Stress-free microinjections in conscious rats

Abstract

Microinjections are a major tool in modern neuroscience. Microinjection techniques in conscious animals typically involve four steps: 1) animal adapts to experimental setup; 2) injection system is filled and the microinjector is carefully inserted; 3) a drug solution is injected; 4) one to two minutes later the microinjector is carefully removed. Steps 2 and 4 are difficult to perform in rodents without disturbing the animal. This disruption can cause stress and accompanying tachycardia and hyperthermia - unwanted artifacts in physiological research. To reduce these effects, we altered the traditional approach. Our procedure of microinjection consisted of the following steps: 1) We filled the injection setup and fixed the microinjector in its guide cannula; 2) allowed an animal to adapt to the setup; 3) performed an experiment including microinjection(s); 4) removed the microinjector after the experiment was complete. The key change we incorporated was a one meter long piece of tubing with a small internal diameter; it allowed us to inject nanoliter volumes through the injector which had been placed into the guide cannula in advance. This way we avoided the usual manipulations related to microinjection, and minimized extraneous disturbances to the rat. In this report we describe the details of this technique in conscious rats and provide examples of the effects and the reproducibility of a 100 nL drug injection on cardiovascular function.