Increased Ca2+ influx through Na+/Ca2+ exchanger during long-term facilitation at crayfish neuromuscular junctions

Intense motor neuron activity induces a long-term facilitation (LTF) of synaptic transmission at crayfish neuromuscular junctions (NMJs) that is accompanied by an increase in the accumulation of presynaptic Ca²⁺ ions during a test train of action potentials. It is natural to assume that the increased Ca²⁺ influx during action potentials is directly responsible for the increased transmitter release in LTF, especially as the magnitudes of LTF and increased Ca²⁺ influx are positively correlated. However, our results indicate that the elevated Ca²⁺ entry occurs through the reverse mode operation of presynaptic Na⁺/Ca²⁺ exchangers that are activated by an LTF-inducing tetanus. Inhibition of Na⁺/Ca²⁺ exchange blocks this additional Ca²⁺ influx without affecting LTF, showing that LTF is not a consequence of the regulation of these transporters and is not directly related to the increase in [Ca²⁺]_i reached during a train of action potentials. Their correlation is probably due to both being induced independently by the strong [Ca²⁺]_i elevation accompanying LTF-inducing stimuli. Our results reveal a new form of regulation of neuronal Na⁺/Ca²⁺ exchange that does not directly alter the strength of synaptic transmission.     

Action potential fidelity during normal and epileptiform activity in paired soma–axon recordings from rat hippocampus

Although action potential initiation and propagation are fundamental to nervous system function, there are few direct electrophysiological observations of propagating action potentials in small unmyelinated fibres, such as the axons within mammalian hippocampus. To circumvent limitations of previous studies that relied on extracellular stimulation, we performed dual recordings: whole-cell recordings from hippocampal CA3 pyramidal cell somas and extracellular recordings from their axons, up to 800 μm away. During brief spike trains under normal conditions, axonal spikes were more resistant to amplitude reduction than somatic spikes. Axonal amplitude depression was greatest at the axon initial segment < 150 μm from the soma, and initiation occurred ∼75 μm from the soma. Although prior studies, which failed to verify spike initiation, suggested substantial axonal depression during seizure-associated extracellular K⁺ ([K⁺]_o) rises, we found that 8 m m [K⁺]_o caused relatively small decreases in axonal spike amplitude during brief spike trains. However, during sustained, epileptiform spiking induced in 8 m m [K⁺]_o, axonal waveforms decreased significantly in peak amplitude. During epileptiform spiking, bursts of two or more action potentials > 20 Hz failed to propagate in most cases. In normal [K⁺]_o at 25 and 32°C, spiking superimposed on sustained somatic depolarization, but not spiking alone, produced similar axonal changes as the epileptiform activity. These results highlight the likely importance of steady-state inactivation of axonal channels in maintaining action potential fidelity. Such changes in axonal propagation properties could encode information and/or serve as an endogenous brake on seizure propagation.     

Multiple mechanisms govern the dynamics of depression at neocortical synapses of young rats

Synaptic transmission between pairs of excitatory neurones in layers V ( N = 38) or IV ( N = 6) of somatosensory cortex was examined in a parasagittal slice preparation obtained from young Wistar rats (14–18 days old). A combined experimental and theoretical approach reveals two characteristics of short-term synaptic depression. Firstly, as well as a release-dependent depression, there is a release-independent component that is evident in smaller postsynaptic responses even following failure to release transmitter. Secondly, recovery from depression is activity dependent and is faster at higher input frequencies. Frequency-dependent recovery is a Ca²⁺-dependent process and does not reflect an underlying augmentation. Frequency-dependent recovery and release-independent depression are correlated, such that at those connections with a large amount of release-independent depression, recovery from depression is faster. In addition, both are more pronounced in experiments performed at physiological temperatures. Simulations demonstrate that these homeostatic properties allow the transfer of rate information at all frequencies, essentially linearizing synaptic responses at high input frequencies.     

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.     

