The spatial distribution of ion channels in different subcellular regions is a
key determinant of neuronal behaviour. Patch clamp electrophysiology allows
characterisation of ion channel activity, but precise localisation is more
difficult. This is particularly true for very small, specialised compartments
such as synaptic terminals, which are inaccessible by conventional, direct
patch recording methods.
Scanning ion conductance microscopy (SICM) generates high resolution topographic
images by using a precisely positioned probe to measure ion currents.
The development of a new ‘hopping’ mode allows convoluted neuronal
networks to be imaged using the SICM probe. Some details of the
implementation of this mode are described. The geometry of SICM pipette
tips is examined, and the interaction between the probe and the cell membrane
is shown to differ from the standard account.
Application of SICM to localised patch clamp recording has previously been
demonstrated in several cell types and here is extended to record from
presynaptic sites. Control experiments are performed and a model is introduced
to explain how the use of fine-tipped SICM pipettes may give rise
to artefacts seen in some of these experiments. The technique is then applied
to synaptic boutons in primary cerebellar culture and a number of
successful recordings are presented, along with some attempts to combine
the advantages of SICM positioning with those of more conventional patch
clamp pipettes. Experimental limitations of these approaches are discussed
