Microelectrode Injection, Axonal Iontophoresis, and the Structure of Neurons

Abstract

This paper is a critique of two methods of filling identified neurons: microelectrode injection and axonal iontophoresis. Fig. 1 illustrates the differences between these two methods. Since most of the work reviewed in this book has been done with microelectrode injection, I am restricting my comments on this approach to generalities and concentrating on several applications of axonal iontophoresis. These demonstrate its limitations and advantages and introduce the methodological variations which have achieved the best results in specific systems. The microelectrode injection approach to filling neurons has some limitations. First, you must be able to penetrate and hold a physiologically-identified cell long enough to fill it. Iontophoresis may require an hour or more, and to hold a small cell this long often demands technical heroics (however, see Chapter 15 for a description of the “one-shot” technique). Second, each neuron must be filled individually. For most of us, that means one at a time. This may not be a problem, since the structure of a particular cell or the points of contact between two cells is frequently the information sought. But when one wants information about the structure and distribution of groups of cells, it can be very tedious to get using micropipettes. The alternative of filling the neurons by iontophoresing stain into their cut axons (lies and Mulloney, 1971) can provide a solution in some cases to each of these difficulties. I shall limit my discussion to invertebrate preparations, since Chapter 15 and the discussion following it detail axonal iontophoresis in vertebrates.