Quantal Characteristics of Synaptic GABA Release under Conditions of Stimulation of Single Synaptic Terminals of Cultured Cortical Cells of the Rat

We studied evoked inhibitory postsynaptic currents (eIPSC) using local electrical stimulation of single presynaptic terminals of cultured rat neocortical neurons. According to pharmacological and kinetic properties, these currents were qualified as GABA_A-activated. Using autocorrelation analysis of distributions of the eIPSC amplitudes, which were in all cases polymodal, we examined quantal characteristics of the above eIPSC. These results were compared with the values of quantal parameters (N, p, Q, and m) of the current families obtained using approximation by binomial distribution. Amplitude histograms of spontaneous miniature IPSC recorded under conditions of the minimum quantal release of the neurotransmitter were normal (close to Gaussian) with the mode within a 10 pA range, which is very close to analogous parameters calculated using autocorrelation and binomial techniques.     

Nifedipine-sensitive low voltage-activated calcium current in neurons of the rat laterodorsal thalamic nucleus

The kinetic and pharmacological properties of low voltage-activated (LVA) Ca²⁺ channels were studied in neurons of the laterodorsal (LD) thalamic nucleus in brain slices from 12-day-old rats. A homogeneous population of LVA Ca²⁺ channels was found in the tested neurons. LVA Ca²⁺ current evoked by a step depolarization from a holding potential more negative than −70 mV was found to be sensitive to nifedipine (K _d=2.6 (M). This current gained its maximum at −55 mV and demonstrated fast monoexponential decay with the time constant of 32.3±4.0 msec (n=15). Lanthanum (1 μM) effectively blocked LVA Ca²⁺ current, while nickel (25 μM) did not affect this current. It is concluded that the channels that, according to their pharmacological properties, provide the studied LVA Ca²⁺ current cannot be regarded as T-type Ca²⁺ channels and belong to some other type of LVA Ca²⁺ channels.     

Assessment of the Reliability of Amplitude Histograms for Inhibitory Synaptic Currents in Rat Cortex Culture

We analyzed in detail the quantum parameters of evoked inhibitory postsynaptic currents (eIPSC) recorded from synaptically connected cultured cortex neurons using a whole-cell patch-clamp technique. The IPSC were evoked using minimum extracellular stimulation of a presynaptic unit with a frequency of 0.2 sec^-1 at the holding potential of -80 mV. Amplitude histograms for eIPSC demonstrated clearly detectable equally spaced peaks. For each histogram, we used a method based on autocorrelation analysis and Monte Carlo simulation to determine whether peaks in the amplitude histograms can result due to finite sampling from the sum of the Gaussian distributions. The autocorrelation function allowed us to measure the peak spacing (and, hence, the mean quantum size) for each histogram; this parameter was found to be 10 pA.     

GABA-Mediated Inhibition in Cultured Neurons of the Rat Brain Cortex

In cultured pyramidal neurons of the rat brain cortex, we recorded (in the whole-cell configuration) postsynaptic currents (PSC) evoked by direct electrical microstimulation of an axon of the interneuron adjacent to the pyramidal cell. Application of 5 M bicuculline rapidly, entirely, and reversibly blocked these currents. Linear changes in the holding potential on the membrane of the postsynaptic cell resulted in linear changes in the amplitude of averaged currents. The currents underwent reversion when the holding potential was –16 mV, which was close to the reversal potential for Cl^- ions at their respective concentrations in the extra- and intracellular solutions. We conclude that the recorded currents are inhibitory PSC (IPSC) mediated by GABA release. The amplitudes of the recorded currents varied from a measurable minimum (8 pA) to more than 150 pA at a holding potential on the postsynaptic cell membrane of –80 mV. Times to peak of the high- and low-amplitude currents showed no significant differences, being about 6.4 msec on average. Decays of the current could be satisfactorily approximated by a monoexponential function with a mean time constant of 17 msec. The time constants of IPSC decay were distributed accordingly to the Gaussian law. In some cases, the amplitude distributions of IPSC were unimodal ((with a rightward asymmetry), but in most cases they were clearly polymodal. The amplitude distribution can be described by the sum of several Gaussian distributions; the distance between modes of the Gaussians was 25 ± 6 pA, on average. The obtained estimates of the amplitude of monoquantal GABA-induced IPSC in neurons of the brain cortex allow us to conclude that in various CNS regions the dimension of the vesicles in GABA-ergic synapses formed by inhibitory interneurons is identical.     

Quantum Properties of GABA Release in Cultured Neurons from the Rat Cortex

We analyzed the properties of inhibitory synaptic transmission between neurons in low-density cultures of cortical cells. Miniature, spontaneous, and evoked inhibitory postsynaptic currents were studied using a whole-cell path-clamp technique at a holding potential of -80 mV. These postsynaptic currents were identified as GABA release-activated Cl^- currents mediated by GABA_A receptors. Fitting amplitude histograms of these currents with Gaussian curves and an autocorrelation technique revealed the presence of equidistant peaks corresponding to a mean quantum size of 10 pA.