Subthreshold inactivation of voltage-gated K+ channels modulates action potentials in neocortical bitufted interneurones from rats

Voltage-gated K⁺ channels perform many functions in integration of synaptic input and action potential (AP) generation. In this study we show that in bitufted interneurones from layer 2/3 of the somatosensory cortex, the height and width of APs recorded at the soma are sensitive to changes in the resting membrane potential, suggesting subthreshold activity of voltage-gated conductances. Attributes of K⁺ currents examined in nucleated patches revealed a fast subthreshold-inactivating K⁺ conductance (K_f) and a slow suprathreshold-inactivating K⁺ conductance (K_s). Simulations of these K⁺ conductances, incorporated into a Hodgkin–Huxley-type model, suggested that during a single AP or during low frequency trains of APs, subthreshold inactivation of K_f was the primary modulator of AP shape, whereas during trains of APs the shape was governed to a larger degree by K_s resulting in the generation of smaller and broader APs. Utilizing the facilitating function of unitary pyramidal-to-bitufted cell synaptic transmission, single back-propagating APs were initiated in a bitufted interneurone by repeated stimulation of a presynaptic pyramidal cell. Ca²⁺ imaging and dendritic whole-cell recordings revealed that modulation of APs, which also affect the shape of back-propagating APs, resulted in a change in dendritic Ca²⁺ influx. Compartmental simulation of the back-propagating AP suggested a mechanism for the modulation of the back-propagating AP height and width by subthreshold activation of K_f. We speculate that this signal may modulate retrograde GABA release and consequently depression of synaptic efficacy of excitatory input from neighbouring pyramidal neurones.     

Activity-dependent excitability changes in hippocampal CA3 cell Schaffer axons

The membrane potential changes following action potentials in thin unmyelinated cortical axons with en passant boutons may be important for synaptic release and conduction abilities of such axons. In the lack of intra-axonal recording techniques we have used extracellular excitability testing as an indirect measure of the after-potentials. We recorded from individual CA3 soma in hippocampal slices and activated the axon with a range of stimulus intensities. When conditioning and test stimuli were given to the same site the excitability changes were partly masked by local effects of the stimulating electrode at intervals < 5 ms. Therefore, we elicited the conditioning action potential from one axonal branch and tested the excitability of another branch. We found that a single action potential reduced the axonal excitability for 15 ms followed by an increased excitability for ∼200 ms at 24°C. Using field recordings of axonal action potentials we show that raising the temperature to 34°C reduced the magnitude and duration of the initial depression. However, the duration of the increased excitability was very similar (time constant 135 ± 20 ms) at 24 and 34°C, and with 2.0 and 0.5 m m Ca²⁺ in the bath. At stimulus rates > 1 Hz, a condition that activates a hyperpolarization-activated current ( I _h) in these axons, the decay was faster than at lower stimulation rates. This effect was reduced by the I _h blocker ZD7288. These data suggest that the decay time course of the action potential-induced hyperexcitability is determined by the membrane time constant.     

Priming of intracellular calcium stores in rat CA1 pyramidal neurons

Repetitive synaptic stimulation evokes large amplitude Ca²⁺ release waves from internal stores in many kinds of pyramidal neurons. The waves result from mGluR mobilization of IP₃ leading to Ca²⁺-induced Ca²⁺ release. In most experiments in slices, regenerative Ca²⁺ release can be evoked for only a few trials. We examined the conditions required for consistent release from the internal stores in hippocampal CA1 pyramidal neurons. We found that priming with action potentials evoked at 0.5–1 Hz for intervals as short as 15 s were sufficient to fill the stores, while sustained subthreshold depolarization or subthreshold synaptic stimulation lasting from 15 s to 2 min was less effective. A single episode of priming was effective for about 2–3 min. Ca²⁺ waves could also be evoked by uncaging IP₃ with a UV flash in the dendrites. Priming was necessary to evoke these waves repetitively; 7–10 spikes in 15 s were again effective for this protocol, indicating that priming acts to refill the stores and not at a site upstream to the production of IP₃. These results suggest that normal spiking activity of pyramidal neurons in vivo should be sufficient to maintain their internal stores in a primed state ready to release Ca²⁺ in response to an appropriate physiological stimulus. This may be a novel form of synaptic plasticity where a cell's capacity to release Ca²⁺ is modulated by its average firing frequency.     

