A mong the variety of roles for nanopores in biology, an important one is enabling polymer transport, for example in gene transfer between bacteria 1 and transport of RNA through the nuclear membrane 2 . Recently, this has inspired the use of protein 3-5 and solid-state 6-10 nanopores as single-molecule sensors for the detection and structural analysis of DNA and RNA by voltage-driven translocation. The magnitude of the force involved is of fundamental importance in understanding and exploiting this translocation mechanism, yet so far it has remained unknown. Here, we demonstrate the first measurements of the force on a single DNA molecule in a solid-state nanopore by combining optical tweezers 11 with ionic-current detection. The opposing force exerted by the optical tweezers can be used to slow down and even arrest the translocation of the DNA molecules. We obtain a value of 0.24 ± 0.02 pN mV −1 for the force on a single DNA molecule, independent of salt concentration from 0.02 to 1 M KCl. This force corresponds to an effective charge of 0.50 ± 0.05 electrons per base pair equivalent to a 75% reduction of the bare DNA charge.It is possible to manipulate DNA molecules using electric fields because DNA is negatively charged in solution. Confining an electrical field to a nanopore enables the study of voltagedriven DNA translocation where the force is only applied to the few monomers that are inserted in the nanopore. We can calculate the electrical force F el on the DNA in the nanopore as F el = (q eff (z)/a)E(z)dz, where q eff is the effective charge of a DNA base pair, E(z) is the position-dependent electrical field in our system, a is the distance between two base pairs, and the integral is taken along the DNA contour. Assuming that q eff is identical for every base pair leads to F el = (q eff /a) E(z)dz = q eff V /a, with V the applied potential across the nanopore. The simplicity of this formula stems from the translational invariance of our system, in which the contour length of the DNA exceeds the length of the nanopore.

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Although feed-forward inhibition onto Purkinje cells was first documented forty years ago, we still understand little of how inhibitory interneurons contribute to cerebellar function in behaving animals. Using a mouse line (PC-Δγ2) in which GABAA receptor-mediated synaptic inhibition was selectively removed from Purkinje cells, we examined how feed-forward inhibition from molecular layer interneurons regulates adaptation of the vestibulo-ocular reflex. Whereas impairment of baseline motor performance was relatively mild, the ability to adapt the phase of the vestibulo-ocular reflex and to consolidate gain adaptations, was strongly compromised. Purkinje cells showed abnormal patterns of simple spikes, both during and in the absence of evoked compensatory eye movements. Based on modeling of the experimental data, we propose that feed-forward inhibition, by controlling the fine scale patterns of Purkinje cell activity, enables the induction of plasticity in neurons of the cerebellar and vestibular nuclei.

The level of electrotonic coupling in the inferior olive is extremely high, but its functional role in cerebellar motor control remains elusive. Here, we subjected mice that lack olivary coupling to paradigms that require learning-dependent timing. Cx36-deficient mice showed impaired timing of both locomotion and eye-blink responses that were conditioned to a tone. The latencies of their olivary spike activities in response to the unconditioned stimulus were significantly more variable than those in wild-types. Whole-cell recordings of olivary neurons in vivo showed that these differences in spike timing result at least in part from altered interactions with their subthreshold oscillations. These results, combined with analyses of olivary activities in computer simulations at both the cellular and systems level, suggest that electrotonic coupling among olivary neurons by gap junctions is essential for proper timing of their action potentials and thereby for learning-dependent timing in cerebellar motor control.