Synergistic Effect of Intracranial and Intraperitoneal Delivery of Analgesic Drugs on Thermal Pain Response in a Rat

Begin with a secured, anesthetized rat with implanted guide cannulas in the prefrontal cortex of its brain, a region associated with pain regulation.

Take a Hamilton syringe containing an analgesic or pain-relieving drug connected with an injector cannula via tubing.

Attach the injector cannula to the guide cannula and administer the drug intracranially over a predetermined period.

Administer a second analgesic drug intraperitoneally to study the synergistic effect of the two drugs.

Allow the animal to recover.

Place the freely moving animal on Hargreave’s apparatus.

Focus an infrared beam through the glass onto the rat’s paw.

As the rat withdraws its paw in response to thermal pain, the withdrawal latency time is recorded by the apparatus.

Analyze the data to evaluate the effect of intracranial injection and the co-administration of analgesic drugs on thermal pain perception.

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 planter 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 1 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.