Douglas Ling

Physiological Properties of Cortical Inhibitory Circuits
Our research efforts focus on the physiology of GABAergic inhibitory circuits and the regulation of excitation in the mammalian cortex. We are studying the properties of inhibitory neurons and circuits which define the operational limits of fast GABA_Aergic inhibition in order to understand the control and modulation of synaptic transmission.

Normal activity in the central nervous system is supported by the balance between excitation and inhibition. Neuropathologies, such as epilepsy, may result from the hyperexcitability which develops when inhibition fails to check excitation. The neuronal circuits in layer V of neocortex seem particularly vulnerable to the generation of unrestrained excitability. One factor contributing to this predisposition may be an intrinsic limit on the recruitment of inhibition in this region. The recruitment of inhibition, specifically fast GABA_Aergic inhibition, appears to be strictly limited relative to excitation.

We are using whole-cell voltage-clamp recordings in cortical slices to examine the restrictions on recruitment of fast inhibition. From recordings of isolated postsynaptic currents in layer V pyramidal cells, we found that the recruitment of evoked GABA_A-mediated inhibition is strictly limited relative to excitation, reaching a maximal peak amplitude at stimulation levels which evoked submaximal levels of excitation. Our results also showed that the limit on inhibition is insensitive to conditions which modulate NMDA-mediated glutamatergic excitation, suggesting that inhibitory circuits in this brain region are driven predominantly by non-NMDA (AMPA/kainate) excitatory transmission.

To elucidate the factors underlying the limit on inhibition in the neocortex, we examined the kinetic properties of evoked fast inhibitory postsynaptic currents (IPSCs). As stimulus levels were increased beyond the point at which maximal-amplitude IPSCs were recruited, the time constant of current decay increased as a function of stimulus strength (with an attendant increase in the total charge flux), while IPSC rise time remained unaffected. Modulation of NMDA transmission to increase (presumed) polysynaptic excitatory drive to interneurons yielded similar results. These findings suggested that the limit on evoked inhibition arises, at least in part, from a saturation of postsynaptic GABA_A receptors. We also examined spontaneous miniature IPSCs, using quantal analysis to obtain functional estimates of the number of inhibitory synapses on target pyramidal cells. The mean amplitude of miniature IPSCs and quantal events were both relatively small (~20 pA and ~7 pA, respectively). Comparisons to previously published data on single channel currents and evoked IPSCs suggested 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.

Most recently, we have begun to investigate the effects of non-NMDA receptor modulators on the recruitment of fast inhibition. The results of these studies to date suggest that excitatory drive to deep layer interneurons is mediated primarily by AMPA receptors and that the recruitment of fast inhibition can be enhanced by modulators of AMPA receptor activity.

Further investigations will involve direct whole-cell recordings from cortical inhibitory interneurons and dual recordings from interneuron-pyramidal cell pairs. Our continuing investigations of fast inhibition and inhibitory interneurons should further our understanding of the control of synaptic inhibition and may provide insights in to the therapeutic regulation of inhibitory strength.

Figure 1. Recruitment of isolated, evoked postsynaptic currents in neocortical layer V pyramidal cells. A, Monopolar cathodal shocks applied in cortical layer VI evoked excitatory and inhibitory postsynaptic currents (EPSCs, IPSCs) in layer V pyramidal cells. Purely outward IPSCs were evoked at holding potentials of 0 mV (top) and purely inward EPSCs were evoked at -75 mV (bottom) with graded stimulus intensities ranging from 2 V to 10 V. B, Input-output relationship of isolated EPSCs (#) and fast IPSCs (!). EPSCs increased in amplitude with increasing stimulus intensity, but IPSCs consistently reached a maximal level at < 2-times the threshold stimulus. C, The ratio of EPSC amplitude to IPSC amplitude was plotted against relative stimulus intensity (mean ± SEM; n=17). Initially the EPSC/IPSC ratio was approximately equal to 1, but once the IPSC plateaued, the ratio approached 2.5 at the highest stimulus intensities tested.

Selected Publications

Ling, D. S. F., Petroski, R. E., and Geller, H. M. (1991). Both survival and development of rat hypothalamic neurons in dissociated culture are dependent on membrane depolarization. Dev. Brain Res. 59, 99-103.

Ling, D. S. F. and Benardo, L. S. (1994). Properties of isolated GABA_B-mediated IPSCs in hippocampal pyramidal cells. Neuroscience 63, 937-944.

Ling, D. S. F. and Benardo, L. S. (1995). Activity-dependent depression of monosynaptic fast IPSCs in hippocampus: contributions from reductions in chloride driving force and conductance. Brain Res. 670, 142-146.

Ling, D. S. F. and Benardo, L. S. Recruitment of GABA_A inhibition in rat neocortex is limited and not NMDA-dependent. (1995). J. Neurophysiol. 74, 2329-2335.

Ling, D. S. F. and Benardo, L. S. (1997). Epilepsies. In Encyclopedia of Human Biology, 2nd ed, vol 3 (R. Dulbecco, ed.). Academic Press, San Diego, pp. 799-805.

Ling, D. S. F. and Benardo, L. S. (1998). Synchronous firing of inhibitory interneurons results in saturation of fast GABA_A IPSC magnitude but not saturation of fast inhibitory efficacy in rat neocortical pyramidal cells. Synapse 28, 91-102.

Ling, D. S. F. and Benardo, L. S. (1999). Restrictions on inhibitory circuits contribute to limited recruitment of fast inhibition in rat neocortical pyramidal cells in vitro. J. Neurophysiol. 82, 1793-1807.

Ling, D. S. F., Benardo, L. S., Serrano, P. A., Blace, N., Kelly, M. T., Crary, J. F., and Sacktor, T. C. (2002). Protein kinase Mζ is necessary and sufficient for LTP maintenance. Nat. Neurosci. 5, 295-296.

Ling, D. S. F. and Benardo, L. S. (2005). Nootropic agents enhance the recruitment of fast GABA_A inhibition in rat neocortex. Cerebral Cortex 15, 921-928.

Ling, D. S. F., Benardo, L. S., and Sacktor, T. C. Protein kinase Mζ enhances excitatory synaptic transmission by increasing the number of active postsynaptic AMPA receptors. Hippocampus, 2006 (ePub).

List of Publications (Pub Med)