A simple method to microinject solid neural tracers into deep structures of the brain

We have developed an instrument to perform microinjections of solid neural tracers into deep structures of the brain. The instrument consists of a thi n hypodermic needle equipped with a movable internal rod, which is connecte d to a pressure chamber. When a pressure pulse is applied to the chamber, t he rod moves forward and back inside the needle, pushing out a solid load p reviously packed inside the needle tip. By attaching a microelectrode to th e instrument, it is also possible to have electrophysiological control of t he injection placement. To test the instrument, we microinjected DiI and rh odamine crystals into selected structures of the visual system of pigeons. The results show small, well-defined injection sites, accurately located in the desired targets, together with well-developed anterogade and retrograd e transport, selectively originated from the injection sites. This method e xtends the usage of solid tracers to most structures in the brain and may, in certain cases, be more advantageous than the conventional method of inje cting tracer solutions. (C) 2001 Elsevier Science B.V. All rights reserved.