Muscarinic Receptors, from Synaptic Plasticity to its Role in Network Activity

Acetylcholine acting via metabotropic receptors plays a key role in learning and memory by regulating synaptic plasticity and circuit activity. However, a recent overall view of the effects of muscarinic acetylcholine receptors (mAChRs) on excitatory and inhibitory long-term synaptic plasticity and on circuit activity is lacking. This review focusses on specific aspects of the regulation of synaptic plasticity and circuit activity by mAChRs in the hippocampus and cortex. Acetylcholine increases the excitability of pyramidal neurons, facilitating the generation of dendritic Ca²⁺-spikes, NMDA-spikes and action potential bursts which provide the main source of Ca²⁺ influx necessary to induce synaptic plasticity. The activation of mAChRs induced Ca²⁺ release from intracellular IP₃-sensitive stores is a major player in the induction of a NMDA independent long-term potentiation (LTP) caused by an increased expression of AMPA receptors in hippocampal pyramidal neuron dendritic spines. In the neocortex, activation of mAChRs also induces a long-term enhancement of excitatory postsynaptic currents. In addition to effects on excitatory synapses, a single brief activation of mAChRs together with short repeated membrane depolarization can induce a long-term enhancement of GABA A type (GABA_A) inhibition through an increased expression of GABA_A receptors in hippocampal pyramidal neurons. By contrast, a long term depression of GABA_A inhibition (iLTD) is induced by muscarinic receptor activation in the absence of postsynaptic depolarizations. This iLTD is caused by an endocannabinoid-mediated presynaptic inhibition that reduces the GABA release probability at the terminals of inhibitory interneurons. This bidirectional long-term plasticity of inhibition may dynamically regulate the excitatory/inhibitory balance depending on the quiescent or active state of the postsynaptic pyramidal neurons. Therefore, acetylcholine can induce varied effects on neuronal activity and circuit behavior that can enhance sensory detection and processing through the modification of circuit activity leading to learning, memory and behavior.

Quantified EEG for the Characterization of Epileptic Seizures versus Periodic Activity in Critically Ill Patients

Epileptic seizures (ES) are frequent in critically ill patients and their detection and treatment are mandatory. However, sometimes it is quite difficult to discriminate between ES and non-epileptic bursts of periodic activity (BPA). Our aim was to characterize ES and BPA by means of quantified electroencephalography (qEEG). Records containing either ES or BPA were visually identified and divided into 1 s windows that were 10% overlapped. Differential channels were grouped by frontal, parieto-occipital and temporal lobes. For every channel and window, the power spectrum was calculated and the area for delta (0–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), and beta (13–30 Hz) bands and spectral entropy (Se) were computed. Mean values of percentage changes normalized to previous basal activity and standardized mean difference (SMD) for every lobe were computed. We have observed that BPA are characterized by a selective increment of delta activity and decrease in Se along the scalp. Focal seizures (FS) always propagated and were similar to generalized seizures (GS). In both cases, although delta and theta bands increased, the faster bands (alpha and beta) showed the highest increments (more than 4 times) without modifications in Se. We have defined the numerical features of ES and BPA, which can facilitate its clinical identification.

Neuromodulation of central pattern generators and its role in the functional recovery of central pattern generator activity

Neuromodulators play an important role in how the nervous system organizes activity that results in behavior. Disruption of the normal patterns of neuromodulatory release or production is known to be related to the onset of severe pathologies such as Parkinson's disease, Rett syndrome, Alzheimer's disease, and affective disorders. Some of these pathologies involve neuronal structures that are called central pattern generators (CPGs), which are involved in the production of rhythmic activities throughout the nervous system. Here I discuss the interplay between CPGs and neuromodulatory activity, with particular emphasis on the potential role of neuromodulators in the recovery of disrupted neuronal activity. I refer to invertebrate and vertebrate model systems and some of the lessons we have learned from research on these systems and propose a few avenues for future research. I make one suggestion that may guide future research in the field: neuromodulators restrict the parameter landscape in which CPG components operate, and the removal of neuromodulators may enable a perturbed CPG in finding a new set of parameter values that can allow it to regain normal function.

[A Potential Mechanism for Spontaneous Oscillation in the Abnormal Retina]

　Rhythmic neural activities are observed in many brain regions, and these are considered to play an important role in neural information processing. On the other hand, distinct rhythmic neural activities emerge under several pathological conditions, suggesting that rhythmic neural activity has a close relation to brain function and dysfunction. In many pathological cases, the intrinsic property of unusual rhythm generation in a neuron or a neuronal network is prevented under normal conditions, but released by the pathological condition. Therefore, it may be useful to explore which conditions determine rhythm generation in order to understand the mechanisms of brain function/dysfunction. The pathological retina in retinal degeneration exhibits rhythmic neural activity not observed in the healthy retina. In this review, we first provide a brief introduction to the possible mechanisms of rhythm generation in a neural system. Then we introduce experimental evidence of rhythm generation in the pathological retina, as well as two hypotheses regarding this mechanism. Finally, we raise several issues to be solved for the further understanding of pathological rhythm generation.

