TRESK two-pore-domain K+ channels constitute a significant component of background potassium currents in murine dorsal root ganglion neurones.

TRESK (TWIK-related spinal cord K(+) channel) is the most recently identified member of the two-pore-domain potassium channel (K(2P)) family, the molecular source of background potassium currents. Human TRESK channels are not affected by external acidification. However, the mouse orthologue displays moderate pH dependence isolated to a single histidine residue adjacent to the GYG selectivity filter. In the human protein, sequence substitution of tyrosine by histidine at this critical position generated a mutant that displays almost identical proton sensitivity compared with mouse TRESK. In contrast to human TRESK, which is specifically located in spinal cord, we detected mouse TRESK (mTRESK) mRNA in several epithelial and neuronal tissues including lung, liver, kidney, brain and spinal cord. As revealed by endpoint and quantitative RT-PCR, mTRESK channels are mainly expressed in dorsal root ganglia (DRG) and on the transcript level represent the most important background potassium channel in this tissue. DRG neurones of all sizes were labelled by in situ hybridizations with TRESK-specific probes. In DRG neurones of TRESK[G339R] functional knock-out (KO) mice the standing outward current IK(so) was significantly reduced compared with TRESK wild-type (WT) littermates. Different responses to K(2P) channel regulators such as bupivacaine, extracellular protons and quinidine corroborated the finding that approximately 20% of IK(so) is carried by TRESK channels. Unexpectedly, we found no difference in resting membrane potential between DRG neurones of TRESK[WT] and TRESK[G339R] functional KO mice. However, in current-clamp recordings we observed significant changes in action potential duration and amplitude of after-hyperpolarization. Most strikingly, cellular excitability of DRG neurones from functional KO mice was significantly augmented as revealed by reduced rheobase current to elicit action potentials.

Synaptic depression and short-term habituation are located in the sensory part of the mammalian startle pathway.

BACKGROUND: Short-term habituation of the startle response represents an elementary form of learning in mammals. The underlying mechanism is located within the primary startle pathway, presumably at sensory synapses on giant neurons in the caudal pontine reticular nucleus (PnC). Short trains of action potentials in sensory afferent fibers induce depression of synaptic responses in PnC giant neurons, a phenomenon that has been proposed to be the cellular correlate for short-term habituation. We address here the question whether both this synaptic depression and the short-term habituation of the startle response are localized at the presynaptic terminals of sensory afferents. If this is confirmed, it would imply that these processes take place prior to multimodal signal integration, rather than occurring at postsynaptic sites on PnC giant neurons that directly drive motor neurons. RESULTS: Patch-clamp recordings in vitro were combined with behavioral experiments; synaptic depression was specific for the input pathway stimulated and did not affect signals elicited by other sensory afferents. Concordant with this, short-term habituation of the acoustic startle response in behavioral experiments did not influence tactile startle response amplitudes and vice versa. Further electrophysiological analysis showed that the passive properties of the postsynaptic neuron were unchanged but revealed some alterations in short-term plasticity during depression. Moreover, depression was induced only by trains of presynaptic action potentials and not by single pulses. There was no evidence for transmitter receptor desensitization. In summary, the data indicates that the synaptic depression mechanism is located presynaptically. CONCLUSION: Our electrophysiological and behavioral data strongly indicate that synaptic depression in the PnC as well as short-term habituation are located in the sensory part of the startle pathway, namely at the axon terminals of sensory afferents in the PnC. Our results further corroborate the link between synaptic depression and short-term habituation of the startle response.

Synaptic plasticity in the acoustic startle pathway: the neuronal basis for short-term habituation?

The aim of the present study was to analyse the cellular mechanism underlying short-term habituation of the acoustic startle response (ASR). We explored distinct synapses of the neuronal startle pathway in rat brain slices by patch-clamp recordings of giant neurons in the caudal pontine reticular formation. Presynaptic stimulation of auditory afferents by repeated bursts at 0.1 and 1 Hz led to an exponential decay of EPSC magnitudes. This homosynaptic depression (HSD) was reversible and repeatedly inducible after recovery. Many parameters of HSD in vitro match those of ASR habituation in vivo. The mechanisms underlying HSD are distinct from classical short-term plasticity: paired-pulse as well as paired-burst stimulation revealed a facilitation of the second EPSC, occurring in a much smaller time window up to interstimulus intervals of 200 ms. Pharmacological experiments demonstrated that HSD could be completely blocked by the group II and III metabotropic glutamate receptor antagonist MPPG. Similar results were obtained by CPPG, another group II and III antagonist. In contrast, HSD was not affected by the group I and II antagonist MCPG. We conclude that we found a form of synaptic depression in synapses within the primary startle pathway which correlates in many respects with short-term habituation of the ASR and which is presumably mediated by group III metabotropic glutamate receptors.