Spontaneous Voltage Oscillations in Striatal Projection Neurons in a Rat Corticostriatal Slice

In a rat corticostriatal slice, brief, suprathreshold, repetitive cortical stimulation evoked long-lasting plateau potentials in neostriatal neurons. Plateau potentials were often followed by spontaneous voltage transitions between two preferred membrane potentials. While the induction of plateau potentials was disrupted by non-NMDA and NMDA glutamate receptor antagonists, the maintenance of spontaneous voltage transitions was only blocked by NMDA receptor and L-type Ca²⁺ channel antagonists. The frequency and duration of depolarized events, resembling up-states described in vivo , were increased by NMDA and L-type Ca²⁺ channel agonists as well as by GABA_A receptor and K⁺ channel antagonists. NMDA created a region of negative slope conductance and a positive slope crossing indicative of membrane bistability in the current-voltage relationship. NMDA-induced bistability was partially blocked by L-type Ca²⁺ channel antagonists. Although evoked by synaptic stimulation, plateau potentials and voltage oscillations could not be evoked by somatic current injection – suggesting a dendritic origin. These data show that NMDA and L-type Ca²⁺ conductances of spiny neurons are capable of rendering them bistable. This may help to support prolonged depolarizations and voltage oscillations under certain conditions.     

Electrophysiological properties of two axonal sodium channels, Nav1.2 and Nav1.6, expressed in mouse spinal sensory neurones

Sodium channels Na_v1.2 and Na_v1.6 are both normally expressed along premyelinated and myelinated axons at different stages of maturation and are also expressed in a subset of demyelinated axons, where coexpression of Na_v1.6 together with the Na⁺/Ca²⁺ exchanger is associated with axonal injury. It has been difficult to distinguish the currents produced by Na_v1.2 and Na_v1.6 in native neurones, and previous studies have not compared these channels within neuronal expression systems. In this study, we have characterized and directly compared Na_v1.2 and Na_v1.6 in a mammalian neuronal cell background and demonstrate differences in their properties that may affect neuronal behaviour. The Na_v1.2 channel displays more depolarized activation and availability properties that may permit conduction of action potentials, even with depolarization. However, Na_v1.2 channels show a greater accumulation of inactivation at higher frequencies of stimulation (20–100 Hz) than Na_v1.6 and thus are likely to generate lower frequencies of firing. Na_v1.6 channels produce a larger persistent current that may play a role in triggering reverse Na⁺/Ca²⁺ exchange, which can injure demyelinated axons where Na_v1.6 and the Na⁺/Ca²⁺ exchanger are colocalized, while selective expression of Na_v1.2 may support action potential electrogenesis, at least at lower frequencies, while producing a smaller persistent current.     

Nerve-evoked purinergic signalling suppresses action potentials, Ca2+ flashes and contractility evoked by muscarinic receptor activation in mouse urinary bladder smooth muscle

Contraction of urinary bladder smooth muscle (UBSM) is caused by the release of ATP and ACh from parasympathetic nerves. Although both purinergic and muscarinic pathways are important to contraction, their relative contributions and signalling mechanisms are not well understood. Here, the contributions of each pathway to urinary bladder contraction and the underlying electrical and Ca²⁺ signalling events were examined in UBSM strips from wild type mice and mice deficient in P2X1 receptors (P2X1^−/−) before and after pharmacological inhibition of purinergic and muscarinic receptors. Electrical field stimulation was used to excite parasympathetic nerves to increase action potentials, Ca²⁺ flash frequency, and force. Loss of P2X1 function not only eliminated action potentials and Ca²⁺ flashes during stimulation, but it also led to a significant increase in Ca²⁺ flashes following stimulation and a corresponding increase in the force transient. Block of muscarinic receptors did not affect action potentials or Ca²⁺ flashes during stimulation, but prevented them following stimulation. These findings indicate that nerve excitation leads to rapid engagement of smooth muscle P2X1 receptors to increase action potentials (Ca²⁺ flashes) during stimulation, and a delayed increase in excitability in response to muscarinic receptor activation. Together, purinergic and muscarinic stimulation shape the time course of force transients. Furthermore, this study reveals a novel inhibitory effect of P2X1 receptor activation on subsequent increases in muscarinic-driven excitability and force generation.     

Kinetics of Mg2+ unblock of NMDA receptors: implications for spike-timing dependent synaptic plasticity

