Dynamic spike threshold reveals a mechanism for synaptic coincidence detection in cortical neurons in vivo.

Cortical neurons are sensitive to the timing of their synaptic inputs. They can synchronize their firing on a millisecond time scale and follow rapid stimulus fluctuations with high temporal precision. These findings suggest that cortical neurons have an enhanced sensitivity to synchronous synaptic inputs that lead to rapid rates of depolarization. The voltage-gated currents underlying action potential generation may provide one mechanism to amplify rapid depolarizations. We have tested this hypothesis by analyzing the relations between membrane potential fluctuations and spike threshold in cat visual cortical neurons recorded intracellularly in vivo. We find that visual stimuli evoke broad variations in spike threshold that are caused in large part by an inverse relation between spike threshold and the rate of membrane depolarization preceding a spike. We also find that spike threshold is inversely related to the rate of rise of the action potential upstroke, suggesting that increases in spike threshold result from a decrease in the availability of Na(+) channels. By using a simple neuronal model, we show that voltage-gated Na(+) and K(+) conductances endow cortical neurons with an enhanced sensitivity to rapid depolarizations that arise from synchronous excitatory synaptic inputs. Thus, the basic mechanism responsible for action potential generation also enhances the sensitivity of cortical neurons to coincident synaptic inputs.

Cellular mechanisms contributing to response variability of cortical neurons in vivo.

Cortical neurons recorded in vivo exhibit highly variable responses to the repeated presentation of the same stimulus. To further understand the cellular mechanisms underlying this phenomenon, we performed intracellular recordings from neurons in cat striate cortex in vivo and examined the relationships between spontaneous activity and visually evoked responses. Activity was assessed on a trial-by-trial basis by measuring the membrane potential (Vm) fluctuations and spike activity during brief epochs immediately before and after the onset of an evoked response. We found that the response magnitude, expressed as a change in Vm relative to baseline, was linearly correlated with the preceding spontaneous Vm. This correlation was enhanced when the cells were hyperpolarized to reduce the activation of voltage-gated conductances. The output of the cells, expressed as spike counts and latencies, was only moderately correlated with fluctuations in the preceding spontaneous Vm. Spike-triggered averaging of Vm revealed that visually evoked action potentials arise from transient depolarizations having a rise time of approximately 10 msec. Consistent with this, evoked spike count was found to be linearly correlated with the magnitude of Vm fluctuations in the gamma (20-70 Hz) frequency band. We also found that the threshold of visually evoked action potentials varied over a range of approximately 10 mV. Examination of simultaneously recorded intracellular and extracellular activity revealed a correlation between Vm depolarization and spike discharges in adjacent cells. Together these results demonstrate that response variability is attributable largely to coherent fluctuations in cortical activity preceding the onset of a stimulus, but also to variations in action potential threshold and the magnitude of high-frequency fluctuations evoked by the stimulus.

Physiological properties of inhibitory interneurons in cat striate cortex.

Physiological and morphological properties of identified interneurons in the striate cortex of the cat were studied in vivo by intracellular recording and staining with biocytin. In conformity with in vitro studies, these non-pyramidal fast spiking cells have very brief action potentials associated with a high rate of fall, and a large hyperpolarizing afterpotential. These cells show high discharge rates, little or no spike frequency adaptation in response to depolarizing current injection, as well as a diverse range of firing patterns. Three of the cells were labeled and were found to be aspiny or sparsely spiny basket cells, with bitufted or radial dendritic arrangements, in layers II-IV. Their axonal arborizations were more dense near their somata and extended horizontally or vertically. Of 13 visually responsive cells tested, the receptive field properties of six cells and the orientation and direction preferences of eight cells were determined. Five of the successfully mapped cells had simple receptive fields while one had a complex receptive field type. The orientation and direction tuning properties of the overlapping set of eight cells showed a broad spectrum ranging from unselective to tightly tuned. The majority exhibited a clear preference for orientation and none of the cells were clearly direction selective. Quantitative analysis of the temporal properties of the spike trains during visual stimulation and spontaneous activity revealed that these cells do not exhibit any significant periodic activity, and fired at rates that were well below their maximum in response to depolarizing current pulses.

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 (pi o). 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 pi o 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 (tau m) increased in low pi o and decreased in high pi o. 3. Reducing pi o converted non-bursting neurones (non-bursters) to bursting neurones (bursters) and decreased the stimulus requirements for evoking burst firing in native bursters. Increasing pi o suppressed endogenous burst firing. 4. Lowering pi o increased the size of the 'active' (i.e. re-depolarizing) component of the spike after-depolarization (ADP). Conversely, increasing pi o suppressed the active ADP component. 5. The sensitivity of spike ADPs and firing patterns of pyramidal cells to the changes in pi o persisted also in Ca(2+)-free saline, indicating that the osmotic effects are not imparted by modulation of Ca2+ and/or Ca(2+)-activated K+ currents. 6. Blocking most K+ currents with Ca(2+)-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 pi o and abated by high pi o. 7. When injected with subthreshold depolarizing current pulses in Ca(2+)-free saline, pyramidal cells displayed a distinct TTX-sensitive inward rectification. This rectification was augmented by low pi o and reduced by high pi o. 8. The various effects of low-pi o and high-pi o saline solutions were reversible upon washing with normosmotic saline. 9. We conclude that pi o 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 in discussed.

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.

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).

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 microM) converted burst firing in these neurons into regular trains of independent spikes. Acetylcholine (5 microM) exerted a similar effect, provided acetylcholine esterase activity was blocked with neostigmine (2 microM). Atropine (1 microM) 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 microM) 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 microM). 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 microM) 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.

Variant firing patterns in rat hippocampal pyramidal cells modulated by extracellular potassium.

1. The distribution of distinctive firing modes within the population of CA1 pyramidal cells and their modulation by the extracellular concentration of potassium ([K+]o) were investigated with intracellular recordings in rat hippocampal slices. 2. Pyramidal cells were injected with long (> 250 ms) and brief (3-5 ms) positive current pulses of increasing intensity. In normal [K+]o (3.5 mM), most cells (38 of 46 cells; 83%) were regular spiking neurons (generating accommodating trains of independent action potentials during long depolarizations and a single spike in response to brief stimuli). The remaining pyramidal cells (8 of 46; 17%) displayed differential tendencies to generate stereotyped clusters of action potentials, or bursts, according to which they were grouped into three subsets of endogenous bursters: grade I, bursting only when stimulated with long depolarizing current pulses (6 of 46; 13%); grade II, bursting also in response to brief stimulation (1 of 46; 2%); grade III, bursting also spontaneously even in absence of synaptic transmission (1 of 46; 2%). 3. Raising [K+]o from 3.5 to 7.5 mM (high [K+]o) significantly reduced resting membrane potential and input impedance but did not change the threshold potential for eliciting an action potential. 4. Raising [K+]o to 7.5 mM reversibly converted many regular spiking cells to bursters. Likewise, the burst tendency of normally bursting pyramidal cells increased to a higher grade in high [K+]o. Consequently, the fraction of bursters in high [K+]o (17 of 41 cells; 42%) was approximately 2.5-fold higher than in normal [K+]o and their differential distribution was shifted toward higher grades of bursting.(ABSTRACT TRUNCATED AT 250 WORDS)