Hydraulically coupled microejection technique for precise local solution delivery in tissues

Hydraulically coupled volume microejection (HCVM) was employed to overcome known drawbacks of conventional pressure ejection of drugs in isolated brain slices and other tissues. For HCVM, a solution-filled glass micropipette is connected to a motor-driven, liquid-filled syringe by low-compliance, liquid-filled tubing, and the system is sealed leaving no gaps or air bubbles. The volume ejected from the micropipette is the same as the volume displaced by movement of the syringe plunger (it is not much influenced by the diameter of the micropipette tip or its clogging by tissue debris), so the ejection rate and duration can be precisely controlled. The HCVM performance was characterized by fluorescent imaging of ejected dyes, its combination with infrared/differential interference contrast (IR/DIC) imaging in brain slices, and by concurrent patch-clamp recording of ejected drug effects in individual neurons. Ejection of varied volumes into a brain slice (1.6–400 nl) formed a globular shape that transiently displaced the tissue adjacent to the micropipette tip. The radial penetration of the ejected dye correlated well with the electrophysiological responses to a concurrently ejected drug, at least for brief (100–200 ms) intervals after the ejection. The penetration distance could be increased by increasing either the ejection volume, or the ejection rate, or both. However, at higher ejection volumes (100–400 nl), a notable fraction of the ejectate was “pushed” out of the slice by tissue resiliency, and escaped to the outside by a back-flow along the ejection pipette. The use of HCVM enabled us to achieve a concentration rise time (10–90%) on the order of 20–40 ms. These and other measurements demonstrated principal capabilities and limitations of HCVM and provided guidance for its practical use, including some potentially unique applications.