Rony Azouz

Ionic basis of spike after‐depolarization and burst generation in adult rat hippocampal CA1 pyramidal cells.

1. Intracellular recordings in adult rat hippocampal slices were used to identify the ionic conductances underlying active spike after‐depolarization (ADP) and intrinsic burst firing in the somata of CA1 pyramidal cells (PCs). To test the ‘Ca2+ hypothesis’, Ca2+ currents were suppressed by replacing the Ca2+ in the saline with either Mn2+ or Mg2+. Alternatively, the inorganic Ca2+ channel blockers Cd2+ (0.5 mM) or Ni2+ (2 mM) were added to the saline. To test the ‘Na+ hypothesis’, Na+ currents were blocked with tetrodotoxin (TTX; 0.5 microM). 2. The suppression of Ca2+ currents blocked the fast after‐hyperpolarization (AHP) generated by the fast Ca(2+)‐gated K+ current Ic, while enhancing the amplitude and duration of active spike ADPS. 3. Evoked and spontaneous burst firing was preserved undiminished following Ca2+ current suppression, while the propensity to fire bursts increased in many cases. The postburst medium AHP (generated primarily by the muscarine‐sensitive voltage‐gated K+ current, IM) was not affected by this treatment, which blocked the slow AHP (generated by the slow Ca(2+)‐gated K+ current, IAHP). 4. TTX strongly suppressed active ADPs and intrinsic bursts before substantially reducing the threshold, rate of rise and amplitude of solitary spikes. 5. In Ca(2+)‐free saline, caesium‐filled PCs generated large, plateau ADPs following an initial burst of fast spikes. Application of TTX suppressed these ADPs before solitary fast spikes appeared to be reduced. 6. Injection of brief, just subthreshold depolarizing current pulses into bursters evoked slow depolarizing potentials lasting up to 50 ms. These persisted after suppression of Ca2+ currents and were entirely blocked by TTX. 7. We conclude that active spike ADPs and intrinsic bursts in the somata of adult CA1 PCs are generated by a low voltage‐gated, persistent Na+ current. Burst termination is mediated by voltage‐gated K+ currents activated during the burst (most likely IM), rather than by the Ca(2+)‐gated K+ currents Ic and IAHP. The latter currents downregulate the innate tendency of CA1 PCs to burst (Ic) and limit the rate of spontaneous burst firing (IAHP).

SPIKE AFTER-DEPOLARIZATION AND BURST GENERATION IN ADULT RAT HIPPOCAMPAL CA1 PYRAMIDAL CELLS

1. Intracellular recordings in adult rat hippocampal slices were used to investigate the properties and origins of intrinsically generated bursts in the somata of CA1 pyramidal cells (PCs). The CA1 PCs were classified as either non‐bursters or bursters according to the firing patterns evoked by intrasomatically applied long ( > or = 100 ms) depolarizing current pulses. Non‐bursters generated stimulus‐graded trains of independent action potentials, whereas bursters generated clusters of three or more closely spaced spikes riding on a distinct depolarizing envelope. 2. In all PCs fast spike repolarization was incomplete and ended at a potential approximately 10 mV more positive than resting potential. Solitary spikes were followed by a distinct after‐depolarizing potential (ADP) lasting 20‐40 ms. The ADP in most non‐bursters declined monotonically to baseline (‘passive’ ADP), whereas in most bursters it remained steady or even re‐depolarized before declining to baseline (‘active’ ADP). 3. Active, but not passive, ADPs were associated with an apparent increase in input conductance. They were maximal in amplitude when the spike was evoked from resting potential and were reduced by mild depolarization or hyperpolarization (+/‐ 2 mV). 4. Evoked and spontaneous burst firing was sensitive to small changes in membrane potential. In most cases maximal bursts were generated at resting potential and were curtailed by small depolarizations or hyperpolarizations (+/‐ 5 mV). 5. Bursts comprising clusters of spikelets (‘d‐spikes’) were observed in 12% of the bursters. Some of the d‐spikes attained threshold for triggering full somatic spikes. Gradually hyperpolarizing these neurones blocked somatic spikes before blocking d‐spikes, suggesting that the latter are generated at more remote sites. 6. The data suggest that active ADPs and intrinsic bursts in the somata of adult CA1 PCs are generated by a slow, voltage‐gated inward current. Bursts arise in neurones in which this current is sufficiently large to generate suprathreshold ADPs, and thereby initiate a regenerative process of spike recruitment and slow depolarization.

