Modelling the postsynaptic location and magnitude of tonic conductance changes resulting from neurotransmitters or drugs

Selective shunting of the NMDA EPSP component by the slow afterhyperpolarization in rat CA1 pyramidal neurons

Pyramidal neuron dendrites express voltage-gated conductances that control synaptic integration and plasticity, but the contribution of the Ca(2+)-activated K(+)-mediated currents to dendritic function is not well understood. Using dendritic and somatic recordings in rat hippocampal CA1 pyramidal neurons in vitro, we analyzed the changes induced by the slow Ca(2+)-activated K(+)-mediated afterhyperpolarization (sAHP) generated by bursts of action potentials on excitatory postsynaptic potentials (EPSPs) evoked at the apical dendrites by perforant path-Schaffer collateral stimulation. Both the amplitude and decay time constants of EPSPs (tau(EPSP)) were reduced by the sAHP in somatic recordings. In contrast, the dendritic EPSP amplitude remained unchanged, whereas tau(EPSP) was reduced. Temporal summation was reduced and spatial summation linearized by the sAHP. The amplitude of the isolated N-methyl-D-aspartate component of EPSPs (EPSP(NMDA)) was reduced, whereas tau(NMDA) was unaffected by the sAHP. In contrast, the sAHP did not modify the amplitude of the isolated EPSP(AMPA) but reduced tau(AMPA) both in dendritic and somatic recordings. These changes are attributable to a conductance increase that acted mainly via a selective "shunt" of EPSP(NMDA) because they were absent under voltage clamp, not present with imposed hyperpolarization simulating the sAHP, missing when the sAHP was inhibited with isoproterenol, and reduced under block of EPSP(NMDA). EPSPs generated at the basal dendrites were similarly modified by the sAHP, suggesting both a somatic and apical dendritic location of the sAHP channels. Therefore the sAHP may play a decisive role in the dendrites by regulating synaptic efficacy and temporal and spatial summation.

Hypocretinergic control of spinal cord motoneurons

Hypocretinergic (orexinergic) neurons in the lateral hypothalamus project to motor columns in the lumbar spinal cord. Consequently, we sought to determine whether the hypocretinergic system modulates the electrical activity of motoneurons. Using in vivo intracellular recording techniques, we examined the response of spinal motoneurons in the cat to electrical stimulation of the lateral hypothalamus. In addition, we examined the membrane potential response to orthodromic stimulation and intracellular current injection before and after both hypothalamic stimulation and the juxtacellular application of hypocretin-1. It was found that (1) hypothalamic stimulation produced a complex sequence of depolarizing- hyperpolarizing potentials in spinal motoneurons; (2) the depolarizing potentials decreased in amplitude after the application of SB-334867, a hypocretin type 1 receptor antagonist; (3) the EPSP induced by dorsal root stimulation was not affected by the application of SB-334867; (4) subthreshold stimulation of dorsal roots and intracellular depolarizing current steps produced spike potentials when applied in concert to stimulation of the hypothalamus or after the local application of hypocretin-1; (5) the juxtacellular application of hypocretin-1 induced motoneuron depolarization and, frequently, high-frequency discharge; (6) hypocretin-1 produced a significant decrease in rheobase (36%), membrane time constant (16.4%), and the equalizing time constant (23.3%); (7) in a small number of motoneurons, hypocretin-1 produced an increase in the synaptic noise; and (8) the input resistance was not affected after hypocretin-1. The juxtacellular application of vehicle (saline) and denatured hypocretin-1 did not produce changes in the preceding electrophysiological properties. We conclude that hypothalamic hypocretinergic neurons are capable of modulating the activity of lumbar motoneurons through presynaptic and postsynaptic mechanisms. The lack of hypocretin-induced facilitation of motoneurons may be a critical component of the pathophysiology of cataplexy.

