The overall goal of this procedure is to evaluate the effects of deep brain stimulation that is temporally and spatially separate from the drug use environment of intravenous methamphetamine in rodents. This method can help answer key questions in addiction psychiatry, such as:can electrical stimulation of discrete brain regions decrease drug abuse, and under what circumstances is this therapy most effective? The main advantage of this technique is that the electrical therapy is delivered at a different time and in a different setting than the drug use environment.
This more closely approximates what will be possible in human patients. Demonstrating this procedure will be Dr.Vinita Batra, one of our post-doctoral fellows in our laboratory, and Mr.Glenn Guerin, our head laboratory technician. Preparations for this experiment are outlined in the text protocol.
To begin, load the rats into the operant chambers as quickly and calmly as possible to minimize behavioral artifacts. Flush the rat's catheter with 0.1 milliliters of 0.9%saline solution to ensure patency of the line prior to the beginning of the experiment. Next, attach a stainless steel spring leash to the guide cannula on the rodent's back.
Connect the other end of the cannula to a leak-proof fluid swivel above the operant chamber. In order for the rats to rapidly learn the self-administration task, run the sessions for six hours per day over four to five consecutive days, and always around the same time of day. For each active lever press, provide one infusion of methamphetamine, followed by a 30 second timeout where the lever supplies nothing.
By the end of the first week, the rodents will be adept at administering themselves methamphetamine. During the second week of training, run the rats on daily two hour sessions, Monday through Friday, to maintain and refine their IV methamphetamine self administration. Continue conducting the sessions on a fixed ratio of one, with thirty second timeouts.
Stable, intense responding is reached when the total number of methamphetamine infusions across each three consecutive sessions varies by less than ten percent. Another indicator of stable intense responding occurs when the cumulative number of infusions across the first thirty minutes is greater than the cumulative number of infusions during the second thirty minutes. When the rats develop this drug loading pattern, it indicates addictive behavior, and not simply casual use.
At the end of each session, prepare a syringe to flush the catheter and disconnect the leash from the rodent's back. Flush the rat's catheter with 0.1 milliliters of 0.9%saline solution containing 800 IU streptokinase to prevent blood clots. Following the flush, insert a sheath onto each guide cannula to prevent clogging.
Then, return the rat to its home cage. See the text protocol on testing the patency of the catheters and how to address common problems with this experiment. Prepare ten to twelve Plexiglas boxes for this experiment.
On each box, cover the outside of three walls with stiff opaque paper to prevent the rats from seeing each other. However, leave the front wall clear to view the animals during the stimulation sessions. Next, partially cover the tops of the boxes with a hard panel to prevent the rats from escaping, but still allowing air flow.
On the top panel, support the commutators for the electrical connection between the rodent head cap and the stimulation system. Use a stimulation system that can deliver constant current to multiple simultaneous animals for the DBS experiments. It should include a programmable interface.
With custom length cables, connect the stimulators'channel ports to the superior electronic pedestal of each commutator. Then, connect the inferior electronic pedestal of the commutator to the implanted electrode pedestal on the rodent's head cap using 16 inch cables encased in a stainless steel spring. The cable should allow free movement for the rat to every area of the enclosure without creating significant tension on the head cap.
A cable that reaches to where the rat's head could go when it is on four feet is usually long enough. To program the system, use a visual programming language to specify which functions each device will perform to meet the experimental endpoints, and which data will be stored and/or projected for viewing in real time. Specify the desired frequency, pulse width, and amplitude into the visual control panel prior to the start of the experiment.
Typical parameters for high frequency stimulation in rats are similar to those used in clinical human deep brain stimulation. A frequency of 130 to 180 Hertz, a pulse width of 60 to 90 milliseconds, and a current amplitude of 100 to 250 microamps. For the brain stimulation experiment, when loading the rats into the boxes, attach the stainless steel spring cable from the commutator to each electrode pedestal on the head cap.
First test the impedance of each electrode using five microamps of current at 1000 Hertz for two seconds. If an electrode's impedance is equal to or less than 125 kilo Ohms, then proceed with the experiment. But if not, consider removing the animal from the experiment because the electrode's resistance may truncate the current to potentially sub-therapeutic levels.
Start with one or two mock sessions to habituate the rats. Don't apply any active therapy during these sessions. Immediately following each mock session, transport the rats to the operant boxes for their daily two hour session of IV methamphetamine self administration.
For the experiment, counterbalance the rats into two groups, an active stimulation cohort and a sham stimulation cohort that gets mock sessions. Perform the daily deep brain stimulation sessions for five days, for three hours a day. Observe animals carefully during a portion of each stimulation session to note if the stimulation is causing any clear alteration in behavior.
Directly following every deep brain stimulation session, start the rats'daily IV methamphetamine self administration session. Following the placement of intravenous jugular catheters and intracranial deep brain stimulation electrodes, rats acquired and escalated drug self administration after two days of extended access to methamphetamine. Next, the rats were moved to a daily two hour schedule of operant training to prevent methamphetamine toxicity and to establish a stable rate of responding that could be manipulated by various therapeutic interventions.
By day six of operant training, the rats developed an increased motivation to take the drug, as indicated by the emergence of a front loading pattern of intake. This pattern was largely sustained over the subsequent sessions. Following the establishment of this stabilized drug abuse pattern, deep brain stimulation was administered per the described protocol.
This resulted in a marked decrease of operant IV methamphetamine self administration. Once mastered, this technique can be completed in a two to four week time frame, using about ten to twelve animals per group. This is ideally suited to test the effects of deep brain stimulation, given the limited lifespan of head caps and IV catheters in rodents using methamphetamine.
This procedure may be used to investigate alternate electrical parameters, different brain targets, and novel delivery patterns, as well as combinations of electrical therapy and pharmaceutical agents that may result in long lasting behavioral modification.