Modulation of endogenous firing patterns by osmolarity in rat hippocampal neurones

1 Intracellular recordings in adult rat hippocampal slices were used to investigate the modulation of endogenous neuronal firing patterns by moderate changes (± 13%) in the extracellular osmotic pressure (πo). The responses of CA1 pyramidal cells to graded depolarizing current pulses were used to differentiate between regular and burst‐firing patterns and to characterize the stimulus requirements for evoking endogenous burst discharge. 2 Decreasing or increasing πo had no significant effects on resting membrane potential and input resistance, spike threshold and amplitude, and the amplitudes of the fast, medium and slow spike after‐hyperpolarizations (AHPs). The apparent membrane time constant (τm) increased in low πo and decreased in high πo. 3 Reducing πo converted non‐bursting neurones (non‐bursters) to bursting neurones (bursters) and decreased the stimulus requirements for evoking burst firing in native bursters. Increasing πo suppressed endogenous burst firing. 4 Lowering πo increased the size of the ‘active’ (i.e. re‐depolarizing) component of the spike after‐depolarization (ADP). Conversely, increasing πo suppressed the active ADP component. 5 The sensitivity of spike ADPs and firing patterns of pyramidal cells to the changes in πo persisted also in Ca2+‐free saline, indicating that the osmotic effects are not imparted by modulation of Ca2+ and/or Ca2+‐activated K+ currents. 6 Blocking most K+ currents with Ca2+‐free, TEA‐containing saline induced large and prolonged (up to 1 s), TTX‐sensitive plateau potentials following the primary fast spikes. These potentials were augmented by low πo and abated by high πo. 7 When injected with subthreshold depolarizing current pulses in Ca2+‐free saline, pyramidal cells displayed a distinct TTX‐sensitive inward rectification. This rectification was augmented by low πo and reduced by high πo. 8 The various effects of low‐πo and high‐πo saline solutions were reversible upon washing with normosmotic saline. 9 We conclude that πo is a critical determinant of the endogenous firing patterns of CA1 pyramidal cells. The data suggest that the osmotic effects are most likely to be mediated by changes in the persistent Na+ current, which underlies the active spike ADP and the burst potential in CA1 pyramidal neurones. The possible contribution of these effects to changes in brain excitability in various abnormal osmotic states is discussed.

Dynamic Translation of Surface Coarseness Into Whisker Vibrations

Rodents in their natural environment use their whiskers to distinguish between surfaces having subtly different textures and shapes. They do so by actively sweeping their whiskers across surfaces in a rhythmic motion. To determine how textures are transformed into vibration signals in whiskers and how these vibrations are expressed in neuronal discharges, we induced active whisking in anesthetized rats, monitored the movement of whiskers across surfaces, and concurrently recorded from trigeminal ganglion (TG) neurons. We show that tactile information is transmitted through high-frequency micromotions superimposed on whisking macro motions. Consistent with this, we find that in most TG neurons, spike activity, and high-frequency micromotions are closely correlated. To determine whether these vibration signals can support texture discrimination, we examined their dependence on surface roughness and found that both vibration signals carry information about surface coarseness. Despite a large variability in this translation process, different textures are translated into distinct vibrations profiles. These profiles depend on whiskers properties, on radial distance to the surface, and on whisking frequency. Using the characteristics of these signals, we employ linear discriminant analysis and found that all whiskers were able to discriminate between different textures. While deteriorating with radial distance, this classification did not depend on whisking frequency. Finally, increasing the number of whisks and integrating tactile information from multiple whiskers improved texture discrimination. These results indicate that surface roughness is translated into distinct whisker vibration signals that result in neuronal discharges. However, due to the dynamic nature of this translation process, we propose that texture discrimination may require the integration of signals from multiple spatial and temporal sensory channels to disambiguate surface roughness.

Muscarinic Modulation of Intrinsic Burst Firing in Rat Hippocampal Neurons

Intracellular recordings in rat hippocampal slices were used to examine how exogenous and endogenous cholinergic agonists modulate the firing pattern of intrinsically burst‐firing pyramidal cells. About 24% of CA1 pyramidal cells generated all‐or‐none, high‐frequency bursts of fast action potentials in response to intracellular injection of long positive current pulses. Application of carbachol (5 μM) converted burst firing in these neurons into regular trains of independent spikes. Acetylcholine (5 μM) exerted a similar effect, provided acetylcholine esterase activity was blocked with neostigmine (2 μM). Atropine (1 μM) reversed this cholinergic effect, indicating its mediation by muscarinic receptors. Cholinergic agonists also caused mild neuronal depolarization but the block of intrinsic burst firing was independent of this effect. Repetitive stimulation of cholinergic fibres in the presence of neostigmine (2 μM) evoked a slow cholinergic excitatory postsynaptic potential (EPSP) lasting about a minute. During the slow EPSP, burst firing could not be evoked by depolarizing pulses and the neurons fired in regular mode. These effects were prevented by pretreatment with atropine (1 μM). Exogenously applied cholinergic agonists and endogenously released acetylcholine also reduced spike frequency accommodation and suppressed the long‐duration afterhyperpolarization in burst‐firing pyramidal cells in an atropine‐sensitive manner. A membrane‐permeable cAMP analogue (8‐bromo‐cAMP; 1 mM) also reduced frequency accommodation and blocked the long‐duration afterhyperpolarization, but did not affect intrinsic burst firing at all. Taken together, the data show that muscarinic receptor stimulation transforms the stereotyped, phasic response of burst‐firing neurons into stimulus‐graded, tonic discharge.

