they are subjected to repetition of use. Here, we describe short-term plasticity at synapses that mediate long-range input to neocortical layer 1 and compare it with that which normally occurs in the hippocampal Schaffer collateral pathway, which also involves projection by remote inputs onto apical dendrites. We isolated tangential inputs to layer 1 in neocortical slices, stimulated these with brief 40-Hz trains, and examined postsynaptic responses by recording extracellularly from layer 1 in somatosensory, prefrontal, and visual neocortex, and intracellularly from visually identified pyramidal cell somata in layer 2/3 in somatosensory and prefrontal neocortex. Train response amplitudes were characterized by calculating paired-pulse ratios, fifth-versus-first amplitude ratios (5th/1st ratios), and a center-of-mass index "M". As expected, the hippocampal train responses facilitated strongly. In contrast, layer-1 responses displayed strong synaptic depression in all regions examined. This depression was reflected in 5th/lst ratios and M scores, but not paired-pulse ratios because it did not consistently begin until the third responses in trains. It persisted unchanged in the presence of partially blocking levels of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but was converted to strong facilitation when slices were bathed in low-Ca++ media. Intracellularly, we observed response-train depression very similar to that recorded extracellularly. These findings show that long-range inputs to neocortical layer 1 display short-term plasticity markedly different from that which normally occurs at hippocampal Schaffer collateral synapses, but similar to that which has been described previously for excitatory inputs to pyramidal cells in deeper neocortical layers.     

Short-term modulation at synapses between neurons in laminae II-V of the rodent spinal dorsal horn

Unitary excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials (PSPs) were evoked between neurons in Rexed's laminae (L)II-V of spinal slices from young hamsters (7-24 days old) at 27°C using paired whole cell recordings. Laminar differences in synaptic efficacy were observed: excitatory connections were more secure than inhibitory connections in LII and inhibitory linkages in LII were less reliable than those in LIII-V. A majority of connections displayed paired-pulse facilitation or depression. Depression was observed for both EPSPs and IPSPs, but facilitation was seen almost exclusively for IPSPs. There were no frequency-dependent shifts between facilitation and depression. Synaptic depression was associated with an increased failure rate and decreased PSP half-width for a majority of connections. However, there were no consistent changes in failure rate or PSP time course at facilitating connections. IPSPs evoked at high-failure synapses had consistently smaller amplitude and showed greater facilitation than low-failure connections. Facilitation at inhibitory connections was positively correlated with synaptic jitter and associated with a decrease in latency. At many connections, the paired-pulse ratio varied from trial to trial and depended on the amplitude of the first PSP; dependence was greater for inhibitory synapses than excitatory synapses. Paired-pulse ratios for connections onto neurons with rapidly adapting, "phasic" discharge to depolarizing current injection were significantly greater than for connections onto neurons with tonic discharge properties. These results are evidence of diversity in synaptic transmission between dorsal horn neurons, the nature of which may depend on the types of linkage, laminar location, and intrinsic firing properties of postsynaptic cells.     

Microcircuits of excitatory and inhibitory neurons in layer 2/3 of mouse barrel cortex

Synaptic interactions between nearby excitatory and inhibitory neurons in the neocortex are thought to play fundamental roles in sensory processing. Here, we have combined optogenetic stimulation, whole cell recordings, and computational modeling to define key functional microcircuits within layer 2/3 of mouse primary somatosensory barrel cortex. In vitro optogenetic stimulation of excitatory layer 2/3 neurons expressing channelrhodopsin-2 evoked a rapid sequence of excitation followed by inhibition. Fast-spiking (FS) GABAergic neurons received large-amplitude, fast-rising depolarizing postsynaptic potentials, often driving action potentials. In contrast, the same optogenetic stimulus evoked small-amplitude, subthreshold postsynaptic potentials in excitatory and non-fast-spiking (NFS) GABAergic neurons. To understand the synaptic mechanisms underlying this network activity, we investigated unitary synaptic connectivity through multiple simultaneous whole cell recordings. FS GABAergic neurons received unitary excitatory postsynaptic potentials with higher probability, larger amplitudes, and faster kinetics compared with NFS GABAergic neurons and other excitatory neurons. Both FS and NFS GABAergic neurons evoked robust inhibition on postsynaptic layer 2/3 neurons. A simple computational model based on the experimentally determined electrophysiological properties of the different classes of layer 2/3 neurons and their unitary synaptic connectivity accounted for key aspects of the network activity evoked by optogenetic stimulation, including the strong recruitment of FS GABAergic neurons acting to suppress firing of excitatory neurons. We conclude that FS GABAergic neurons play an important role in neocortical microcircuit function through their strong local synaptic connectivity, which might contribute to driving sparse coding in excitatory layer 2/3 neurons of mouse barrel cortex in vivo.     

