Relative contributions of passive equilibrium and active transport to the distribution of chloride in mammalian cortical neurons

Abstract

1. Active and passive factors affecting the chloride gradient of cortical neurons were assessed using intracellular recordings from neurons in slices of cingulate cortex maintained in vitro. The chloride equilibrium potential (ECl-) was estimated indirectly from the reversal potentials of responses to perisomatic gamma-aminobutyric acid (GABA) application and the Cl(-)-dependent inhibitory postsynaptic potential (IPSP). Under control conditions the mean resting potential (Vm; -69.7 mV) was not significantly different than the mean IPSP reversal potential (EIPSP; -70.1 mV). 2. Increasing the external potassium concentration ([K+]o) from 1 to 10 mM shifted the mean EIPSP from -80.4 to -61.8 mV. The mean EIPSP was approximately equal to the mean Vm at all [K+]oS. The conditions of Donnan equilibrium are not met in [K+]o less than 10 mM. 3. Polarization of Vm up to 20 mV away from EIPSP for 4 min with maintained current injection had no significant effect on EIPSP. 4. The GABA reversal potential was maintained 37-52 mV less negative than Vm after equilibration in saline in which the external chloride concentration had been reduced from 133 to 5 mM by substitution with isethionate. Vm and input resistance were not significantly different from control values in cells recorded under these conditions. 5. We conclude that Cl- is not passively distributed in cortical neurons, perhaps due to a low resting Cl- permeability. 6. Impalement with electrodes containing 2 M KCl resulted in a rapid 10 mV depolarizing shift in EIPSP that then remained relatively constant. Intracellular iontophoresis of Cl- resulted in a further depolarizing shift of EIPSP of 5-10 mV that returned to control in less than 1 min. The time course of recovery of IPSP amplitude could be fit with a single exponential having a mean time constant of 6.9 +/- 1.5 s and was independent of the amount of Cl- injected or stimulation frequency. 7. Reductions in temperature from 37 to 32 degrees C significantly increased the mean time constant of IPSP recovery from Cl- injection to 11.1 +/- 3.3 s, corresponding to Q10 = 2.6.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency‐dependent depression of inhibition in guinea‐pig neocortex in vitro by GABAB receptor feed‐back on GABA release.

“…two distinct components of inhibition can be evoked by orthodromic stimulation of sufficient intensity. The differences in reversal potentials and conductances found here corroborate previous conclusions that these events are mediated by entirely different conductance mechanisms, an early Cl--dependent and a late K+-dependent component (see Thompson, Deisz & Prince, 1988b). Before discussing the mechanisms contributing to the pronounced frequency dependence, we would like to emphasize some features of JPSPs pertinent to the issues addressed here.…”

“…1988a). The rather low experimental temperature used in earlier experiments (McCarren & Alger, 1985) might have impaired temperature-sensitive transport mnechanisms such as Cl-extrusion (Thompson et al , 1988b) and the Na'-K' pump Contribution of postsynaptic sensitivity changes to IPSP decline A crucial experiment for excluding postsynaptic sensitivity changes as a cause for the decline in IPSP conductance would be its comparison with GABA-induced conductance before and after a series of IPSPs. Such a comparison is not possible within the required accuracy, since GABA application reduced IPSP.…”

Optical Current Source Density Analysis in Hippocampal Organotypic Culture Shows That Spreading Depression Occurs with Uniquely Reversing Currents

“…Our typical SD induction paradigm includes transient exposure to Ringer's solution in which chloride is replaced with acetate (Kunkler and Kraig, 1998a). This likely prompts a reversal of IPSPs (Thompson et al, 1988), which when coupled to a single bipolar electrical pulse from the dentate gyrus, triggers SD in CA3 Kraig, 1998a, 2004). Furthermore, the P/Q calcium channel blocker -agatoxin-IVA stops SD in HOTCs .…”

Simulations of cortical pyramidal neurons synchronized by inhibitory interneurons

“…A maximal IPSP conductance of 90 nS was used, corresponding to the value measured by changes in input impedance in layer II/III of cat visual cortex slices in response to cathode stimulation of underlying white matter (Connors et al 1988). This was assumed to be a chloride-mediated a-aminobutyric acid-A (GABA,) conductance with a reversal potential of -75 mV, 10 mV below resting membrane potential (Thompson et al 1988).…”