Electrical coupling and neuronal synchronization in the Mammalian brain

The sensory neuron and the triumph of Camillo Golgi

While Golgi's concept of the sensory neuron provided sound reasons for his rejection of the polarity principles underlying the 'neuron doctrine', it is now apparent that his concern about recovery of function after injury and the vast modern findings of ephemerality of connexin-clustered connections in the cerebral cortex and elsewhere in the central nervous system, and credibly termed 'reticularist', has somewhat eclipsed the polarized neuron doctrine of reflex physiology with the "fixed and immutable" connections championed by Cajal. Although Golgi's view was not the result of incisive reasoning based on subsequently confirmed observation, both principles espoused by these combatant Nobel laureate partners have proven robustly operative in different spheres and time frames of neural activity that have vastly enhanced contemporary understanding of neural connectivity.

Viscosity-dependent flow reversal in a density oscillator

The density oscillator is a simple system that exhibits self-sustained oscillation. It alternately exhibits up and down flow through a pipe which connects two containers filled with fluids of different densities. However, the mechanism of the flow reversal has not yet been fully understood. From the detailed measurements, we have found that flow reversal begins with an intrusion of fluid, which is followed by rapid growth. This process is definitely sensitive to the viscosities of the fluids, and as a consequence, the critical heights leading to flow reversal are clearly viscosity dependent. These experimental results are explained by a simple model, derived by considering forces acting on a unit volume element located at the tip of the intrusion. Using this model, we can successfully explain the mechanism of flow reversal, which is the most essential process in a density oscillator.

Retinal encoding of ultrabrief shape recognition cues

Shape encoding mechanisms can be probed by the sequential brief display of dots that mark the boundary of the shape, and delays of less that a millisecond between successive dots can impair recognition. It is not entirely clear whether this is accomplished by preserving stimulus timing in the signal being sent to the brain, or calls for a retinal binding mechanism. Two experiments manipulated the degree of simultaneity among and within dot pairs, requiring also that the pair members be in the same half of the visual field or on opposite halves, i.e., across the midline from one another. Recognition performance was impaired the same for these two conditions. The results make it likely that simultaneity of cues is being registered within the retina. A potential mechanism is suggested, calling for linkage of stimulated sites through activation of PA1 cells. A third experiment confirmed a prior finding that the overall level of recognition deficit is partly a function of display-set size, and affirmed submillisecond resolution in binding dot pairs into effective shape-recognition cues.

Identification of connexin36 in gap junctions between neurons in rodent locus coeruleus

Locus coeruleus neurons are strongly coupled during early postnatal development, and it has been proposed that these neurons are linked by extraordinarily abundant gap junctions consisting of connexin32 (Cx32) and connexin26 (Cx26), and that those same connexins abundantly link neurons to astrocytes. Based on the controversial nature of those claims, immunofluorescence imaging and freeze-fracture replica immunogold labeling were used to re-investigate the abundance and connexin composition of neuronal and glial gap junctions in developing and adult rat and mouse locus coeruleus. In early postnatal development, connexin36 (Cx36) and connexin43 (Cx43) immunofluorescent puncta were densely distributed in the locus coeruleus, whereas Cx32 and Cx26 were not detected. By freeze-fracture replica immunogold labeling, Cx36 was found in ultrastructurally-defined neuronal gap junctions, whereas Cx32 and Cx26 were not detected in neurons and only rarely detected in glia. In 28-day postnatal (adult) rat locus coeruleus, immunofluorescence labeling for Cx26 was always co-localized with the glial gap junction marker Cx43; Cx32 was associated with the oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase); and Cx36 was never co-localized with Cx26, Cx32 or Cx43. Ultrastructurally, Cx36 was localized to gap junctions between neurons, whereas Cx32 was detected only in oligodendrocyte gap junctions; and Cx26 was found only rarely in astrocyte junctions but abundantly in pia mater. Thus, in developing and adult locus coeruleus, neuronal gap junctions contain Cx36 but do not contain detectable Cx32 or Cx26, suggesting that the locus coeruleus has the same cell-type specificity of connexin expression as observed ultrastructurally in other regions of the CNS. Moreover, in both developing and adult locus coeruleus, no evidence was found for gap junctions or connexins linking neurons with astrocytes or oligodendrocytes, indicating that neurons in this nucleus are not linked to the pan-glial syncytium by Cx32- or Cx26-containing gap junctions or by abundant free connexons composed of those connexins.

Modafinil enhances thalamocortical activity by increasing neuronal electrotonic coupling

Modafinil (Provigil, Modiodal), an antinarcoleptic and mood-enhancing drug, is shown here to sharpen thalamocortical activity and to increase electrical coupling between cortical interneurons and between nerve cells in the inferior olivary nucleus. After irreversible pharmacological block of connexin permeability (i.e., by using either 18beta-glycyrrhetinic derivatives or mefloquine), modafinil restored electrotonic coupling within 30 min. It was further established that this restoration is implemented through a Ca(2+)/calmodulin protein kinase II-dependent step.

An innexin-dependent cell network establishes left-right neuronal asymmetry in C. elegans

Gap junctions are widespread in immature neuronal circuits, but their functional significance is poorly understood. We show here that a transient network formed by the innexin gap-junction protein NSY-5 coordinates left-right asymmetry in the developing nervous system of Caenorhabditis elegans. nsy-5 is required for the left and right AWC olfactory neurons to establish stochastic, asymmetric patterns of gene expression during embryogenesis. nsy-5-dependent gap junctions in the embryo transiently connect the AWC cell bodies with those of numerous other neurons. Both AWCs and several other classes of nsy-5-expressing neurons participate in signaling that coordinates left-right AWC asymmetry. The right AWC can respond to nsy-5 directly, but the left AWC requires nsy-5 function in multiple cells of the network. NSY-5 forms hemichannels and intercellular gap-junction channels in Xenopus oocytes, consistent with a combination of cell-intrinsic and network functions. These results provide insight into gap-junction activity in developing circuits.

UNC-1 regulates gap junctions important to locomotion in C. elegans

In C. elegans, loss-of-function (lf) mutations of the stomatin-like protein (SLP) UNC-1 and the innexin UNC-9 inhibit locomotion [1, 2] and modulate sensitivity to volatile anesthetics [3, 4]. It was unknown why unc-1(lf) and unc-9(lf) mutants have similar phenotypes. We tested the hypothesis that UNC-1 is a regulator of gap junctions formed by UNC-9. Analyses of junctional currents between body-wall muscle cells showed that electrical coupling was inhibited to a similar degree in unc-1(lf), unc-9(lf), and unc-1(lf);unc-9(lf) double mutants, suggesting that UNC-1 and UNC-9 function together. Expression of Punc-1::DsRED2 and Punc-9::GFP transcriptional fusions suggests that unc-1 and unc-9 are coexpressed in neurons and body-wall muscle cells. Immunohistochemistry showed that UNC-1 and UNC-9 colocalized at intercellular junctions and that unc-1(lf) did not alter UNC-9 expression or subcellular localization. Bimolecular fluorescence complementation (BiFC) assays suggest that UNC-1 and UNC-9 are physically very close at intercellular junctions. Targeted rescue experiments suggest that UNC-9 and UNC-1 function predominantly in neurons to control locomotion. Thus, in addition to the recently reported function of regulating mechanosensitive ion channels [5, 6], SLPs might have a novel function of regulating gap junctions.