A balance of outward and linear inward ionic currents is required for generation of slow-wave oscillations

Regenerative inward currents help produce slow oscillations through a negative-slope conductance region of their current-voltage relationship that is well approximated by a linear negative conductance. We used dynamic-clamp injections of a linear current with such conductance, I_NL, to explore why some neurons can generate intrinsic slow oscillations whereas others cannot. We addressed this question in synaptically isolated neurons of the crab Cancer borealis after blocking action potentials. The pyloric network consists of a distinct pacemaker and follower neurons, all of which express the same complement of ionic currents. When the pyloric dilator (PD) neuron, a member of the pacemaker group, was injected with I_NL with dynamic clamp, it consistently produced slow oscillations. In contrast, all follower neurons failed to oscillate with I_NL To understand these distinct behaviors, we compared outward current levels of PD with those of follower lateral pyloric (LP) and ventral pyloric (VD) neurons. We found that LP and VD neurons had significantly larger high-threshold potassium currents (I_HTK) than PD and LP had lower-transient potassium current (I_A). Reducing I_HTK pharmacologically enabled both LP and VD neurons to produce I_NL-induced oscillations, whereas modifying I_A levels did not affect I_NL-induced oscillations. Using phase-plane and bifurcation analysis of a simplified model cell, we demonstrate that large levels of I_HTK can block I_NL-induced oscillatory activity whereas generation of oscillations is almost independent of I_A levels. These results demonstrate the general importance of a balance between inward pacemaking currents and high-threshold K⁺ current levels in determining slow oscillatory activity.NEW & NOTEWORTHY Pacemaker neuron-generated rhythmic activity requires the activation of at least one inward and one outward current. We have previously shown that the inward current can be a linear current (with negative conductance). Using this simple mechanism, here we demonstrate that the inward current conductance must be in relative balance with the outward current conductances to generate oscillatory activity. Surprisingly, an excess of outward conductances completely precludes the possibility of achieving such a balance.

Na(+)/K(+) pump interacts with the h-current to control bursting activity in central pattern generator neurons of leeches

The dynamics of different ionic currents shape the bursting activity of neurons and networks that control motor output. Despite being ubiquitous in all animal cells, the contribution of the Na(+)/K(+) pump current to such bursting activity has not been well studied. We used monensin, a Na(+)/H(+) antiporter, to examine the role of the pump on the bursting activity of oscillator heart interneurons in leeches. When we stimulated the pump with monensin, the period of these neurons decreased significantly, an effect that was prevented or reversed when the h-current was blocked by Cs(+). The decreased period could also occur if the pump was inhibited with strophanthidin or K(+)-free saline. Our monensin results were reproduced in model, which explains the pump's contributions to bursting activity based on Na(+) dynamics. Our results indicate that a dynamically oscillating pump current that interacts with the h-current can regulate the bursting activity of neurons and networks.

Motoneuron activity patterns related to the earliest behavior of the zebrafish embryo

As a first step in the study of the developing motor circuitry of the embryonic zebrafish spinal cord, we obtained patch-clamp recordings in vivo from identified motoneurons in curarized embryos from the onset of the first motor behavior. At an early developmental stage in which embryos showed slow and repetitive spontaneous contractions of the trunk, motoneurons showed periodic depolarizations that triggered rhythmic bursts of action potentials with a frequency and duration that were consistent with those of the spontaneous contractions. The periodic depolarizations were blocked by tetrodotoxin or Cd(2+). Surprisingly, the contractions and periodic depolarizations were insensitive to general blockade of synaptic transmission (by elevated Mg(2+) and reduced Ca(2+), or by Co(2+)) and to selective blockade of the major neurotransmitter receptors of the mature spinal cord (acetylcholine, GABA(A), NMDA, AMPA/kainate, and glycine). The periodic depolarizations were suppressed by heptanol or by intracellular acidification, treatments that are known to uncouple gap junctions, indicating that electrotonic synapses could underlie the earliest motor behavior. A few hours later, most motoneurons already showed a new pattern of repetitive activity consisting of bursts of glycinergic synaptic events, but these were not necessary for the spontaneous contractions. Transecting the spinal cord at the hindbrain border did not affect the rhythmic activity patterns of the motoneurons. We suggest that spontaneous contractions of the zebrafish embryo are mediated by an early spinal circuit that is independent of the main neurotransmitter systems and descending hindbrain projections that are required for locomotion in the mature vertebrate spinal cord.

