Intracranial Pharmacotherapy and Pain Assays in Rodents

Here we present a protocol to perform intracranial pharmacological experiments followed by pain behavior assays in rodents. This protocol allows researchers to deliver molecular and cellular targets in the brain, for pharmacologic agents in the treatment of pain.

The main advantage of this technique is that it can be used to further explore pain mechanisms and drug actions in the brain. To study the effect of intracranial injections on acute pain behavior in rats, one week after the surgery, place the rat onto the glass plate of a Hargreaves apparatus, and use the apparatus to focus an infrared beam through the glass plane under the plantar area of one of the rat's feet. To study the synergistic pharmacological effects between two different study drugs, use a piece of PE-50 tubing attached to a 10-microliter Hamilton syringe equipped with a 33-gauge injector cannula that extends one millimeter beyond the implanted guides.

Inject 0.5 microliters or less of the study drug into one intracranial cannula over a period of 100 seconds. Keep the injector cannula in place for an additional 60 seconds after the injection to facilitate a slow diffusion of the solution. Then, intraperitoneally inject the second drug of interest.

20 to 30 minutes after the injection, perform the plantar reflex test five more times as demonstrated. In this representative experiment, the intracranial delivery of an AMPAkine demonstrated an additive analgesic effect when co-administered with intraperitoneally delivered morphine, suggesting that intracranial injections have the power, at least in part, to uncover a mechanism for drug activation in the context of pain. While attempting this procedure, it's important to remember to closely follow the timing of the procedure.

Following this procedure, intracranial implants can be used in conjunction with pain assays to answer additional questions about the region of the brain responsible for a given pain phenotype, possible sites of drug action, or possible pain drug mechanisms.