Gap junctions mediate large-scale Turing structures in a mean-field cortex driven by subcortical noise

One of the grand puzzles in neuroscience is establishing the link between cognition and the disparate patterns of spontaneous and task-induced brain activity that can be measured clinically using a wide range of detection modalities such as scalp electrodes and imaging tomography. High-level brain function is not a single-neuron property, yet emerges as a cooperative phenomenon of multiply-interacting populations of neurons. Therefore a fruitful modeling approach is to picture the cerebral cortex as a continuum characterized by parameters that have been averaged over a small volume of cortical tissue. Such mean-field cortical models have been used to investigate gross patterns of brain behavior such as anesthesia, the cycles of natural sleep, memory and erasure in slow-wave sleep, and epilepsy. There is persuasive and accumulating evidence that direct gap-junction connections between inhibitory neurons promote synchronous oscillatory behavior both locally and across distances of some centimeters, but, to date, continuum models have ignored gap-junction connectivity. In this paper we employ simple mean-field arguments to derive an expression for D2, the diffusive coupling strength arising from gap-junction connections between inhibitory neurons. Using recent neurophysiological measurements reported by Fukuda [J. Neurosci. 26, 3434 (2006)], we estimate an upper limit of D2 approximately 0.6cm2. We apply a linear stability analysis to a standard mean-field cortical model, augmented with gap-junction diffusion, and find this value for the diffusive coupling strength to be close to the critical value required to destabilize the homogeneous steady state. Computer simulations demonstrate that larger values of D2 cause the noise-driven model cortex to spontaneously crystalize into random mazelike Turing structures: centimeter-scale spatial patterns in which regions of high-firing activity are intermixed with regions of low-firing activity. These structures are consistent with the spatial variations in brain activity patterns detected with the BOLD (blood oxygen-level-dependent) signal detected with magnetic resonance imaging, and may provide a natural substrate for synchronous gamma-band rhythms observed across separated EEG (electroencephalogram) electrodes.

Mechanism of spontaneous and self-sustained oscillations in networks connected through axo-axonal gap junctions

Electrical coupling of clusters of neurons via axo-axonal gap junctions is a candidate mechanism underlying the ultra-fast (> 100 Hz) oscillations recorded in various in vitro and in vivo normal and pathological conditions [Traub et al. (1999)Neuroscience, 92, 407-426]. The poor characterization of axo-axonal gap junctions, however, limits experimental verification of this mechanism. We simulated networks of prototype multi-compartmental neurons in order to identify the parameters constraining the production and frequency of ultra-fast oscillations. Weak axo-axonal coupling was found to synchronize networks preferentially at the gamma-range frequency (30-100 Hz). Networks with strong axo-axonal coupling were able to produce 200 Hz oscillations, a finding we extended with several new observations. Ultra-fast oscillations arose spontaneously during dendritic excitation, i.e. in the absence of extrinsic axonal background spikes, as the spike trigger zone concomitantly shifted from soma to axon. The all-or-none oscillations could be transitory or self-sustained, and lasted longer in larger networks. They occurred more likely during low to modest soma firing rates, as strong afterhyperpolarizing currents tended to impair them. The rate of the rhythm was independent of network size and of the level of excitation, but inversely proportional to the distance of the junctions from the soma. As a matter of fact, the resulting axonal firing rate was the highest one at which antidromic spikes would not collide with spikes reflected from the soma. Taken together, the observed model dynamics lends further credibility to axo-axonal coupling as a mechanism of ultra-fast oscillations.

Electrical synapses in basal ganglia

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

UNC-4 represses CEH-12/HB9 to specify synaptic inputs to VA motor neurons in C. elegans

In Caenorhabditis elegans, VA and VB motor neurons arise as lineal sisters but synapse with different interneurons to regulate locomotion. VA-specific inputs are defined by the UNC-4 homeoprotein and its transcriptional corepressor, UNC-37/Groucho, which function in the VAs to block the creation of chemical synapses and gap junctions with interneurons normally reserved for VBs. To reveal downstream genes that control this choice, we have employed a cell-specific microarray strategy that has now identified unc-4-regulated transcripts. One of these genes, ceh-12, a member of the HB9 family of homeoproteins, is normally restricted to VBs. We show that expression of CEH-12/HB9 in VA motor neurons in unc-4 mutants imposes VB-type inputs. Thus, this work reveals a developmental switch in which motor neuron input is defined by differential expression of transcription factors that select alternative presynaptic partners. The conservation of UNC-4, HB9, and Groucho expression in the vertebrate motor circuit argues that similar mechanisms may regulate synaptic specificity in the spinal cord.

Pyramid power: principal cells of the hippocampus unite!

Electrical transmission in the mammalian brain is now well established. A new study by Thomson and colleagues elegantly demonstrates coupling between CA1 hippocampal pyramidal cells, which is far more common than previously supposed. Although the history of coupling is extensive, doubt, predjudice, and technical issues long kept it from wide acceptance. Here "spikelets" or "fast prepotentials" are found when two cells are coupled and in this situation result from electrical transmission of impulses from one coupled cell to the other. Interesting questions remain as to whether connexin or pannexin gap junctions serve as the molecular substrate of transmission, and the role of electrical transmission in hippocampal physiology is uncertain. Increased coupling could well contribute to the known tendency of the hippocampus to exhibit seizure activity.

Synaptic mechanisms of synchronized gamma oscillations in inhibitory interneuron networks

Gamma frequency oscillations are thought to provide a temporal structure for information processing in the brain. They contribute to cognitive functions, such as memory formation and sensory processing, and are disturbed in some psychiatric disorders. Fast-spiking, parvalbumin-expressing, soma-inhibiting interneurons have a key role in the generation of these oscillations. Experimental analysis in the hippocampus and the neocortex reveals that synapses among these interneurons are highly specialized. Computational analysis further suggests that synaptic specialization turns interneuron networks into robust gamma frequency oscillators.

