Rate maintenance and resonance in the entorhinal cortex

Coapplication of noisy patterned electrical stimuli and NMDA plus serotonin facilitates fictive locomotion in the rat spinal cord

A new stimulating protocol [fictive locomotion-induced stimulation (FL istim)], consisting of intrinsically variable weak waveforms applied to a single dorsal root is very effective (though not optimal as it eventually wanes away) in activating the locomotor program of the isolated rat spinal cord. The present study explored whether combination of FL istim with low doses of pharmacological agents that raise network excitability might further improve the functional outcome, using this in vitro model. FL istim was applied together with N-methyl-d-aspartate (NMDA) + serotonin, while fictive locomotion (FL) was electrophysiologically recorded from lumbar ventral roots. Superimposing FL istim on FL evoked by these neurochemicals persistently accelerated locomotor-like cycles to a set periodicity and modulated cycle amplitude depending on FL istim rate. Trains of stereotyped rectangular pulses failed to replicate this phenomenon. The GABAB agonist baclofen dose dependently inhibited, in a reversible fashion, FL evoked by either FL istim or square pulses. Sustained episodes of FL emerged when FL istim was delivered, at an intensity subthreshold for FL, in conjunction with subthreshold pharmacological stimulation. Such an effect was, however, not found when high potassium solution instead of NMDA + serotonin was used. These results suggest that the combined action of subthreshold FL istim (e.g., via epidural stimulation) and neurochemicals should be tested in vivo to improve locomotor rehabilitation after injury. In fact, reactivation of spinal locomotor circuits by conventional electrical stimulation of afferent fibers is difficult, while pharmacological activation of spinal networks is clinically impracticable due to concurrent unwanted effects. We speculate that associating subthreshold chemical and electrical inputs might decrease side effects when attempting to evoke human locomotor patterns.

State-dependent modulation of gap junction signaling by the persistent sodium current

Thalamic neurons fluctuate between two states: a hyperpolarized state associated with burst firing and sleep spindles, and a depolarized state associated with tonic firing and rapid, reliable information transmission between the sensory periphery and cortex. The thalamic reticular nucleus (TRN) plays a central role in thalamocortical processing by providing feed-forward and feedback inhibition to thalamic relay cells; TRN cells participate in the generation of sleep spindles, and have been suggested to focus the neural “searchlight” of attention. The mechanisms underlying synchrony in the TRN during different behavioral states are largely unknown. TRN cells are densely interconnected by electrical synapses. Here we show that activation of the persistent sodium current (I_NaP) by depolarization causes up to fourfold changes in electrical synaptic efficacy between TRN neurons. We further show that amplification of electrical synaptic responses strongly enhances tonic spike synchrony but, surprisingly, does not affect burst coordination. We use a Hodgkin–Huxley model to gain insight into the differences between the effects of burstlets, spikelets, and amplification on burst and spike times.

Using computer simulations to determine the limitations of dynamic clamp stimuli applied at the soma in mimicking distributed conductance sources

In previous studies we used the technique of dynamic clamp to study how temporal modulation of inhibitory and excitatory inputs control the frequency and precise timing of spikes in neurons of the deep cerebellar nuclei (DCN). Although this technique is now widely used, it is limited to interpreting conductance inputs as being location independent; i.e., all inputs that are biologically distributed across the dendritic tree are applied to the soma. We used computer simulations of a morphologically realistic model of DCN neurons to compare the effects of purely somatic vs. distributed dendritic inputs in this cell type. We applied the same conductance stimuli used in our published experiments to the model. To simulate variability in neuronal responses to repeated stimuli, we added a somatic white current noise to reproduce subthreshold fluctuations in the membrane potential. We were able to replicate our dynamic clamp results with respect to spike rates and spike precision for different patterns of background synaptic activity. We found only minor differences in the spike pattern generation between focal or distributed input in this cell type even when strong inhibitory or excitatory bursts were applied. However, the location dependence of dynamic clamp stimuli is likely to be different for each cell type examined, and the simulation approach developed in the present study will allow a careful assessment of location dependence in all cell types.