Parameter-extraction of a two-compartment model for whole-cell data analysis

Spike encoding of neurotransmitter release timing by spiral ganglion neurons of the cochlea

Mammalian cochlear spiral ganglion neurons (SGNs) encode sound with microsecond precision. Spike triggering relies upon input from a single ribbon-type active zone of a presynaptic inner hair cell (IHC). Using patch-clamp recordings of rat SGN postsynaptic boutons innervating the modiolar face of IHCs from the cochlear apex, at room temperature, we studied how spike generation contributes to spike timing relative to synaptic input. SGNs were phasic, firing a single short-latency spike for sustained currents of sufficient onset slope. Almost every EPSP elicited a spike, but latency (300-1500 μs) varied with EPSP size and kinetics. When current-clamp stimuli approximated the mean physiological EPSC (≈300 pA), several times larger than threshold current (rheobase, ≈50 pA), spikes were triggered rapidly (latency, ≈500 μs) and precisely (SD, <50 μs). This demonstrated the significance of strong synaptic input. However, increasing EPSC size beyond the physiological mean resulted in less-potent reduction of latency and jitter. Differences in EPSC charge and SGN baseline potential influenced spike timing less as EPSC onset slope and peak amplitude increased. Moreover, the effect of baseline potential on relative threshold was small due to compensatory shift of absolute threshold potential. Experimental first-spike latencies in response to a broad range of stimuli were predicted by a two-compartment exponential integrate-and-fire model, with latency prediction error of <100 μs. In conclusion, the close anatomical coupling between a strong synapse and spike generator along with the phasic firing property lock SGN spikes to IHC exocytosis timing to generate the auditory temporal code with high fidelity.

A simple method for characterizing passive and active neuronal properties: application to striatal neurons

The study of active and passive neuronal dynamics usually relies on a sophisticated array of electrophysiological, staining and pharmacological techniques. We describe here a simple complementary method that recovers many findings of these more complex methods but relies only on a basic patch-clamp recording approach. Somatic short and long current pulses were applied in vitro to striatal medium spiny (MS) and fast spiking (FS) neurons from juvenile rats. The passive dynamics were quantified by fitting two-compartment models to the short current pulse data. Lumped conductances for the active dynamics were then found by compensating this fitted passive dynamics within the current-voltage relationship from the long current pulse data. These estimated passive and active properties were consistent with previous more complex estimations of the neuron properties, supporting the approach. Relationships within the MS and FS neuron types were also evident, including a graduation of MS neuron properties consistent with recent findings about D1 and D2 dopamine receptor expression. Application of the method to simulated neuron data supported the hypothesis that it gives reasonable estimates of membrane properties and gross morphology. Therefore detailed information about the biophysics can be gained from this simple approach, which is useful for both classification of neuron type and biophysical modelling. Furthermore, because these methods rely upon no manipulations to the cell other than patch clamping, they are ideally suited to in vivo electrophysiology.

Serotoninergic control of glycinergic inhibitory postsynaptic currents in rat hypoglossal motoneurons

This report presents the results of a study of the frequency potentiation of inhibitory postsynaptic currents (IPSCs) in hypoglossal motoneurons and its modulation by serotonin. A release-site model of synaptic plasticity was used to characterize the frequency-related potentiation of evoked IPSCs. Data were obtained to determine if the frequency potentiation of IPSCs occurs as a consequence of a low baseline quantal content of evoked IPSCs using whole cell patch-clamp recordings from hypoglossal motoneurons in the neonatal rat brainstem slice preparation. In these motoneurons, EPSCs and GABAergic IPSCs were blocked by the application of CNQX, AP-5 and bicuculline. Glycinergic IPSCs were evoked by threshold stimulation of inhibitory neurons in the nucleus of Roller, which is located ventro-lateral to the hypoglossal nucleus. IPSC responses to trains of stimuli were recorded in control solutions and in solutions containing serotonin, which is known to reduce IPSPs in this preparation. The amplitude of non-potentiated IPSCs was reduced and their frequency potentiation was enhanced when serotonin was added to the bath. These data were examined using a release-site model of synaptic plasticity in which facilitation is attributed to a time-dependent increase in the probability of transmitter release; depression is attributed to a time-dependent decrease in the number of sites available for release. Using this model, the effect of serotonin on frequency potentiation was explained by a combination of a reduction in the baseline probability of transmitter release and an increase in the time constant of decay of the increase in probability of release that follows a stimulus.