Evolutionary selection pressure and family relationships among connexin genes

We suggest an extension ofconnexinorthology relationships across the major vertebrate lineages. We first show that the conserved domains of mammalianconnexins(encoding the N-terminus, four transmembrane domains and two extracellular loops) are subjected to a considerably more strict selection pressure than the full-length sequences or the variable domains (the intracellular loop and C-terminal tail). Therefore, the conserved domains are more useful for the study of family relationships over larger evolutionary distances. The conserved domains ofconnexinswere collected from chicken,Xenopus tropicalis, zebrafish, pufferfish, green spotted pufferfish,Ciona intestinalisandHalocynthia pyriformis(two tunicates). A total of 305connexinsequences were included in this analysis. Phylogenetic trees were constructed, from which the orthologies and the presumed evolutionary relationships between the sequences were deduced. The tunicateconnexinsstudied had the closest, but still distant, relationships to vertebrateconnexin36,39.2,43.4,45and47. The main structure in theconnexinfamily known from mammals pre-dates the divergence of bony fishes, but some additional losses and gains ofconnexinsequences have occurred in the evolutionary lineages of subsequent vertebrates. Thus, theconnexingene family probably originated in the early evolution of chordates, and underwent major restructuring with regard to gene and subfamily structures (including the number of genes in each subfamily) during early vertebrate evolution.

Altered olivocerebellar activity patterns in the connexin36 knockout mouse

The inferior olive (IO) has among the highest densities of neuronal gap junctions in the nervous system. These gap junctions are proposed to be the underlying mechanism for generating synchronous Purkinje cell complex spike (CS) activity. Gap junctions between neurons are formed mostly by connexin36 proteins. Thus, the connexin36 knockout (Cx36KO) mouse provides an opportunity to test whether gap junction coupling between IO neurons is the basis of CS synchrony. Multiple electrode recordings of crus 2 CSs were obtained from wildtype (Wt) and Cx36KO mice. Wts showed statistically significant levels of CS synchrony, with the same spatial distribution as has been reported for other species: high CS synchrony levels occurred mostly among Purkinje cells within the same parasagittally-oriented cortical strip. In contrast, in Cx36KOs, synchrony was at chance levels and had no preferential spatial orientation, supporting the gap junction hypothesis. CS firing rates for Cx36KOs were significantly lower than for Wts, suggesting that electrical coupling is an important determinant of IO excitability. Rhythmic CS activity was present in both Wt and Cx36KOs, suggesting that individual IO cells can act as intrinsic oscillators. In addition, the climbing fiber reflex was absent in the Cx36KOs, validating its use as a tool for assessing electrical coupling of IO neurons. Zebrin II staining and anterograde tracing showed that cerebellar cortical organization and the topography of the olivocerebellar projection are normal in the Cx36KO. Thus, the differences in CS activity between Wts and Cx36KOs likely reflect the loss of electrical coupling of IO cells.

Induction of synchronous oscillatory activity in the rat lateral amygdala in vitro is dependent on gap junction activity

Synchronized and rhythmic activity within the amygdala is thought to play a pivotal role in the generation of fear- and anxiety-related behaviour. The aim here was to determine the validity of the in vitro amygdala slice preparation to investigate the generation of rhythmic activity similar to that observed in vivo. Extracellular population activity recorded from the lateral nucleus of the amygdala in vitro showed significant enhancement of activity within the theta-band frequency (3-9 Hz) in the presence of kainic acid (100 nm; n=18). Alterations in the patterns of oscillatory activity within the gamma frequency band (20-40 Hz) were observed in the presence of (RS)-3,5-dihydroxyphenylglycine (10 microm; n=7) or carbachol (50 microm; n=5). Theta frequency oscillatory activity was blocked in the presence of the gap junction blocker carbenoxolone (100 mm), whereas gamma frequency oscillatory activity showed increased variability in the dominant frequency of rhythmic activity. The results suggest that the neuronal circuitry of the amygdala in vitro is capable of generating and sustaining rhythmic activity and that intercellular communication via gap junctions may play a role in the synchronization of population activity underlying this oscillatory activity.

Whither withered Golgi? A retrospective evaluation of reticularist and synaptic constructs

The 100th anniversary of the shared first Nobel prize in neuroscience by Camillo Golgi and Ramon y Cajal invites reappraisal of the merits of the arguments adduced by these two combative scientists in the light of contemporary knowledge. Guided by cogent reasons for reluctance in accepting the inviolable polarity principle of the neuron doctrine and concern for explaining cerebral recovery of function, Golgi joined the 'reticularists' of his generation. Modern observations of axo-axonic and dendro-dendritic synapses, gap-junction interconnections, rules for the direction and mode of analog or impulse conduction, the myriad diversity of ion channels and gating principles and the complexities of synaptic plasticity have eclipsed the polarized neuron doctrine explanations of reflex physiology and the 'fixed and immutable' connections successfully championed by Cajal. Without violating the cell theory, expanded modes of neuronal and glial communication have encompassed reticularist notions and provided insight into the long-term changes underlying synaptic and extra-synaptic neural patterns. Both laureates espoused operative principles that have survived in different modes and distinctive temporal domains. Together, they reflect the roots of our contemporary understanding of neural interaction.

Identification and dynamics of spontaneous burst initiation zones in unidimensional neuronal cultures

Spontaneous activity is typical of in vitro neural networks, often in the form of large population bursts. The origins of this activity are attributed to intrinsically bursting neurons and to noisy backgrounds as well as to recurrent network connections. Spontaneous activity is often observed to emanate from localized sources or initiation zones, propagating from there to excite large populations of neurons. In this study, we use unidimensional cultures to overcome experimental difficulties in identifying initiation zones in vivo and in dissociated two-dimensional cultures. We found that spontaneous activity in these cultures is initiated exclusively in localized zones that are characterized by high neuronal density but also by recurrent and inhibitory network connections. We demonstrate that initiation zones compete in driving network activity in a winner-takes-most scenario.

Relating the neuron doctrine to the cell theory. Should contemporary knowledge change our view of the neuron doctrine?

The neuron doctrine, formulated in 1891, attacked in 1906 by Golgi and fiercely defended by Cajal, provided a powerful tool for analyzing the pathways of the brain. It has often been described as though it were merely the cell theory applied to nervous systems. In this essay I show that the neuron doctrine claims more than does the cell theory, and that in many instances, where it goes beyond the cell theory, it can no longer be defended on the basis of contemporary evidence. The neuron doctrine should be seen as a practical tool that is particularly useful for understanding the long pathways of the brain; it cannot be regarded as providing an accurate account of what nerve cells in general are really like.

