Weak Electrical Coupling Research Articles

Electrical coupling between model midbrain dopamine neurons: effects on firing pattern and synchrony.

The role of gap junctions between midbrain dopamine (DA) neurons in mechanisms of firing pattern generation and synchronization has not been well characterized experimentally. We modified a multi-compartment model of DA neuron by adding a spike-generating mechanism and electrically coupling the dendrites of two such neurons through gap junctions. The burst-generating mechanism in the model neuron results from the interaction of a N-methyl-D-aspartate (NMDA)-induced current and the sodium pump. The firing patterns exhibited by the two model neurons included low frequency (2-7 Hz) spiking, high-frequency (13-20 Hz) spiking, irregular spiking, regular bursting, irregular bursting, and leader/follower bursting, depending on the parameter values used for the permeability for NMDA-induced current and the conductance for electrical coupling. All of these firing patterns have been observed in physiological neurons, but a systematic dependence of the firing pattern on the covariation of these two parameters has not been established experimentally. Our simulations indicate that electrical coupling facilitates NMDA-induced burst firing via two mechanisms. The first can be observed in a pair of identical cells. At low frequencies (low NMDA), as coupling strength was increased, only a transition from asynchronous to synchronous single-spike firing was observed. At high frequencies (high NMDA), increasing the strength of the electrical coupling in an identical pair resulted in a transition from high-frequency single-spike firing to burst firing, and further increases led to synchronous high-frequency spiking. Weak electrical coupling destabilizes the synchronous solution of the fast spiking subsystems, and in the presence of a slowly varying sodium concentration, the desynchronized spiking solution leads to bursts that are approximately in phase with spikes that are not in phase. Thus this transitional mechanism depends critically on action potential dynamics. The second mechanism for the induction of burst firing requires a heterogeneous pair that is, respectively, too depolarized and too hyperpolarized to burst. The net effect of the coupling is to bias at least one cell into an endogenously burst firing regime. In this case, action potential dynamics are not critical to the transitional mechanism. If electrical coupling is indeed more prominent in vivo due to basal level of modulation of gap junctions in vivo, these results may indicate why NMDA-induced burst firing is easier to observe in vivo as compared in vitro.

Characterization of the Aplysia californica cerebral ganglion F cluster.

The cerebral ganglia neurons of Aplysia californica are involved in the development and modulation of many behaviors. The medially located F cluster has been characterized using morphological, electrophysiological and biochemical techniques and contains at least three previously uncharacterized neuronal population. As the three subtypes are located in three distinct layers, they are designated as top, middle, and bottom layer F-cluster neurons (CFT, CFM, and CFB). The CFT cells are large (92 +/- 25 microm), white, nonuniformly shaped, and located partially in the sheath surrounding the ganglion. These neurons exhibit weak electrical coupling, the presence of synchronized spontaneous changes in membrane potential, and a generalized inhibitory input upon electrical stimulation of the anterior tentacular (AT) nerve. Similar to the CFT neurons, the CFM neurons (46 +/- 12 microm) are mainly silent but do not show electrical coupling or synchronized changes in membrane potential. Unlike the CFT neurons, the CFM neurons exhibit weak action potential broadening during constant current injection. Comparison of the peptide profiles of CFT, CFM, and CFB (10-30 microm) neurons using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry demonstrates distinct peptide molecular weights for each neuronal subtype with the masses of these peptides not matching any previously characterized peptides from A. californica. The mass spectra obtained from the AT nerve are similar to the CFT neuron mass spectra, while upper labial nerve contains many peptides observed in the CFM neurons located in nongranular neuron region.

