Localization and function of Ih channels in a small neural network

Subthreshold ionic currents, which activate below the firing threshold and shape the cell's firing properties, play important roles in shaping neural network activity. We examined the distribution and synaptic roles of the hyperpolarization-activated inward current ( Ih) in the pyloric network of the lobster stomatogastric ganglion (STG). Ih channels are expressed throughout the STG in a patchy distribution and are highly expressed in the fine neuropil, an area that is rich in synaptic contacts. We performed double labeling for Ih protein and for the presynaptic marker synaptotagmin. The large majority of labeling in the fine neuropil was adjacent but nonoverlapping, suggesting that Ih is localized in close proximity to synapses but not in the presynaptic terminals. We compared the pattern of Ih localization with Shal transient potassium channels, whose expression is coregulated with Ih in many STG neurons. Unlike Ih, we found significant levels of Shal protein in the soma membrane and the primary neurite. Both proteins were found in the synaptic fine neuropil, but with little evidence of colocalization in individual neurites. We performed electrophysiological experiments to study a potential role for Ih in regulating synaptic transmission. At a synapse between two identified pyloric neurons, the amplitude of inhibitory postsynaptic potentials (IPSPs) decreased with increasing postsynaptic activation of Ih. Pharmacological block of Ih restored IPSP amplitudes to levels seen when Ih was not activated. These experiments suggest that modulation of postsynaptic Ih might play an important role in the control of synaptic strength in this rhythmogenic neural network.

EEG Generator—A Model of Potentials in a Volume Conductor

EEG generator—a model of potentials in a volume conductor. The potential recorded over the cortex electro-corticogram (ECoG) or over the scalp [electroencephalograph (EEG)] derives from the activity of many sources known as “EEG generators.” The recorded amplitude is basically a function of the unitary potential of a generator and the statistical relationship between different EEG generators in the recorded population. In this study, we first suggest a new definition of the EEG generator. We use the theory of potentials in a volume conductor and model the contribution of a single synapse activated to the surface potential. We then model the contribution of the generator to the surface potential. Once the generator and its contribution are well defined, we can quantitatively assess the degree of synchronization among generators. The measures obtained by the model for a real life scenario of a group of generators organized in a specific statistical way were consistent with the expected values that were reported experimentally. The study sheds new light on macroscopic modeling approaches which make use of mean soma membrane potential. We showed major contribution of activity of superficial apical synapses to the ECoG signal recorded relative to lower somatic or basal synapses activity.

Estimating the electrotonic structure of neurons with compartmental models

1. A procedure based on compartmental modeling called the "constrained inverse computation" was developed for estimating the electrotonic structure of neurons. With the constrained inverse computation, a set of N electrotonic parameters are estimated iteratively with use of a Newton-Raphson algorithm given values of N parameters that can be measured or estimated from experimental data. 2. The constrained inverse computation is illustrated by several applications to the basic example of a neuron represented as one cylinder coupled to a soma. The number of unknown parameters estimated was different (ranging from 2 to 6) when different sets of constraints were chosen. The unknowns were chosen from the following: dendritic membrane resistivity Rmd, soma membrane resistivity Rms, intracellular resistivity Ri, membrane capacity Cm, dendritic membrane area AD, soma membrane area As, electrotonic length L, and resistivity-free length, rfl (rfl = 2l/d1/2 where l and d are length and diameter of the cylinder). The values of the unknown parameters were estimated from the values of an equal number of known parameters, which were chosen from the following: the time constants and coefficients of a voltage transient tau 0, tau 1, ..., C0, C1, ..., voltage-clamp time constants tau vc1, tau vc2, ..., and input resistance RN. Note that initially, morphological data were treated as unknown, rather than known. 3. When complete morphology was not known, parameters from voltage and current transients, combined with the input resistance were not sufficient to completely specify the electrotonic structure of the neuron. For a neuron represented as a cylinder coupled to a soma, there were an infinite number of combinations of Rmd, Rms, Ri, Cm, AS, AD, and L that could be fitted to the same voltage and current transients and input resistance. 4. One reason for the nonuniqueness when complete morphology was not specified is that the Ri estimate is intrinsically bound to the morphology. Ri enters the inverse computation only in the calculation of the electrotonic length of a compartment. The electrotonic length of a compartment is l[4 Ri/(dRmd)]1/2, where l and d are the length and diameter of the compartment. Without complete morphology, the inverse computation cannot distinguish between a change in d or l and a change in Ri. Even when morphology is known, the accuracy of the Ri estimate obtained by any fitting procedure is affected by systematic errors in length and diameter measurements (i.e., tissue shrinkage); the Ri estimate is inversely proportional to the length measurement and proportional to the square root of the diameter measurement.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Mechanisms of Spiking in Normally ‘Non-Spiking’ Motoneurone Somata in Crayfish

The soma membrane of the abdominal giant motor neurone (MoG) of the crayfish was studied by use of current injection and by measurements of total and local membrane current under voltage-clamp. Depolarization of the soma from the recorded resting potential (about −70 mV) did not produce a regenerative potential. Sodium and calcium currents were observed under voltage-clamp. It was shown that the sodium spike cannot develop at the usual resting potential since sodium channels are already partially inactivated; the spike can appear after the removal of inactivation by a hyperpolarizing prepulse. It was also shown that an early outward current prevents the Ca2+; regenerative potential; the potential can appear after inactivation of this outward current by a depolarizing prepulse.

The effects of axotomy upon the extrasynaptic acetylcholine sensitivity of an identified motoneurone in the cockroach Periplaneta americana

The effects of axotomy on the sensitivity of the fast coxal depressor motoneurone (Df) of the cockroach (Periplaneta americana) to applied acetylcholine (ACh) and carbamylcholine (CCh) have been investigated. ACh and CCh applied to the soma membrane either by bath perfusion or by ionophoresis caused depolarization; repeated application of large doses of these agonists resulted in a relatively rapid desensitization and depression of the response. Axotomy performed 3–10 days before recordings were made caused an approximately threefold increase in the sensitivity to ACh but had no affect upon the sensitivity to CCh. The resting potential, input resistance and membrane time-constant remained within the normal range. In addition there was no change in the rise-times of the responses or of the ACh reversal potential, or in the apparent number of ACh molecules needed to combine with individual cholinoceptors to produce a response. The anticholinesterases physostigmine and neostigmine potentiated the ACh response at relatively low concentrations (10(−7) M-10(−6) M). This potentiation was significantly greater in the normal cells than in the axotomized cells. It is therefore concluded that the increase in ACh sensitivity of this motoneurone results at least partly from a fall in the activity of cholinesterase in the region of the applied drug.