The time course of Mg²⁺ block and unblock of NMDA receptors (NMDARs) determines the extent they are activated by depolarization. Here, we directly measure the rate of NMDAR channel opening in response to depolarizations at different times after brief (1 ms) and sustained (4.6 s) applications of glutamate to nucleated patches from neocortical pyramidal neurons. The kinetics of Mg²⁺ unblock were found to be non-instantaneous and complex, consisting of a prominent fast component (time constant ∼100 μs) and slower components (time constants 4 and ∼300 ms), the relative amplitudes of which depended on the timing of the depolarizing pulse. Fitting a kinetic model to these data indicated that Mg²⁺ not only blocks the NMDAR channel, but reduces both the open probability and affinity for glutamate, while enhancing desensitization. These effects slow the rate of NMDAR channel opening in response to depolarization in a time-dependent manner such that the slower components of Mg²⁺ unblock are enhanced during depolarizations at later times after glutamate application. One physiological consequence of this is that brief depolarizations occurring earlier in time after glutamate application are better able to open NMDAR channels. This finding has important implications for spike-timing-dependent synaptic plasticity (STDP), where the precise (millisecond) timing of action potentials relative to synaptic inputs determines the magnitude and sign of changes in synaptic strength. Indeed, we find that STDP timing curves of NMDAR channel activation elicited by realistic dendritic action potential waveforms are narrower than expected assuming instantaneous Mg²⁺ unblock, indicating that slow Mg²⁺ unblock of NMDAR channels makes the STDP timing window more precise.     

Sodium and calcium currents shape action potentials in immature mouse inner hair cells

Before the onset of hearing at postnatal day 12, mouse inner hair cells (IHCs) produce spontaneous and evoked action potentials. These spikes are likely to induce neurotransmitter release onto auditory nerve fibres. Since immature IHCs express both α1D (Ca_v1.3) Ca²⁺ and Na⁺ currents that activate near the resting potential, we examined whether these two conductances are involved in shaping the action potentials. Both had extremely rapid activation kinetics, followed by fast and complete voltage-dependent inactivation for the Na⁺ current, and slower, partially Ca²⁺-dependent inactivation for the Ca²⁺ current. Only the Ca²⁺ current is necessary for spontaneous and induced action potentials, and 29 % of cells lacked a Na⁺ current. The Na⁺ current does, however, shorten the time to reach the action-potential threshold, whereas the Ca²⁺ current is mainly involved, together with the K⁺ currents, in determining the speed and size of the spikes. Both currents increased in size up to the end of the first postnatal week. After this, the Ca²⁺ current reduced to about 30 % of its maximum size and persisted in mature IHCs. The Na⁺ current was downregulated around the onset of hearing, when the spiking is also known to disappear. Although the Na⁺ current was observed as early as embryonic day 16.5, its role in action-potential generation was only evident from just after birth, when the resting membrane potential became sufficiently negative to remove a sizeable fraction of the inactivation (half inactivation was at −71 mV). The size of both currents was positively correlated with the developmental change in action-potential frequency.     

Stimulation-induced changes in NADH fluorescence and mitochondrial membrane potential in lizard motor nerve terminals

To investigate mitochondrial responses to repetitive stimulation, we measured changes in NADH fluorescence and mitochondrial membrane potential (Ψ_m) produced by trains of action potentials (50 Hz for 10–50 s) delivered to motor nerve terminals innervating external intercostal muscles. Stimulation produced a rapid decrease in NADH fluorescence and partial depolarization of Ψ_m. These changes were blocked when Ca²⁺ was removed from the bath or when N-type Ca²⁺ channels were inhibited with ω-conotoxin GVIA, but were not blocked when bath Ca²⁺ was replaced by Sr²⁺, or when vesicular release was inhibited with botulinum toxin A. When stimulation stopped, NADH fluorescence and Ψ_m returned to baseline values much faster than mitochondrial [Ca²⁺]. In contrast to findings in other tissues, there was usually little or no poststimulation overshoot of NADH fluorescence. These findings suggest that the major change in motor terminal mitochondrial function brought about by repetitive stimulation is a rapid acceleration of electron transport chain (ETC) activity due to the Ψ_m depolarization produced by mitochondrial Ca²⁺ (or Sr²⁺) influx. After partial inhibition of complex I of the ETC with amytal, stimulation produced greater Ψ_m depolarization and a greater elevation of cytosolic [Ca²⁺]. These results suggest that the ability to accelerate ETC activity is important for normal mitochondrial sequestration of stimulation-induced Ca²⁺ loads.     