Granular cells of the mormyrid electrosensory lobe and postsynaptic control over presynaptic spike occurrence and amplitude through an electrical synapse

Primary afferent fibers from the electroreceptors of mormyrid electric fish use a latency code to signal the intensity of electrical current evoked by the fish's own electric organ discharge (EOD). The afferent fibers terminate centrally in the deep and superficial granular layers of the electrosensory lobe with morphologically mixed chemical-electrical synapses. The granular cells in these layers seem to decode afferent latency through an interaction between primary afferent input and a corollary discharge input associated with the EOD motor command. We studied the physiology of deep and superficial granular cells in a slice preparation with whole cell patch recording and electrical stimulation of afferent fibers. Afferent stimulation evoked large all-or-none electrical excitatory postsynaptic potentials (EPSPs) and large all or none GABAergic inhibitory postsynaptic potentials (IPSPs) in both superficial and deep granular cells. The amplitudes of the electrical EPSPs depended on postsynaptic membrane potential, with maximum amplitudes at membrane potentials between -65 and -110 mV. Hyperpolarization beyond this level resulted in either the abrupt disappearance of EPSPs, a step-like reduction to a smaller EPSP, or a graded reduction in EPSP amplitude. Depolarization to membrane potentials lower than that yielding a maximum caused a linear decrease in EPSP amplitude, with EPSP amplitude reaching 0 mV at potentials between -55 and -40 mV. We suggest that the dependence of EPSP size on postsynaptic membrane potential is caused by close linkage of pre- and postsynaptic membrane potentials through a high-conductance gap junction. We also suggest that this dependence may result in functionally important nonlinear interactions between synaptic inputs.

What is hidden in the pannexin treasure trove: the sneak peek and the guesswork

Connexins had been considered to be the only class of the vertebrate proteins capable of gap junction formation; however, new candidates for this function with no homology to connexins, termed pannexins were discovered. So far three pannexins were described in rodent and human genomes: Panx1, Panx2 and Panx3. Expressions of pannexins can be detected in numerous brain structures, and now found both in neuronal and glial cells. Hypothetical roles of pannexins in the nervous system include participating in sensory processing, hippocampal plasticity, synchronization between hippocampus and cortex, and propagation of the calcium waves supported by glial cells, which help maintain and modulate neuronal metabolism. Pannexin also may participate in pathological reactions of the neural cells, including their damage after ischemia and subsequent cell death. Recent study revealed non-gap junction function of Panx1 hemichannels in erythrocytes, where they serve as the conduits for the ATP release in response to the osmotic stress. High-throughput studies produced some evidences of the pannexin involvement in the process of tumorigenesis. According to brain cancer gene expression database REMBRANDT, PANX2 expression levels can predict post diagnosis survival for patients with glial tumors. Further investigations are needed to verify or reject hypotheses listed.

Transient electrical coupling regulates formation of neuronal networks

Electrical synapses are abundant before and during developmental windows of intense chemical synapse formation, and might therefore contribute to the establishment of neuronal networks. Transient electrical coupling develops and is then eliminated between regenerating Helisoma motoneurons 110 and 19 during a period of 48-72 h in vivo and in vitro following nerve injury. An inverse relationship exists between electrical coupling and chemical synaptic transmission at these synapses, such that the decline in electrical coupling is coincident with the emergence of cholinergic synaptic transmission. In this study, we have generated two- and three-cell neuronal networks to test whether predicted synaptogenic capabilities were affected by previous synaptic interactions. Electrophysiological analyses demonstrated that synapses formed in three-cell neuronal networks were not those predicted based on synaptogenic outcomes in two-cell networks. Thus, new electrical and chemical synapse formation within a neuronal network is dependent on existing connectivity of that network. In addition, new contacts formed with established networks have little impact on these existing connections. These results suggest that network-dependent mechanisms, particularly those mediated by gap junctional coupling, regulate synapse formation within simple neural networks.

Connexin mRNA expression in single dopaminergic neurons of substantia nigra pars compacta

Dopaminergic neurons of the substantia nigra pars compacta play a major role in goal-directed behavior and reinforcement learning. The study of their local interactions has revealed that they are connected by electrical synapses. Connexins, the molecular substrate of electrical synapses, constitute a multigenic family of 20 proteins in rodents. The permeability and regulation properties of electrical synapses depend on their connexin composition. Therefore, the knowledge of the molecular composition of electrical synapses is fundamental to the understanding of their specific functions. We have investigated the connexin mRNA expression pattern of dopaminergic neurons by single-cell RT-PCR analysis, during two periods in which dopaminergic neurons are electrically coupled in vitro (P7-P10 and P17-P21). Our results show that dopaminergic neurons express mRNAs of various connexins (Cx26, Cx30, Cx31.1, Cx32, Cx36 and Cx43) in a developmentally regulated manner. Furthermore, we have observed that dopaminergic neurons display different connexin expression patterns, and that multiple connexins can be expressed in a single dopaminergic neuron. These observations underline the importance of electrical coupling in the development of dopaminergic neurons and raise the question of the existence of functionally distinct electrically coupled networks in the substantia nigra pars compacta.

Pannexin expression in the cerebellum

Pannexin1 and pannexin2 are members of the pannexin gene family which are widely expressed in the central nervous system. Here we present an overview of pannexin expression and distribution in the mouse cerebellum. Pannexin1 and pannexin2 are expressed in the Purkinje cells and in some cells of the granule cell layer. Pannexin2 is also expressed in the stellate cells of the molecular layer. A differential expression of pannexin1 and pannexin2 mRNA is observed during cerebellar development. These findings constitute the first indication of the involvement of pannexin molecules in the developing cerebellum. Although the functional relevance of these molecules remains currently unknown, the abundance of pannexins in the Purkinje cells suggests that they may contribute to the generation of cerebellar rhythms.

Propagation of postsynaptic currents and potentials via gap junctions in GABAergic networks of the rat hippocampus

The integration of synaptic signalling in the mammalian hippocampus underlies higher cognitive functions such as learning and memory. We have studied the gap junction-mediated cell-to-cell and network propagation of GABA(A) receptor-mediated events in stratum lacunosum moleculare interneurons of the rat hippocampus. Propagated events were identified both in voltage- and current-clamp configurations. After blockade of ionotropic excitatory synaptic transmission, voltage-clamp recordings with chloride-loaded electrodes (predicted GABA(A) receptor reversal potential: 0 mV) at -15 mV revealed the unexpected presence of spontaneous events of opposite polarities. Inward events were larger and kinetically faster when compared to outward currents. Both types of events were blocked by gabazine, but only outward currents were significantly affected by the gap junction blocker carbenoxolone, indicating that outward events originated in electrically coupled neurons. These results were in agreement with computational modelling showing that propagated events were modulated in size and shape by their relative distance to the gap junction site. Paired recordings from electrically coupled interneurons performed with high- and low-chloride pipettes (predicted GABA(A) receptor reversal potentials: 0 mV and -80 mV, respectively) directly demonstrated that depolarizing postsynaptic events could propagate to the cell recorded with the low-chloride solution. Cell-to-cell propagation was abolished by carbenoxolone, and was not observed in uncoupled pairs. Application of 4-aminopyridine on slices resulted in spontaneous network activation of interneurons, which was driven by excitatory GABA(A) receptor-mediated input. Population activity was greatly depressed by carbenoxolone, suggesting that propagation of depolarizing synaptic GABAergic potentials may be a critical determinant of interneuronal synchronous bursting in the hippocampus.