The ac-voltammetric study of fast charge transfer processes in benzene solutions

Abstract Oxidation of ferrocene (Fc) and reduction of p-dicyanobenzene (DCB) were examined by ac-voltammetry in benzene solutions containing tetra-n-hexylammonium perchlorate at a hemispherical (Hg) and a disk (Au) μm-electrode, respectively. The specific conductivity of benzene solutions indicates that the degree of dissociation of the supporting electrolyte is less than 1%. The low concentration of free ions combined with the low dielectric permittivity of the solvent cause large double layer effects evident from the values of the charge transfer coefficients (0.35 and 0.69 for Fc and DCB, respectively). In addition, the low concentration of ions causes problems in measurements of charge transfer kinetics if the concentration of the depolarizer exceeds 0.2 mM. The standard rate constants obtained in benzene solutions (0.22 and 0.15 cm s−1 for Fc and DCB, respectively) are not much different from values observed in common polar solvents. Apparently, in benzene solutions, very weak electrical coupling of the activated state with solvent molecules is compensated by very strong interactions with counter ions and large double layer effects. The experimental results indicate the necessity of taking into account ionic relaxation processes in a theoretical description of interfacial charge transfer dynamics.

Electrical coupling, without dye coupling, between mammalian astrocytes and oligodendrocytes in cell culture

Evidence of electrical and dye coupling between oligodendrocytes and astrocytes was sought in cultures of mouse spinal cord. Cell identity was verified using cell specific antigenic markers. In most experiments current was injected into oligodendrocytes while recording voltage in nearby astrocytes. Nine of 17 oligodendrocyte-astrocyte cell pairs showed weak electrical coupling; the average estimated coupling ratio was 0.03 +/- 0.06 (cf. 0.11 for oligodendrocyte-oligodendrocyte and 0.44 for astrocyte-astrocyte pairs; Kettenmann and Ransom: Glia, 1: 64-73, 1988). Application of 0.5 mM BaCl2 or 44.6 mM CsCl depolarized astrocytes and oligodendrocytes and was estimated to increase the coupling ratio between these cells 3-5-fold; these effects were rapid in onset and completely reversible. In 5 of 7 cases, oligodendrocyte-astrocyte pairs that appeared uncoupled in normal solution exhibited coupling during Ba++ or Cs+ exposure. The actions of these cations are believed to be mediated by blockade of glial K+ channels. Depolarization, per se, as induced by increasing [K+]o, did not increase coupling ratio. The fluorescent dye lucifer yellow (LY) was injected into 10 oligodendrocytes, 8 of which were electrically coupled to nearby astrocytes, and never passed into astrocytes in detectable quantities. Likewise, astrocytes injected with LY stained other astrocytes, but never oligodendrocytes. These findings document the presence of weak electrical coupling between astrocytes and oligodendrocytes, in the absence of dye coupling. Weak coupling of this sort could subserve metabolic interactions between these cells mediated by the passage of small but important molecules such as cyclic AMP, but would not allow strong electrical interactions. If such coupling among glial cells is widespread, it would constitute a "metabolic syncytium" that could serve to coordinate glial behavior.

Signal clipping by the rod output synapse.

The properties of synapses between retinal neurons make an essential contribution to early visual processing. Light produces a graded hyperpolarization in photoreceptors, up to 25 mV in amplitude, and it is conventionally assumed that all of this response range is available for coding visual information. We report here, however, that the rod output synapse rectifies strongly, so that only potential changes within 5 mV of the rod dark potential are transmitted effectively to postsynaptic horizontal cells. This finding is consistent with the voltage-dependence of the calcium current presumed to control neurotransmitter release from rods. It suggests functional roles for the strong electrical coupling of adjacent rods and the weak electrical coupling of adjacent rods and cones. The existence of photoreceptor coupling resolves the apparent paradox that rods have a 25 mV response range, while signals greater than 5 mV in amplitude are clipped during synaptic transmission. We predict that the strengths of rod-rod and rod-cone coupling are quantitatively linked to the relationship between the rod response range and the synapse operating range.