Multiple modes of GABAergic inhibition of rat cerebellar granule cells

Cerebellar granule cells are inhibited phasically by GABA released synaptically from Golgi cells, but are inhibited more powerfully by tonic activity of high affinity α₆ subunit-containing GABA_A receptors. During development the tonic activity is generated by the accumulation of GABA released by action potentials, but in the adult the tonic activity is independent of action potentials. Here we show that in adult rats the tonic activation of GABA_A receptors is produced by non-vesicular transmitter release and is reduced by the activity of GAT-1 and GAT-3 GABA transporters, demonstrating that alterations of GABA uptake will modulate information flow through granule cells. Acetylcholine (ACh) evokes a large Ca²⁺-dependent but action potential-independent release of GABA, which activates α₆ subunit-containing GABA_A receptors. These data show that three separate modes of transmitter release can activate GABA_A receptors in adult cerebellar granule cells: action potential-evoked exocytotic GABA release, non-vesicular release, and ACh-evoked Ca²⁺-dependent release independent of action potentials. The relative magnitudes of the inhibitory charge transfers generated by action potential-evoked release (during high frequency stimulation of the mossy fibres), tonic inhibition and superfused ACh are 1:3:12, indicating that tonic and ACh-mediated inhibition may play a major role in regulating granule cell firing.     

Presynaptic short-term depression is maintained during regulation of transmitter release at a GABAergic synapse in rat hippocampus

To examine possible interactions between fast depression and modulation of inhibitory synaptic transmission in the hippocampus, we recorded from pairs of synaptically connected basket cells (BCs) and granule cells (GCs) in the dentate gyrus of rat brain slices at 34 °C. Multiple-pulse depression (MPD) was examined in trains of 5 or 10 inhibitory postsynaptic currents (IPSCs) evoked at frequencies of 10–00 Hz under several conditions that inhibit transmitter release: block of voltage-dependent Ca²⁺ channels by Cd²⁺ (10 μ m ), activation of γ-amino-butyric acid type B receptors (GABA_BRs) by baclofen (10 μ m ) and activation of muscarinic acetylcholine receptors (mAchRs) by carbachol (2 μ m ). All manipulations led to a substantial inhibition of synaptic transmission, reducing the amplitude of the first IPSC in the train (IPSC₁) by 72 %, 61 % and 29 %, respectively. However, MPD was largely preserved under these conditions (0.34 in control versus 0.31, 0.50 and 0.47 in the respective conditions at 50 Hz). Similarly, a theta burst stimulation (TBS) protocol reduced IPSC₁ by 54 %, but left MPD unchanged (0.40 in control and 0.39 during TBS). Analysis of both fractions of transmission failures and coefficients of variation (CV) of IPSC peak amplitudes suggested that MPD had a presynaptic expression site, independent of release probability. In conclusion, different types of presynaptic modulation of inhibitory synaptic transmission converge on a reduction of synaptic strength, while short-term dynamics are largely unchanged.     

Action potential initiation in the peripheral terminals of cold-sensitive neurones innervating the guinea-pig cornea

The site at which action potentials initiate within the terminal region of unmyelinated sensory axons has not been resolved. Combining recordings of nerve terminal impulses (NTIs) and collision analysis, the site of action potential initiation in guinea-pig corneal cold receptors was determined. For most receptors (77%), initiation mapped to a point in the time domain that was closer to the nerve terminal than to the site of electrical stimulation at the back of the eye. Guinea-pig corneal cold receptors are Aδ-neurones that lose their myelin sheath at the point where they enter the cornea, and therefore their axons conduct more slowly within the cornea. Allowing for this inhomogeneity in conduction speed, the resulting spatial estimates of action potential initiation sites correlated with changes in NTI shape predicted by simulation of action potentials initiating within a nerve terminal. In some receptors, more than one NTI shape was observed. Simulations of NTI shape suggest that the origin of differing NTI shapes result from action potentials initiating at different, spatially discrete, locations within the nerve terminal. Importantly, the relative incidence of NTI shapes resulting from action potential initiation close to the nerve termination increased during warming when nerve activity decreased, indicating that the favoured site of action potential initiation shifts toward the nerve terminal when it hyperpolarizes. This finding can be explained by a hyperpolarization-induced relief of Na⁺ channel inactivation in the nerve terminal. The results provide direct evidence that the molecular entities responsible for stimulus transduction and action potential initiation reside in parallel with one another in the unmyelinated nerve terminals of cold receptors.     