White-noise susceptibility and critical slowing in neurons near spiking threshold

We present mathematical and simulation analyses of the below-threshold noisy response of two biophysically motivated models for excitable membrane due to H. R. Wilson: a squid axon ("resonator") and a human cortical neuron ("integrator"). When stimulated with a low-intensity white noise superimposed on a dc control current, both membrane types generate voltage fluctuations that exhibit critical slowing down--that is, the voltage responsiveness to noisy input currents grows in amplitude while slowing in frequency--as the membrane approaches spiking threshold from below. We define threshold unambiguously as that dc current that renders a zero real eigenvalue for the Jacobian matrix for the integrator neuron, and, for the resonator neuron, as the dc current that gives a complex eigenvalue pair whose real part is zero. Using a linear Ornstein-Uhlenbeck analysis, we give exact small-noise expressions for the variance, power spectrum, and correlation function of the voltage fluctuations, and we derive the scaling laws for the divergence of susceptibility and correlation times for approach to threshold. We compare these predictions with numerical simulations of the nonlinear stochastic equations, and demonstrate that, provided the white-noise perturbations are kept sufficiently small, the linearized theory works well. These predictions should be testable in the laboratory using a current-clamped cell configuration. If confirmed, then the proximity of a neuron to its spike-transition point can be judged by measuring its subthreshold susceptibility to white-noise stimulation. We postulate that such temporally correlated fluctuations could provide a means of subthreshold signaling via gap-junction connections with neighboring neurons.

A single evoked afterdischarge produces rapid time-dependent changes in connexin36 protein expression in adult rat dorsal hippocampus

Gap junctions between neurons contribute to synchronous neuronal firing and may play a role in the pathophysiology of epilepsy. We examined the expression of a number of gap junction subunits, including the neuronal gap junction forming protein connexin36 (Cx36), in the hippocampus at various time points following an electrically stimulated afterdischarge (AD) in freely-moving animals. Once recovered from electrode implantation, animals were tested with an escalating series of stimulations until an AD was evoked. Suprathreshold stimulation produced a brief AD with no convulsion. Groups of animals were sacrificed at 3, 12, and 24h post-stimulation, and connexin expression was assessed via semiquantitative immunoblotting. Compared to implanted non-stimulated controls, a significant decrease in Cx36 expression was observed in the stimulated dorsal hippocampus at 3h post-stimulation, which returned to control levels by 24h. No changes were seen in the ventral hippocampus. As well, no changes were seen in other selected connexin proteins including Cx26, Cx32, and Cx43, thought to be expressed primarily in glia, in either dorsal or ventral hippocampus. These data suggest that a relatively brief hypersynchronous neuronal discharge can produce rapid and specific changes in Cx36 expression, which may have implications for both normal brain function and the pathophysiology of epilepsy.

Localization of heterotypic gap junctions composed of connexin45 and connexin36 in the rod pathway of the mouse retina

The primary rod pathway in mammals contains gap junctions between AII amacrine cells and ON cone bipolar cells which relay the rod signal into the cone pathway under scotopic conditions. Two gap junctional proteins, connexin36 (Cx36) and connexin45 (Cx45), appear to play a pivotal role in this pathway because lack of either protein leads to an impairment of visual transmission under scotopic conditions. To investigate whether these connexins form heterotypic gap junctions between ON cone bipolar and AII amacrine cells, we used newly developed Cx45 antibodies and studied the cellular and subcellular distribution of this protein in the mouse retina. Specificity of the Cx45 antibodies was determined, among others, by Western blot and immunostaining of mouse heart, where Cx45 is abundantly expressed. In mouse retina, Cx45 immunosignals were detected in both plexiform layers and the ganglion cell layer. Double staining for Cx45 and Cx36 revealed a partial overlap in the punctate patterns in the ON sublamina of the inner plexiform layer of the retina. We quantified the distributions of these two connexins in the ON sublamina, and detected 30% of the Cx45 signals to be co-localized with or in close apposition to Cx36 signals. Combining immunostaining and intracellular dye injection revealed an overlap or tight association of Cx36 and Cx45 signals on the terminals of injected AII amacrine and two types of ON cone bipolar cells. Our results provide direct evidence for heterotypic gap junctions composed of Cx36 and Cx45 between AII amacrine and certain types of ON cone bipolar cells.

Synapse development: still looking for the forest, still lost in the trees

Synapse development in the vertebrate central nervous system is a highly orchestrated process occurring not only during early stages of brain development, but also (to a lesser extent) in the mature nervous system. During development, the formation of synapses is intimately linked to the differentiation of neuronal cells, the extension of their axons and dendrites, and the course wiring of the nervous system. Subsequently, the stabilization, elimination, and strengthening of synaptic contacts is coupled to the refinement of axonal and dendritic arbors, to the establishment of functionally meaningful connections, and probably also to the day-to-day acquisition, storage, and retrieval of memories, higher order thought processes, and behavioral patterns.

Cell-cell communication beyond connexins: the pannexin channels

Direct cell-to-cell communication through specialized intercellular channels is a characteristic feature of virtually all multi-cellular organisms. The remarkable functional conservation of cell-to-cell coupling throughout the animal kingdom, however, is not matched at the molecular level of the structural protein components. Thus protostomes (including nematodes and flies) and deuterostomes (including all vertebrates) utilize two unrelated families of gap-junction genes, innexins and connexins, respectively. The recent discovery that pannexins, a novel group of proteins expressed by several organisms, are able to form intercellular channels has started a quest to understand their evolutionary relationship and functional contribution to cell communication in vivo. There are three pannexin genes in mammals, two of which are co-expressed in the developing and adult brain. Of note, pannexin1 can also form Ca2+-activated hemichannels that open at physiological extracellular Ca2+ concentrations and exhibit distinct pharmacological properties.

Structure and function of gap junctions in the developing brain

Gap-junction-dependent neuronal communication is widespread in the developing brain, and the prevalence of gap-junctional coupling is well correlated with specific developmental events. We summarize here our current knowledge of the contribution of gap junctions to brain development and propose that they carry out this role by taking advantage of the full complement of their functional properties. Thus, hemichannel activation may represent a key step in the initiation of Ca(2+) waves that coordinate cell cycle events during early prenatal neurogenesis, whereas both hemichannels and/or gap junctions may control the division and migration of cohorts of precursor cells during late prenatal neurogenesis. Finally, the recent discovery that pannexins, a novel group of proteins prominently expressed in the brain, are able to form both hemichannels and gap-junction channels suggests that we need to seek more than just connexins with respect to these junctions.