Extrinsic control of phasic supraoptic neurones in vitro: burst initiation and termination following brief changes in excitatory drive

Experiments were performed to investigate the responses of phasic supraoptic cells in hypothalamic slices to brief (2-3 sec) changes in excitatory drive, induced by varying the level of local glutamate stimulation. A brief increase in excitation during a silent period immediately triggered a prolonged burst of firing, similar in duration to the ongoing phasic bursts. A transient decrease in excitation during a phasic burst evoked a protracted quiescence, reminiscent of a typical silent period. These results support the idea that in phasic cells the effects of brief synaptic inputs may be amplified in time to produce a prolonged change in electrical activity and associated hormone release.     

Frequency-selective augmenting responses by short-term synaptic depression in cat neocortex

Thalamic stimulation at frequencies between 5 and 15 Hz elicits incremental or 'augmenting' cortical responses. Augmenting responses can also be evoked in cortical slices and isolated cortical slabs in vivo . Here we show that a realistic network model of cortical pyramidal cells and interneurones including short-term plasticity of inhibitory and excitatory synapses replicates the main features of augmenting responses as obtained in isolated slabs in vivo . Repetitive stimulation of synaptic inputs at frequencies around 10 Hz produced postsynaptic potentials that grew in size and carried an increasing number of action potentials resulting from the depression of inhibitory synaptic currents. Frequency selectivity was obtained through the relatively weak depression of inhibitory synapses at low frequencies, and strong depression of excitatory synapses together with activation of a calcium-activated potassium current at high frequencies. This network resonance is a consequence of short-term synaptic plasticity in a network of neurones without intrinsic resonances. These results suggest that short-term plasticity of cortical synapses could shape the dynamics of synchronized oscillations in the brain.     

Dopamine modulation of phasing of activity in a rhythmic motor network: contribution of synaptic and intrinsic modulatory actions

The phasing of neuronal activity in a rhythmic motor network is determined by a neuron's intrinsic firing properties and synaptic inputs; these could vary in their relative importance under different modulatory conditions. In the lobster pyloric network, the firing of eight follower pyloric (PY) neurons is shaped by their intrinsic rebound after pacemaker inhibition and by synaptic input from the lateral pyloric (LP) neuron, which inhibits all PY neurons and is electrically coupled to a subset of them. Under control conditions, LP inhibition is weak and has little influence on PY firing. We examined modulation that could theoretically enhance the LP's synaptic contribution to PY firing. We measured the effects of dopamine (DA) on LP-->PY synapses, driving the LP neuron with trains of realistic waveforms constructed from prerecorded control and DA LP oscillations, which differed in shape and duration. Under control conditions, chemical inhibition underwent severe depression and disappeared; in the mixed synapses, electrical coupling dominated. Switching between control and DA LP waveforms (with or without DA present) caused only subtle changes in synaptic transmission. DA markedly enhanced synaptic inhibition, reduced synaptic depression and weakened electrical coupling, reversing the sign of the mixed synapses. Despite this, removal of the LP from the intact network still had only weak effects on PY firing. DA also enhances PY intrinsic rebound properties, which still control the onset of PY firing. Thus in a rhythmic network, the functional importance of synaptic modulation can only be understood in the context of parallel modulation of intrinsic properties.