Inspiratory bursts in the preBötzinger complex depend on a calcium-activated non-specific cation current linked to glutamate receptors in neonatal mice

Inspiratory neurons of the preBötzinger complex (preBötC) form local excitatory networks and display 10–30 mV transient depolarizations, dubbed inspiratory drive potentials , with superimposed spiking. AMPA receptors are critical for rhythmogenesis under normal conditions in vitro but whether other postsynaptic mechanisms contribute to drive potential generation remains unknown. We examined synaptic and intrinsic membrane properties that generate inspiratory drive potentials in preBötC neurons using neonatal mouse medullary slice preparations that generate respiratory rhythm. We found that NMDA receptors, group I metabotropic glutamate receptors (mGluRs), but not group II mGluRs, contributed to inspiratory drive potentials. Subtype 1 of the group I mGluR family (mGluR1) probably regulates a K⁺ channel, whereas mGluR5 operates via an inositol 1,4,5-trisphosphate (IP₃) receptor-dependent mechanism to augment drive potential generation. We tested for and verified the presence of a Ca²⁺-activated non-specific cation current ( I _CAN) in preBötC neurons. We also found that high concentrations of intracellular BAPTA, a high-affinity Ca²⁺ chelator, and the I _CAN antagonist flufenamic acid (FFA) decreased the magnitude of drive potentials. We conclude that I _CAN underlies robust inspiratory drive potentials in preBötC neurons, and is only fully evoked by ionotropic and metabotropic glutamatergic synaptic inputs, i.e. by network activity.     

Contribution of a Calcium-Activated Non-Specific Conductance to NMDA Receptor-Mediated Synaptic Potentials in Granule Cells of the Frog Olfactory Bulb

We studied granule cells (GCs) in the intact frog olfactory bulb (OB) by combining whole-cell recordings and functional two-photon Ca²⁺ imaging in an in vitro nose-brain preparation. GCs are local interneurones that shape OB output via distributed dendrodendritic inhibition of OB projection neurones, the mitral-tufted cells (MTCs). In contrast to MTCs, GCs exhibited a Ca²⁺-activated non-specific cation conductance ( I _CAN) that could be evoked through strong synaptic stimulation or suprathreshold current injection. Photolysis of the caged Ca²⁺ chelator o -nitrophenol-EGTA resulted in activation of an inward current with a reversal potential within the range -20 to +10 mV. I _CAN in GCs was suppressed by the intracellular Ca²⁺ chelator BAPTA (0.5–5.0 mM), but not by EGTA (up to 5 mM). The current persisted in whole-cell recordings for up to 1.5 h post-breakthrough, was observed during perforated-patch recordings and was independent of ionotropic glutamate and GABA_A receptor activity. In current-clamp mode, GC responses to synaptic stimulation consisted of an initial AMPA-mediated conductance followed by a late-phase APV-sensitive plateau (100–500 ms). BAPTA-mediated suppression of I _CAN resulted in a selective reduction of the late component of the evoked synaptic potential, consistent with a positive feedback relationship between NMDA receptor (NMDAR) current and I _CAN. I _CAN requires Ca²⁺ influx either through voltage-gated Ca²⁺ channels or possibly NMDARs, both of which have a high threshold for activation in GCs, predicting a functional role for this current in the selective enhancement of strong synaptic inputs to GCs.     

Functional connectivity in layer IV local excitatory circuits of rat somatosensory cortex

There are two types of excitatory neurons within layer IV of rat somatosensory cortex: star pyramidal (SP) and spiny stellate cells (SS). We examined the intrinsic properties and connectivity between these neurons to determine differences in function. Eighty-four whole cell recordings of pairs of neurons were examined in slices of rat barrel cortex at 36 +/- 1 degrees C. Only minimal differences in intrinsic properties were found; however, differences in synaptic strength could clearly be shown. Connections between homonymous pairs (SS-SS or SP-SP) had a higher efficacy than heteronymous connections. This difference was mainly a result of quantal content. In 42 pairs, synaptic dynamics were examined. Sequences of action potentials (3-20 Hz) in the presynaptic neuron consistently caused synaptic depression (E2/E1=0.53+/-0.18). The dominant component of depression was release-independent; this depression occurred even when preceding action potentials had failed to cause a response. The release-dependence of depression was target specific; in addition, release-independence was greater for postsynaptic SPs. In a subset of connections formed only between SP and any other cell type (43%), synaptic efficacy was dependent on the presynaptic membrane potential (Vm); at -55 mV, the connections were almost silent, whereas at -85 mV, transmission was very reliable. We suggest that, within layer IV, there is stronger efficacy between homonymous than between heteronymous excitatory connections. Under dynamic conditions, the functional connectivity is shaped by synaptic efficacy at individual connections, by Vm, and by the specificity in the types of synaptic depression.