A novel fluorescent tracer for visualizing coupled cells in neural circuits of living tissue

Gap junctions have diverse roles in a wide variety of tissues and have recently become a subject of intense investigation in neural circuits where synchrony and oscillations may play an important part. In circuits where gap junctions are present, the possibility arises of identifying intercommunicating cells via introduction of tracer into one cell and observing its spread into its coupled neighbors. Staining the coupled cells by this means opens the door to many vital techniques including paired-cell electrophysiology, RT-PCR, and morphological characterization of previously unknown coupled cells. Tracers commonly used at the present time are not generally suitable for these purposes in many tissues, including neurons. This paper describes how a fluorescent nuclear tracer, Po-pro-1, can be used to visualize coupled cells in several types of retinal neurons thought to be comprised of different connexin proteins including Cx36, Cx45, Cx50, and Cx57.

Timing in cognition and EEG brain dynamics: discreteness versus continuity

This article provides an overview of recent developments in solving the timing problem (discreteness vs. continuity) in cognitive neuroscience. Both theoretical and empirical studies have been considered, with an emphasis on the framework of operational architectonics (OA) of brain functioning (Fingelkurts and Fingelkurts in Brain Mind 2:291-29, 2001; Neurosci Biobehav Rev 28:827-836, 2005). This framework explores the temporal structure of information flow and interarea interactions within the network of functional neuronal populations by examining topographic sharp transition processes in the scalp EEG, on the millisecond scale. We conclude, based on the OA framework, that brain functioning is best conceptualized in terms of continuity-discreteness unity which is also the characteristic property of cognition. At the end we emphasize where one might productively proceed for the future research.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Association of connexin36 and zonula occludens-1 with zonula occludens-2 and the transcription factor zonula occludens-1-associated nucleic acid-binding protein at neuronal gap junctions in rodent retina

Most gap junctions between neurons in mammalian retina contain abundant connexin36, often in association with the scaffolding protein zonula occludens-1. We now investigate co-association of connexin36, zonula occludens-1, zonula occludens-2 and Y-box transcription factor 3 (zonula occludens-1-associated nucleic acid-binding protein) in mouse and rat retina. By immunoblotting, zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were both detected in retina, and zonula occludens-2 in retina was found to co-immunoprecipitate with connexin36. By immunofluorescence, the four proteins appeared as puncta distributed in the plexiform layers. In the inner plexiform layer, most connexin36-puncta were co-localized with zonula occludens-1, and many were co-localized with zonula occludens-1-associated nucleic acid-binding protein. Moreover, zonula occludens-1-associated nucleic acid-binding protein was often co-localized with zonula occludens-1. Nearly all zonula occludens-2-puncta were positive for connexin36, zonula occludens-1 and zonula occludens-1-associated nucleic acid-binding protein. In the outer plexiform layer, connexin36 was also often co-localized with zonula occludens-1-associated nucleic acid-binding protein. In connexin36 knockout mice, labeling of zonula occludens-1 was slightly reduced in the inner plexiform layer, zonula occludens-1-associated nucleic acid-binding protein was decreased in the outer plexiform layer, and both zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were markedly decreased in the inner sublamina of the inner plexiform layer, whereas zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein puncta persisted and remained co-localized in the outer sublamina of the inner plexiform layer. By freeze-fracture replica immunogold labeling, connexin36 was found to be co-localized with zonula occludens-2 within individual neuronal gap junctions. In addition, zonula occludens-1-associated nucleic acid-binding protein was abundant in a portion of ultrastructurally-defined gap junctions throughout the inner plexiform layer, and some of these junctions contained both connexin36 and zonula occludens-1-associated nucleic acid-binding protein. These distinct patterns of connexin36 association with zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein in different sublaminae of retina, and differential responses of these proteins to connexin36 gene deletion suggest differential regulatory and scaffolding roles of these gap junction accessory proteins. Further, the persistence of a subpopulation of zonula occludens-1/zonula occludens-2/zonula occludens-1-associated nucleic acid-binding protein co-localized puncta in the outer part of the inner plexiform layer of connexin36 knockout mice suggests close association of these proteins with other structures in retina, possibly including gap junctions composed of an as-yet-unidentified connexin.

Chlorpromazine reduces the intercellular communication via gap junctions in mammalian cells

In the work presented herein, we evaluated the effect of chlorpromazine (CPZ) on gap junctions expressed by two mammalian cell types; Gn-11 cells (cell line derived from mouse LHRH neurons) and rat cortical astrocytes maintained in culture. We also attempted to elucidate possible mechanisms of action of CPZ effects on gap junctions. CPZ, in concentrations comparable with doses used to treat human diseases, was found to reduce the intercellular communication via gap junctions as evaluated with measurements of dye coupling (Lucifer yellow). In both cell types, maximal inhibition of functional gap junctions was reached within about 1 h of treatment with CPZ, an recovery was almost complete at about 5 h after CPZ wash out. In both cell types, CPZ treatment increased the phosphorylation state of connexin43 (Cx43), a gap junction protein subunit. Moreover, CPZ reduced the reactivity of Cx43 (immunofluorescence) at cell interfaces and concomitantly increased its reactivity in intracellular vesicles, suggesting an increased retrieval from and/or reduced insertion into the plasma membrane. CPZ also caused cellular retraction reducing cell-cell contacts in a reversible manner. The reduction in contact area might destabilize existing gap junctions and abrogate formation of new ones. Moreover, the CPZ-induced reduction in gap junctional communication may depend on the connexins (Cxs) forming the junctions. If Cx43 were the only connexin expressed, MAPK-dependent phosphorylation of this connexin would induce closure of gap junction channels.

Effects of gap junction blockers on human neocortical synchronization

Field potentials and intracellular recordings were obtained from human neocortical slices to study the role of gap junctions (GJ) in neuronal network synchronization. First, we examined the effects of GJ blockers (i.e., carbenoxolone, octanol, quinine, and quinidine) on the spontaneous synchronous events (duration = 0.2-1.1 s; intervals of occurrence = 3-27 s) generated by neocortical slices obtained from temporal lobe epileptic patients during application of 4-aminopyridine (4AP, 50 muM) and glutamatergic receptor antagonists. The synchronicity of these potentials (recorded at distances up to 5 mm) was decreased by GJ blockers within 20 min of application, while prolonged GJ blockers treatment at higher doses made them disappear with different time courses. Second, we found that slices from patients with focal cortical dysplasia (FCD) could generate in normal medium spontaneous synchronous discharges (duration = 0.4-8 s; intervals of occurrence = 0.5-90 s) that were (i) abolished by NMDA receptor antagonists and (ii) slowed down by carbenoxolone. Finally, octanol or carbenoxolone blocked 4AP-induced ictal-like discharges (duration = up to 35 s) in FCD slices. These data indicate that GJ play a role in synchronizing human neocortical networks and may implement epileptiform activity in FCD.