Determination of the Location and Magnitude of Synaptic Conductance Changes in Spinal Motoneurons by Impedance Measurements

The relation between impedance change and the location and magnitude of a tonic synaptic conductance was examined in compartmental motoneuron models based on previously published data. The dependency of motoneuron impedance on system time constant (τ), electrotonic length (L), and dendritic-to-somatic conductance ratio (ρ) was examined, showing that the relation between impedance phase and ρ differed markedly between models with uniform and nonuniform membrane resistivity. Dendritic synaptic conductances decreased impedance magnitude at low frequencies; at higher frequencies, impedance magnitude increased. The frequency at which the change in impedance magnitude reversed from a decrease to an increase—the reversal frequency, Fr—was a good estimator of electrotonic synaptic location. A measure of the average normalized impedance change at frequencies less than Fr, cuΔZ, estimated relative synaptic conductance. Fr and cuΔZ provided useful estimates of synaptic location and conductance in models with nonuniform (step, sigmoidal) and uniform membrane resistivity. Fr also provided good estimates of spatial synaptic location on the equivalent cable in both step and sigmoidal models. Variability in relations between Fr, cuΔZ, and conductance location and magnitude between neurons was reduced by normalization with ρ and τ. The effects on Fr and cuΔZ of noise in experimental recordings, different synaptic distributions, and voltage-dependent conductances were also assessed. This study indicates that location and conductance of tonic dendritic conductances can be estimated from Fr, cuΔZ, and basic electrotonic motoneuron parameters with the exercise of suitable precautions.

Changes in electrophysiological properties of cat hypoglossal motoneurons during carbachol-induced motor inhibition

The control of hypoglossal motoneurons during sleep is important from a basic science perspective as well as to understand the bases for pharyngeal occlusion which results in the obstructive sleep apnea syndrome. In the present work, we used intracellular recording techniques to determine changes in membrane properties in adult cats in which atonia was produced by the injection of carbachol into the pontine tegmentum (AS-carbachol). During AS-carbachol, 86% of the recorded hypoglossal motoneurons were found to be postsynaptically inhibited on the basis of analyses of their electrical properties; the electrical properties of the remaining 14% were similar to motoneurons recorded during control conditions. Those cells that exhibited changes in their electrical properties during AS-carbachol also displayed large-amplitude inhibitory synaptic potentials. Following sciatic nerve stimulation, hypoglossal motoneurons which responded with a depolarizing potential during control conditions exhibited a hyperpolarizing potential during AS-carbachol. Both spontaneous and evoked inhibitory potentials recorded during AS-carbachol were comparable to those that have been previously observed in trigeminal and spinal cord motoneurons under similar experimental conditions as well as during naturally occurring active sleep. Calculations based on modeling the changes that we found in input resistance and membrane time constant with a three-compartment neuron model suggest that shunts are present in all three compartments of the hypoglossal motoneuron model. Taken together, these data indicate that postsynaptic inhibitory drives are widely distributed on the soma-dendritic tree of hypoglossal motoneurons during AS-carbachol. These postsynaptic inhibitory actions are likely to be involved in the pathophysiology of obstructive sleep apnea.

Postnatal changes in electrophysiological properties of rat nucleus tractus solitarii neurons

Whole-cell recordings in brainstem slices revealed postnatal changes in passive and firing properties in the rat caudal nucleus tractus solitarii (cNTS) neurons. Membrane potential, threshold for Na+ spike and degree of sag were unchanged during development. In the adult, the rheobase was twice that found at birth. The input resistance decreased over the period studied, while time constants declined markedly after the third postnatal week. At all postnatal ages, Na(+)-dependent action potentials (APs) were elicited in response to depolarization. Nevertheless, AP duration gradually decreased by 40% over the developmental period studied. Spike amplitude was smaller at birth than at any other ages and reached a peak two weeks after birth. At all ages, Na(+)-dependent APs were blocked by application of tetrodotoxin. Full APs were replaced by an initial slow oscillation in young cells and by oscillations in older cells. The TTX-resistant oscillations were altered by cobalt (2 mM) and cadmium (100 microM). The spike afterhyperpolarization (AHP) was not altered during development, but was observed in less neurons in adult cells when measured at a holding potential of -60 mV. Neurons were subdivided into one of three classes based on their responses to a hyperpolarizing prepulse: 1) post-inhibitory rebound (PIR) cells, 2) delayed excitation (DE) cells and 3) NON cells expressing neither PIR nor DE. The relative proportions of different cell types varied with age. The mean maximum duration of DE increased three times. Voltage-clamp experiments revealed that the DE was due to the activation of an A-current. In addition, a three-fold increase in its inactivation rate was observed postnatally. The physiological significance of these results is discussed.

