The role of Ca2+-dependent cationic current in generating gamma frequency rhythmic bursts: modeling study

Calcium-activated sustained firing responses distinguish accessory from main olfactory bulb mitral cells

Many mammals rely on pheromones for mediating social interactions. Recent studies indicate that both the main olfactory system (MOS) and accessory olfactory system (AOS) detect and process pheromonal stimuli, yet the functional difference between these two chemosensory systems remains unclear. We hypothesized that the main functional distinction between the MOS and AOS is the type of sensory information processing performed by each system. Here we compared the electrophysiological responses of mitral cells recorded from the accessory olfactory bulb (AOB) and main olfactory bulb (MOB) in acute mouse brain slices to various stimuli and found them markedly different. The response of MOB mitral cells to brief (0.1 ms, 1-100 V) stimulation of their sensory afferents remained transient regardless of stimulus strength, whereas sufficiently strong stimuli evoked sustained firing in AOB mitral cells lasting up to several minutes. Using EPSC-like current injections (10-100 pA, 10 ms rise time constant, 5 s decay time constant) in the presence of various synaptic blockers (picrotoxin, CGP55845, APV, DNQX, E4CPG, and MSPG), we demonstrated that this difference is attributable to distinct intrinsic properties of the two neuronal populations. The AOB sustained responses were found to be mediated by calcium-activated nonselective cationic current induced by transient intense firing. This current was found to be at least partially mediated by TRPM4 channels activated by calcium influx. We hypothesize that the sustained activity of the AOS induces a new sensory state in the animal, reflecting its social context.

Neurophysiological and computational principles of cortical rhythms in cognition

Synchronous rhythms represent a core mechanism for sculpting temporal coordination of neural activity in the brain-wide network. This review focuses on oscillations in the cerebral cortex that occur during cognition, in alert behaving conditions. Over the last two decades, experimental and modeling work has made great strides in elucidating the detailed cellular and circuit basis of these rhythms, particularly gamma and theta rhythms. The underlying physiological mechanisms are diverse (ranging from resonance and pacemaker properties of single cells to multiple scenarios for population synchronization and wave propagation), but also exhibit unifying principles. A major conceptual advance was the realization that synaptic inhibition plays a fundamental role in rhythmogenesis, either in an interneuronal network or in a reciprocal excitatory-inhibitory loop. Computational functions of synchronous oscillations in cognition are still a matter of debate among systems neuroscientists, in part because the notion of regular oscillation seems to contradict the common observation that spiking discharges of individual neurons in the cortex are highly stochastic and far from being clocklike. However, recent findings have led to a framework that goes beyond the conventional theory of coupled oscillators and reconciles the apparent dichotomy between irregular single neuron activity and field potential oscillations. From this perspective, a plethora of studies will be reviewed on the involvement of long-distance neuronal coherence in cognitive functions such as multisensory integration, working memory, and selective attention. Finally, implications of abnormal neural synchronization are discussed as they relate to mental disorders like schizophrenia and autism.

Weighted spike-triggered average of a fluctuating stimulus yielding the phase response curve

We demonstrate that the phase response curve (PRC) can be reconstructed using a weighted spike-triggered average of an injected fluctuating input. The key idea is to choose the weight to be proportional to the magnitude of the fluctuation of the oscillatory period. Particularly, when a neuron exhibits random switching behavior between two bursting modes, two corresponding PRCs can be simultaneously reconstructed, even from the data of a single trial. This method offers an efficient alternative to the experimental investigation of oscillatory systems, without the need for detailed modeling.

Somato-dendritic mechanisms underlying the electrophysiological properties of hypothalamic magnocellular neuroendocrine cells: a multicompartmental model study

Magnocellular neuroendocrine cells (MNCs) of the hypothalamus synthesize the neurohormones vasopressin and oxytocin, which are released into the blood and exert a wide spectrum of actions, including the regulation of cardiovascular and reproductive functions. Vasopressin- and oxytocin-secreting neurons have similar morphological structure and electrophysiological characteristics. A realistic multicompartmental model of a MNC with a bipolar branching structure was developed and calibrated based on morphological and in vitro electrophysiological data in order to explore the roles of ion currents and intracellular calcium dynamics in the intrinsic electrical MNC properties. The model was used to determine the likely distributions of ion conductances in morphologically distinct parts of the MNCs: soma, primary dendrites and secondary dendrites. While reproducing the general electrophysiological features of MNCs, the model demonstrates that the differential spatial distributions of ion channels influence the functional expression of MNC properties, and reveals the potential importance of dendritic conductances in these properties.