Gap junction blockade does not alter eupnea or gasping in the juvenile rat

The role of gap junctions in the brainstem respiratory control system is ambiguous. In the present study, we used juvenile rats to determine whether blocking gap junctions altered eupnea or gasping in the in situ, arterially perfused rat preparation. Blockade of gap junctions with 100 microM carbenoxolone or 300 microM octanol did not produce any consistent changes in the timing or amplitude of integrated phrenic discharge or in the peak frequency in the power spectrum of phrenic nerve discharge during eupnea or ischemic gasping beyond those changes seen in time-control animals. These findings do not rule out a role for gap junctions in the expression of eupnea or gasping, but they do demonstrate that these intermembrane channels are not obligatory for either rhythm to occur.

Effects in vitro and in vivo of a gap junction blocker on epileptiform activities in a genetic model of absence epilepsy

We investigated the effects of carbenoxolone (CBX), a gap junctions (GJ) blocker, on epileptiform activities in vivo and in vitro. In a first series of experiments, i.p. CBX decreased the cumulative duration of cortical spike-wave discharges (SWD) in adult Genetic Absence Epilepsy Rats from Strasbourg (GAERS) without reduction in the SW amplitude or frequency. Since SWD are generated in thalamocortical networks, we studied the effect of CBX on thalamic and cortical activities elicited by 4-aminopyridine (4AP) in thalamocortical slices from GAERS or non-epileptic rats (NER). Spontaneous ictal-like activities (ILA) were recorded simultaneously in thalamus and somatosensory cortex. However, experiments where these structures were surgically separated showed that ILA were generated in the cortex and recorded by volume conduction in the thalamus. GABA-dependent negative field potentials were also recorded in the cortex, either isolated or initiating ILA. After bath-applying CBX (100 microM), the frequency and cumulative duration of ILA decreased but less rapidly in GAERS than in NER slices and they disappeared at a time point when GABA-dependent negative potentials remained. These data suggest that GJ do not mediate the 4AP induced interneuronal synchronisation but may be implicated in the spreading of the synchronised activities from interneuronal networks to principal neurones. Our results show that CBX exerts an antiepileptic action in vivo, and that GJ blockers limits spread of synchronised activities in vitro. They may represent an appropriate target for development of new antiepileptic drugs.

Nerve growth factor increases connexin43 phosphorylation and gap junctional intercellular communication

The function of gap junctions is regulated by the phosphorylation state of their connexin subunits. Numerous growth factors are known to regulate connexin phosphorylation; however, the effect of nerve growth factor on gap junction function is not understood. The phosphorylation of connexin subunits is a key event during many aspects of the lifecycle of a connexin, including open/close states, assembly/trafficking, and degradation, and thus affects the functionality of the channel. PC12 cells infected with connexin43 (Cx43) retrovirus were used as a neuronal model to characterize the signal transduction pathways activated by nerve growth factor (NGF) that potentially affect the functional state of Cx43. Immunoblot analysis demonstrated that Cx43 and the mitogen-activated protein kinase (MAPK), ERK-1/2, were phosphorylated in response to TrkA activation via NGF and that phosphorylation could be prevented by treatment with the MEK-1/2 inhibitor U0126. The effects of NGF on gap junction intercellular communication were examined by monitoring fluorescence recovery after photobleaching PC12-Cx43 cells preloaded with calcein. Fluorescence recovery in the photobleached area increased after NGF treatment and decreased when pretreated with the MEK-1/2 inhibitor U0126. These data are the first to show a direct signaling link between neurotrophins and the phosphorylation of connexin proteins through the MAPK pathway resulting in increased gap junctional intercellular communication. Neurotrophic regulation of connexin activity provides a novel mechanism of regulating intercellular communication between neurons during nervous system development and repair.

Gap junctional coupling and connexin immunoreactivity in rabbit retinal glia

Gap junctions provide a pathway for the direct intercellular exchange of ions and small signaling molecules. Gap junctional coupling between retinal astrocytes and between astrocytes and Müller cells, the principal glia of vertebrate retinas, has been previously demonstrated by the intercellular transfer of gap-junction permeant tracers. However, functional gap junctions have yet to be demonstrated between mammalian Müller cells. In the present study, when the gap-junction permeant tracers Neurobiotin and Lucifer yellow were injected into a Müller cellviaa patch pipette, the tracers transferred to at least one additional cell in more than half of the cases examined. Simultaneous whole-cell recordings from pairs of Müller cells in the isolated rabbit retina revealed electrical coupling between closely neighboring cells, confirming the presence of functional gap junctions between rabbit Müller cells. The limited degree of this coupling suggests that Müller cell–Müller cell gap junctions may coordinate the functions of small ensembles of these glial cells. Immunohistochemistry and immunoblotting were used to identify the connexins in rabbit retinal glia. Connexin30 (Cx30) and connexin43 (Cx43) immunoreactivities were associated with astrocytes in the medullary ray region of the retinas of both pigmented and albino rabbits. Connexin43 was also found in Müller cells, but antibody recognition differed between astrocytic and Müller cell connexin43.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Low Conductance Gap Junctions Mediate Specific Electrical Coupling in Body-wall Muscle Cells of Caenorhabditis elegans

Invertebrate innexins and their mammalian homologues, the pannexins, are gap junction proteins. Although a large number of such proteins have been identified, few of the gap junctions that they form have been characterized to provide combined information of biophysical properties, coupling pattern, and molecular compositions. We adapted the dual whole cell voltage clamp technique to in situ analysis of electrical coupling in Caenorhabditis elegans body-wall muscle. We found that body-wall muscle cells were electrically coupled in a highly organized and specific pattern. The coupling was characterized by small (350 pS or less) junctional conductance (G(j)), which showed a bell-shaped relationship with junctional potential (V(j)) but was independent of membrane potential (V(m)). Injection of currents comparable to the junctional current (I(j)) into body-wall muscle cells caused significant depolarization, suggesting important functional relevance. The innexin UNC-9 appeared to be a key component of the gap junctions. Both Myc- and green fluorescent protein-tagged UNC-9 was localized to muscle intercellular junctions. G(j) was greatly inhibited in unc-9(fc16), a putative null mutant. Specific inhibition of UNC-9 function in muscle cells reduced locomotion velocity. Despite UNC-9 expression in both motor neurons and body-wall muscle cells, analyses of miniature and evoked postsynaptic currents in the unc-9 mutant showed normal neuromuscular transmission. These analyses provide a relatively detailed description of innexin-based gap junctions in a native tissue and suggest that innexin-based small conductance gap junctions can play an important role in processes such as locomotion.