Dorsal spinocerebellar tract neurons are not subjected to postsynaptic inhibition during carbachol-induced motor inhibition

Dorsal spinocerebellar tract (DSCT) neurons in Clarke's column in the lumbar spinal cord of cats anesthetized with alpha-chloralose were recorded intracellularly. The membrane potential activity and electrophysiological properties of these neurons were examined before and during the state of active-sleep-like motor inhibition induced by the injection of carbachol into the nucleus pontis oralis. The synaptic activity of DSCT neurons during carbachol-induced motor inhibition did not change compared with that during control conditions. In particular, there was an absence of inhibitory postsynaptic potentials (IPSPs) in high-gain recordings from DSCT neurons and the resting membrane potential of DSCT neurons was not significantly hyperpolarized during carbachol-induced motor inhibition. The mean amplitude of both monosynaptic excitatory postsynaptic potentials and disynaptic IPSPs evoked in DSCT neurons following stimulation of group I muscle afferents after the injection of carbachol was similar to that evoked before the injection of carbachol. There were no significant changes in the mean input resistance and membrane time constant of DSCT neurons during carbachol-induced motor inhibition. We conclude that, in contrast to lumbar motoneurons, DSCT neurons in Clarke's column are not postsynaptically inhibited during carbachol-induced motor inhibition. Therefore the population of spinal cord Ib interneurons that inhibit both DSCT neurons and lumbar motoneurons is not likely to be the interneurons that are responsible for the postsynaptic inhibition of motoneurons that occurs during carbachol-induced motor inhibition. The present findings also indicate that transmission through the DSCT is not modulated by postsynaptic inhibition at the level of DSCT neurons during carbachol-induced motor inhibition.

Electrophysiological properties of lumbar motoneurons in the alpha-chloralose-anesthetized cat during carbachol-induced motor inhibition

The present study was undertaken 1) to examine the neuronal mechanisms responsible for the inhibition of spinal cord motoneurons that occurs in alpha-chloralose-anesthetized cats following the microinjection of carbachol into the nucleus pontis oralis (NPO), and 2) to determine whether the inhibitory mechanisms are the same as those that are responsible for the postsynaptic inhibition of motoneurons that is present during naturally occurring active sleep. Accordingly, the basic electrophysiological properties of lumbar motoneurons were examined, with the use of intracellular recording techniques, in cats anesthetized with alpha-chloralose and compared with those present during naturally occurring active sleep. The intrapontine administration of carbachol resulted in a sustained reduction in the amplitude of the spinal cord Ia monosynaptic reflex. Discrete large-amplitude inhibitory postsynaptic potentials (IPSPs), which are only present during the state of active sleep in the chronic cat, were also observed in high-gain recordings from lumbar motoneurons after the injection of carbachol. During carbachol-induced motor inhibition, lumbar motoneurons exhibited a statistically significant decrease in input resistance, membrane time constant and a reduction in the amplitude of the action potential's afterhyperpolarization. In addition, there was a statistically significant increase in rheobase and in the delay between the initial-segment (IS) and somadendritic (SD) portions of the action potential (IS-SD delay). There was a significant increase in the mean motoneuron resting membrane potential (i.e., hyperpolarization). The preceding changes in the electrophysiological properties of motoneurons, as well as the development of discrete IPSPs, indicate that lumbar motoneurons are postsynaptically inhibited after the intrapontine administration of carbachol in cats that are anesthetized with alpha-chloralose. These changes in the electrophysiological properties of lumbar motoneurons were found to be comparable with those that take place during the atonia of active (rapid-eye-movement) sleep in chronic cats. The present results support the conclusion that the neural system that is responsible for motor inhibition during naturally occurring active sleep can also be activated in alpha-chloralose-anesthetized cats following the injection of carbachol into the NPO.