Texture coarseness responsive neurons and their mapping in layer 2–3 of the rat barrel cortex in vivo

Texture discrimination is a fundamental function of somatosensory systems, yet the manner by which texture is coded and spatially represented in the barrel cortex are largely unknown. Using in vivo two-photon calcium imaging in the rat barrel cortex during artificial whisking against different surface coarseness or controlled passive whisker vibrations simulating different coarseness, we show that layer 2–3 neurons within barrel boundaries differentially respond to specific texture coarsenesses, while only a minority of neurons responded monotonically with increased or decreased surface coarseness. Neurons with similar preferred texture coarseness were spatially clustered. Multi-contact single unit recordings showed a vertical columnar organization of texture coarseness preference in layer 2–3. These findings indicate that layer 2–3 neurons perform high hierarchical processing of tactile information, with surface coarseness embodied by distinct neuronal subpopulations that are spatially mapped onto the barrel cortex. DOI: http://dx.doi.org/10.7554/eLife.03405.001

Stimulus‐selective spiking is driven by the relative timing of synchronous excitation and disinhibition in cat striate neurons in vivo

What patterns of synaptic input cause cortical neurons to fire action potentials? Are they stochastic in nature, or do action potentials arise from the specific timing of synaptic input? We addressed these questions by measuring the membrane potential fluctuations associated with the generation of visually evoked action potentials in cat striate cortical neurons in vivo. In response to visual stimulation, action potentials occurred at the crest of large‐amplitude, transient depolarizations (TDs) riding on sustained depolarization of the membrane potential. The magnitude, duration and rate of depolarization of these transient events were tuned for stimulus orientation. Using numerical simulations, we find that these transient events can arise from the temporal interplay between synchronous excitation and inhibition. To validate these findings, we made conductance measurements, at the preferred stimulus orientation, and showed that the TDs arise either from an increase in excitatory conductance, or from a combination of increased excitatory and decreased inhibitory conductance, both riding on sustained changes in synaptic conductances. The properties of the TDs and their underlying conductance suggest that they arise from a specific temporal interplay between synchronous excitatory and inhibitory synaptic inputs. Our results illustrate a mechanism by which the timing of synaptic inputs determines much of the spiking activity in striate cortical neurons.

Microvibrissae-Based Texture Discrimination

Rodents use their whiskers to detect a variety of tactile features of their environment. They do so by using two functionally distinct whisker systems: the macrovibrissae and microvibrissae. To determine the functional role of unexplored microvibrissae, we recorded from the cortical area representing the frontobuccal pad in anesthetized rats while presenting moving textures of varying coarseness. We find that surface coarseness is coded by the discharge rates of frontobuccal pad cortical neurons. Cortical neurons can use this response measure to robustly and reliably discriminate between the different textures. While neuronal discharge rates carry tactile information, the highly reproducible firing patterns of these neurons suggest that a single spike train may contain sufficient information to encode the stimulus. Together, these results indicate that rodents may use their microvibrissae to distinguish between surfaces having subtly different textures and shapes.

Effects of stimulus transformations on estimated functional properties of mechanosensory neurons

The functional properties of a neural sensory cell or small ensembles are often characterized by analyzing response-conditioned stimulus ensembles. Many widely used analytical methods, like STA, Wiener kernels or STRF, rely on simple statistics of those ensembles. They also tend to rely on simple noise models for the residuals of the conditional ensemble. However, in many cases the response-conditioned stimulus set has a more complex structure than any simple noise model. If not taken explicitly into account, this difference can bias the estimates of many simple statistics, and distort the estimated functionality of a neural sensory system. Here we present analysis of two transformation-based noise sources in the rat vibrissal system: temporal jitter and temporal dilation invariance. We analyze the perturbations for several cells and correct their effect on the spike-triggered average.