Gap-junctional coupling between neurogliaform cells and various interneuron types in the neocortex

Electrical synapses contribute to the generation of synchronous activity in neuronal networks. Several types of cortical GABAergic neurons acting via postsynaptic GABA(A) receptors also form electrical synapses with interneurons of the same class, suggesting that synchronization through gap junctions could be limited to homogenous interneuron populations. Neurogliaform cells elicit combined GABA(A) and GABA(B) receptor-mediated postsynaptic responses in cortical pyramidal cells, but it is not clear whether neurogliaform cells are involved in networks linked by electrical coupling. We recorded from pairs, triplets, and quadruplets of cortical neurons in layers 2 and 3 of rat somatosensory cortex (postnatal day 20-35). Neurogliaform cells eliciting slow IPSPs on pyramidal cells also triggered divergent electrical coupling potentials on interneurons. Neurogliaform cells were electrically coupled to other neurogliaform cells, basket cells, regular-spiking nonpyramidal cells, to an axoaxonic cell, and to various unclassified interneurons showing diverse firing patterns and morphology. Electrical interactions were mediated by one or two electron microscopically verified gap junctions linking the somatodendritic domain of the coupled cells. Our results suggest that neurogliaform cells have a unique position in the cortical circuit. Apart from eliciting combined GABA(A) and GABA(B) receptor-mediated inhibition on pyramidal cells, neurogliaform cells establish electrical synapses and link multiple networks formed by gap junctions restricted to a particular class of interneuron. Widespread electrical connections might enable neurogliaform cells to monitor the activity of different interneurons acting on GABA(A) receptors at various regions of target cells.

Lateral excitation within the olfactory bulb

Lateral inhibition is a common feature of cortical networks, serving such functions as contrast enhancement. In the olfactory bulb, inhibition is imbedded in the local connectivity at dendrodendritic synapses between mitral cells and interneurons. However, there is also evidence for excitatory interactions between mitral cells despite the lack of direct synaptic connections. This lateral excitation, although a less well recognized feature of the circuit, provides a potentially powerful mechanism to enhance coordinated activity. We examined lateral excitation in paired recordings between mitral cells projecting to the same glomerulus. Trains of action potentials in one mitral cell evoked autoexcitation in the stimulated cell and a prolonged depolarization in the second cell. This lateral excitation was absent in connexin36(-/-) mice, which lack mitral-mitral cell gap junctions. However, spillover of dendritically released glutamate contributed to lateral excitation during concerted mitral cell excitation or by single-cell activity if glutamate uptake was blocked. Our results suggest that electrical coupling and spillover create a lateral excitatory network within the glomerulus, thus markedly amplifying the sensitivity of each glomerulus to incoming sensory input.

Neurogliaform neurons form a novel inhibitory network in the hippocampal CA1 area

We studied neurogliaform neurons in the stratum lacunosum moleculare of the CA1 hippocampal area. These interneurons have short stellate dendrites and an extensive axonal arbor mainly located in the stratum lacunosum moleculare. Single-cell reverse transcription-PCR showed that these neurons were GABAergic and that the majority expressed mRNA for neuropeptide Y. Most neurogliaform neurons tested were immunoreactive for alpha-actinin-2, and many stratum lacunosum moleculare interneurons coexpressed alpha-actinin-2 and neuropeptide Y. Neurogliaform neurons received monosynaptic, DNQX-sensitive excitatory input from the perforant path, and 40 Hz stimulation of this input evoked EPSCs displaying either depression or initial facilitation, followed by depression. Paired recordings performed between neurogliaform neurons showed that 85% of pairs were electrically connected and 70% were also connected via GABAergic synapses. Injection of sine waveforms into neurons during paired recordings resulted in transmission of the waveforms through the electrical synapse. Unitary IPSCs recorded from neurogliaform pairs readily fatigued, had a slow decay, and had a strong depression of the synaptic response at a 5 Hz stimulation frequency that was antagonized by the GABA(B) antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid (CGP55845). The amplitude of the first IPSC during the 5 Hz stimulation was also increased by CGP55845, suggesting a tonic inhibition of synaptic transmission. A small unitary GABA(B)-mediated IPSC could also be detected, providing the first evidence for such a component between GABAergic interneurons. Electron microscopic localization of the GABA(B1) subunit at neurogliaform synapses revealed the protein in both presynaptic and postsynaptic membranes. Our data disclose a novel interneuronal network well suited for modulating the flow of information between the entorhinal cortex and CA1 hippocampus.

Representation of auditory signals in the M-cell: role of electrical synapses

The teleost Mauthner (M-) cell mediates a sound-evoked escape behavior. A major component of the auditory input is transmitted by large myelinated club endings of the posterior VIIIth nerve. Paradoxically, although nerve stimulations revealed these afferents have mixed electrical and glutamatergic synapses on the M-cell's distal lateral dendrite, paired pre- and postsynaptic recordings indicated most individual connections are chemically silent. To determine the sensory information encoded and the relative contributions of these two transmission modes, M-cell responses to acoustic stimuli in air were recorded intracellularly. Excitatory postsynaptic potentials (EPSPs) evoked by both short 100- to 900-Hz "pips" and longer-lasting amplitude- and frequency-modulated sounds were dominated by fast, repetitive EPSPs superimposed on an underlying slow depolarization. Fast EPSPs 1) have kinetics comparable to presynaptic action potentials, 2) are maximal on the distal lateral dendrite, and 3) are insensitive to GluR antagonists. They presumably are coupling potentials, and power spectral analysis indicated they constitute a high-pass signal that accurately tracks sound frequency and amplitude. The spatial profile of the slow EPSP suggests both proximal and distal dendritic sources, a result supported by predictions of a multicompartmental model and the effects of AMPAR antagonists, which preferentially reduced the proximal component. Thus a second class of afferents generates a portion of the slow EPSP that, with sound stimuli, demonstrate that the dominant mode of transmission at LMCE synapses is electrical. The slow EPSP is a dynamic, low-pass representation of stimulus strength. Accordingly, amplitude and phase information, which are segregated in other systems, are faithfully represented in the M-cell.

What is the function of the claustrum?

The claustrum is a thin, irregular, sheet-like neuronal structure hidden beneath the inner surface of the neocortex in the general region of the insula. Its function is enigmatic. Its anatomy is quite remarkable in that it receives input from almost all regions of cortex and projects back to almost all regions of cortex. We here briefly summarize what is known about the claustrum, speculate on its possible relationship to the processes that give rise to integrated conscious percepts, propose mechanisms that enable information to travel widely within the claustrum and discuss experiments to address these questions.

Long-term modulation of electrical synapses in the mammalian thalamus

Electrical synapses are common between inhibitory neurons in the mammalian thalamus and neocortex. Synaptic modulation, which allows flexibility of communication between neurons, has been studied extensively at chemical synapses, but modulation of electrical synapses in the mammalian brain has barely been examined. We found that the activation of metabotropic glutamate receptors, via endogenous neurotransmitter or by agonist, causes long-term reduction of electrical synapse strength between the inhibitory neurons of the rat thalamic reticular nucleus.