Neural Control of Heart Beat in the African Giant Snail, Achatina Fulica Férussac: II. Interconnections Among the Heart Regulatory Neurones

ABSTRACT The synaptic connections between identified heart regulatory neurones were examined in the central nervous system of the African giant snail, Achatina fulica Férussac. Two cerebral ganglion cells, the dorsal right and left cerebral distinct neurones (d-RCDN and d-LCDN), were found to have excitatory connections with several neurones in the suboesophageal ganglia (the periodically oscillating neurone, PON, the tonically autoactive neurones, TAN, TAN-2 and TAN-3, and the visceral intermittent firing neurone, VIN) and the connections are probably monosynaptic. VIN had a weak electrical coupling with PON. VIN inhibited TAN, TAN-2 and TAN-3, and the connections were considered to be monosynaptic. At the same time, TAN, TAN-2, TAN-3 and the visceral ganglion neurone (VG1) inhibited PON and VIN although the connections are unlikely to be monosynaptic. Another neurone in the pedal ganglia, the dorsal left pedal large neurone (d-LPeLN), was found to excite PON, VIN, TAN, TAN-2 and TAN-3. These connections were not monosynaptic. These results are interpreted in relation to heart regulation in Achatina.

Electrophysiological and pharmacological properties of cultured rat thyroid cells.

The electrophysiological properties of a hormone-dependent, differentiated thyroid epithelial cell strain were studied using intracellular microelectrodes. The average membrane potential of solitary, isolated cells was -78.4 +/- 1.3 mV. The membrane potential depolarized 55 mV per tenfold increase in extracellular potassium concentration. Weak electrical coupling was recorded between contiguous cells. Like thyroid cells in vivo, these cells did not generate action potentials. In some cells a spontaneous, slow transition in the membrane potential from -80mV to -30 mV was accompanied by an increase in input resistance. Membrane potential transitions could be induced by perfusing cells with isotonic Hanks solutions saturated with CO2 (pH = 5.5) or by perfusing cells with hypotonic Hanks solutions (190-290 mOsm/kg). Membrane potential transitions were due to a decreased potassium permeability. Noradrenaline elicited both a fast depolarization and a slow depolarization. The fast depolarization was due to an increase in conductance of Na+ channels and of Cl- channels. Intracellular injection of Ca++ elicited the fast depolarization. Intracellular injection of EGTA or cobalt abolished the fast depolarization. Replacement of extracellular Ca++ by Mg++ did not affect the fast depolarization. Thus, the fast depolarization was due to accumulation of intracellular Ca++. The fast depolarization was abolished by the alpha adrenergic blocker phentolamine (10(-6) M), and was not abolished by the beta adrenergic blocker propranolol (10(-5) M).

Electrophysiology of touch sensitive neurons in a mollusc

1. The somata of first-order tactile sensory neurons in the surf clam are located in the central nervous system. 2. Some of these neurons show extensive bilateral branching, and the cell bodies may be multipolar structures. 3. The peripheral receptive fields of the tactile sensory neurons can be either limited to a unique region or may consist of several discrete regions on either side of the animal. 4. Central interaction amoung the different sensory neurons occurs and seems to include a weak electrical coupling as well as inhibitory feedback. In some cases the latter is sufficiently strong to prevent the invasion of a cell body by an orthodromic impulse.

Synaptic control of electronic coupling between neurons

Neurons of the inferior olive (IO) are electrically coupled and show subthreshold oscillations (STOs) in membrane potential that enhance spike synchrony. Spike synchrony among IO neurons is relevant for the fine temporal control of movement and the activation of muscles as collectives, or as “muscle synergies,” during complex movements. Multiple lines of evidence indicate that STOs are an emergent property of the IO network enabled by weak, but prevalent, electrical coupling. We have demonstrated N-methyl-d-aspartate (NMDA) receptor-mediated strengthening of electrical coupling between IO neurons that synchronizes STOs and increases their amplitude. We argue that: (1) weak electrical coupling in the IO makes available a wide dynamic range upon which a strengthening process can modulate STO amplitude and synchrony to have a strong functional impact; and (2) the upward modulation of STO amplitude and synchrony by NMDA receptor-mediated strengthening of electrical synapses may underlie the ability to acquire, through motor learning, the highest degree of temporal precision of synergistic muscle activity during skilled movement.