Temporal integration by stochastic recurrent network dynamics with bimodal neurons

Temporal integration of externally or internally driven information is required for a variety of cognitive processes. This computation is generally linked with graded rate changes in cortical neurons, which typically appear during a delay period of cognitive task in the prefrontal and other cortical areas. Here, we present a neural network model to produce graded (climbing or descending) neuronal activity. Model neurons are interconnected randomly by AMPA-receptor-mediated fast excitatory synapses and are subject to noisy background excitatory and inhibitory synaptic inputs. In each neuron, a prolonged afterdepolarizing potential follows every spike generation. Then, driven by an external input, the individual neurons display bimodal rate changes between a baseline state and an elevated firing state, with the latter being sustained by regenerated afterdepolarizing potentials. When the variance of background input and the uniform weight of recurrent synapses are adequately tuned, we show that stochastic noise and reverberating synaptic input organize these bimodal changes into a sequence that exhibits graded population activity with a nearly constant slope. To test the validity of the proposed mechanism, we analyzed the graded activity of anterior cingulate cortex neurons in monkeys performing delayed conditional Go/No-go discrimination tasks. The delay-period activities of cingulate neurons exhibited bimodal activity patterns and trial-to-trial variability that are similar to those predicted by the proposed model.

Synchronous and asynchronous bursting states: role of intrinsic neural dynamics

Brain signals such as local field potentials often display gamma-band oscillations (30-70 Hz) in a variety of cognitive tasks. These oscillatory activities possibly reflect synchronization of cell assemblies that are engaged in a cognitive function. A type of pyramidal neurons, i.e., chattering neurons, show fast rhythmic bursting (FRB) in the gamma frequency range, and may play an active role in generating the gamma-band oscillations in the cerebral cortex. Our previous phase response analyses have revealed that the synchronization between the coupled bursting neurons significantly depends on the bursting mode that is defined as the number of spikes in each burst. Namely, a network of neurons bursting through a Ca(2+)-dependent mechanism exhibited sharp transitions between synchronous and asynchronous firing states when the neurons exchanged the bursting mode between singlet, doublet and so on. However, whether a broad class of bursting neuron models commonly show such a network behavior remains unclear. Here, we analyze the mechanism underlying this network behavior using a mathematically tractable neuron model. Then we extend our results to a multi-compartment version of the NaP current-based neuron model and prove a similar tight relationship between the bursting mode changes and the network state changes in this model. Thus, the synchronization behavior couples tightly to the bursting mode in a wide class of networks of bursting neurons.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Synchrony of fast-spiking interneurons interconnected by GABAergic and electrical synapses

Fast-spiking (FS) interneurons have specific types (Kv3.1/3.2 type) of the delayed potassium channel, which differ from the conventional Hodgkin-Huxley (HH) type potassium channel (Kv1.3 type) in several aspects. In this study, we show dramatic effects of the Kv3.1/3.2 potassium channel on the synchronization of the FS interneurons. We show analytically that two identical electrically coupled FS interneurons modeled with Kv3.1/3.2 channel fire synchronously at arbitrary firing frequencies, unlike similarly coupled FS neurons modeled with Kv1.3 channel that show frequency-dependent synchronous and antisynchronous firing states. Introducing GABA(A) receptor-mediated synaptic connections into an FS neuron pair tends to induce an antisynchronous firing state, even if the chemical synapses are bidirectional. Accordingly, an FS neuron pair connected simultaneously by electrical and chemical synapses achieves both synchronous firing state and antisynchronous firing state in a physiologically plausible range of the conductance ratio between electrical and chemical synapses. Moreover, we find that a large-scale network of FS interneurons connected by gap junctions and bidirectional GABAergic synapses shows similar bistability in the range of gamma frequencies (30-70 Hz).

Gamma rhythmic bursts: coherence control in networks of cortical pyramidal neurons

Much evidence indicates that synchronized gamma-frequency (20-70 Hz) oscillation plays a significant functional role in the neocortex and hippocampus. Chattering neuron is a possible neocortical pacemaker for the gamma oscillation. Based on our recent model of chattering neurons, here we study how gamma-frequency bursting is synchronized in a network of these neurons. Using a phase oscillator description, we first examine how two coupled chattering neurons are synchronized. The analysis reveals that an incremental change of the bursting mode, such as from singlet to doublet, always accompanies a rapid transition from antisynchronous to synchronous firing. The state transition occurs regardless of what changes the bursting mode. Within each bursting mode, the neuronal activity undergoes a gradual change from synchrony to antisynchrony. Since the sensitivity to Ca(2+) and the maximum conductance of Ca(2+)-dependent cationic current as well as the intensity of input current systematically control the bursting mode, these quantities may be crucial for the regulation of the coherence of local cortical activity. Numerical simulations demonstrate that the modulations of the calcium sensitivity and the amplitude of the cationic current can induce rapid transitions between synchrony and asynchrony in a large-scale network of chattering neurons. The rapid synchronization of chattering neurons is shown to synchronize the activities of regular spiking pyramidal neurons at the gamma frequencies, as may be necessary for selective attention or binding processing in object recognition.