Restrictions on Inhibitory Circuits Contribute to Limited Recruitment of Fast Inhibition in Rat Neocortical Pyramidal Cells

Abstract

To further define the operational boundaries on fast inhibition in neocortex, whole cell recordings were made from layer V pyramidal neurons in neocortical slices to evaluate evoked inhibitory postsynaptic currents (IPSCs) and spontaneous miniature IPSCs (mIPSCs). Stimulating electrodes were placed in layers VI and I/II to determine whether simultaneous stimulation of deep and superficial laminae could extend the magnitude of maximal IPSCs evoked by deep-layer stimulation alone. The addition of superficial-layer stimulation did not increase maximal IPSC amplitude, confirming the strict limit on fast inhibition. Spontaneous miniature IPSCs were recorded in the presence of tetrodotoxin. The frequency of spontaneous mIPSCs ranged from 10.0 to 33.1 Hz. mIPSC amplitude varied considerably, with a range of 5. 0-128.2 pA and a mean value of 20.7+/-4.1 pA (n = 12 cells). The decay phase of miniature IPSCs was best fit by a single exponential, similar to evoked IPSCs. The mean time constant of decay was 6.4+/-0.6 ms, with a range of 0.2-20.1 ms. The mean 10-90% rise time was 1.9+/-0.2 ms, ranging from 0.2 to 6.3 ms. Evaluation of mIPSC kinetics revealed no evidence of dendritic filtering. Amplitude histograms of mIPSCs exhibited skewed distributions with several discernable peaks that, when fit with Gaussian curves, appeared to be spaced equidistantly, suggesting that mIPSC amplitudes varied quantally. The mean separation of Gaussian peaks ranged from 6.1 to 7.8 pA. The quantal distributions did not appear to be artifacts of noise. Exposure to saline containing low Ca(2+) and high Mg(2+) concentrations reduced the number of histogram peaks, but did not affect the quantal size. Mean mIPSC amplitude and quantal size varied with cell holding potential in a near-linear manner. Statistical evaluation of amplitude histograms verified the multimodality of mIPSC amplitude distributions and corroborated the equidistant spacing of peaks. Comparison of mIPSC values with published data from single GABA channel recordings suggests that the mean mIPSC conductance corresponds to the activation of 10-20 GABA(A) receptor channels, and that the release of a single inhibitory quantum opens 3-6 channels. Further comparison of mIPSCs with evoked inhibitory events suggests that a single interneuron may form, on average, 4-12 functional synapses with a pyramidal cell, and that 10-12 individual interneurons are engaged during recruitment of maximal population IPSCs. This suggests that inhibitory circuits are much more restricted in both the size of the unit events and effective number of connections when compared with excitatory inputs.

A Reconfiguration of CaV2 Ca2+ Channel Current and Its Dopaminergic D2 Modulation in Developing Neostriatal Neurons

“…7B). These events may come from interneurons or spiny cell terminals and fitted to Gaussian functions with a first modal amplitude of 8 pA (range, 5-12 pA), similar to that reported for other GABAergic synapses (e.g., Ling and Benardo 1999). The second mode was 16 pA (range, 12-20 pA) (cf., Paulsen and Heggelund 1994).…”

“…The amplitudes histogram of these eIPSCs was multimodal (Fig. 7, C and D), obtaining values of 9 and 17 pA for the first modes; the interval between peaks was 8 -9 pA (Ling and Benardo 1999). Similarity between sIPSCs and eIPSCs histograms supports the notion that terminals that interconnect spiny neurons are fairly homogeneous , and their modal values can now be used to independently ascertain the results of the mean-variance analysis (Clements and Silver 2000).…”

Dopamine–Glutamate Interactions Controlling Prefrontal Cortical Pyramidal Cell Excitability Involve Multiple Signaling Mechanisms

“…First, interneurons could be selectively activated by NMDA. This is unlikely because spontaneous and evoked IPSCs in pyramidal neurons are blocked by AMPA and not NMDA antagonists (Salin and Prince, 1996a;Ling and Benardo, 1999). Another possibility is that D2-activated interneurons release GABA acting on receptors located on NMDA-rich areas of the pyramidal cell membrane, enabling a GABA shunting effect only when NMDA receptors are activated.…”

Acute Injury to Superficial Cortex Leads to a Decrease in Synaptic Inhibition and Increase in Excitation in Neocortical Layer V Pyramidal Cells

“…As a routine check, all events detected were visually inspected and those that did not exhibit the general shape expected for synaptic currents were rejected for analysis. To select for events generated more proximally and thus minimize potential contributions from space-clamp error and dendritic filtering, only events with 10 -90% rise times Ͻ1.5 ms were included in our analyses (Ling and Benardo 1999). Due to the variability in threshold stimulus values among neurons, input-output relationships were constructed using relative stimulus intensities normalized to the threshold stimulus value (T) of each cell (Li and Prince 2002), defined as the intensity that recruits the smallest discernable evoked synaptic current.…”