Differential expression of group I metabotropic glutamate receptors in functionally distinct hippocampal interneurons

Metabotropic glutamate receptors (mGluRs) have been proposed to be involved in oscillatory rhythmic activity in the hippocampus. However, the subtypes of mGluRs involved and their precise distribution in different populations of interneurons is unclear. In this study, we combined functional analysis of mGluR-mediated inward currents in CA1 oriens-alveus interneurons with anatomical and immunocytochemical identification of these interneurons and expression analysis of group I mGluR using single-cell reverse transcription-PCR (RT-PCR). Four major interneuron subtypes could be distinguished based on the mGluR-mediated inward current induced by the application of 100 microm trans-(1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) under voltage-clamp conditions and the action potential firing pattern under current-clamp conditions. Type I interneurons responded with a large inward current of approximately 224 pA, were positive for somatostatin, and the majority expressed both mGluR1 and mGluR5. Type II interneurons responded with an inward current of approximately 80 pA, contained calbindin, and expressed mainly mGluR1. Type III interneurons responded with an inward current of approximately 60 pA. These interneurons were fast-spiking, contained parvalbumin, and expressed mainly mGluR5. Type IV interneurons did not respond with an inward current upon application of ACPD, yet they expressed group I mGluRs. Activation of group I mGluRs under current-clamp conditions increased spike frequency and resulted in rhythmic firing activity in type I and II, but not in type III and IV, interneurons. RT-PCR results suggest that activation of mGluR1 in the subsets of GABAergic interneurons, classified here as type I and II, may play an important role in mediating synchronous activity.

Activity patterns and synaptic organization of ventrally located interneurons in the embryonic chick spinal cord

To investigate the origin of spontaneous activity in developing spinal networks, we examined the activity patterns and synaptic organization of ventrally located lumbosacral interneurons, including those whose axons project into the ventrolateral funiculus (VLF), in embryonic day 9 (E9)-E12 chick embryos. During spontaneous episodes, rhythmic synaptic potentials were recorded from the VLF and from spinal interneurons that were synchronized, cycle by cycle, with rhythmic ventral root potentials. At the beginning of an episode, ventral root potentials started before the VLF discharge and the firing of individual interneurons. However, pharmacological blockade of recurrent motoneuron collaterals did not prevent or substantially delay interneuron recruitment during spontaneous episodes. The synaptic connections of interneurons were examined by stimulating the VLF and recording the potentials evoked in the ventral roots, in the VLF, or in individual interneurons. Low-intensity stimulation of the VLF evoked a short-latency depolarizing potential in the ventral roots, or in interneurons, that was probably mediated mono- or disynaptically. At higher intensities, long-latency responses were recruited in a highly nonlinear manner, eventually culminating in the activation of an episode. VLF-evoked potentials were reversibly blocked by extracellular Co2+, indicating that they were mediated by chemical synaptic transmission. Collectively, these findings indicate that ventral interneurons are rhythmically active, project to motoneurons, and are likely to be interconnected by recurrent excitatory synaptic connections. This pattern of organization may explain the synchronous activation of spinal neurons and the regenerative activation of spinal networks when provided with a suprathreshold stimulus.

Spontaneous network activity transiently depresses synaptic transmission in the embryonic chick spinal cord

We examined the effects of spontaneous or evoked episodes of rhythmic activity on synaptic transmission in several spinal pathways of embryonic day 9-12 chick embryos. We compared the amplitude of synaptic potentials evoked by stimulation of the ventrolateral funiculus (VLF), the dorsal or ventral roots, before and after episodes of activity. With the exception of the short-latency responses evoked by dorsal root stimulation, the potentials were briefly potentiated and then reduced for several minutes after an episode of rhythmic activity. Their amplitude progressively recovered in the interval between successive episodes. The lack of post-episode depression in the short-latency component of the dorsal root evoked responses is probably attributable to the absence of firing in cut muscle afferents during an episode of activity. The post-episode depression of VLF-evoked potentials was mimicked by prolonged stimulation of the VLF, subthreshold for an episode of activity. By contrast, antidromically induced motoneuron firing and the accompanying calcium entry did not depress VLF-evoked potentials recorded from the stimulated ventral root. In addition, post-episode depression of VLF-evoked synaptic currents was observed in voltage-clamped spinal neurons. Collectively, these findings suggest that somatic postsynaptic activity and calcium entry are not required for the depression. We propose instead that the mechanism may involve a form of long-lasting activity-induced synaptic depression, possibly a combination of transmitter depletion and ligand-induced changes in the postsynaptic current accompanying transmitter release. This activity-dependent depression appears to be an important mechanism underlying the occurrence of spontaneous activity in developing spinal networks.