Apical dendritic location of slow afterhyperpolarization current in hippocampal pyramidal neurons: implications for the integration of long-term potentiation

Trains of action potentials in hippocampal pyramidal neurons are followed by a prolonged afterhyperpolarization (AHP) lasting several seconds, which is attributable to the activation of a slow calcium-activated potassium current ((sI)AHP). Here we examine the location of (sI)AHP on CA1 pyramidal neurons by comparing it with two GABAergic inhibitory postsynaptic currents (IPSCs) with known somatic and dendritic locations. Whole-cell patch-clamp recordings were made for CA1 pyramidal neurons in acute hippocampal slices. Stepping the membrane potential at the peak of (sI)AHP produced a relaxation ("switchoff") of the AHP current with a time constant of 7.4 +/- 0.4 msec (mean +/- SEM). The switchoff time constants for somatic and dendritic GABAA IPSCs were 3.5 +/- 0.5 msec and 8.8 +/- 0.3 msec, respectively. This data, together with cable modeling, indicates that active (sI)AHP channels are distributed over the proximal dendrites within approximately 200 micrometers of the soma. Excitatory postsynaptic potentials (EPSPs) evoked in stratum (s.) radiatum had their amplitudes shunted more by the AHP than did EPSPs evoked in s. oriens, suggesting that active AHP channels are restricted to the apical dendritic tree. Blockade of the AHP during a tetanus, which in control conditions elicited a decremental short-term potentiation (STP), converted STP to long-term potentiation (LTP). Thus, activation of the AHP increases the threshold for induction of LTP. These results suggest that in addition to its established role in spike frequency adaptation, the AHP works as an adjustable gain control, variably hyperpolarizing and shunting synaptic potentials arising in the apical dendrites.

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Large reductions in composite monosynaptic EPSP amplitude following conditioning stimulation are not accounted for by increased postsynaptic conductances in motoneurons

A compartmental neuronal model was used to show that increased compartmental leak conductances distal to the site of excitatory postsynaptic potential (EPSP) generation have little effect on EPSP amplitude but decrease half-width markedly. Using intracellular recording from cat hindlimb motoneurons, reductions of composite Ia EPSP amplitude by up to 70% unaccompanied by reductions in half-width were seen following conditioning stimuli to hindlimb nerves. Appropriate condition-test intervals produced large reductions in EPSP amplitude that were unaccompanied by detectable increases in motoneuron conductance. These observations suggest that presynaptic inhibition and not increased postsynaptic motoneuron conductances is responsible for large EPSP amplitude reductions following conditioning stimulation.

A system model for investigating passive electrical properties of neurons

Passive membrane properties of neurons, characterized by a linear voltage response to constant current stimulation, were investigated by busing a system model approach. This approach utilizes the derived expression for the input impedance of a network, which simulates the passive properties of neurons, to correlate measured intracellular recordings with the response of network models. In this study, the input impedances of different network configurations and of dentate granule neurons, were derived as a function of the network elements and were validated with computer simulations. The parameters of the system model, which are the values of the network elements, were estimated using an optimization strategy. The system model provides for better estimation of the network elements than the previously described signal model, due to its explicit nature. In contrast, the signal model is an implicit function of the network elements which requires intermediate steps to estimate some of the passive properties.

Computation of the passive electrical parameters of neurons using a system model

Time-domain analysis of voltage responses to current pulse stimulation has been used to estimate the electrotonic parameters of neurons using the signal model. Errors are likely to accumulate from various steps of the analysis due to noise and electrode artifacts. A system model, which has inherent noise immunity and filtering properties, is presented here. This model employs frequency-domain analysis of the input impedance of a neuronal model (an RC cable). The resistances and capacitances of the system model are estimated from the cell-input impedance using an optimization strategy. Using the expression for the input impedance, any specified number of equalizing time constants can be computed exactly. The accessibility to these equalizing time constants 1) provides greater insight into the charge equalization along the length and circumference of the cable, and 2) improves the estimation of all other passive parameters including the electrotonic length. Thus, the system model approach allows information to be extracted more directly and accurately than the signal model approach.

Electrotonic properties of neostriatal neurons are modulated by extracellular potassium

In order to assess the effects of [K+]o on the passive membrane properties of neostriatal neurons, the cable properties of these cells were determined at two extracellular potassium concentrations (6.25 and 3.0 mM). The effect of tetraethylammonium (TEA) on cable properties was also studied at 6.25 [K+]o. At 6.25 mM [K+]o, the mean input resistance at the resting membrane potential (RMP), and the mean membrane time constant (tau o) were 27 +/- 1.5 M omega and 6.9 +/- 0.5 ms respectively (n = 17), while at 3 mM [K+]o they were 62.9 +/- 4.8 M omega and 14.3 +/- 0.6 ms (n = 15) (mean +/- SEM). With one of the methods used to calculate the electronic parameters, the total electrotonic length of the dendrites (L) and the dendritic to somatic conductance ratio (rho) were 1.3 +/- 0.05 and 5 +/- 0.8 at the higher [K+]o respectively, while they were 0.95 +/- 0.04 and 3 +/- 0.7 at the lower [K+]o. Cells were depolarized in 6.25 as compared to 3 mM [K+]o (RMP = -66 +/- 1.3 mV vs RMP = -80.5 +/- 1.4 mV). After one hour exposure to TEA (10 mM), the input resistance and time constant tripled at 6.25 mM [K+]o. TEA slightly depolarized the cells bathed in 6.25 mM [K+]o. The results suggest that changes in [K+]o, within the physiological range, markedly affect the cable properties of neostriatal neurons, possibly modifying subthreshold, voltage-dependent K+-conductances. TEA seems to block some of these channels.

Measurements of dendritic conductance changes to GABA in granule cells of the rat dentate gyrus

The magnitude of dendritic conductance changes occurring distantly from the somatic site of recording can be difficult to measure. We have used measurements of the neuronal time constant, tau 0, instead of the neuronal input resistance, RN, to estimate the resistance decrease that accompanies the depolarizing response of the dendrites of granule cells when GABA is applied. From the changes in tau 0, we estimated the reversal potential of the response and found that the conductance change accompanying a given GABA-mediated voltage response as measured at the cell body was the same regardless of where in the dendritic tree the drug was applied. On the other hand, RN changes underestimated the increase in conductance of the GABA responses in the distal dendrites and were not accurate for determining the reversal potential.

The somatic shunt cable model for neurons

The derivation of the equations for an electrical model of nerve cells is presented. The model consists of an equivalent cylinder, a lumped somatic impedance, and a variable shunt at the soma. This shunt was introduced to take into account the fast voltage decays observed following the injections of current pulses in some motoneurons and hippocampal granule cells that could not be explained by existing models. The shunt can be interpreted either by penetration damage with the electrode or by a lower membrane specific resistance at the soma than in the dendrites. A solution of the model equations is presented that allows the estimation of the electrotonic length L, the membrane time constant tau m, the dendritic dominance ratio rho, and the shunt parameter epsilon, based only on the measurement of the first two coefficients and time constants in the multiexponential voltage response to injected current pulses.

Unequal diameters and their effects on time-varying voltages in branched neurons

A theoretical method, developed in a previous paper, enables one to calculate analytical expressions for time-varying voltages at specific locations in branching dendritic systems in response to synaptic current inputs at other sites. Exact results were obtained for a number of dendritic trees that possessed certain symmetries: all branch lengths had to be integral multiples of one another, and all branch diameters had to be equal. Because the second of these conditions is unrealistic, the method has been generalized to treat dendritic trees whose branches differ in diameter. The method entails adding onto the symmetric results a sum of correction terms. It is found that the correction terms, as well as the symmetric results, can be expressed as combinations of two families of functions. These functions, generalizations of those found in our earlier paper, provide a precise formalism for analyzing how voltage transients depend on the geometrical structure of the dendritic tree. Examples are given that show how the correction terms affect the value of the voltage, and how variations in branch diameters alter the behavior of the propagated postsynaptic potential. The implications of these results for our understanding of neuronal functioning are discussed.